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"page_content": "**Respiratory System. Figure 1-1** shows that each time blood passes through the body, it also flows through the lungs. The blood picks up *oxygen* in alveoli, thus acquiring the oxygen needed by cells. The membrane between the alveoli and the lumen of the pulmonary capillaries, the *alveolar membrane*, is only 0.4 to 2.0 micrometers thick, and oxygen rapidly diffuses by molecular motion through this membrane into the blood. \n**Gastrointestinal Tract.** A large portion of the blood pumped by the heart also passes through the walls of the gastrointestinal tract. Here different dissolved nutrients, including *carbohydrates*, *fatty acids*, and *amino acids*, are absorbed from ingested food into the extracellular fluid of the blood. \nLiver and Other Organs That Perform Primarily Metabolic Functions. Not all substances absorbed from the gastrointestinal tract can be used in their absorbed form by the cells. The liver changes the chemical compositions of many of these substances to more usable forms, and other tissues of the body—fat cells, gastrointestinal mucosa, kidneys, and endocrine glands—help modify the absorbed substances or store them until they are needed. The liver also eliminates certain waste products produced in the body and toxic substances that are ingested. \n**Musculoskeletal System.** How does the musculoskeletal system contribute to homeostasis? The answer is obvious and simple. Were it not for the muscles, the body could not move to obtain the foods required for nutrition. The musculoskeletal system also provides motility for protection against adverse surroundings, without which the entire body, along with its homeostatic mechanisms, could be destroyed. \n#### **REMOVAL OF METABOLIC END PRODUCTS** \n**Removal of Carbon Dioxide by the Lungs.** At the same time that blood picks up oxygen in the lungs, *carbon dioxide* is released from the blood into lung alveoli; the respiratory movement of air into and out of the lungs carries carbon dioxide to the atmosphere. Carbon dioxide is the most abundant of all the metabolism products. \n**Kidneys.** Passage of blood through the kidneys removes most of the other substances from the plasma besides carbon dioxide that are not needed by cells. These substances include different end products of cellular metabolism, such as urea and uric acid; they also include excesses of ions and water from the food that accumulate in the extracellular fluid. \nThe kidneys perform their function first by filtering large quantities of plasma through the glomerular capillaries into the tubules and then reabsorbing into the blood substances needed by the body, such as glucose, amino acids, appropriate amounts of water, and many of the ions. Most of the other substances that are not needed by the body, especially metabolic waste products such as urea and creatinine, are reabsorbed poorly and pass through the renal tubules into the urine. \n**Gastrointestinal Tract.** Undigested material that enters the gastrointestinal tract and some waste products of metabolism are eliminated in the feces. \n**Liver.** Among the many functions of the liver is detoxification or removal of ingested drugs and chemicals. The liver secretes many of these wastes into the bile to be eventually eliminated in the feces. \n#### **REGULATION OF BODY FUNCTIONS** \n**Nervous System.** The nervous system is composed of three major parts—the *sensory input portion,* the *central nervous system* (or *integrative portion*), and the *motor output portion.* Sensory receptors detect the state of the body and its surroundings. For example, receptors in the skin alert us whenever an object touches the skin. The eyes are sensory organs that give us a visual image of the surrounding area. The ears are also sensory organs. The central nervous system is composed of the brain and spinal cord. The brain stores information, generates thoughts, creates ambition, and determines reactions that the body performs in response to the sensations. Appropriate signals are then transmitted through the motor output portion of the nervous system to carry out one's desires. \nAn important segment of the nervous system is called the *autonomic system.* It operates at a subconscious level and controls many functions of internal organs, including the level of pumping activity by the heart, movements of the gastrointestinal tract, and secretion by many of the body's glands. \n**Hormone Systems.** Located in the body are *endocrine glands,* organs and tissues that secrete chemical substances called *hormones.* Hormones are transported in the extracellular fluid to other parts of the body to help regulate cellular function. For example, *thyroid hormone* increases the rates of most chemical reactions in all cells, thus helping set the tempo of bodily activity. *Insulin* controls glucose metabolism, *adrenocortical hormones* control sodium and potassium ions and protein metabolism, and *parathyroid hormone* controls bone calcium and phosphate. Thus, the hormones provide a regulatory system that complements the nervous system. The nervous system controls many muscular and secretory activities of the body, whereas the hormonal system regulates many metabolic functions. The nervous and hormonal systems normally work together in a coordinated manner to control essentially all the organ systems of the body. \n#### **PROTECTION OF THE BODY** \n**Immune System.** The immune system includes white blood cells, tissue cells derived from white blood cells, the thymus, lymph nodes, and lymph vessels that protect the body from pathogens such as bacteria, viruses, parasites, and fungi. The immune system provides a mechanism for the body to carry out the following: (1) distinguish its own cells from harmful foreign cells and substances; and (2) destroy the invader by *phagocytosis* or by producing *sensitized lymphocytes* or specialized proteins (e.g., *antibodies*) that destroy or neutralize the invader. \n**Integumentary System.** The skin and its various appendages (including the hair, nails, glands, and other structures) cover, cushion, and protect the deeper tissues and organs of the body and generally provide a boundary between the body's internal environment and the outside world. The integumentary system is also important for temperature regulation and excretion of wastes, and it provides a sensory interface between the body and the external environment. The skin generally comprises about 12% to 15% of body weight. \n#### **REPRODUCTION** \nAlthough reproduction is sometimes not considered a homeostatic function, it helps maintain homeostasis by generating new beings to take the place of those that are dying. This may sound like a permissive usage of the term *homeostasis,* but it illustrates that in the final analysis, essentially all body structures are organized to help maintain the automaticity and continuity of life. \n#### CONTROL SYSTEMS OF THE BODY \nThe human body has thousands of control systems. Some of the most intricate of these systems are genetic control systems that operate in all cells to help regulate intracellular and extracellular functions. This subject is discussed in Chapter 3. \nMany other control systems operate *within the organs* to regulate functions of the individual parts of the organs; others operate throughout the entire body *to control the interrelationships between the organs.* For example, the respiratory system, operating in association with the nervous system, regulates the concentration of carbon dioxide in the extracellular fluid. The liver and pancreas control glucose concentration in the extracellular fluid, and the kidneys regulate concentrations of hydrogen, sodium, potassium, phosphate, and other ions in the extracellular fluid. \n#### **EXAMPLES OF CONTROL MECHANISMS** \n**Regulation of Oxygen and Carbon Dioxide Concentrations in the Extracellular Fluid.** Because oxygen is one of the major substances required for chemical reactions in cells, the body has a special control mechanism to maintain an almost exact and constant oxygen concentration in the extracellular fluid. This mechanism depends principally on the chemical characteristics of *hemoglobin,* which is present in red blood cells. Hemoglobin combines with oxygen as the blood passes through the lungs. Then, as the blood passes through the tissue capillaries, hemoglobin, because of its own strong chemical affinity for oxygen, does not release oxygen into the tissue fluid if too much oxygen is already there. However, if oxygen concentration in the tissue fluid is too low, sufficient oxygen is released to re-establish an adequate concentration. Thus, regulation of oxygen concentration in the tissues relies to a great extent on the chemical characteristics of hemoglobin. This regulation is called the *oxygen-buffering function of hemoglobin.* \nCarbon dioxide concentration in the extracellular fluid is regulated in a much different way. Carbon dioxide is a major end product of oxidative reactions in cells. If all the carbon dioxide formed in the cells continued to accumulate in the tissue fluids, all energy-giving reactions of the cells would cease. Fortunately, a higher than normal carbon dioxide concentration in the blood *excites the respiratory center,* causing a person to breathe rapidly and deeply. This deep rapid breathing increases expiration of carbon dioxide and, therefore, removes excess carbon dioxide from the blood and tissue fluids. This process continues until the concentration returns to normal. \n \n**Figure 1-3.** Negative feedback control of arterial pressure by the arterial baroreceptors. Signals from the sensor (baroreceptors) are sent to the medulla of the brain, where they are compared with a reference set point. When arterial pressure increases above normal, this abnormal pressure increases nerve impulses from the baroreceptors to the medulla of the brain, where the input signals are compared with the set point, generating an error signal that leads to decreased sympathetic nervous system activity. Decreased sympathetic activity causes dilation of blood vessels and reduced pumping activity of the heart, which return arterial pressure toward normal. \n**Regulation of Arterial Blood Pressure.** Several systems contribute to arterial blood pressure regulation. One of these, the *baroreceptor system,* is an excellent example of a rapidly acting control mechanism (**[Figure 1-3](#page-11-0)**). In the walls of the bifurcation region of the carotid arteries in the neck, and also in the arch of the aorta in the thorax, are many nerve receptors called *baroreceptors* that are stimulated by stretch of the arterial wall. When arterial pressure rises too high, the baroreceptors send barrages of nerve impulses to the medulla of the brain. Here, these impulses inhibit the *vasomotor center,* which in turn decreases the number of impulses transmitted from the vasomotor center through the sympathetic nervous system to the heart and blood vessels. Lack of these impulses causes diminished pumping activity by the heart and dilation of peripheral blood vessels, allowing increased blood flow through the vessels. Both these effects decrease the arterial pressure, moving it back toward normal. \nConversely, a decrease in arterial pressure below normal relaxes the stretch receptors, allowing the vasomotor center to become more active than usual, thereby causing vasoconstriction and increased heart pumping. The initial decrease in arterial pressure thus initiates negative feedback mechanisms that raise arterial pressure back toward normal. \n#### **Normal Ranges and Physical Characteristics of Important Extracellular Fluid Constituents** \n**[Table 1-1](#page-12-0)** lists some important constituents and physical characteristics of extracellular fluid, along with their normal values, normal ranges, and maximum limits without causing death. Note the narrowness of the normal range for each one. Values outside these ranges are often caused by illness, injury, or major environmental challenges. \n| Constituent | Normal Value | Normal Range | Approximate Short-Term Nonlethal Limit | Unit |\n|-------------------------|--------------|----------------|----------------------------------------|---------|\n| Oxygen (venous) | 40 | 25–40 | 10–1000 | mm Hg |\n| Carbon dioxide (venous) | 45 | 41–51 | 5–80 | mm Hg |\n| Sodium ion | 142 | 135–145 | 115–175 | mmol/L |\n| Potassium ion | 4.2 | 3.5-5.3 | 1.5–9.0 | mmol/L |\n| Calcium ion | 1.2 | 1.0-1.4 | 0.5–2.0 | mmol/L |\n| Chloride ion | 106 | 98–108 | 70–130 | mmol/L |\n| Bicarbonate ion | 24 | 22–29 | 8–45 | mmol/L |\n| Glucose | 90 | 70–115 | 20–1500 | mg/dl |\n| Body temperature | 98.4 (37.0) | 98-98.8 (37.0) | 65–110 (18.3–43.3) | °F (°C) |\n| Acid-base (venous) | 7.4 | 7.3–7.5 | 6.9–8.0 | рН | \nTable 1-1 Important Constituents and Physical Characteristics of Extracellular Fluid \nMost important are the limits beyond which abnormalities can cause death. For example, an increase in the body temperature of only 11°F (7°C) above normal can lead to a vicious cycle of increasing cellular metabolism that destroys the cells. Note also the narrow range for acid-base balance in the body, with a normal pH value of 7.4 and lethal values only about 0.5 on either side of normal. Whenever the potassium ion concentration decreases to less than one-third normal, paralysis may result from the inability of the nerves to carry signals. Alternatively, if potassium ion concentration increases to two or more times normal, the heart muscle is likely to be severely depressed. Also, when the calcium ion concentration falls below about one-half normal, a person is likely to experience tetanic contraction of muscles throughout the body because of the spontaneous generation of excess nerve impulses in peripheral nerves. When the glucose concentration falls below one-half normal, a person frequently exhibits extreme mental irritability and sometimes even has convulsions. \nThese examples should give one an appreciation for the necessity of the vast numbers of control systems that keep the body operating in health. In the absence of any one of these controls, serious body malfunction or death can result. \n#### **CHARACTERISTICS OF CONTROL SYSTEMS** \nThe aforementioned examples of homeostatic control mechanisms are only a few of the many thousands in the body, all of which have some common characteristics, as explained in this section.",
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"page_content": "Most control systems of the body act by *negative feed-back*, which can be explained by reviewing some of the homeostatic control systems mentioned previously. In the regulation of carbon dioxide concentration, a high concentration of carbon dioxide in the extracellular fluid increases pulmonary ventilation. This, in turn, decreases \nthe extracellular fluid carbon dioxide concentration because the lungs expire greater amounts of carbon dioxide from the body. Thus, the high concentration of carbon dioxide initiates events that decrease the concentration toward normal, which is *negative* to the initiating stimulus. Conversely, a carbon dioxide concentration that falls too low results in feedback to increase the concentration. This response is also negative to the initiating stimulus. \nIn the arterial pressure—regulating mechanisms, a high pressure causes a series of reactions that promote reduced pressure, or a low pressure causes a series of reactions that promote increased pressure. In both cases, these effects are negative with respect to the initiating stimulus. \nTherefore, in general, if some factor becomes excessive or deficient, a control system initiates *negative feedback*, which consists of a series of changes that return the factor toward a certain mean value, thus maintaining homeostasis. \nGain of a Control System. The degree of effectiveness with which a control system maintains constant conditions is determined by the gain of negative feedback. For example, let us assume that a large volume of blood is transfused into a person whose baroreceptor pressure control system is not functioning, and the arterial pressure rises from the normal level of 100 mm Hg up to 175 mm Hg. Then, let us assume that the same volume of blood is injected into the same person when the baroreceptor system is functioning, and this time the pressure increases by only 25 mm Hg. Thus, the feedback control system has caused a \"correction\" of -50 mm Hg, from 175 mm Hg to 125 mm Hg. There remains an increase in pressure of +25 mm Hg, called the \"error,\" which means that the control system is not 100% effective in preventing change. The gain of the system is then calculated by using the following formula: \n$$Gain = \\frac{Correction}{Frror}$$ \nThus, in the baroreceptor system example, the correction is -50 mm Hg, and the error persisting is +25 mm Hg. Therefore, the gain of the person's baroreceptor system \n \n**Figure 1-4.** Recovery of heart pumping caused by negative feedback after 1 liter of blood is removed from the circulation. Death is caused by positive feedback when 2 liters or more blood is removed. \nfor control of arterial pressure is −50 divided by +25, or −2. That is, a disturbance that increases or decreases the arterial pressure does so only one-third as much as would occur if this control system were not present. \nThe gains of some other physiological control systems are much greater than that of the baroreceptor system. For example, the gain of the system controlling internal body temperature when a person is exposed to moderately cold weather is about −33. Therefore, one can see that the temperature control system is much more effective than the baroreceptor pressure control system. \n#### **Positive Feedback May Cause Vicious Cycles and Death** \nWhy do most control systems of the body operate by negative feedback rather than by positive feedback? If one considers the nature of positive feedback, it is obvious that positive feedback leads to instability rather than stability and, in some cases, can cause death. \n**[Figure 1-4](#page-13-0)** shows an example in which death can ensue from positive feedback. This figure depicts the pumping effectiveness of the heart, showing the heart of a healthy human pumping about 5 liters of blood per minute. If the person suddenly bleeds a total of 2 liters, the amount of blood in the body is decreased to such a low level that not enough blood is available for the heart to pump effectively. As a result, the arterial pressure falls, and the flow of blood to the heart muscle through the coronary vessels diminishes. This scenario results in weakening of the heart, further diminished pumping, a further decrease in coronary blood flow, and still more weakness of the heart; the cycle repeats itself again and again until death occurs. Note that each cycle in the feedback results in further weakening of the heart. In other words, the initiating stimulus causes more of the same, which is *positive feedback.* \nPositive feedback is sometimes known as a \"vicious cycle,\" but a mild degree of positive feedback can be overcome by the negative feedback control mechanisms of the body, and the vicious cycle then fails to develop. For example, if the person in the aforementioned example bleeds only 1 liter instead of 2 liters, the normal negative feedback mechanisms for controlling cardiac output and arterial pressure can counterbalance the positive feedback and the person can recover, as shown by the dashed curve of **[Figure 1-4](#page-13-0)**. \n**Positive Feedback Can Sometimes Be Useful.** The body sometimes uses positive feedback to its advantage. Blood clotting is an example of a valuable use of positive feedback. When a blood vessel is ruptured, and a clot begins to form, multiple enzymes called *clotting factors* are activated within the clot. Some of these enzymes act on other inactivated enzymes of the immediately adjacent blood, thus causing more blood clotting. This process continues until the hole in the vessel is plugged and bleeding no longer occurs. On occasion, this mechanism can get out of hand and cause formation of unwanted clots. In fact, this is what initiates most acute heart attacks, which can be caused by a clot beginning on the inside surface of an atherosclerotic plaque in a coronary artery and then growing until the artery is blocked. \nChildbirth is another situation in which positive feedback is valuable. When uterine contractions become strong enough for the baby's head to begin pushing through the cervix, stretching of the cervix sends signals through the uterine muscle back to the body of the uterus, causing even more powerful contractions. Thus, the uterine contractions stretch the cervix, and cervical stretch causes stronger contractions. When this process becomes powerful enough, the baby is born. If they are not powerful enough, the contractions usually die out, and a few days pass before they begin again. \nAnother important use of positive feedback is for the generation of nerve signals. Stimulation of the membrane of a nerve fiber causes slight leakage of sodium ions through sodium channels in the nerve membrane to the fiber's interior. The sodium ions entering the fiber then change the membrane potential, which, in turn, causes more opening of channels, more change of potential, still more opening of channels, and so forth. Thus, a slight leak becomes an explosion of sodium entering the interior of the nerve fiber, which creates the nerve action potential. This action potential, in turn, causes electrical current to flow along the outside and inside of the fiber and initiates additional action potentials. This process continues until the nerve signal goes all the way to the end of the fiber. \nIn each case in which positive feedback is useful, the positive feedback is part of an overall negative feedback process. For example, in the case of blood clotting, the positive feedback clotting process is a negative feedback process for the maintenance of normal blood volume. Also, the positive feedback that causes nerve signals allows the nerves to participate in thousands of negative feedback nervous control systems. \n#### **More Complex Types of Control Systems—Feed-Forward and Adaptive Control** \nLater in this text, when we study the nervous system, we shall see that this system contains great numbers of interconnected control mechanisms. Some are simple feedback systems similar to those already discussed. Many are not. For example, some movements of the body occur so rapidly that there is not enough time for nerve signals to travel from the peripheral parts of the body all the way to the brain and then back to the periphery again to control the movement. Therefore, the brain uses a mechanism called *feed-forward control* to cause required muscle contractions. Sensory nerve signals from the moving parts apprise the brain about whether the movement is performed correctly. If not, the brain corrects the feed-forward signals that it sends to the muscles the *next* time the movement is required. Then, if still further correction is necessary, this process will be performed again for subsequent movements. This process is called *adaptive control.* Adaptive control, in a sense, is delayed negative feedback. \nThus, one can see how complex the feedback control systems of the body can be. A person's life depends on all of them. Therefore, much of this text is devoted to discussing these life-giving mechanisms. \n#### **PHYSIOLOGICAL VARIABILITY** \nAlthough some physiological variables, such as plasma concentrations of potassium, calcium, and hydrogen ions, are tightly regulated, others, such as body weight and adiposity, show wide variation among different individuals and even in the same individual at different stages of life. Blood pressure, cardiac pumping, metabolic rate, nervous system activity, hormones, and other physiological variables change throughout the day as we move about and engage in normal daily activities. Therefore, when we discuss \"normal\" values, it is with the understanding that many of the body's control systems are constantly reacting to perturbations, and that variability may exist among different individuals, depending on body weight and height, diet, age, sex, environment, genetics, and other factors. \nFor simplicity, discussion of physiological functions often focuses on the \"average\" 70-kg young, lean male. However, the American male no longer weighs an average of 70 kg; he now weighs over 88 kg, and the average American female weighs over 76 kg, more than the average man in the 1960s. Body weight has also increased substantially in most other industrialized countries during the past 40 to 50 years. \nExcept for reproductive and hormonal functions, many other physiological functions and normal values are often discussed in terms of male physiology. However, there are clearly differences in male and female physiology beyond the obvious differences that relate to reproduction. These differences can have important consequences for understanding normal physiology as well as for treatment of diseases. \nAge-related and ethnic or racial differences in physiology also have important influences on body composition, physiological control systems, and pathophysiology of diseases. For example, in a lean young male the total body water is about 60% of body weight. As a person grows and ages, this percentage gradually decreases, partly because aging is usually associated with declining skeletal muscle mass and increasing fat mass. Aging may also cause a decline in the function and effectiveness of some organs and physiological control systems. \nThese sources of physiological variability—sex differences, aging, ethnic, and racial—are complex but important considerations when discussing normal physiology and the pathophysiology of diseases. \n#### SUMMARY—AUTOMATICITY OF THE BODY \nThe main purpose of this chapter has been to discuss briefly the overall organization of the body and the means whereby the different parts of the body operate in harmony. To summarize, the body is actually a *social order of about 35 to 40 trillion cells* organized into different functional structures, some of which are called *organs.* Each functional structure contributes its share to the maintenance of homeostasis in the extracellular fluid, which is called the *internal environment.* As long as normal conditions are maintained in this internal environment, the cells of the body continue to live and function properly. Each cell benefits from homeostasis and, in turn, each cell contributes its share toward the maintenance of homeostasis. This reciprocal interplay provides continuous automaticity of the body until one or more functional systems lose their ability to contribute their share of function. When this happens, all the cells of the body suffer. Extreme dysfunction leads to death; moderate dysfunction leads to sickness. \n#### Bibliography \nAdolph EF: Physiological adaptations: hypertrophies and superfunctions. Am Sci 60:608, 1972. \nBentsen MA, Mirzadeh Z, Schwartz MW: Revisiting how the brain senses glucose-and why. Cell Metab 29:11, 2019. \nBernard C: Lectures on the Phenomena of Life Common to Animals and Plants. Springfield, IL: Charles C Thomas, 1974. \nCannon WB: Organization for physiological homeostasis. Physiol Rev 9:399, 1929. \nChien S: Mechanotransduction and endothelial cell homeostasis: the wisdom of the cell. Am J Physiol Heart Circ Physiol 292:H1209, 2007. \nDiBona GF: Physiology in perspective: the wisdom of the body. Neural control of the kidney. Am J Physiol Regul Integr Comp Physiol 289:R633, 2005. \nDickinson MH, Farley CT, Full RJ, et al: How animals move: an integrative view. Science 288:100, 2000. \nEckel-Mahan K, Sassone-Corsi P: Metabolism and the circadian clock converge. Physiol Rev 93:107, 2013. \n- Guyton AC: Arterial Pressure and Hypertension. Philadelphia: WB Saunders, 1980.\n- Herman MA, Kahn BB: Glucose transport and sensing in the maintenance of glucose homeostasis and metabolic harmony. J Clin Invest 116:1767, 2006.\n- Kabashima K, Honda T, Ginhoux F, Egawa G: The immunological anatomy of the skin. Nat Rev Immunol 19:19, 2019.\n- Khramtsova EA, Davis LK, Stranger BE: The role of sex in the genomics of human complex traits. Nat Rev Genet 20: 173, 2019.\n- Kim KS, Seeley RJ, Sandoval DA: Signalling from the periphery to the brain that regulates energy homeostasis. Nat Rev Neurosci 19:185, 2018.\n- Nishida AH, Ochman H: A great-ape view of the gut microbiome. Nat Rev Genet 20:185, 2019.\n- Orgel LE: The origin of life on the earth. Sci Am 271:76,1994.\n- Reardon C, Murray K, Lomax AE: Neuroimmune communication in health and disease. Physiol Rev 98:2287-2316, 2018.\n- Sender R, Fuchs S, Milo R: Revised estimates for the number of human and bacteria cells in the body. PLoS Biol 14(8):e1002533, 2016.\n- Smith HW: From Fish to Philosopher. New York: Doubleday, 1961. \n",
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"page_content": "Physiology is the science that seeks to explain the physical and chemical mechanisms that are responsible for the origin, development, and progression of life. Each type of life, from the simplest virus to the largest tree or the complicated human being, has its own functional characteristics. Therefore, the vast field of physiology can be divided into viral physiology, bacterial physiology, cellular physiology, plant physiology, invertebrate physiology, vertebrate physiology, mammalian physiology, human physiology, and many more subdivisions. \n**Human Physiology.** The science of human physiology attempts to explain the specific characteristics and mechanisms of the human body that make it a living being. The fact that we remain alive is the result of complex control systems. Hunger makes us seek food, and fear makes us seek refuge. Sensations of cold make us look for warmth. Other forces cause us to seek fellowship and to reproduce. The fact that we are sensing, feeling, and knowledgeable beings is part of this automatic sequence of life; these special attributes allow us to exist under widely varying conditions that otherwise would make life impossible. \nHuman physiology links the basic sciences with medicine and integrates multiple functions of the cells, tissues, and organs into the functions of the living human being. This integration requires communication and coordination by a vast array of control systems that operate at every level—from the genes that program synthesis of molecules to the complex nervous and hormonal systems that coordinate functions of cells, tissues, and organs throughout the body. Thus, the coordinated functions of the human body are much more than the sum of its parts, and life in health, as well as in disease states, relies on this total function. Although the main focus of this book is on normal human physiology, we will also discuss, to some extent, *pathophysiology,* which is the study of disordered body function and the basis for clinical medicine. \n#### CELLS ARE THE LIVING UNITS OF THE BODY \nThe basic living unit of the body is the cell. Each tissue or organ is an aggregate of many different cells held together by intercellular supporting structures. \nEach type of cell is specially adapted to perform one or a few particular functions. For example, the red blood cells, numbering about 25 trillion in each person, transport oxygen from the lungs to the tissues. Although the red blood cells are the most abundant of any single type of cell in the body, there are also trillions of additional cells of other types that perform functions different from those of the red blood cell. The entire body, then, contains about 35 to 40 trillion human cells. \nThe many cells of the body often differ markedly from one another but all have certain basic characteristics that are alike. For example, oxygen reacts with carbohydrate, fat, and protein to release the energy required for all cells to function. Furthermore, the general chemical mechanisms for changing nutrients into energy are basically the same in all cells, and all cells deliver products of their chemical reactions into the surrounding fluids. \nAlmost all cells also have the ability to reproduce additional cells of their own type. Fortunately, when cells of a particular type are destroyed, the remaining cells of this type usually generate new cells until the supply is replenished. \n**Microorganisms Living in the Body Outnumber Human Cells.** In addition to human cells, trillions of microbes inhabit the body, living on the skin and in the mouth, gut, and nose. The gastrointestinal tract, for example, normally contains a complex and dynamic population of 400 to 1000 species of microorganisms that outnumber our human cells. Communities of microorganisms that inhabit the body, often called *microbiota,* can cause diseases, but most of the time they live in harmony with their human hosts and provide vital functions that are essential for survival of their hosts. Although the importance of gut microbiota in the digestion of foodstuffs is widely recognized, additional roles for the body's microbes in nutrition, immunity, and other functions are just beginning to be appreciated and represent an intensive area of biomedical research. \n#### EXTRACELLULAR FLUID—THE \"INTERNAL ENVIRONMENT\" \nAbout 50% to 70% of the adult human body is fluid, mainly a water solution of ions and other substances. Although most of this fluid is inside the cells and is called *intracellular fluid,* about one-third is in the spaces outside the cells and is called *extracellular fluid.* This extracellular fluid is in constant motion throughout the body. It is transported rapidly in the circulating blood and then mixed between the blood and tissue fluids by diffusion through the capillary walls. \nIn the extracellular fluid are the ions and nutrients needed by the cells to maintain life. Thus, all cells live in essentially the same environment—the extracellular fluid. For this reason, the extracellular fluid is also called the *internal environment* of the body, or the *milieu intérieur,* a term introduced by the great 19th-century French physiologist Claude Bernard (1813–1878). \nCells are capable of living and performing their special functions as long as the proper concentrations of oxygen, glucose, different ions, amino acids, fatty substances, and other constituents are available in this internal environment. \n#### **Differences in Extracellular and Intracellular Fluids.** \nThe extracellular fluid contains large amounts of sodium, chloride, and bicarbonate ions plus nutrients for the cells, such as oxygen, glucose, fatty acids, and amino acids. It also contains carbon dioxide that is being transported from the cells to the lungs to be excreted, plus other cellular waste products that are being transported to the kidneys for excretion. \nThe intracellular fluid contains large amounts of potassium, magnesium, and phosphate ions instead of the sodium and chloride ions found in the extracellular fluid. Special mechanisms for transporting ions through the cell membranes maintain the ion concentration differences between the extracellular and intracellular fluids. These transport processes are discussed in Chapter 4. \n#### HOMEOSTASIS—MAINTENANCE OF A NEARLY CONSTANT INTERNAL ENVIRONMENT \nIn 1929, the American physiologist Walter Cannon (1871–1945) coined the term *homeostasis* to describe the *maintenance of nearly constant conditions in the internal environment*. Essentially, all organs and tissues of the body perform functions that help maintain these relatively constant conditions. For example, the lungs provide oxygen to the extracellular fluid to replenish the oxygen used by the cells, the kidneys maintain constant ion concentrations, and the gastrointestinal system provides nutrients while eliminating waste from the body. \nThe various ions, nutrients, waste products, and other constituents of the body are normally regulated within a range of values, rather than at fixed values. For some of the body's constituents, this range is extremely small. Variations in the blood hydrogen ion concentration, for example, are normally less than 5 *nanomoles/L* (0.000000005 moles/L). The blood sodium concentration is also tightly regulated, normally varying only a few *millimoles* per liter, even with large changes in sodium intake, but these variations of sodium concentration are at least 1 million times greater than for hydrogen ions. \nPowerful control systems exist for maintaining concentrations of sodium and hydrogen ions, as well as for most of the other ions, nutrients, and substances in the body at levels that permit the cells, tissues, and organs to perform their normal functions, despite wide environmental variations and challenges from injury and diseases. \nMuch of this text is concerned with how each organ or tissue contributes to homeostasis. Normal body functions require integrated actions of cells, tissues, organs, and multiple nervous, hormonal, and local control systems that together contribute to homeostasis and good health. \n**Homeostatic Compensations in Diseases.** *Disease* is often considered to be a state of disrupted homeostasis. However, even in the presence of disease, homeostatic mechanisms continue to operate and maintain vital functions through multiple compensations. In some cases, these compensations may lead to major deviations of the body's functions from the normal range, making it difficult to distinguish the primary cause of the disease from the compensatory responses. For example, diseases that impair the kidneys' ability to excrete salt and water may lead to high blood pressure, which initially helps return excretion to normal so that a balance between intake and renal excretion can be maintained. This balance is needed to maintain life, but, over long periods of time, the high blood pressure can damage various organs, including the kidneys, causing even greater increases in blood pressure and more renal damage. Thus, homeostatic compensations that ensue after injury, disease, or major environmental challenges to the body may represent trade-offs that are necessary to maintain vital body functions but, in the long term, contribute to additional abnormalities of body function. The discipline of *pathophysiology* seeks to explain how the various physiological processes are altered in diseases or injury. \nThis chapter outlines the different functional systems of the body and their contributions to homeostasis. We then briefly discuss the basic theory of the body's control systems that allow the functional systems to operate in support of one another. \n#### **EXTRACELLULAR FLUID TRANSPORT AND MIXING SYSTEM—THE BLOOD CIRCULATORY SYSTEM** \nExtracellular fluid is transported through the body in two stages. The first stage is movement of blood through the body in the blood vessels. The second is movement of fluid between the blood capillaries and the *intercellular spaces* between the tissue cells. \n**[Figure 1-1](#page-9-0)** shows the overall circulation of blood. All the blood in the circulation traverses the entire circuit an average \n \n**Figure 1-1.** General organization of the circulatory system. \nof once each minute when the body is at rest and as many as six times each minute when a person is extremely active. \nAs blood passes through blood capillaries, continual exchange of extracellular fluid occurs between the plasma portion of the blood and the interstitial fluid that fills the intercellular spaces. This process is shown in **Figure 1-2**. The capillary walls are permeable to most molecules in the blood plasma, with the exception of plasma proteins, which are too large to pass through capillaries readily. Therefore, large amounts of fluid and its dissolved constituents *diffuse* back and forth between the blood and the tissue spaces, as shown by the arrows in **Figure 1-2**. \nThis process of diffusion is caused by kinetic motion of the molecules in the plasma and the interstitial fluid. \n \n**Figure 1-2.** Diffusion of fluid and dissolved constituents through the capillary walls and interstitial spaces. \nThat is, the fluid and dissolved molecules are continually moving and bouncing in all directions in the plasma and fluid in the intercellular spaces, as well as through capillary pores. Few cells are located more than 50 micrometers from a capillary, which ensures diffusion of almost any substance from the capillary to the cell within a few seconds. Thus, the extracellular fluid everywhere in the body—both that of the plasma and that of the interstitial fluid—is continually being mixed, thereby maintaining homogeneity of extracellular fluid throughout the body.\n\n**Respiratory System. Figure 1-1** shows that each time blood passes through the body, it also flows through the lungs. The blood picks up *oxygen* in alveoli, thus acquiring the oxygen needed by cells. The membrane between the alveoli and the lumen of the pulmonary capillaries, the *alveolar membrane*, is only 0.4 to 2.0 micrometers thick, and oxygen rapidly diffuses by molecular motion through this membrane into the blood. \n**Gastrointestinal Tract.** A large portion of the blood pumped by the heart also passes through the walls of the gastrointestinal tract. Here different dissolved nutrients, including *carbohydrates*, *fatty acids*, and *amino acids*, are absorbed from ingested food into the extracellular fluid of the blood. \nLiver and Other Organs That Perform Primarily Metabolic Functions. Not all substances absorbed from the gastrointestinal tract can be used in their absorbed form by the cells. The liver changes the chemical compositions of many of these substances to more usable forms, and other tissues of the body—fat cells, gastrointestinal mucosa, kidneys, and endocrine glands—help modify the absorbed substances or store them until they are needed. The liver also eliminates certain waste products produced in the body and toxic substances that are ingested. \n**Musculoskeletal System.** How does the musculoskeletal system contribute to homeostasis? The answer is obvious and simple. Were it not for the muscles, the body could not move to obtain the foods required for nutrition. The musculoskeletal system also provides motility for protection against adverse surroundings, without which the entire body, along with its homeostatic mechanisms, could be destroyed. \n#### **REMOVAL OF METABOLIC END PRODUCTS** \n**Removal of Carbon Dioxide by the Lungs.** At the same time that blood picks up oxygen in the lungs, *carbon dioxide* is released from the blood into lung alveoli; the respiratory movement of air into and out of the lungs carries carbon dioxide to the atmosphere. Carbon dioxide is the most abundant of all the metabolism products. \n**Kidneys.** Passage of blood through the kidneys removes most of the other substances from the plasma besides carbon dioxide that are not needed by cells. These substances include different end products of cellular metabolism, such as urea and uric acid; they also include excesses of ions and water from the food that accumulate in the extracellular fluid. \nThe kidneys perform their function first by filtering large quantities of plasma through the glomerular capillaries into the tubules and then reabsorbing into the blood substances needed by the body, such as glucose, amino acids, appropriate amounts of water, and many of the ions. Most of the other substances that are not needed by the body, especially metabolic waste products such as urea and creatinine, are reabsorbed poorly and pass through the renal tubules into the urine. \n**Gastrointestinal Tract.** Undigested material that enters the gastrointestinal tract and some waste products of metabolism are eliminated in the feces. \n**Liver.** Among the many functions of the liver is detoxification or removal of ingested drugs and chemicals. The liver secretes many of these wastes into the bile to be eventually eliminated in the feces. \n#### **REGULATION OF BODY FUNCTIONS** \n**Nervous System.** The nervous system is composed of three major parts—the *sensory input portion,* the *central nervous system* (or *integrative portion*), and the *motor output portion.* Sensory receptors detect the state of the body and its surroundings. For example, receptors in the skin alert us whenever an object touches the skin. The eyes are sensory organs that give us a visual image of the surrounding area. The ears are also sensory organs. The central nervous system is composed of the brain and spinal cord. The brain stores information, generates thoughts, creates ambition, and determines reactions that the body performs in response to the sensations. Appropriate signals are then transmitted through the motor output portion of the nervous system to carry out one's desires. \nAn important segment of the nervous system is called the *autonomic system.* It operates at a subconscious level and controls many functions of internal organs, including the level of pumping activity by the heart, movements of the gastrointestinal tract, and secretion by many of the body's glands. \n**Hormone Systems.** Located in the body are *endocrine glands,* organs and tissues that secrete chemical substances called *hormones.* Hormones are transported in the extracellular fluid to other parts of the body to help regulate cellular function. For example, *thyroid hormone* increases the rates of most chemical reactions in all cells, thus helping set the tempo of bodily activity. *Insulin* controls glucose metabolism, *adrenocortical hormones* control sodium and potassium ions and protein metabolism, and *parathyroid hormone* controls bone calcium and phosphate. Thus, the hormones provide a regulatory system that complements the nervous system. The nervous system controls many muscular and secretory activities of the body, whereas the hormonal system regulates many metabolic functions. The nervous and hormonal systems normally work together in a coordinated manner to control essentially all the organ systems of the body. \n#### **PROTECTION OF THE BODY** \n**Immune System.** The immune system includes white blood cells, tissue cells derived from white blood cells, the thymus, lymph nodes, and lymph vessels that protect the body from pathogens such as bacteria, viruses, parasites, and fungi. The immune system provides a mechanism for the body to carry out the following: (1) distinguish its own cells from harmful foreign cells and substances; and (2) destroy the invader by *phagocytosis* or by producing *sensitized lymphocytes* or specialized proteins (e.g., *antibodies*) that destroy or neutralize the invader. \n**Integumentary System.** The skin and its various appendages (including the hair, nails, glands, and other structures) cover, cushion, and protect the deeper tissues and organs of the body and generally provide a boundary between the body's internal environment and the outside world. The integumentary system is also important for temperature regulation and excretion of wastes, and it provides a sensory interface between the body and the external environment. The skin generally comprises about 12% to 15% of body weight. \n#### **REPRODUCTION** \nAlthough reproduction is sometimes not considered a homeostatic function, it helps maintain homeostasis by generating new beings to take the place of those that are dying. This may sound like a permissive usage of the term *homeostasis,* but it illustrates that in the final analysis, essentially all body structures are organized to help maintain the automaticity and continuity of life. \n#### CONTROL SYSTEMS OF THE BODY \nThe human body has thousands of control systems. Some of the most intricate of these systems are genetic control systems that operate in all cells to help regulate intracellular and extracellular functions. This subject is discussed in Chapter 3. \nMany other control systems operate *within the organs* to regulate functions of the individual parts of the organs; others operate throughout the entire body *to control the interrelationships between the organs.* For example, the respiratory system, operating in association with the nervous system, regulates the concentration of carbon dioxide in the extracellular fluid. The liver and pancreas control glucose concentration in the extracellular fluid, and the kidneys regulate concentrations of hydrogen, sodium, potassium, phosphate, and other ions in the extracellular fluid. \n#### **EXAMPLES OF CONTROL MECHANISMS** \n**Regulation of Oxygen and Carbon Dioxide Concentrations in the Extracellular Fluid.** Because oxygen is one of the major substances required for chemical reactions in cells, the body has a special control mechanism to maintain an almost exact and constant oxygen concentration in the extracellular fluid. This mechanism depends principally on the chemical characteristics of *hemoglobin,* which is present in red blood cells. Hemoglobin combines with oxygen as the blood passes through the lungs. Then, as the blood passes through the tissue capillaries, hemoglobin, because of its own strong chemical affinity for oxygen, does not release oxygen into the tissue fluid if too much oxygen is already there. However, if oxygen concentration in the tissue fluid is too low, sufficient oxygen is released to re-establish an adequate concentration. Thus, regulation of oxygen concentration in the tissues relies to a great extent on the chemical characteristics of hemoglobin. This regulation is called the *oxygen-buffering function of hemoglobin.* \nCarbon dioxide concentration in the extracellular fluid is regulated in a much different way. Carbon dioxide is a major end product of oxidative reactions in cells. If all the carbon dioxide formed in the cells continued to accumulate in the tissue fluids, all energy-giving reactions of the cells would cease. Fortunately, a higher than normal carbon dioxide concentration in the blood *excites the respiratory center,* causing a person to breathe rapidly and deeply. This deep rapid breathing increases expiration of carbon dioxide and, therefore, removes excess carbon dioxide from the blood and tissue fluids. This process continues until the concentration returns to normal. \n \n**Figure 1-3.** Negative feedback control of arterial pressure by the arterial baroreceptors. Signals from the sensor (baroreceptors) are sent to the medulla of the brain, where they are compared with a reference set point. When arterial pressure increases above normal, this abnormal pressure increases nerve impulses from the baroreceptors to the medulla of the brain, where the input signals are compared with the set point, generating an error signal that leads to decreased sympathetic nervous system activity. Decreased sympathetic activity causes dilation of blood vessels and reduced pumping activity of the heart, which return arterial pressure toward normal. \n**Regulation of Arterial Blood Pressure.** Several systems contribute to arterial blood pressure regulation. One of these, the *baroreceptor system,* is an excellent example of a rapidly acting control mechanism (**[Figure 1-3](#page-11-0)**). In the walls of the bifurcation region of the carotid arteries in the neck, and also in the arch of the aorta in the thorax, are many nerve receptors called *baroreceptors* that are stimulated by stretch of the arterial wall. When arterial pressure rises too high, the baroreceptors send barrages of nerve impulses to the medulla of the brain. Here, these impulses inhibit the *vasomotor center,* which in turn decreases the number of impulses transmitted from the vasomotor center through the sympathetic nervous system to the heart and blood vessels. Lack of these impulses causes diminished pumping activity by the heart and dilation of peripheral blood vessels, allowing increased blood flow through the vessels. Both these effects decrease the arterial pressure, moving it back toward normal. \nConversely, a decrease in arterial pressure below normal relaxes the stretch receptors, allowing the vasomotor center to become more active than usual, thereby causing vasoconstriction and increased heart pumping. The initial decrease in arterial pressure thus initiates negative feedback mechanisms that raise arterial pressure back toward normal. \n#### **Normal Ranges and Physical Characteristics of Important Extracellular Fluid Constituents** \n**[Table 1-1](#page-12-0)** lists some important constituents and physical characteristics of extracellular fluid, along with their normal values, normal ranges, and maximum limits without causing death. Note the narrowness of the normal range for each one. Values outside these ranges are often caused by illness, injury, or major environmental challenges. \n| Constituent | Normal Value | Normal Range | Approximate Short-Term Nonlethal Limit | Unit |\n|-------------------------|--------------|----------------|----------------------------------------|---------|\n| Oxygen (venous) | 40 | 25–40 | 10–1000 | mm Hg |\n| Carbon dioxide (venous) | 45 | 41–51 | 5–80 | mm Hg |\n| Sodium ion | 142 | 135–145 | 115–175 | mmol/L |\n| Potassium ion | 4.2 | 3.5-5.3 | 1.5–9.0 | mmol/L |\n| Calcium ion | 1.2 | 1.0-1.4 | 0.5–2.0 | mmol/L |\n| Chloride ion | 106 | 98–108 | 70–130 | mmol/L |\n| Bicarbonate ion | 24 | 22–29 | 8–45 | mmol/L |\n| Glucose | 90 | 70–115 | 20–1500 | mg/dl |\n| Body temperature | 98.4 (37.0) | 98-98.8 (37.0) | 65–110 (18.3–43.3) | °F (°C) |\n| Acid-base (venous) | 7.4 | 7.3–7.5 | 6.9–8.0 | рН | \nTable 1-1 Important Constituents and Physical Characteristics of Extracellular Fluid \nMost important are the limits beyond which abnormalities can cause death. For example, an increase in the body temperature of only 11°F (7°C) above normal can lead to a vicious cycle of increasing cellular metabolism that destroys the cells. Note also the narrow range for acid-base balance in the body, with a normal pH value of 7.4 and lethal values only about 0.5 on either side of normal. Whenever the potassium ion concentration decreases to less than one-third normal, paralysis may result from the inability of the nerves to carry signals. Alternatively, if potassium ion concentration increases to two or more times normal, the heart muscle is likely to be severely depressed. Also, when the calcium ion concentration falls below about one-half normal, a person is likely to experience tetanic contraction of muscles throughout the body because of the spontaneous generation of excess nerve impulses in peripheral nerves. When the glucose concentration falls below one-half normal, a person frequently exhibits extreme mental irritability and sometimes even has convulsions. \nThese examples should give one an appreciation for the necessity of the vast numbers of control systems that keep the body operating in health. In the absence of any one of these controls, serious body malfunction or death can result. \n#### **CHARACTERISTICS OF CONTROL SYSTEMS** \nThe aforementioned examples of homeostatic control mechanisms are only a few of the many thousands in the body, all of which have some common characteristics, as explained in this section.\n\nMost control systems of the body act by *negative feed-back*, which can be explained by reviewing some of the homeostatic control systems mentioned previously. In the regulation of carbon dioxide concentration, a high concentration of carbon dioxide in the extracellular fluid increases pulmonary ventilation. This, in turn, decreases \nthe extracellular fluid carbon dioxide concentration because the lungs expire greater amounts of carbon dioxide from the body. Thus, the high concentration of carbon dioxide initiates events that decrease the concentration toward normal, which is *negative* to the initiating stimulus. Conversely, a carbon dioxide concentration that falls too low results in feedback to increase the concentration. This response is also negative to the initiating stimulus. \nIn the arterial pressure—regulating mechanisms, a high pressure causes a series of reactions that promote reduced pressure, or a low pressure causes a series of reactions that promote increased pressure. In both cases, these effects are negative with respect to the initiating stimulus. \nTherefore, in general, if some factor becomes excessive or deficient, a control system initiates *negative feedback*, which consists of a series of changes that return the factor toward a certain mean value, thus maintaining homeostasis. \nGain of a Control System. The degree of effectiveness with which a control system maintains constant conditions is determined by the gain of negative feedback. For example, let us assume that a large volume of blood is transfused into a person whose baroreceptor pressure control system is not functioning, and the arterial pressure rises from the normal level of 100 mm Hg up to 175 mm Hg. Then, let us assume that the same volume of blood is injected into the same person when the baroreceptor system is functioning, and this time the pressure increases by only 25 mm Hg. Thus, the feedback control system has caused a \"correction\" of -50 mm Hg, from 175 mm Hg to 125 mm Hg. There remains an increase in pressure of +25 mm Hg, called the \"error,\" which means that the control system is not 100% effective in preventing change. The gain of the system is then calculated by using the following formula: \n$$Gain = \\frac{Correction}{Frror}$$ \nThus, in the baroreceptor system example, the correction is -50 mm Hg, and the error persisting is +25 mm Hg. Therefore, the gain of the person's baroreceptor system \n \n**Figure 1-4.** Recovery of heart pumping caused by negative feedback after 1 liter of blood is removed from the circulation. Death is caused by positive feedback when 2 liters or more blood is removed. \nfor control of arterial pressure is −50 divided by +25, or −2. That is, a disturbance that increases or decreases the arterial pressure does so only one-third as much as would occur if this control system were not present. \nThe gains of some other physiological control systems are much greater than that of the baroreceptor system. For example, the gain of the system controlling internal body temperature when a person is exposed to moderately cold weather is about −33. Therefore, one can see that the temperature control system is much more effective than the baroreceptor pressure control system. \n#### **Positive Feedback May Cause Vicious Cycles and Death** \nWhy do most control systems of the body operate by negative feedback rather than by positive feedback? If one considers the nature of positive feedback, it is obvious that positive feedback leads to instability rather than stability and, in some cases, can cause death. \n**[Figure 1-4](#page-13-0)** shows an example in which death can ensue from positive feedback. This figure depicts the pumping effectiveness of the heart, showing the heart of a healthy human pumping about 5 liters of blood per minute. If the person suddenly bleeds a total of 2 liters, the amount of blood in the body is decreased to such a low level that not enough blood is available for the heart to pump effectively. As a result, the arterial pressure falls, and the flow of blood to the heart muscle through the coronary vessels diminishes. This scenario results in weakening of the heart, further diminished pumping, a further decrease in coronary blood flow, and still more weakness of the heart; the cycle repeats itself again and again until death occurs. Note that each cycle in the feedback results in further weakening of the heart. In other words, the initiating stimulus causes more of the same, which is *positive feedback.* \nPositive feedback is sometimes known as a \"vicious cycle,\" but a mild degree of positive feedback can be overcome by the negative feedback control mechanisms of the body, and the vicious cycle then fails to develop. For example, if the person in the aforementioned example bleeds only 1 liter instead of 2 liters, the normal negative feedback mechanisms for controlling cardiac output and arterial pressure can counterbalance the positive feedback and the person can recover, as shown by the dashed curve of **[Figure 1-4](#page-13-0)**. \n**Positive Feedback Can Sometimes Be Useful.** The body sometimes uses positive feedback to its advantage. Blood clotting is an example of a valuable use of positive feedback. When a blood vessel is ruptured, and a clot begins to form, multiple enzymes called *clotting factors* are activated within the clot. Some of these enzymes act on other inactivated enzymes of the immediately adjacent blood, thus causing more blood clotting. This process continues until the hole in the vessel is plugged and bleeding no longer occurs. On occasion, this mechanism can get out of hand and cause formation of unwanted clots. In fact, this is what initiates most acute heart attacks, which can be caused by a clot beginning on the inside surface of an atherosclerotic plaque in a coronary artery and then growing until the artery is blocked. \nChildbirth is another situation in which positive feedback is valuable. When uterine contractions become strong enough for the baby's head to begin pushing through the cervix, stretching of the cervix sends signals through the uterine muscle back to the body of the uterus, causing even more powerful contractions. Thus, the uterine contractions stretch the cervix, and cervical stretch causes stronger contractions. When this process becomes powerful enough, the baby is born. If they are not powerful enough, the contractions usually die out, and a few days pass before they begin again. \nAnother important use of positive feedback is for the generation of nerve signals. Stimulation of the membrane of a nerve fiber causes slight leakage of sodium ions through sodium channels in the nerve membrane to the fiber's interior. The sodium ions entering the fiber then change the membrane potential, which, in turn, causes more opening of channels, more change of potential, still more opening of channels, and so forth. Thus, a slight leak becomes an explosion of sodium entering the interior of the nerve fiber, which creates the nerve action potential. This action potential, in turn, causes electrical current to flow along the outside and inside of the fiber and initiates additional action potentials. This process continues until the nerve signal goes all the way to the end of the fiber. \nIn each case in which positive feedback is useful, the positive feedback is part of an overall negative feedback process. For example, in the case of blood clotting, the positive feedback clotting process is a negative feedback process for the maintenance of normal blood volume. Also, the positive feedback that causes nerve signals allows the nerves to participate in thousands of negative feedback nervous control systems. \n#### **More Complex Types of Control Systems—Feed-Forward and Adaptive Control** \nLater in this text, when we study the nervous system, we shall see that this system contains great numbers of interconnected control mechanisms. Some are simple feedback systems similar to those already discussed. Many are not. For example, some movements of the body occur so rapidly that there is not enough time for nerve signals to travel from the peripheral parts of the body all the way to the brain and then back to the periphery again to control the movement. Therefore, the brain uses a mechanism called *feed-forward control* to cause required muscle contractions. Sensory nerve signals from the moving parts apprise the brain about whether the movement is performed correctly. If not, the brain corrects the feed-forward signals that it sends to the muscles the *next* time the movement is required. Then, if still further correction is necessary, this process will be performed again for subsequent movements. This process is called *adaptive control.* Adaptive control, in a sense, is delayed negative feedback. \nThus, one can see how complex the feedback control systems of the body can be. A person's life depends on all of them. Therefore, much of this text is devoted to discussing these life-giving mechanisms. \n#### **PHYSIOLOGICAL VARIABILITY** \nAlthough some physiological variables, such as plasma concentrations of potassium, calcium, and hydrogen ions, are tightly regulated, others, such as body weight and adiposity, show wide variation among different individuals and even in the same individual at different stages of life. Blood pressure, cardiac pumping, metabolic rate, nervous system activity, hormones, and other physiological variables change throughout the day as we move about and engage in normal daily activities. Therefore, when we discuss \"normal\" values, it is with the understanding that many of the body's control systems are constantly reacting to perturbations, and that variability may exist among different individuals, depending on body weight and height, diet, age, sex, environment, genetics, and other factors. \nFor simplicity, discussion of physiological functions often focuses on the \"average\" 70-kg young, lean male. However, the American male no longer weighs an average of 70 kg; he now weighs over 88 kg, and the average American female weighs over 76 kg, more than the average man in the 1960s. Body weight has also increased substantially in most other industrialized countries during the past 40 to 50 years. \nExcept for reproductive and hormonal functions, many other physiological functions and normal values are often discussed in terms of male physiology. However, there are clearly differences in male and female physiology beyond the obvious differences that relate to reproduction. These differences can have important consequences for understanding normal physiology as well as for treatment of diseases. \nAge-related and ethnic or racial differences in physiology also have important influences on body composition, physiological control systems, and pathophysiology of diseases. For example, in a lean young male the total body water is about 60% of body weight. As a person grows and ages, this percentage gradually decreases, partly because aging is usually associated with declining skeletal muscle mass and increasing fat mass. Aging may also cause a decline in the function and effectiveness of some organs and physiological control systems. \nThese sources of physiological variability—sex differences, aging, ethnic, and racial—are complex but important considerations when discussing normal physiology and the pathophysiology of diseases. \n#### SUMMARY—AUTOMATICITY OF THE BODY \nThe main purpose of this chapter has been to discuss briefly the overall organization of the body and the means whereby the different parts of the body operate in harmony. To summarize, the body is actually a *social order of about 35 to 40 trillion cells* organized into different functional structures, some of which are called *organs.* Each functional structure contributes its share to the maintenance of homeostasis in the extracellular fluid, which is called the *internal environment.* As long as normal conditions are maintained in this internal environment, the cells of the body continue to live and function properly. Each cell benefits from homeostasis and, in turn, each cell contributes its share toward the maintenance of homeostasis. This reciprocal interplay provides continuous automaticity of the body until one or more functional systems lose their ability to contribute their share of function. When this happens, all the cells of the body suffer. Extreme dysfunction leads to death; moderate dysfunction leads to sickness. \n#### Bibliography \nAdolph EF: Physiological adaptations: hypertrophies and superfunctions. Am Sci 60:608, 1972. \nBentsen MA, Mirzadeh Z, Schwartz MW: Revisiting how the brain senses glucose-and why. Cell Metab 29:11, 2019. \nBernard C: Lectures on the Phenomena of Life Common to Animals and Plants. Springfield, IL: Charles C Thomas, 1974. \nCannon WB: Organization for physiological homeostasis. Physiol Rev 9:399, 1929. \nChien S: Mechanotransduction and endothelial cell homeostasis: the wisdom of the cell. Am J Physiol Heart Circ Physiol 292:H1209, 2007. \nDiBona GF: Physiology in perspective: the wisdom of the body. Neural control of the kidney. Am J Physiol Regul Integr Comp Physiol 289:R633, 2005. \nDickinson MH, Farley CT, Full RJ, et al: How animals move: an integrative view. Science 288:100, 2000. \nEckel-Mahan K, Sassone-Corsi P: Metabolism and the circadian clock converge. Physiol Rev 93:107, 2013. \n- Guyton AC: Arterial Pressure and Hypertension. Philadelphia: WB Saunders, 1980.\n- Herman MA, Kahn BB: Glucose transport and sensing in the maintenance of glucose homeostasis and metabolic harmony. J Clin Invest 116:1767, 2006.\n- Kabashima K, Honda T, Ginhoux F, Egawa G: The immunological anatomy of the skin. Nat Rev Immunol 19:19, 2019.\n- Khramtsova EA, Davis LK, Stranger BE: The role of sex in the genomics of human complex traits. Nat Rev Genet 20: 173, 2019.\n- Kim KS, Seeley RJ, Sandoval DA: Signalling from the periphery to the brain that regulates energy homeostasis. Nat Rev Neurosci 19:185, 2018.\n- Nishida AH, Ochman H: A great-ape view of the gut microbiome. Nat Rev Genet 20:185, 2019.\n- Orgel LE: The origin of life on the earth. Sci Am 271:76,1994.\n- Reardon C, Murray K, Lomax AE: Neuroimmune communication in health and disease. Physiol Rev 98:2287-2316, 2018.\n- Sender R, Fuchs S, Milo R: Revised estimates for the number of human and bacteria cells in the body. PLoS Biol 14(8):e1002533, 2016.\n- Smith HW: From Fish to Philosopher. New York: Doubleday, 1961. \n\n\nEach of the trillions of cells in a human being is a living structure that can survive for months or years, provided its surrounding fluids contain appropriate nutrients. Cells are the building blocks of the body, providing structure for the body's tissues and organs, ingesting nutrients and converting them to energy, and performing specialized functions. Cells also contain the body's hereditary code, which controls the substances synthesized by the cells and permits them to make copies of themselves. \n#### ORGANIZATION OF THE CELL \nA schematic drawing of a typical cell, as seen by the light microscope, is shown in **[Figure 2-1](#page-16-0)**. Its two major parts are the *nucleus* and the *cytoplasm.* The nucleus is separated from the cytoplasm by a *nuclear membrane,* and the cytoplasm is separated from the surrounding fluids by a *cell membrane,* also called the *plasma membrane.* \nThe different substances that make up the cell are collectively called *protoplasm.* Protoplasm is composed mainly of five basic substances—water, electrolytes, proteins, lipids, and carbohydrates. \n**Water.** Most cells, except for fat cells, are comprised mainly of water in a concentration of 70% to 85%. Many cellular chemicals are dissolved in the water. Others are suspended in the water as solid particulates. Chemical reactions take place among the dissolved chemicals or at the surfaces of the suspended particles or membranes. \n**Ions.** Important ions in the cell include *potassium, magnesium, phosphate, sulfate, bicarbonate,* and smaller quantities of *sodium, chloride,* and *calcium.* These ions are all discussed in Chapter 4, which considers the interrelations between the intracellular and extracellular fluids. \nThe ions provide inorganic chemicals for cellular reactions and are necessary for the operation of some cellular control mechanisms. For example, ions acting at the cell membrane are required for the transmission of electrochemical impulses in nerve and muscle fibers. \n**Proteins.** After water, the most abundant substances in most cells are proteins, which normally constitute 10% to 20% of the cell mass. These proteins can be divided into two types, *structural proteins* and *functional proteins.* \nStructural proteins are present in the cell mainly in the form of long filaments that are polymers of many individual protein molecules. A prominent use of such intracellular filaments is to form *microtubules,* which provide the cytoskeletons of cellular organelles such as cilia, nerve axons, the mitotic spindles of cells undergoing mitosis, and a tangled mass of thin filamentous tubules that hold the parts of the cytoplasm and nucleoplasm together in their respective compartments. Fibrillar proteins are found outside the cell, especially in the collagen and elastin fibers of connective tissue, and elsewhere, such as in blood vessel walls, tendons, and ligaments. \nThe *functional proteins* are usually composed of combinations of a few molecules in tubular-globular form. These proteins are mainly the *enzymes* of the cell and, in contrast to the fibrillar proteins, are often mobile in the cell fluid. Also, many of them are adherent to membranous structures inside the cell and catalyze specific intracellular chemical reactions. For example, the chemical reactions that split glucose into its component parts and then combine these with oxygen to form carbon dioxide and water while simultaneously providing energy for cellular function are all catalyzed by a series of protein enzymes. \n**Lipids.** Lipids are several types of substances that are grouped together because of their common property of being soluble in fat solvents. Especially important lipids \n \n**Figure 2-1.** Illustration of cell structures visible with a light microscope. \n \n**Figure 2-2.** Reconstruction of a typical cell, showing the internal organelles in the cytoplasm and nucleus. \nare *phospholipids* and *cholesterol,* which together constitute only about 2% of the total cell mass. Phospholipids and cholesterol are mainly insoluble in water and therefore are used to form the cell membrane and intracellular membrane barriers that separate the different cell compartments. \nIn addition to phospholipids and cholesterol, some cells contain large quantities of *triglycerides,* also called *neutral fats.* In *fat cells (adipocytes),* triglycerides often account for as much as 95% of the cell mass. The fat stored in these cells represents the body's main storehouse of energy-giving nutrients that can later be used to provide energy wherever it is needed in the body. \n**Carbohydrates.** Carbohydrates play a major role in cell nutrition and, as parts of glycoprotein molecules, have structural functions. Most human cells do not maintain large stores of carbohydrates; the amount usually averages only about 1% of their total mass but increases to as much as 3% in muscle cells and, occasionally, to 6% in liver cells. However, carbohydrate in the form of dissolved glucose is always present in the surrounding extracellular fluid so that it is readily available to the cell. Also, a small amount of carbohydrate is stored in cells as *glycogen,* an insoluble polymer of glucose that can be depolymerized and used rapidly to supply the cell's energy needs. \n#### CELL STRUCTURE \nThe cell contains highly organized physical structures called *intracellular organelles,* which are critical for cell function. For example, without one of the organelles, the *mitochondria,* more than 95% of the cell's energy release from nutrients would cease immediately. The most important organelles and other structures of the cell are shown in **[Figure 2-2](#page-17-0)**. \n#### **MEMBRANOUS STRUCTURES OF THE CELL** \nMost organelles of the cell are covered by membranes composed primarily of lipids and proteins. These membranes include the *cell membrane, nuclear membrane, membrane of the endoplasmic reticulum,* and *membranes of the mitochondria, lysosomes,* and *Golgi apparatus.* \n \n**Figure 2-3.** Structure of the cell membrane showing that it is composed mainly of a lipid bilayer of phospholipid molecules, but with large numbers of protein molecules protruding through the layer. Also, carbohydrate moieties are attached to the protein molecules on the outside of the membrane and to additional protein molecules on the inside. \nThe lipids in membranes provide a barrier that impedes movement of water and water-soluble substances from one cell compartment to another because water is not soluble in lipids. However, protein molecules often penetrate all the way through membranes, thus providing specialized pathways, often organized into actual *pores,* for passage of specific substances through membranes. Also, many other membrane proteins are *enzymes,* which catalyze a multitude of different chemical reactions, discussed here and in subsequent chapters. \n#### **Cell Membrane** \nThe cell membrane (also called the *plasma membrane*) envelops the cell and is a thin, pliable, elastic structure only 7.5 to 10 nanometers thick. It is composed almost entirely of proteins and lipids. The approximate composition is 55% proteins, 25% phospholipids, 13% cholesterol, 4% other lipids, and 3% carbohydrates. \n**The Cell Membrane Lipid Barrier Impedes Penetration by Water-Soluble Substances. [Figure 2-3](#page-18-0)** shows the structure of the cell membrane. Its basic structure is a *lipid bilayer,* which is a thin, double-layered film of lipids—each layer only one molecule thick—that is continuous over the entire cell surface. Interspersed in this lipid film are large globular proteins. \nThe basic lipid bilayer is composed of three main types of lipids—*phospholipids, sphingolipids,* and *cholesterol*. Phospholipids are the most abundant cell membrane lipids. One end of each phospholipid molecule is *hydrophilic* and soluble in water*.* The other end is *hydrophobic* and soluble only in fats. The phosphate end of the phospholipid is hydrophilic, and the fatty acid portion is hydrophobic. \nBecause the hydrophobic portions of the phospholipid molecules are repelled by water but are mutually attracted to one another, they have a natural tendency to attach to one another in the middle of the membrane, as shown in **[Figure 2-3](#page-18-0)**. The hydrophilic phosphate portions then constitute the two surfaces of the complete cell membrane, in contact with *intracellular* water on the inside of the membrane and *extracellular* water on the outside surface. \nThe lipid layer in the middle of the membrane is impermeable to the usual water-soluble substances, such as ions, glucose, and urea. Conversely, fat-soluble substances, such as oxygen, carbon dioxide, and alcohol, can penetrate this portion of the membrane with ease. \nSphingolipids, derived from the amino alcohol *sphingosine*, also have hydrophobic and hydrophilic groups and are present in small amounts in the cell membranes, especially nerve cells. Complex sphingolipids in cell membranes are thought to serve several functions, including protection from harmful environmental factors, signal transmission, and adhesion sites for extracellular proteins. \nCholesterol molecules in membranes are also lipids because their steroid nuclei are highly fat-soluble. These molecules, in a sense, are dissolved in the bilayer of the membrane. They mainly help determine the degree of permeability (or impermeability) of the bilayer to watersoluble constituents of body fluids. Cholesterol controls much of the fluidity of the membrane as well. \n#### **Integral and Peripheral Cell Membrane Proteins.** \n**[Figure 2-3](#page-18-0)** also shows globular masses floating in the lipid bilayer. These membrane proteins are mainly *glycoproteins.* There are two types of cell membrane proteins, *integral proteins,* which protrude all the way through the membrane, and *peripheral proteins,* which are attached only to one surface of the membrane and do not penetrate all the way through. \nMany of the integral proteins provide structural *channels* (or *pores*) through which water molecules and watersoluble substances, especially ions, can diffuse between extracellular and intracellular fluids. These protein channels also have selective properties that allow preferential diffusion of some substances over others. \nOther integral proteins act as *carrier proteins* for transporting substances that otherwise could not penetrate the lipid bilayer. Sometimes, these carrier proteins even transport substances in the direction opposite to their electrochemical gradients for diffusion, which is called *active transport.* Still others act as *enzymes.* \nIntegral membrane proteins can also serve as *receptors* for water-soluble chemicals, such as peptide hormones, that do not easily penetrate the cell membrane. Interaction of cell membrane receptors with specific *ligands* that bind to the receptor causes conformational changes in the receptor protein. This process, in turn, enzymatically activates the intracellular part of the protein or induces interactions between the receptor and proteins in the cytoplasm that act as *second messengers,* relaying the signal from the extracellular part of the receptor to the interior of the cell. In this way, integral proteins spanning the cell membrane provide a means of conveying information about the environment to the cell interior. \nPeripheral protein molecules are often attached to integral proteins. These peripheral proteins function almost entirely as enzymes or as controllers of transport of substances through cell membrane *pores.* \n#### **Membrane Carbohydrates—The Cell \"Glycocalyx.\"** \nMembrane carbohydrates occur almost invariably in combination with proteins or lipids in the form of *glycoproteins* or *glycolipids.* In fact, most of the integral proteins are glycoproteins, and about one-tenth of the membrane lipid molecules are glycolipids. The *glyco-* portions of these molecules almost invariably protrude to the outside of the cell, dangling outward from the cell surface. Many other carbohydrate compounds, called *proteoglycans* which are mainly carbohydrates bound to small protein cores—are loosely attached to the outer surface of the cell as well. Thus, the entire outside surface of the cell often has a loose carbohydrate coat called the *glycocalyx.* \nThe carbohydrate moieties attached to the outer surface of the cell have several important functions: \n- 1. Many of them have a negative electrical charge, which gives most cells an overall negative surface charge that repels other negatively charged objects.\n- 2. The glycocalyx of some cells attaches to the glycocalyx of other cells, thus attaching cells to one another.\n- 3. Many of the carbohydrates act as *receptors* for binding hormones, such as insulin. When bound, this combination activates attached internal proteins that in turn activate a cascade of intracellular enzymes.\n- 4. Some carbohydrate moieties enter into immune reactions, as discussed in Chapter 35. \n#### **CYTOPLASM AND ITS ORGANELLES** \nThe cytoplasm is filled with minute and large dispersed particles and organelles. The jelly-like fluid portion of the cytoplasm in which the particles are dispersed is called *cytosol* and contains mainly dissolved proteins, electrolytes, and glucose. \nDispersed in the cytoplasm are neutral fat globules, glycogen granules, ribosomes, secretory vesicles, and five especially important organelles—the *endoplasmic reticulum,* the *Golgi apparatus, mitochondria, lysosomes,* and *peroxisomes.* \n#### **Endoplasmic Reticulum** \n**[Figure 2-2](#page-17-0)** shows the *endoplasmic reticulum,* a network of tubular structures called *cisternae* and flat vesicular structures in the cytoplasm. This organelle helps process molecules made by the cell and transports them to their specific destinations inside or outside the cell. The tubules and vesicles interconnect. Also, their walls are constructed of lipid bilayer membranes that contain large amounts of proteins, similar to the cell membrane. The total surface area of this structure in some cells—the liver cells, for example—can be as much as 30 to 40 times the cell membrane area. \nThe detailed structure of a small portion of endoplasmic reticulum is shown in **[Figure 2-4](#page-20-0)**. The space inside the tubules and vesicles is filled with *endoplasmic matrix,* a watery medium that is different from fluid in the cytosol outside the endoplasmic reticulum. Electron micrographs show that the space inside the endoplasmic reticulum is connected with the space between the two membrane surfaces of the nuclear membrane. \nSubstances formed in some parts of the cell enter the space of the endoplasmic reticulum and are then directed to other parts of the cell. Also, the vast surface area of this \n \n**Figure 2-4.** Structure of the endoplasmic reticulum. \nreticulum and the multiple enzyme systems attached to its membranes provide the mechanisms for a major share of the cell's metabolic functions. \n**Ribosomes and the Rough (Granular) Endoplasmic Reticulum.** Attached to the outer surfaces of many parts of the endoplasmic reticulum are large numbers of minute granular particles called *ribosomes.* Where these particles are present, the reticulum is called the *rough (granular) endoplasmic reticulum.* The ribosomes are composed of a mixture of RNA and proteins; they function to synthesize new protein molecules in the cell, as discussed later in this chapter and in Chapter 3. \n**Smooth (Agranular) Endoplasmic Reticulum.** Part of the endoplasmic reticulum has no attached ribosomes. This part is called the *smooth,* or *agranular, endoplasmic reticulum.* The smooth reticulum functions for the synthesis of lipid substances and for other processes of the cells promoted by intrareticular enzymes. \n#### **Golgi Apparatus** \nThe Golgi apparatus, shown in **[Figure 2-5](#page-20-1)**, is closely related to the endoplasmic reticulum. It has membranes similar to those of the smooth endoplasmic reticulum. The Golgi apparatus is usually composed of four or more stacked layers of thin, flat, enclosed vesicles lying near one side of the nucleus. This apparatus is prominent in secretory cells, where it is located on the side of the cell from which secretory substances are extruded. \nThe Golgi apparatus functions in association with the endoplasmic reticulum. As shown in **[Figure 2-5](#page-20-1)**, small *transport vesicles* (also called *endoplasmic reticulum vesicles* [*ER vesicles*]) continually pinch off from the endoplasmic reticulum and shortly thereafter fuse with the Golgi apparatus. In this way, substances entrapped in ER \n \n**Figure 2-5.** A typical Golgi apparatus and its relationship to the endoplasmic reticulum (ER) and the nucleus. \nvesicles are transported from the endoplasmic reticulum to the Golgi apparatus. The transported substances are then processed in the Golgi apparatus to form lysosomes, secretory vesicles, and other cytoplasmic components (discussed later in this chapter). \n#### **Lysosomes** \nLysosomes, shown in **[Figure 2-2](#page-17-0)**, are vesicular organelles that form by breaking off from the Golgi apparatus; they then disperse throughout the cytoplasm. The lysosomes provide an *intracellular digestive system* that allows the cell to digest the following: (1) damaged cellular structures; (2) food particles that have been ingested by the cell; and (3) unwanted matter such as bacteria. Lysosome are different in various cell types but are usually 250 to 750 nanometers in diameter. They are surrounded by typical lipid bilayer membranes and are filled with large numbers of small granules, 5 to 8 nanometers in diameter, which are protein aggregates of as many as 40 different *hydrolase (digestive) enzymes.* A hydrolytic enzyme is capable of splitting an organic compound into two or more parts by combining hydrogen from a water molecule with one part of the compound and combining the hydroxyl portion of the water molecule with the other part of the compound. For example, protein is hydrolyzed to form amino acids, glycogen is hydrolyzed to form glucose, and lipids are hydrolyzed to form fatty acids and glycerol. \nHydrolytic enzymes are highly concentrated in lysosomes. Ordinarily, the membrane surrounding the lysosome prevents the enclosed hydrolytic enzymes from coming into contact with other substances in the cell and therefore prevents their digestive actions. However, some conditions of the cell break the membranes of lysosomes, allowing release of the digestive enzymes. These enzymes then split the organic substances with which they come in contact into small, highly diffusible substances such as \n \n**Figure 2-6.** Secretory granules (secretory vesicles) in acinar cells of the pancreas. \namino acids and glucose. Some of the specific functions of lysosomes are discussed later in this chapter. \n#### **Peroxisomes** \nPeroxisomes are physically similar to lysosomes, but they are different in two important ways. First, they are believed to be formed by self-replication (or perhaps by budding off from the smooth endoplasmic reticulum) rather than from the Golgi apparatus. Second, they contain *oxidases* rather than hydrolases. Several of the oxidases are capable of combining oxygen with hydrogen ions derived from different intracellular chemicals to form hydrogen peroxide (H2O2). Hydrogen peroxide is a highly oxidizing substance and is used in association with *catalase,* another oxidase enzyme present in large quantities in peroxisomes, to oxidize many substances that might otherwise be poisonous to the cell. For example, about half the alcohol that a person drinks is detoxified into acetaldehyde by the peroxisomes of the liver cells in this manner. A major function of peroxisomes is to catabolize long-chain fatty acids. \n#### **Secretory Vesicles** \nOne of the important functions of many cells is secretion of special chemical substances. Almost all such secretory substances are formed by the endoplasmic reticulum– Golgi apparatus system and are then released from the Golgi apparatus into the cytoplasm in the form of storage vesicles called *secretory vesicles* or *secretory granules.* **[Figure 2-6](#page-21-0)** shows typical secretory vesicles inside pancreatic acinar cells; these vesicles store protein proenzymes (enzymes that are not yet activated). The proenzymes are secreted later through the outer cell membrane into the pancreatic duct and then into the duodenum, where they become activated and perform digestive functions on the food in the intestinal tract. \n#### **Mitochondria** \nThe mitochondria, shown in **[Figure 2-2](#page-17-0)** and **[Figure 2-7](#page-21-1)**, are called the *powerhouses* of the cell. Without them, cells would be unable to extract enough energy from the nutrients, and essentially all cellular functions would cease. \n \n**Figure 2-7.** Structure of a mitochondrion. \nMitochondria are present in all areas of each cell's cytoplasm, but the total number per cell varies from less than 100 up to several thousand, depending on the energy requirements of the cell. Cardiac muscle cells (cardiomyocytes), for example, use large amounts of energy and have far more mitochondria than fat cells (adipocytes), which are much less active and use less energy. Furthermore, the mitochondria are concentrated in those portions of the cell responsible for the major share of its energy metabolism. They are also variable in size and shape. Some mitochondria are only a few hundred nanometers in diameter and are globular in shape, whereas others are elongated and are as large as 1 micrometer in diameter and 7 micrometers long. Still others are branching and filamentous. \nThe basic structure of the mitochondrion, shown in **[Figure 2-7](#page-21-1)**, is composed mainly of two lipid bilayerprotein membranes, an *outer membrane* and an *inner membrane.* Many infoldings of the inner membrane form shelves or tubules called *cristae* onto which oxidative enzymes are attached. The cristae provide a large surface area for chemical reactions to occur. In addition, the inner cavity of the mitochondrion is filled with a *matrix* that contains large quantities of dissolved enzymes necessary for extracting energy from nutrients. These enzymes operate in association with oxidative enzymes on the cristae to cause oxidation of nutrients, thereby forming carbon dioxide and water and, at the same time, releasing energy. The liberated energy is used to synthesize a high-energy substance called *adenosine triphosphate* (ATP). ATP is then transported out of the mitochondrion and diffuses throughout the cell to release its own energy wherever it is needed for performing cellular functions. The chemical details of ATP formation by the mitochondrion are provided in Chapter 68, but some basic functions of ATP in the cell are introduced later in this chapter. \nMitochondria are self-replicative, which means that one mitochondrion can form a second one, a third one, and so on whenever the cell needs increased amounts of ATP. Indeed, the mitochondria contain DNA similar to that found in the cell nucleus. In Chapter 3, we will see that DNA is the basic constituent of the nucleus that \n \n**Figure 2-8.** Cell cytoskeleton composed of protein fibers called microfilaments, intermediate filaments, and microtubules. \ncontrols replication of the cell. The DNA of the mitochondrion plays a similar role, controlling replication of the mitochondrion. Cells that are faced with increased energy demands—for example, in skeletal muscles subjected to chronic exercise training—may increase the density of mitochondria to supply the additional energy required. \n#### **Cell Cytoskeleton—Filament and Tubular Structures** \nThe cell cytoskeleton is a network of fibrillar proteins organized into filaments or tubules. These originate as precursor proteins synthesized by ribosomes in the cytoplasm. The precursor molecules then polymerize to form *filaments* (**[Figure 2-8](#page-22-0)**). As an example, large numbers of actin *microfilaments* frequently occur in the outer zone of the cytoplasm, called the *ectoplasm,* to form an elastic support for the cell membrane. Also, in muscle cells, actin and myosin filaments are organized into a special contractile machine that is the basis for muscle contraction, as discussed in Chapter 6. \n*Intermediate filaments* are generally strong ropelike filaments that often work together with microtubules, providing strength and support for the fragile tubulin structures. They are called *intermediate* because their average diameter is between that of narrower actin microfilaments and wider myosin filaments found in muscle cells. Their functions are mainly mechanical, and they are less dynamic than actin microfilaments or microtubules. All cells have intermediate filaments, although the protein subunits of these structures vary, depending on the cell type. Specific intermediate filaments found in various cells include desmin filaments in muscle cells, neurofilaments in neurons, and keratins in epithelial cells. \nA special type of stiff filament composed of polymerized *tubulin* molecules is used in all cells to construct strong tubular structures, the *microtubules.* **[Figure 2-8](#page-22-0)** shows typical microtubules of a cell. \nAnother example of microtubules is the tubular skeletal structure in the center of each cilium that radiates upward from the cell cytoplasm to the tip of the cilium. This structure is discussed later in the chapter (see **[Figure 2-18](#page-31-0)**). Also, both the *centrioles* and *mitotic spindles* of cells undergoing mitosis are composed of stiff microtubules. \nA major function of microtubules is to act as a *cytoskeleton,* providing rigid physical structures for certain parts of cells. The cell cytoskeleton not only determines cell shape but also participates in cell division, allows cells to move, and provides a tracklike system that directs the movement of organelles in the cells. Microtubules serve as the conveyor belts for the intracellular transport of vesicles, granules, and organelles such as mitochondria. \n#### **Nucleus** \nThe nucleus is the control center of the cell and sends messages to the cell to grow and mature, replicate, or die. Briefly, the nucleus contains large quantities of DNA, \n \n**Figure 2-9.** Structure of the nucleus. \nwhich comprise the *genes.* The genes determine the characteristics of the cell's proteins, including the structural proteins, as well as the intracellular enzymes that control cytoplasmic and nuclear activities. \nThe genes also control and promote cell reproduction. The genes first reproduce to create two identical sets of genes; then the cell splits by a special process called *mitosis* to form two daughter cells, each of which receives one of the two sets of DNA genes. All these activities of the nucleus are discussed in Chapter 3. \nUnfortunately, the appearance of the nucleus under the microscope does not provide many clues to the mechanisms whereby the nucleus performs its control activities. **[Figure 2-9](#page-23-0)** shows the light microscopic appearance of the *interphase* nucleus (during the period between mitoses), revealing darkly staining *chromatin material* throughout the nucleoplasm. During mitosis, the chromatin material organizes in the form of highly structured *chromosomes,* which can then be easily identified using the light microscope, as illustrated in Chapter 3. \n**Nuclear Membrane.** The *nuclear membrane,* also called the *nuclear envelope,* is actually two separate bilayer membranes, one inside the other. The outer membrane is continuous with the endoplasmic reticulum of the cell cytoplasm, and the space between the two nuclear membranes is also continuous with the space inside the endoplasmic reticulum, as shown in **[Figure 2-9](#page-23-0)**. \nThe nuclear membrane is penetrated by several thousand *nuclear pores.* Large complexes of proteins are attached at the edges of the pores so that the central area of each pore is only about 9 nanometers in diameter. Even this size is large enough to allow molecules up to a molecular weight of 44,000 to pass through with reasonable ease. \n**Nucleoli and Formation of Ribosomes.** The nuclei of most cells contain one or more highly staining structures called *nucleoli.* The nucleolus, unlike most other organelles discussed here, does not have a limiting membrane. Instead, it is simply an accumulation of large amounts of \n \n**Figure 2-10.** Comparison of sizes of precellular organisms with that of the average cell in the human body. \nRNA and proteins of the types found in ribosomes. The nucleolus enlarges considerably when the cell is actively synthesizing proteins. \nFormation of the nucleoli (and of the ribosomes in the cytoplasm outside the nucleus) begins in the nucleus. First, specific DNA genes in the chromosomes cause RNA to be synthesized. Some of this synthesized RNA is stored in the nucleoli, but most of it is transported outward through the nuclear pores into the cytoplasm. Here it is used in conjunction with specific proteins to assemble \"mature\" ribosomes that play an essential role in forming cytoplasmic proteins, as discussed in Chapter 3. \n#### COMPARISON OF THE ANIMAL CELL WITH PRECELLULAR FORMS OF LIFE \nThe cell is a complicated organism that required many hundreds of millions of years to develop after the earliest forms of life, microorganisms that may have been similar to present-day *viruses,* first appeared on earth. **[Figure 2-10](#page-23-1)** shows the relative sizes of the following: (1) the smallest known virus; (2) a large virus; (3) a *Rickettsia;* (4) a *bacterium;* and (5) a *nucleated cell,* This demonstrates that the cell has a diameter about 1000 times that of the smallest virus and therefore a volume about 1 billion times that of the smallest virus. Correspondingly, the functions and anatomical organization of the cell are also far more complex than those of the virus. \nThe essential life-giving constituent of the small virus is a *nucleic acid* embedded in a coat of protein. This nucleic acid is composed of the same basic nucleic acid constituents (DNA or RNA) found in mammalian cells and is capable of reproducing itself under appropriate conditions. Thus, the virus propagates its lineage from generation to generation and is therefore a living structure in the same way that cells and humans are living structures. \nAs life evolved, other chemicals in addition to nucleic acid and simple proteins became integral parts of the organism, and specialized functions began to develop in different parts of the virus. A membrane formed around the virus and, inside the membrane, a fluid matrix appeared. Specialized chemicals then developed inside the fluid to perform special functions; many protein enzymes appeared that were capable of catalyzing chemical reactions, thus determining the organism's activities. \nIn still later stages of life, particularly in the rickettsial and bacterial stages, *organelles* developed inside the organism. These represent physical structures of chemical aggregates that perform functions in a more efficient manner than what can be achieved by dispersed chemicals throughout the fluid matrix. \nFinally, in the nucleated cell, still more complex organelles developed, the most important of which is the *nucleus*. The nucleus distinguishes this type of cell from all lower forms of life; it provides a control center for all cellular activities and for reproduction of new cells generation after generation, with each new cell having almost exactly the same structure as its progenitor. \n#### FUNCTIONAL SYSTEMS OF THE CELL \nIn the remainder of this chapter, we discuss some functional systems of the cell that make it a living organism. \n#### **ENDOCYTOSIS—INGESTION BY THE CELL** \nIf a cell is to live and grow and reproduce, it must obtain nutrients and other substances from the surrounding fluids. Most substances pass through the cell membrane by the processes of diffusion and *active transport.* \nDiffusion involves simple movement through the membrane caused by the random motion of the molecules of the substance. Substances move through cell membrane pores or, in the case of lipid-soluble substances, through the lipid matrix of the membrane. \nActive transport involves actually carrying a substance through the membrane by a physical protein structure that penetrates all the way through the membrane. These active transport mechanisms are so important to cell function that they are presented in detail in Chapter 4. \nLarge particles enter the cell by a specialized function of the cell membrane called *endocytosis* (Video 2-1). The principal forms of endocytosis are *pinocytosis* and *phagocytosis.* Pinocytosis means the ingestion of minute particles that form vesicles of extracellular fluid and particulate constituents inside the cell cytoplasm. Phagocytosis means the ingestion of large particles, such as bacteria, whole cells, or portions of degenerating tissue. \n**Pinocytosis.** Pinocytosis occurs continually in the cell membranes of most cells, but is especially rapid in some cells. For example, it occurs so rapidly in macrophages that about 3% of the total macrophage membrane is engulfed in the form of vesicles each minute. Even so, the pinocytotic vesicles are so small—usually only 100 to 200 nanometers in diameter—that most of them can be seen only with an electron microscope. \n \n**Figure 2-11.** Mechanism of pinocytosis. \nPinocytosis is the only means whereby most large macromolecules, such as most proteins, can enter cells. In fact, the rate at which pinocytotic vesicles form is usually enhanced when such macromolecules attach to the cell membrane. \n**[Figure 2-11](#page-24-0)** demonstrates the successive steps of pinocytosis *(A–D),* showing three molecules of protein attaching to the membrane. These molecules usually attach to specialized protein *receptors* on the surface of the membrane that are specific for the type of protein that is to be absorbed. The receptors generally are concentrated in small pits on the outer surface of the cell membrane, called *coated pits.* On the inside of the cell membrane beneath these pits is a latticework of fibrillar protein called *clathrin,* as well as other proteins, perhaps including contractile filaments of *actin* and *myosin.* Once the protein molecules have bound with the receptors, the surface properties of the local membrane change in such a way that the entire pit invaginates inward, and fibrillar proteins surrounding the invaginating pit cause its borders to close over the attached proteins, as well as over a small amount of extracellular fluid. Immediately thereafter, the invaginated portion of the membrane breaks away from the surface of the cell, forming a *pinocytotic vesicle* inside the cytoplasm of the cell. \nWhat causes the cell membrane to go through the necessary contortions to form pinocytotic vesicles is still unclear. This process requires energy from within the cell, which is supplied by ATP, a high-energy substance discussed later in this chapter. This process also requires the presence of calcium ions in the extracellular fluid, which probably react with contractile protein filaments beneath the coated pits to provide the force for pinching the vesicles away from the cell membrane. \n**Phagocytosis.** Phagocytosis occurs in much the same way as pinocytosis, except that it involves large particles rather than molecules. Only certain cells have the capability of phagocytosis—notably, tissue macrophages and some white blood cells. \n \n**Figure 2-12.** Digestion of substances in pinocytotic or phagocytic vesicles by enzymes derived from lysosomes. \nPhagocytosis is initiated when a particle such as a bacterium, dead cell, or tissue debris binds with receptors on the surface of the phagocyte. In the case of bacteria, each bacterium is usually already attached to a specific antibody; it is the antibody that attaches to the phagocyte receptors, dragging the bacterium along with it. This intermediation of antibodies is called *opsonization,* which is discussed in Chapters 34 and 35. \nPhagocytosis occurs in the following steps: \n- 1. The cell membrane receptors attach to the surface ligands of the particle.\n- 2. The edges of the membrane around the points of attachment evaginate outward within a fraction of a second to surround the entire particle; then, progressively more and more membrane receptors attach to the particle ligands. All this occurs suddenly in a zipper-like manner to form a closed *phagocytic vesicle.*\n- 3. Actin and other contractile fibrils in the cytoplasm surround the phagocytic vesicle and contract around its outer edge, pushing the vesicle to the interior.\n- 4. The contractile proteins then pinch the stem of the vesicle so completely that the vesicle separates from the cell membrane, leaving the vesicle in the cell interior in the same way that pinocytotic vesicles are formed. \n#### **LYSOSOMES DIGEST PINOCYTOTIC AND PHAGOCYTIC FOREIGN SUBSTANCES INSIDE THE CELL** \nAlmost immediately after a pinocytotic or phagocytic vesicle appears inside a cell, one or more *lysosomes* become attached to the vesicle and empty their *acid hydrolases* to the inside of the vesicle, as shown in **[Figure 2-12](#page-25-0)**. Thus, a *digestive vesicle* is formed inside the cell cytoplasm in which the vesicular hydrolases begin hydrolyzing the proteins, carbohydrates, lipids, and other substances in the vesicle. The products of digestion are small molecules of substances such as amino acids, glucose, and phosphates that can diffuse through the membrane of the vesicle into the cytoplasm. What is left of the digestive vesicle, called the *residual body,* represents indigestible substances. In most cases, the residual body is finally excreted through the cell membrane by a process called *exocytosis,* which is essentially the opposite of endocytosis. Thus, the pinocytotic and phagocytic vesicles containing lysosomes can be called the *digestive organs* of the cells. \n**Lysosomes and Regression of Tissues and Autolysis of Damaged Cells.** Tissues of the body often regress to a smaller size. For example, this regression occurs in the uterus after pregnancy, in muscles during long periods of inactivity, and in mammary glands at the end of lactation. Lysosomes are responsible for much of this regression. \nAnother special role of the lysosomes is the removal of damaged cells or damaged portions of cells from tissues. Damage to the cell—caused by heat, cold, trauma, chemicals, or any other factor—induces lysosomes to rupture. The released hydrolases immediately begin to digest the surrounding organic substances. If the damage is slight, only a portion of the cell is removed, and the cell is then repaired. If the damage is severe, the entire cell is digested, a process called *autolysis.* In this way, the cell is completely removed, and a new cell of the same type is formed, ordinarily by mitotic reproduction of an adjacent cell to take the place of the old one. \nThe lysosomes also contain bactericidal agents that can kill phagocytized bacteria before they cause cellular damage. These agents include the following: (1) *lysozyme,* which dissolves the bacterial cell wall; (2) *lysoferrin,* which binds iron and other substances before they can promote bacterial growth; and (3) acid at a pH of about 5.0, which activates the hydrolases and inactivates bacterial metabolic systems. \n#### **Autophagy and Recycling of Cell Organelles.** \nLysosomes play a key role in the process of *autophagy,* which literally means \"to eat oneself.\" Autophagy is a housekeeping process whereby obsolete organelles and large protein aggregates are degraded and recycled (**[Figure 2-13](#page-26-0)**). Worn-out cell organelles are transferred to lysosomes by double-membrane structures called *autophagosomes,* which are formed in the cytosol. Invagination of the lysosomal membrane and the formation of vesicles provides another pathway for cytosolic structures to be transported into the lumen of lysosomes. Once inside the lysosomes, the organelles are digested, and the nutrients are reused by the cell. Autophagy contributes to the routine turnover of cytoplasmic components; it is a key mechanism for tissue development, cell survival when nutrients are scarce, and maintenance of homeostasis. In liver cells, for example, the average mitochondrion normally has a life span of only about 10 days before it is destroyed. \n \n**Figure 2-13.** Schematic diagram of autophagy steps. \n#### **SYNTHESIS OF CELLULAR STRUCTURES BY ENDOPLASMIC RETICULUM AND GOLGI APPARATUS** \n#### **Endoplasmic Reticulum Functions** \nThe extensiveness of the endoplasmic reticulum and Golgi apparatus in secretory cells has already been emphasized. These structures are formed primarily of lipid bilayer membranes, similar to the cell membrane, and their walls are loaded with protein enzymes that catalyze the synthesis of many substances required by the cell. \nMost synthesis begins in the endoplasmic reticulum. The products formed there are then passed on to the Golgi apparatus, where they are further processed before being released into the cytoplasm. First, however, let us note the specific products that are synthesized in specific portions of the endoplasmic reticulum and Golgi apparatus. \n#### **Proteins Synthesis by the Rough Endoplasmic Reticu-** \n**lum.** The rough endoplasmic reticulum is characterized by large numbers of ribosomes attached to the outer surfaces of the endoplasmic reticulum membrane. As discussed in Chapter 3, protein molecules are synthesized within the structures of the ribosomes. The ribosomes extrude some of the synthesized protein molecules directly into the cytosol, but they also extrude many more through the wall of the endoplasmic reticulum to the interior of the endoplasmic vesicles and tubules into the *endoplasmic matrix.* \n#### **Lipid Synthesis by the Smooth Endoplasmic Reticu-** \n**lum.** The endoplasmic reticulum also synthesizes lipids, especially phospholipids and cholesterol. These lipids are rapidly incorporated into the lipid bilayer of the endoplasmic reticulum, thus causing the endoplasmic reticulum to grow more extensive. This process occurs mainly in the smooth portion of the endoplasmic reticulum. \nTo keep the endoplasmic reticulum from growing beyond the needs of the cell, small vesicles called *ER vesicles* or *transport vesicles* continually break away from the smooth reticulum; most of these vesicles then migrate rapidly to the Golgi apparatus. \n#### **Other Functions of the Endoplasmic Reticulum.** \nOther significant functions of the endoplasmic reticulum, especially the smooth reticulum, include the following: \n- 1. It provides the enzymes that control glycogen breakdown when glycogen is to be used for energy.\n- 2. It provides a vast number of enzymes that are capable of detoxifying substances, such as drugs, that might damage the cell. It achieves detoxification by processes such as coagulation, oxidation, hydrolysis, and conjugation with glycuronic acid. \n#### **Golgi Apparatus Functions** \n**Synthetic Functions of the Golgi Apparatus.** Although a major function of the Golgi apparatus is to provide additional processing of substances already formed in the endoplasmic reticulum, it can also synthesize certain carbohydrates that cannot be formed in the endoplasmic reticulum. This is especially true for the formation of large saccharide polymers bound with small amounts of protein; important examples include *hyaluronic acid* and *chondroitin sulfate.* \nA few of the many functions of hyaluronic acid and chondroitin sulfate in the body are as follows: (1) they are the major components of proteoglycans secreted in mucus and other glandular secretions; (2) they are the major components of the *ground substance*, or nonfibrous components of the extracellular matrix, outside the cells in the interstitial spaces, which act as fillers between collagen fibers and cells; (3) they are principal components of the organic matrix in both cartilage and bone; and (4) they are important in many cell activities, including migration and proliferation. \n \n**Figure 2-14.** Formation of proteins, lipids, and cellular vesicles by the endoplasmic reticulum and Golgi apparatus. \nProcessing of Endoplasmic Secretions by the Golgi Apparatus—Formation of Vesicles. Figure 2-14 summarizes the major functions of the endoplasmic reticulum and Golgi apparatus. As substances are formed in the endoplasmic reticulum, especially proteins, they are transported through the tubules toward portions of the smooth endoplasmic reticulum that lie nearest to the Golgi apparatus. At this point, transport vesicles composed of small envelopes of smooth endoplasmic reticulum continually break away and diffuse to the deepest layer of the Golgi apparatus. Inside these vesicles are synthesized proteins and other products from the endoplasmic reticulum. \nThe transport vesicles instantly fuse with the Golgi apparatus and empty their contained substances into the vesicular spaces of the Golgi apparatus. Here, additional carbohydrate moieties are added to the secretions. Also, an important function of the Golgi apparatus is to compact the endoplasmic reticular secretions into highly concentrated packets. As the secretions pass toward the outermost layers of the Golgi apparatus, the compaction and processing proceed. Finally, both small and large vesicles continually break away from the Golgi apparatus, carrying with them the compacted secretory substances and diffusing throughout the cell. \nThe following example provides an idea of the timing of these processes. When a glandular cell is bathed in amino acids, newly formed protein molecules can be detected in the granular endoplasmic reticulum within 3 to 5 minutes. Within 20 minutes, newly formed proteins are already present in the Golgi apparatus and, within 1 to 2 hours, the proteins are secreted from the surface of the cell. \nTypes of Vesicles Formed by the Golgi Apparatus—Secretory Vesicles and Lysosomes. In a highly secretory cell, the vesicles formed by the Golgi apparatus are mainly secretory vesicles containing proteins that are secreted through the surface of the cell membrane. These secretory vesicles first diffuse to the cell membrane and then fuse with it and empty their substances to the exterior by the mechanism called exocytosis. Exocytosis, in most cases, is stimulated by entry of calcium ions into the cell. Calcium ions interact with the vesicular membrane and cause its fusion with the cell membrane, followed by exocytosis—opening of the membrane's outer surface and extrusion of its contents outside the cell. Some vesicles, however, are destined for intracellular use. \n**Use of Intracellular Vesicles to Replenish Cellular Membranes.** Some intracellular vesicles formed by the Golgi apparatus fuse with the cell membrane or with the membranes of intracellular structures such as the mitochondria and even the endoplasmic reticulum. This fusion increases the expanse of these membranes and replenishes the membranes as they are used up. For example, the cell membrane loses much of its substance every time it forms a phagocytic or pinocytotic vesicle, and the vesicular membranes of the Golgi apparatus continually replenish the cell membrane. \nIn summary, the membranous system of the endoplasmic reticulum and Golgi apparatus are highly metabolic and capable of forming new intracellular structures and secretory substances to be extruded from the cell.\n\nThe principal substances from which cells extract energy are foods that react chemically with oxygen—carbohydrates, fats, and proteins. In the human body, essentially all carbohydrates are converted into *glucose* by the digestive tract and liver before they reach the other cells of the body. Similarly, proteins are converted into *amino acids*, and fats are converted into *fatty acids*. **Figure 2-15** shows oxygen and the foodstuffs—glucose, fatty acids, and amino acids—all entering the cell. Inside the cell, they react chemically with oxygen under the influence of enzymes that control the reactions and channel the energy released in the proper direction. The details of all these digestive and metabolic functions are provided in Chapters 63 through 73. \nBriefly, almost all these oxidative reactions occur inside the mitochondria, and the energy that is released is used to form the high-energy compound ATP. Then, ATP, not the original food, is used throughout the cell to energize almost all the subsequent intracellular metabolic reactions. \n \n**Figure 2-15.** Formation of adenosine triphosphate (ATP) in the cell showing that most of the ATP is formed in the mitochondria. (ADP, Adenosine diphosphate; CoA, coenzyme A.) \n#### **Functional Characteristics of Adenosine Triphosphate** \n$$\\begin{array}{c|ccccccccccccccccccccccccccccccccccc$$ \n**Adenosine triphosphate** \nATP is a nucleotide composed of the following: (1) the nitrogenous base *adenine;* (2) the pentose sugar *ribose;* and (3) three *phosphate radicals.* The last two phosphate radicals are connected with the remainder of the molecule by *high-energy phosphate bonds,* which are represented in the formula shown by the symbol ∼. *Under the physical and chemical conditions of the body,* each of these high-energy bonds contains about 12,000 calories of energy per mole of ATP, which is many times greater than the energy stored in the average chemical bond, thus giving rise to the term *high-energy bond.* Furthermore, the high-energy phosphate bond is very labile, so that it can be split instantly on demand whenever energy is required to promote other intracellular reactions. \nWhen ATP releases its energy, a phosphoric acid radical is split away, and *adenosine diphosphate* (ADP) is formed. This released energy is used to energize many of the cell's other functions, such as syntheses of substances and muscular contraction. \nTo reconstitute the cellular ATP as it is used up, energy derived from the cellular nutrients causes ADP and phosphoric acid to recombine to form new ATP, and the entire process is repeated over and over. For these reasons, ATP has been called the *energy currency* of the cell because it can be spent and reformed continually, having a turnover time of only a few minutes. \n**Chemical Processes in the Formation of ATP—Role of the Mitochondria.** On entry into the cells, glucose is converted by enzymes in the *cytoplasm* into *pyruvic acid* (a process called *glycolysis*). A small amount of ADP is changed into ATP by the energy released during this conversion, but this amount accounts for less than 5% of the overall energy metabolism of the cell. \nAbout 95% of the cell's ATP formation occurs in the mitochondria. The pyruvic acid derived from carbohydrates, fatty acids from lipids, and amino acids from proteins is eventually converted into the compound *acetyl-coenzyme A* (CoA) in the matrix of mitochondria. This substance, in turn, is further dissolved (for the purpose of extracting its energy) by another series of enzymes in the mitochondrion matrix, undergoing dissolution in a sequence of chemical reactions called the *citric acid cycle,* or *Krebs cycle.* These chemical reactions are so important that they are explained in detail in Chapter 68. \nIn this citric acid cycle, acetyl-CoA is split into its component parts, *hydrogen atoms* and *carbon dioxide.* The carbon dioxide diffuses out of the mitochondria and eventually out of the cell; finally, it is excreted from the body through the lungs. \nThe hydrogen atoms, conversely, are highly reactive; they combine with oxygen that has also diffused into the mitochondria. This combination releases a tremendous amount of energy, which is used by mitochondria to convert large amounts of ADP to ATP. The processes of these reactions are complex, requiring the participation of many protein enzymes that are integral parts of mitochondrial *membranous shelves* that protrude into the mitochondrial matrix. The initial event is the removal of an electron from the hydrogen atom, thus converting it to a hydrogen ion. The terminal event is the combination of hydrogen ions with oxygen to form water and the release of large amounts of energy to globular proteins that protrude like knobs from the membranes of the mitochondrial shelves; these proteins are called *ATP synthetase*. Finally, the enzyme ATP synthetase uses the energy from the hydrogen ions to convert ADP to ATP. The newly formed ATP is transported out of the mitochondria into all parts of the cell cytoplasm and nucleoplasm, where it energizes multiple cell functions. \nThis overall process for formation of ATP is called the *chemiosmotic mechanism* of ATP formation. The chemical and physical details of this mechanism are presented \n \n**Figure 2-16.** Use of adenosine triphosphate (ATP; formed in the mitochondrion) to provide energy for three major cellular functions—membrane transport, protein synthesis, and muscle contraction. (ADP, Adenosine diphosphate.) \nin Chapter 68, and many of the detailed metabolic functions of ATP in the body are discussed in Chapters 68 through 72. \n**Uses of ATP for Cellular Function.** Energy from ATP is used to promote three major categories of cellular functions: (1) *transport* of substances through multiple cell membranes; (2) *synthesis of chemical compounds* throughout the cell; and (3) *mechanical work.* These uses of ATP are illustrated by the examples in **Figure 2-16**: (1) to supply energy for the transport of sodium through the cell membrane; (2) to promote protein synthesis by the ribosomes; and (3) to supply the energy needed during muscle contraction. \nIn addition to the membrane transport of sodium, energy from ATP is required for the membrane transport of potassium, calcium, magnesium, phosphate, chloride, urate, and hydrogen ions and many other ions, as well as various organic substances. Membrane transport is so important to cell function that some cells—the renal tubular cells, for example—use as much as 80% of the ATP that they form for this purpose alone. \nIn addition to synthesizing proteins, cells make phospholipids, cholesterol, purines, pyrimidines, and many other substances. Synthesis of almost any chemical compound requires energy. For example, a single protein molecule might be composed of as many as several thousand amino acids attached to one another by peptide linkages. The formation of each of these linkages requires energy derived from the breakdown of four high-energy bonds; thus, many thousand ATP molecules must release their energy as each protein molecule is formed. Indeed, some cells use as much as 75% of all the ATP formed in the cell \n \nFigure 2-17. Ameboid motion by a cell. \nsimply to synthesize new chemical compounds, especially protein molecules; this is particularly true during the growth phase of cells. \nAnother use of ATP is to supply energy for special cells to perform mechanical work. We discuss in Chapter 6 that each contraction of a muscle fiber requires the expenditure of large quantities of ATP energy. Other cells perform mechanical work in other ways, especially by *ciliary* and *ameboid motion*, described later in this chapter. The source of energy for all these types of mechanical work is ATP. \nIn summary, ATP is readily available to release its energy rapidly wherever it is needed in the cell. To replace ATP used by the cell, much slower chemical reactions break down carbohydrates, fats, and proteins and use the energy derived from these processes to form new ATP. More than 95% of this ATP is formed in the mitochondria, which is why the mitochondria are called the *powerhouses* of the cell. \n#### **LOCOMOTION OF CELLS** \nThe most obvious type of movement in the body is that which occurs in skeletal, cardiac, and smooth muscle cells, which constitute almost 50% of the entire body mass. The specialized functions of these cells are discussed in Chapters 6 through 9. Two other types of movement—ameboid locomotion and ciliary movement—occur in other cells. \n#### AMEBOID MOVEMENT \nAmeboid movement is a crawling-like movement of an entire cell in relation to its surroundings, such as movement of white blood cells through tissues. This type of movement gets its name from the fact that amebae move in this manner, and amebae have provided an excellent tool for studying the phenomenon. \nTypically, ameboid locomotion begins with the protrusion of a *pseudopodium* from one end of the cell. The pseudopodium projects away from the cell body and partially secures itself in a new tissue area; then the remainder of the cell is pulled toward the pseudopodium. **Figure 2-17** \ndemonstrates this process, showing an elongated cell, the right-hand end of which is a protruding pseudopodium. The membrane of this end of the cell is continually moving forward, and the membrane at the left-hand end of the cell is continually following along as the cell moves. \n**Mechanism of Ameboid Locomotion. [Figure 2-17](#page-29-1)** shows the general principle of ameboid motion. Basically, this results from the continual formation of new cell membrane at the leading edge of the pseudopodium and continual absorption of the membrane in the mid and rear portions of the cell. Two other effects are also essential for forward movement of the cell. The first is attachment of the pseudopodium to surrounding tissues so that it becomes fixed in its leading position while the remainder of the cell body is being pulled forward toward the point of attachment. This attachment is caused by *receptor proteins* that line the insides of exocytotic vesicles. When the vesicles become part of the pseudopodial membrane, they open so that their insides evert to the outside, and the receptors now protrude to the outside and attach to ligands in the surrounding tissues. \nAt the opposite end of the cell, the receptors pull away from their ligands and form new endocytotic vesicles. Then, inside the cell, these vesicles stream toward the pseudopodial end of the cell, where they are used to form new membrane for the pseudopodium. \nThe second essential effect for locomotion is to provide the energy required to pull the cell body in the direction of the pseudopodium. A moderate to large amount of the protein *actin* is in the cytoplasm of all cells*.* Much of the actin is in the form of single molecules that do not provide any motive power; however, these molecules polymerize to form a filamentous network, and the network contracts when it binds with an actin-binding protein such as *myosin.* The entire process is energized by the high-energy compound ATP. This is what occurs in the pseudopodium of a moving cell, where such a network of actin filaments forms anew inside the enlarging pseudopodium. Contraction also occurs in the ectoplasm of the cell body, where a preexisting actin network is already present beneath the cell membrane. \n#### **Types of Cells That Exhibit Ameboid Locomotion.** \nThe most common cells to exhibit ameboid locomotion in the human body are the *white blood cells* when they move out of the blood into the tissues to form *tissue macrophages.* Other types of cells can also move by ameboid locomotion under certain circumstances. For example, fibroblasts move into a damaged area to help repair the damage, and even the germinal cells of the skin, although ordinarily completely sessile cells, move toward a cut area to repair the opening. Cell locomotion is also especially important in the development of the embryo and fetus after fertilization of an ovum. For example, embryonic cells often must migrate long distances from their sites of origin to new areas during the development of special structures. \nSome types of cancer cells, such as sarcomas, which arise from connective tissue cells, are especially proficient at ameboid movement. This partially accounts for their relatively rapid spreading from one part of the body to another, known as *metastasis.* \n**Control of Ameboid Locomotion—Chemotaxis.** An important initiator of ameboid locomotion is the process called *chemotaxis,* which results from the appearance of certain chemical substances in the tissues. Any chemical substance that causes chemotaxis to occur is called a *chemotactic substance.* Most cells that exhibit ameboid locomotion move toward the source of a chemotactic substance—that is, from an area of lower concentration toward an area of higher concentration. This is called *positive chemotaxis.* Some cells move away from the source, which is called *negative chemotaxis.* \nHow does chemotaxis control the direction of ameboid locomotion? Although the answer is not certain, it is known that the side of the cell most exposed to the chemotactic substance develops membrane changes that cause pseudopodial protrusion. \n#### **CILIA AND CILIARY MOVEMENTS** \nThere are two types of cilia, *motile* and *nonmotile*, or *primary*, cilia. Motile cilia can undergo a whiplike movement on the surfaces of cells. This movement occurs mainly in two places in the human body, on the surfaces of the respiratory airways and on the inside surfaces of the uterine tubes (fallopian tubes) of the reproductive tract. In the nasal cavity and lower respiratory airways, the whiplike motion of motile cilia causes a layer of mucus to move at a rate of about 1 cm/min toward the pharynx, in this way continually clearing these passageways of mucus and particles that have become trapped in the mucus. In the uterine tubes, cilia cause slow movement of fluid from the ostium of the uterine tube toward the uterus cavity; this movement of fluid transports the ovum from the ovary to the uterus. \nAs shown in **[Figure 2-18](#page-31-0)**, a cilium has the appearance of a sharp-pointed straight or curved hair that projects 2 to 4 micrometers from the surface of the cell. Often, many motile cilia project from a single cell—for example, as many as 200 cilia on the surface of each epithelial cell inside the respiratory passageways. The cilium is covered by an outcropping of the cell membrane, and it is supported by 11 microtubules—nine double tubules located around the periphery of the cilium and two single tubules down the center, as demonstrated in the cross section shown in **[Figure 2-18](#page-31-0)**. Each cilium is an outgrowth of a structure that lies immediately beneath the cell membrane, called the *basal body* of the cilium. \nThe *flagellum of a sperm* is similar to a motile cilium; in fact, it has much the same type of structure and same type of contractile mechanism. The flagellum, however, is much longer and moves in quasisinusoidal waves instead of whiplike movements. \n \n**Figure 2-18.** Structure and function of the cilium. *(Modified from Satir P: Cilia. Sci Am 204:108, 1961.)* \nIn the inset of **[Figure 2-18](#page-31-0)**, movement of the motile cilium is shown. The cilium moves forward with a sudden, rapid whiplike stroke 10 to 20 times per second, bending sharply where it projects from the surface of the cell. Then it moves backward slowly to its initial position. The rapid, forward-thrusting, whiplike movement pushes the fluid lying adjacent to the cell in the direction that the cilium moves; the slow dragging movement in the backward direction has almost no effect on fluid movement. As a result, the fluid is continually propelled in the direction of the fast-forward stroke. Because most motile ciliated cells have large numbers of cilia on their surfaces, and because all the cilia are oriented in the same direction, this is an effective means for moving fluids from one part of the surface to another. \n**Mechanism of Ciliary Movement.** Although not all aspects of ciliary movement are known, we are aware of the following elements. First, the nine double tubules and two single tubules are all linked to one another by a complex of protein cross-linkages; this total complex of tubules and cross-linkages is called the *axoneme.* Second, even after removal of the membrane and destruction of other elements of the cilium in addition to the axoneme, the cilium can still beat under appropriate conditions. Third, two conditions are necessary for continued beating of the axoneme after removal of the other structures of the cilium: (1) the availability of ATP; and (2) appropriate ionic conditions, especially appropriate concentrations of magnesium and calcium. Fourth, during forward motion of the cilium, the double tubules on the front edge of the cilium slide outward toward the tip of the cilium, whereas those on the back edge remain in place. Fifth, multiple protein arms composed of the protein *dynein,* which has adenosine triphosphatase (ATPase) enzymatic activity, project from each double tubule toward an adjacent double tubule. \nGiven this basic information, it has been determined that the release of energy from ATP in contact with the ATPase dynein arms causes the heads of these arms to \"crawl\" rapidly along the surface of the adjacent double tubule. If the front tubules crawl outward while the back tubules remain stationary, bending occurs. \nThe way in which cilia contraction is controlled is not well understood. The cilia of some genetically abnormal cells do not have the two central single tubules, and these cilia fail to beat. Therefore, it is presumed that some signal, perhaps an electrochemical signal, is transmitted along these two central tubules to activate the dynein arms. \n**Nonmotile Primary Cilia Serve as Cell Sensory \"Antennae.\"** *Primary cilia* are nonmotile and generally occur only as a single cilium on each cell. Although the physiological functions of primary cilia are not fully understood, current evidence indicates that they function as cellular ''sensory antennae,\" which coordinate cellular signaling pathways involved in chemical and mechanical sensation, signal transduction, and cell growth. In the kidneys, for example, primary cilia are found in most epithelial cells of the tubules, projecting into the tubule lumen and acting as a flow sensor. In response to fluid flow over the tubular epithelial cells, the primary cilia bend and cause flow-induced changes in intracellular calcium signaling. These signals, in turn, initiate multiple effects on the cells. Defects in signaling by primary cilia in renal tubular epithelial cells are thought to contribute to various disorders, including the development of large fluid-filled cysts, a condition called *polycystic kidney disease*. \n#### Bibliography \nAlberts B, Johnson A, Lewis J, et al: Molecular Biology of the Cell, 6th ed. New York: Garland Science, 2014. \nBrandizzi F, Barlowe C: Organization of the ER-Golgi interface for membrane traffic control. Nat Rev Mol Cell Biol 14:382, 2013. \nDikic I, Elazar Z. Mechanism and medical implications of mammalian autophagy. Nat Rev Mol Cell Biol 19:349, 2018. \nEisner V, Picard M, Hajnóczky G. Mitochondrial dynamics in adaptive and maladaptive cellular stress responses. Nat Cell Biol 20:755, 2018. \nGalluzzi L, Yamazaki T, Kroemer G. Linking cellular stress responses to systemic homeostasis. Nat Rev Mol Cell Biol 19:731, 2018. \n- Guerriero CJ, Brodsky JL: The delicate balance between secreted protein folding and endoplasmic reticulum-associated degradation in human physiology. Physiol Rev 92:537, 2012.\n- Harayama T, Riezman H. Understanding the diversity of membrane lipid composition. Nat Rev Mol Cell Biol 19:281, 2018.\n- Insall R: The interaction between pseudopods and extracellular signalling during chemotaxis and directed migration. Curr Opin Cell Biol 25:526, 2013.\n- Kaksonen M, Roux A. Mechanisms of clathrin-mediated endocytosis. Nat Rev Mol Cell Biol 19:313, 2018.\n- Lawrence RE, Zoncu R. The lysosome as a cellular centre for signalling, metabolism and quality control. Nat Cell Biol 21: 133, 2019.\n- Nakamura N, Wei JH, Seemann J: Modular organization of the mammalian Golgi apparatus. Curr Opin Cell Biol 24:467, 2012. \n- Palikaras K, Lionaki E, Tavernarakis N. Mechanisms of mitophagy in cellular homeostasis, physiology and pathology. Nat Cell Biol 20:1013, 2018.\n- Sezgin E, Levental I, Mayor S, Eggeling C. The mystery of membrane organization: composition, regulation and roles of lipid rafts. Nat Rev Mol Cell Biol 18:361, 2017.\n- Spinelli JB, Haigis MC. The multifaceted contributions of mitochondria to cellular metabolism. Nat Cell Biol. 20:745, 2018.\n- Walker CL, Pomatto LCD, Tripathi DN, Davies KJA. Redox regulation of homeostasis and proteostasis in peroxisomes. Physiol Rev 98:89, 2018.\n- Zhou K, Gaullier G, Luger K. Nucleosome structure and dynamics are coming of age. Nat Struct Mol Biol 26:3, 2019. \n\n\nGenes, which are located in the nuclei of all cells of the body, control heredity from parents to children, as well as the daily functioning of all the body's cells. The genes control cell function by determining which structures, enzymes, and chemicals are synthesized within the cell. \n**Figure 3-1** shows the general schema of genetic control. Each gene, which is composed of *deoxyribonucleic acid* (DNA), controls the formation of another nucleic acid, *ribonucleic acid* (RNA); this RNA then spreads throughout the cell to control formation of a specific protein. The entire process, from *transcription* of the genetic code in the nucleus to *translation* of the RNA code and the formation of proteins in the cell cytoplasm, is often referred to as *gene expression.* \nBecause the human body has approximately 20,000 to 25,000 different genes that code for proteins in each cell, it is possible to form a large number of different cellular proteins. In fact, RNA molecules transcribed from the same segment of DNA—the same gene—can be processed in more than one way by the cell, giving rise to alternate versions of the protein. The total number of different proteins produced by the various cell types in humans is estimated to be at least 100,000. \nSome of the cellular proteins are *structural proteins,* which, in association with various lipids and carbohydrates, form structures of the various intracellular organelles discussed in Chapter 2. However, most of the proteins are *enzymes* that catalyze different chemical reactions in the cells. For example, enzymes promote all the oxidative reactions that supply energy to the cell, along with synthesis of all the cell chemicals, such as lipids, glycogen, and adenosine triphosphate (ATP). \n#### CELL NUCLEUS GENES CONTROL PROT[EIN](#page-34-0) SYNTHESIS \nIn the cell nucleus, large numbers of genes are attached end on end in extremely long, double-stranded helical molecules of DNA having molecular weights measured in the billions. A very short segment of such a molecule is shown in **Figure 3-2**. This molecule is composed of several simple chemical compounds bound together in a regular pattern, the details of which are explained in the next few paragraphs. \n#### **Building Blocks of DNA** \n**Figure 3-3** shows the basic chemical compounds involved in the formation of DNA. These compounds include the following: (1) *phosphoric acid;* (2) a sugar [called](#page-34-0) *deoxyribose;* and (3) four nitrogenous *bases* (two purines, *adenine* and *guanine,* and two pyrimidines, *thymine* and *cytosine*). The phosphoric acid and deoxyribose form the two helical strands that are the backbone of the DNA molecule, and the nitrogenous bases lie between the two strands and connect them, as illustrated in **Figure 3-2**. \n#### **Nucleotides** \nThe first stage of DNA formation is to combine one molecule of phosphoric acid, one molecule of deoxyribose, and one of the four bases to form an acidic nucleotide. Four separate nucleotides are thus formed, one for each of the four bases: *deoxyadenylic, deoxythymidylic, deoxyguanylic,* and *deoxycytidylic acids*. **Figure 3-4** shows the chemical \n \n**Figure 3-1** The general schema whereby genes control cell function. *mRNA,* Messenger RNA. \n \n**Figure 3-2** The helical double-stranded structure of the gene. The outside strands are composed of phosphoric acid and the sugar deoxyribose. The internal molecules connecting the two strands of the helix are purine and pyrimidine bases, which determine the \"code\" of the gene. \n \nFigure 3-3 The basic building blocks of DNA. \nstructure of deoxyadenylic acid, and **Figure 3-5** shows simple symbols for the four nucleotides that form DNA. \n#### Nucleotides Are Organized to Form Two Strands of DNA Loosely Bound to Each Other \n**Figure 3-2** shows the manner in which multiple nucleotides are bound together to form two strands of DNA. The two strands are, in turn, loosely bonded with each other by weak cross-linkages, as illustrated in **Figure 3-6** by the central dashed lines. Note that the backbone of each DNA strand is composed of alternating phosphoric acid and deoxyribose molecules. In turn, purine and pyrimidine bases are attached to the sides of the deoxyribose molecules. Then, by means of loose *hydrogen bonds* (dashed \n**Figure 3-4.** Deoxyadenylic acid, one of the nucleotides that make up DNA. \n \n**Figure 3-5.** Symbols for the four nucleotides that combine to form DNA. Each nucleotide contains phosphoric acid (P), deoxyribose (D), and one of the four nucleotide bases: adenine (A); thymine (T); guanine (G); or cytosine (C). \nlines) between the purine and pyrimidine bases, the two respective DNA strands are held together. Note the following caveats, however: \n- 1. Each purine base *adenine* of one strand always bonds with a pyrimidine base *thymine* of the other strand.\n- 2. Each purine base *guanine* always bonds with a pyrimidine base *cytosine*. \nThus, in **Figure 3**-6, the sequence of complementary pairs of bases is CG, CG, GC, TA, CG, TA, GC, AT, and AT. Because of the looseness of the hydrogen bonds, the two strands can pull apart with ease, and they do so many times during the course of their function in the cell. \nTo put the DNA of **Figure 3**-6 into its proper physical perspective, one could merely pick up the two ends and twist them into a helix. Ten pairs of nucleotides are present in each full turn of the helix in the DNA molecule. \n#### **GENETIC CODE** \nThe importance of DNA lies in its ability to control the formation of proteins in the cell, which it achieves by means of a *genetic code*. That is, when the two strands of a DNA molecule are split apart, the purine and pyrimidine bases projecting to the side of each DNA strand are exposed, as shown by the top strand in **Figure 3-7**. It is these projecting bases that form the genetic code. \nThe genetic code consists of successive \"triplets\" of bases—that is, each three successive bases is a *code word*. The successive triplets eventually control the sequence of amino acids in a protein molecule that is to be synthesized in the cell. Note in **Figure 3**-6 that the top strand of DNA, reading from left to right, has the genetic code GGC, AGA, CTT, with the triplets being separated from one another by the arrows. As we follow this genetic code through **Figure 3**-7 and **Figure 3**-8, we see that these three respective triplets are responsible for successive placement of the three amino acids, *proline, serine,* and *glutamic acid,* in a newly formed molecule of protein.\n\nBecause DNA is located in the cell nucleus, yet most of the cell functions are carried out in the cytoplasm, there must be some means for DNA genes of the nucleus to control chemical reactions of the cytoplasm. This control \n \n**Figure 3-6.** Arrangement of deoxyribose nucleotides in a double strand of DNA. \n \nR C \nP \nP R C P R G P R U \n**Figure 3-7.** Combination of ribose nucleotides with a strand of DNA to form a molecule of RNA that carries the genetic code from the gene to the cytoplasm. The *RNA polymerase* enzyme moves along the DNA strand and builds the RNA molecule. \n**Figure 3-8.** A portion of an RNA molecule showing thre[e](#page-36-0) RNA codons—CCG, UCU, and GAA—that control attachment of the three amino acids, proline, serine, and glutamic acid, respectively, to the growing RNA chain. **Proline Serine Glutamic acid** \nis achieved through the intermediary of another type of nucleic acid, RNA, the formation of which is controlled by DNA of the nucleus. Thus, as shown in **Figure 3-7**, the code is transferred to RNA in a process called *transcription.* The RNA, in turn, diffuses from the nucleus through nuclear pores into the cytoplasmic compartment, where it controls protein synthesis. \n#### **RNA IS SYNTHESIZED IN THE NUCLEUS FROM A DNA TEMPLATE** \nDuring RNA synthesis, the two strands of DNA separate temporarily; one of these strands is used as a template for synthesis of an RNA molecule. The code triplets in the DNA result in the formation of *complementary* code triplets (called *codons*) in the RNA. These codons, in turn, will control the sequence of amino acids in a protein to be synthesized in the cell cytoplasm. \n**Building Blocks of RNA.** The basic building blocks of RNA are almost the same as those of DNA, except for two differences. First, the sugar deoxyribose is not used in RNA formation. In its place is another sugar of slightly different composition, *ribose,* which contains an extra hydroxyl ion appended to the ribose ring structure. Second, thymine is replaced by another pyrimidine, *uracil.* \n**Formation of RNA Nucleotides.** The basic building blocks of RNA form *RNA nucleotides,* exactly as described previously for DNA synthesis. Here again, four separate nucleotides are used to form RNA. These nucleotides contain the bases *adenine, guanine, cytosine,* and *uracil.* Note that these bases are the same as in DNA, except that uracil in RNA replaces thymine in DNA. \n**\"Activation\" of RNA Nucleotides.** The next step in the synthesis of RNA is \"activation\" of RNA nucleotides by an enzyme, *RNA polymerase.* This activation occurs by adding two extra phosphate radicals to each nucleotide to form \ntriphosphates (shown in **Figure 3-7** by the two RNA nucleotides to the far right during RNA chain formation). These last two phosphates are combined with the nucleotide by \nP R U P R G P R A P R A \nP R C \n*high-energy phosphate bonds* derived from ATP in the cell. The result of this activation process is that large quantities of ATP energy are made available to each of the nucleotides. This energy is used to promote chemical reactions that add each new RNA nucleotide at the end of the developing RNA [chain.](#page-36-0) \n#### **RNA CHAIN ASSEMBLY FROM ACTIVATED NUCLEOTIDES USING THE DNA STRAND AS A TEMPLATE** \nAs shown in **Figure 3-7,** assembly of RNA is accomplished under the influence of an enzyme, *RNA polymerase.* This large protein enzyme has many functional properties necessary for formation of RNA, as follows: \n- 1. In the DNA strand immediately ahead of the gene to be transcribed is a sequence of nucleotides called the *promoter.* The RNA polymerase has an appropriate complementary structure that recognizes this promoter and becomes attached to it, which is the essential step for initiating the formation of RNA.\n- 2. After the RNA polymerase attaches to the promoter, the polymerase causes unwinding of about two turns of the DNA helix and separation of the unwound portions of the two strands.\n- 3. The polymerase then moves along the DNA strand, temporarily unwinding and separating the two DNA strands at each stage of its movement. As it moves along, at each stage it adds a new activated RNA nucleotide to the end of the newly forming RNA chain through the following steps:\n- a. First, it causes a hydrogen bond to form between the end base of the DNA strand and the base of an RNA nucleotide in the nucleoplasm. \n- b. Then, one at a time, the RNA polymerase breaks two of the three phosphate radicals away from each of these RNA nucleotides, liberating large amounts of energy from the broken high-energy phosphate bonds. This energy is used to cause covalent linkage of the remaining phosphate on the nucleotide with the ribose on the end of the growing RNA chain.\n- c. When the RNA polymerase reaches the end of the DNA gene, it encounters a new sequence of DNA nucleotides called the *chain-terminating sequence,* which causes the polymerase and the newly formed RNA chain to break away from the DNA strand. The polymerase then can be used again and again to form more new RNA chains.\n- d. As the new RNA strand is formed, its weak hydrogen bonds with the DNA template break away because the DNA has a high affinity for rebonding with its own complementary DNA strand. Thus, the RNA chain is forced away from the DNA and is released into the nucleoplasm. \nTherefore, the code that is present in the DNA strand is eventually transmitted in *complementary* form to the RNA chain. The ribose nucleotide bases always combine with the deoxyribose bases in the following combinations: \n| DNA Base | RNA Base |\n|----------|----------|\n| guanine | Cytosine |\n| cytosine | Guanine |\n| adenine | Uracil |\n| thymine | adenine | \n**There Are Several Different Types of RNA.** As research on RNA has continued to advance, many different types of RNA have been discovered. Some types of RNA are involved in protein synthesis, whereas other types serve gene regulatory functions or are involved in posttranscriptional modification of RNA. The functions of some types of RNA, especially those that do not appear to code for proteins, are still mysterious. The following six types of RNA play independent and different roles in protein synthesis: \n- 1. *Precursor messenger RNA* (pre-mRNA) is a large, immature, single strand of RNA that is processed in the nucleus to form mature messenger RNA (mRNA). The pre-RNA includes two different types of segments, called *introns,* which are removed by a process called splicing, and *exons,* which are retained in the final mRNA.\n- 2. *Small nuclear RNA* (snRNA) directs the splicing of pre-mRNA to form mRNA.\n- 3. *Messenger RNA* (mRNA) carries the genetic code to the cytoplasm for controlling the type of protein formed.\n- 4. *Transfer RNA* (tRNA) transports activated amino acids to the ribosomes to be used in assembling the protein molecule.\n- 5. *Ribosomal RNA,* along with about 75 different proteins, forms *ribosomes,* the physical and chemical \n- structures on which protein molecules are actually assembled.\n- 6. *MicroRNAs* (miRNAs) are single-stranded RNA molecules of 21 to 23 nucleotides that can regulate gene transcription and translation. \n#### **MESSENGER RN[A—THE C](#page-36-1)ODONS** \n*Messenger RNA* molecules are long single RNA strands that are suspended in the cytoplasm. These molecules are composed of several hundred to several thousand RNA nucleotides in unpaired strands, and they contain *c[odons](#page-36-0)* that are exactly complementary to the code triplets of t[he](#page-37-0) DNA genes. **Figure 3-8** shows a small segment of mRNA. Its codons are CCG, UCU, and GAA, which are the codons for the amino acids proline, serine, and glutamic acid. The transcription of these codons from the DNA molecule to the RNA molecule is shown in **Figure 3-7**. \n**RNA Codons for the Different Amino Acids. [Table 3](#page-37-0)-1** lists the RNA codons for the 20 common amino acids found in protein molecules. Note that most of the amino acids are represented by more than one codon; also, one codon represents the signal \"start manufacturing the protein molecule,\" and three codons represent \"stop manufacturing the protein molecule.\" In **Table 3-1**, these two \n**Table 3-1** RNA Codons for Amino Acids and for Start and Stop \n| Amino Acid | | | RNA Codons | | | |\n|---------------|-----|-----|------------|-----|-----|-----|\n| Alanine | GCU | GCC | GCA | GCG | | |\n| Arginine | CGU | CGC | CGA | CGG | AGA | AGG |\n| Asparagine | AAU | AAC | | | | |\n| Aspartic acid | GAU | GAC | | | | |\n| Cysteine | UGU | UGC | | | | |\n| Glutamic acid | GAA | GAG | | | | |\n| Glutamine | CAA | CAG | | | | |\n| Glycine | GGU | GGC | GGA | GGG | | |\n| Histidine | CAU | CAC | | | | |\n| Isoleucine | AUU | AUC | AUA | | | |\n| Leucine | CUU | CUC | CUA | CUG | UUA | UUG |\n| Lysine | AAA | AAG | | | | |\n| Methionine | AUG | | | | | |\n| Phenylalanine | UUU | UUC | | | | |\n| Proline | CCU | CCC | CCA | CCG | | |\n| Serine | UCU | UCC | UCA | UCG | AGC | AGU |\n| Threonine | ACU | ACC | ACA | ACG | | |\n| Tryptophan | UGG | | | | | |\n| Tyrosine | UAU | UAC | | | | |\n| Valine | GUU | GUC | GUA | GUG | | |\n| Start (CI) | AUG | | | | | |\n| Stop (CT) | UAA | UAG | UGA | | | | \n*CI,* Chain-initiating; *CT,* chain-terminating. \ntypes of codons are designated CI for \"chain-initiating\" or \"start\" codon and CT for \"chain-terminating\" or \"stop\" codon. \n#### **TRANSFER RNA—THE ANTICODONS** \nAnother type of RNA that is essential for protein synthesis is called transfer RNA (tRNA) because it transfers amino acids to protein molecules as the protein is being synthesized. Each type of tRNA combines specifically with 1 of the 20 amino acids that are to be incorporated into proteins. The tRNA then acts as a *carrier* to transport its specific type of amino acid to the ribosomes, where protein molecules are forming. In the ribosomes, each specific type of tRNA recognizes a particular codon on the mRNA (described later) and thereby delivers [the appro](#page-38-0)priate amino acid to the appropriate place in the chain of the newly forming protein molecule. \nTransfer RNA, which contains only about 80 nucleotides, is a relatively small molecule in comparison with mRNA. It is a folded chain of nucleotides with a cloverleaf appearance similar to that shown in **Figure 3-9**. At one end of the molecule there is always an adenylic acid to which the transported amino acid attaches at a hydroxyl group of the ribose in the adenylic acid. \nBecause the function of tRNA is to cause attachment of a specific amino acid to a forming protein chain, it is essential that each type of t[RNA also ha](#page-38-0)ve specificity for a particular codon in the mRNA. The specific code in the tRNA that allows it to recognize a specific codon is again a triplet of nucleotide bases and is called an *anticodon.* This anticodon is located approximately in the middle of the tRNA molecule (at the bottom of the cloverleaf configuration shown in **Figure 3-9**). During formation of the protein molecule, the anticodon bases combine loosely by hydrogen bonding with the codon bases of the mRNA. In this way, the respective amino acids are lined up one after another along the mRNA chain, thus establishing the \n \n**Figure 3-9.** A messenger RNA strand is moving through two ribosomes. As each codon passes through, an amino acid is added to the growing protein chain, which is shown in the right-hand ribosome. The transfer RNA molecule transports each specific amino acid to the newly forming protein. \nappropriate sequence of amino acids in the newly forming protein molecule. \n#### **RIBOSOMAL RNA** \nThe third type of RNA in the cell is ribosomal RNA, which constitutes about 60% of the *ribosome.* The remainder of the ribosome is protein, including about 75 types of proteins that are both structural proteins and enzymes needed to manufacture proteins. \nThe ribosome is the physical structure in the cytoplasm on which proteins are actually synthesized. However, it always functions in association with the other two types of RNA; *tRNA* transports amino acids to the ribosome for incorporation into the developing protein, whereas *mRNA* provides the information necessary for sequencing the amino acids in proper order for each specific type of protein to be manufactured. Thus, the ribosome acts as a manufacturing plant in which the protein molecules are formed. \n**Formation of Ribosomes in the Nucleolus.** The DNA genes for the formation of ribosomal RNA are located in five pairs of chromosomes in the nucleus. Each of these chromosomes contains many duplicates of these particular genes because of the large amounts of ribosomal RNA required for cellular function. \nAs the ribosomal RNA forms, it collects in the *nucleolus,* a specialized structure lying adjacent to the chromosomes. When large amounts of ribosomal RNA are being synthesized, as occurs in cells that manufacture large amounts of protein, the nucleolus is a large structure, whereas in cells that synthesize little protein, the nucleolus may not even be seen. Ribosomal RNA is specially processed in the nucleolus, where it binds with ribosomal proteins to form granular condensation products that are primordial subunits of ribosomes. These subunits are then released from the nucleolus and transported through the large pores of the nuclear envelope to almost all parts of the cytoplasm. After the subunits enter the cytoplasm, they are assembled to form mature functional ribosomes. Therefore, proteins are formed in the cytoplasm of the cell, but not in the cell nucleus, because the nucleus does not cont[ain m](#page-39-0)ature ribosomes. \n#### **miRNA AND SMALL INTERFERING RNA** \nA fourth type of RNA in the cell is *microRNA* (miRNA); miRNA are short (21 to 23 nucleotides) single-stranded RNA fragments that regulate gene expression **(Figure 3-10).** The miRNAs are encoded from the transcribed DNA of genes, but they are not translated into proteins and are therefore often called *noncoding RNA*. The miRNAs are processed by the cell into molecules that are complementary to mRNA and act to decrease gene expression. The generation of miRNAs involves special processing of longer primary precursor RNAs called *primiRNAs,* which are the primary transcripts of the gene. \n \n**Figure 3-10.** Regulation of gene expression by microRNA (miRNA). Primary miRNA (pri-miRNA), the primary transcripts of a gene processed in the cell nucleus by the microprocessor complex, are converted to pre-miRNAs. These pre-miRNAs are then further processed in the cytoplasm by *dicer,* an enzyme that helps assemble an RNAinduced silencing complex (RISC) and generates miRNAs. The miRNAs regulate gene expression by binding to the complementary region of the RNA and repressing translation or promoting degradation of the messenger RNA (mRNA) before it can be translated by the ribosome. \nThe pri-miRNAs are then processed in the cell nucleus by the *microprocessor complex* to pre-miRNAs, which are 70-nucleotide, stem loop structures. These pre-miRNAs are then further processed in the cytoplasm by a specific *dicer enzyme* that helps assemble an *RNA-induced silencing complex* (RISC) and generates miRNAs. \nThe miRNAs regulate gene expression by binding to the complementary region of the RNA and promoting repression of translation or degradation of the mRNA before it can be translated by the ribosome. miRNAs are believed to play an important role in normal regulation of cell function, and alterations in miRNA function have been associated with diseases such as cancer and heart disease. \nAnother type of miRNA is *small interfering RNA* (siRNA), also called *silencing RNA* or *short interfering RNA.* The siRNAs are short, double-stranded RNA molecules, comprised of 20 to 25 nucleotides, that interfere with expression of specific genes. siRNAs generally refer to synthetic miRNAs and can be administered to silence expression of specific genes. They are designed to avoid nuclear processing by the microprocessor complex and, after the siRNA enters the cytoplasm, it activates the RISC silencing complex, blocking the translation of mRNA. Because siRNAs can be tailored for any specific sequence in the gene, they can be used to block translation of any mRNA and therefore expression by any gene for which the nucleotide sequence is known. Researchers have proposed that siRNAs may become useful therapeutic tools to silence genes that contribute to the pathophysiology of diseases. \n#### TRANSLATION—FORMATION O[F](#page-38-0) PROTEINS ON THE RIBOSOMES \nWhen a molecule of mRNA comes in contact with a ribosome, it travels through the ribosome, beginning at a predetermined end of the RNA molecule specified by an appropriate sequence of RNA bases called the *chaininitiating codon.* Then, as shown in **Figure 3-9**, while the mRNA travels through the ribosome, a protein molecule is formed, a process called *translation.* Thus, the ribosome reads the codons of the mRNA in much the same way that a tape is read as it passes through the playback head of a tape recorder. Then, when a \"stop\" (or \"chainterminating\") codon slips past the ribosome, the end of a protein molecule is sig[naled, and t](#page-38-0)he p[rotein molecu](#page-40-0)le is freed into the cytoplasm. \n**Polyribosomes.** A single mRNA molecule can form protein molecules in several ribosomes at the same time because the initial end of the RNA strand can pass to a successive ribosome as it leaves the first, as shown at the bottom left in **Figure 3-9** and **Figure 3-11**. The protein molecules are in different stages of development in each ribosome. As a result, clusters of ribosomes frequently occur, with 3 to 10 ribosomes being attached to a single mRNA at the same time. These clusters are called *polyribosomes.* \nAn mRNA can cause formation of a protein molecule in any ribosome; there is no specificity of ribosomes for given types of protein. The ribosome is simply the physical manufacturing plant in which the chemical reactions take place. \n**Many Ribosomes Attach to the Endoplasmic Reticulum.** In Chapter 2, we noted that many ribosomes become attached to the endoplasmic reticulum. This attachment occurs because the initial ends of many forming protein molecules have amino acid sequences that immediately attach to specific receptor sites on the endoplasmic reticulum, causing these molecules to penetrate the \n \n**Figure 3-11.** The physical structure of the ribosomes, as well as their functional relationship to messenger RNA, transfer RNA, and the endoplasmic reticulum during the formation of protein molecules. \nreticulum wall and enter the endoplasmic reticulum matrix. This process gives a granular appearance to the portions of the reticulum where proteins are being formed and are entering the matrix of the reticulum. \n**Figure 3-11** shows the functional relationship of mRNA to the ribosomes and the manner in which the ribosomes attach to the membrane of the endoplasmic reticulum. Note the process of translation occurring in several ribosomes at the same time in response to the same strand of mRNA. Note also the newly forming polypeptide (protein) chains passing through the endoplasmic reticulum membrane into the endoplasmic matrix. \nIt should be noted that except in glandular cells, in which large amounts of protein-containing secretory vesicles are formed, most proteins synthesized by the ribosomes are released directly into the cytosol instead of into the endoplasmic reticulum. These proteins are enzymes and internal structural proteins of the cell. \n**Chemical Steps in Protein Synthesis.** Some of the chemical events that occur in the synthesis of a protein molecule are shown in **Figure 3-12**. This Fig. shows representative reactions for three separate amino acids, $AA_1$ , $AA_2$ , and $AA_{20}$ . The stages of the reactions are as follows: \n- Each amino acid is activated by a chemical process in which ATP combines with the amino acid to form an adenosine monophosphate complex with the amino acid, giving up two high-energy phosphate bonds in the process.\n- 2. The activated amino acid, having an excess of energy, then *combines with its specific tRNA to form an amino acid–tRNA complex* and, at the same time, releases the adenosine monophosphate.\n- 3. The tRNA carrying the amino acid complex then comes in contact with the mRNA molecule in the ribosome, where the anticodon of the tRNA attaches temporarily to its specific codon of the mRNA, thus lining up the amino acid in the appropriate sequence to form a protein molecule. \nThen, under the influence of the enzyme *peptidyl transferase* (one of the proteins in the ribosome), *peptide bonds* are formed between the successive amino acids, thus adding progressively to the protein chain. These \nchemical events require energy from two additional high-energy phosphate bonds, making a total of four high-energy bonds used for each amino acid added to the protein chain. Thus, the synthesis of proteins is one of the most energy-consuming processes of the cell. \n**Peptide Linkage—Combination of Amino Acids.** The successive amino acids in the protein chain combine with one another according to the typical reaction. \n$$\\begin{array}{cccccccccccccccccccccccccccccccccccc$$ \nIn this chemical reaction, a hydroxyl radical (OH-) is removed from the COOH portion of the first amino acid, and a hydrogen (H+) of the NH2 portion of the other amino acid is removed. These combine to form water, and the two reactive sites left on the two successive amino acids bond with each other, resulting in a single molecule. This process is called *peptide linkage*. As each additional amino acid is added, an additional peptide linkage is formed.\n\nMany thousand protein enzymes formed in the manner just described control essentially all the other chemical reactions that take place in cells. These enzymes promote synthesis of lipids, glycogen, purines, pyrimidines, and hundreds of other substances. We discuss many of these synthetic processes in relation to carbohydrate, lipid, and protein metabolism in Chapters 68 through 70. These substances each contribute to the various functions of the cells.\n\nFrom our discussion thus far, it is clear that the genes control both the physical and chemical functions of the cells. However, the degree of activation of respective genes must also be \n \n**Figure 3-12.** Chemical events in the formation of a protein molecule. AMP, Adenosine monophosphate; ATP, adenosine triphosphate; tRNA, transfer RNA. \ncontrolled; otherwise, some parts of the cell might overgrow or some chemical reactions might overact until they kill the cell. Each cell has powerful internal feedback control mechanisms that keep the various functional operations of the cell in step with one another. For each gene ( $\\approx 20,000-25,000$ genes in all), at least one such feedback mechanism exists. \nThere are basically two methods whereby the biochemical activities in the cell are controlled: (1) *genetic regulation,* in which the degree of activation of the genes and the formation of gene products are themselves controlled, and (2) *enzyme regulation,* in which the activity levels of already formed enzymes in the cell are controlled. \n#### **GENETIC REGULATION** \nGenetic regulation, or regulation of gene expression, covers the entire process from transcription of the genetic code in the nucleus to the formation of proteins in the cytoplasm. Regulation of gene expression provides all living organisms with the ability to respond to changes in their environment. In animals that have many different types of cells, tissues, and organs, differential regulation of gene expression also permits the different cell types in the body to each perform their specialized functions. Although a cardiac myocyte contains the same genetic code as a renal tubular epithelial cell, many genes are expressed in cardiac cells that are not expressed in renal tubular cells. The ultimate measure of gene \"expression\" is whether (and how much) of the gene products (proteins) are produced because proteins carry out cell functions specified by the genes. Regulation of gene expression can occur at any point in the pathways of transcription, RNA processing, and translation. \n**The Promoter Controls Gene Expression.** Synthesis of cellular proteins is a complex process that starts with transcription of DNA into RNA. Transcription of DNA is \n \n**Figure 3-13.** Gene transcription in eukaryotic cells. A complex arrangement of multiple clustered enhancer modules is interspersed with insulator elements, which can be located upstream or downstream of a basal promoter containing TATA box (TATA), proximal promoter elements (response elements, RE), and initiator sequences (INR). \ncontrolled by regulatory elements found in the promoter of a gene (Figure 3-13). In eukaryotes, which includes all mammals, the basal promoter consists of a sequence of bases (TATAAA) called the TATA box, the binding site for the TATA-binding protein and several other important transcription factors that are collectively referred to as the transcription factor IID complex. In addition to the transcription factor IID complex, this region is where transcription factor IIB binds to both the DNA and RNA polymerase 2 to facilitate transcription of the DNA into RNA. This basal promoter is found in all protein-coding genes, and the polymerase must bind with this basal promoter before it can begin traveling along the DNA strand to synthesize RNA. The upstream promoter is located farther upstream from the transcription start site and contains several binding sites for positive or negative transcription factors that can affect transcription through interactions with proteins bound to the basal promoter. The structure and transcription factor binding sites in the upstream promoter vary from gene to gene to give rise to the different expression patterns of genes in different tissues. \nTranscription of genes in eukaryotes is also influenced by *enhancers,* which are regions of DNA that can bind transcription factors. Enhancers can be located a great distance from the gene they act on or even on a different chromosome. They can also be located upstream or downstream of the gene that they regulate. Although enhancers may be located far from their target gene, they may be relatively close when DNA is coiled in the nucleus. It is estimated that there are more than 100,000 gene enhancer sequences in the human genome. \nIn the organization of the chromosome, it is important to separate active genes that are being transcribed from genes that are repressed. This separation can be challenging because multiple genes may be located close together on the chromosome. The separation is achieved by chromosomal *insulators.* These insulators are gene sequences that provide a barrier so that a specific gene is isolated against transcriptional influences from surrounding genes. Insulators can vary greatly in their DNA sequence and the proteins that bind to them. One way an insulator activity can be modulated is by *DNA methylation*, which is the case for the mammalian insulin-like growth factor 2 (IGF-2) gene. The mother's allele has an insulator between the enhancer and promoter of the gene that allows for the binding of a transcriptional repressor. However, the paternal DNA sequence is methylated such that the transcriptional repressor cannot bind to the insulator, and the IGF-2 gene is expressed from the paternal copy of the gene. \n#### **Other Mechanisms for Control of Transcription by the Promoter.** Variations in the basic mechanism for control of the promoter have been discovered in the past three decades. Without giving details, let us list some of them: \n- 1. A promoter is frequently controlled by transcription factors located elsewhere in the genome. That is, the regulatory gene causes the formation of a regulatory protein that in turn acts as an activator or repressor of transcription.\n- 2. Occasionally, many different promoters are controlled at the same time by the same regulatory protein. In some cases, the same regulatory protein functions as an activator for one promoter and as a repressor for another promoter.\n- 3. Some proteins are controlled not at the starting point of transcription on the DNA strand but farther along the strand. Sometimes, the control is not even at the DNA strand itself but occurs during the processing of the RNA molecules in the nucleus before they are released into the cytoplasm. Control may also occur at the level of protein formation in the cytoplasm during RNA translation by the ribosomes.\n- 4. In nucleated cells, the nuclear DNA is packaged in specific structural units, the *chromosomes.* Within \neach chromosome, the DNA is wound around small proteins called *histones,* which in turn are held tightly together in a compacted state by still other proteins. As long as the DNA is in this compacted state, it cannot function to form RNA. However, multiple control mechanisms are being discovered that can cause selected areas of chromosomes to become decompacted one part at a time, so that partial RNA transcription can occur. Even then, specific *transcriptor factor*s control the actual rate of transcription by the promoter in the chromosome. Thus, still higher orders of control are used to establish proper cell function. In addition, signals from outside the cell, such as some of the body's hormones, can activate specific chromosomal areas and specific transcription factors, therefore controlling the chemical machinery for function of the cell. \nBecause there are many thousands of different genes in each human cell, the large number of ways in which genetic activity can be controlled is not surprising. The gene control systems are especially important for controlling intracellular concentrations of amino acids, amino acid derivatives, and intermediate substrates and products of carbohydrate, lipid, and protein metabolism. \n#### **CONTROL OF INTRACELLULAR FUNCTION BY ENZYME REGULATION** \nIn addition to control of cell function by genetic regulation, cell activities are also controlled by intracellular inhibitors or activators that act directly on specific intracellular enzymes. Thus, enzyme regulation represents a second category of mechanisms whereby cellular biochemical functions can be controlled. \n**Enzyme Inhibition.** Some chemical substances formed in the cell have direct feedback effects to inhibit the specific enzyme systems that synthesize them. Almost always, the synthesized product acts on the first enzyme in a sequence, rather than on the subsequent enzymes, usually binding directly with the enzyme and causing an allosteric conformational change that inactivates it. One can readily recognize the importance of inactivating the first enzyme because this prevents buildup of intermediary products that are not used. \nEnzyme inhibition is another example of negative feedback control. It is responsible for controlling intracellular concentrations of multiple amino acids, purines, pyrimidines, vitamins, and other substances. \n**Enzyme Activation.** Enzymes that are normally inactive often can be activated when needed. An example of this phenomenon occurs when most of the ATP has been depleted in a cell. In this case, a considerable amount of cyclic adenosine monophosphate (cAMP) begins to be formed as a breakdown product of ATP. The presence of this cAMP, in turn, immediately activates the glycogensplitting enzyme phosphorylase, liberating glucose molecules that are rapidly metabolized, with their energy used for replenishment of the ATP stores. Thus, cAMP acts as an enzyme activator for the enzyme phosphorylase and thereby helps control intracellular ATP concentration. \nAnother interesting example of both enzyme inhibition and enzyme activation occurs in the formation of the purines and pyrimidines. These substances are needed by the cell in approximately equal quantities for the formation of DNA and RNA. When purines are formed, they *inhibit* the enzymes that are required for formation of additional purines. However, they *activate* the enzymes for formation of pyrimidines. Conversely, the pyrimidines inhibit their own enzymes but activate the purine enzymes. In this way, there is continual cross-talk between the synthesizing systems for these two substances, resulting in almost exactly equal amounts of the two substances in the cells at all times. \n**Summary.** There are two principal mechanisms whereby cells control proper proportions and quantities of different cellular constituents: (1) genetic regulation; and (2) enzyme regulation. The genes can be activated or inhibited, and likewise, the enzyme systems can be activated or inhibited. These regulatory mechanisms usually function as feedback control systems that continually monitor the cell's biochemical composition and make corrections as needed. However, on occasion, substances from outside the cell (especially some of the hormones discussed in this text) also control the intracellular biochemical reactions by activating or inhibiting one or more of the intracellular control systems. \n#### THE DNA–GENETIC SYSTEM CONTROLS CELL REPRODUCTION \nCell reproduction is another example of the ubiquitous role that the DNA–genetic system plays in all life processes. The genes and their regulatory mechanisms determine cell growth characteristics and when or whether cells will divide to form new cells. In this way, the allimportant genetic system controls each stage in the development of the human, from the single-cell fertilized ovum to the whole functioning body. Thus, if there is any central theme to life, it is the DNA–genetic system. \n#### **Life Cycle of the Cell** \nThe life cycle of a cell is the period from cell repr[oduction](#page-43-0) to the next cell reproduction. When mammalian cells *are not inhibited and are reproducing as rapidly as they can,* this life cycle may be as little as 10 to 30 hours. It is terminated by a series of distinct physical events called *mitosis* that cause division of the cell into two new daughter cells. The events of mitosis are shown in **Figure 3-14** and described later. The actual stage of mitosis, however, lasts for only about 30 minutes, and thus more than 95% of the life cycle of even rapidly reproducing cells is represented by the interval between mitosis, called *interphase.* \nExcept in special conditions of rapid cellular reproduction, inhibitory factors almost always slow or stop the \n \n**Figure 3-14.** Stages of cell reproduction. *A, B, C,* Prophase. *D,* Prometaphase. *E,* Metaphase. *F,* Anaphase. *G, H,* Telophase. \nuninhibited life cycle of the cell. Therefore, different cells of the body actually have life cycle periods that vary from as little as 10 hours for highly stimulated bone marrow cells to an entire lifetime of the human body for many nerve cells. \n#### **Cell Reproduction Begins with Replication of DNA** \nThe first step of cell reproduction is *replication (duplication) of all DNA in the chromosomes.* It is only after this replication has occurred that mitosis can take place. \nThe DNA begins to be duplicated 5 to 10 hours before mitosis, and the duplication is completed in 4 to 8 hours. The net result is two exact *replicas* of all DNA. These replicas become the DNA in the two new daughter cells that will be formed at mitosis. After replication of the DNA, there is another period of 1 to 2 hours before mitosis begins abruptly. Even during this period, preliminary changes that will lead to the mitotic process are beginning to take place. \n**DNA Replication.** DNA is replicated in much the same way that RNA is transcribed from DNA, except for a few important differences: \n1. Both strands of the DNA in each chromosome are replicated, not just one of them. \n \nFigure 3-15. DNA replication, showing the replication fork and leading and lagging strands of DNA. \n- 2. Both entire strands of the DNA helix are replicated from end to end, rather than small portions of them, as occurs in the transcription of RNA.\n- 3. Multiple enzymes called *DNA polymerase*, which is comparable to RNA polymerase, are essential for replicating DNA. DNA polymerase attaches to and moves along the DNA template strand, adding nucleotides in the 5′ to 3′ direction. Another enzyme, *DNA ligase*, causes bonding of successive DNA nucleotides to one another, using high-energy phosphate bonds to energize these attachments.\n- 4. Replication fork formation. Before DNA can be replicated, the double-stranded molecule must be \"unzipped\" into two single strands (Figure 3-15). Because the DNA helixes in each chromosome are approximately 6 centimeters in length and have millions of helical turns, it would be impossible for the two newly formed DNA helixes to uncoil from each other were it not for some special mechanism. This uncoiling is achieved by DNA helicase enzymes that break the hydrogen bonding between the base pairs of the DNA, permitting the two strands to separate into a Y shape known as the replication fork, the area that will be the template for replication to begin. \nDNA is directional in both strands, signified by a 5′ and 3′ end (see **Figure 3-15**). Replication progresses only in the 5′ to 3′ direction. At the replication fork one strand, the *leading strand*, is oriented in the 3′ to 5′ direction, toward the replication fork, while the *lagging strand* is oriented 5′ to 3′, away from the replication fork. Because of their different orientations, the two strands are replicated differently. \n5. *Primer binding*. Once the DNA strands have been separated, a short piece of RNA called an *RNA primer* binds to the 3' end of the leading strand. Primers are generated by the enzyme *DNA primase*. \n- Primers always bind as the starting point for DNA replication.\n- 6. Elongation. DNA polymerases are responsible for creating the new strand by a process called *elongation*. Because replication proceeds in the 5′ to 3′ direction on the leading strand, the newly formed strand is continuous. The lagging strand begins replication by binding with multiple primers that are only several bases apart. DNA polymerase then adds pieces of DNA, called *Okazaki fragments*, to the strand between primers. This process of replication is discontinuous because the newly created Okazaki fragments are not yet connected. An enzyme, *DNA ligase*, joins the Okazaki fragments to form a single unified strand.\n- 7. Termination. After the continuous and discontinuous strands are both formed, the enzyme exonuclease removes the RNA primers from the original strands, and the primers are replaced with appropriate bases. Another exonuclease \"proofreads\" the newly formed DNA, checking and clipping off any mismatched or unpaired residues. \nAnother enzyme, *topoisomerase*, can transiently break the phosphodiester bond in the backbone of the DNA strand to prevent the DNA in front of the replication fork from being overwound. This reaction is reversible, and the phosphodiester bond reforms as the topoisomerase leaves. \nOnce completed, the parent strand and its complementary DNA strand coils into the double helix shape. The process of replication therefore produces two DNA molecules, each with one strand from the parent DNA and one new strand. For this reason, DNA replication is often described as *semiconservative*; half of the chain is part of the original DNA molecule and half is brand new. \n**DNA Repair, DNA \"Proofreading,\" and \"Mutation.\"**During the hour or so between DNA replication and \nthe beginning of mitosis, there is a period of active repair and \"proofreading\" of the DNA strands. Wherever inappropriate DNA nucleotides have been matched up with the nucleotides of the original template strand, special enzymes cut out the defective areas and replace them with appropriate complementary nucleotides. This repair process, which is achieved by the same DNA polymerases and DNA ligases that are used in replication, is referred to as *DNA proofreading.* \nBecause of repair and proofreading, mistakes are rarely made in the DNA replication process. When a mistake is made, it is called a *mutation.* The mutation may cause formation of some abnormal protein in the cell rather than a needed protein, which may lead to abnormal cellular function and sometimes even cell death. Given that many thousands of genes exist in the human genome, and that the period from one human generation to another is about 30 years, one would expect as many as 10 or many more mutations in the passage of the genome from parent to offspring. As a further protection, however, each human genome is represented by two separate sets of chromosomes, one derived from each parent, with almost identical genes. Therefore, one functional gene of each pair is almost always available to the child, despite mutations. \n#### **CHROMOSOMES AND THEIR REPLICATION** \nThe DNA helixes of the nucleus are packaged in chromosomes. The human cell contains 46 chromosomes arranged in 23 pairs. Most of the genes in the two chromosomes of each pair are identical or almost identical to each other, so it is usually stated that the different genes also exist in pairs, although occasionally this is not the case. \nIn addition to DNA, there is a large amount of protein in the chromosome, composed mainly of many small molecules of electropositively charged *histones.* The histones are organized into vast numbers of small, bobbinlike cores. Small segments of each DNA helix are coiled sequentially around one core after another. \nThe histone cores play an important role in regulation of DNA activity because as long as the DNA is packaged tightly, it cannot function as a template for formation of RNA or replication of new DNA. Furthermore, some of the regulatory proteins *decondense* the histone packaging of the DNA and allow small segments at a time to form RNA. \nSeveral nonhistone proteins are also major components of chromosomes, functioning as chromosomal structural proteins and, in connection with the genetic regulatory machinery, as activators, inhibitors, and enzymes. \nReplication of the chromosomes in their entirety occurs during the next few minutes after replication of the DNA helixes has been completed; the new DNA helixes collect new protein molecules as needed. The two newly formed chromosomes remain attached to each other (until time for mitosis) at a point called the *centromere* located near their center. These duplicated but still attached chromosomes are called *chromatids.* \n#### **CELL MITOSIS** \nThe actual process whereby the cell splits into two new cells is called *mitosis.* Once each chromosome has been replicated to form the two chromatids, mitos[is follows](#page-43-0) automatically within 1 or 2 hours in many cells. \n**Mitotic Apparatus: Function of the Centrioles.** One of the first events of mitosis takes place in the cytoplasm in or around the small structures called *centrioles* during the latter part of interphase*.* As shown in **Figure 3-**14, two pairs of centrioles lie close to each other near one pole of the nucleus. These centrioles, like the DNA and chromosomes, are also replicated during interphase, usually shortly before replication of the DNA. Each centriole is a small cylindrical body about 0.4 micrometer long and about 0.15 micrometer in diameter, consisting mainly of nine parallel tubular structures arranged in the form of a cylinder. The two centrioles of each pair lie at right angles to each other. Each pair of centrioles, along with attached *pericentriolar material,* is called a *centrosome.* \nShortly before mitosis takes place, the two pairs of centrioles begin to move apart from each other. This movement is caused by polymerization of protein microtubules growing between the respective centriole pairs and actually pushing them apart. At the same time, other microtubules grow radially away from each of the centriole pairs, forming a spiny star called the *aster,* in each end of the cell. Some of the spines of the aster penetrate the nuclear membrane and h[elp separate t](#page-43-0)he two sets of chromatids during mitosis. The complex of microtubules extending between the two new centriole pairs is called the *spindle,* and the entire set of microtubules plus the two pairs of centrioles is called the *mitotic apparatus.* \n**Prophase.** [The fi](#page-43-0)rst stage of mitosis, called *prophase,* is shown in **Figure 3-14***A, B,* and *C.* While the spindle is forming, the chromosomes of the nucleus (which in interphase consist of loosely coiled strands) become condensed into well-defined chromosomes. \n**Prometaphase.** During the prometaphase stage (see **Figure 3-14***D*), the growing microtubular spines of the aster fragment the nuclear envelope. At the same time, multiple mic[rotubu](#page-43-0)les from the aster attach to the chromatids at the centromeres, where the paired chromatids are still bound to each other. The tubules then pull one chromatid of each pair toward one cellular pole and its partner toward the opposite pole. \n**Metaphase.** During the metaphase stage (see **Figure 3-14***E*), the two asters of the mitotic apparatus are pushed farther apart. This pushing is believed to occur because the microtubular spines from the two asters, where they interdigitate with each other to form the mitotic spindle, push each other away. Minute contractile protein molecules called *\"molecular motors,\"* which may be composed of the muscle protein *actin,* extend between the respective spines a[nd, us](#page-43-0)ing a stepping action as in muscle, actively slide the spines in a reverse direction along each other. Simultaneously, the chromatids are pulled tightly by their attached microtubules to the very center of the cell, lining up to form the *equatorial plate* of the mitotic spindle. \n**Anaphase.** During the anaphase stage (see **Figure 3-14***F*), the two chromatids of each chromosome are pulled apart at the centromere. All 46 pairs of c[hromatids](#page-43-0) are sepa[rat](#page-43-0)ed, forming two separate sets of 46 *daughter chromosomes.* One of these sets is pulled toward one mitotic aster, and the other is pulled toward the other aster, as the two respective poles of the dividing cell are pushed still farther apart. \n**Telophase.** In the telophase stage (see **Figure 3-14***G* and *H*), the two sets of daughter chromosomes are pushed completely apart. Then, the mitotic apparatus dissipates, and a new nuclear membrane develops around each set of chromosomes. This membrane is formed from portions of the endoplasmic reticulum that are already present in the cytoplasm. Shortly thereafter, the cell pinches in two, midway between the two nuclei. This pinching is caused by the formation of a contractile ring of *microfilaments* composed of *actin* and probably *myosin* (the two contractile proteins of muscle) at the juncture of the newly developing cells that pinches them off from each other. \n#### **CONTROL OF CELL GROWTH AND CELL REPRODUCTION** \nSome cells grow and reproduce all the time, such as the blood-forming cells of the bone marrow, the germinal layers of the skin, and the epithelium of the gut. Many other cells, however, such as smooth muscle cells, may not reproduce for many years. A few cells, such as the neurons and most striated muscle cells, do not reproduce during the entire life of a person, except during the original period of fetal life. \nIn certain tissues, an insufficiency of some types of cells causes them to grow and reproduce rapidly until appropriate numbers of these cells are again available. For example, in some young animals, seven-eighths of the liver can be removed surgically, and the cells of the remaining one-eighth will grow and divide until the liver mass returns to almost normal. The same phenomenon occurs for many glandular cells and most cells of the bone marrow, subcutaneous tissue, intestinal epithelium, and almost any other tissue except highly differentiated cells such as nerve and muscle cells. \nThe mechanisms that maintain proper numbers of the different types of cells in the body are still poorly understood. However, experiments have shown at least three ways in which growth can be controlled. First, growth often is controlled by *growth factors* that come from other parts of the body. Some of these growth factors circulate in the blood, but others originate in adjacent tissues. For [exam](#page-43-0)ple, the epithelial cells of some glands, such as the pancreas, fail to grow without a growth factor from the underlying connective tissue of the gland. Second, most normal cells stop growing when they have run out of space for growth. This phenomenon occurs when cells are grown in tissue culture; the cells grow until they contact a solid object, and then growth stops. Third, cells grown in tissue culture often stop growing when minute am[ounts](#page-46-0) of their own secretions are allowed to collect in the culture medium. This mechanism, too, could provide a means for negative feedback control of growth. \n**Telomeres Prevent the Degradation of Chromosomes.** A *telomere* is a region of repetitive nucleotide sequences located at each end of a chromatid (**Figure 3-16**). Telomeres serve as protective caps that prevent the chromosome from deterioration during cell division. During cell division, a short piece of \"primer\" RNA attaches to the DNA strand to start the replication. However, because the primer does not attach at the very end of the DNA strand, the copy is missing a small section of the DNA. With each cell division, the copied DNA loses additional nucleotides from the telomere region. The nucleotide sequences provided by the telomeres therefore prevent the degradation of genes near the ends of chromosomes. Without telomeres, the genomes would progressively lose information and be truncated after each cell division. Thus, the telomeres can be considered to be disposable chromosomal buffers that help maintain stability of the genes but are gradually consumed during repeated cell divisions. \n \n**Figure 3-16.** Control of cell replication by telomeres and telomerase. The cells' chromosomes are capped by telomeres, which, in the absence of telomerase activity, shorten with each cell division until the cell stops replicating. Therefore, most cells of the body cannot replicate indefinitely. In cancer cells, telomerase is activated, and telomere length is maintained so that the cells continue to replicate themselves uncontrollably. \nEach time a cell divides, an average person loses 30 to 200 base pairs from the ends of that cell's telomeres. In human blood cells, the length of telomeres ranges from 8000 base pairs at birth to as low as 1500 in older people. Eventually, when the telomeres shorten to a critical length, the chromosomes become unstable, and the cells die. This process of telomere shortening is believed to be an important reason for some of the physiological changes associated with aging. Telomere erosion can also occur as a result of diseases, especially those associated with oxidative stress and inflammation. \nIn some cells, such as stem cells of the bone marrow or skin that must be replenished throughout life, or germ cells in the ovaries and testes, the enzyme *telomerase* adds bases to the ends of the telomeres so that many more generations of cells can be produced. However, telomerase activity is usually low in most cells of the body, and after many generations the descendent cells will inherit defective chromoso[mes, become](#page-46-0) *senescent,* and cease dividing. This process of telomere shortening is important in regulating cell proliferation and maintaining gene stability. In cancer cells, telomerase activity is abnormally activated so that telomere length is maintained, making it possible for the cells to replicate over and over again uncontrollably (see **Figure 3-16**). Some scientists have therefore proposed that telomere shortening protects us from cancer and other proliferative diseases. \n**Regulation of Cell Size.** Cell size is determined almost entirely by the amount of functioning DNA in the nucleus. If replication of the DNA does not occur, the cell grows to a certain size and thereafter remains at that size. Conversely, use of the chemical *colchicine* makes it possible to prevent formation of the mitotic spindle and therefore prevent mitosis, even though replication of the DNA continues. In this event, the nucleus contains far greater quantities of DNA than it normally does, and the cell grows proportionately larger. It is assumed that this cell growth results from increased production of RNA and cell proteins, which, in turn, cause the cell to grow larger. \n#### CELL DIFFERENTIATION \nA special characteristic of cell growth and cell division is *cell differentiation,* which refers to changes in the physical and functional properties of cells as they proliferate in the embryo to form the different body structures and organs. The following description of an especially interesting experiment helps explain these processes. \nWhen the nucleus from an intestinal mucosal cell of a frog is surgically implanted into a frog ovum from which the original ovum nucleus was removed, the result is often the formation of a normal frog. This experiment demonstrates that even the intestinal mucosal cell, which is a well-differentiated cell, carries all the necessary genetic information for development of all structures required in the frog's body. \nTherefore, it has become clear that differentiation results not from loss of genes but from selective repression of different gene promoters. In fact, electron micrographs suggest that some segments of DNA helixes that are wound around histone cores become so condensed that they no longer uncoil to form RNA molecules. One explanation for this is as follows. It has been supposed that the cellular genome begins at a certain stage of cell differentiation to produce a regulatory *protein* that forever after represses a select group of genes. Therefore, the repressed genes never function again. Regardless of the mechanism, mature human cells each produce a maximum of about 8000 to 10,000 proteins rather than the potential 20,000 to 25,000 or more that would be produced if all genes were active. \nEmbryological experiments have shown that certain cells in an embryo control differentiation of adjacent cells. For example, the *primordial chordamesoderm* is called the *primary organizer* of the embryo because it forms a focus around which the remainder of the embryo develops. It differentiates into a *mesodermal axis* that contains segmentally arranged *somites* and, as a result of *inductions* in the surrounding tissues, causes the formation of essentially all the organs of the body. \nAnother instance of induction occurs when the developing eye vesicles come into contact with the ectoderm of the head and cause the ectoderm to thicken into a lens plate that folds inward to form the lens of the eye. Therefore, a large share of the embryo develops as a result of such inductions, with one part of the body affecting another part, and this part affecting still other parts. \nThus, although our understanding of cell differentiation is still hazy, we are aware of many control mechanisms whereby differentiation *could* occur. \n#### APOPTOSIS—PROGRAMMED CELL DEATH \nThe many trillions of the body's cells are members of a highly organized community in which the total number of cells is regulated not only by controlling the rate of cell division, but also by controlling the rate of cell death. When cells are no longer needed or become a threat to the organism, they undergo a suicidal *programmed cell death,* or *apoptosis.* This process involves a specific proteolytic cascade that causes the cell to shrink and condense, disassemble its cytoskeleton, and alter its cell surface so that a neighboring phagocytic cell, such as a macrophage, can attach to the cell membrane and digest the cell. \nIn contrast to programmed death, cells that die as a result of an acute injury usually swell and burst due to loss of cell membrane integrity, a process called cell *necrosis.* Necrotic cells may spill their contents, causing inflammation and injury to neighboring cells. Apoptosis, however, is an orderly cell death that results in disassembly and phagocytosis of the cell before any leakage of its contents occurs, and neighboring cells usually remain healthy. \nApoptosis is initiated by activation of a family of proteases called *caspases*, which are enzymes that are synthesized and stored in the cell as inactive *procaspases*. The mechanisms of activation of caspases are complex but, once activated, the enzymes cleave and activate other procaspases, triggering a cascade that rapidly breaks down proteins within the cell. The cell thus dismantles itself, and its remains are rapidly digested by neighboring phagocytic cells. \nA tremendous amount of apoptosis occurs in tissues that are being remodeled during development. Even in adult humans, billions of cells die each hour in tissues such as the intestine and bone marrow and are replaced by new cells. Programmed cell death, however, is normally balanced by formation of new cells in healthy adults. Otherwise, the body's tissues would shrink or grow excessively. Abnormalities of apoptosis may play a key role in neurodegenerative diseases such as Alzheimer disease, as well as in cancer and autoimmune disorders. Some drugs that have been used successfully for chemotherapy appear to induce apoptosis in cancer cells. \n#### CANCER \nCancer may be caused by *mutation* or by some other *abnormal activation* of cellular genes that control cell growth and cell mitosis. *Proto-oncogenes* are normal genes that code for various proteins that control cell adhesion, growth and division. If mutated or excessively activated, proto-oncogenes can become abnormally functioning *oncogenes* capable of causing cancer*.* As many as 100 different oncogenes have been discovered in human cancers. \nAlso present in all cells are *antioncogenes,* also called *tumor suppressor genes*, which suppress the activation of specific oncogenes. Therefore, loss or inactivation of antioncogenes can allow activation of oncogenes that lead to cancer. \nFor several reasons, only a minute fraction of the cells that mutate in the body ever lead to cancer: \n- • First, most mutated cells have less survival capability than normal cells, and they simply die.\n- • Second, only a few of the mutated cells that survive become cancerous because most mutated cells still have normal feedback controls that prevent excessive growth.\n- • Third, cells that are potentially cancerous are often destroyed by the body's immune system before they grow into a cancer. \nMost mutated cells form abnormal proteins within their cell bodies because of their altered genes, and these proteins activate the body's immune system, causing it to form antibodies or sensitized lymphocytes that react against the cancerous cells, destroying them. In people whose immune systems have been suppressed, such as in persons taking immunosuppressant drugs after kidney or heart transplantation, the probability that a cancer will develop is multiplied as much as fivefold. \n• Fourth, the simultaneous presence of several different activated oncogenes is usually required to cause a cancer. For example, one such gene might promote rapid reproduction of a cell line, but no cancer occurs because another mutant gene is not present simultaneously to form the needed blood vessels. \nWhat is it that causes the altered genes? Considering that many trillions of new cells are formed each year in humans, a better question might be to ask why all of us do not develop millions or billions of mutant cancerous cells. The answer is the incredible precision with which DNA chromosomal strands are replicated in each cell before mitosis can take place, along with the proofreading process that cuts and repairs any abnormal DNA strand before the mitotic process is allowed to proceed. Yet, despite these inherited cellular precautions, probably one newly formed cell in every few million still has significant mutant characteristics. \nThus, chance alone is all that is required for mutations to take place, so we can suppose that a large number of cancers are merely the result of an unlucky occurrence. However, the probability of mutations can be greatly increased when a person is exposed to certain chemical, physical, or biological factors, including the following: \n- 1. *Ionizing radiation,* such as x-rays, gamma rays, particle radiation from radioactive substances, and even ultraviolet light, can predispose individuals to cancer. Ions formed in tissue cells under the influence of such radiation are highly reactive and can rupture DNA strands, causing many mutations.\n- 2. *Chemical substances* of certain types may also cause mutations. It was discovered long ago that various aniline dye derivatives are likely to cause cancer, and thus workers in chemical plants producing such substances, if unprotected, have a special predisposition to cancer. Chemical substances that can cause mutation are called *carcinogens.* The carcinogens that currently cause the greatest number of deaths are those in cigarette smoke. These carcinogens cause over 30% of all cancer deaths and at least 85% of lung cancer deaths.\n- 3. *Physical irritants* can also lead to cancer, such as continued abrasion of the linings of the intestinal tract by some types of food. The damage to the tissues leads to rapid mitotic replacement of the cells; the more rapid the mitosis, the greater the chance for mutation.\n- 4. *Hereditary tendency* to cancer occurs in some families. This hereditary tendency results from the fact that most cancers require not one mutation but two or more mutations before cancer occurs. In families that are particularly predisposed to cancer, it is presumed that one or more cancerous genes are already \n- mutated in the inherited genome. Therefore, far fewer additional mutations must take place in such family members before a cancer begins to grow.\n- 5. *Certain types of oncoviruses* can cause various types of cancer. Some examples of viruses associated with cancers in humans include *human papilloma virus* (HPV), *hepatitis B and hepatitis C virus,* Epstein-Barr virus, human immunodeficiency virus (HIV), human T-cell leukemia virus, Kaposi sarcoma–associated herpes virus (KSHV), and Merkel cell polyomavirus. Although the mechanisms whereby oncoviruses cause cancer are not fully understood, there are at least two potential ways. In the case of DNA viruses, the DNA strand of the virus can insert itself directly into one of the chromosomes, thereby causing a mutation that leads to cancer. In the case of RNA viruses, some of these viruses carry with them an enzyme called *reverse transcriptase* that causes DNA to be transcribed from the RNA. The transcribed DNA then inserts itself into the animal cell genome, leading to cancer. \n**Invasive Characteristic of the Cancer Cell.** The major differences between a cancer cell and a normal cell are as follows: \n- 1. The cancer cell does not respect usual cellular growth limits because these cells presumably do not require all the same growth factors that are necessary to cause growth of normal cells.\n- 2. Cancer cells are often far less adhesive to one another than are normal cells. Therefore, they tend to wander through the tissues, enter the blood stream, and be transported all through the body, where they form nidi for numerous new cancerous growths.\n- 3. Some cancers also produce *angiogenic factors* that cause many new blood vessels to grow into the cancer, thus supplying the nutrients required for cancer growth. \n**Why Do Cancer Cells Kill?** Cancer tissue competes with normal tissues for nutrients. Because cancer cells continue to proliferate indefinitely, with their numbers multiplying every day, cancer cells soon demand essentially all the nutrition available to the body or to an essential part of the body. As a result, normal tissues gradually sustain nutritive death. \nSome cancers cause disruption of vital organ functions. For example, a lung cancer might replace healthy tissue to the extent that the lungs cannot absorb enough oxygen to maintain tissues in the rest of the body. \n#### Bibliography \nAlberts B, Johnson A, Lewis J, et al: Molecular Biology of the Cell, 6th ed. New York: Garland Science 2014. \n- Armanios M: Telomeres and age-related disease: how telomere biology informs clinical paradigms. J Clin Invest 123:996, 2013.\n- Bickmore WA, van Steensel B: Genome architecture: domain organization of interphase chromosomes. Cell 152:1270, 2013.\n- Calcinotto A, Kohli J, Zagato E, Pellegrini L, Demaria M, Alimonti A: Cellular senescence: aging, cancer, and injury. Physiol Rev 99:1047-1078, 2019.\n- Clift D, Schuh M: Restarting life: fertilization and the transition from meiosis to mitosis. Nat Rev Mol Cell Biol 14:549, 2013.\n- Coppola CJ, C Ramaker R, Mendenhall EM: Identification and function of enhancers in the human genome. Hum Mol Genet 25(R2):R190-R197, 2016.\n- Feinberg AP: The key role of epigenetics in human disease prevention and mitigation. N Engl J Med 378:1323-1334, 2018.\n- Fyodorov DV, Zhou BR, Skoultchi AI, Bai Y: Emerging roles of linker histones in regulating chromatin structure and function. Nat Rev Mol Cell Biol 19:192-206, 2018.\n- Haberle V, Stark A: Eukaryotic core promoters and the functional basis of transcription initiation. Nat Rev Mol Cell Biol 19:621-637, 2018.\n- Kaushik S, Cuervo AM: The coming of age of chaperone-mediated autophagy. Nat Rev Mol Cell Biol 19:365-381, 2018.\n- Krump NA, You J: Molecular mechanisms of viral oncogenesis in humans. Nat Rev Microbiol 16:684-698, 2018.\n- Leidal AM, Levine B, Debnath J: Autophagy and the cell biology of age-related disease. Nat Cell Biol 20:1338-1348, 2018.\n- Maciejowski J, de Lange T: Telomeres in cancer: tumour suppression and genome instability. Nat Rev Mol Cell Biol 18:175-186, 2017.\n- McKinley KL, Cheeseman IM: The molecular basis for centromere identity and function. Nat Rev Mol Cell Biol 17:16-29, 2016.\n- Monk D, Mackay DJG, Eggermann T, Maher ER, Riccio A: Genomic imprinting disorders: lessons on how genome, epigenome and environment interact. Nat Rev Genet 10:235, 2019.\n- Müller S, Almouzni G: Chromatin dynamics during the cell cycle at centromeres. Nat Rev Genet 18:192-208, 2017.\n- Nigg EA, Holland AJ: Once and only once: mechanisms of centriole duplication and their deregulation in disease. Nat Rev Mol Cell Biol 19:297-312, 2018.\n- Palozola KC, Lerner J, Zaret KS: A changing paradigm of transcriptional memory propagation through mitosis. Nat Rev Mol Cell Biol 20:55-64, 2019.\n- Perez MF, Lehner B: Intergenerational and transgenerational epigenetic inheritance in animals. Nat Cell Biol 21:143, 2019.\n- Prosser SL, Pelletier L: Mitotic spindle assembly in animal cells: a fine balancing act. Nat Rev Mol Cell Biol 18:187-201, 2017.\n- Schmid M, Jensen TH. Controlling nuclear RNA levels. Nat Rev Genet 19:518-529, 2018.\n- Treiber T, Treiber N, Meister G: Regulation of microRNA biogenesis and its crosstalk with other cellular pathways. Nat Rev Mol Cell Biol 20:5-20, 2019. \n\n\nFigure 4-1 lists the approximate concentrations of important electrolytes and other substances in the *extracellular fluid* and *intracellular fluid*. Note that the extracellular fluid contains a large amount of *sodium* but only a small amount of *potassium*. The opposite is true of the intracellular fluid. Also, the extracellular fluid contains a large amount of *chloride* ions, whereas the intracellular fluid contains very little of these ions. However, the concentrations of *phosphates* and *proteins* in the intracellular fluid are considerably greater than those in the extracellular fluid. These differences are extremely important to the life of the cell. The purpose of this chapter is to explain how the differences are brought about by the cell membrane transport mechanisms. \n \n**Figure 4-1.** Chemical compositions of extracellular and intracellular fluids. The question marks indicate that the precise values for intracellular fluid are unknown. The *red line* indicates the cell membrane.\n\nThe structure of the membrane covering the outside of every cell of the body is discussed in Chapter 2 and illustrated in Figure 2-3 and Figure 4-2. This membrane consists almost entirely of a *lipid bilayer* with large numbers of protein molecules in the lipid, many of which penetrate all the way through the membrane. \nThe lipid bilayer is not miscible with the extracellular fluid or the intracellular fluid. Therefore, it constitutes a barrier against movement of water molecules and water-soluble substances between the extracellular and intracellular fluid compartments. However, as shown in **Figure 4-2** by the leftmost arrow, lipid-soluble substances can diffuse directly through the lipid substance. \nThe membrane protein molecules interrupt the continuity of the lipid bilayer, constituting an alternative pathway through the cell membrane. Many of these penetrating proteins can function as *transport proteins*. Some proteins have watery spaces all the way through the molecule and allow free movement of water, as well as selected ions or molecules; these proteins are called *channel proteins*. Other proteins, called *carrier proteins*, bind with molecules or ions that are to be transported, and conformational changes in the protein molecules then move the substances through the interstices of the protein to the \n \n**Figure 4-2.** Transport pathways through the cell membrane and the basic mechanisms of transport. \n \n**Figure 4-3.** Diffusion of a fluid molecule during one thousandth of a second. \nother side of the membrane. Channel proteins and carrier proteins are usually selective for the types of molecules or ions that are allowed to cross the membrane. \n**\"Diffusion\" Versus \"Active Transport.\"** Transport through the cell membrane, either directly through the lipid bilayer or through the proteins, occurs via one of two basic processes, *diffusion* or *active transport.* \nAlthough many variations of these basic mechanisms exist, *diffusion* means random molecular movement of substances molecule by molecule, either through intermolecular spaces in the membrane or in combination with a carrier protein. The energy that causes diffusion is the energy of the normal kinetic motion of matter. \nIn contrast, *active transport* means movement of ions or other substances across the membrane in combination with a carrier protein in such a way that the carrier protein causes the substance to move against an energy gradient, such as from a low-concentration state to a highconcentration state. This movement requires an additional source of energy besides kinetic energy. A more detailed explanation of the basic physics and physical chemistry of these two processes is provided later in this chapter. \n#### DIFFUSION \nAll molecules and ions in the body fluids, including water molecules and dissolved substances, are in constant motion, with each particle moving in its separate way. The motion of these particles is what physicists call \"heat\" the greater the motion, the higher the temperature—and the motion never ceases, except at absolute zero temperature. When a moving molecule, [A, approac](#page-51-0)hes a stationary molecule, B, the electrostatic and other nuclear forces of molecule A repel molecule B, transferring some of the energy of motion of molecule A to molecule B. Consequently, molecule B gains kinetic energy of motion, whereas molecule A slows down, losing some of its kinetic energy. As shown in **Figure 4-3**, a single molecule in a solution bounces among the other molecules—first in one direction, then another, then another, and so forth randomly bouncing thousands of times each second. This continual movement of molecules among one another in liquids or gases is called *diffusion.* \nIons diffuse in the same manner as whole molecules, and even suspended colloid particles diffuse in a similar manner, except that the colloids diffuse far less rapidly than molecular substances because of their large size. \n#### **DIFFUSION THROUGH THE CELL MEMBRANE** \nDiffusion through the cell membrane is divided into two subtypes, called *simple diffusion* and *facilitated diffusion.* Simple diffusion means that kinetic movement of molecules or ions occurs through a membrane opening or through intermolecular spaces without interaction with carrier proteins in the membrane. The rate of diffusion is determined by the amount of substance available, the velocity of kinetic motion, and the number and sizes of openings in the membrane through which the molecules or ions can move. \nFacilitated diffusion requires interaction of a carrier protein. The carrier protein aids passage of molecules or ions through the membrane by binding chemically with them and shuttlin[g them thro](#page-50-0)ugh the membrane in this form. \nSimple diffusion can occur through the cell membrane by two pathways: (1) through the interstices of the lipid bilayer if the diffusing substance is lipid-soluble; and (2) through watery channels that penetrate all the way through some of the large transport proteins, as shown to the left in **Figure 4-2**. \n**Diffusion of Lipid-Soluble Substances Through the Lipid Bilayer.** The *lipid solubility* of a substance is an important factor for determining how rapidly it diffuses through the lipid bilayer. For example, the lipid solubilities of oxygen, nitrogen, carbon dioxide, and alcohols are high, and all these substances can dissolve directly in the lipid bilayer and diffuse through the cell membrane in the same manner that diffusion of water solutes occurs in a watery solution. The rate of diffusion of each of these substances through the membrane is directly proportional to its lipid solubility. Especially large amounts of oxygen can be transported in this way; therefore, oxygen can be delivered to the interior of the cell almost as though the cell membrane did not exist. \n**Diffusion of Water and Other Lipid-Insoluble Molecules Through Protein Channels.** Even though water is highly insoluble in the membrane lipids, it readily passes through channels in protein molecules that penetrate all the way through the membrane. Many of the body's cell membranes contain protein \"pores\" called *aquaporins* that selectively permit rapid passage of water through the membrane. The aquaporins are highly specialized, and there are at least 13 different types in various cells of mammals. \nThe rapidity with which water molecules can diffuse through most cell membranes is astounding. For example, the total amount of water that diffuses in each direction through the red blood cell membrane during each second is about 100 times as great as the volume of the red blood cell. \nOther lipid-insoluble molecules can pass through the protein pore channels in the same way as water molecules if they are water-soluble and small enough. However, as they become larger, their penetration falls off rapidly. For example, the diameter of the urea molecule is only 20% greater than that of water, yet its penetration through the cell membrane pores is about 1000 times less than that of water. Even so, given the astonishing rate of water penetration, this amount of urea penetration still allows rapid transport of urea through the membrane within minutes. \n#### **DIFFUSION THROUGH PROTEIN PORES AND CHANNELS—SELECTIVE PERMEABILITY AND \"GATING\" OF CHANNELS** \nComputerized three-dimensional reconstructions of protein pores and channels have demonstrated tubular pathways all the way from the extracellular to the intracellular fluid. Therefore, substances can move by simple diffusion directly along these pores and channels from one side of the membrane to the other. \nPores are composed of integral cell membrane proteins that form open tubes through the membrane and are always open. However, the diameter of a pore and its electrical charges provide selectivity that permits only certain molecules to pass through. For example, *aquaporins* permit rapid passage of water through cell membranes but exclude other molecules. Aquaporins have a narrow pore that permits water molecules to diffuse through the membrane in single file. The pore is too narrow to permit passage of any hydrated ions. As discussed in Chapters 28 and 76, the density of some aquaporins (e.g., aquaporin-2) in cell membranes is not static but is altered in different physiological conditions. \nThe protein channels are distinguished by two important characteristics: (1) they are often *selectively permeable* to certain substances; and (2) many of the channels can be opened or closed by *gates* that are regulated by electrical signals *(voltage-gated channels)* or chemicals that bind to the channel proteins *(ligand-gated channels).* Thus, ion channels are flexible dynamic structures, and subtle conformational changes influence gating and ion selectivity. \n**Selective Permeability of Protein Channels.** Many protein channels are highly selective for transport of one or more specific ions or molecules. This selectivity results from specific characteristics of the channel, such as its diameter, shape, and the nature of the electrical charges and chemical bonds along its inside surfaces. \n*Potassium channels* permit passage of potassium ions across the cell membrane about 1000 times more readily than they permit passage of sodium ions. This high degree of selectivity cannot be explained entirely by the \n \n**Figure 4-4.** The structure of a potassium channel. The channel is composed of four subunits (only two of which are shown), each with two transmembrane helices. A narrow selectivity filter is formed from the pore loops, and carbonyl oxygens line the walls of the selectivity filter, forming sites for transiently binding dehydrated potassium ions. The interaction of the potassium ions with carbonyl oxygens causes the potassium ions to shed their bound water molecules, permitting the dehydrated potassium ions to pass through the pore. \nmolecular diameters of the ions because potassium ions are slightly larger than sodium ions. Using x-ray crystallography, potassium channels were found to have a *tetrameric structure* consisting of four identical protein subunits surrounding a central pore (**Figure 4-4**). At the top of the channel pore are *pore loops* that form a narrow *selectivity filter*. Lining the selectivity filter are *carbonyl oxygens.* When hydrated potassium ions enter the selectivity filter, they interact with the carbonyl oxygens and shed most of their bound water molecules, permitting the dehydrated potassium ions to pass through the channel. The carbonyl oxygens are too far apart, however, to enable them to interact closely with the smaller sodium ions, which are therefore effectively excluded by the selectivity filter from passing through the pore. \nDifferent selectivity filters for the various ion channels are believed to determine, in large part, the specificity of various channels for cations or anions or for particular ions, such as sodium (Na+), potassium (K+), and calcium (Ca2+), that gain access to the channels. \nOne of the most important of the protein channels, the *sodium channel,* is only 0.3 to 0.5 nanometer in diameter, but the ability of sodium channels to discriminate sodium ions among other competing ions in the surrounding fluids is crucial for proper cellular function. \n \n**Figure 4-5.** Transport of sodium and potassium ions through protein channels. Also shown are conformational changes in the protein molecules to open or close the \"gates\" guarding the channels. \nThe narrowest part of the sodium channel's open pore, the *selectivity filter*, is lined with *strongly negatively charged* amino acid residues, as shown in the top panel of **Figure 4-5**. These strong negative charges can pull small *dehydrated* sodium ions away from their hydrating water molecules into these channels, although the ions do not need to be fully dehydrated to pass through the channels. Once in the channel, the sodium ions diffuse in either direction according to the usual laws of diffusion. Thus, the sodium channel is highly selective for passage of sodium ions. \n**Gating of Protein Channels.** Gating of protein channels provides a means of controlling ion permeability of the channels. This mechanism is shown in both panels of **Figure 4-5** for selective gating of sodium and potassium ions. Some of the gates are thought to be gatelike extensions of the transport protein molecule, which can close the opening of the channel or can be lifted away from the opening by a conformational change in the shape of the protein molecule. \nThe opening and closing of gates are controlled in two principal ways: \n1. Voltage gating. In the case of voltage gating, the molecular conformation of the gate or its chemical bonds responds to the electrical potential across the cell membrane. For example, in the top panel of Figure 4-5, a strong negative charge on the inside of the cell membrane may cause the outside sodium gates to remain tightly closed. Conversely, when the inside of the membrane loses its negative charge, these gates open suddenly and allow sodium to pass inward through the sodium pores. This process is the basic mechanism for eliciting action potentials in nerves that are responsible for nerve signals. In \n- the bottom panel of **Figure 4-5**, the potassium gates are on the intracellular ends of the potassium channels, and they open when the inside of the cell membrane becomes positively charged. The opening of these gates is partly responsible for terminating the action potential, a process discussed in Chapter 5.\n- 2. Chemical (ligand) gating. Some protein channel gates are opened by the binding of a chemical substance (a ligand) with the protein, which causes a conformational or chemical bonding change in the protein molecule that opens or closes the gate. One of the most important instances of chemical gating is the effect of the neurotransmitter acetylcholine on the acetylcholine receptor which serves as a ligand-gated ion channel. Acetylcholine opens the gate of this channel, providing a negatively charged pore about 0.65 nanometer in diameter that allows uncharged molecules or positive ions smaller than this diameter to pass through. This gate is exceedingly important for the transmission of nerve signals from one nerve cell to another (see Chapter 46) and from nerve cells to muscle cells to cause muscle contraction (see Chapter 7). \n#### Open-State Versus Closed-State of Gated Channels. \nFigure 4-6A shows two recordings of electrical current flowing through a single sodium channel when there was an approximately 25-millivolt potential gradient across the membrane. Note that the channel conducts current in an all-or-none fashion. That is, the gate of the channel snaps open and then snaps closed, with each open state lasting for only a fraction of a millisecond, up to several milliseconds, demonstrating the rapidity with which changes can occur during the opening and closing of the protein gates. At one voltage potential, the channel may remain closed all the time or almost all the time, whereas at another voltage, it may remain open either all or most of the time. At in-between voltages, as shown in the figure, the gates tend to snap open and closed intermittently, resulting in an average current flow somewhere between the minimum and maximum. \nPatch Clamp Method for Recording Ion Current Flow Through Single Channels. The patch clamp method for recording ion current flow through single protein channels is illustrated in Figure 4-6B. A micropipette with a tip diameter of only 1 or 2 micrometers is abutted against the outside of a cell membrane. Suction is then applied inside the pipette to pull the membrane against the tip of the pipette, which creates a seal where the edges of the pipette touch the cell membrane. The result is a minute membrane \"patch\" at the tip of the pipette through which electrical current flow can be recorded. \nAlternatively, as shown at the bottom right in **Figure 4-6B**, the small cell membrane patch at the end of the pipette can be torn away from the cell. The pipette with its sealed patch is then inserted into a free solution, which \n \n**Figure 4-6. A**, Recording of current flow through a single voltagegated sodium channel, demonstrating the all or none principle for opening and closing of the channel. **B**, Patch clamp method for recording current flow through a single protein channel. To the left, the recording is performed from a \"patch\" of a living cell membrane. To the right, the recording is from a membrane patch that has been torn away from the cell. \nallows the concentrations of ions both inside the micropipette and in the outside solution to be altered as desired. Also, the voltage between the two sides of the membrane can be set, or \"clamped,\" to a given voltage. \nIt has been possible to make such patches small enough so that only a single channel protein is found in the membrane patch being studied. By varying the concentrations of different ions, as well as the voltage across the membrane, one can determine the transport characteristics of the single channel, along with its gating properties. \n \n**Figure 4-7.** Effect of concentration of a substance on the rate of diffusion through a membrane by simple diffusion and facilitated diffusion. This graph shows that facilitated diffusion approaches a maximum rate, called the *Vmax.* \n#### **FACILITATED DIFFUSION REQUIRES MEMBRANE CARRIER PROTEINS** \nFacilitated diffusion is also called *carrier-mediated diffusion* because a substance transported in this manner diffuses through the membrane with the help of a specific carrier protein. That is, the carrier *facilitates* diffusion of the substance to the other side. \nFacilitated diffusion differs from simple diffusion in the following important way. Although the rat[e of simple](#page-54-1) diffusion through an open channel increases proportionately with the concentration of the diffusing substance, in facilitated diffusion the rate of diffusion approaches a maximum, called Vmax, as the concentration of the diffusing substance increases. This difference between simple diffusion and facilitated diffusion is demonstrated in **Figure 4-7**. The figure shows t[hat a](#page-55-0)s the concentration of the diffusing substance increases, the rate of simple diffusion continues to increase proportionately but, in the case of facilitated diffusion, the rate of diffusion cannot rise higher than the Vmax level. \nWhat is it that limits the rate of facilitated diffusion? A probable answer is the mechanism illustrated in **Figure 4-8**. This Figure shows a carrier protein with a pore large enough to transport a specific molecule partway through. It also shows a binding receptor on the inside of the protein carrier. The molecule to be transported enters the pore and becomes bound. Then, in a fraction of a second, a conformational or chemical change occurs in the carrier protein, so that the pore now opens to the opposite side of the membrane. Because the binding force of the receptor is weak, the thermal motion of the attached molecule causes it to break away and be released on the opposite side of the membrane. The rate at which molecules can be transported by this mechanism can never be greater than the rate at which the carrier protein molecule can undergo change back and forth between its two states. Note specifically, though, that this mechanism allows the transported molecule to move—that is, diffuse—in either direction through the membrane. \n \nFigure 4-8. Postulated mechanism for facilitated diffusion. \nAmong the many substances that cross cell membranes by facilitated diffusion are *glucose* and most of the *amino acids*. In the case of glucose, at least 14 members of a family of membrane proteins (called *GLUT*) that transport glucose molecules have been discovered in various tissues. Some of these GLUT proteins transport other monosaccharides that have structures similar to that of glucose, including galactose and fructose. One of these, glucose transporter 4 (GLUT4), is activated by insulin, which can increase the rate of facilitated diffusion of glucose as much as 10- to 20-fold in insulin-sensitive tissues. This is the principal mechanism whereby insulin controls glucose use in the body, as discussed in Chapter 79.\n\nBy now, it is evident that many substances can diffuse through the cell membrane. What is usually important is the *net* rate of diffusion of a substance in the desired direction. This net rate is determined by several factors. \n**Net Diffusion Rate Is Proportional to the Concentration Difference Across a Membrane. Figure 4-9.4** shows a cell membrane with a high concentration of a substance on the outside and a low concentration of a substance on the inside. The rate at which the substance diffuses *inward* is proportional to the concentration of molecules on the *outside* because this concentration determines how many molecules strike the outside of the membrane each second. Conversely, the rate at which molecules diffuse *outward* is proportional to their concentration *inside* the membrane. Therefore, the rate of net diffusion into the cell is proportional to the concentration on the outside *minus* the concentration on the inside: \nNet diffusion $\\propto (C_o - C_i)$ \n \n**Figure 4-9.** Effect of concentration difference (**A**), electrical potential difference affecting negative ions (**B**), and pressure difference (**C**) to cause diffusion of molecules and ions through a cell membrane. $C_o$ , concentration outside the cell; $C_i$ , concentration inside the cell; $P_1$ pressure 1; $P_2$ pressure 2. \nin which $C_0$ is the concentration outside and $C_i$ is the concentration inside the cell. \nMembrane Electrical Potential and Diffusion of lons-The \"Nernst Potential.\" If an electrical potential is applied across the membrane, as shown in Figure **4-9B**, the electrical charges of the ions cause them to move through the membrane even though no concentration difference exists to cause movement. Thus, in the left panel of Figure 4-9B, the concentration of negative ions is the same on both sides of the membrane, but a positive charge has been applied to the right side of the membrane, and a negative charge has been applied to the left, creating an electrical gradient across the membrane. The positive charge attracts the negative ions, whereas the negative charge repels them. Therefore, net diffusion occurs from left to right. After some time, large quantities of negative ions have moved to the right, creating the condition shown in the right panel of Figure 4-9B, in which a concentration difference of the ions has developed in the direction opposite to the electrical potential difference. The concentration difference now tends to move the ions to the left, whereas the electrical difference tends to move them to the right. When the concentration difference rises high enough, the two effects balance each other. At normal body temperature (98.6°F; 37°C), the electrical difference that will balance a given concentration difference \nof *univalent* ions—such as Na+ ions—can be determined from the following formula, called the *Nernst equation*: \nEMF (in millivolts) =\n$$\\pm 61\\log \\frac{C_1}{C_2}$$ \nin which EMF is the electromotive force (voltage) between side 1 and side 2 of the membrane, $C_1$ is the concentration on side 1, and $C_2$ is the concentration on side 2. This equation is extremely important in understanding the transmission of nerve impulses and is discussed in Chapter 5. \n#### Effect of a Pressure Difference Across the Membrane. \nAt times, a considerable pressure difference develops between the two sides of a diffusible membrane. This pressure difference occurs, for example, at the blood capillary membranes in all tissues of the body. The pressure in many capillaries is about 20 mm Hg greater inside than outside. \nPressure actually means the sum of all the forces of the different molecules striking a unit surface area at a given instant. Therefore, having a higher pressure on one side of a membrane than on the other side means that the sum of all the forces of the molecules striking the channels on that side of the membrane is greater than on the other side. In most cases, this situation is caused by greater numbers of molecules striking the membrane per second on one side than on the other side. The result is that increased amounts of energy are available to cause a net movement of molecules from the high-pressure side toward the low-pressure side. This effect is demonstrated in **Figure 4-9C**, which shows a piston developing high pressure on one side of a pore, thereby causing more molecules to strike the pore on this side and, therefore, more molecules to diffuse to the other side.\n\nBy far, the most abundant substance that diffuses through the cell membrane is water. Enough water ordinarily diffuses in each direction through the red blood cell membrane per second to equal about 100 times the volume of the cell itself. Yet, the amount that normally diffuses in the two directions is balanced so precisely that zero net movement of water occurs. Therefore, the volume of the cell remains constant. However, under certain conditions, a concentration difference for water can develop across a membrane. When this concentration difference for water develops, net movement of water does occur across the cell membrane, causing the cell to swell or shrink, depending on the direction of the water movement. This process of net movement of water caused by a concentration difference of water is called osmosis. \nTo illustrate osmosis, let us assume the conditions shown in Figure 4-10, with pure water on one side of the cell membrane and a solution of sodium chloride on the \n \n**Figure 4-10.** Osmosis at a cell membrane when a sodium chloride solution is placed on one side of the membrane and water is placed on the other side. \nother side. Water molecules pass through the cell membrane with ease, whereas sodium and chloride ions pass through only with difficulty. Therefore, sodium chloride solution is actually a mixture of permeant water molecules and nonpermeant sodium and chloride ions, and the membrane is said to be *selectively permeable* to water but much less so to sodium and chloride ions. Yet, the presence of the sodium and chloride has displaced some of the water molecules on the side of the membrane where these ions are present and, therefore, has reduced the concentration of water molecules to less than that of pure water. As a result, in the example shown in Figure 4-10, more water molecules strike the channels on the left side, where there is pure water, than on the right side, where the water concentration has been reduced. Thus, net movement of water occurs from left to right-that is, osmosis occurs from the pure water into the sodium chloride solution. \n#### **Osmotic Pressure** \nIf in **Figure 4-10** pressure were applied to the sodium chloride solution, osmosis of water into this solution would be slowed, stopped, or even reversed. The amount of pressure required to stop osmosis is called the *osmotic pressure* of the sodium chloride solution. \nThe principle of a pressure difference opposing osmosis is demonstrated in **Figure 4-11**, which shows a selectively permeable membrane separating two columns of fluid, one containing pure water and the other containing a solution of water and any solute that will not penetrate the membrane. Osmosis of water from chamber B into chamber A causes the levels of the fluid columns to become farther and farther apart, until eventually a pressure difference develops between the two sides of the membrane that is great enough to oppose the osmotic effect. The pressure difference across the membrane at this point is equal to the osmotic pressure of the solution that contains the nondiffusible solute. \n \n**Figure 4-11.** Demonstration of osmotic pressure caused by osmosis at a semipermeable membrane. \n#### **Importance of Number of Osmotic Particles (Molar Concentration) in Determining Osmotic Pressure.** \nThe osmotic pressure exerted by particles in a solution, whether they are molecules or ions, is determined by the number of particles per unit volume of fluid, not by the mass of the particles. The reason for this is that each particle in a solution, regardless of its mass, exerts, on average, the same amount of pressure against the membrane. That is, large particles, which have greater mass (m) than small particles, move at a slower velocity (v). The small particles move at higher velocities in such a way that their average kinetic energies (k), as determined by the following equation, \n$$k = \\frac{mv^2}{2}$$ \nare the same for each small particle as for each large particle. Consequently, the factor that determines the osmotic pressure of a solution is the concentration of the solution in terms of the number of particles (which is the same as its *molar concentration* if it is a nondissociated molecule), not in terms of mass of the solute. \n**Osmolality—The Osmole.** To express the concentration of a solution in terms of numbers of particles, a unit called the *osmole* is used in place of grams. \nOne osmole is 1 gram molecular weight of osmotically active solute. Thus, 180 grams of glucose, which is 1 gram molecular weight of glucose, is equal to 1 osmole of glucose because glucose does not dissociate into ions. If a solute dissociates into two ions, 1 gram molecular weight of the solute will become 2 osmoles because the number of osmotically active particles is now twice as great as for the nondissociated solute. Therefore, when fully dissociated, 1 gram molecular weight of sodium chloride, 58.5 grams, is equal to 2 osmoles. \nThus, a solution that has *1 osmole of solute dissolved in each kilogram of water* is said to have an *osmolality of 1 osmole per kilogram,* and a solution that has 1/1000 osmole dissolved per kilogram has an osmolality of 1 milliosmole per kilogram. The normal osmolality of the extracellular and intracellular fluids is about *300 milliosmoles per kilogram of water.* \n**Relationship of Osmolality to Osmotic Pressure.** At normal body temperature, 37°C (98.6°F), a concentration of 1 osmole per liter will cause 19,300 mm Hg osmotic pressure in the solution. Likewise, *1 milliosmole* per liter concentration is equivalent to *19.3 mm Hg* osmotic pressure. Multiplying this value by the 300-milliosmolar concentration of the body fluids gives a total calculated osmotic pressure of the body fluids of 5790 mm Hg. The measured value for this, however, averages only about 5500 mm Hg. The reason for this difference is that many ions in the body fluids, such as sodium and chloride ions, are highly attracted to one another; consequently, they cannot move entirely unrestrained in the fluids and create their full osmotic pressure potential. Therefore, on average, the actual osmotic pressure of the body fluids is about 0.93 times the calculated value. \n**The Term** *Osmolarity***.** *Osmolarity* is the osmolar concentration expressed as *osmoles per liter of solution* rather than osmoles per kilogram of water. Although, strictly speaking, it is osmoles per kilogram of water (osmolality) that determines osmotic pressure, the quantitative differences between osmolarity and osmolality are less than 1% for dilute solutions such as those in the body. Because it is far more practical to measure osmolarity than osmolality, measuring osmolarity is the usual practice in physiological studies. \n#### ACTIVE TRANSPORT OF SUBSTANCES THROUGH MEMBRANES \nAt times, a large concentration of a substance is required in the intracellular fluid, even though the extracellular fluid contains only a small concentration. This situation is true, for example, for potassium ions. Conversely, it is important to keep the concentrations of other ions very low inside the cell, even though their concentrations in the extracellular fluid are high. This situation is especially true for sodium ions. Neither of these two effects could occur by simple diffusion because simple diffusion eventually equilibrates concentrations on the two sides of the membrane. Instead, some energy source must cause excess movement of potassium ions to the inside of cells and excess movement of sodium ions to the outside of cells. When a cell membrane moves molecules or ions uphill against a concentration gradient (or uphill against an electrical or pressure gradient), the process is called *active transport.* \nSome examples of substances that are actively transported through at least some cell membranes include sodium, potassium, calcium, iron, hydrogen, chloride, iodide, and urate ions, several different sugars, and most of the amino acids. \n**Primary Active Transport and Secondary Active Transport.** Active transport is divided into two types according to the source of the energy used to facilitate the transport, *primary active transport* and *secondary active transport.* In primary active transport, the energy is derived directly from the breakdown of adenosine triphosphate (ATP) or some other high-energy phosphate compound. In secondary active transport, the energy is derived secondarily from energy that has been stored in the form of ionic concentration differences of secondary molecular or ionic substances between the two sides of a cell membrane, created originally by primary active transport. In both cases, transport depends on *carrier proteins* that penetrate through the cell membrane, as is true for facilitated diffusion. However, in active transport, the carrier protein functions differently from the carrier in facilitated diffusion because it is capable of imparting energy to the transported substance to move it against the electrochemical gradient. The following sections provide some examples of primary active transport and secondary active transport, with more detailed explanations of their principles of function. \n#### **PRIMARY ACTIVE TRANSPORT** \n#### **Sodium-Potassium Pump Transports Sodium Ions Out of Cells and Potassium Ions into Cells** \nAmong the substances that are transported by primary active transport are sodium, potassium, calcium, hydrogen, chloride, and a few other ions. The active transport mechanism that has been studied in greatest detail is the *sodium-potassium* (Na+-K+) pump, a transporter that pumps sodium ions outward through the cell membrane of all cells and, at the same time, pumps potassium ions from the outside to the inside. This pump is responsible for mainta[ining the so](#page-58-0)dium and potassium concentration differences across the cell membrane, as well as for establishing a negative electrical voltage inside the cells. Indeed, Chapter 5 shows that this pump is also the basis of nerve function, transmitting nerve signals throughout the nervous system. \n**Figure 4-12** shows the basic physical components of the Na+-K+ pump. The *carrier protein* is a complex of two separate globular proteins—a larger one called the α subunit, with a molecular weight of about 100,000, and a smaller one called the β subunit, with a molecular weight of about 55,000. Although the function of the smaller protein is not known (except that it might anchor the protein complex in the lipid membrane), the larger protein has three specific features that are important for the functioning of the pump: \n1. It has three *binding sites for sodium ions* on the portion of the protein that protrudes to the inside of the cell. \n \n**Figure 4-12.** Postulated mechanism of the sodium-potassium pump. ADP, Adenosine diphosphate; ATP, adenosine triphosphate; Pi, phosphate ion. \n- 2. It has two *binding sites for potassium ions* on the outside.\n- 3. The inside portion of this protein near the sodium binding sites has adenosine triphosphatase (AT-Pase) activity. \nWhen two potassium ions bind on the outside of the carrier protein and three sodium ions bind on the inside, the ATPase function of the protein becomes activated. Activation of the ATPase function leads to cleavage of one molecule of ATP, splitting it to adenosine diphosphate (ADP) and liberating a high-energy phosphate bond of energy. This liberated energy is believed to cause a chemical and conformational change in the protein carrier molecule, extruding three sodium ions to the outside and two potassium ions to the inside. \nAs with other enzymes, the Na+-K+ ATPase pump can run in reverse. If the electrochemical gradients for Na+ and K+ are experimentally increased to the degree that the energy stored in their gradients is greater than the chemical energy of ATP hydrolysis, these ions will move down their concentration gradients, and the Na+-K+ pump will synthesize ATP from ADP and phosphate. The phosphorylated form of the Na+-K+ pump, therefore, can either donate its phosphate to ADP to produce ATP or use the energy to change its conformation and pump Na+ out of the cell and K+ into the cell. The relative concentrations of ATP, ADP, and phosphate, as well as the electrochemical gradients for Na+ and K+, determine the direction of the enzyme reaction. For some cells, such as electrically active nerve cells, 60% to 70% of the cell's energy requirement may be devoted to pumping Na+ out of the cell and K+ into the cell. \n**The Na+-K+ Pump Is Important for Controlling Cell Volume.** One of the most important functions of the Na+-K+ pump is to control the cell volume. Without function of this pump, most cells of the body would swell until they burst. \nThe mechanism for controlling the volume is as follows. Inside the cell are large numbers of proteins and other organic molecules that cannot escape from the cell. Most of these proteins and other organic molecules are negatively charged and, therefore, attract large numbers of potassium, sodium, and other positive ions. All these molecules and ions then cause osmosis of water to the interior of the cell. Unless this process is checked, the cell will swell indefinitely until it bursts. The normal mechanism for preventing this outcome is the Na+-K+ pump. Note again that this mechanism pumps three Na+ ions to the outside of the cell for every two K+ ions pumped to the interior. Also, the membrane is far less permeable to sodium ions than to potassium ions and, once the sodium ions are on the outside, they have a strong tendency to stay there. This process thus represents a net loss of ions out the cell, which also initiates osmosis of water out of the cell. \nIf a cell begins to swell for any reason, the Na+-K+ pump is automatically activated, moving still more ions to the exterior and carrying water with them. Therefore, the Na+-K+ pump performs a continual surveillance role in maintaining normal cell volume. \n**Electrogenic Nature of the Na+-K+ Pump.** The fact that the Na+-K+ pump moves three Na+ ions to the exterior for every two K+ ions that are moved to the interior means that a net of one positive charge is moved from the interior of the cell to the exterior of the cell for each cycle of the pump. This action creates positivity outside the cell but results in a deficit of positive ions inside the cell; that is, it causes negativity on the inside. Therefore, the Na+-K+ pump is said to be *electrogenic* because it creates an electrical potential across the cell membrane. As discussed in Chapter 5, this electrical potential is a basic requirement in nerve and muscle fibers for transmitting nerve and muscle signals. \n#### **Primary Active Transport of Calcium Ions** \nAnother important primary active transport mechanism is the *calcium pump*. Calcium ions are normally maintained at an extremely low concentration in the intracellular cytosol of virtually all cells in the body, at a concentration about 10,000 times less than that in the extracellular fluid. This level of maintenance is achieved mainly by two primary active transport calcium pumps. One, which is in the cell membrane, pumps calcium to the outside of the cell. The other pumps calcium ions into one or more of the intracellular vesicular organelles of the cell, such as the sarcoplasmic reticulum of muscle cells and the mitochondria in all cells. In each of these cases, the carrier protein penetrates the membrane and functions as an enzyme ATPase, with the same capability to cleave ATP as the ATPase of the sodium carrier protein. The difference is that this protein has a highly specific binding site for calcium instead of for sodium.\n\nPrimary active transport of hydrogen ions is especially important at two places in the body: (1) in the gastric glands of the stomach; and (2) in the late distal tubules and cortical collecting ducts of the kidneys. \nIn the gastric glands, the deep-lying *parietal cells* have the most potent primary active mechanism for transporting hydrogen ions of any part of the body. This mechanism is the basis for secreting hydrochloric acid in stomach digestive secretions. At the secretory ends of the gastric gland parietal cells, the hydrogen ion concentration is increased as much as a million-fold and then is released into the stomach, along with chloride ions, to form hydrochloric acid. \nIn the renal tubules, special *intercalated cells* found in the late distal tubules and cortical collecting ducts also transport hydrogen ions by primary active transport. In this case, large amounts of hydrogen ions are secreted from the blood into the renal tubular fluid for the purpose of eliminating excess hydrogen ions from the body fluids. The hydrogen ions can be secreted into the renal tubular fluid against a concentration gradient of about 900-fold. Yet, as discussed in Chapter 31, most of these hydrogen ions combine with tubular fluid buffers before they are eliminated in the urine \n#### **Energetics of Primary Active Transport** \nThe amount of energy required to transport a substance actively through a membrane is determined by how much the substance is concentrated during transport. Compared with the energy required to concentrate a substance 10-fold, concentrating it 100-fold requires twice as much energy, and concentrating it 1000-fold requires three times as much energy. In other words, the energy required is proportional to the *logarithm* of the degree that the substance is concentrated, as expressed by the following formula: \nEnergy (in calories per osmole) = 1400 log\n$$\\frac{C_1}{C_2}$$ \nThus, in terms of calories, the amount of energy required to concentrate 1 osmole of a substance 10-fold is about 1400 calories, whereas to concentrate it 100-fold, 2800 calories are required. One can see that the energy expenditure for concentrating substances in cells or for removing substances from cells against a concentration gradient can be tremendous. Some cells, such as those lining the renal tubules and many glandular cells, expend as much as 90% of their energy for this purpose alone.\n\nWhen sodium ions are transported out of cells by primary active transport, a large concentration gradient of \nsodium ions across the cell membrane usually develops, with a high concentration outside the cell and a low concentration inside. This gradient represents a storehouse of energy, because the excess sodium outside the cell membrane is always attempting to diffuse to the interior. Under appropriate conditions, this diffusion energy of sodium can pull other substances along with the sodium through the cell membrane. This phenomenon, called *cotransport,* is one form of *secondary active transport.* \nFor sodium to pull another substance along with it, a coupling mechanism is required; this is achieved by means of still another carrier protein in the cell membrane. The carrier in this case serves as an attachment point for both the sodium ion and the substance to be co-transported. Once they are both attached, the energy gradient of the sodium ion causes the sodium ion and the other substance to be transported together to the interior of the cell. \nIn *counter-transport,* sodium ions again attempt to diffuse to the interior of the cell because of their large concentration gradient. However, this time, the substance to be transported is on the inside of the cell and is transported to the outside. Therefore, the sodium ion binds to the carrier protein, where it projects to the exterior surface of the membrane, and the substance to be countertransported binds to the interior projection of the carrier protein. Once both have become bound, a conformational change occurs, and energy released by the action of the sodium ion moving to the interior causes the other substance to move to the exterior. \n#### **Co-Transport o[f Glucose a](#page-60-0)nd Amino Acids Along with Sodium Ions** \nGlucose and many amino acids are transported into most cells against large concentration gradients; the mechanism of this action is entirely by co-transport, as shown in **Figure 4-13**. Note that the transport carrier protein has two binding sites on its exterior side, one for sodium and one for glucose. Also, the concentration of sodium ions is high on the outside and low on the inside, which provides energy for the transport. A special property of the transport protein is that a conformational change to allow sodium movement to the interior will not occur until a glucose molecule also attaches. When they both become attached, the conformational change takes place, and the sodium and glucose are transported to the inside of the cell at the same time. Hence, this is a *sodium-glucose co-transporter*. Sodium-glucose cotransporters are especially important for transporting glucose across renal and intestinal epithelial cells, as discussed in Chapters 28 and 66. \n*Sodium co-transport of amino acids* occurs in the same manner as for glucose, except that it uses a different set of transport proteins. At least five *amino acid transport proteins* have been identified, each of which is responsible for transporting one subset of amino acids with specific molecular characteristics. \n \n**Figure 4-13** Postulated mechanism for sodium co-transport of glucose. \n \n**Figure 4-14.** Sodium counter-transport of calcium and hydrogen ions. \nSodium co-transport of glucose and amino acids occurs especially through the epithelial cells of the intestinal tract and the renal tubules of the kidneys to promote absorption of these substances into the blood. This process will be discussed in later chapters. \nOther important co-transport mechanisms in at least some cells include co-transport of potassium, chloride, bicarbonate, phosphate, iodine, iron, and urate ions. \n#### **Sodium Counter-Tran[sport of Ca](#page-60-1)lcium and Hydrogen Ions** \nTwo especially important counter-transporters (i.e., transport in a direction opposite to the primary ion) are *sodium-calcium counter-transport* and *sodium-hydrogen counter-transport* (**Figure 4-14**). \nSodium-calcium counter-transport occurs through all or almost all cell membranes, with sodium ions moving to the interior and calcium ions to the exterior; both are bound to the same transport protein in a countertransport mode. This mechanism is in addition to the primary active transport of calcium that occurs in some cells. \nSodium-hydrogen counter-transport occurs in several tissues. An especially important example is in the *proximal tubules* of the kidneys, where sodium ions move from the lumen of the tubule to the interior of the tubular cell and hydrogen ions are counter-transported into the tubule lumen. As a mechanism for concentrating hydrogen ions, counter-transport is not nearly as powerful as the primary active transport of hydrogen ions that occurs in the more distal renal tubules, but it can transport extremely *large numbers of hydrogen ions,* thus making it a key to hydrogen ion control in the body fluids, as discussed in detail in Chapter 31. \n \n**Figure 4-15.** Basic mechanism of active transport across a layer of cells. \n#### **ACTIVE TRANSPORT THROUGH CELLULAR SHEETS** \nAt many places in the body, substances must be transported all the way through a cellular sheet instead of simply through the cell membrane. Transport of this type occurs through the following: (1) intestinal epithelium; (2) epithelium of the renal tubules; (3) epithelium of all exocrine glands; (4) epithelium of the gallbladder; and (5) membrane of the choroid plexus of the brain, along with other membranes. \nThe bas[ic mechanism](#page-61-0) for transport of a substance through a cellular sheet is as follows: (1) *active transport* through the cell membrane *on one side* of the transporting cells in the sheet; and then (2) either *simple diffusion* or *facilitated diffusion* through the membrane *on the opposite side* of the cell. \n**Figure 4-15** shows a mechanism for the transport of sodium ions through the epithelial sheet of the intestines, gallbladder, and renal tubules. This figure shows that the epithelial cells are connected together tightly at the luminal pole by means of junctions. The brush border on the luminal surfaces of the cells is permeable to both sodium ions and water. Therefore, sodium and water diffuse readily from the lumen into the interior of the cell. Then, at the basal and lateral membranes of the cells, sodium ions are actively transported into the extracellular fluid of the surrounding connective tissue and blood vessels. This action creates a high sodium ion concentration gradient across these membranes, which in turn causes osmosis of water. Thus, active transport of sodium ions at the basolateral sides of the epithelial cells results in the transport not only of sodium ions but also of water. \nIt is through these mechanisms that almost all nutrients, ions, and other substances are absorbed into the blood from the intestine. These mechanisms are also how the same substances are reabsorbed from the glomerular filtrate by the renal tubules. \nNumerous examples of the different types of transport discussed in this chapter are provided throughout this text. \n#### Bibliography \nAgre P, Kozono D: Aquaporin water channels: molecular mechanisms for human diseases. FEBS Lett 555:72, 2003. \nBröer S: Amino acid transport across mammalian intestinal and renal epithelia. Physiol Rev 88:249, 2008. \nDeCoursey TE: Voltage-gated proton channels: molecular biology, physiology, and pathophysiology of the H(V) family. Physiol Rev 93:599, 2013. \nDiPolo R, Beaugé L: Sodium/calcium exchanger: influence of metabolic regulation on ion carrier interactions. Physiol Rev 86:155, 2006. \nDrummond HA, Jernigan NL, Grifoni SC: Sensing tension: epithelial sodium channel/acid-sensing ion channel proteins in cardiovascular homeostasis. Hypertension 51:1265, 2008. \nEastwood AL, Goodman MB: Insight into DEG/ENaC channel gating from genetics and structure. Physiology (Bethesda) 27:282, 2012. \nFischbarg J: Fluid transport across leaky epithelia: central role of the tight junction and supporting role of aquaporins. Physiol Rev 90:1271, 2010. \nGadsby DC: Ion channels versus ion pumps: the principal difference, in principle. Nat Rev Mol Cell Biol 10:344, 2009. \nGhezzi C, Loo DDF, Wright EM. Physiology of renal glucose handling via SGLT1, SGLT2 and GLUT2. Diabetologia 61:2087-2097, 2018. \nHilge M: Ca2+ regulation of ion transport in the Na+/Ca2+ exchanger. J Biol Chem 287:31641, 2012. \nJentsch TJ, Pusch M. CLC Chloride channels and transporters: structure, function, physiology, and disease. Physiol Rev 2018 98:1493- 1590, 2018. \nKaksonen M, Roux A. Mechanisms of clathrin-mediated endocytosis. Nat Rev Mol Cell Biol 19:313-326, 2018. \nKandasamy P, Gyimesi G, Kanai Y, Hediger MA. Amino acid transporters revisited: new views in health and disease. Trends Biochem Sci 43:752-789, 2018. \nPapadopoulos MC, Verkman AS: Aquaporin water channels in the nervous system. Nat Rev Neurosci 14:265, 2013. \nRieg T, Vallon V. Development of SGLT1 and SGLT2 inhibitors. Diabetologia 61:2079-2086, 2018. \nSachs F: Stretch-activated ion channels: what are they? Physiology 25:50, 2010. \nSchwab A, Fabian A, Hanley PJ, Stock C: Role of ion channels and transporters in cell migration. Physiol Rev 92:1865, 2012. \nStransky L, Cotter K, Forgac M. The function of V-ATPases in cancer. Physiol Rev 96:1071-1091, 2016 \nTian J, Xie ZJ: The Na-K-ATPase and calcium-signaling microdomains. Physiology (Bethesda) 23:205, 2008. \nVerkman AS, Anderson MO, Papadopoulos MC. Aquaporins: important but elusive drug targets. Nat Rev Drug Discov 13:259-277, 2014. \nWright EM, Loo DD, Hirayama BA: Biology of human sodium glucose transporters. Physiol Rev 91:733, 2011. \n\n\nElectrical potentials exist across the membranes of virtually all cells of the body. Some cells, such as nerve and muscle cells, generate rapidly changing electrochemical impulses at their membranes, and these impulses are used to transmit signals along the nerve or muscle membranes. In other types of cells, such as glandular cells, macrophages, and ciliated cells, local changes in membrane potentials also activate many of the cell's functions. This chapter reviews the basic mechanisms whereby membrane potentials are generated at rest and during action by nerve and muscle cells. See Video 5-1. \n\n\nIn Figure 5-1A, the potassium concentration is great inside a nerve fiber membrane but very low outside the membrane. Let us assume that the membrane in this case is permeable to the potassium ions but not to any other ions. Because of the large potassium concentration gradient from the inside toward the outside, there is a strong tendency for potassium ions to diffuse outward through the membrane. As they do so, they carry positive electrical charges to the outside, thus creating electropositivity outside the membrane and electronegativity inside the membrane because of negative anions that remain behind and do not diffuse outward with the potassium. Within about 1 millisecond, the potential difference between the inside and outside, called the diffusion potential, becomes great enough to block further net potassium diffusion to the exterior, despite the high potassium ion concentration gradient. In the normal mammalian nerve fiber, the potential difference is about 94 millivolts, with negativity inside the fiber membrane. \n**Figure 5-1***B* shows the same phenomenon as in **Figure 5-1***A*, but this time with a high concentration of sodium ions *outside* the membrane and a low concentration of sodium ions *inside*. These ions are also positively charged. This time, the membrane is highly permeable to the sodium ions but is impermeable to all other ions. Diffusion of the positively charged sodium ions to the inside \ncreates a membrane potential of opposite polarity to that in **Figure 5-1***A*, with negativity outside and positivity inside. Again, the membrane potential rises high enough within milliseconds to block further net diffusion of sodium ions to the inside; however, this time, in the mammalian nerve fiber, *the potential is about 61 millivolts positive inside the fiber.* \nThus, in both parts of **Figure 5-1**, we see that a concentration difference of ions across a selectively permeable membrane can, under appropriate conditions, create a membrane potential. Later in this chapter, we show that many of the rapid changes in membrane potentials observed during nerve and muscle impulse transmission result from such rapidly changing diffusion potentials. \nThe Nernst Equation Describes the Relationship of Diffusion Potential to the lon Concentration Difference Across a Membrane. The diffusion potential across a membrane that exactly opposes the net diffusion of a particular ion through the membrane is called the *Nernst potential* for that ion, a term that was introduced in Chapter 4. The magnitude of the Nernst potential is determined by the *ratio* of the concentrations of that specific ion on the two sides of the membrane. The greater this ratio, the greater the tendency for the ion to diffuse in one direction and therefore the greater the Nernst potential required to prevent additional net diffusion. The following equation, called the *Nernst equation*, can be used to calculate the Nernst potential for any univalent ion at the normal body temperature of 98.6°F (37°C): \nEMF (millivolts) =\n$$\\pm \\frac{61}{z} \\times log \\frac{Concentration\\ inside}{Concentration\\ outside}$$ \nwhere EMF is the electromotive force and z is the electrical charge of the ion (e.g., +1 for $K^+$ ). \nWhen using this formula, it is usually assumed that the potential in the extracellular fluid outside the membrane remains at zero potential, and the Nernst potential is the potential inside the membrane. Also, the sign of the potential is positive (+) if the ion diffusing from inside to outside is a negative ion, and it is negative (–) if the ion is positive. Thus, when the concentration of positive potassium ions on the inside is 10 times that on the outside, the log of 10 is 1, so the Nernst potential calculates to be -61 millivolts inside the membrane.\n\n**Figure 5-1 A**, Establishment of a diffusion potential across a nerve fiber membrane, caused by diffusion of potassium ions from inside the cell to outside the cell through a membrane that is selectively permeable only to potassium. **B**, Establishment of a diffusion potential when the nerve fiber membrane is permeable only to sodium ions. Note that the internal membrane potential is negative when potassium ions diffuse and positive when sodium ions diffuse because of opposite concentration gradients of these two ions. \nThe Goldman Equation Is Used to Calculate the Diffusion Potential When the Membrane Is Permeable to Several Different Ions. When a membrane is permeable to several different ions, the diffusion potential that develops depends on three factors: (1) the polarity of the electrical charge of each ion; (2) the permeability of the membrane (P) to each ion; and (3) the concentration (C) of the respective ions on the inside (i) and outside (o) of the membrane. Thus, the following formula, called the *Goldman equation* or the *Goldman-Hodgkin-Katz equation*, gives the calculated membrane potential on the *inside* of the membrane when two univalent positive ions, sodium (Na+) and potassium (K+), and one univalent negative ion, chloride (Cl-), are involved: \n$$EMF \\ (millivolts) = -61 \\times log \\frac{C_{Na_{i}^{+}}P_{Na^{+}} + C_{K_{i}^{+}}P_{K^{+}} + C_{Cl_{0}}P_{Cl^{-}}}{C_{Na_{0}^{+}}P_{Na^{+}} + C_{K_{0}^{+}}P_{K^{+}} + C_{Cl_{i}^{-}}P_{Cl^{-}}}$$ \nSeveral key points become evident from the Goldman equation. First, sodium, potassium, and chloride ions are the most important ions involved in the development of membrane potentials in nerve and muscle fibers, as well as in the neuronal cells. The concentration gradient of each of these ions across the membrane helps determine the voltage of the membrane potential. \nSecond, the quantitative importance of each of the ions in determining the voltage is proportional to the membrane permeability for that particular ion. If the membrane has zero permeability to sodium and chloride ions, the membrane potential becomes entirely dominated by the concentration gradient of potassium ions alone, and the resulting potential will be equal to the Nernst potential for potassium. The same holds true for each of the other two ions if the membrane should become selectively permeable for either one of them alone. \nThird, a positive ion concentration gradient from *inside* the membrane to the *outside* causes electronegativity \n**Table 5-1** Resting Membrane Potential in Different Cell Types \n| Cell Type | Resting Potential (mV) |\n|-----------------|---------------------------|\n| Neurons | −60 to −70 |\n| Skeletal muscle | −85 to −95 |\n| Smooth muscle | −50 to −60 |\n| Cardiac muscle | −80 to −90 |\n| Hair (cochlea) | –15 to –40 |\n| Astrocyte | -80 to -90 |\n| Erythrocyte | −8 to −12 |\n| Photoreceptor | –40 (dark) to –70 (light) | \ninside the membrane. The reason for this phenomenon is that excess positive ions diffuse to the outside when their concentration is higher inside than outside the membrane. This diffusion carries positive charges to the outside but leaves the nondiffusible negative anions on the inside, thus creating electronegativity on the inside. The opposite effect occurs when there is a gradient for a negative ion. That is, a chloride ion gradient from the outside to the inside causes negativity inside the cell because excess negatively charged chloride ions diffuse to the inside while leaving the nondiffusible positive ions on the outside. \nFourth, as explained later, the permeability of the sodium and potassium channels undergoes rapid changes during transmission of a nerve impulse, whereas the permeability of the chloride channels does not change greatly during this process. Therefore, rapid changes in sodium and potassium permeability are primarily responsible for signal transmission in neurons, which is the subject of most of the remainder of this chapter. \nResting Membrane Potential of Different Cell Types. In some cells, such as the cardiac pacemaker cells discussed in Chapter 10, the membrane potential is continuously changing, and the cells are never \"resting\". In many other cells, even excitable cells, there is a quiescent period in which a resting membrane potential can be measured. Table 5-1 shows the approximate resting membrane potentials of some different types of cells. The membrane potential is obviously very dynamic in excitable cells such as neurons, in which action potentials occur. However, even in nonexcitable cells, the membrane potential (voltage) also changes in response to various stimuli, which alter activities for the various ion transporters, ion channels, and membrane permeability for sodium, potassium, calcium, and chloride ions. The resting membrane potential is, therefore, only a brief transient state for many cells. \n**Electrochemical Driving Force.** When multiple ions contribute to the membrane potential, the equilibrium potential for any of the contributing ions will differ from the membrane potential, and there will be an *electrochemical driving force* ( $V_{df}$ ) for each ion that tends to cause net \n \n**Figure 5-2** Measurement of the membrane potential of the nerve fiber using a microelectrode. \nmovement of the ion across the membrane. This driving force is equal to the difference between the membrane potential $(V_m)$ and the equilibrium potential of the ion $(V_{eq})$ Thus, $V_{df} = V_m - V_{eq}.$ \nThe arithmetic sign of $V_{df}$ (positive or negative) and the valence of the ion (cation or anion) can be used to predict the direction of ion flow across the membrane, into or out of the cell. For cations such as $Na^+$ and $K^+$ , a positive $V_{df}$ predicts ion movement out of the cell down its electrochemical gradient, and a negative $V_{df}$ predicts ion movement into the cell. For anions, such as $Cl^-$ , a positive $V_{df}$ predicts ion movement into the cell, and a negative $V_{df}$ predicts ion movement out of the cell. When $V_m = V_{eq}$ , there is no net movement of the ion into or out of the cell. Also, the direction of ion flux through the membrane reverses as $V_m$ becomes greater than or less than $V_{eq}$ ; hence, the equilibrium potential ( $V_{eq}$ ) is also called the *reversal potential*. \n#### Measuring the Membrane Potential \nThe method for measuring the membrane potential is simple in theory but often difficult in practice because of the small size of most of the cells and fibers. Figure 5-2 shows a small micropipette filled with an electrolyte solution. The micropipette is impaled through the cell membrane to the interior of the fiber. Another electrode, called the indifferent electrode, is then placed in the extracellular fluid, and the potential difference between the inside and outside of the fiber is measured using an appropriate voltmeter. This voltmeter is a highly sophisticated electronic apparatus that is capable of measuring small voltages despite extremely high resistance to electrical flow through the tip of the micropipette, which has a lumen diameter usually less than 1 micrometer and a resistance of more than 1 million ohms. For recording rapid changes in the membrane potential during transmission of nerve impulses, the microelectrode is connected to an oscilloscope, as explained later in the chapter. \nThe lower part of **Figure 5-3** shows the electrical potential that is measured at each point in or near the nerve fiber membrane, beginning at the left side of the figure and passing to the right. As long as the electrode is outside the neuronal membrane, the recorded potential \n \n**Figure 5-3** Distribution of positively and negatively charged ions in the extracellular fluid surrounding a nerve fiber and in the fluid inside the fiber. Note the alignment of negative charges along the inside surface of the membrane and positive charges along the outside surface. The *lower panel* displays the abrupt changes in membrane potential that occur at the membranes on the two sides of the fiber. \nis zero, which is the potential of the extracellular fluid. Then, as the recording electrode passes through the voltage change area at the cell membrane (called the *electrical dipole layer*), the potential decreases abruptly to –70 millivolts. Moving across the center of the fiber, the potential remains at a steady –70-millivolt level but reverses back to zero the instant it passes through the membrane on the opposite side of the fiber. \nTo create a negative potential inside the membrane, only enough positive ions to develop the electrical dipole layer at the membrane itself must be transported outward. The remaining ions inside the nerve fiber can be both positive and negative, as shown in the upper panel of Figure 5-3. Therefore, transfer of an incredibly small number of ions through the membrane can establish the normal resting potential of -70 millivolts inside the nerve fiber, which means that only about 1/3,000,000 to 1/100,000,000 of the total positive charges inside the fiber must be transferred. Also, an equally small number of positive ions moving from outside to inside the fiber can reverse the potential from -70 millivolts to as much as +35 millivolts within as little as 1/10,000 of a second. Rapid shifting of ions in this manner causes the nerve signals discussed in subsequent sections of this chapter.\n\nThe resting membrane potential of large nerve fibers when they are not transmitting nerve signals is about -70 millivolts. That is, the potential *inside the fiber* is 70 millivolts more negative than the potential in the extracellular fluid on the outside of the fiber. In the next few paragraphs, the transport properties of the resting nerve membrane for \n \n**Figure 5-4** Functional characteristics of the Na+-K+ pump and the K+ \"leak\" channels. The K+ leak channels also leak Na+ ions into the cell slightly but are much more permeable to K+. ADP, Adenosine diphosphate; ATP, adenosine triphosphate. \nsodium and potassium and the factors that determine the level of this resting potential are explained. \nActive Transport of Sodium and Potassium lons Through the Membrane—the Sodium-Potassium (Na+-K+) Pump. Recall from Chapter 4 that all cell membranes of the body have a powerful Na+-K+ pump that continually transports sodium ions to the outside of the cell and potassium ions to the inside, as illustrated on the left side in Figure 5-4. Note that this is an *electrogenic pump* because three Na+ ions are pumped to the outside for each two K+ ions to the inside, leaving a net deficit of positive ions on the inside and causing a negative potential inside the cell membrane. \nThe $Na^+-K^+$ pump also causes large concentration gradients for sodium and potassium across the resting nerve membrane. These gradients are as follows: \nNa+ (outside): 142 mEq/L \nNa+ (inside): 14 mEq/L \nK+(outside): 4 mEq/L \nK+(inside): 140 mEq/L \nThe ratios of these two respective ions from the inside to the outside are as follows: \n$$Na_{inside}^{+}/Na_{outside}^{+}=0.1$$ \n$$K^{+}_{inside}/K^{+}_{outside} = 35.0$$ \n**Leakage of Potassium Through the Nerve Cell Membrane.** The right side of **Figure 5-4** shows a channel protein (sometimes called a *tandem pore domain, potassium channel*, or *potassium* $[K^+]$ *\"leak\" channel*) in the nerve membrane through which potassium ions can leak, even in a resting cell. The basic structure of potassium channels was described in Chapter 4 (**Figure 4-4**). These $K^+$ leak \n \n**Figure 5-5** Establishment of resting membrane potentials under three conditions. **A**, When the membrane potential is caused entirely by potassium diffusion alone. **B**, When the membrane potential is caused by diffusion of both sodium and potassium ions. **C**, When the membrane potential is caused by diffusion of both sodium and potassium ions plus pumping of both these ions by the Na+-K+ pump. \nchannels may also leak sodium ions slightly but are far more permeable to potassium than to sodium, normally about 100 times as permeable. As discussed later, this differential in permeability is a key factor in determining the level of the normal resting membrane potential.\n\n**Figure 5-5** shows the important factors in the establishment of the normal resting membrane potential. They are as follows. \n#### Contribution of the Potassium Diffusion Potential. \nIn **Figure 5-5A**, we assume that the only movement of ions through the membrane is diffusion of potassium ions, as demonstrated by the open channels between the potassium symbol $(K^+)$ inside and outside the mem- \nbrane. Because of the high ratio of potassium ions inside to outside, 35:1, the Nernst potential corresponding to this ratio is –94 millivolts because the logarithm of 35 is 1.54, and this, multiplied by –61 millivolts, is –94 millivolts. Therefore, if potassium ions were the only factor causing the resting potential, the resting potential *inside the fiber* would be equal to –94 millivolts, as shown in the figure. \nContribution of Sodium Diffusion Through the **Nerve Membrane. Figure 5-5***B* shows the addition of slight permeability of the nerve membrane to sodium ions, caused by the minute diffusion of sodium ions through the K+-Na+ leak channels. The ratio of sodium ions from inside to outside the membrane is 0.1, which gives a calculated Nernst potential for the inside of the membrane of +61 millivolts. Also shown in **Figure 5-5***B* is the Nernst potential for potassium diffusion of -94 millivolts. How do these interact with each other, and what will be the summated potential? This question can be answered by using the Goldman equation described previously. Intuitively, one can see that if the membrane is highly permeable to potassium but only slightly permeable to sodium, the diffusion of potassium contributes far more to the membrane potential than the diffusion of sodium. In the normal nerve fiber, the permeability of the membrane to potassium is about 100 times as great as its permeability to sodium. Using this value in the Goldman equation, and considering only sodium and potassium, gives a potential inside the membrane of -86 millivolts, which is near the potassium potential shown in the figure. \n**Contribution of the Na**+-**K**+ **Pump.** In **Figure 5-5***C*, the Na+-K+ pump is shown to provide an additional contribution to the resting potential. This figure shows that continuous pumping of three sodium ions to the outside occurs for each two potassium ions pumped to the inside of the membrane. The pumping of more sodium ions to the outside than the potassium ions being pumped to the inside causes a continual loss of positive charges from inside the membrane, creating an additional degree of negativity (about –4 millivolts additional) on the inside, beyond that which can be accounted for by diffusion alone. \nTherefore, as shown in **Figure 5-5***C*, the net membrane potential when all these factors are operative at the same time is about –90 millivolts. However, additional ions, such as chloride, must also be considered in calculating the membrane potential. \nIn summary, the diffusion potentials alone caused by potassium and sodium diffusion would give a membrane potential of about -86 millivolts, with almost all of this being determined by potassium diffusion. An additional -4 millivolts is then contributed to the membrane potential by the continuously acting electrogenic Na+-K+ pump, and there is a contribution of chloride ions. As mentioned previously, the resting membrane potential \n \n \n**Figure 5-6** Typical action potential recorded by the method shown in the *upper panel*. \nvaries in different cells from as low as around -10 millivolts in erythrocytes to as high as -90 millivolts in skeletal muscle cells. \n#### **NEURON ACTION POTENTIAL** \nNerve signals are transmitted by *action potentials*, which are rapid changes in the membrane potential that spread rapidly along the nerve fiber membrane. Each action potential begins with a sudden change from the normal resting negative membrane potential to a positive potential and ends with an almost equally rapid change back to the negative potential. To conduct a nerve signal, the action potential moves along the nerve fiber until it comes to the fiber's end. \nThe upper panel of **Figure 5-6** shows the changes that occur at the membrane during the action potential, with the transfer of positive charges to the interior of the fiber at its onset and the return of positive charges to the exterior at its end. The lower panel shows graphically the successive changes in membrane potential over a few 10,000ths of a second, illustrating the explosive onset of the action potential and the almost equally rapid recovery. \nThe successive stages of the action potential are as follows. \n**Resting Stage.** The resting stage is the resting membrane potential before the action potential begins. The membrane is said to be \"polarized\" during this stage because of the –70 millivolts negative membrane potential that is present. \n**Depolarization Stage.** At this time, the membrane suddenly becomes permeable to sodium ions, allowing rapid diffusion of positively charged sodium ions to the interior of the axon. The normal polarized state of −70 millivolts is immediately neutralized by the inflowing, positively charged sodium ions, with the potential rising rapidly in the positive direction—a process called *depolarization.* In large nerve fibers, the great excess of positive sodium ions moving to the inside causes the membrane potential to actually overshoot beyond the zero level and to become somewhat positive. In some smaller fibers, as well as in many central nervous system neurons, the potential merely approaches the zero level and does not overshoot to the positive state. \n**Repolarization Stage.** Within a few 10,000ths of a second after the membrane becomes highly permeable to sodium ions, the sodium channels begin to close, and the potassium channels open to a greater degree than normal. Then, rapid diffusion of potassium ions to the exterior reestablishes the normal negative resting membrane potential, which is called *repolarization* of the membrane. \nTo explain more fully the factors that cause both depolarization and repolarization, we will describe the special characteristics of two other types of transport channels through the nerve membrane, the voltage-gated sodium and potassium channels. \n#### **VOLTAGE-GATED SODIUM AND POTASSIUM CHANNELS** \nThe necessary factor in causing both depolarization and repolarization of the nerve membrane during the action potential is the *voltage-gated sodium channel.* A *voltagegated potassium channel* also plays an important role in increasing the rapidity of repolarization of the membrane. *These two voltage-gated channels are in addition to the Na*+*-K*+ *pump and the K*+ *leak channels.* \n#### **Activation and Inactivation of the Voltage-Gated Sodium Channel** \nThe upper panel of **[Figure 5-7](#page-67-0)** shows the voltage-gated sodium channel in three separate states. This channel has two *gates*—one near the outside of the channel called the *activation gate,* and another near the inside called the *inactivation gate.* The upper left of the figure depicts the state of these two gates in the normal resting membrane when the membrane potential is −70 millivolts. In this state, the activation gate is closed, which prevents any entry of sodium ions to the interior of the fiber through these sodium channels. \n**Activation of the Sodium Channel.** When the membrane potential becomes less negative than during the resting state, rising from −70 millivolts toward zero, it finally reaches a voltage—usually somewhere around −55 millivolts—that causes a sudden conformational \n \n**Figure 5-7** Characteristics of the voltage-gated sodium *(top)* and potassium *(bottom)* channels, showing successive activation and inactivation of the sodium channels and delayed activation of the potassium channels when the membrane potential is changed from the normal resting negative value to a positive value. \nchange in the activation gate, flipping it all the way to the open position. During this *activated state,* sodium ions can pour inward through the channel, increasing the sodium permeability of the membrane as much as 500- to 5000-fold. \n**Inactivation of the Sodium Channel.** The upper right panel of **[Figure 5-7](#page-67-0)** shows a third state of the sodium channel. The same increase in voltage that opens the activation gate also closes the inactivation gate. The inactivation gate, however, closes a few 10,000ths of a second after the activation gate opens. That is, the conformational change that flips the inactivation gate to the closed state is a slower process than the conformational change that opens the activation gate. Therefore, after the sodium channel has remained open for a few 10,000ths of a second, the inactivation gate closes, and sodium ions no longer can pour to the inside of the membrane. At this point, the membrane potential begins to return toward the resting membrane state, which is the repolarization process. \nAnother important characteristic of the sodium channel inactivation process is that the inactivation gate will not reopen until the membrane potential returns to or near the original resting membrane potential level. Therefore, it is usually not possible for the sodium channels to open again without first repolarizing the nerve fiber. \n#### **Voltage-Gated Potassium Channel and Its Activation** \nThe lower panel of **[Figure 5-7](#page-67-0)** shows the voltage-gated potassium channel in two states—during the resting state \n \n**Figure 5-8** Voltage clamp method for studying flow of ions through specific channels. \n(left) and toward the end of the action potential (right). During the resting state, the gate of the potassium channel is closed, and potassium ions are prevented from passing through this channel to the exterior. When the membrane potential rises from −70 millivolts toward zero, this voltage change causes a conformational opening of the gate and allows increased potassium diffusion outward through the channel. However, because of the slight delay in opening of the potassium channels, they open, for the most part, at about the same time that the sodium channels are beginning to close because of inactivation. Thus, the decrease in sodium entry to the cell and the simultaneous increase in potassium exit from the cell combine to speed the repolarization process, leading to full recovery of the resting membrane potential within another few 10,000ths of a second. \n**The Voltage Clamp Method for Measuring the Effect of Voltage on Opening and Closing of Voltage-Gated Channels.** The original research that led to quantitative understanding of the sodium and potassium channels was so ingenious that it led to Nobel Prizes for the scientists responsible, Hodgkin and Huxley, in 1963. The essence of these studies is shown in **[Figures. 5-8 and 5-9](#page-68-0)**. \n**[Figure 5-8](#page-68-0)** shows the *voltage clamp method,* which is used to measure the flow of ions through the different channels. In using this apparatus, two electrodes are inserted into the nerve fiber. One of these electrodes is used to measure the voltage of the membrane potential, and the other is used to conduct electrical current into or out of the nerve fiber. \nThis apparatus is used in the following way. The investigator decides which voltage to establish inside the nerve fiber. The electronic portion of the apparatus is then adjusted to the desired voltage, automatically injecting either positive or negative electricity through the current electrode at whatever rate is required to hold the voltage, as measured \n \n**Figure 5-9** Typical changes in conductance of sodium and potassium ion channels when the membrane potential is suddenly increased from the normal resting value of −70 millivolts to a positive value of +10 millivolts for 2 milliseconds. This figure shows that the sodium channels open (activate) and then close (inactivate) before the end of the 2 milliseconds, whereas the potassium channels only open (activate), and the rate of opening is much slower than that of the sodium channels. \nby the voltage electrode, at the level set by the operator. When the membrane potential is suddenly increased by this voltage clamp from −70 millivolts to zero, the voltagegated sodium and potassium channels open, and sodium and potassium ions begin to pour through the channels. To counterbalance the effect of these ion movements on the desired setting of the intracellular voltage, electrical current is injected automatically through the current electrode of the voltage clamp to maintain the intracellular voltage at the required steady zero level. To achieve this level, the current injected must be equal to but of opposite polarity to the net current flow through the membrane channels. To measure how much current flow is occurring at each instant, the current electrode is connected to an ampere meter that records the current flow, as demonstrated in **[Figure 5-8](#page-68-0)**. \nFinally, the investigator adjusts the concentrations of the ions to other than normal levels both inside and outside the nerve fiber and repeats the study. This experiment can be performed easily when using large nerve fibers removed from some invertebrates, especially the giant squid axon, which in some cases is as large as 1 millimeter in diameter. When sodium is the only permeant ion in the solutions inside and outside the squid axon, the voltage clamp measures current flow only through the sodium channels. When potassium is the only permeant ion, current flow only through the potassium channels is measured. \nAnother means for studying the flow of ions through an individual type of channel is to block one type of channel at a time. For example, the sodium channels can be blocked by a toxin called tetrodotoxin when it is applied to the outside of the cell membrane where the sodium activation gates are located. Conversely, tetraethylammonium ion blocks the potassium channels when it is applied to the interior of the nerve fiber. \n**[Figure 5-9](#page-68-1)** shows typical changes in conductance of the voltage-gated sodium and potassium channels when the membrane potential is suddenly changed through use of the voltage clamp, from −70 millivolts to +10 millivolts and then, 2 milliseconds later, back to −70 millivolts. Note the sudden opening of the sodium channels (the activation stage) within a small fraction of a millisecond after the membrane potential is increased to the positive value. However, during the next millisecond or so, the sodium channels automatically close (the inactivation stage). \nNote the opening (activation) of the potassium channels, which open less rapidly and reach their full open state only after the sodium channels have almost completely closed. Furthermore, once the potassium channels open, they remain open for the entire duration of the positive membrane potential and do not close again until after the membrane potential is decreased back to a negative value. \n#### **SUMMARY OF EVENTS THAT CAUSE THE ACTION POTENTIAL** \n**[Figure 5-10](#page-69-0)** summarizes the sequential events that occur during and shortly after the action potential. The bottom of the figure shows the changes in membrane conductance for sodium and potassium ions. During the resting state, before the action potential begins, the conductance for potassium ions is 50 to 100 times as great as the conductance for sodium ions. This disparity is caused by much greater leakage of potassium ions than sodium ions through the leak channels. However, at the onset of the action potential, the sodium channels almost instantaneously become activated and allow up to a 5000-fold increase in sodium conductance. The inactivation process then closes the sodium channels \n \n**Figure 5-10** Changes in sodium and potassium conductance during the course of the action potential. Sodium conductance increases several thousand–fold during the early stages of the action potential, whereas potassium conductance increases only about 30-fold during the latter stages of the action potential and for a short period thereafter. (These curves were constructed from theory presented in papers by Hodgkin and Huxley but transposed from a squid axon to apply to the membrane potentials of large mammalian nerve fibers.) \nwithin another fraction of a millisecond. The onset of the action potential also initiates voltage gating of the potassium channels, causing them to begin opening more slowly, a fraction of a millisecond after the sodium channels open. At the end of the action potential, the return of the membrane potential to the negative state causes the potassium channels to close back to their original status but, again, only after an additional millisecond or more delay. \nThe middle portion of **[Figure 5-10](#page-69-0)** shows the ratio of sodium to potassium conductance at each instant during the action potential, and above this depiction is the action potential itself. During the early portion of the action potential, the ratio of sodium to potassium conductance increases more than 1000-fold. Therefore, far more sodium ions flow to the interior of the fiber than potassium ions to the exterior. This is what causes the membrane potential to become positive at the action potential onset. Then, the sodium channels begin to close, and the potassium channels begin to open; thus, the ratio of conductance shifts far in favor of high potassium conductance but low sodium conductance. This shift allows for a very rapid loss of potassium ions to the exterior but virtually zero flow of sodium ions to the interior. Consequently, the action potential quickly returns to its baseline level. \n#### **Roles of Other Ions During the Action Potential** \nThus far, we have considered only the roles of sodium and potassium ions in generating the action potential. At least two other types of ions must be considered, negative anions and calcium ions. \n**Impermeant Negatively Charged Ions (Anions) Inside the Nerve Axon.** Inside the axon are many negatively charged ions that cannot go through the membrane channels. They include the anions of protein molecules and of many organic phosphate compounds and sulfate compounds, among others. Because these ions cannot leave the interior of the axon, any deficit of positive ions inside the membrane leaves an excess of these impermeant negative anions. Therefore, these impermeant negative ions are responsible for the negative charge inside the fiber when there is a net deficit of positively charged potassium ions and other positive ions. \n**Calcium Ions.** The membranes of almost all cells of the body have a calcium pump similar to the sodium pump, and calcium serves along with (or instead of) sodium in some cells to cause most of the action potential. Like the sodium pump, the calcium pump transports calcium ions from the interior to the exterior of the cell membrane (or into the endoplasmic reticulum of the cell), creating a calcium ion gradient of about 10,000-fold. This process leaves an internal cell concentration of calcium ions of about 10−7 molar, in contrast to an external concentration of about 10−3 molar. \nIn addition, there are *voltage-gated calcium channels*. Because the calcium ion concentration is more than 10,000 times greater in the extracellular fluid than in the intracellular fluid, there is a tremendous diffusion gradient and electrochemical driving force for the passive flow of calcium ions into the cells. These channels are slightly permeable to sodium ions and calcium ions, but their permeability to calcium is about 1000-fold greater than to sodium under normal physiological conditions. When the channels open in response to a stimulus that depolarizes the cell membrane, calcium ions flow to the interior of the cell. \nA major function of the voltage-gated calcium ion channels is to contribute to the depolarizing phase on the action potential in some cells. The gating of calcium channels, however, is relatively slow, requiring 10 to 20 times as long for activation as for the sodium channels. For this reason, they are often called *slow channels,* in contrast to the sodium channels, which are called *fast channels.* Therefore, the opening of calcium channels provides a more sustained depolarization, whereas the sodium channels play a key role in initiating action potentials. \nCalcium channels are numerous in cardiac muscle and smooth muscle. In fact, in some types of smooth muscle, the fast sodium channels are hardly present; therefore, the action potentials are caused almost entirely by the activation of slow calcium channels. \n**Increased Permeability of the Sodium Channels When There Is a Deficit of Calcium Ions.** The concentration of calcium ions in the extracellular fluid also has a profound effect on the voltage level at which the sodium channels become activated. When there is a deficit of calcium ions, the sodium channels become activated (opened) by a small increase of the membrane potential from its normal, very negative level. Therefore, the nerve fiber becomes highly excitable, sometimes discharging repetitively without provocation, rather than remaining in the resting state. In fact, the calcium ion concentration needs to fall only 50% below normal before spontaneous discharge occurs in some peripheral nerves, often causing *muscle \"tetany*.*\"* Muscle tetany is sometimes lethal because of tetanic contraction of the respiratory muscles. \nThe probable way in which calcium ions affect the sodium channels is as follows. These ions appear to bind to the exterior surfaces of the sodium channel protein. The positive charges of these calcium ions, in turn, alter the electrical state of the sodium channel protein, thus altering the voltage level required to open the sodium gate. \n#### **INITIATION OF THE ACTION POTENTIAL** \nThus far, we have explained the changing sodium and potassium permeability of the membrane, as well as the development of the action potential, but we have not explained what initiates the action potential. \n**A Positive-Feedback Cycle Opens the Sodium Channels.** As long as the membrane of the nerve fiber remains undisturbed, no action potential occurs in the normal nerve. However, if any event causes enough initial rise in the membrane potential from −70 millivolts toward the zero level, the rising voltage will cause many voltage-gated sodium channels to begin opening. This occurrence allows for the rapid inflow of sodium ions, which causes a further rise in the membrane potential, thus opening still more voltage-gated sodium channels and allowing more streaming of sodium ions to the interior of the fiber. This process is a positive feedback cycle that, once the feedback is strong enough, continues until all the voltage-gated sodium channels have become activated (opened). Then, within another fraction of a millisecond, the rising membrane potential causes closure of the sodium channels and opening of potassium channels, and the action potential soon terminates. \n**Initiation of the Action Potential Occurs Only After the Threshold Potential is Reached.** An action potential will not occur until the initial rise in membrane potential is great enough to create the positive feedback described in the preceding paragraph. This occurs when the number of sodium ions entering the fiber is greater than the number of potassium ions leaving the fiber. A sudden rise in membrane potential of 15 to 30 millivolts is usually required. Therefore, a sudden increase in the membrane potential in a large nerve fiber, from −70 millivolts up to about −55 millivolts, usually causes the explosive development of an action potential. This level of −55 millivolts is said to be the *threshold* for stimulation. \n#### PROPAGATION OF THE ACTION POTENTIAL \nIn the preceding paragraphs, we discussed the action potential as though it occurs at one spot on the membrane. However, an action potential elicited at any one point on an excitable membrane usually excites adjacent portions of the membrane, resulting in propagation of the action potential along the membrane. This mechanism is demonstrated in **[Figure 5-11.](#page-71-0)** \n**[Figure 5-11](#page-71-0)***A* shows a normal resting nerve fiber, and **[Figure 5-11](#page-71-0)***B* shows a nerve fiber that has been excited in its midportion, which suddenly develops increased permeability to sodium. The *arrows* show a local circuit of current flow from the depolarized areas of the membrane to the adjacent resting membrane areas. That is, positive electrical charges are carried by the inward-diffusing sodium ions through the depolarized membrane and then for several millimeters in both directions along the core of the axon. These positive charges increase the voltage for a distance of 1 to 3 millimeters inside the large myelinated fiber to above the threshold voltage value for initiating an action potential. Therefore, the sodium channels in these new areas immediately open, as shown in **[Figure 5-11](#page-71-0)***C* [and](#page-71-0) *D*, and the explosive action potential spreads. These newly depolarized areas produce still more local circuits of current flow farther along the membrane, causing progressively more and more depolarization. Thus, the depolarization process travels along the entire length of the fiber. This transmission of the depolarization process along a nerve or muscle fiber is called a *nerve* or *muscle impulse.* \n**Direction of Propagation.** As demonstrated in **[Figure 5-](#page-71-0) [11](#page-71-0)**, an excitable membrane has no single direction of propagation, but the action potential travels in all directions away from the stimulus—even along all branches of a nerve fiber—until the entire membrane has become depolarized. \n**All-or-Nothing Principle.** Once an action potential has been elicited at any point on the membrane of a normal fiber, the depolarization process travels over the entire membrane if conditions are right, but it does not travel at all if conditions are not right. This principle is called the *allor-nothing principle,* and it applies to all normal excitable tissues. Occasionally, the action potential reaches a point on the membrane at which it does not generate sufficient voltage to stimulate the next area of the membrane. When this situation occurs, the spread of depolarization stops. Therefore, for continued propagation of an impulse to occur, the ratio of action potential to threshold for excitation must at all times be greater than 1. This \"greater than 1\" requirement is called the *safety factor* for propagation. \n#### RE-ESTABLISHING SODIUM AND POTASSIUM IONIC GRADIENTS AFTER ACTION POTENTIALS ARE COMPLETED—IMPORTANCE OF ENERGY METABOLISM \nTransmission of each action potential along a nerve fiber slightly reduces the concentration differences of sodium and potassium inside and outside the membrane because sodium ions diffuse to the inside during depolarization, and potassium ions diffuse to the outside during \n+ + + + + + + + + + + + – – + + + + + + + + + + + + + + + + + + + + + – – + + + + + + + + + + + + + + + + + + + – – – – + + + + + + + + + + + + + + + + + + – – – – + + + + + + + + – – + + + + + + + + + + + + + + + + + + – – – – + + + + + + + + + + + + + + + + + + – – + + – – – – – – – – – – – – – – – – – – + + + + – – – – – – – – – – – – – – – – – – + + – – – – – – – – – – – – + + – – – – – – – – – + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – + + – – – – – – – – – – – – – – – – – – – + + + + – – – – – – – – – – – – – – – – – – + + + + – – – – – – – – A B C D \n**Figure 5-11** A–D, Propagation of action potentials in both directions along a conductive fiber. \nrepolarization. For a single action potential, this effect is so minute that it cannot be measured. Indeed, 100,000 to 50 million impulses can be transmitted by large nerve fibers before the concentration differences reach the point that action potential conduction ceases. With time, however, it becomes necessary to re-establish the sodium and potassium membrane concentration differences, which is achieved by action of the Na+-K+ pump in the same way as described previously for the original establishment of the resting potential. That is, sodium ions that have diffused to the interior of the cell during the action potentials and potassium ions that have diffused to the exterior must be returned to their original state by the Na+-K+ pump. Because this pump requires energy for operation, this \"recharging\" of the nerve fiber is an active metabolic process, using energy derived from the adenosine triphosphate (ATP) energy system of the cell. **[Figure 5-12](#page-71-1)** shows that the nerve fiber produces increased heat during recharging, which is a measure of energy expenditure when the nerve impulse frequency increases. \nA special feature of the Na+-K+ ATP pump is that its degree of activity is strongly stimulated when excess sodium ions accumulate inside the cell membrane. In fact, the pumping activity increases approximately in proportion to the third power of this intracellular sodium concentration. As the internal sodium concentration rises from 10 to 20 mEq/L, the activity of the pump does not merely double but increases about eightfold. Therefore, it is easy to understand how the recharging process of the nerve fiber can be set rapidly into motion whenever the concentration differences of sodium and potassium ions across the membrane begin to run down. \n#### PLATEAU IN SOME ACTION POTENTIALS \nIn some cases, the excited membrane does not repolarize immediately after depolarization; instead, the potential remains on a plateau near the peak of the spike potential for many milliseconds before repolarization begin. Such a plateau is shown in **[Figure 5-13](#page-72-0)**; one can readily see that \n \n**Figure 5-12** Heat production in a nerve fiber at rest and at progressively increasing rates of stimulation. \nthe plateau greatly prolongs the period of depolarization. This type of action potential occurs in heart muscle fibers, where the plateau lasts for as long as 0.2 to 0.3 second and causes contraction of heart muscle to last for this same long period. \nThe cause of the plateau is a combination of several factors. First, in heart muscle, two types of channels contribute to the depolarization process: (1) the usual voltage-activated sodium channels, called *fast channels;* and (2) voltageactivated calcium-sodium channels *(L-type calcium channels)*, which are slow to open and therefore are called *slow channels.* Opening of fast channels causes the spike portion of the action potential, whereas the prolonged opening of the slow calcium-sodium channels mainly allows calcium ions to enter the fiber, which is largely responsible for the plateau portion of the action potential. \nAnother factor that may be partly responsible for the plateau is that the voltage-gated potassium channels are slower to open than usual, often not opening much until the end of the plateau. This factor delays the return of the membrane potential toward its normal negative value of −70 millivolts. The plateau ends when the calciumsodium channels close, and permeability to potassium ions increases. \n#### RHYTHMICITY OF SOME EXCITABLE TISSUES—REPETITIVE DISCHARGE \nRepetitive self-induced discharges occur normally in the heart, in most smooth muscle, and in many of the neurons of the central nervous system. These rhythmical discharges cause the following: (1) rhythmical beat of the heart; (2) rhythmical peristalsis of the intestines; and (3) neuronal events such as the rhythmical control of breathing. \nIn addition, almost all other excitable tissues can discharge repetitively if the threshold for stimulation of the tissue cells is reduced to a low enough level. For example, even large nerve fibers and skeletal muscle fibers, which normally are highly stable, discharge repetitively when they \n \n**Figure 5-13** Action potential (in millivolts) from a Purkinje fiber of the heart, showing a plateau. \nare placed in a solution that contains the drug *veratridine*, which activates sodium ion channels, or when the calcium ion concentration decreases below a critical value, which increases the sodium permeability of the membrane. \n**Re-Excitation Process Necessary for Spontaneous Rhythmicity.** For spontaneous rhythmicity to occur, the membrane—even in its natural state—must be permeable enough to sodium ions (or to calcium and sodium ions through the slow calcium-sodium channels) to allow automatic membrane depolarization. Thus, **[Figure 5-14](#page-72-1)** shows that the resting membrane potential in the rhythmical control center of the heart is only −60 to −70 millivolts, which is not enough negative voltage to keep the sodium and calcium channels totally closed. Therefore, the following sequence occurs: (1) some sodium and calcium ions flow inward; (2) this activity increases the membrane voltage in the positive direction, which further increases membrane permeability; (3) still more ions flow inward; and (4) the permeability increases more, and so on, until an action potential is generated. Then, at the end of the action potential, the membrane repolarizes. After another delay of milliseconds or seconds, spontaneous excitability causes depolarization again, and a new action potential occurs spontaneously. This cycle continues over and over and causes self-induced rhythmical excitation of the excitable tissue. \nWhy does the membrane of the heart control center not depolarize immediately after it has become repolarized, rather than delaying for nearly 1 second before the onset of the next action potential? The answer can be found by observing the curve labeled \"potassium conductance\" in **[Figure 5-14](#page-72-1)**. This curve shows that toward the end of each action potential, and continuing for a short period thereafter, the membrane becomes more permeable to potassium ions. The increased outflow of potassium ions carries tremendous numbers of positive charges to the outside of the membrane, leaving considerably more negativity inside the fiber than would otherwise occur. This continues for nearly 1 second after the preceding action potential is over, thus drawing the membrane potential \n \n**Figure 5-14** Rhythmical action potentials (in millivolts) similar to those recorded in the rhythmical control center of the heart. Note their relationship to potassium conductance and to the state of hyperpolarization. \nnearer to the potassium Nernst potential. This state, called *hyperpolarization,* is also shown in **[Figure 5-14](#page-72-1)**. As long as this state exists, self–re-excitation will not occur. However, the increased potassium conductance (and the state of hyperpolarization) gradually disappears, as shown after each action potential is completed in the figure, thereby again allowing the membrane potential to increase up to the *threshold* for excitation. Then, suddenly, a new action potential results and the process occurs again and again. \n#### SPECIAL CHARACTERISTICS OF SIGNAL TRANSMISSION IN NERVE TRUNKS \n**Myelinated and Unmyelinated Nerve Fibers. [Figure](#page-73-0) [5-15](#page-73-0)** shows a cross section of a typical small nerve, revealing many large nerve fibers that constitute most of the cross-sectional area. However, a more careful look reveals many more small fibers lying between the large ones. The large fibers are *myelinated,* and the small ones are *unmyelinated.* The average nerve trunk contains about twice as many unmyelinated fibers as myelinated fibers. \n**[Figure 5-16](#page-73-1)** illustrates schematically the features of a typical myelinated fiber. The central core of the fiber is the *axon,* and the membrane of the axon is the membrane that actually conducts the action potential. The axon is filled in its center with *axoplasm,* which is a viscid intracellular fluid. Surrounding the axon is a *myelin sheath* that is often much thicker than the axon itself. About once every 1 to 3 millimeters along the length of the myelin sheath is a *node of Ranvier.* \nThe myelin sheath is deposited around the axon by *Schwann cells* in the following manner. The membrane of a Schwann cell first envelops the axon. The Schwann cell then rotates around the axon many times, laying down multiple layers of Schwann cell membrane containing the lipid substance *sphingomyelin.* This substance is an excellent \n \n**Figure 5-15** Cross section of a small nerve trunk containing both myelinated and unmyelinated fibers. \nelectrical insulator that decreases ion flow through the membrane about 5000-fold. At the juncture between each two successive Schwann cells along the axon, a small uninsulated area only 2 to 3 micrometers in length remains where ions still can flow with ease through the axon membrane between the extracellular fluid and intracellular fluid inside the axon. This area is called the *node of Ranvier.* \n#### **Saltatory Conduction in Myelinated Fibers from Node** \n**to Node.** Even though almost no ions can flow through the thick myelin sheaths of myelinated nerves, they can flow with ease through the nodes of Ranvier. Therefore, action potentials occur *only at the nodes.* Yet, the action potentials are conducted from node to node by *saltatory conduction*, as shown in **[Figure 5-17](#page-74-0)**. That is, electrical current flows through the surrounding extracellular fluid outside the myelin sheath, as well as through the axoplasm inside the axon from node to node, exciting successive nodes one after another. Thus, the nerve impulse jumps along the fiber, which is the origin of the term *saltatory.* \nSaltatory conduction is of value for two reasons: \n1. First, by causing the depolarization process to jump long intervals along the axis of the nerve fiber, this mechanism increases the velocity of nerve transmission in myelinated fibers as much as 5- to 50-fold. \n \n**Figure 5-16** Function of the Schwann cell to insulate nerve fibers. **A**, Wrapping of a Schwann cell membrane around a large axon to form the myelin sheath of the myelinated nerve fiber. **B**, Partial wrapping of the membrane and cytoplasm of a Schwann cell around multiple unmyelinated nerve fibers (shown in cross section). *(A, Modified from Leeson TS, Leeson R: Histology. Philadelphia: WB Saunders, 1979.)* \n2. Second, saltatory conduction conserves energy for the axon because only the nodes depolarize, allowing perhaps 100 times less loss of ions than would otherwise be necessary, and therefore requiring much less energy expenditure for re-establishing the sodium and potassium concentration differences across the membrane after a series of nerve impulses. \nThe excellent insulation afforded by the myelin membrane and the 50-fold decrease in membrane capacitance also allow repolarization to occur with little transfer of ions. \n**Velocity of Conduction in Nerve Fibers.** The velocity of action potential conduction in nerve fibers varies from as little as 0.25 m/sec in small unmyelinated fibers to as much as 100 m/sec—more than the length of a football field in 1 second—in large myelinated fibers. \n#### EXCITATION—THE PROCESS OF ELICITING THE ACTION POTENTIAL \nBasically, any factor that causes sodium ions to begin to diffuse inward through the membrane in sufficient numbers can set off automatic regenerative opening of the sodium channels. This automatic regenerative opening can result from *mechanical* disturbance of the membrane, *chemical* effects on the membrane, or passage of *electricity* through the membrane. All these approaches are used at different points in the body to elicit nerve or muscle action potentials: mechanical pressure to excite sensory nerve endings in the skin, chemical neurotransmitters to transmit signals from one neuron to the next in the brain, and electrical current to transmit signals between successive muscle cells in the heart and intestine. \n**Excitation of a Nerve Fiber by a Negatively Charged Metal Electrode.** The usual means for exciting a nerve or muscle in the experimental laboratory is to apply electricity to the nerve or muscle surface through two small electrodes, one of which is negatively charged and the other positively charged. When electricity is applied in this manner, the excitable membrane becomes stimulated at the negative electrode. \nRemember that the action potential is initiated by the opening of voltage-gated sodium channels. Furthermore, these channels are opened by a decrease in the normal resting electrical voltage across the membrane—that is, negative current from the electrode decreases the voltage on the outside of the membrane to a negative value nearer to the voltage of the negative potential inside the fiber. This effect decreases the electrical voltage across the membrane and allows the sodium channels to open, resulting in an action potential. Conversely, at the positive electrode, the injection of positive charges on the outside of the nerve membrane heightens the voltage difference across the membrane, rather than lessening it. This effect causes a state of hyperpolarization, which actually decreases the excitability of the fiber rather than causing an action potential. \n \n**Figure 5-17** Saltatory conduction along a myelinated axon. The flow of electrical current from node to node is illustrated by the *arrows.* \n#### Threshold for Excitation and Acute Local Potentials. \nA weak negative electrical stimulus may not be able to excite a fiber. However, when the voltage of the stimulus is increased, there comes a point at which excitation does take place. Figure 5-18 shows the effects of successively applied stimuli of progressing strength. A weak stimulus at point A causes the membrane potential to change from -70 to -65 millivolts, but this change is not sufficient for the automatic regenerative processes of the action potential to develop. At point B, the stimulus is greater, but the intensity is still not enough. The stimulus does, however, disturb the membrane potential locally for as long as 1 millisecond or more after both of these weak stimuli. These local potential changes are called acute local potentials and, when they fail to elicit an action potential, they are called acute subthreshold potentials. \nAt point C in **Figure 5-18**, the stimulus is even stronger. Now, the local potential has barely reached the *threshold level* required to elicit an action potential, but this occurs only after a short \"latent period.\" At point D, the stimulus is still stronger, the acute local potential is also stronger, and the action potential occurs after less of a latent period. \nThus, this figure shows that even a weak stimulus causes a local potential change at the membrane, but the intensity of the local potential must rise to a threshold level before the action potential is set off.",
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"Header 1": "**The Cell and Its Functions**",
"Header 2": "Introduction to Cells",
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"page_content": "Genes, which are located in the nuclei of all cells of the body, control heredity from parents to children, as well as the daily functioning of all the body's cells. The genes control cell function by determining which structures, enzymes, and chemicals are synthesized within the cell. \n**Figure 3-1** shows the general schema of genetic control. Each gene, which is composed of *deoxyribonucleic acid* (DNA), controls the formation of another nucleic acid, *ribonucleic acid* (RNA); this RNA then spreads throughout the cell to control formation of a specific protein. The entire process, from *transcription* of the genetic code in the nucleus to *translation* of the RNA code and the formation of proteins in the cell cytoplasm, is often referred to as *gene expression.* \nBecause the human body has approximately 20,000 to 25,000 different genes that code for proteins in each cell, it is possible to form a large number of different cellular proteins. In fact, RNA molecules transcribed from the same segment of DNA—the same gene—can be processed in more than one way by the cell, giving rise to alternate versions of the protein. The total number of different proteins produced by the various cell types in humans is estimated to be at least 100,000. \nSome of the cellular proteins are *structural proteins,* which, in association with various lipids and carbohydrates, form structures of the various intracellular organelles discussed in Chapter 2. However, most of the proteins are *enzymes* that catalyze different chemical reactions in the cells. For example, enzymes promote all the oxidative reactions that supply energy to the cell, along with synthesis of all the cell chemicals, such as lipids, glycogen, and adenosine triphosphate (ATP). \n#### CELL NUCLEUS GENES CONTROL PROT[EIN](#page-34-0) SYNTHESIS \nIn the cell nucleus, large numbers of genes are attached end on end in extremely long, double-stranded helical molecules of DNA having molecular weights measured in the billions. A very short segment of such a molecule is shown in **Figure 3-2**. This molecule is composed of several simple chemical compounds bound together in a regular pattern, the details of which are explained in the next few paragraphs. \n#### **Building Blocks of DNA** \n**Figure 3-3** shows the basic chemical compounds involved in the formation of DNA. These compounds include the following: (1) *phosphoric acid;* (2) a sugar [called](#page-34-0) *deoxyribose;* and (3) four nitrogenous *bases* (two purines, *adenine* and *guanine,* and two pyrimidines, *thymine* and *cytosine*). The phosphoric acid and deoxyribose form the two helical strands that are the backbone of the DNA molecule, and the nitrogenous bases lie between the two strands and connect them, as illustrated in **Figure 3-2**. \n#### **Nucleotides** \nThe first stage of DNA formation is to combine one molecule of phosphoric acid, one molecule of deoxyribose, and one of the four bases to form an acidic nucleotide. Four separate nucleotides are thus formed, one for each of the four bases: *deoxyadenylic, deoxythymidylic, deoxyguanylic,* and *deoxycytidylic acids*. **Figure 3-4** shows the chemical \n \n**Figure 3-1** The general schema whereby genes control cell function. *mRNA,* Messenger RNA. \n \n**Figure 3-2** The helical double-stranded structure of the gene. The outside strands are composed of phosphoric acid and the sugar deoxyribose. The internal molecules connecting the two strands of the helix are purine and pyrimidine bases, which determine the \"code\" of the gene. \n \nFigure 3-3 The basic building blocks of DNA. \nstructure of deoxyadenylic acid, and **Figure 3-5** shows simple symbols for the four nucleotides that form DNA. \n#### Nucleotides Are Organized to Form Two Strands of DNA Loosely Bound to Each Other \n**Figure 3-2** shows the manner in which multiple nucleotides are bound together to form two strands of DNA. The two strands are, in turn, loosely bonded with each other by weak cross-linkages, as illustrated in **Figure 3-6** by the central dashed lines. Note that the backbone of each DNA strand is composed of alternating phosphoric acid and deoxyribose molecules. In turn, purine and pyrimidine bases are attached to the sides of the deoxyribose molecules. Then, by means of loose *hydrogen bonds* (dashed \n**Figure 3-4.** Deoxyadenylic acid, one of the nucleotides that make up DNA. \n \n**Figure 3-5.** Symbols for the four nucleotides that combine to form DNA. Each nucleotide contains phosphoric acid (P), deoxyribose (D), and one of the four nucleotide bases: adenine (A); thymine (T); guanine (G); or cytosine (C). \nlines) between the purine and pyrimidine bases, the two respective DNA strands are held together. Note the following caveats, however: \n- 1. Each purine base *adenine* of one strand always bonds with a pyrimidine base *thymine* of the other strand.\n- 2. Each purine base *guanine* always bonds with a pyrimidine base *cytosine*. \nThus, in **Figure 3**-6, the sequence of complementary pairs of bases is CG, CG, GC, TA, CG, TA, GC, AT, and AT. Because of the looseness of the hydrogen bonds, the two strands can pull apart with ease, and they do so many times during the course of their function in the cell. \nTo put the DNA of **Figure 3**-6 into its proper physical perspective, one could merely pick up the two ends and twist them into a helix. Ten pairs of nucleotides are present in each full turn of the helix in the DNA molecule. \n#### **GENETIC CODE** \nThe importance of DNA lies in its ability to control the formation of proteins in the cell, which it achieves by means of a *genetic code*. That is, when the two strands of a DNA molecule are split apart, the purine and pyrimidine bases projecting to the side of each DNA strand are exposed, as shown by the top strand in **Figure 3-7**. It is these projecting bases that form the genetic code. \nThe genetic code consists of successive \"triplets\" of bases—that is, each three successive bases is a *code word*. The successive triplets eventually control the sequence of amino acids in a protein molecule that is to be synthesized in the cell. Note in **Figure 3**-6 that the top strand of DNA, reading from left to right, has the genetic code GGC, AGA, CTT, with the triplets being separated from one another by the arrows. As we follow this genetic code through **Figure 3**-7 and **Figure 3**-8, we see that these three respective triplets are responsible for successive placement of the three amino acids, *proline, serine,* and *glutamic acid,* in a newly formed molecule of protein.\n\nBecause DNA is located in the cell nucleus, yet most of the cell functions are carried out in the cytoplasm, there must be some means for DNA genes of the nucleus to control chemical reactions of the cytoplasm. This control \n \n**Figure 3-6.** Arrangement of deoxyribose nucleotides in a double strand of DNA. \n \nR C \nP \nP R C P R G P R U \n**Figure 3-7.** Combination of ribose nucleotides with a strand of DNA to form a molecule of RNA that carries the genetic code from the gene to the cytoplasm. The *RNA polymerase* enzyme moves along the DNA strand and builds the RNA molecule. \n**Figure 3-8.** A portion of an RNA molecule showing thre[e](#page-36-0) RNA codons—CCG, UCU, and GAA—that control attachment of the three amino acids, proline, serine, and glutamic acid, respectively, to the growing RNA chain. **Proline Serine Glutamic acid** \nis achieved through the intermediary of another type of nucleic acid, RNA, the formation of which is controlled by DNA of the nucleus. Thus, as shown in **Figure 3-7**, the code is transferred to RNA in a process called *transcription.* The RNA, in turn, diffuses from the nucleus through nuclear pores into the cytoplasmic compartment, where it controls protein synthesis. \n#### **RNA IS SYNTHESIZED IN THE NUCLEUS FROM A DNA TEMPLATE** \nDuring RNA synthesis, the two strands of DNA separate temporarily; one of these strands is used as a template for synthesis of an RNA molecule. The code triplets in the DNA result in the formation of *complementary* code triplets (called *codons*) in the RNA. These codons, in turn, will control the sequence of amino acids in a protein to be synthesized in the cell cytoplasm. \n**Building Blocks of RNA.** The basic building blocks of RNA are almost the same as those of DNA, except for two differences. First, the sugar deoxyribose is not used in RNA formation. In its place is another sugar of slightly different composition, *ribose,* which contains an extra hydroxyl ion appended to the ribose ring structure. Second, thymine is replaced by another pyrimidine, *uracil.* \n**Formation of RNA Nucleotides.** The basic building blocks of RNA form *RNA nucleotides,* exactly as described previously for DNA synthesis. Here again, four separate nucleotides are used to form RNA. These nucleotides contain the bases *adenine, guanine, cytosine,* and *uracil.* Note that these bases are the same as in DNA, except that uracil in RNA replaces thymine in DNA. \n**\"Activation\" of RNA Nucleotides.** The next step in the synthesis of RNA is \"activation\" of RNA nucleotides by an enzyme, *RNA polymerase.* This activation occurs by adding two extra phosphate radicals to each nucleotide to form \ntriphosphates (shown in **Figure 3-7** by the two RNA nucleotides to the far right during RNA chain formation). These last two phosphates are combined with the nucleotide by \nP R U P R G P R A P R A \nP R C \n*high-energy phosphate bonds* derived from ATP in the cell. The result of this activation process is that large quantities of ATP energy are made available to each of the nucleotides. This energy is used to promote chemical reactions that add each new RNA nucleotide at the end of the developing RNA [chain.](#page-36-0) \n#### **RNA CHAIN ASSEMBLY FROM ACTIVATED NUCLEOTIDES USING THE DNA STRAND AS A TEMPLATE** \nAs shown in **Figure 3-7,** assembly of RNA is accomplished under the influence of an enzyme, *RNA polymerase.* This large protein enzyme has many functional properties necessary for formation of RNA, as follows: \n- 1. In the DNA strand immediately ahead of the gene to be transcribed is a sequence of nucleotides called the *promoter.* The RNA polymerase has an appropriate complementary structure that recognizes this promoter and becomes attached to it, which is the essential step for initiating the formation of RNA.\n- 2. After the RNA polymerase attaches to the promoter, the polymerase causes unwinding of about two turns of the DNA helix and separation of the unwound portions of the two strands.\n- 3. The polymerase then moves along the DNA strand, temporarily unwinding and separating the two DNA strands at each stage of its movement. As it moves along, at each stage it adds a new activated RNA nucleotide to the end of the newly forming RNA chain through the following steps:\n- a. First, it causes a hydrogen bond to form between the end base of the DNA strand and the base of an RNA nucleotide in the nucleoplasm. \n- b. Then, one at a time, the RNA polymerase breaks two of the three phosphate radicals away from each of these RNA nucleotides, liberating large amounts of energy from the broken high-energy phosphate bonds. This energy is used to cause covalent linkage of the remaining phosphate on the nucleotide with the ribose on the end of the growing RNA chain.\n- c. When the RNA polymerase reaches the end of the DNA gene, it encounters a new sequence of DNA nucleotides called the *chain-terminating sequence,* which causes the polymerase and the newly formed RNA chain to break away from the DNA strand. The polymerase then can be used again and again to form more new RNA chains.\n- d. As the new RNA strand is formed, its weak hydrogen bonds with the DNA template break away because the DNA has a high affinity for rebonding with its own complementary DNA strand. Thus, the RNA chain is forced away from the DNA and is released into the nucleoplasm. \nTherefore, the code that is present in the DNA strand is eventually transmitted in *complementary* form to the RNA chain. The ribose nucleotide bases always combine with the deoxyribose bases in the following combinations: \n| DNA Base | RNA Base |\n|----------|----------|\n| guanine | Cytosine |\n| cytosine | Guanine |\n| adenine | Uracil |\n| thymine | adenine | \n**There Are Several Different Types of RNA.** As research on RNA has continued to advance, many different types of RNA have been discovered. Some types of RNA are involved in protein synthesis, whereas other types serve gene regulatory functions or are involved in posttranscriptional modification of RNA. The functions of some types of RNA, especially those that do not appear to code for proteins, are still mysterious. The following six types of RNA play independent and different roles in protein synthesis: \n- 1. *Precursor messenger RNA* (pre-mRNA) is a large, immature, single strand of RNA that is processed in the nucleus to form mature messenger RNA (mRNA). The pre-RNA includes two different types of segments, called *introns,* which are removed by a process called splicing, and *exons,* which are retained in the final mRNA.\n- 2. *Small nuclear RNA* (snRNA) directs the splicing of pre-mRNA to form mRNA.\n- 3. *Messenger RNA* (mRNA) carries the genetic code to the cytoplasm for controlling the type of protein formed.\n- 4. *Transfer RNA* (tRNA) transports activated amino acids to the ribosomes to be used in assembling the protein molecule.\n- 5. *Ribosomal RNA,* along with about 75 different proteins, forms *ribosomes,* the physical and chemical \n- structures on which protein molecules are actually assembled.\n- 6. *MicroRNAs* (miRNAs) are single-stranded RNA molecules of 21 to 23 nucleotides that can regulate gene transcription and translation. \n#### **MESSENGER RN[A—THE C](#page-36-1)ODONS** \n*Messenger RNA* molecules are long single RNA strands that are suspended in the cytoplasm. These molecules are composed of several hundred to several thousand RNA nucleotides in unpaired strands, and they contain *c[odons](#page-36-0)* that are exactly complementary to the code triplets of t[he](#page-37-0) DNA genes. **Figure 3-8** shows a small segment of mRNA. Its codons are CCG, UCU, and GAA, which are the codons for the amino acids proline, serine, and glutamic acid. The transcription of these codons from the DNA molecule to the RNA molecule is shown in **Figure 3-7**. \n**RNA Codons for the Different Amino Acids. [Table 3](#page-37-0)-1** lists the RNA codons for the 20 common amino acids found in protein molecules. Note that most of the amino acids are represented by more than one codon; also, one codon represents the signal \"start manufacturing the protein molecule,\" and three codons represent \"stop manufacturing the protein molecule.\" In **Table 3-1**, these two \n**Table 3-1** RNA Codons for Amino Acids and for Start and Stop \n| Amino Acid | | | RNA Codons | | | |\n|---------------|-----|-----|------------|-----|-----|-----|\n| Alanine | GCU | GCC | GCA | GCG | | |\n| Arginine | CGU | CGC | CGA | CGG | AGA | AGG |\n| Asparagine | AAU | AAC | | | | |\n| Aspartic acid | GAU | GAC | | | | |\n| Cysteine | UGU | UGC | | | | |\n| Glutamic acid | GAA | GAG | | | | |\n| Glutamine | CAA | CAG | | | | |\n| Glycine | GGU | GGC | GGA | GGG | | |\n| Histidine | CAU | CAC | | | | |\n| Isoleucine | AUU | AUC | AUA | | | |\n| Leucine | CUU | CUC | CUA | CUG | UUA | UUG |\n| Lysine | AAA | AAG | | | | |\n| Methionine | AUG | | | | | |\n| Phenylalanine | UUU | UUC | | | | |\n| Proline | CCU | CCC | CCA | CCG | | |\n| Serine | UCU | UCC | UCA | UCG | AGC | AGU |\n| Threonine | ACU | ACC | ACA | ACG | | |\n| Tryptophan | UGG | | | | | |\n| Tyrosine | UAU | UAC | | | | |\n| Valine | GUU | GUC | GUA | GUG | | |\n| Start (CI) | AUG | | | | | |\n| Stop (CT) | UAA | UAG | UGA | | | | \n*CI,* Chain-initiating; *CT,* chain-terminating. \ntypes of codons are designated CI for \"chain-initiating\" or \"start\" codon and CT for \"chain-terminating\" or \"stop\" codon. \n#### **TRANSFER RNA—THE ANTICODONS** \nAnother type of RNA that is essential for protein synthesis is called transfer RNA (tRNA) because it transfers amino acids to protein molecules as the protein is being synthesized. Each type of tRNA combines specifically with 1 of the 20 amino acids that are to be incorporated into proteins. The tRNA then acts as a *carrier* to transport its specific type of amino acid to the ribosomes, where protein molecules are forming. In the ribosomes, each specific type of tRNA recognizes a particular codon on the mRNA (described later) and thereby delivers [the appro](#page-38-0)priate amino acid to the appropriate place in the chain of the newly forming protein molecule. \nTransfer RNA, which contains only about 80 nucleotides, is a relatively small molecule in comparison with mRNA. It is a folded chain of nucleotides with a cloverleaf appearance similar to that shown in **Figure 3-9**. At one end of the molecule there is always an adenylic acid to which the transported amino acid attaches at a hydroxyl group of the ribose in the adenylic acid. \nBecause the function of tRNA is to cause attachment of a specific amino acid to a forming protein chain, it is essential that each type of t[RNA also ha](#page-38-0)ve specificity for a particular codon in the mRNA. The specific code in the tRNA that allows it to recognize a specific codon is again a triplet of nucleotide bases and is called an *anticodon.* This anticodon is located approximately in the middle of the tRNA molecule (at the bottom of the cloverleaf configuration shown in **Figure 3-9**). During formation of the protein molecule, the anticodon bases combine loosely by hydrogen bonding with the codon bases of the mRNA. In this way, the respective amino acids are lined up one after another along the mRNA chain, thus establishing the \n \n**Figure 3-9.** A messenger RNA strand is moving through two ribosomes. As each codon passes through, an amino acid is added to the growing protein chain, which is shown in the right-hand ribosome. The transfer RNA molecule transports each specific amino acid to the newly forming protein. \nappropriate sequence of amino acids in the newly forming protein molecule. \n#### **RIBOSOMAL RNA** \nThe third type of RNA in the cell is ribosomal RNA, which constitutes about 60% of the *ribosome.* The remainder of the ribosome is protein, including about 75 types of proteins that are both structural proteins and enzymes needed to manufacture proteins. \nThe ribosome is the physical structure in the cytoplasm on which proteins are actually synthesized. However, it always functions in association with the other two types of RNA; *tRNA* transports amino acids to the ribosome for incorporation into the developing protein, whereas *mRNA* provides the information necessary for sequencing the amino acids in proper order for each specific type of protein to be manufactured. Thus, the ribosome acts as a manufacturing plant in which the protein molecules are formed. \n**Formation of Ribosomes in the Nucleolus.** The DNA genes for the formation of ribosomal RNA are located in five pairs of chromosomes in the nucleus. Each of these chromosomes contains many duplicates of these particular genes because of the large amounts of ribosomal RNA required for cellular function. \nAs the ribosomal RNA forms, it collects in the *nucleolus,* a specialized structure lying adjacent to the chromosomes. When large amounts of ribosomal RNA are being synthesized, as occurs in cells that manufacture large amounts of protein, the nucleolus is a large structure, whereas in cells that synthesize little protein, the nucleolus may not even be seen. Ribosomal RNA is specially processed in the nucleolus, where it binds with ribosomal proteins to form granular condensation products that are primordial subunits of ribosomes. These subunits are then released from the nucleolus and transported through the large pores of the nuclear envelope to almost all parts of the cytoplasm. After the subunits enter the cytoplasm, they are assembled to form mature functional ribosomes. Therefore, proteins are formed in the cytoplasm of the cell, but not in the cell nucleus, because the nucleus does not cont[ain m](#page-39-0)ature ribosomes. \n#### **miRNA AND SMALL INTERFERING RNA** \nA fourth type of RNA in the cell is *microRNA* (miRNA); miRNA are short (21 to 23 nucleotides) single-stranded RNA fragments that regulate gene expression **(Figure 3-10).** The miRNAs are encoded from the transcribed DNA of genes, but they are not translated into proteins and are therefore often called *noncoding RNA*. The miRNAs are processed by the cell into molecules that are complementary to mRNA and act to decrease gene expression. The generation of miRNAs involves special processing of longer primary precursor RNAs called *primiRNAs,* which are the primary transcripts of the gene. \n \n**Figure 3-10.** Regulation of gene expression by microRNA (miRNA). Primary miRNA (pri-miRNA), the primary transcripts of a gene processed in the cell nucleus by the microprocessor complex, are converted to pre-miRNAs. These pre-miRNAs are then further processed in the cytoplasm by *dicer,* an enzyme that helps assemble an RNAinduced silencing complex (RISC) and generates miRNAs. The miRNAs regulate gene expression by binding to the complementary region of the RNA and repressing translation or promoting degradation of the messenger RNA (mRNA) before it can be translated by the ribosome. \nThe pri-miRNAs are then processed in the cell nucleus by the *microprocessor complex* to pre-miRNAs, which are 70-nucleotide, stem loop structures. These pre-miRNAs are then further processed in the cytoplasm by a specific *dicer enzyme* that helps assemble an *RNA-induced silencing complex* (RISC) and generates miRNAs. \nThe miRNAs regulate gene expression by binding to the complementary region of the RNA and promoting repression of translation or degradation of the mRNA before it can be translated by the ribosome. miRNAs are believed to play an important role in normal regulation of cell function, and alterations in miRNA function have been associated with diseases such as cancer and heart disease. \nAnother type of miRNA is *small interfering RNA* (siRNA), also called *silencing RNA* or *short interfering RNA.* The siRNAs are short, double-stranded RNA molecules, comprised of 20 to 25 nucleotides, that interfere with expression of specific genes. siRNAs generally refer to synthetic miRNAs and can be administered to silence expression of specific genes. They are designed to avoid nuclear processing by the microprocessor complex and, after the siRNA enters the cytoplasm, it activates the RISC silencing complex, blocking the translation of mRNA. Because siRNAs can be tailored for any specific sequence in the gene, they can be used to block translation of any mRNA and therefore expression by any gene for which the nucleotide sequence is known. Researchers have proposed that siRNAs may become useful therapeutic tools to silence genes that contribute to the pathophysiology of diseases. \n#### TRANSLATION—FORMATION O[F](#page-38-0) PROTEINS ON THE RIBOSOMES \nWhen a molecule of mRNA comes in contact with a ribosome, it travels through the ribosome, beginning at a predetermined end of the RNA molecule specified by an appropriate sequence of RNA bases called the *chaininitiating codon.* Then, as shown in **Figure 3-9**, while the mRNA travels through the ribosome, a protein molecule is formed, a process called *translation.* Thus, the ribosome reads the codons of the mRNA in much the same way that a tape is read as it passes through the playback head of a tape recorder. Then, when a \"stop\" (or \"chainterminating\") codon slips past the ribosome, the end of a protein molecule is sig[naled, and t](#page-38-0)he p[rotein molecu](#page-40-0)le is freed into the cytoplasm. \n**Polyribosomes.** A single mRNA molecule can form protein molecules in several ribosomes at the same time because the initial end of the RNA strand can pass to a successive ribosome as it leaves the first, as shown at the bottom left in **Figure 3-9** and **Figure 3-11**. The protein molecules are in different stages of development in each ribosome. As a result, clusters of ribosomes frequently occur, with 3 to 10 ribosomes being attached to a single mRNA at the same time. These clusters are called *polyribosomes.* \nAn mRNA can cause formation of a protein molecule in any ribosome; there is no specificity of ribosomes for given types of protein. The ribosome is simply the physical manufacturing plant in which the chemical reactions take place. \n**Many Ribosomes Attach to the Endoplasmic Reticulum.** In Chapter 2, we noted that many ribosomes become attached to the endoplasmic reticulum. This attachment occurs because the initial ends of many forming protein molecules have amino acid sequences that immediately attach to specific receptor sites on the endoplasmic reticulum, causing these molecules to penetrate the \n \n**Figure 3-11.** The physical structure of the ribosomes, as well as their functional relationship to messenger RNA, transfer RNA, and the endoplasmic reticulum during the formation of protein molecules. \nreticulum wall and enter the endoplasmic reticulum matrix. This process gives a granular appearance to the portions of the reticulum where proteins are being formed and are entering the matrix of the reticulum. \n**Figure 3-11** shows the functional relationship of mRNA to the ribosomes and the manner in which the ribosomes attach to the membrane of the endoplasmic reticulum. Note the process of translation occurring in several ribosomes at the same time in response to the same strand of mRNA. Note also the newly forming polypeptide (protein) chains passing through the endoplasmic reticulum membrane into the endoplasmic matrix. \nIt should be noted that except in glandular cells, in which large amounts of protein-containing secretory vesicles are formed, most proteins synthesized by the ribosomes are released directly into the cytosol instead of into the endoplasmic reticulum. These proteins are enzymes and internal structural proteins of the cell. \n**Chemical Steps in Protein Synthesis.** Some of the chemical events that occur in the synthesis of a protein molecule are shown in **Figure 3-12**. This Fig. shows representative reactions for three separate amino acids, $AA_1$ , $AA_2$ , and $AA_{20}$ . The stages of the reactions are as follows: \n- Each amino acid is activated by a chemical process in which ATP combines with the amino acid to form an adenosine monophosphate complex with the amino acid, giving up two high-energy phosphate bonds in the process.\n- 2. The activated amino acid, having an excess of energy, then *combines with its specific tRNA to form an amino acid–tRNA complex* and, at the same time, releases the adenosine monophosphate.\n- 3. The tRNA carrying the amino acid complex then comes in contact with the mRNA molecule in the ribosome, where the anticodon of the tRNA attaches temporarily to its specific codon of the mRNA, thus lining up the amino acid in the appropriate sequence to form a protein molecule. \nThen, under the influence of the enzyme *peptidyl transferase* (one of the proteins in the ribosome), *peptide bonds* are formed between the successive amino acids, thus adding progressively to the protein chain. These \nchemical events require energy from two additional high-energy phosphate bonds, making a total of four high-energy bonds used for each amino acid added to the protein chain. Thus, the synthesis of proteins is one of the most energy-consuming processes of the cell. \n**Peptide Linkage—Combination of Amino Acids.** The successive amino acids in the protein chain combine with one another according to the typical reaction. \n$$\\begin{array}{cccccccccccccccccccccccccccccccccccc$$ \nIn this chemical reaction, a hydroxyl radical (OH-) is removed from the COOH portion of the first amino acid, and a hydrogen (H+) of the NH2 portion of the other amino acid is removed. These combine to form water, and the two reactive sites left on the two successive amino acids bond with each other, resulting in a single molecule. This process is called *peptide linkage*. As each additional amino acid is added, an additional peptide linkage is formed.",
"metadata": {
"Header 1": "**Genetic Control of Protein Synthesis, Cell Function, and [Cell](#page-34-1) Reproduction**",
"Header 2": "Genetic Control of Protein Synthesis",
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"source_pdf": "/share/project/bs_scaling/lmse/self-evolution-explore/datasets/websources/Bio/1671268744mpp.pdf"
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"page_content": "Many thousand protein enzymes formed in the manner just described control essentially all the other chemical reactions that take place in cells. These enzymes promote synthesis of lipids, glycogen, purines, pyrimidines, and hundreds of other substances. We discuss many of these synthetic processes in relation to carbohydrate, lipid, and protein metabolism in Chapters 68 through 70. These substances each contribute to the various functions of the cells.",
"metadata": {
"Header 1": "**Genetic Control of Protein Synthesis, Cell Function, and [Cell](#page-34-1) Reproduction**",
"Header 2": "Synthesis of Other Substances in the Cell",
"is_merged_section": true,
"source_pdf": "/share/project/bs_scaling/lmse/self-evolution-explore/datasets/websources/Bio/1671268744mpp.pdf"
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"page_content": "From our discussion thus far, it is clear that the genes control both the physical and chemical functions of the cells. However, the degree of activation of respective genes must also be \n \n**Figure 3-12.** Chemical events in the formation of a protein molecule. AMP, Adenosine monophosphate; ATP, adenosine triphosphate; tRNA, transfer RNA. \ncontrolled; otherwise, some parts of the cell might overgrow or some chemical reactions might overact until they kill the cell. Each cell has powerful internal feedback control mechanisms that keep the various functional operations of the cell in step with one another. For each gene ( $\\approx 20,000-25,000$ genes in all), at least one such feedback mechanism exists. \nThere are basically two methods whereby the biochemical activities in the cell are controlled: (1) *genetic regulation,* in which the degree of activation of the genes and the formation of gene products are themselves controlled, and (2) *enzyme regulation,* in which the activity levels of already formed enzymes in the cell are controlled. \n#### **GENETIC REGULATION** \nGenetic regulation, or regulation of gene expression, covers the entire process from transcription of the genetic code in the nucleus to the formation of proteins in the cytoplasm. Regulation of gene expression provides all living organisms with the ability to respond to changes in their environment. In animals that have many different types of cells, tissues, and organs, differential regulation of gene expression also permits the different cell types in the body to each perform their specialized functions. Although a cardiac myocyte contains the same genetic code as a renal tubular epithelial cell, many genes are expressed in cardiac cells that are not expressed in renal tubular cells. The ultimate measure of gene \"expression\" is whether (and how much) of the gene products (proteins) are produced because proteins carry out cell functions specified by the genes. Regulation of gene expression can occur at any point in the pathways of transcription, RNA processing, and translation. \n**The Promoter Controls Gene Expression.** Synthesis of cellular proteins is a complex process that starts with transcription of DNA into RNA. Transcription of DNA is \n \n**Figure 3-13.** Gene transcription in eukaryotic cells. A complex arrangement of multiple clustered enhancer modules is interspersed with insulator elements, which can be located upstream or downstream of a basal promoter containing TATA box (TATA), proximal promoter elements (response elements, RE), and initiator sequences (INR). \ncontrolled by regulatory elements found in the promoter of a gene (Figure 3-13). In eukaryotes, which includes all mammals, the basal promoter consists of a sequence of bases (TATAAA) called the TATA box, the binding site for the TATA-binding protein and several other important transcription factors that are collectively referred to as the transcription factor IID complex. In addition to the transcription factor IID complex, this region is where transcription factor IIB binds to both the DNA and RNA polymerase 2 to facilitate transcription of the DNA into RNA. This basal promoter is found in all protein-coding genes, and the polymerase must bind with this basal promoter before it can begin traveling along the DNA strand to synthesize RNA. The upstream promoter is located farther upstream from the transcription start site and contains several binding sites for positive or negative transcription factors that can affect transcription through interactions with proteins bound to the basal promoter. The structure and transcription factor binding sites in the upstream promoter vary from gene to gene to give rise to the different expression patterns of genes in different tissues. \nTranscription of genes in eukaryotes is also influenced by *enhancers,* which are regions of DNA that can bind transcription factors. Enhancers can be located a great distance from the gene they act on or even on a different chromosome. They can also be located upstream or downstream of the gene that they regulate. Although enhancers may be located far from their target gene, they may be relatively close when DNA is coiled in the nucleus. It is estimated that there are more than 100,000 gene enhancer sequences in the human genome. \nIn the organization of the chromosome, it is important to separate active genes that are being transcribed from genes that are repressed. This separation can be challenging because multiple genes may be located close together on the chromosome. The separation is achieved by chromosomal *insulators.* These insulators are gene sequences that provide a barrier so that a specific gene is isolated against transcriptional influences from surrounding genes. Insulators can vary greatly in their DNA sequence and the proteins that bind to them. One way an insulator activity can be modulated is by *DNA methylation*, which is the case for the mammalian insulin-like growth factor 2 (IGF-2) gene. The mother's allele has an insulator between the enhancer and promoter of the gene that allows for the binding of a transcriptional repressor. However, the paternal DNA sequence is methylated such that the transcriptional repressor cannot bind to the insulator, and the IGF-2 gene is expressed from the paternal copy of the gene. \n#### **Other Mechanisms for Control of Transcription by the Promoter.** Variations in the basic mechanism for control of the promoter have been discovered in the past three decades. Without giving details, let us list some of them: \n- 1. A promoter is frequently controlled by transcription factors located elsewhere in the genome. That is, the regulatory gene causes the formation of a regulatory protein that in turn acts as an activator or repressor of transcription.\n- 2. Occasionally, many different promoters are controlled at the same time by the same regulatory protein. In some cases, the same regulatory protein functions as an activator for one promoter and as a repressor for another promoter.\n- 3. Some proteins are controlled not at the starting point of transcription on the DNA strand but farther along the strand. Sometimes, the control is not even at the DNA strand itself but occurs during the processing of the RNA molecules in the nucleus before they are released into the cytoplasm. Control may also occur at the level of protein formation in the cytoplasm during RNA translation by the ribosomes.\n- 4. In nucleated cells, the nuclear DNA is packaged in specific structural units, the *chromosomes.* Within \neach chromosome, the DNA is wound around small proteins called *histones,* which in turn are held tightly together in a compacted state by still other proteins. As long as the DNA is in this compacted state, it cannot function to form RNA. However, multiple control mechanisms are being discovered that can cause selected areas of chromosomes to become decompacted one part at a time, so that partial RNA transcription can occur. Even then, specific *transcriptor factor*s control the actual rate of transcription by the promoter in the chromosome. Thus, still higher orders of control are used to establish proper cell function. In addition, signals from outside the cell, such as some of the body's hormones, can activate specific chromosomal areas and specific transcription factors, therefore controlling the chemical machinery for function of the cell. \nBecause there are many thousands of different genes in each human cell, the large number of ways in which genetic activity can be controlled is not surprising. The gene control systems are especially important for controlling intracellular concentrations of amino acids, amino acid derivatives, and intermediate substrates and products of carbohydrate, lipid, and protein metabolism. \n#### **CONTROL OF INTRACELLULAR FUNCTION BY ENZYME REGULATION** \nIn addition to control of cell function by genetic regulation, cell activities are also controlled by intracellular inhibitors or activators that act directly on specific intracellular enzymes. Thus, enzyme regulation represents a second category of mechanisms whereby cellular biochemical functions can be controlled. \n**Enzyme Inhibition.** Some chemical substances formed in the cell have direct feedback effects to inhibit the specific enzyme systems that synthesize them. Almost always, the synthesized product acts on the first enzyme in a sequence, rather than on the subsequent enzymes, usually binding directly with the enzyme and causing an allosteric conformational change that inactivates it. One can readily recognize the importance of inactivating the first enzyme because this prevents buildup of intermediary products that are not used. \nEnzyme inhibition is another example of negative feedback control. It is responsible for controlling intracellular concentrations of multiple amino acids, purines, pyrimidines, vitamins, and other substances. \n**Enzyme Activation.** Enzymes that are normally inactive often can be activated when needed. An example of this phenomenon occurs when most of the ATP has been depleted in a cell. In this case, a considerable amount of cyclic adenosine monophosphate (cAMP) begins to be formed as a breakdown product of ATP. The presence of this cAMP, in turn, immediately activates the glycogensplitting enzyme phosphorylase, liberating glucose molecules that are rapidly metabolized, with their energy used for replenishment of the ATP stores. Thus, cAMP acts as an enzyme activator for the enzyme phosphorylase and thereby helps control intracellular ATP concentration. \nAnother interesting example of both enzyme inhibition and enzyme activation occurs in the formation of the purines and pyrimidines. These substances are needed by the cell in approximately equal quantities for the formation of DNA and RNA. When purines are formed, they *inhibit* the enzymes that are required for formation of additional purines. However, they *activate* the enzymes for formation of pyrimidines. Conversely, the pyrimidines inhibit their own enzymes but activate the purine enzymes. In this way, there is continual cross-talk between the synthesizing systems for these two substances, resulting in almost exactly equal amounts of the two substances in the cells at all times. \n**Summary.** There are two principal mechanisms whereby cells control proper proportions and quantities of different cellular constituents: (1) genetic regulation; and (2) enzyme regulation. The genes can be activated or inhibited, and likewise, the enzyme systems can be activated or inhibited. These regulatory mechanisms usually function as feedback control systems that continually monitor the cell's biochemical composition and make corrections as needed. However, on occasion, substances from outside the cell (especially some of the hormones discussed in this text) also control the intracellular biochemical reactions by activating or inhibiting one or more of the intracellular control systems. \n#### THE DNA–GENETIC SYSTEM CONTROLS CELL REPRODUCTION \nCell reproduction is another example of the ubiquitous role that the DNA–genetic system plays in all life processes. The genes and their regulatory mechanisms determine cell growth characteristics and when or whether cells will divide to form new cells. In this way, the allimportant genetic system controls each stage in the development of the human, from the single-cell fertilized ovum to the whole functioning body. Thus, if there is any central theme to life, it is the DNA–genetic system. \n#### **Life Cycle of the Cell** \nThe life cycle of a cell is the period from cell repr[oduction](#page-43-0) to the next cell reproduction. When mammalian cells *are not inhibited and are reproducing as rapidly as they can,* this life cycle may be as little as 10 to 30 hours. It is terminated by a series of distinct physical events called *mitosis* that cause division of the cell into two new daughter cells. The events of mitosis are shown in **Figure 3-14** and described later. The actual stage of mitosis, however, lasts for only about 30 minutes, and thus more than 95% of the life cycle of even rapidly reproducing cells is represented by the interval between mitosis, called *interphase.* \nExcept in special conditions of rapid cellular reproduction, inhibitory factors almost always slow or stop the \n \n**Figure 3-14.** Stages of cell reproduction. *A, B, C,* Prophase. *D,* Prometaphase. *E,* Metaphase. *F,* Anaphase. *G, H,* Telophase. \nuninhibited life cycle of the cell. Therefore, different cells of the body actually have life cycle periods that vary from as little as 10 hours for highly stimulated bone marrow cells to an entire lifetime of the human body for many nerve cells. \n#### **Cell Reproduction Begins with Replication of DNA** \nThe first step of cell reproduction is *replication (duplication) of all DNA in the chromosomes.* It is only after this replication has occurred that mitosis can take place. \nThe DNA begins to be duplicated 5 to 10 hours before mitosis, and the duplication is completed in 4 to 8 hours. The net result is two exact *replicas* of all DNA. These replicas become the DNA in the two new daughter cells that will be formed at mitosis. After replication of the DNA, there is another period of 1 to 2 hours before mitosis begins abruptly. Even during this period, preliminary changes that will lead to the mitotic process are beginning to take place. \n**DNA Replication.** DNA is replicated in much the same way that RNA is transcribed from DNA, except for a few important differences: \n1. Both strands of the DNA in each chromosome are replicated, not just one of them. \n \nFigure 3-15. DNA replication, showing the replication fork and leading and lagging strands of DNA. \n- 2. Both entire strands of the DNA helix are replicated from end to end, rather than small portions of them, as occurs in the transcription of RNA.\n- 3. Multiple enzymes called *DNA polymerase*, which is comparable to RNA polymerase, are essential for replicating DNA. DNA polymerase attaches to and moves along the DNA template strand, adding nucleotides in the 5′ to 3′ direction. Another enzyme, *DNA ligase*, causes bonding of successive DNA nucleotides to one another, using high-energy phosphate bonds to energize these attachments.\n- 4. Replication fork formation. Before DNA can be replicated, the double-stranded molecule must be \"unzipped\" into two single strands (Figure 3-15). Because the DNA helixes in each chromosome are approximately 6 centimeters in length and have millions of helical turns, it would be impossible for the two newly formed DNA helixes to uncoil from each other were it not for some special mechanism. This uncoiling is achieved by DNA helicase enzymes that break the hydrogen bonding between the base pairs of the DNA, permitting the two strands to separate into a Y shape known as the replication fork, the area that will be the template for replication to begin. \nDNA is directional in both strands, signified by a 5′ and 3′ end (see **Figure 3-15**). Replication progresses only in the 5′ to 3′ direction. At the replication fork one strand, the *leading strand*, is oriented in the 3′ to 5′ direction, toward the replication fork, while the *lagging strand* is oriented 5′ to 3′, away from the replication fork. Because of their different orientations, the two strands are replicated differently. \n5. *Primer binding*. Once the DNA strands have been separated, a short piece of RNA called an *RNA primer* binds to the 3' end of the leading strand. Primers are generated by the enzyme *DNA primase*. \n- Primers always bind as the starting point for DNA replication.\n- 6. Elongation. DNA polymerases are responsible for creating the new strand by a process called *elongation*. Because replication proceeds in the 5′ to 3′ direction on the leading strand, the newly formed strand is continuous. The lagging strand begins replication by binding with multiple primers that are only several bases apart. DNA polymerase then adds pieces of DNA, called *Okazaki fragments*, to the strand between primers. This process of replication is discontinuous because the newly created Okazaki fragments are not yet connected. An enzyme, *DNA ligase*, joins the Okazaki fragments to form a single unified strand.\n- 7. Termination. After the continuous and discontinuous strands are both formed, the enzyme exonuclease removes the RNA primers from the original strands, and the primers are replaced with appropriate bases. Another exonuclease \"proofreads\" the newly formed DNA, checking and clipping off any mismatched or unpaired residues. \nAnother enzyme, *topoisomerase*, can transiently break the phosphodiester bond in the backbone of the DNA strand to prevent the DNA in front of the replication fork from being overwound. This reaction is reversible, and the phosphodiester bond reforms as the topoisomerase leaves. \nOnce completed, the parent strand and its complementary DNA strand coils into the double helix shape. The process of replication therefore produces two DNA molecules, each with one strand from the parent DNA and one new strand. For this reason, DNA replication is often described as *semiconservative*; half of the chain is part of the original DNA molecule and half is brand new. \n**DNA Repair, DNA \"Proofreading,\" and \"Mutation.\"**During the hour or so between DNA replication and \nthe beginning of mitosis, there is a period of active repair and \"proofreading\" of the DNA strands. Wherever inappropriate DNA nucleotides have been matched up with the nucleotides of the original template strand, special enzymes cut out the defective areas and replace them with appropriate complementary nucleotides. This repair process, which is achieved by the same DNA polymerases and DNA ligases that are used in replication, is referred to as *DNA proofreading.* \nBecause of repair and proofreading, mistakes are rarely made in the DNA replication process. When a mistake is made, it is called a *mutation.* The mutation may cause formation of some abnormal protein in the cell rather than a needed protein, which may lead to abnormal cellular function and sometimes even cell death. Given that many thousands of genes exist in the human genome, and that the period from one human generation to another is about 30 years, one would expect as many as 10 or many more mutations in the passage of the genome from parent to offspring. As a further protection, however, each human genome is represented by two separate sets of chromosomes, one derived from each parent, with almost identical genes. Therefore, one functional gene of each pair is almost always available to the child, despite mutations. \n#### **CHROMOSOMES AND THEIR REPLICATION** \nThe DNA helixes of the nucleus are packaged in chromosomes. The human cell contains 46 chromosomes arranged in 23 pairs. Most of the genes in the two chromosomes of each pair are identical or almost identical to each other, so it is usually stated that the different genes also exist in pairs, although occasionally this is not the case. \nIn addition to DNA, there is a large amount of protein in the chromosome, composed mainly of many small molecules of electropositively charged *histones.* The histones are organized into vast numbers of small, bobbinlike cores. Small segments of each DNA helix are coiled sequentially around one core after another. \nThe histone cores play an important role in regulation of DNA activity because as long as the DNA is packaged tightly, it cannot function as a template for formation of RNA or replication of new DNA. Furthermore, some of the regulatory proteins *decondense* the histone packaging of the DNA and allow small segments at a time to form RNA. \nSeveral nonhistone proteins are also major components of chromosomes, functioning as chromosomal structural proteins and, in connection with the genetic regulatory machinery, as activators, inhibitors, and enzymes. \nReplication of the chromosomes in their entirety occurs during the next few minutes after replication of the DNA helixes has been completed; the new DNA helixes collect new protein molecules as needed. The two newly formed chromosomes remain attached to each other (until time for mitosis) at a point called the *centromere* located near their center. These duplicated but still attached chromosomes are called *chromatids.* \n#### **CELL MITOSIS** \nThe actual process whereby the cell splits into two new cells is called *mitosis.* Once each chromosome has been replicated to form the two chromatids, mitos[is follows](#page-43-0) automatically within 1 or 2 hours in many cells. \n**Mitotic Apparatus: Function of the Centrioles.** One of the first events of mitosis takes place in the cytoplasm in or around the small structures called *centrioles* during the latter part of interphase*.* As shown in **Figure 3-**14, two pairs of centrioles lie close to each other near one pole of the nucleus. These centrioles, like the DNA and chromosomes, are also replicated during interphase, usually shortly before replication of the DNA. Each centriole is a small cylindrical body about 0.4 micrometer long and about 0.15 micrometer in diameter, consisting mainly of nine parallel tubular structures arranged in the form of a cylinder. The two centrioles of each pair lie at right angles to each other. Each pair of centrioles, along with attached *pericentriolar material,* is called a *centrosome.* \nShortly before mitosis takes place, the two pairs of centrioles begin to move apart from each other. This movement is caused by polymerization of protein microtubules growing between the respective centriole pairs and actually pushing them apart. At the same time, other microtubules grow radially away from each of the centriole pairs, forming a spiny star called the *aster,* in each end of the cell. Some of the spines of the aster penetrate the nuclear membrane and h[elp separate t](#page-43-0)he two sets of chromatids during mitosis. The complex of microtubules extending between the two new centriole pairs is called the *spindle,* and the entire set of microtubules plus the two pairs of centrioles is called the *mitotic apparatus.* \n**Prophase.** [The fi](#page-43-0)rst stage of mitosis, called *prophase,* is shown in **Figure 3-14***A, B,* and *C.* While the spindle is forming, the chromosomes of the nucleus (which in interphase consist of loosely coiled strands) become condensed into well-defined chromosomes. \n**Prometaphase.** During the prometaphase stage (see **Figure 3-14***D*), the growing microtubular spines of the aster fragment the nuclear envelope. At the same time, multiple mic[rotubu](#page-43-0)les from the aster attach to the chromatids at the centromeres, where the paired chromatids are still bound to each other. The tubules then pull one chromatid of each pair toward one cellular pole and its partner toward the opposite pole. \n**Metaphase.** During the metaphase stage (see **Figure 3-14***E*), the two asters of the mitotic apparatus are pushed farther apart. This pushing is believed to occur because the microtubular spines from the two asters, where they interdigitate with each other to form the mitotic spindle, push each other away. Minute contractile protein molecules called *\"molecular motors,\"* which may be composed of the muscle protein *actin,* extend between the respective spines a[nd, us](#page-43-0)ing a stepping action as in muscle, actively slide the spines in a reverse direction along each other. Simultaneously, the chromatids are pulled tightly by their attached microtubules to the very center of the cell, lining up to form the *equatorial plate* of the mitotic spindle. \n**Anaphase.** During the anaphase stage (see **Figure 3-14***F*), the two chromatids of each chromosome are pulled apart at the centromere. All 46 pairs of c[hromatids](#page-43-0) are sepa[rat](#page-43-0)ed, forming two separate sets of 46 *daughter chromosomes.* One of these sets is pulled toward one mitotic aster, and the other is pulled toward the other aster, as the two respective poles of the dividing cell are pushed still farther apart. \n**Telophase.** In the telophase stage (see **Figure 3-14***G* and *H*), the two sets of daughter chromosomes are pushed completely apart. Then, the mitotic apparatus dissipates, and a new nuclear membrane develops around each set of chromosomes. This membrane is formed from portions of the endoplasmic reticulum that are already present in the cytoplasm. Shortly thereafter, the cell pinches in two, midway between the two nuclei. This pinching is caused by the formation of a contractile ring of *microfilaments* composed of *actin* and probably *myosin* (the two contractile proteins of muscle) at the juncture of the newly developing cells that pinches them off from each other. \n#### **CONTROL OF CELL GROWTH AND CELL REPRODUCTION** \nSome cells grow and reproduce all the time, such as the blood-forming cells of the bone marrow, the germinal layers of the skin, and the epithelium of the gut. Many other cells, however, such as smooth muscle cells, may not reproduce for many years. A few cells, such as the neurons and most striated muscle cells, do not reproduce during the entire life of a person, except during the original period of fetal life. \nIn certain tissues, an insufficiency of some types of cells causes them to grow and reproduce rapidly until appropriate numbers of these cells are again available. For example, in some young animals, seven-eighths of the liver can be removed surgically, and the cells of the remaining one-eighth will grow and divide until the liver mass returns to almost normal. The same phenomenon occurs for many glandular cells and most cells of the bone marrow, subcutaneous tissue, intestinal epithelium, and almost any other tissue except highly differentiated cells such as nerve and muscle cells. \nThe mechanisms that maintain proper numbers of the different types of cells in the body are still poorly understood. However, experiments have shown at least three ways in which growth can be controlled. First, growth often is controlled by *growth factors* that come from other parts of the body. Some of these growth factors circulate in the blood, but others originate in adjacent tissues. For [exam](#page-43-0)ple, the epithelial cells of some glands, such as the pancreas, fail to grow without a growth factor from the underlying connective tissue of the gland. Second, most normal cells stop growing when they have run out of space for growth. This phenomenon occurs when cells are grown in tissue culture; the cells grow until they contact a solid object, and then growth stops. Third, cells grown in tissue culture often stop growing when minute am[ounts](#page-46-0) of their own secretions are allowed to collect in the culture medium. This mechanism, too, could provide a means for negative feedback control of growth. \n**Telomeres Prevent the Degradation of Chromosomes.** A *telomere* is a region of repetitive nucleotide sequences located at each end of a chromatid (**Figure 3-16**). Telomeres serve as protective caps that prevent the chromosome from deterioration during cell division. During cell division, a short piece of \"primer\" RNA attaches to the DNA strand to start the replication. However, because the primer does not attach at the very end of the DNA strand, the copy is missing a small section of the DNA. With each cell division, the copied DNA loses additional nucleotides from the telomere region. The nucleotide sequences provided by the telomeres therefore prevent the degradation of genes near the ends of chromosomes. Without telomeres, the genomes would progressively lose information and be truncated after each cell division. Thus, the telomeres can be considered to be disposable chromosomal buffers that help maintain stability of the genes but are gradually consumed during repeated cell divisions. \n \n**Figure 3-16.** Control of cell replication by telomeres and telomerase. The cells' chromosomes are capped by telomeres, which, in the absence of telomerase activity, shorten with each cell division until the cell stops replicating. Therefore, most cells of the body cannot replicate indefinitely. In cancer cells, telomerase is activated, and telomere length is maintained so that the cells continue to replicate themselves uncontrollably. \nEach time a cell divides, an average person loses 30 to 200 base pairs from the ends of that cell's telomeres. In human blood cells, the length of telomeres ranges from 8000 base pairs at birth to as low as 1500 in older people. Eventually, when the telomeres shorten to a critical length, the chromosomes become unstable, and the cells die. This process of telomere shortening is believed to be an important reason for some of the physiological changes associated with aging. Telomere erosion can also occur as a result of diseases, especially those associated with oxidative stress and inflammation. \nIn some cells, such as stem cells of the bone marrow or skin that must be replenished throughout life, or germ cells in the ovaries and testes, the enzyme *telomerase* adds bases to the ends of the telomeres so that many more generations of cells can be produced. However, telomerase activity is usually low in most cells of the body, and after many generations the descendent cells will inherit defective chromoso[mes, become](#page-46-0) *senescent,* and cease dividing. This process of telomere shortening is important in regulating cell proliferation and maintaining gene stability. In cancer cells, telomerase activity is abnormally activated so that telomere length is maintained, making it possible for the cells to replicate over and over again uncontrollably (see **Figure 3-16**). Some scientists have therefore proposed that telomere shortening protects us from cancer and other proliferative diseases. \n**Regulation of Cell Size.** Cell size is determined almost entirely by the amount of functioning DNA in the nucleus. If replication of the DNA does not occur, the cell grows to a certain size and thereafter remains at that size. Conversely, use of the chemical *colchicine* makes it possible to prevent formation of the mitotic spindle and therefore prevent mitosis, even though replication of the DNA continues. In this event, the nucleus contains far greater quantities of DNA than it normally does, and the cell grows proportionately larger. It is assumed that this cell growth results from increased production of RNA and cell proteins, which, in turn, cause the cell to grow larger. \n#### CELL DIFFERENTIATION \nA special characteristic of cell growth and cell division is *cell differentiation,* which refers to changes in the physical and functional properties of cells as they proliferate in the embryo to form the different body structures and organs. The following description of an especially interesting experiment helps explain these processes. \nWhen the nucleus from an intestinal mucosal cell of a frog is surgically implanted into a frog ovum from which the original ovum nucleus was removed, the result is often the formation of a normal frog. This experiment demonstrates that even the intestinal mucosal cell, which is a well-differentiated cell, carries all the necessary genetic information for development of all structures required in the frog's body. \nTherefore, it has become clear that differentiation results not from loss of genes but from selective repression of different gene promoters. In fact, electron micrographs suggest that some segments of DNA helixes that are wound around histone cores become so condensed that they no longer uncoil to form RNA molecules. One explanation for this is as follows. It has been supposed that the cellular genome begins at a certain stage of cell differentiation to produce a regulatory *protein* that forever after represses a select group of genes. Therefore, the repressed genes never function again. Regardless of the mechanism, mature human cells each produce a maximum of about 8000 to 10,000 proteins rather than the potential 20,000 to 25,000 or more that would be produced if all genes were active. \nEmbryological experiments have shown that certain cells in an embryo control differentiation of adjacent cells. For example, the *primordial chordamesoderm* is called the *primary organizer* of the embryo because it forms a focus around which the remainder of the embryo develops. It differentiates into a *mesodermal axis* that contains segmentally arranged *somites* and, as a result of *inductions* in the surrounding tissues, causes the formation of essentially all the organs of the body. \nAnother instance of induction occurs when the developing eye vesicles come into contact with the ectoderm of the head and cause the ectoderm to thicken into a lens plate that folds inward to form the lens of the eye. Therefore, a large share of the embryo develops as a result of such inductions, with one part of the body affecting another part, and this part affecting still other parts. \nThus, although our understanding of cell differentiation is still hazy, we are aware of many control mechanisms whereby differentiation *could* occur. \n#### APOPTOSIS—PROGRAMMED CELL DEATH \nThe many trillions of the body's cells are members of a highly organized community in which the total number of cells is regulated not only by controlling the rate of cell division, but also by controlling the rate of cell death. When cells are no longer needed or become a threat to the organism, they undergo a suicidal *programmed cell death,* or *apoptosis.* This process involves a specific proteolytic cascade that causes the cell to shrink and condense, disassemble its cytoskeleton, and alter its cell surface so that a neighboring phagocytic cell, such as a macrophage, can attach to the cell membrane and digest the cell. \nIn contrast to programmed death, cells that die as a result of an acute injury usually swell and burst due to loss of cell membrane integrity, a process called cell *necrosis.* Necrotic cells may spill their contents, causing inflammation and injury to neighboring cells. Apoptosis, however, is an orderly cell death that results in disassembly and phagocytosis of the cell before any leakage of its contents occurs, and neighboring cells usually remain healthy. \nApoptosis is initiated by activation of a family of proteases called *caspases*, which are enzymes that are synthesized and stored in the cell as inactive *procaspases*. The mechanisms of activation of caspases are complex but, once activated, the enzymes cleave and activate other procaspases, triggering a cascade that rapidly breaks down proteins within the cell. The cell thus dismantles itself, and its remains are rapidly digested by neighboring phagocytic cells. \nA tremendous amount of apoptosis occurs in tissues that are being remodeled during development. Even in adult humans, billions of cells die each hour in tissues such as the intestine and bone marrow and are replaced by new cells. Programmed cell death, however, is normally balanced by formation of new cells in healthy adults. Otherwise, the body's tissues would shrink or grow excessively. Abnormalities of apoptosis may play a key role in neurodegenerative diseases such as Alzheimer disease, as well as in cancer and autoimmune disorders. Some drugs that have been used successfully for chemotherapy appear to induce apoptosis in cancer cells. \n#### CANCER \nCancer may be caused by *mutation* or by some other *abnormal activation* of cellular genes that control cell growth and cell mitosis. *Proto-oncogenes* are normal genes that code for various proteins that control cell adhesion, growth and division. If mutated or excessively activated, proto-oncogenes can become abnormally functioning *oncogenes* capable of causing cancer*.* As many as 100 different oncogenes have been discovered in human cancers. \nAlso present in all cells are *antioncogenes,* also called *tumor suppressor genes*, which suppress the activation of specific oncogenes. Therefore, loss or inactivation of antioncogenes can allow activation of oncogenes that lead to cancer. \nFor several reasons, only a minute fraction of the cells that mutate in the body ever lead to cancer: \n- • First, most mutated cells have less survival capability than normal cells, and they simply die.\n- • Second, only a few of the mutated cells that survive become cancerous because most mutated cells still have normal feedback controls that prevent excessive growth.\n- • Third, cells that are potentially cancerous are often destroyed by the body's immune system before they grow into a cancer. \nMost mutated cells form abnormal proteins within their cell bodies because of their altered genes, and these proteins activate the body's immune system, causing it to form antibodies or sensitized lymphocytes that react against the cancerous cells, destroying them. In people whose immune systems have been suppressed, such as in persons taking immunosuppressant drugs after kidney or heart transplantation, the probability that a cancer will develop is multiplied as much as fivefold. \n• Fourth, the simultaneous presence of several different activated oncogenes is usually required to cause a cancer. For example, one such gene might promote rapid reproduction of a cell line, but no cancer occurs because another mutant gene is not present simultaneously to form the needed blood vessels. \nWhat is it that causes the altered genes? Considering that many trillions of new cells are formed each year in humans, a better question might be to ask why all of us do not develop millions or billions of mutant cancerous cells. The answer is the incredible precision with which DNA chromosomal strands are replicated in each cell before mitosis can take place, along with the proofreading process that cuts and repairs any abnormal DNA strand before the mitotic process is allowed to proceed. Yet, despite these inherited cellular precautions, probably one newly formed cell in every few million still has significant mutant characteristics. \nThus, chance alone is all that is required for mutations to take place, so we can suppose that a large number of cancers are merely the result of an unlucky occurrence. However, the probability of mutations can be greatly increased when a person is exposed to certain chemical, physical, or biological factors, including the following: \n- 1. *Ionizing radiation,* such as x-rays, gamma rays, particle radiation from radioactive substances, and even ultraviolet light, can predispose individuals to cancer. Ions formed in tissue cells under the influence of such radiation are highly reactive and can rupture DNA strands, causing many mutations.\n- 2. *Chemical substances* of certain types may also cause mutations. It was discovered long ago that various aniline dye derivatives are likely to cause cancer, and thus workers in chemical plants producing such substances, if unprotected, have a special predisposition to cancer. Chemical substances that can cause mutation are called *carcinogens.* The carcinogens that currently cause the greatest number of deaths are those in cigarette smoke. These carcinogens cause over 30% of all cancer deaths and at least 85% of lung cancer deaths.\n- 3. *Physical irritants* can also lead to cancer, such as continued abrasion of the linings of the intestinal tract by some types of food. The damage to the tissues leads to rapid mitotic replacement of the cells; the more rapid the mitosis, the greater the chance for mutation.\n- 4. *Hereditary tendency* to cancer occurs in some families. This hereditary tendency results from the fact that most cancers require not one mutation but two or more mutations before cancer occurs. In families that are particularly predisposed to cancer, it is presumed that one or more cancerous genes are already \n- mutated in the inherited genome. Therefore, far fewer additional mutations must take place in such family members before a cancer begins to grow.\n- 5. *Certain types of oncoviruses* can cause various types of cancer. Some examples of viruses associated with cancers in humans include *human papilloma virus* (HPV), *hepatitis B and hepatitis C virus,* Epstein-Barr virus, human immunodeficiency virus (HIV), human T-cell leukemia virus, Kaposi sarcoma–associated herpes virus (KSHV), and Merkel cell polyomavirus. Although the mechanisms whereby oncoviruses cause cancer are not fully understood, there are at least two potential ways. In the case of DNA viruses, the DNA strand of the virus can insert itself directly into one of the chromosomes, thereby causing a mutation that leads to cancer. In the case of RNA viruses, some of these viruses carry with them an enzyme called *reverse transcriptase* that causes DNA to be transcribed from the RNA. The transcribed DNA then inserts itself into the animal cell genome, leading to cancer. \n**Invasive Characteristic of the Cancer Cell.** The major differences between a cancer cell and a normal cell are as follows: \n- 1. The cancer cell does not respect usual cellular growth limits because these cells presumably do not require all the same growth factors that are necessary to cause growth of normal cells.\n- 2. Cancer cells are often far less adhesive to one another than are normal cells. Therefore, they tend to wander through the tissues, enter the blood stream, and be transported all through the body, where they form nidi for numerous new cancerous growths.\n- 3. Some cancers also produce *angiogenic factors* that cause many new blood vessels to grow into the cancer, thus supplying the nutrients required for cancer growth. \n**Why Do Cancer Cells Kill?** Cancer tissue competes with normal tissues for nutrients. Because cancer cells continue to proliferate indefinitely, with their numbers multiplying every day, cancer cells soon demand essentially all the nutrition available to the body or to an essential part of the body. As a result, normal tissues gradually sustain nutritive death. \nSome cancers cause disruption of vital organ functions. For example, a lung cancer might replace healthy tissue to the extent that the lungs cannot absorb enough oxygen to maintain tissues in the rest of the body. \n#### Bibliography \nAlberts B, Johnson A, Lewis J, et al: Molecular Biology of the Cell, 6th ed. New York: Garland Science 2014. \n- Armanios M: Telomeres and age-related disease: how telomere biology informs clinical paradigms. J Clin Invest 123:996, 2013.\n- Bickmore WA, van Steensel B: Genome architecture: domain organization of interphase chromosomes. Cell 152:1270, 2013.\n- Calcinotto A, Kohli J, Zagato E, Pellegrini L, Demaria M, Alimonti A: Cellular senescence: aging, cancer, and injury. Physiol Rev 99:1047-1078, 2019.\n- Clift D, Schuh M: Restarting life: fertilization and the transition from meiosis to mitosis. Nat Rev Mol Cell Biol 14:549, 2013.\n- Coppola CJ, C Ramaker R, Mendenhall EM: Identification and function of enhancers in the human genome. Hum Mol Genet 25(R2):R190-R197, 2016.\n- Feinberg AP: The key role of epigenetics in human disease prevention and mitigation. N Engl J Med 378:1323-1334, 2018.\n- Fyodorov DV, Zhou BR, Skoultchi AI, Bai Y: Emerging roles of linker histones in regulating chromatin structure and function. Nat Rev Mol Cell Biol 19:192-206, 2018.\n- Haberle V, Stark A: Eukaryotic core promoters and the functional basis of transcription initiation. Nat Rev Mol Cell Biol 19:621-637, 2018.\n- Kaushik S, Cuervo AM: The coming of age of chaperone-mediated autophagy. Nat Rev Mol Cell Biol 19:365-381, 2018.\n- Krump NA, You J: Molecular mechanisms of viral oncogenesis in humans. Nat Rev Microbiol 16:684-698, 2018.\n- Leidal AM, Levine B, Debnath J: Autophagy and the cell biology of age-related disease. Nat Cell Biol 20:1338-1348, 2018.\n- Maciejowski J, de Lange T: Telomeres in cancer: tumour suppression and genome instability. Nat Rev Mol Cell Biol 18:175-186, 2017.\n- McKinley KL, Cheeseman IM: The molecular basis for centromere identity and function. Nat Rev Mol Cell Biol 17:16-29, 2016.\n- Monk D, Mackay DJG, Eggermann T, Maher ER, Riccio A: Genomic imprinting disorders: lessons on how genome, epigenome and environment interact. Nat Rev Genet 10:235, 2019.\n- Müller S, Almouzni G: Chromatin dynamics during the cell cycle at centromeres. Nat Rev Genet 18:192-208, 2017.\n- Nigg EA, Holland AJ: Once and only once: mechanisms of centriole duplication and their deregulation in disease. Nat Rev Mol Cell Biol 19:297-312, 2018.\n- Palozola KC, Lerner J, Zaret KS: A changing paradigm of transcriptional memory propagation through mitosis. Nat Rev Mol Cell Biol 20:55-64, 2019.\n- Perez MF, Lehner B: Intergenerational and transgenerational epigenetic inheritance in animals. Nat Cell Biol 21:143, 2019.\n- Prosser SL, Pelletier L: Mitotic spindle assembly in animal cells: a fine balancing act. Nat Rev Mol Cell Biol 18:187-201, 2017.\n- Schmid M, Jensen TH. Controlling nuclear RNA levels. Nat Rev Genet 19:518-529, 2018.\n- Treiber T, Treiber N, Meister G: Regulation of microRNA biogenesis and its crosstalk with other cellular pathways. Nat Rev Mol Cell Biol 20:5-20, 2019. \n",
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"Header 1": "**Genetic Control of Protein Synthesis, Cell Function, and [Cell](#page-34-1) Reproduction**",
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"page_content": "Physiology is the science that seeks to explain the physical and chemical mechanisms that are responsible for the origin, development, and progression of life. Each type of life, from the simplest virus to the largest tree or the complicated human being, has its own functional characteristics. Therefore, the vast field of physiology can be divided into viral physiology, bacterial physiology, cellular physiology, plant physiology, invertebrate physiology, vertebrate physiology, mammalian physiology, human physiology, and many more subdivisions. \n**Human Physiology.** The science of human physiology attempts to explain the specific characteristics and mechanisms of the human body that make it a living being. The fact that we remain alive is the result of complex control systems. Hunger makes us seek food, and fear makes us seek refuge. Sensations of cold make us look for warmth. Other forces cause us to seek fellowship and to reproduce. The fact that we are sensing, feeling, and knowledgeable beings is part of this automatic sequence of life; these special attributes allow us to exist under widely varying conditions that otherwise would make life impossible. \nHuman physiology links the basic sciences with medicine and integrates multiple functions of the cells, tissues, and organs into the functions of the living human being. This integration requires communication and coordination by a vast array of control systems that operate at every level—from the genes that program synthesis of molecules to the complex nervous and hormonal systems that coordinate functions of cells, tissues, and organs throughout the body. Thus, the coordinated functions of the human body are much more than the sum of its parts, and life in health, as well as in disease states, relies on this total function. Although the main focus of this book is on normal human physiology, we will also discuss, to some extent, *pathophysiology,* which is the study of disordered body function and the basis for clinical medicine. \n#### CELLS ARE THE LIVING UNITS OF THE BODY \nThe basic living unit of the body is the cell. Each tissue or organ is an aggregate of many different cells held together by intercellular supporting structures. \nEach type of cell is specially adapted to perform one or a few particular functions. For example, the red blood cells, numbering about 25 trillion in each person, transport oxygen from the lungs to the tissues. Although the red blood cells are the most abundant of any single type of cell in the body, there are also trillions of additional cells of other types that perform functions different from those of the red blood cell. The entire body, then, contains about 35 to 40 trillion human cells. \nThe many cells of the body often differ markedly from one another but all have certain basic characteristics that are alike. For example, oxygen reacts with carbohydrate, fat, and protein to release the energy required for all cells to function. Furthermore, the general chemical mechanisms for changing nutrients into energy are basically the same in all cells, and all cells deliver products of their chemical reactions into the surrounding fluids. \nAlmost all cells also have the ability to reproduce additional cells of their own type. Fortunately, when cells of a particular type are destroyed, the remaining cells of this type usually generate new cells until the supply is replenished. \n**Microorganisms Living in the Body Outnumber Human Cells.** In addition to human cells, trillions of microbes inhabit the body, living on the skin and in the mouth, gut, and nose. The gastrointestinal tract, for example, normally contains a complex and dynamic population of 400 to 1000 species of microorganisms that outnumber our human cells. Communities of microorganisms that inhabit the body, often called *microbiota,* can cause diseases, but most of the time they live in harmony with their human hosts and provide vital functions that are essential for survival of their hosts. Although the importance of gut microbiota in the digestion of foodstuffs is widely recognized, additional roles for the body's microbes in nutrition, immunity, and other functions are just beginning to be appreciated and represent an intensive area of biomedical research. \n#### EXTRACELLULAR FLUID—THE \"INTERNAL ENVIRONMENT\" \nAbout 50% to 70% of the adult human body is fluid, mainly a water solution of ions and other substances. Although most of this fluid is inside the cells and is called *intracellular fluid,* about one-third is in the spaces outside the cells and is called *extracellular fluid.* This extracellular fluid is in constant motion throughout the body. It is transported rapidly in the circulating blood and then mixed between the blood and tissue fluids by diffusion through the capillary walls. \nIn the extracellular fluid are the ions and nutrients needed by the cells to maintain life. Thus, all cells live in essentially the same environment—the extracellular fluid. For this reason, the extracellular fluid is also called the *internal environment* of the body, or the *milieu intérieur,* a term introduced by the great 19th-century French physiologist Claude Bernard (1813–1878). \nCells are capable of living and performing their special functions as long as the proper concentrations of oxygen, glucose, different ions, amino acids, fatty substances, and other constituents are available in this internal environment. \n#### **Differences in Extracellular and Intracellular Fluids.** \nThe extracellular fluid contains large amounts of sodium, chloride, and bicarbonate ions plus nutrients for the cells, such as oxygen, glucose, fatty acids, and amino acids. It also contains carbon dioxide that is being transported from the cells to the lungs to be excreted, plus other cellular waste products that are being transported to the kidneys for excretion. \nThe intracellular fluid contains large amounts of potassium, magnesium, and phosphate ions instead of the sodium and chloride ions found in the extracellular fluid. Special mechanisms for transporting ions through the cell membranes maintain the ion concentration differences between the extracellular and intracellular fluids. These transport processes are discussed in Chapter 4. \n#### HOMEOSTASIS—MAINTENANCE OF A NEARLY CONSTANT INTERNAL ENVIRONMENT \nIn 1929, the American physiologist Walter Cannon (1871–1945) coined the term *homeostasis* to describe the *maintenance of nearly constant conditions in the internal environment*. Essentially, all organs and tissues of the body perform functions that help maintain these relatively constant conditions. For example, the lungs provide oxygen to the extracellular fluid to replenish the oxygen used by the cells, the kidneys maintain constant ion concentrations, and the gastrointestinal system provides nutrients while eliminating waste from the body. \nThe various ions, nutrients, waste products, and other constituents of the body are normally regulated within a range of values, rather than at fixed values. For some of the body's constituents, this range is extremely small. Variations in the blood hydrogen ion concentration, for example, are normally less than 5 *nanomoles/L* (0.000000005 moles/L). The blood sodium concentration is also tightly regulated, normally varying only a few *millimoles* per liter, even with large changes in sodium intake, but these variations of sodium concentration are at least 1 million times greater than for hydrogen ions. \nPowerful control systems exist for maintaining concentrations of sodium and hydrogen ions, as well as for most of the other ions, nutrients, and substances in the body at levels that permit the cells, tissues, and organs to perform their normal functions, despite wide environmental variations and challenges from injury and diseases. \nMuch of this text is concerned with how each organ or tissue contributes to homeostasis. Normal body functions require integrated actions of cells, tissues, organs, and multiple nervous, hormonal, and local control systems that together contribute to homeostasis and good health. \n**Homeostatic Compensations in Diseases.** *Disease* is often considered to be a state of disrupted homeostasis. However, even in the presence of disease, homeostatic mechanisms continue to operate and maintain vital functions through multiple compensations. In some cases, these compensations may lead to major deviations of the body's functions from the normal range, making it difficult to distinguish the primary cause of the disease from the compensatory responses. For example, diseases that impair the kidneys' ability to excrete salt and water may lead to high blood pressure, which initially helps return excretion to normal so that a balance between intake and renal excretion can be maintained. This balance is needed to maintain life, but, over long periods of time, the high blood pressure can damage various organs, including the kidneys, causing even greater increases in blood pressure and more renal damage. Thus, homeostatic compensations that ensue after injury, disease, or major environmental challenges to the body may represent trade-offs that are necessary to maintain vital body functions but, in the long term, contribute to additional abnormalities of body function. The discipline of *pathophysiology* seeks to explain how the various physiological processes are altered in diseases or injury. \nThis chapter outlines the different functional systems of the body and their contributions to homeostasis. We then briefly discuss the basic theory of the body's control systems that allow the functional systems to operate in support of one another. \n#### **EXTRACELLULAR FLUID TRANSPORT AND MIXING SYSTEM—THE BLOOD CIRCULATORY SYSTEM** \nExtracellular fluid is transported through the body in two stages. The first stage is movement of blood through the body in the blood vessels. The second is movement of fluid between the blood capillaries and the *intercellular spaces* between the tissue cells. \n**[Figure 1-1](#page-9-0)** shows the overall circulation of blood. All the blood in the circulation traverses the entire circuit an average \n \n**Figure 1-1.** General organization of the circulatory system. \nof once each minute when the body is at rest and as many as six times each minute when a person is extremely active. \nAs blood passes through blood capillaries, continual exchange of extracellular fluid occurs between the plasma portion of the blood and the interstitial fluid that fills the intercellular spaces. This process is shown in **Figure 1-2**. The capillary walls are permeable to most molecules in the blood plasma, with the exception of plasma proteins, which are too large to pass through capillaries readily. Therefore, large amounts of fluid and its dissolved constituents *diffuse* back and forth between the blood and the tissue spaces, as shown by the arrows in **Figure 1-2**. \nThis process of diffusion is caused by kinetic motion of the molecules in the plasma and the interstitial fluid. \n \n**Figure 1-2.** Diffusion of fluid and dissolved constituents through the capillary walls and interstitial spaces. \nThat is, the fluid and dissolved molecules are continually moving and bouncing in all directions in the plasma and fluid in the intercellular spaces, as well as through capillary pores. Few cells are located more than 50 micrometers from a capillary, which ensures diffusion of almost any substance from the capillary to the cell within a few seconds. Thus, the extracellular fluid everywhere in the body—both that of the plasma and that of the interstitial fluid—is continually being mixed, thereby maintaining homogeneity of extracellular fluid throughout the body.\n\n**Respiratory System. Figure 1-1** shows that each time blood passes through the body, it also flows through the lungs. The blood picks up *oxygen* in alveoli, thus acquiring the oxygen needed by cells. The membrane between the alveoli and the lumen of the pulmonary capillaries, the *alveolar membrane*, is only 0.4 to 2.0 micrometers thick, and oxygen rapidly diffuses by molecular motion through this membrane into the blood. \n**Gastrointestinal Tract.** A large portion of the blood pumped by the heart also passes through the walls of the gastrointestinal tract. Here different dissolved nutrients, including *carbohydrates*, *fatty acids*, and *amino acids*, are absorbed from ingested food into the extracellular fluid of the blood. \nLiver and Other Organs That Perform Primarily Metabolic Functions. Not all substances absorbed from the gastrointestinal tract can be used in their absorbed form by the cells. The liver changes the chemical compositions of many of these substances to more usable forms, and other tissues of the body—fat cells, gastrointestinal mucosa, kidneys, and endocrine glands—help modify the absorbed substances or store them until they are needed. The liver also eliminates certain waste products produced in the body and toxic substances that are ingested. \n**Musculoskeletal System.** How does the musculoskeletal system contribute to homeostasis? The answer is obvious and simple. Were it not for the muscles, the body could not move to obtain the foods required for nutrition. The musculoskeletal system also provides motility for protection against adverse surroundings, without which the entire body, along with its homeostatic mechanisms, could be destroyed. \n#### **REMOVAL OF METABOLIC END PRODUCTS** \n**Removal of Carbon Dioxide by the Lungs.** At the same time that blood picks up oxygen in the lungs, *carbon dioxide* is released from the blood into lung alveoli; the respiratory movement of air into and out of the lungs carries carbon dioxide to the atmosphere. Carbon dioxide is the most abundant of all the metabolism products. \n**Kidneys.** Passage of blood through the kidneys removes most of the other substances from the plasma besides carbon dioxide that are not needed by cells. These substances include different end products of cellular metabolism, such as urea and uric acid; they also include excesses of ions and water from the food that accumulate in the extracellular fluid. \nThe kidneys perform their function first by filtering large quantities of plasma through the glomerular capillaries into the tubules and then reabsorbing into the blood substances needed by the body, such as glucose, amino acids, appropriate amounts of water, and many of the ions. Most of the other substances that are not needed by the body, especially metabolic waste products such as urea and creatinine, are reabsorbed poorly and pass through the renal tubules into the urine. \n**Gastrointestinal Tract.** Undigested material that enters the gastrointestinal tract and some waste products of metabolism are eliminated in the feces. \n**Liver.** Among the many functions of the liver is detoxification or removal of ingested drugs and chemicals. The liver secretes many of these wastes into the bile to be eventually eliminated in the feces. \n#### **REGULATION OF BODY FUNCTIONS** \n**Nervous System.** The nervous system is composed of three major parts—the *sensory input portion,* the *central nervous system* (or *integrative portion*), and the *motor output portion.* Sensory receptors detect the state of the body and its surroundings. For example, receptors in the skin alert us whenever an object touches the skin. The eyes are sensory organs that give us a visual image of the surrounding area. The ears are also sensory organs. The central nervous system is composed of the brain and spinal cord. The brain stores information, generates thoughts, creates ambition, and determines reactions that the body performs in response to the sensations. Appropriate signals are then transmitted through the motor output portion of the nervous system to carry out one's desires. \nAn important segment of the nervous system is called the *autonomic system.* It operates at a subconscious level and controls many functions of internal organs, including the level of pumping activity by the heart, movements of the gastrointestinal tract, and secretion by many of the body's glands. \n**Hormone Systems.** Located in the body are *endocrine glands,* organs and tissues that secrete chemical substances called *hormones.* Hormones are transported in the extracellular fluid to other parts of the body to help regulate cellular function. For example, *thyroid hormone* increases the rates of most chemical reactions in all cells, thus helping set the tempo of bodily activity. *Insulin* controls glucose metabolism, *adrenocortical hormones* control sodium and potassium ions and protein metabolism, and *parathyroid hormone* controls bone calcium and phosphate. Thus, the hormones provide a regulatory system that complements the nervous system. The nervous system controls many muscular and secretory activities of the body, whereas the hormonal system regulates many metabolic functions. The nervous and hormonal systems normally work together in a coordinated manner to control essentially all the organ systems of the body. \n#### **PROTECTION OF THE BODY** \n**Immune System.** The immune system includes white blood cells, tissue cells derived from white blood cells, the thymus, lymph nodes, and lymph vessels that protect the body from pathogens such as bacteria, viruses, parasites, and fungi. The immune system provides a mechanism for the body to carry out the following: (1) distinguish its own cells from harmful foreign cells and substances; and (2) destroy the invader by *phagocytosis* or by producing *sensitized lymphocytes* or specialized proteins (e.g., *antibodies*) that destroy or neutralize the invader. \n**Integumentary System.** The skin and its various appendages (including the hair, nails, glands, and other structures) cover, cushion, and protect the deeper tissues and organs of the body and generally provide a boundary between the body's internal environment and the outside world. The integumentary system is also important for temperature regulation and excretion of wastes, and it provides a sensory interface between the body and the external environment. The skin generally comprises about 12% to 15% of body weight. \n#### **REPRODUCTION** \nAlthough reproduction is sometimes not considered a homeostatic function, it helps maintain homeostasis by generating new beings to take the place of those that are dying. This may sound like a permissive usage of the term *homeostasis,* but it illustrates that in the final analysis, essentially all body structures are organized to help maintain the automaticity and continuity of life. \n#### CONTROL SYSTEMS OF THE BODY \nThe human body has thousands of control systems. Some of the most intricate of these systems are genetic control systems that operate in all cells to help regulate intracellular and extracellular functions. This subject is discussed in Chapter 3. \nMany other control systems operate *within the organs* to regulate functions of the individual parts of the organs; others operate throughout the entire body *to control the interrelationships between the organs.* For example, the respiratory system, operating in association with the nervous system, regulates the concentration of carbon dioxide in the extracellular fluid. The liver and pancreas control glucose concentration in the extracellular fluid, and the kidneys regulate concentrations of hydrogen, sodium, potassium, phosphate, and other ions in the extracellular fluid. \n#### **EXAMPLES OF CONTROL MECHANISMS** \n**Regulation of Oxygen and Carbon Dioxide Concentrations in the Extracellular Fluid.** Because oxygen is one of the major substances required for chemical reactions in cells, the body has a special control mechanism to maintain an almost exact and constant oxygen concentration in the extracellular fluid. This mechanism depends principally on the chemical characteristics of *hemoglobin,* which is present in red blood cells. Hemoglobin combines with oxygen as the blood passes through the lungs. Then, as the blood passes through the tissue capillaries, hemoglobin, because of its own strong chemical affinity for oxygen, does not release oxygen into the tissue fluid if too much oxygen is already there. However, if oxygen concentration in the tissue fluid is too low, sufficient oxygen is released to re-establish an adequate concentration. Thus, regulation of oxygen concentration in the tissues relies to a great extent on the chemical characteristics of hemoglobin. This regulation is called the *oxygen-buffering function of hemoglobin.* \nCarbon dioxide concentration in the extracellular fluid is regulated in a much different way. Carbon dioxide is a major end product of oxidative reactions in cells. If all the carbon dioxide formed in the cells continued to accumulate in the tissue fluids, all energy-giving reactions of the cells would cease. Fortunately, a higher than normal carbon dioxide concentration in the blood *excites the respiratory center,* causing a person to breathe rapidly and deeply. This deep rapid breathing increases expiration of carbon dioxide and, therefore, removes excess carbon dioxide from the blood and tissue fluids. This process continues until the concentration returns to normal. \n \n**Figure 1-3.** Negative feedback control of arterial pressure by the arterial baroreceptors. Signals from the sensor (baroreceptors) are sent to the medulla of the brain, where they are compared with a reference set point. When arterial pressure increases above normal, this abnormal pressure increases nerve impulses from the baroreceptors to the medulla of the brain, where the input signals are compared with the set point, generating an error signal that leads to decreased sympathetic nervous system activity. Decreased sympathetic activity causes dilation of blood vessels and reduced pumping activity of the heart, which return arterial pressure toward normal. \n**Regulation of Arterial Blood Pressure.** Several systems contribute to arterial blood pressure regulation. One of these, the *baroreceptor system,* is an excellent example of a rapidly acting control mechanism (**[Figure 1-3](#page-11-0)**). In the walls of the bifurcation region of the carotid arteries in the neck, and also in the arch of the aorta in the thorax, are many nerve receptors called *baroreceptors* that are stimulated by stretch of the arterial wall. When arterial pressure rises too high, the baroreceptors send barrages of nerve impulses to the medulla of the brain. Here, these impulses inhibit the *vasomotor center,* which in turn decreases the number of impulses transmitted from the vasomotor center through the sympathetic nervous system to the heart and blood vessels. Lack of these impulses causes diminished pumping activity by the heart and dilation of peripheral blood vessels, allowing increased blood flow through the vessels. Both these effects decrease the arterial pressure, moving it back toward normal. \nConversely, a decrease in arterial pressure below normal relaxes the stretch receptors, allowing the vasomotor center to become more active than usual, thereby causing vasoconstriction and increased heart pumping. The initial decrease in arterial pressure thus initiates negative feedback mechanisms that raise arterial pressure back toward normal. \n#### **Normal Ranges and Physical Characteristics of Important Extracellular Fluid Constituents** \n**[Table 1-1](#page-12-0)** lists some important constituents and physical characteristics of extracellular fluid, along with their normal values, normal ranges, and maximum limits without causing death. Note the narrowness of the normal range for each one. Values outside these ranges are often caused by illness, injury, or major environmental challenges. \n| Constituent | Normal Value | Normal Range | Approximate Short-Term Nonlethal Limit | Unit |\n|-------------------------|--------------|----------------|----------------------------------------|---------|\n| Oxygen (venous) | 40 | 25–40 | 10–1000 | mm Hg |\n| Carbon dioxide (venous) | 45 | 41–51 | 5–80 | mm Hg |\n| Sodium ion | 142 | 135–145 | 115–175 | mmol/L |\n| Potassium ion | 4.2 | 3.5-5.3 | 1.5–9.0 | mmol/L |\n| Calcium ion | 1.2 | 1.0-1.4 | 0.5–2.0 | mmol/L |\n| Chloride ion | 106 | 98–108 | 70–130 | mmol/L |\n| Bicarbonate ion | 24 | 22–29 | 8–45 | mmol/L |\n| Glucose | 90 | 70–115 | 20–1500 | mg/dl |\n| Body temperature | 98.4 (37.0) | 98-98.8 (37.0) | 65–110 (18.3–43.3) | °F (°C) |\n| Acid-base (venous) | 7.4 | 7.3–7.5 | 6.9–8.0 | рН | \nTable 1-1 Important Constituents and Physical Characteristics of Extracellular Fluid \nMost important are the limits beyond which abnormalities can cause death. For example, an increase in the body temperature of only 11°F (7°C) above normal can lead to a vicious cycle of increasing cellular metabolism that destroys the cells. Note also the narrow range for acid-base balance in the body, with a normal pH value of 7.4 and lethal values only about 0.5 on either side of normal. Whenever the potassium ion concentration decreases to less than one-third normal, paralysis may result from the inability of the nerves to carry signals. Alternatively, if potassium ion concentration increases to two or more times normal, the heart muscle is likely to be severely depressed. Also, when the calcium ion concentration falls below about one-half normal, a person is likely to experience tetanic contraction of muscles throughout the body because of the spontaneous generation of excess nerve impulses in peripheral nerves. When the glucose concentration falls below one-half normal, a person frequently exhibits extreme mental irritability and sometimes even has convulsions. \nThese examples should give one an appreciation for the necessity of the vast numbers of control systems that keep the body operating in health. In the absence of any one of these controls, serious body malfunction or death can result. \n#### **CHARACTERISTICS OF CONTROL SYSTEMS** \nThe aforementioned examples of homeostatic control mechanisms are only a few of the many thousands in the body, all of which have some common characteristics, as explained in this section.\n\nMost control systems of the body act by *negative feed-back*, which can be explained by reviewing some of the homeostatic control systems mentioned previously. In the regulation of carbon dioxide concentration, a high concentration of carbon dioxide in the extracellular fluid increases pulmonary ventilation. This, in turn, decreases \nthe extracellular fluid carbon dioxide concentration because the lungs expire greater amounts of carbon dioxide from the body. Thus, the high concentration of carbon dioxide initiates events that decrease the concentration toward normal, which is *negative* to the initiating stimulus. Conversely, a carbon dioxide concentration that falls too low results in feedback to increase the concentration. This response is also negative to the initiating stimulus. \nIn the arterial pressure—regulating mechanisms, a high pressure causes a series of reactions that promote reduced pressure, or a low pressure causes a series of reactions that promote increased pressure. In both cases, these effects are negative with respect to the initiating stimulus. \nTherefore, in general, if some factor becomes excessive or deficient, a control system initiates *negative feedback*, which consists of a series of changes that return the factor toward a certain mean value, thus maintaining homeostasis. \nGain of a Control System. The degree of effectiveness with which a control system maintains constant conditions is determined by the gain of negative feedback. For example, let us assume that a large volume of blood is transfused into a person whose baroreceptor pressure control system is not functioning, and the arterial pressure rises from the normal level of 100 mm Hg up to 175 mm Hg. Then, let us assume that the same volume of blood is injected into the same person when the baroreceptor system is functioning, and this time the pressure increases by only 25 mm Hg. Thus, the feedback control system has caused a \"correction\" of -50 mm Hg, from 175 mm Hg to 125 mm Hg. There remains an increase in pressure of +25 mm Hg, called the \"error,\" which means that the control system is not 100% effective in preventing change. The gain of the system is then calculated by using the following formula: \n$$Gain = \\frac{Correction}{Frror}$$ \nThus, in the baroreceptor system example, the correction is -50 mm Hg, and the error persisting is +25 mm Hg. Therefore, the gain of the person's baroreceptor system \n \n**Figure 1-4.** Recovery of heart pumping caused by negative feedback after 1 liter of blood is removed from the circulation. Death is caused by positive feedback when 2 liters or more blood is removed. \nfor control of arterial pressure is −50 divided by +25, or −2. That is, a disturbance that increases or decreases the arterial pressure does so only one-third as much as would occur if this control system were not present. \nThe gains of some other physiological control systems are much greater than that of the baroreceptor system. For example, the gain of the system controlling internal body temperature when a person is exposed to moderately cold weather is about −33. Therefore, one can see that the temperature control system is much more effective than the baroreceptor pressure control system. \n#### **Positive Feedback May Cause Vicious Cycles and Death** \nWhy do most control systems of the body operate by negative feedback rather than by positive feedback? If one considers the nature of positive feedback, it is obvious that positive feedback leads to instability rather than stability and, in some cases, can cause death. \n**[Figure 1-4](#page-13-0)** shows an example in which death can ensue from positive feedback. This figure depicts the pumping effectiveness of the heart, showing the heart of a healthy human pumping about 5 liters of blood per minute. If the person suddenly bleeds a total of 2 liters, the amount of blood in the body is decreased to such a low level that not enough blood is available for the heart to pump effectively. As a result, the arterial pressure falls, and the flow of blood to the heart muscle through the coronary vessels diminishes. This scenario results in weakening of the heart, further diminished pumping, a further decrease in coronary blood flow, and still more weakness of the heart; the cycle repeats itself again and again until death occurs. Note that each cycle in the feedback results in further weakening of the heart. In other words, the initiating stimulus causes more of the same, which is *positive feedback.* \nPositive feedback is sometimes known as a \"vicious cycle,\" but a mild degree of positive feedback can be overcome by the negative feedback control mechanisms of the body, and the vicious cycle then fails to develop. For example, if the person in the aforementioned example bleeds only 1 liter instead of 2 liters, the normal negative feedback mechanisms for controlling cardiac output and arterial pressure can counterbalance the positive feedback and the person can recover, as shown by the dashed curve of **[Figure 1-4](#page-13-0)**. \n**Positive Feedback Can Sometimes Be Useful.** The body sometimes uses positive feedback to its advantage. Blood clotting is an example of a valuable use of positive feedback. When a blood vessel is ruptured, and a clot begins to form, multiple enzymes called *clotting factors* are activated within the clot. Some of these enzymes act on other inactivated enzymes of the immediately adjacent blood, thus causing more blood clotting. This process continues until the hole in the vessel is plugged and bleeding no longer occurs. On occasion, this mechanism can get out of hand and cause formation of unwanted clots. In fact, this is what initiates most acute heart attacks, which can be caused by a clot beginning on the inside surface of an atherosclerotic plaque in a coronary artery and then growing until the artery is blocked. \nChildbirth is another situation in which positive feedback is valuable. When uterine contractions become strong enough for the baby's head to begin pushing through the cervix, stretching of the cervix sends signals through the uterine muscle back to the body of the uterus, causing even more powerful contractions. Thus, the uterine contractions stretch the cervix, and cervical stretch causes stronger contractions. When this process becomes powerful enough, the baby is born. If they are not powerful enough, the contractions usually die out, and a few days pass before they begin again. \nAnother important use of positive feedback is for the generation of nerve signals. Stimulation of the membrane of a nerve fiber causes slight leakage of sodium ions through sodium channels in the nerve membrane to the fiber's interior. The sodium ions entering the fiber then change the membrane potential, which, in turn, causes more opening of channels, more change of potential, still more opening of channels, and so forth. Thus, a slight leak becomes an explosion of sodium entering the interior of the nerve fiber, which creates the nerve action potential. This action potential, in turn, causes electrical current to flow along the outside and inside of the fiber and initiates additional action potentials. This process continues until the nerve signal goes all the way to the end of the fiber. \nIn each case in which positive feedback is useful, the positive feedback is part of an overall negative feedback process. For example, in the case of blood clotting, the positive feedback clotting process is a negative feedback process for the maintenance of normal blood volume. Also, the positive feedback that causes nerve signals allows the nerves to participate in thousands of negative feedback nervous control systems. \n#### **More Complex Types of Control Systems—Feed-Forward and Adaptive Control** \nLater in this text, when we study the nervous system, we shall see that this system contains great numbers of interconnected control mechanisms. Some are simple feedback systems similar to those already discussed. Many are not. For example, some movements of the body occur so rapidly that there is not enough time for nerve signals to travel from the peripheral parts of the body all the way to the brain and then back to the periphery again to control the movement. Therefore, the brain uses a mechanism called *feed-forward control* to cause required muscle contractions. Sensory nerve signals from the moving parts apprise the brain about whether the movement is performed correctly. If not, the brain corrects the feed-forward signals that it sends to the muscles the *next* time the movement is required. Then, if still further correction is necessary, this process will be performed again for subsequent movements. This process is called *adaptive control.* Adaptive control, in a sense, is delayed negative feedback. \nThus, one can see how complex the feedback control systems of the body can be. A person's life depends on all of them. Therefore, much of this text is devoted to discussing these life-giving mechanisms. \n#### **PHYSIOLOGICAL VARIABILITY** \nAlthough some physiological variables, such as plasma concentrations of potassium, calcium, and hydrogen ions, are tightly regulated, others, such as body weight and adiposity, show wide variation among different individuals and even in the same individual at different stages of life. Blood pressure, cardiac pumping, metabolic rate, nervous system activity, hormones, and other physiological variables change throughout the day as we move about and engage in normal daily activities. Therefore, when we discuss \"normal\" values, it is with the understanding that many of the body's control systems are constantly reacting to perturbations, and that variability may exist among different individuals, depending on body weight and height, diet, age, sex, environment, genetics, and other factors. \nFor simplicity, discussion of physiological functions often focuses on the \"average\" 70-kg young, lean male. However, the American male no longer weighs an average of 70 kg; he now weighs over 88 kg, and the average American female weighs over 76 kg, more than the average man in the 1960s. Body weight has also increased substantially in most other industrialized countries during the past 40 to 50 years. \nExcept for reproductive and hormonal functions, many other physiological functions and normal values are often discussed in terms of male physiology. However, there are clearly differences in male and female physiology beyond the obvious differences that relate to reproduction. These differences can have important consequences for understanding normal physiology as well as for treatment of diseases. \nAge-related and ethnic or racial differences in physiology also have important influences on body composition, physiological control systems, and pathophysiology of diseases. For example, in a lean young male the total body water is about 60% of body weight. As a person grows and ages, this percentage gradually decreases, partly because aging is usually associated with declining skeletal muscle mass and increasing fat mass. Aging may also cause a decline in the function and effectiveness of some organs and physiological control systems. \nThese sources of physiological variability—sex differences, aging, ethnic, and racial—are complex but important considerations when discussing normal physiology and the pathophysiology of diseases. \n#### SUMMARY—AUTOMATICITY OF THE BODY \nThe main purpose of this chapter has been to discuss briefly the overall organization of the body and the means whereby the different parts of the body operate in harmony. To summarize, the body is actually a *social order of about 35 to 40 trillion cells* organized into different functional structures, some of which are called *organs.* Each functional structure contributes its share to the maintenance of homeostasis in the extracellular fluid, which is called the *internal environment.* As long as normal conditions are maintained in this internal environment, the cells of the body continue to live and function properly. Each cell benefits from homeostasis and, in turn, each cell contributes its share toward the maintenance of homeostasis. This reciprocal interplay provides continuous automaticity of the body until one or more functional systems lose their ability to contribute their share of function. When this happens, all the cells of the body suffer. Extreme dysfunction leads to death; moderate dysfunction leads to sickness. \n#### Bibliography \nAdolph EF: Physiological adaptations: hypertrophies and superfunctions. Am Sci 60:608, 1972. \nBentsen MA, Mirzadeh Z, Schwartz MW: Revisiting how the brain senses glucose-and why. Cell Metab 29:11, 2019. \nBernard C: Lectures on the Phenomena of Life Common to Animals and Plants. Springfield, IL: Charles C Thomas, 1974. \nCannon WB: Organization for physiological homeostasis. Physiol Rev 9:399, 1929. \nChien S: Mechanotransduction and endothelial cell homeostasis: the wisdom of the cell. Am J Physiol Heart Circ Physiol 292:H1209, 2007. \nDiBona GF: Physiology in perspective: the wisdom of the body. Neural control of the kidney. Am J Physiol Regul Integr Comp Physiol 289:R633, 2005. \nDickinson MH, Farley CT, Full RJ, et al: How animals move: an integrative view. Science 288:100, 2000. \nEckel-Mahan K, Sassone-Corsi P: Metabolism and the circadian clock converge. Physiol Rev 93:107, 2013. \n- Guyton AC: Arterial Pressure and Hypertension. Philadelphia: WB Saunders, 1980.\n- Herman MA, Kahn BB: Glucose transport and sensing in the maintenance of glucose homeostasis and metabolic harmony. J Clin Invest 116:1767, 2006.\n- Kabashima K, Honda T, Ginhoux F, Egawa G: The immunological anatomy of the skin. Nat Rev Immunol 19:19, 2019.\n- Khramtsova EA, Davis LK, Stranger BE: The role of sex in the genomics of human complex traits. Nat Rev Genet 20: 173, 2019.\n- Kim KS, Seeley RJ, Sandoval DA: Signalling from the periphery to the brain that regulates energy homeostasis. Nat Rev Neurosci 19:185, 2018.\n- Nishida AH, Ochman H: A great-ape view of the gut microbiome. Nat Rev Genet 20:185, 2019.\n- Orgel LE: The origin of life on the earth. Sci Am 271:76,1994.\n- Reardon C, Murray K, Lomax AE: Neuroimmune communication in health and disease. Physiol Rev 98:2287-2316, 2018.\n- Sender R, Fuchs S, Milo R: Revised estimates for the number of human and bacteria cells in the body. PLoS Biol 14(8):e1002533, 2016.\n- Smith HW: From Fish to Philosopher. New York: Doubleday, 1961. \n\n\nEach of the trillions of cells in a human being is a living structure that can survive for months or years, provided its surrounding fluids contain appropriate nutrients. Cells are the building blocks of the body, providing structure for the body's tissues and organs, ingesting nutrients and converting them to energy, and performing specialized functions. Cells also contain the body's hereditary code, which controls the substances synthesized by the cells and permits them to make copies of themselves. \n#### ORGANIZATION OF THE CELL \nA schematic drawing of a typical cell, as seen by the light microscope, is shown in **[Figure 2-1](#page-16-0)**. Its two major parts are the *nucleus* and the *cytoplasm.* The nucleus is separated from the cytoplasm by a *nuclear membrane,* and the cytoplasm is separated from the surrounding fluids by a *cell membrane,* also called the *plasma membrane.* \nThe different substances that make up the cell are collectively called *protoplasm.* Protoplasm is composed mainly of five basic substances—water, electrolytes, proteins, lipids, and carbohydrates. \n**Water.** Most cells, except for fat cells, are comprised mainly of water in a concentration of 70% to 85%. Many cellular chemicals are dissolved in the water. Others are suspended in the water as solid particulates. Chemical reactions take place among the dissolved chemicals or at the surfaces of the suspended particles or membranes. \n**Ions.** Important ions in the cell include *potassium, magnesium, phosphate, sulfate, bicarbonate,* and smaller quantities of *sodium, chloride,* and *calcium.* These ions are all discussed in Chapter 4, which considers the interrelations between the intracellular and extracellular fluids. \nThe ions provide inorganic chemicals for cellular reactions and are necessary for the operation of some cellular control mechanisms. For example, ions acting at the cell membrane are required for the transmission of electrochemical impulses in nerve and muscle fibers. \n**Proteins.** After water, the most abundant substances in most cells are proteins, which normally constitute 10% to 20% of the cell mass. These proteins can be divided into two types, *structural proteins* and *functional proteins.* \nStructural proteins are present in the cell mainly in the form of long filaments that are polymers of many individual protein molecules. A prominent use of such intracellular filaments is to form *microtubules,* which provide the cytoskeletons of cellular organelles such as cilia, nerve axons, the mitotic spindles of cells undergoing mitosis, and a tangled mass of thin filamentous tubules that hold the parts of the cytoplasm and nucleoplasm together in their respective compartments. Fibrillar proteins are found outside the cell, especially in the collagen and elastin fibers of connective tissue, and elsewhere, such as in blood vessel walls, tendons, and ligaments. \nThe *functional proteins* are usually composed of combinations of a few molecules in tubular-globular form. These proteins are mainly the *enzymes* of the cell and, in contrast to the fibrillar proteins, are often mobile in the cell fluid. Also, many of them are adherent to membranous structures inside the cell and catalyze specific intracellular chemical reactions. For example, the chemical reactions that split glucose into its component parts and then combine these with oxygen to form carbon dioxide and water while simultaneously providing energy for cellular function are all catalyzed by a series of protein enzymes. \n**Lipids.** Lipids are several types of substances that are grouped together because of their common property of being soluble in fat solvents. Especially important lipids \n \n**Figure 2-1.** Illustration of cell structures visible with a light microscope. \n \n**Figure 2-2.** Reconstruction of a typical cell, showing the internal organelles in the cytoplasm and nucleus. \nare *phospholipids* and *cholesterol,* which together constitute only about 2% of the total cell mass. Phospholipids and cholesterol are mainly insoluble in water and therefore are used to form the cell membrane and intracellular membrane barriers that separate the different cell compartments. \nIn addition to phospholipids and cholesterol, some cells contain large quantities of *triglycerides,* also called *neutral fats.* In *fat cells (adipocytes),* triglycerides often account for as much as 95% of the cell mass. The fat stored in these cells represents the body's main storehouse of energy-giving nutrients that can later be used to provide energy wherever it is needed in the body. \n**Carbohydrates.** Carbohydrates play a major role in cell nutrition and, as parts of glycoprotein molecules, have structural functions. Most human cells do not maintain large stores of carbohydrates; the amount usually averages only about 1% of their total mass but increases to as much as 3% in muscle cells and, occasionally, to 6% in liver cells. However, carbohydrate in the form of dissolved glucose is always present in the surrounding extracellular fluid so that it is readily available to the cell. Also, a small amount of carbohydrate is stored in cells as *glycogen,* an insoluble polymer of glucose that can be depolymerized and used rapidly to supply the cell's energy needs. \n#### CELL STRUCTURE \nThe cell contains highly organized physical structures called *intracellular organelles,* which are critical for cell function. For example, without one of the organelles, the *mitochondria,* more than 95% of the cell's energy release from nutrients would cease immediately. The most important organelles and other structures of the cell are shown in **[Figure 2-2](#page-17-0)**. \n#### **MEMBRANOUS STRUCTURES OF THE CELL** \nMost organelles of the cell are covered by membranes composed primarily of lipids and proteins. These membranes include the *cell membrane, nuclear membrane, membrane of the endoplasmic reticulum,* and *membranes of the mitochondria, lysosomes,* and *Golgi apparatus.* \n \n**Figure 2-3.** Structure of the cell membrane showing that it is composed mainly of a lipid bilayer of phospholipid molecules, but with large numbers of protein molecules protruding through the layer. Also, carbohydrate moieties are attached to the protein molecules on the outside of the membrane and to additional protein molecules on the inside. \nThe lipids in membranes provide a barrier that impedes movement of water and water-soluble substances from one cell compartment to another because water is not soluble in lipids. However, protein molecules often penetrate all the way through membranes, thus providing specialized pathways, often organized into actual *pores,* for passage of specific substances through membranes. Also, many other membrane proteins are *enzymes,* which catalyze a multitude of different chemical reactions, discussed here and in subsequent chapters. \n#### **Cell Membrane** \nThe cell membrane (also called the *plasma membrane*) envelops the cell and is a thin, pliable, elastic structure only 7.5 to 10 nanometers thick. It is composed almost entirely of proteins and lipids. The approximate composition is 55% proteins, 25% phospholipids, 13% cholesterol, 4% other lipids, and 3% carbohydrates. \n**The Cell Membrane Lipid Barrier Impedes Penetration by Water-Soluble Substances. [Figure 2-3](#page-18-0)** shows the structure of the cell membrane. Its basic structure is a *lipid bilayer,* which is a thin, double-layered film of lipids—each layer only one molecule thick—that is continuous over the entire cell surface. Interspersed in this lipid film are large globular proteins. \nThe basic lipid bilayer is composed of three main types of lipids—*phospholipids, sphingolipids,* and *cholesterol*. Phospholipids are the most abundant cell membrane lipids. One end of each phospholipid molecule is *hydrophilic* and soluble in water*.* The other end is *hydrophobic* and soluble only in fats. The phosphate end of the phospholipid is hydrophilic, and the fatty acid portion is hydrophobic. \nBecause the hydrophobic portions of the phospholipid molecules are repelled by water but are mutually attracted to one another, they have a natural tendency to attach to one another in the middle of the membrane, as shown in **[Figure 2-3](#page-18-0)**. The hydrophilic phosphate portions then constitute the two surfaces of the complete cell membrane, in contact with *intracellular* water on the inside of the membrane and *extracellular* water on the outside surface. \nThe lipid layer in the middle of the membrane is impermeable to the usual water-soluble substances, such as ions, glucose, and urea. Conversely, fat-soluble substances, such as oxygen, carbon dioxide, and alcohol, can penetrate this portion of the membrane with ease. \nSphingolipids, derived from the amino alcohol *sphingosine*, also have hydrophobic and hydrophilic groups and are present in small amounts in the cell membranes, especially nerve cells. Complex sphingolipids in cell membranes are thought to serve several functions, including protection from harmful environmental factors, signal transmission, and adhesion sites for extracellular proteins. \nCholesterol molecules in membranes are also lipids because their steroid nuclei are highly fat-soluble. These molecules, in a sense, are dissolved in the bilayer of the membrane. They mainly help determine the degree of permeability (or impermeability) of the bilayer to watersoluble constituents of body fluids. Cholesterol controls much of the fluidity of the membrane as well. \n#### **Integral and Peripheral Cell Membrane Proteins.** \n**[Figure 2-3](#page-18-0)** also shows globular masses floating in the lipid bilayer. These membrane proteins are mainly *glycoproteins.* There are two types of cell membrane proteins, *integral proteins,* which protrude all the way through the membrane, and *peripheral proteins,* which are attached only to one surface of the membrane and do not penetrate all the way through. \nMany of the integral proteins provide structural *channels* (or *pores*) through which water molecules and watersoluble substances, especially ions, can diffuse between extracellular and intracellular fluids. These protein channels also have selective properties that allow preferential diffusion of some substances over others. \nOther integral proteins act as *carrier proteins* for transporting substances that otherwise could not penetrate the lipid bilayer. Sometimes, these carrier proteins even transport substances in the direction opposite to their electrochemical gradients for diffusion, which is called *active transport.* Still others act as *enzymes.* \nIntegral membrane proteins can also serve as *receptors* for water-soluble chemicals, such as peptide hormones, that do not easily penetrate the cell membrane. Interaction of cell membrane receptors with specific *ligands* that bind to the receptor causes conformational changes in the receptor protein. This process, in turn, enzymatically activates the intracellular part of the protein or induces interactions between the receptor and proteins in the cytoplasm that act as *second messengers,* relaying the signal from the extracellular part of the receptor to the interior of the cell. In this way, integral proteins spanning the cell membrane provide a means of conveying information about the environment to the cell interior. \nPeripheral protein molecules are often attached to integral proteins. These peripheral proteins function almost entirely as enzymes or as controllers of transport of substances through cell membrane *pores.* \n#### **Membrane Carbohydrates—The Cell \"Glycocalyx.\"** \nMembrane carbohydrates occur almost invariably in combination with proteins or lipids in the form of *glycoproteins* or *glycolipids.* In fact, most of the integral proteins are glycoproteins, and about one-tenth of the membrane lipid molecules are glycolipids. The *glyco-* portions of these molecules almost invariably protrude to the outside of the cell, dangling outward from the cell surface. Many other carbohydrate compounds, called *proteoglycans* which are mainly carbohydrates bound to small protein cores—are loosely attached to the outer surface of the cell as well. Thus, the entire outside surface of the cell often has a loose carbohydrate coat called the *glycocalyx.* \nThe carbohydrate moieties attached to the outer surface of the cell have several important functions: \n- 1. Many of them have a negative electrical charge, which gives most cells an overall negative surface charge that repels other negatively charged objects.\n- 2. The glycocalyx of some cells attaches to the glycocalyx of other cells, thus attaching cells to one another.\n- 3. Many of the carbohydrates act as *receptors* for binding hormones, such as insulin. When bound, this combination activates attached internal proteins that in turn activate a cascade of intracellular enzymes.\n- 4. Some carbohydrate moieties enter into immune reactions, as discussed in Chapter 35. \n#### **CYTOPLASM AND ITS ORGANELLES** \nThe cytoplasm is filled with minute and large dispersed particles and organelles. The jelly-like fluid portion of the cytoplasm in which the particles are dispersed is called *cytosol* and contains mainly dissolved proteins, electrolytes, and glucose. \nDispersed in the cytoplasm are neutral fat globules, glycogen granules, ribosomes, secretory vesicles, and five especially important organelles—the *endoplasmic reticulum,* the *Golgi apparatus, mitochondria, lysosomes,* and *peroxisomes.* \n#### **Endoplasmic Reticulum** \n**[Figure 2-2](#page-17-0)** shows the *endoplasmic reticulum,* a network of tubular structures called *cisternae* and flat vesicular structures in the cytoplasm. This organelle helps process molecules made by the cell and transports them to their specific destinations inside or outside the cell. The tubules and vesicles interconnect. Also, their walls are constructed of lipid bilayer membranes that contain large amounts of proteins, similar to the cell membrane. The total surface area of this structure in some cells—the liver cells, for example—can be as much as 30 to 40 times the cell membrane area. \nThe detailed structure of a small portion of endoplasmic reticulum is shown in **[Figure 2-4](#page-20-0)**. The space inside the tubules and vesicles is filled with *endoplasmic matrix,* a watery medium that is different from fluid in the cytosol outside the endoplasmic reticulum. Electron micrographs show that the space inside the endoplasmic reticulum is connected with the space between the two membrane surfaces of the nuclear membrane. \nSubstances formed in some parts of the cell enter the space of the endoplasmic reticulum and are then directed to other parts of the cell. Also, the vast surface area of this \n \n**Figure 2-4.** Structure of the endoplasmic reticulum. \nreticulum and the multiple enzyme systems attached to its membranes provide the mechanisms for a major share of the cell's metabolic functions. \n**Ribosomes and the Rough (Granular) Endoplasmic Reticulum.** Attached to the outer surfaces of many parts of the endoplasmic reticulum are large numbers of minute granular particles called *ribosomes.* Where these particles are present, the reticulum is called the *rough (granular) endoplasmic reticulum.* The ribosomes are composed of a mixture of RNA and proteins; they function to synthesize new protein molecules in the cell, as discussed later in this chapter and in Chapter 3. \n**Smooth (Agranular) Endoplasmic Reticulum.** Part of the endoplasmic reticulum has no attached ribosomes. This part is called the *smooth,* or *agranular, endoplasmic reticulum.* The smooth reticulum functions for the synthesis of lipid substances and for other processes of the cells promoted by intrareticular enzymes. \n#### **Golgi Apparatus** \nThe Golgi apparatus, shown in **[Figure 2-5](#page-20-1)**, is closely related to the endoplasmic reticulum. It has membranes similar to those of the smooth endoplasmic reticulum. The Golgi apparatus is usually composed of four or more stacked layers of thin, flat, enclosed vesicles lying near one side of the nucleus. This apparatus is prominent in secretory cells, where it is located on the side of the cell from which secretory substances are extruded. \nThe Golgi apparatus functions in association with the endoplasmic reticulum. As shown in **[Figure 2-5](#page-20-1)**, small *transport vesicles* (also called *endoplasmic reticulum vesicles* [*ER vesicles*]) continually pinch off from the endoplasmic reticulum and shortly thereafter fuse with the Golgi apparatus. In this way, substances entrapped in ER \n \n**Figure 2-5.** A typical Golgi apparatus and its relationship to the endoplasmic reticulum (ER) and the nucleus. \nvesicles are transported from the endoplasmic reticulum to the Golgi apparatus. The transported substances are then processed in the Golgi apparatus to form lysosomes, secretory vesicles, and other cytoplasmic components (discussed later in this chapter). \n#### **Lysosomes** \nLysosomes, shown in **[Figure 2-2](#page-17-0)**, are vesicular organelles that form by breaking off from the Golgi apparatus; they then disperse throughout the cytoplasm. The lysosomes provide an *intracellular digestive system* that allows the cell to digest the following: (1) damaged cellular structures; (2) food particles that have been ingested by the cell; and (3) unwanted matter such as bacteria. Lysosome are different in various cell types but are usually 250 to 750 nanometers in diameter. They are surrounded by typical lipid bilayer membranes and are filled with large numbers of small granules, 5 to 8 nanometers in diameter, which are protein aggregates of as many as 40 different *hydrolase (digestive) enzymes.* A hydrolytic enzyme is capable of splitting an organic compound into two or more parts by combining hydrogen from a water molecule with one part of the compound and combining the hydroxyl portion of the water molecule with the other part of the compound. For example, protein is hydrolyzed to form amino acids, glycogen is hydrolyzed to form glucose, and lipids are hydrolyzed to form fatty acids and glycerol. \nHydrolytic enzymes are highly concentrated in lysosomes. Ordinarily, the membrane surrounding the lysosome prevents the enclosed hydrolytic enzymes from coming into contact with other substances in the cell and therefore prevents their digestive actions. However, some conditions of the cell break the membranes of lysosomes, allowing release of the digestive enzymes. These enzymes then split the organic substances with which they come in contact into small, highly diffusible substances such as \n \n**Figure 2-6.** Secretory granules (secretory vesicles) in acinar cells of the pancreas. \namino acids and glucose. Some of the specific functions of lysosomes are discussed later in this chapter. \n#### **Peroxisomes** \nPeroxisomes are physically similar to lysosomes, but they are different in two important ways. First, they are believed to be formed by self-replication (or perhaps by budding off from the smooth endoplasmic reticulum) rather than from the Golgi apparatus. Second, they contain *oxidases* rather than hydrolases. Several of the oxidases are capable of combining oxygen with hydrogen ions derived from different intracellular chemicals to form hydrogen peroxide (H2O2). Hydrogen peroxide is a highly oxidizing substance and is used in association with *catalase,* another oxidase enzyme present in large quantities in peroxisomes, to oxidize many substances that might otherwise be poisonous to the cell. For example, about half the alcohol that a person drinks is detoxified into acetaldehyde by the peroxisomes of the liver cells in this manner. A major function of peroxisomes is to catabolize long-chain fatty acids. \n#### **Secretory Vesicles** \nOne of the important functions of many cells is secretion of special chemical substances. Almost all such secretory substances are formed by the endoplasmic reticulum– Golgi apparatus system and are then released from the Golgi apparatus into the cytoplasm in the form of storage vesicles called *secretory vesicles* or *secretory granules.* **[Figure 2-6](#page-21-0)** shows typical secretory vesicles inside pancreatic acinar cells; these vesicles store protein proenzymes (enzymes that are not yet activated). The proenzymes are secreted later through the outer cell membrane into the pancreatic duct and then into the duodenum, where they become activated and perform digestive functions on the food in the intestinal tract. \n#### **Mitochondria** \nThe mitochondria, shown in **[Figure 2-2](#page-17-0)** and **[Figure 2-7](#page-21-1)**, are called the *powerhouses* of the cell. Without them, cells would be unable to extract enough energy from the nutrients, and essentially all cellular functions would cease. \n \n**Figure 2-7.** Structure of a mitochondrion. \nMitochondria are present in all areas of each cell's cytoplasm, but the total number per cell varies from less than 100 up to several thousand, depending on the energy requirements of the cell. Cardiac muscle cells (cardiomyocytes), for example, use large amounts of energy and have far more mitochondria than fat cells (adipocytes), which are much less active and use less energy. Furthermore, the mitochondria are concentrated in those portions of the cell responsible for the major share of its energy metabolism. They are also variable in size and shape. Some mitochondria are only a few hundred nanometers in diameter and are globular in shape, whereas others are elongated and are as large as 1 micrometer in diameter and 7 micrometers long. Still others are branching and filamentous. \nThe basic structure of the mitochondrion, shown in **[Figure 2-7](#page-21-1)**, is composed mainly of two lipid bilayerprotein membranes, an *outer membrane* and an *inner membrane.* Many infoldings of the inner membrane form shelves or tubules called *cristae* onto which oxidative enzymes are attached. The cristae provide a large surface area for chemical reactions to occur. In addition, the inner cavity of the mitochondrion is filled with a *matrix* that contains large quantities of dissolved enzymes necessary for extracting energy from nutrients. These enzymes operate in association with oxidative enzymes on the cristae to cause oxidation of nutrients, thereby forming carbon dioxide and water and, at the same time, releasing energy. The liberated energy is used to synthesize a high-energy substance called *adenosine triphosphate* (ATP). ATP is then transported out of the mitochondrion and diffuses throughout the cell to release its own energy wherever it is needed for performing cellular functions. The chemical details of ATP formation by the mitochondrion are provided in Chapter 68, but some basic functions of ATP in the cell are introduced later in this chapter. \nMitochondria are self-replicative, which means that one mitochondrion can form a second one, a third one, and so on whenever the cell needs increased amounts of ATP. Indeed, the mitochondria contain DNA similar to that found in the cell nucleus. In Chapter 3, we will see that DNA is the basic constituent of the nucleus that \n \n**Figure 2-8.** Cell cytoskeleton composed of protein fibers called microfilaments, intermediate filaments, and microtubules. \ncontrols replication of the cell. The DNA of the mitochondrion plays a similar role, controlling replication of the mitochondrion. Cells that are faced with increased energy demands—for example, in skeletal muscles subjected to chronic exercise training—may increase the density of mitochondria to supply the additional energy required. \n#### **Cell Cytoskeleton—Filament and Tubular Structures** \nThe cell cytoskeleton is a network of fibrillar proteins organized into filaments or tubules. These originate as precursor proteins synthesized by ribosomes in the cytoplasm. The precursor molecules then polymerize to form *filaments* (**[Figure 2-8](#page-22-0)**). As an example, large numbers of actin *microfilaments* frequently occur in the outer zone of the cytoplasm, called the *ectoplasm,* to form an elastic support for the cell membrane. Also, in muscle cells, actin and myosin filaments are organized into a special contractile machine that is the basis for muscle contraction, as discussed in Chapter 6. \n*Intermediate filaments* are generally strong ropelike filaments that often work together with microtubules, providing strength and support for the fragile tubulin structures. They are called *intermediate* because their average diameter is between that of narrower actin microfilaments and wider myosin filaments found in muscle cells. Their functions are mainly mechanical, and they are less dynamic than actin microfilaments or microtubules. All cells have intermediate filaments, although the protein subunits of these structures vary, depending on the cell type. Specific intermediate filaments found in various cells include desmin filaments in muscle cells, neurofilaments in neurons, and keratins in epithelial cells. \nA special type of stiff filament composed of polymerized *tubulin* molecules is used in all cells to construct strong tubular structures, the *microtubules.* **[Figure 2-8](#page-22-0)** shows typical microtubules of a cell. \nAnother example of microtubules is the tubular skeletal structure in the center of each cilium that radiates upward from the cell cytoplasm to the tip of the cilium. This structure is discussed later in the chapter (see **[Figure 2-18](#page-31-0)**). Also, both the *centrioles* and *mitotic spindles* of cells undergoing mitosis are composed of stiff microtubules. \nA major function of microtubules is to act as a *cytoskeleton,* providing rigid physical structures for certain parts of cells. The cell cytoskeleton not only determines cell shape but also participates in cell division, allows cells to move, and provides a tracklike system that directs the movement of organelles in the cells. Microtubules serve as the conveyor belts for the intracellular transport of vesicles, granules, and organelles such as mitochondria. \n#### **Nucleus** \nThe nucleus is the control center of the cell and sends messages to the cell to grow and mature, replicate, or die. Briefly, the nucleus contains large quantities of DNA, \n \n**Figure 2-9.** Structure of the nucleus. \nwhich comprise the *genes.* The genes determine the characteristics of the cell's proteins, including the structural proteins, as well as the intracellular enzymes that control cytoplasmic and nuclear activities. \nThe genes also control and promote cell reproduction. The genes first reproduce to create two identical sets of genes; then the cell splits by a special process called *mitosis* to form two daughter cells, each of which receives one of the two sets of DNA genes. All these activities of the nucleus are discussed in Chapter 3. \nUnfortunately, the appearance of the nucleus under the microscope does not provide many clues to the mechanisms whereby the nucleus performs its control activities. **[Figure 2-9](#page-23-0)** shows the light microscopic appearance of the *interphase* nucleus (during the period between mitoses), revealing darkly staining *chromatin material* throughout the nucleoplasm. During mitosis, the chromatin material organizes in the form of highly structured *chromosomes,* which can then be easily identified using the light microscope, as illustrated in Chapter 3. \n**Nuclear Membrane.** The *nuclear membrane,* also called the *nuclear envelope,* is actually two separate bilayer membranes, one inside the other. The outer membrane is continuous with the endoplasmic reticulum of the cell cytoplasm, and the space between the two nuclear membranes is also continuous with the space inside the endoplasmic reticulum, as shown in **[Figure 2-9](#page-23-0)**. \nThe nuclear membrane is penetrated by several thousand *nuclear pores.* Large complexes of proteins are attached at the edges of the pores so that the central area of each pore is only about 9 nanometers in diameter. Even this size is large enough to allow molecules up to a molecular weight of 44,000 to pass through with reasonable ease. \n**Nucleoli and Formation of Ribosomes.** The nuclei of most cells contain one or more highly staining structures called *nucleoli.* The nucleolus, unlike most other organelles discussed here, does not have a limiting membrane. Instead, it is simply an accumulation of large amounts of \n \n**Figure 2-10.** Comparison of sizes of precellular organisms with that of the average cell in the human body. \nRNA and proteins of the types found in ribosomes. The nucleolus enlarges considerably when the cell is actively synthesizing proteins. \nFormation of the nucleoli (and of the ribosomes in the cytoplasm outside the nucleus) begins in the nucleus. First, specific DNA genes in the chromosomes cause RNA to be synthesized. Some of this synthesized RNA is stored in the nucleoli, but most of it is transported outward through the nuclear pores into the cytoplasm. Here it is used in conjunction with specific proteins to assemble \"mature\" ribosomes that play an essential role in forming cytoplasmic proteins, as discussed in Chapter 3. \n#### COMPARISON OF THE ANIMAL CELL WITH PRECELLULAR FORMS OF LIFE \nThe cell is a complicated organism that required many hundreds of millions of years to develop after the earliest forms of life, microorganisms that may have been similar to present-day *viruses,* first appeared on earth. **[Figure 2-10](#page-23-1)** shows the relative sizes of the following: (1) the smallest known virus; (2) a large virus; (3) a *Rickettsia;* (4) a *bacterium;* and (5) a *nucleated cell,* This demonstrates that the cell has a diameter about 1000 times that of the smallest virus and therefore a volume about 1 billion times that of the smallest virus. Correspondingly, the functions and anatomical organization of the cell are also far more complex than those of the virus. \nThe essential life-giving constituent of the small virus is a *nucleic acid* embedded in a coat of protein. This nucleic acid is composed of the same basic nucleic acid constituents (DNA or RNA) found in mammalian cells and is capable of reproducing itself under appropriate conditions. Thus, the virus propagates its lineage from generation to generation and is therefore a living structure in the same way that cells and humans are living structures. \nAs life evolved, other chemicals in addition to nucleic acid and simple proteins became integral parts of the organism, and specialized functions began to develop in different parts of the virus. A membrane formed around the virus and, inside the membrane, a fluid matrix appeared. Specialized chemicals then developed inside the fluid to perform special functions; many protein enzymes appeared that were capable of catalyzing chemical reactions, thus determining the organism's activities. \nIn still later stages of life, particularly in the rickettsial and bacterial stages, *organelles* developed inside the organism. These represent physical structures of chemical aggregates that perform functions in a more efficient manner than what can be achieved by dispersed chemicals throughout the fluid matrix. \nFinally, in the nucleated cell, still more complex organelles developed, the most important of which is the *nucleus*. The nucleus distinguishes this type of cell from all lower forms of life; it provides a control center for all cellular activities and for reproduction of new cells generation after generation, with each new cell having almost exactly the same structure as its progenitor. \n#### FUNCTIONAL SYSTEMS OF THE CELL \nIn the remainder of this chapter, we discuss some functional systems of the cell that make it a living organism. \n#### **ENDOCYTOSIS—INGESTION BY THE CELL** \nIf a cell is to live and grow and reproduce, it must obtain nutrients and other substances from the surrounding fluids. Most substances pass through the cell membrane by the processes of diffusion and *active transport.* \nDiffusion involves simple movement through the membrane caused by the random motion of the molecules of the substance. Substances move through cell membrane pores or, in the case of lipid-soluble substances, through the lipid matrix of the membrane. \nActive transport involves actually carrying a substance through the membrane by a physical protein structure that penetrates all the way through the membrane. These active transport mechanisms are so important to cell function that they are presented in detail in Chapter 4. \nLarge particles enter the cell by a specialized function of the cell membrane called *endocytosis* (Video 2-1). The principal forms of endocytosis are *pinocytosis* and *phagocytosis.* Pinocytosis means the ingestion of minute particles that form vesicles of extracellular fluid and particulate constituents inside the cell cytoplasm. Phagocytosis means the ingestion of large particles, such as bacteria, whole cells, or portions of degenerating tissue. \n**Pinocytosis.** Pinocytosis occurs continually in the cell membranes of most cells, but is especially rapid in some cells. For example, it occurs so rapidly in macrophages that about 3% of the total macrophage membrane is engulfed in the form of vesicles each minute. Even so, the pinocytotic vesicles are so small—usually only 100 to 200 nanometers in diameter—that most of them can be seen only with an electron microscope. \n \n**Figure 2-11.** Mechanism of pinocytosis. \nPinocytosis is the only means whereby most large macromolecules, such as most proteins, can enter cells. In fact, the rate at which pinocytotic vesicles form is usually enhanced when such macromolecules attach to the cell membrane. \n**[Figure 2-11](#page-24-0)** demonstrates the successive steps of pinocytosis *(A–D),* showing three molecules of protein attaching to the membrane. These molecules usually attach to specialized protein *receptors* on the surface of the membrane that are specific for the type of protein that is to be absorbed. The receptors generally are concentrated in small pits on the outer surface of the cell membrane, called *coated pits.* On the inside of the cell membrane beneath these pits is a latticework of fibrillar protein called *clathrin,* as well as other proteins, perhaps including contractile filaments of *actin* and *myosin.* Once the protein molecules have bound with the receptors, the surface properties of the local membrane change in such a way that the entire pit invaginates inward, and fibrillar proteins surrounding the invaginating pit cause its borders to close over the attached proteins, as well as over a small amount of extracellular fluid. Immediately thereafter, the invaginated portion of the membrane breaks away from the surface of the cell, forming a *pinocytotic vesicle* inside the cytoplasm of the cell. \nWhat causes the cell membrane to go through the necessary contortions to form pinocytotic vesicles is still unclear. This process requires energy from within the cell, which is supplied by ATP, a high-energy substance discussed later in this chapter. This process also requires the presence of calcium ions in the extracellular fluid, which probably react with contractile protein filaments beneath the coated pits to provide the force for pinching the vesicles away from the cell membrane. \n**Phagocytosis.** Phagocytosis occurs in much the same way as pinocytosis, except that it involves large particles rather than molecules. Only certain cells have the capability of phagocytosis—notably, tissue macrophages and some white blood cells. \n \n**Figure 2-12.** Digestion of substances in pinocytotic or phagocytic vesicles by enzymes derived from lysosomes. \nPhagocytosis is initiated when a particle such as a bacterium, dead cell, or tissue debris binds with receptors on the surface of the phagocyte. In the case of bacteria, each bacterium is usually already attached to a specific antibody; it is the antibody that attaches to the phagocyte receptors, dragging the bacterium along with it. This intermediation of antibodies is called *opsonization,* which is discussed in Chapters 34 and 35. \nPhagocytosis occurs in the following steps: \n- 1. The cell membrane receptors attach to the surface ligands of the particle.\n- 2. The edges of the membrane around the points of attachment evaginate outward within a fraction of a second to surround the entire particle; then, progressively more and more membrane receptors attach to the particle ligands. All this occurs suddenly in a zipper-like manner to form a closed *phagocytic vesicle.*\n- 3. Actin and other contractile fibrils in the cytoplasm surround the phagocytic vesicle and contract around its outer edge, pushing the vesicle to the interior.\n- 4. The contractile proteins then pinch the stem of the vesicle so completely that the vesicle separates from the cell membrane, leaving the vesicle in the cell interior in the same way that pinocytotic vesicles are formed. \n#### **LYSOSOMES DIGEST PINOCYTOTIC AND PHAGOCYTIC FOREIGN SUBSTANCES INSIDE THE CELL** \nAlmost immediately after a pinocytotic or phagocytic vesicle appears inside a cell, one or more *lysosomes* become attached to the vesicle and empty their *acid hydrolases* to the inside of the vesicle, as shown in **[Figure 2-12](#page-25-0)**. Thus, a *digestive vesicle* is formed inside the cell cytoplasm in which the vesicular hydrolases begin hydrolyzing the proteins, carbohydrates, lipids, and other substances in the vesicle. The products of digestion are small molecules of substances such as amino acids, glucose, and phosphates that can diffuse through the membrane of the vesicle into the cytoplasm. What is left of the digestive vesicle, called the *residual body,* represents indigestible substances. In most cases, the residual body is finally excreted through the cell membrane by a process called *exocytosis,* which is essentially the opposite of endocytosis. Thus, the pinocytotic and phagocytic vesicles containing lysosomes can be called the *digestive organs* of the cells. \n**Lysosomes and Regression of Tissues and Autolysis of Damaged Cells.** Tissues of the body often regress to a smaller size. For example, this regression occurs in the uterus after pregnancy, in muscles during long periods of inactivity, and in mammary glands at the end of lactation. Lysosomes are responsible for much of this regression. \nAnother special role of the lysosomes is the removal of damaged cells or damaged portions of cells from tissues. Damage to the cell—caused by heat, cold, trauma, chemicals, or any other factor—induces lysosomes to rupture. The released hydrolases immediately begin to digest the surrounding organic substances. If the damage is slight, only a portion of the cell is removed, and the cell is then repaired. If the damage is severe, the entire cell is digested, a process called *autolysis.* In this way, the cell is completely removed, and a new cell of the same type is formed, ordinarily by mitotic reproduction of an adjacent cell to take the place of the old one. \nThe lysosomes also contain bactericidal agents that can kill phagocytized bacteria before they cause cellular damage. These agents include the following: (1) *lysozyme,* which dissolves the bacterial cell wall; (2) *lysoferrin,* which binds iron and other substances before they can promote bacterial growth; and (3) acid at a pH of about 5.0, which activates the hydrolases and inactivates bacterial metabolic systems. \n#### **Autophagy and Recycling of Cell Organelles.** \nLysosomes play a key role in the process of *autophagy,* which literally means \"to eat oneself.\" Autophagy is a housekeeping process whereby obsolete organelles and large protein aggregates are degraded and recycled (**[Figure 2-13](#page-26-0)**). Worn-out cell organelles are transferred to lysosomes by double-membrane structures called *autophagosomes,* which are formed in the cytosol. Invagination of the lysosomal membrane and the formation of vesicles provides another pathway for cytosolic structures to be transported into the lumen of lysosomes. Once inside the lysosomes, the organelles are digested, and the nutrients are reused by the cell. Autophagy contributes to the routine turnover of cytoplasmic components; it is a key mechanism for tissue development, cell survival when nutrients are scarce, and maintenance of homeostasis. In liver cells, for example, the average mitochondrion normally has a life span of only about 10 days before it is destroyed. \n \n**Figure 2-13.** Schematic diagram of autophagy steps. \n#### **SYNTHESIS OF CELLULAR STRUCTURES BY ENDOPLASMIC RETICULUM AND GOLGI APPARATUS** \n#### **Endoplasmic Reticulum Functions** \nThe extensiveness of the endoplasmic reticulum and Golgi apparatus in secretory cells has already been emphasized. These structures are formed primarily of lipid bilayer membranes, similar to the cell membrane, and their walls are loaded with protein enzymes that catalyze the synthesis of many substances required by the cell. \nMost synthesis begins in the endoplasmic reticulum. The products formed there are then passed on to the Golgi apparatus, where they are further processed before being released into the cytoplasm. First, however, let us note the specific products that are synthesized in specific portions of the endoplasmic reticulum and Golgi apparatus. \n#### **Proteins Synthesis by the Rough Endoplasmic Reticu-** \n**lum.** The rough endoplasmic reticulum is characterized by large numbers of ribosomes attached to the outer surfaces of the endoplasmic reticulum membrane. As discussed in Chapter 3, protein molecules are synthesized within the structures of the ribosomes. The ribosomes extrude some of the synthesized protein molecules directly into the cytosol, but they also extrude many more through the wall of the endoplasmic reticulum to the interior of the endoplasmic vesicles and tubules into the *endoplasmic matrix.* \n#### **Lipid Synthesis by the Smooth Endoplasmic Reticu-** \n**lum.** The endoplasmic reticulum also synthesizes lipids, especially phospholipids and cholesterol. These lipids are rapidly incorporated into the lipid bilayer of the endoplasmic reticulum, thus causing the endoplasmic reticulum to grow more extensive. This process occurs mainly in the smooth portion of the endoplasmic reticulum. \nTo keep the endoplasmic reticulum from growing beyond the needs of the cell, small vesicles called *ER vesicles* or *transport vesicles* continually break away from the smooth reticulum; most of these vesicles then migrate rapidly to the Golgi apparatus. \n#### **Other Functions of the Endoplasmic Reticulum.** \nOther significant functions of the endoplasmic reticulum, especially the smooth reticulum, include the following: \n- 1. It provides the enzymes that control glycogen breakdown when glycogen is to be used for energy.\n- 2. It provides a vast number of enzymes that are capable of detoxifying substances, such as drugs, that might damage the cell. It achieves detoxification by processes such as coagulation, oxidation, hydrolysis, and conjugation with glycuronic acid. \n#### **Golgi Apparatus Functions** \n**Synthetic Functions of the Golgi Apparatus.** Although a major function of the Golgi apparatus is to provide additional processing of substances already formed in the endoplasmic reticulum, it can also synthesize certain carbohydrates that cannot be formed in the endoplasmic reticulum. This is especially true for the formation of large saccharide polymers bound with small amounts of protein; important examples include *hyaluronic acid* and *chondroitin sulfate.* \nA few of the many functions of hyaluronic acid and chondroitin sulfate in the body are as follows: (1) they are the major components of proteoglycans secreted in mucus and other glandular secretions; (2) they are the major components of the *ground substance*, or nonfibrous components of the extracellular matrix, outside the cells in the interstitial spaces, which act as fillers between collagen fibers and cells; (3) they are principal components of the organic matrix in both cartilage and bone; and (4) they are important in many cell activities, including migration and proliferation. \n \n**Figure 2-14.** Formation of proteins, lipids, and cellular vesicles by the endoplasmic reticulum and Golgi apparatus. \nProcessing of Endoplasmic Secretions by the Golgi Apparatus—Formation of Vesicles. Figure 2-14 summarizes the major functions of the endoplasmic reticulum and Golgi apparatus. As substances are formed in the endoplasmic reticulum, especially proteins, they are transported through the tubules toward portions of the smooth endoplasmic reticulum that lie nearest to the Golgi apparatus. At this point, transport vesicles composed of small envelopes of smooth endoplasmic reticulum continually break away and diffuse to the deepest layer of the Golgi apparatus. Inside these vesicles are synthesized proteins and other products from the endoplasmic reticulum. \nThe transport vesicles instantly fuse with the Golgi apparatus and empty their contained substances into the vesicular spaces of the Golgi apparatus. Here, additional carbohydrate moieties are added to the secretions. Also, an important function of the Golgi apparatus is to compact the endoplasmic reticular secretions into highly concentrated packets. As the secretions pass toward the outermost layers of the Golgi apparatus, the compaction and processing proceed. Finally, both small and large vesicles continually break away from the Golgi apparatus, carrying with them the compacted secretory substances and diffusing throughout the cell. \nThe following example provides an idea of the timing of these processes. When a glandular cell is bathed in amino acids, newly formed protein molecules can be detected in the granular endoplasmic reticulum within 3 to 5 minutes. Within 20 minutes, newly formed proteins are already present in the Golgi apparatus and, within 1 to 2 hours, the proteins are secreted from the surface of the cell. \nTypes of Vesicles Formed by the Golgi Apparatus—Secretory Vesicles and Lysosomes. In a highly secretory cell, the vesicles formed by the Golgi apparatus are mainly secretory vesicles containing proteins that are secreted through the surface of the cell membrane. These secretory vesicles first diffuse to the cell membrane and then fuse with it and empty their substances to the exterior by the mechanism called exocytosis. Exocytosis, in most cases, is stimulated by entry of calcium ions into the cell. Calcium ions interact with the vesicular membrane and cause its fusion with the cell membrane, followed by exocytosis—opening of the membrane's outer surface and extrusion of its contents outside the cell. Some vesicles, however, are destined for intracellular use. \n**Use of Intracellular Vesicles to Replenish Cellular Membranes.** Some intracellular vesicles formed by the Golgi apparatus fuse with the cell membrane or with the membranes of intracellular structures such as the mitochondria and even the endoplasmic reticulum. This fusion increases the expanse of these membranes and replenishes the membranes as they are used up. For example, the cell membrane loses much of its substance every time it forms a phagocytic or pinocytotic vesicle, and the vesicular membranes of the Golgi apparatus continually replenish the cell membrane. \nIn summary, the membranous system of the endoplasmic reticulum and Golgi apparatus are highly metabolic and capable of forming new intracellular structures and secretory substances to be extruded from the cell.\n\nThe principal substances from which cells extract energy are foods that react chemically with oxygen—carbohydrates, fats, and proteins. In the human body, essentially all carbohydrates are converted into *glucose* by the digestive tract and liver before they reach the other cells of the body. Similarly, proteins are converted into *amino acids*, and fats are converted into *fatty acids*. **Figure 2-15** shows oxygen and the foodstuffs—glucose, fatty acids, and amino acids—all entering the cell. Inside the cell, they react chemically with oxygen under the influence of enzymes that control the reactions and channel the energy released in the proper direction. The details of all these digestive and metabolic functions are provided in Chapters 63 through 73. \nBriefly, almost all these oxidative reactions occur inside the mitochondria, and the energy that is released is used to form the high-energy compound ATP. Then, ATP, not the original food, is used throughout the cell to energize almost all the subsequent intracellular metabolic reactions. \n \n**Figure 2-15.** Formation of adenosine triphosphate (ATP) in the cell showing that most of the ATP is formed in the mitochondria. (ADP, Adenosine diphosphate; CoA, coenzyme A.) \n#### **Functional Characteristics of Adenosine Triphosphate** \n$$\\begin{array}{c|ccccccccccccccccccccccccccccccccccc$$ \n**Adenosine triphosphate** \nATP is a nucleotide composed of the following: (1) the nitrogenous base *adenine;* (2) the pentose sugar *ribose;* and (3) three *phosphate radicals.* The last two phosphate radicals are connected with the remainder of the molecule by *high-energy phosphate bonds,* which are represented in the formula shown by the symbol ∼. *Under the physical and chemical conditions of the body,* each of these high-energy bonds contains about 12,000 calories of energy per mole of ATP, which is many times greater than the energy stored in the average chemical bond, thus giving rise to the term *high-energy bond.* Furthermore, the high-energy phosphate bond is very labile, so that it can be split instantly on demand whenever energy is required to promote other intracellular reactions. \nWhen ATP releases its energy, a phosphoric acid radical is split away, and *adenosine diphosphate* (ADP) is formed. This released energy is used to energize many of the cell's other functions, such as syntheses of substances and muscular contraction. \nTo reconstitute the cellular ATP as it is used up, energy derived from the cellular nutrients causes ADP and phosphoric acid to recombine to form new ATP, and the entire process is repeated over and over. For these reasons, ATP has been called the *energy currency* of the cell because it can be spent and reformed continually, having a turnover time of only a few minutes. \n**Chemical Processes in the Formation of ATP—Role of the Mitochondria.** On entry into the cells, glucose is converted by enzymes in the *cytoplasm* into *pyruvic acid* (a process called *glycolysis*). A small amount of ADP is changed into ATP by the energy released during this conversion, but this amount accounts for less than 5% of the overall energy metabolism of the cell. \nAbout 95% of the cell's ATP formation occurs in the mitochondria. The pyruvic acid derived from carbohydrates, fatty acids from lipids, and amino acids from proteins is eventually converted into the compound *acetyl-coenzyme A* (CoA) in the matrix of mitochondria. This substance, in turn, is further dissolved (for the purpose of extracting its energy) by another series of enzymes in the mitochondrion matrix, undergoing dissolution in a sequence of chemical reactions called the *citric acid cycle,* or *Krebs cycle.* These chemical reactions are so important that they are explained in detail in Chapter 68. \nIn this citric acid cycle, acetyl-CoA is split into its component parts, *hydrogen atoms* and *carbon dioxide.* The carbon dioxide diffuses out of the mitochondria and eventually out of the cell; finally, it is excreted from the body through the lungs. \nThe hydrogen atoms, conversely, are highly reactive; they combine with oxygen that has also diffused into the mitochondria. This combination releases a tremendous amount of energy, which is used by mitochondria to convert large amounts of ADP to ATP. The processes of these reactions are complex, requiring the participation of many protein enzymes that are integral parts of mitochondrial *membranous shelves* that protrude into the mitochondrial matrix. The initial event is the removal of an electron from the hydrogen atom, thus converting it to a hydrogen ion. The terminal event is the combination of hydrogen ions with oxygen to form water and the release of large amounts of energy to globular proteins that protrude like knobs from the membranes of the mitochondrial shelves; these proteins are called *ATP synthetase*. Finally, the enzyme ATP synthetase uses the energy from the hydrogen ions to convert ADP to ATP. The newly formed ATP is transported out of the mitochondria into all parts of the cell cytoplasm and nucleoplasm, where it energizes multiple cell functions. \nThis overall process for formation of ATP is called the *chemiosmotic mechanism* of ATP formation. The chemical and physical details of this mechanism are presented \n \n**Figure 2-16.** Use of adenosine triphosphate (ATP; formed in the mitochondrion) to provide energy for three major cellular functions—membrane transport, protein synthesis, and muscle contraction. (ADP, Adenosine diphosphate.) \nin Chapter 68, and many of the detailed metabolic functions of ATP in the body are discussed in Chapters 68 through 72. \n**Uses of ATP for Cellular Function.** Energy from ATP is used to promote three major categories of cellular functions: (1) *transport* of substances through multiple cell membranes; (2) *synthesis of chemical compounds* throughout the cell; and (3) *mechanical work.* These uses of ATP are illustrated by the examples in **Figure 2-16**: (1) to supply energy for the transport of sodium through the cell membrane; (2) to promote protein synthesis by the ribosomes; and (3) to supply the energy needed during muscle contraction. \nIn addition to the membrane transport of sodium, energy from ATP is required for the membrane transport of potassium, calcium, magnesium, phosphate, chloride, urate, and hydrogen ions and many other ions, as well as various organic substances. Membrane transport is so important to cell function that some cells—the renal tubular cells, for example—use as much as 80% of the ATP that they form for this purpose alone. \nIn addition to synthesizing proteins, cells make phospholipids, cholesterol, purines, pyrimidines, and many other substances. Synthesis of almost any chemical compound requires energy. For example, a single protein molecule might be composed of as many as several thousand amino acids attached to one another by peptide linkages. The formation of each of these linkages requires energy derived from the breakdown of four high-energy bonds; thus, many thousand ATP molecules must release their energy as each protein molecule is formed. Indeed, some cells use as much as 75% of all the ATP formed in the cell \n \nFigure 2-17. Ameboid motion by a cell. \nsimply to synthesize new chemical compounds, especially protein molecules; this is particularly true during the growth phase of cells. \nAnother use of ATP is to supply energy for special cells to perform mechanical work. We discuss in Chapter 6 that each contraction of a muscle fiber requires the expenditure of large quantities of ATP energy. Other cells perform mechanical work in other ways, especially by *ciliary* and *ameboid motion*, described later in this chapter. The source of energy for all these types of mechanical work is ATP. \nIn summary, ATP is readily available to release its energy rapidly wherever it is needed in the cell. To replace ATP used by the cell, much slower chemical reactions break down carbohydrates, fats, and proteins and use the energy derived from these processes to form new ATP. More than 95% of this ATP is formed in the mitochondria, which is why the mitochondria are called the *powerhouses* of the cell. \n#### **LOCOMOTION OF CELLS** \nThe most obvious type of movement in the body is that which occurs in skeletal, cardiac, and smooth muscle cells, which constitute almost 50% of the entire body mass. The specialized functions of these cells are discussed in Chapters 6 through 9. Two other types of movement—ameboid locomotion and ciliary movement—occur in other cells. \n#### AMEBOID MOVEMENT \nAmeboid movement is a crawling-like movement of an entire cell in relation to its surroundings, such as movement of white blood cells through tissues. This type of movement gets its name from the fact that amebae move in this manner, and amebae have provided an excellent tool for studying the phenomenon. \nTypically, ameboid locomotion begins with the protrusion of a *pseudopodium* from one end of the cell. The pseudopodium projects away from the cell body and partially secures itself in a new tissue area; then the remainder of the cell is pulled toward the pseudopodium. **Figure 2-17** \ndemonstrates this process, showing an elongated cell, the right-hand end of which is a protruding pseudopodium. The membrane of this end of the cell is continually moving forward, and the membrane at the left-hand end of the cell is continually following along as the cell moves. \n**Mechanism of Ameboid Locomotion. [Figure 2-17](#page-29-1)** shows the general principle of ameboid motion. Basically, this results from the continual formation of new cell membrane at the leading edge of the pseudopodium and continual absorption of the membrane in the mid and rear portions of the cell. Two other effects are also essential for forward movement of the cell. The first is attachment of the pseudopodium to surrounding tissues so that it becomes fixed in its leading position while the remainder of the cell body is being pulled forward toward the point of attachment. This attachment is caused by *receptor proteins* that line the insides of exocytotic vesicles. When the vesicles become part of the pseudopodial membrane, they open so that their insides evert to the outside, and the receptors now protrude to the outside and attach to ligands in the surrounding tissues. \nAt the opposite end of the cell, the receptors pull away from their ligands and form new endocytotic vesicles. Then, inside the cell, these vesicles stream toward the pseudopodial end of the cell, where they are used to form new membrane for the pseudopodium. \nThe second essential effect for locomotion is to provide the energy required to pull the cell body in the direction of the pseudopodium. A moderate to large amount of the protein *actin* is in the cytoplasm of all cells*.* Much of the actin is in the form of single molecules that do not provide any motive power; however, these molecules polymerize to form a filamentous network, and the network contracts when it binds with an actin-binding protein such as *myosin.* The entire process is energized by the high-energy compound ATP. This is what occurs in the pseudopodium of a moving cell, where such a network of actin filaments forms anew inside the enlarging pseudopodium. Contraction also occurs in the ectoplasm of the cell body, where a preexisting actin network is already present beneath the cell membrane. \n#### **Types of Cells That Exhibit Ameboid Locomotion.** \nThe most common cells to exhibit ameboid locomotion in the human body are the *white blood cells* when they move out of the blood into the tissues to form *tissue macrophages.* Other types of cells can also move by ameboid locomotion under certain circumstances. For example, fibroblasts move into a damaged area to help repair the damage, and even the germinal cells of the skin, although ordinarily completely sessile cells, move toward a cut area to repair the opening. Cell locomotion is also especially important in the development of the embryo and fetus after fertilization of an ovum. For example, embryonic cells often must migrate long distances from their sites of origin to new areas during the development of special structures. \nSome types of cancer cells, such as sarcomas, which arise from connective tissue cells, are especially proficient at ameboid movement. This partially accounts for their relatively rapid spreading from one part of the body to another, known as *metastasis.* \n**Control of Ameboid Locomotion—Chemotaxis.** An important initiator of ameboid locomotion is the process called *chemotaxis,* which results from the appearance of certain chemical substances in the tissues. Any chemical substance that causes chemotaxis to occur is called a *chemotactic substance.* Most cells that exhibit ameboid locomotion move toward the source of a chemotactic substance—that is, from an area of lower concentration toward an area of higher concentration. This is called *positive chemotaxis.* Some cells move away from the source, which is called *negative chemotaxis.* \nHow does chemotaxis control the direction of ameboid locomotion? Although the answer is not certain, it is known that the side of the cell most exposed to the chemotactic substance develops membrane changes that cause pseudopodial protrusion. \n#### **CILIA AND CILIARY MOVEMENTS** \nThere are two types of cilia, *motile* and *nonmotile*, or *primary*, cilia. Motile cilia can undergo a whiplike movement on the surfaces of cells. This movement occurs mainly in two places in the human body, on the surfaces of the respiratory airways and on the inside surfaces of the uterine tubes (fallopian tubes) of the reproductive tract. In the nasal cavity and lower respiratory airways, the whiplike motion of motile cilia causes a layer of mucus to move at a rate of about 1 cm/min toward the pharynx, in this way continually clearing these passageways of mucus and particles that have become trapped in the mucus. In the uterine tubes, cilia cause slow movement of fluid from the ostium of the uterine tube toward the uterus cavity; this movement of fluid transports the ovum from the ovary to the uterus. \nAs shown in **[Figure 2-18](#page-31-0)**, a cilium has the appearance of a sharp-pointed straight or curved hair that projects 2 to 4 micrometers from the surface of the cell. Often, many motile cilia project from a single cell—for example, as many as 200 cilia on the surface of each epithelial cell inside the respiratory passageways. The cilium is covered by an outcropping of the cell membrane, and it is supported by 11 microtubules—nine double tubules located around the periphery of the cilium and two single tubules down the center, as demonstrated in the cross section shown in **[Figure 2-18](#page-31-0)**. Each cilium is an outgrowth of a structure that lies immediately beneath the cell membrane, called the *basal body* of the cilium. \nThe *flagellum of a sperm* is similar to a motile cilium; in fact, it has much the same type of structure and same type of contractile mechanism. The flagellum, however, is much longer and moves in quasisinusoidal waves instead of whiplike movements. \n \n**Figure 2-18.** Structure and function of the cilium. *(Modified from Satir P: Cilia. Sci Am 204:108, 1961.)* \nIn the inset of **[Figure 2-18](#page-31-0)**, movement of the motile cilium is shown. The cilium moves forward with a sudden, rapid whiplike stroke 10 to 20 times per second, bending sharply where it projects from the surface of the cell. Then it moves backward slowly to its initial position. The rapid, forward-thrusting, whiplike movement pushes the fluid lying adjacent to the cell in the direction that the cilium moves; the slow dragging movement in the backward direction has almost no effect on fluid movement. As a result, the fluid is continually propelled in the direction of the fast-forward stroke. Because most motile ciliated cells have large numbers of cilia on their surfaces, and because all the cilia are oriented in the same direction, this is an effective means for moving fluids from one part of the surface to another. \n**Mechanism of Ciliary Movement.** Although not all aspects of ciliary movement are known, we are aware of the following elements. First, the nine double tubules and two single tubules are all linked to one another by a complex of protein cross-linkages; this total complex of tubules and cross-linkages is called the *axoneme.* Second, even after removal of the membrane and destruction of other elements of the cilium in addition to the axoneme, the cilium can still beat under appropriate conditions. Third, two conditions are necessary for continued beating of the axoneme after removal of the other structures of the cilium: (1) the availability of ATP; and (2) appropriate ionic conditions, especially appropriate concentrations of magnesium and calcium. Fourth, during forward motion of the cilium, the double tubules on the front edge of the cilium slide outward toward the tip of the cilium, whereas those on the back edge remain in place. Fifth, multiple protein arms composed of the protein *dynein,* which has adenosine triphosphatase (ATPase) enzymatic activity, project from each double tubule toward an adjacent double tubule. \nGiven this basic information, it has been determined that the release of energy from ATP in contact with the ATPase dynein arms causes the heads of these arms to \"crawl\" rapidly along the surface of the adjacent double tubule. If the front tubules crawl outward while the back tubules remain stationary, bending occurs. \nThe way in which cilia contraction is controlled is not well understood. The cilia of some genetically abnormal cells do not have the two central single tubules, and these cilia fail to beat. Therefore, it is presumed that some signal, perhaps an electrochemical signal, is transmitted along these two central tubules to activate the dynein arms. \n**Nonmotile Primary Cilia Serve as Cell Sensory \"Antennae.\"** *Primary cilia* are nonmotile and generally occur only as a single cilium on each cell. Although the physiological functions of primary cilia are not fully understood, current evidence indicates that they function as cellular ''sensory antennae,\" which coordinate cellular signaling pathways involved in chemical and mechanical sensation, signal transduction, and cell growth. In the kidneys, for example, primary cilia are found in most epithelial cells of the tubules, projecting into the tubule lumen and acting as a flow sensor. In response to fluid flow over the tubular epithelial cells, the primary cilia bend and cause flow-induced changes in intracellular calcium signaling. These signals, in turn, initiate multiple effects on the cells. Defects in signaling by primary cilia in renal tubular epithelial cells are thought to contribute to various disorders, including the development of large fluid-filled cysts, a condition called *polycystic kidney disease*. \n#### Bibliography \nAlberts B, Johnson A, Lewis J, et al: Molecular Biology of the Cell, 6th ed. New York: Garland Science, 2014. \nBrandizzi F, Barlowe C: Organization of the ER-Golgi interface for membrane traffic control. Nat Rev Mol Cell Biol 14:382, 2013. \nDikic I, Elazar Z. Mechanism and medical implications of mammalian autophagy. Nat Rev Mol Cell Biol 19:349, 2018. \nEisner V, Picard M, Hajnóczky G. Mitochondrial dynamics in adaptive and maladaptive cellular stress responses. Nat Cell Biol 20:755, 2018. \nGalluzzi L, Yamazaki T, Kroemer G. Linking cellular stress responses to systemic homeostasis. Nat Rev Mol Cell Biol 19:731, 2018. \n- Guerriero CJ, Brodsky JL: The delicate balance between secreted protein folding and endoplasmic reticulum-associated degradation in human physiology. Physiol Rev 92:537, 2012.\n- Harayama T, Riezman H. Understanding the diversity of membrane lipid composition. Nat Rev Mol Cell Biol 19:281, 2018.\n- Insall R: The interaction between pseudopods and extracellular signalling during chemotaxis and directed migration. Curr Opin Cell Biol 25:526, 2013.\n- Kaksonen M, Roux A. Mechanisms of clathrin-mediated endocytosis. Nat Rev Mol Cell Biol 19:313, 2018.\n- Lawrence RE, Zoncu R. The lysosome as a cellular centre for signalling, metabolism and quality control. Nat Cell Biol 21: 133, 2019.\n- Nakamura N, Wei JH, Seemann J: Modular organization of the mammalian Golgi apparatus. Curr Opin Cell Biol 24:467, 2012. \n- Palikaras K, Lionaki E, Tavernarakis N. Mechanisms of mitophagy in cellular homeostasis, physiology and pathology. Nat Cell Biol 20:1013, 2018.\n- Sezgin E, Levental I, Mayor S, Eggeling C. The mystery of membrane organization: composition, regulation and roles of lipid rafts. Nat Rev Mol Cell Biol 18:361, 2017.\n- Spinelli JB, Haigis MC. The multifaceted contributions of mitochondria to cellular metabolism. Nat Cell Biol. 20:745, 2018.\n- Walker CL, Pomatto LCD, Tripathi DN, Davies KJA. Redox regulation of homeostasis and proteostasis in peroxisomes. Physiol Rev 98:89, 2018.\n- Zhou K, Gaullier G, Luger K. Nucleosome structure and dynamics are coming of age. Nat Struct Mol Biol 26:3, 2019. \n\n\nGenes, which are located in the nuclei of all cells of the body, control heredity from parents to children, as well as the daily functioning of all the body's cells. The genes control cell function by determining which structures, enzymes, and chemicals are synthesized within the cell. \n**Figure 3-1** shows the general schema of genetic control. Each gene, which is composed of *deoxyribonucleic acid* (DNA), controls the formation of another nucleic acid, *ribonucleic acid* (RNA); this RNA then spreads throughout the cell to control formation of a specific protein. The entire process, from *transcription* of the genetic code in the nucleus to *translation* of the RNA code and the formation of proteins in the cell cytoplasm, is often referred to as *gene expression.* \nBecause the human body has approximately 20,000 to 25,000 different genes that code for proteins in each cell, it is possible to form a large number of different cellular proteins. In fact, RNA molecules transcribed from the same segment of DNA—the same gene—can be processed in more than one way by the cell, giving rise to alternate versions of the protein. The total number of different proteins produced by the various cell types in humans is estimated to be at least 100,000. \nSome of the cellular proteins are *structural proteins,* which, in association with various lipids and carbohydrates, form structures of the various intracellular organelles discussed in Chapter 2. However, most of the proteins are *enzymes* that catalyze different chemical reactions in the cells. For example, enzymes promote all the oxidative reactions that supply energy to the cell, along with synthesis of all the cell chemicals, such as lipids, glycogen, and adenosine triphosphate (ATP). \n#### CELL NUCLEUS GENES CONTROL PROT[EIN](#page-34-0) SYNTHESIS \nIn the cell nucleus, large numbers of genes are attached end on end in extremely long, double-stranded helical molecules of DNA having molecular weights measured in the billions. A very short segment of such a molecule is shown in **Figure 3-2**. This molecule is composed of several simple chemical compounds bound together in a regular pattern, the details of which are explained in the next few paragraphs. \n#### **Building Blocks of DNA** \n**Figure 3-3** shows the basic chemical compounds involved in the formation of DNA. These compounds include the following: (1) *phosphoric acid;* (2) a sugar [called](#page-34-0) *deoxyribose;* and (3) four nitrogenous *bases* (two purines, *adenine* and *guanine,* and two pyrimidines, *thymine* and *cytosine*). The phosphoric acid and deoxyribose form the two helical strands that are the backbone of the DNA molecule, and the nitrogenous bases lie between the two strands and connect them, as illustrated in **Figure 3-2**. \n#### **Nucleotides** \nThe first stage of DNA formation is to combine one molecule of phosphoric acid, one molecule of deoxyribose, and one of the four bases to form an acidic nucleotide. Four separate nucleotides are thus formed, one for each of the four bases: *deoxyadenylic, deoxythymidylic, deoxyguanylic,* and *deoxycytidylic acids*. **Figure 3-4** shows the chemical \n \n**Figure 3-1** The general schema whereby genes control cell function. *mRNA,* Messenger RNA. \n \n**Figure 3-2** The helical double-stranded structure of the gene. The outside strands are composed of phosphoric acid and the sugar deoxyribose. The internal molecules connecting the two strands of the helix are purine and pyrimidine bases, which determine the \"code\" of the gene. \n \nFigure 3-3 The basic building blocks of DNA. \nstructure of deoxyadenylic acid, and **Figure 3-5** shows simple symbols for the four nucleotides that form DNA. \n#### Nucleotides Are Organized to Form Two Strands of DNA Loosely Bound to Each Other \n**Figure 3-2** shows the manner in which multiple nucleotides are bound together to form two strands of DNA. The two strands are, in turn, loosely bonded with each other by weak cross-linkages, as illustrated in **Figure 3-6** by the central dashed lines. Note that the backbone of each DNA strand is composed of alternating phosphoric acid and deoxyribose molecules. In turn, purine and pyrimidine bases are attached to the sides of the deoxyribose molecules. Then, by means of loose *hydrogen bonds* (dashed \n**Figure 3-4.** Deoxyadenylic acid, one of the nucleotides that make up DNA. \n \n**Figure 3-5.** Symbols for the four nucleotides that combine to form DNA. Each nucleotide contains phosphoric acid (P), deoxyribose (D), and one of the four nucleotide bases: adenine (A); thymine (T); guanine (G); or cytosine (C). \nlines) between the purine and pyrimidine bases, the two respective DNA strands are held together. Note the following caveats, however: \n- 1. Each purine base *adenine* of one strand always bonds with a pyrimidine base *thymine* of the other strand.\n- 2. Each purine base *guanine* always bonds with a pyrimidine base *cytosine*. \nThus, in **Figure 3**-6, the sequence of complementary pairs of bases is CG, CG, GC, TA, CG, TA, GC, AT, and AT. Because of the looseness of the hydrogen bonds, the two strands can pull apart with ease, and they do so many times during the course of their function in the cell. \nTo put the DNA of **Figure 3**-6 into its proper physical perspective, one could merely pick up the two ends and twist them into a helix. Ten pairs of nucleotides are present in each full turn of the helix in the DNA molecule. \n#### **GENETIC CODE** \nThe importance of DNA lies in its ability to control the formation of proteins in the cell, which it achieves by means of a *genetic code*. That is, when the two strands of a DNA molecule are split apart, the purine and pyrimidine bases projecting to the side of each DNA strand are exposed, as shown by the top strand in **Figure 3-7**. It is these projecting bases that form the genetic code. \nThe genetic code consists of successive \"triplets\" of bases—that is, each three successive bases is a *code word*. The successive triplets eventually control the sequence of amino acids in a protein molecule that is to be synthesized in the cell. Note in **Figure 3**-6 that the top strand of DNA, reading from left to right, has the genetic code GGC, AGA, CTT, with the triplets being separated from one another by the arrows. As we follow this genetic code through **Figure 3**-7 and **Figure 3**-8, we see that these three respective triplets are responsible for successive placement of the three amino acids, *proline, serine,* and *glutamic acid,* in a newly formed molecule of protein.\n\nBecause DNA is located in the cell nucleus, yet most of the cell functions are carried out in the cytoplasm, there must be some means for DNA genes of the nucleus to control chemical reactions of the cytoplasm. This control \n \n**Figure 3-6.** Arrangement of deoxyribose nucleotides in a double strand of DNA. \n \nR C \nP \nP R C P R G P R U \n**Figure 3-7.** Combination of ribose nucleotides with a strand of DNA to form a molecule of RNA that carries the genetic code from the gene to the cytoplasm. The *RNA polymerase* enzyme moves along the DNA strand and builds the RNA molecule. \n**Figure 3-8.** A portion of an RNA molecule showing thre[e](#page-36-0) RNA codons—CCG, UCU, and GAA—that control attachment of the three amino acids, proline, serine, and glutamic acid, respectively, to the growing RNA chain. **Proline Serine Glutamic acid** \nis achieved through the intermediary of another type of nucleic acid, RNA, the formation of which is controlled by DNA of the nucleus. Thus, as shown in **Figure 3-7**, the code is transferred to RNA in a process called *transcription.* The RNA, in turn, diffuses from the nucleus through nuclear pores into the cytoplasmic compartment, where it controls protein synthesis. \n#### **RNA IS SYNTHESIZED IN THE NUCLEUS FROM A DNA TEMPLATE** \nDuring RNA synthesis, the two strands of DNA separate temporarily; one of these strands is used as a template for synthesis of an RNA molecule. The code triplets in the DNA result in the formation of *complementary* code triplets (called *codons*) in the RNA. These codons, in turn, will control the sequence of amino acids in a protein to be synthesized in the cell cytoplasm. \n**Building Blocks of RNA.** The basic building blocks of RNA are almost the same as those of DNA, except for two differences. First, the sugar deoxyribose is not used in RNA formation. In its place is another sugar of slightly different composition, *ribose,* which contains an extra hydroxyl ion appended to the ribose ring structure. Second, thymine is replaced by another pyrimidine, *uracil.* \n**Formation of RNA Nucleotides.** The basic building blocks of RNA form *RNA nucleotides,* exactly as described previously for DNA synthesis. Here again, four separate nucleotides are used to form RNA. These nucleotides contain the bases *adenine, guanine, cytosine,* and *uracil.* Note that these bases are the same as in DNA, except that uracil in RNA replaces thymine in DNA. \n**\"Activation\" of RNA Nucleotides.** The next step in the synthesis of RNA is \"activation\" of RNA nucleotides by an enzyme, *RNA polymerase.* This activation occurs by adding two extra phosphate radicals to each nucleotide to form \ntriphosphates (shown in **Figure 3-7** by the two RNA nucleotides to the far right during RNA chain formation). These last two phosphates are combined with the nucleotide by \nP R U P R G P R A P R A \nP R C \n*high-energy phosphate bonds* derived from ATP in the cell. The result of this activation process is that large quantities of ATP energy are made available to each of the nucleotides. This energy is used to promote chemical reactions that add each new RNA nucleotide at the end of the developing RNA [chain.](#page-36-0) \n#### **RNA CHAIN ASSEMBLY FROM ACTIVATED NUCLEOTIDES USING THE DNA STRAND AS A TEMPLATE** \nAs shown in **Figure 3-7,** assembly of RNA is accomplished under the influence of an enzyme, *RNA polymerase.* This large protein enzyme has many functional properties necessary for formation of RNA, as follows: \n- 1. In the DNA strand immediately ahead of the gene to be transcribed is a sequence of nucleotides called the *promoter.* The RNA polymerase has an appropriate complementary structure that recognizes this promoter and becomes attached to it, which is the essential step for initiating the formation of RNA.\n- 2. After the RNA polymerase attaches to the promoter, the polymerase causes unwinding of about two turns of the DNA helix and separation of the unwound portions of the two strands.\n- 3. The polymerase then moves along the DNA strand, temporarily unwinding and separating the two DNA strands at each stage of its movement. As it moves along, at each stage it adds a new activated RNA nucleotide to the end of the newly forming RNA chain through the following steps:\n- a. First, it causes a hydrogen bond to form between the end base of the DNA strand and the base of an RNA nucleotide in the nucleoplasm. \n- b. Then, one at a time, the RNA polymerase breaks two of the three phosphate radicals away from each of these RNA nucleotides, liberating large amounts of energy from the broken high-energy phosphate bonds. This energy is used to cause covalent linkage of the remaining phosphate on the nucleotide with the ribose on the end of the growing RNA chain.\n- c. When the RNA polymerase reaches the end of the DNA gene, it encounters a new sequence of DNA nucleotides called the *chain-terminating sequence,* which causes the polymerase and the newly formed RNA chain to break away from the DNA strand. The polymerase then can be used again and again to form more new RNA chains.\n- d. As the new RNA strand is formed, its weak hydrogen bonds with the DNA template break away because the DNA has a high affinity for rebonding with its own complementary DNA strand. Thus, the RNA chain is forced away from the DNA and is released into the nucleoplasm. \nTherefore, the code that is present in the DNA strand is eventually transmitted in *complementary* form to the RNA chain. The ribose nucleotide bases always combine with the deoxyribose bases in the following combinations: \n| DNA Base | RNA Base |\n|----------|----------|\n| guanine | Cytosine |\n| cytosine | Guanine |\n| adenine | Uracil |\n| thymine | adenine | \n**There Are Several Different Types of RNA.** As research on RNA has continued to advance, many different types of RNA have been discovered. Some types of RNA are involved in protein synthesis, whereas other types serve gene regulatory functions or are involved in posttranscriptional modification of RNA. The functions of some types of RNA, especially those that do not appear to code for proteins, are still mysterious. The following six types of RNA play independent and different roles in protein synthesis: \n- 1. *Precursor messenger RNA* (pre-mRNA) is a large, immature, single strand of RNA that is processed in the nucleus to form mature messenger RNA (mRNA). The pre-RNA includes two different types of segments, called *introns,* which are removed by a process called splicing, and *exons,* which are retained in the final mRNA.\n- 2. *Small nuclear RNA* (snRNA) directs the splicing of pre-mRNA to form mRNA.\n- 3. *Messenger RNA* (mRNA) carries the genetic code to the cytoplasm for controlling the type of protein formed.\n- 4. *Transfer RNA* (tRNA) transports activated amino acids to the ribosomes to be used in assembling the protein molecule.\n- 5. *Ribosomal RNA,* along with about 75 different proteins, forms *ribosomes,* the physical and chemical \n- structures on which protein molecules are actually assembled.\n- 6. *MicroRNAs* (miRNAs) are single-stranded RNA molecules of 21 to 23 nucleotides that can regulate gene transcription and translation. \n#### **MESSENGER RN[A—THE C](#page-36-1)ODONS** \n*Messenger RNA* molecules are long single RNA strands that are suspended in the cytoplasm. These molecules are composed of several hundred to several thousand RNA nucleotides in unpaired strands, and they contain *c[odons](#page-36-0)* that are exactly complementary to the code triplets of t[he](#page-37-0) DNA genes. **Figure 3-8** shows a small segment of mRNA. Its codons are CCG, UCU, and GAA, which are the codons for the amino acids proline, serine, and glutamic acid. The transcription of these codons from the DNA molecule to the RNA molecule is shown in **Figure 3-7**. \n**RNA Codons for the Different Amino Acids. [Table 3](#page-37-0)-1** lists the RNA codons for the 20 common amino acids found in protein molecules. Note that most of the amino acids are represented by more than one codon; also, one codon represents the signal \"start manufacturing the protein molecule,\" and three codons represent \"stop manufacturing the protein molecule.\" In **Table 3-1**, these two \n**Table 3-1** RNA Codons for Amino Acids and for Start and Stop \n| Amino Acid | | | RNA Codons | | | |\n|---------------|-----|-----|------------|-----|-----|-----|\n| Alanine | GCU | GCC | GCA | GCG | | |\n| Arginine | CGU | CGC | CGA | CGG | AGA | AGG |\n| Asparagine | AAU | AAC | | | | |\n| Aspartic acid | GAU | GAC | | | | |\n| Cysteine | UGU | UGC | | | | |\n| Glutamic acid | GAA | GAG | | | | |\n| Glutamine | CAA | CAG | | | | |\n| Glycine | GGU | GGC | GGA | GGG | | |\n| Histidine | CAU | CAC | | | | |\n| Isoleucine | AUU | AUC | AUA | | | |\n| Leucine | CUU | CUC | CUA | CUG | UUA | UUG |\n| Lysine | AAA | AAG | | | | |\n| Methionine | AUG | | | | | |\n| Phenylalanine | UUU | UUC | | | | |\n| Proline | CCU | CCC | CCA | CCG | | |\n| Serine | UCU | UCC | UCA | UCG | AGC | AGU |\n| Threonine | ACU | ACC | ACA | ACG | | |\n| Tryptophan | UGG | | | | | |\n| Tyrosine | UAU | UAC | | | | |\n| Valine | GUU | GUC | GUA | GUG | | |\n| Start (CI) | AUG | | | | | |\n| Stop (CT) | UAA | UAG | UGA | | | | \n*CI,* Chain-initiating; *CT,* chain-terminating. \ntypes of codons are designated CI for \"chain-initiating\" or \"start\" codon and CT for \"chain-terminating\" or \"stop\" codon. \n#### **TRANSFER RNA—THE ANTICODONS** \nAnother type of RNA that is essential for protein synthesis is called transfer RNA (tRNA) because it transfers amino acids to protein molecules as the protein is being synthesized. Each type of tRNA combines specifically with 1 of the 20 amino acids that are to be incorporated into proteins. The tRNA then acts as a *carrier* to transport its specific type of amino acid to the ribosomes, where protein molecules are forming. In the ribosomes, each specific type of tRNA recognizes a particular codon on the mRNA (described later) and thereby delivers [the appro](#page-38-0)priate amino acid to the appropriate place in the chain of the newly forming protein molecule. \nTransfer RNA, which contains only about 80 nucleotides, is a relatively small molecule in comparison with mRNA. It is a folded chain of nucleotides with a cloverleaf appearance similar to that shown in **Figure 3-9**. At one end of the molecule there is always an adenylic acid to which the transported amino acid attaches at a hydroxyl group of the ribose in the adenylic acid. \nBecause the function of tRNA is to cause attachment of a specific amino acid to a forming protein chain, it is essential that each type of t[RNA also ha](#page-38-0)ve specificity for a particular codon in the mRNA. The specific code in the tRNA that allows it to recognize a specific codon is again a triplet of nucleotide bases and is called an *anticodon.* This anticodon is located approximately in the middle of the tRNA molecule (at the bottom of the cloverleaf configuration shown in **Figure 3-9**). During formation of the protein molecule, the anticodon bases combine loosely by hydrogen bonding with the codon bases of the mRNA. In this way, the respective amino acids are lined up one after another along the mRNA chain, thus establishing the \n \n**Figure 3-9.** A messenger RNA strand is moving through two ribosomes. As each codon passes through, an amino acid is added to the growing protein chain, which is shown in the right-hand ribosome. The transfer RNA molecule transports each specific amino acid to the newly forming protein. \nappropriate sequence of amino acids in the newly forming protein molecule. \n#### **RIBOSOMAL RNA** \nThe third type of RNA in the cell is ribosomal RNA, which constitutes about 60% of the *ribosome.* The remainder of the ribosome is protein, including about 75 types of proteins that are both structural proteins and enzymes needed to manufacture proteins. \nThe ribosome is the physical structure in the cytoplasm on which proteins are actually synthesized. However, it always functions in association with the other two types of RNA; *tRNA* transports amino acids to the ribosome for incorporation into the developing protein, whereas *mRNA* provides the information necessary for sequencing the amino acids in proper order for each specific type of protein to be manufactured. Thus, the ribosome acts as a manufacturing plant in which the protein molecules are formed. \n**Formation of Ribosomes in the Nucleolus.** The DNA genes for the formation of ribosomal RNA are located in five pairs of chromosomes in the nucleus. Each of these chromosomes contains many duplicates of these particular genes because of the large amounts of ribosomal RNA required for cellular function. \nAs the ribosomal RNA forms, it collects in the *nucleolus,* a specialized structure lying adjacent to the chromosomes. When large amounts of ribosomal RNA are being synthesized, as occurs in cells that manufacture large amounts of protein, the nucleolus is a large structure, whereas in cells that synthesize little protein, the nucleolus may not even be seen. Ribosomal RNA is specially processed in the nucleolus, where it binds with ribosomal proteins to form granular condensation products that are primordial subunits of ribosomes. These subunits are then released from the nucleolus and transported through the large pores of the nuclear envelope to almost all parts of the cytoplasm. After the subunits enter the cytoplasm, they are assembled to form mature functional ribosomes. Therefore, proteins are formed in the cytoplasm of the cell, but not in the cell nucleus, because the nucleus does not cont[ain m](#page-39-0)ature ribosomes. \n#### **miRNA AND SMALL INTERFERING RNA** \nA fourth type of RNA in the cell is *microRNA* (miRNA); miRNA are short (21 to 23 nucleotides) single-stranded RNA fragments that regulate gene expression **(Figure 3-10).** The miRNAs are encoded from the transcribed DNA of genes, but they are not translated into proteins and are therefore often called *noncoding RNA*. The miRNAs are processed by the cell into molecules that are complementary to mRNA and act to decrease gene expression. The generation of miRNAs involves special processing of longer primary precursor RNAs called *primiRNAs,* which are the primary transcripts of the gene. \n \n**Figure 3-10.** Regulation of gene expression by microRNA (miRNA). Primary miRNA (pri-miRNA), the primary transcripts of a gene processed in the cell nucleus by the microprocessor complex, are converted to pre-miRNAs. These pre-miRNAs are then further processed in the cytoplasm by *dicer,* an enzyme that helps assemble an RNAinduced silencing complex (RISC) and generates miRNAs. The miRNAs regulate gene expression by binding to the complementary region of the RNA and repressing translation or promoting degradation of the messenger RNA (mRNA) before it can be translated by the ribosome. \nThe pri-miRNAs are then processed in the cell nucleus by the *microprocessor complex* to pre-miRNAs, which are 70-nucleotide, stem loop structures. These pre-miRNAs are then further processed in the cytoplasm by a specific *dicer enzyme* that helps assemble an *RNA-induced silencing complex* (RISC) and generates miRNAs. \nThe miRNAs regulate gene expression by binding to the complementary region of the RNA and promoting repression of translation or degradation of the mRNA before it can be translated by the ribosome. miRNAs are believed to play an important role in normal regulation of cell function, and alterations in miRNA function have been associated with diseases such as cancer and heart disease. \nAnother type of miRNA is *small interfering RNA* (siRNA), also called *silencing RNA* or *short interfering RNA.* The siRNAs are short, double-stranded RNA molecules, comprised of 20 to 25 nucleotides, that interfere with expression of specific genes. siRNAs generally refer to synthetic miRNAs and can be administered to silence expression of specific genes. They are designed to avoid nuclear processing by the microprocessor complex and, after the siRNA enters the cytoplasm, it activates the RISC silencing complex, blocking the translation of mRNA. Because siRNAs can be tailored for any specific sequence in the gene, they can be used to block translation of any mRNA and therefore expression by any gene for which the nucleotide sequence is known. Researchers have proposed that siRNAs may become useful therapeutic tools to silence genes that contribute to the pathophysiology of diseases. \n#### TRANSLATION—FORMATION O[F](#page-38-0) PROTEINS ON THE RIBOSOMES \nWhen a molecule of mRNA comes in contact with a ribosome, it travels through the ribosome, beginning at a predetermined end of the RNA molecule specified by an appropriate sequence of RNA bases called the *chaininitiating codon.* Then, as shown in **Figure 3-9**, while the mRNA travels through the ribosome, a protein molecule is formed, a process called *translation.* Thus, the ribosome reads the codons of the mRNA in much the same way that a tape is read as it passes through the playback head of a tape recorder. Then, when a \"stop\" (or \"chainterminating\") codon slips past the ribosome, the end of a protein molecule is sig[naled, and t](#page-38-0)he p[rotein molecu](#page-40-0)le is freed into the cytoplasm. \n**Polyribosomes.** A single mRNA molecule can form protein molecules in several ribosomes at the same time because the initial end of the RNA strand can pass to a successive ribosome as it leaves the first, as shown at the bottom left in **Figure 3-9** and **Figure 3-11**. The protein molecules are in different stages of development in each ribosome. As a result, clusters of ribosomes frequently occur, with 3 to 10 ribosomes being attached to a single mRNA at the same time. These clusters are called *polyribosomes.* \nAn mRNA can cause formation of a protein molecule in any ribosome; there is no specificity of ribosomes for given types of protein. The ribosome is simply the physical manufacturing plant in which the chemical reactions take place. \n**Many Ribosomes Attach to the Endoplasmic Reticulum.** In Chapter 2, we noted that many ribosomes become attached to the endoplasmic reticulum. This attachment occurs because the initial ends of many forming protein molecules have amino acid sequences that immediately attach to specific receptor sites on the endoplasmic reticulum, causing these molecules to penetrate the \n \n**Figure 3-11.** The physical structure of the ribosomes, as well as their functional relationship to messenger RNA, transfer RNA, and the endoplasmic reticulum during the formation of protein molecules. \nreticulum wall and enter the endoplasmic reticulum matrix. This process gives a granular appearance to the portions of the reticulum where proteins are being formed and are entering the matrix of the reticulum. \n**Figure 3-11** shows the functional relationship of mRNA to the ribosomes and the manner in which the ribosomes attach to the membrane of the endoplasmic reticulum. Note the process of translation occurring in several ribosomes at the same time in response to the same strand of mRNA. Note also the newly forming polypeptide (protein) chains passing through the endoplasmic reticulum membrane into the endoplasmic matrix. \nIt should be noted that except in glandular cells, in which large amounts of protein-containing secretory vesicles are formed, most proteins synthesized by the ribosomes are released directly into the cytosol instead of into the endoplasmic reticulum. These proteins are enzymes and internal structural proteins of the cell. \n**Chemical Steps in Protein Synthesis.** Some of the chemical events that occur in the synthesis of a protein molecule are shown in **Figure 3-12**. This Fig. shows representative reactions for three separate amino acids, $AA_1$ , $AA_2$ , and $AA_{20}$ . The stages of the reactions are as follows: \n- Each amino acid is activated by a chemical process in which ATP combines with the amino acid to form an adenosine monophosphate complex with the amino acid, giving up two high-energy phosphate bonds in the process.\n- 2. The activated amino acid, having an excess of energy, then *combines with its specific tRNA to form an amino acid–tRNA complex* and, at the same time, releases the adenosine monophosphate.\n- 3. The tRNA carrying the amino acid complex then comes in contact with the mRNA molecule in the ribosome, where the anticodon of the tRNA attaches temporarily to its specific codon of the mRNA, thus lining up the amino acid in the appropriate sequence to form a protein molecule. \nThen, under the influence of the enzyme *peptidyl transferase* (one of the proteins in the ribosome), *peptide bonds* are formed between the successive amino acids, thus adding progressively to the protein chain. These \nchemical events require energy from two additional high-energy phosphate bonds, making a total of four high-energy bonds used for each amino acid added to the protein chain. Thus, the synthesis of proteins is one of the most energy-consuming processes of the cell. \n**Peptide Linkage—Combination of Amino Acids.** The successive amino acids in the protein chain combine with one another according to the typical reaction. \n$$\\begin{array}{cccccccccccccccccccccccccccccccccccc$$ \nIn this chemical reaction, a hydroxyl radical (OH-) is removed from the COOH portion of the first amino acid, and a hydrogen (H+) of the NH2 portion of the other amino acid is removed. These combine to form water, and the two reactive sites left on the two successive amino acids bond with each other, resulting in a single molecule. This process is called *peptide linkage*. As each additional amino acid is added, an additional peptide linkage is formed.\n\nMany thousand protein enzymes formed in the manner just described control essentially all the other chemical reactions that take place in cells. These enzymes promote synthesis of lipids, glycogen, purines, pyrimidines, and hundreds of other substances. We discuss many of these synthetic processes in relation to carbohydrate, lipid, and protein metabolism in Chapters 68 through 70. These substances each contribute to the various functions of the cells.\n\nFrom our discussion thus far, it is clear that the genes control both the physical and chemical functions of the cells. However, the degree of activation of respective genes must also be \n \n**Figure 3-12.** Chemical events in the formation of a protein molecule. AMP, Adenosine monophosphate; ATP, adenosine triphosphate; tRNA, transfer RNA. \ncontrolled; otherwise, some parts of the cell might overgrow or some chemical reactions might overact until they kill the cell. Each cell has powerful internal feedback control mechanisms that keep the various functional operations of the cell in step with one another. For each gene ( $\\approx 20,000-25,000$ genes in all), at least one such feedback mechanism exists. \nThere are basically two methods whereby the biochemical activities in the cell are controlled: (1) *genetic regulation,* in which the degree of activation of the genes and the formation of gene products are themselves controlled, and (2) *enzyme regulation,* in which the activity levels of already formed enzymes in the cell are controlled. \n#### **GENETIC REGULATION** \nGenetic regulation, or regulation of gene expression, covers the entire process from transcription of the genetic code in the nucleus to the formation of proteins in the cytoplasm. Regulation of gene expression provides all living organisms with the ability to respond to changes in their environment. In animals that have many different types of cells, tissues, and organs, differential regulation of gene expression also permits the different cell types in the body to each perform their specialized functions. Although a cardiac myocyte contains the same genetic code as a renal tubular epithelial cell, many genes are expressed in cardiac cells that are not expressed in renal tubular cells. The ultimate measure of gene \"expression\" is whether (and how much) of the gene products (proteins) are produced because proteins carry out cell functions specified by the genes. Regulation of gene expression can occur at any point in the pathways of transcription, RNA processing, and translation. \n**The Promoter Controls Gene Expression.** Synthesis of cellular proteins is a complex process that starts with transcription of DNA into RNA. Transcription of DNA is \n \n**Figure 3-13.** Gene transcription in eukaryotic cells. A complex arrangement of multiple clustered enhancer modules is interspersed with insulator elements, which can be located upstream or downstream of a basal promoter containing TATA box (TATA), proximal promoter elements (response elements, RE), and initiator sequences (INR). \ncontrolled by regulatory elements found in the promoter of a gene (Figure 3-13). In eukaryotes, which includes all mammals, the basal promoter consists of a sequence of bases (TATAAA) called the TATA box, the binding site for the TATA-binding protein and several other important transcription factors that are collectively referred to as the transcription factor IID complex. In addition to the transcription factor IID complex, this region is where transcription factor IIB binds to both the DNA and RNA polymerase 2 to facilitate transcription of the DNA into RNA. This basal promoter is found in all protein-coding genes, and the polymerase must bind with this basal promoter before it can begin traveling along the DNA strand to synthesize RNA. The upstream promoter is located farther upstream from the transcription start site and contains several binding sites for positive or negative transcription factors that can affect transcription through interactions with proteins bound to the basal promoter. The structure and transcription factor binding sites in the upstream promoter vary from gene to gene to give rise to the different expression patterns of genes in different tissues. \nTranscription of genes in eukaryotes is also influenced by *enhancers,* which are regions of DNA that can bind transcription factors. Enhancers can be located a great distance from the gene they act on or even on a different chromosome. They can also be located upstream or downstream of the gene that they regulate. Although enhancers may be located far from their target gene, they may be relatively close when DNA is coiled in the nucleus. It is estimated that there are more than 100,000 gene enhancer sequences in the human genome. \nIn the organization of the chromosome, it is important to separate active genes that are being transcribed from genes that are repressed. This separation can be challenging because multiple genes may be located close together on the chromosome. The separation is achieved by chromosomal *insulators.* These insulators are gene sequences that provide a barrier so that a specific gene is isolated against transcriptional influences from surrounding genes. Insulators can vary greatly in their DNA sequence and the proteins that bind to them. One way an insulator activity can be modulated is by *DNA methylation*, which is the case for the mammalian insulin-like growth factor 2 (IGF-2) gene. The mother's allele has an insulator between the enhancer and promoter of the gene that allows for the binding of a transcriptional repressor. However, the paternal DNA sequence is methylated such that the transcriptional repressor cannot bind to the insulator, and the IGF-2 gene is expressed from the paternal copy of the gene. \n#### **Other Mechanisms for Control of Transcription by the Promoter.** Variations in the basic mechanism for control of the promoter have been discovered in the past three decades. Without giving details, let us list some of them: \n- 1. A promoter is frequently controlled by transcription factors located elsewhere in the genome. That is, the regulatory gene causes the formation of a regulatory protein that in turn acts as an activator or repressor of transcription.\n- 2. Occasionally, many different promoters are controlled at the same time by the same regulatory protein. In some cases, the same regulatory protein functions as an activator for one promoter and as a repressor for another promoter.\n- 3. Some proteins are controlled not at the starting point of transcription on the DNA strand but farther along the strand. Sometimes, the control is not even at the DNA strand itself but occurs during the processing of the RNA molecules in the nucleus before they are released into the cytoplasm. Control may also occur at the level of protein formation in the cytoplasm during RNA translation by the ribosomes.\n- 4. In nucleated cells, the nuclear DNA is packaged in specific structural units, the *chromosomes.* Within \neach chromosome, the DNA is wound around small proteins called *histones,* which in turn are held tightly together in a compacted state by still other proteins. As long as the DNA is in this compacted state, it cannot function to form RNA. However, multiple control mechanisms are being discovered that can cause selected areas of chromosomes to become decompacted one part at a time, so that partial RNA transcription can occur. Even then, specific *transcriptor factor*s control the actual rate of transcription by the promoter in the chromosome. Thus, still higher orders of control are used to establish proper cell function. In addition, signals from outside the cell, such as some of the body's hormones, can activate specific chromosomal areas and specific transcription factors, therefore controlling the chemical machinery for function of the cell. \nBecause there are many thousands of different genes in each human cell, the large number of ways in which genetic activity can be controlled is not surprising. The gene control systems are especially important for controlling intracellular concentrations of amino acids, amino acid derivatives, and intermediate substrates and products of carbohydrate, lipid, and protein metabolism. \n#### **CONTROL OF INTRACELLULAR FUNCTION BY ENZYME REGULATION** \nIn addition to control of cell function by genetic regulation, cell activities are also controlled by intracellular inhibitors or activators that act directly on specific intracellular enzymes. Thus, enzyme regulation represents a second category of mechanisms whereby cellular biochemical functions can be controlled. \n**Enzyme Inhibition.** Some chemical substances formed in the cell have direct feedback effects to inhibit the specific enzyme systems that synthesize them. Almost always, the synthesized product acts on the first enzyme in a sequence, rather than on the subsequent enzymes, usually binding directly with the enzyme and causing an allosteric conformational change that inactivates it. One can readily recognize the importance of inactivating the first enzyme because this prevents buildup of intermediary products that are not used. \nEnzyme inhibition is another example of negative feedback control. It is responsible for controlling intracellular concentrations of multiple amino acids, purines, pyrimidines, vitamins, and other substances. \n**Enzyme Activation.** Enzymes that are normally inactive often can be activated when needed. An example of this phenomenon occurs when most of the ATP has been depleted in a cell. In this case, a considerable amount of cyclic adenosine monophosphate (cAMP) begins to be formed as a breakdown product of ATP. The presence of this cAMP, in turn, immediately activates the glycogensplitting enzyme phosphorylase, liberating glucose molecules that are rapidly metabolized, with their energy used for replenishment of the ATP stores. Thus, cAMP acts as an enzyme activator for the enzyme phosphorylase and thereby helps control intracellular ATP concentration. \nAnother interesting example of both enzyme inhibition and enzyme activation occurs in the formation of the purines and pyrimidines. These substances are needed by the cell in approximately equal quantities for the formation of DNA and RNA. When purines are formed, they *inhibit* the enzymes that are required for formation of additional purines. However, they *activate* the enzymes for formation of pyrimidines. Conversely, the pyrimidines inhibit their own enzymes but activate the purine enzymes. In this way, there is continual cross-talk between the synthesizing systems for these two substances, resulting in almost exactly equal amounts of the two substances in the cells at all times. \n**Summary.** There are two principal mechanisms whereby cells control proper proportions and quantities of different cellular constituents: (1) genetic regulation; and (2) enzyme regulation. The genes can be activated or inhibited, and likewise, the enzyme systems can be activated or inhibited. These regulatory mechanisms usually function as feedback control systems that continually monitor the cell's biochemical composition and make corrections as needed. However, on occasion, substances from outside the cell (especially some of the hormones discussed in this text) also control the intracellular biochemical reactions by activating or inhibiting one or more of the intracellular control systems. \n#### THE DNA–GENETIC SYSTEM CONTROLS CELL REPRODUCTION \nCell reproduction is another example of the ubiquitous role that the DNA–genetic system plays in all life processes. The genes and their regulatory mechanisms determine cell growth characteristics and when or whether cells will divide to form new cells. In this way, the allimportant genetic system controls each stage in the development of the human, from the single-cell fertilized ovum to the whole functioning body. Thus, if there is any central theme to life, it is the DNA–genetic system. \n#### **Life Cycle of the Cell** \nThe life cycle of a cell is the period from cell repr[oduction](#page-43-0) to the next cell reproduction. When mammalian cells *are not inhibited and are reproducing as rapidly as they can,* this life cycle may be as little as 10 to 30 hours. It is terminated by a series of distinct physical events called *mitosis* that cause division of the cell into two new daughter cells. The events of mitosis are shown in **Figure 3-14** and described later. The actual stage of mitosis, however, lasts for only about 30 minutes, and thus more than 95% of the life cycle of even rapidly reproducing cells is represented by the interval between mitosis, called *interphase.* \nExcept in special conditions of rapid cellular reproduction, inhibitory factors almost always slow or stop the \n \n**Figure 3-14.** Stages of cell reproduction. *A, B, C,* Prophase. *D,* Prometaphase. *E,* Metaphase. *F,* Anaphase. *G, H,* Telophase. \nuninhibited life cycle of the cell. Therefore, different cells of the body actually have life cycle periods that vary from as little as 10 hours for highly stimulated bone marrow cells to an entire lifetime of the human body for many nerve cells. \n#### **Cell Reproduction Begins with Replication of DNA** \nThe first step of cell reproduction is *replication (duplication) of all DNA in the chromosomes.* It is only after this replication has occurred that mitosis can take place. \nThe DNA begins to be duplicated 5 to 10 hours before mitosis, and the duplication is completed in 4 to 8 hours. The net result is two exact *replicas* of all DNA. These replicas become the DNA in the two new daughter cells that will be formed at mitosis. After replication of the DNA, there is another period of 1 to 2 hours before mitosis begins abruptly. Even during this period, preliminary changes that will lead to the mitotic process are beginning to take place. \n**DNA Replication.** DNA is replicated in much the same way that RNA is transcribed from DNA, except for a few important differences: \n1. Both strands of the DNA in each chromosome are replicated, not just one of them. \n \nFigure 3-15. DNA replication, showing the replication fork and leading and lagging strands of DNA. \n- 2. Both entire strands of the DNA helix are replicated from end to end, rather than small portions of them, as occurs in the transcription of RNA.\n- 3. Multiple enzymes called *DNA polymerase*, which is comparable to RNA polymerase, are essential for replicating DNA. DNA polymerase attaches to and moves along the DNA template strand, adding nucleotides in the 5′ to 3′ direction. Another enzyme, *DNA ligase*, causes bonding of successive DNA nucleotides to one another, using high-energy phosphate bonds to energize these attachments.\n- 4. Replication fork formation. Before DNA can be replicated, the double-stranded molecule must be \"unzipped\" into two single strands (Figure 3-15). Because the DNA helixes in each chromosome are approximately 6 centimeters in length and have millions of helical turns, it would be impossible for the two newly formed DNA helixes to uncoil from each other were it not for some special mechanism. This uncoiling is achieved by DNA helicase enzymes that break the hydrogen bonding between the base pairs of the DNA, permitting the two strands to separate into a Y shape known as the replication fork, the area that will be the template for replication to begin. \nDNA is directional in both strands, signified by a 5′ and 3′ end (see **Figure 3-15**). Replication progresses only in the 5′ to 3′ direction. At the replication fork one strand, the *leading strand*, is oriented in the 3′ to 5′ direction, toward the replication fork, while the *lagging strand* is oriented 5′ to 3′, away from the replication fork. Because of their different orientations, the two strands are replicated differently. \n5. *Primer binding*. Once the DNA strands have been separated, a short piece of RNA called an *RNA primer* binds to the 3' end of the leading strand. Primers are generated by the enzyme *DNA primase*. \n- Primers always bind as the starting point for DNA replication.\n- 6. Elongation. DNA polymerases are responsible for creating the new strand by a process called *elongation*. Because replication proceeds in the 5′ to 3′ direction on the leading strand, the newly formed strand is continuous. The lagging strand begins replication by binding with multiple primers that are only several bases apart. DNA polymerase then adds pieces of DNA, called *Okazaki fragments*, to the strand between primers. This process of replication is discontinuous because the newly created Okazaki fragments are not yet connected. An enzyme, *DNA ligase*, joins the Okazaki fragments to form a single unified strand.\n- 7. Termination. After the continuous and discontinuous strands are both formed, the enzyme exonuclease removes the RNA primers from the original strands, and the primers are replaced with appropriate bases. Another exonuclease \"proofreads\" the newly formed DNA, checking and clipping off any mismatched or unpaired residues. \nAnother enzyme, *topoisomerase*, can transiently break the phosphodiester bond in the backbone of the DNA strand to prevent the DNA in front of the replication fork from being overwound. This reaction is reversible, and the phosphodiester bond reforms as the topoisomerase leaves. \nOnce completed, the parent strand and its complementary DNA strand coils into the double helix shape. The process of replication therefore produces two DNA molecules, each with one strand from the parent DNA and one new strand. For this reason, DNA replication is often described as *semiconservative*; half of the chain is part of the original DNA molecule and half is brand new. \n**DNA Repair, DNA \"Proofreading,\" and \"Mutation.\"**During the hour or so between DNA replication and \nthe beginning of mitosis, there is a period of active repair and \"proofreading\" of the DNA strands. Wherever inappropriate DNA nucleotides have been matched up with the nucleotides of the original template strand, special enzymes cut out the defective areas and replace them with appropriate complementary nucleotides. This repair process, which is achieved by the same DNA polymerases and DNA ligases that are used in replication, is referred to as *DNA proofreading.* \nBecause of repair and proofreading, mistakes are rarely made in the DNA replication process. When a mistake is made, it is called a *mutation.* The mutation may cause formation of some abnormal protein in the cell rather than a needed protein, which may lead to abnormal cellular function and sometimes even cell death. Given that many thousands of genes exist in the human genome, and that the period from one human generation to another is about 30 years, one would expect as many as 10 or many more mutations in the passage of the genome from parent to offspring. As a further protection, however, each human genome is represented by two separate sets of chromosomes, one derived from each parent, with almost identical genes. Therefore, one functional gene of each pair is almost always available to the child, despite mutations. \n#### **CHROMOSOMES AND THEIR REPLICATION** \nThe DNA helixes of the nucleus are packaged in chromosomes. The human cell contains 46 chromosomes arranged in 23 pairs. Most of the genes in the two chromosomes of each pair are identical or almost identical to each other, so it is usually stated that the different genes also exist in pairs, although occasionally this is not the case. \nIn addition to DNA, there is a large amount of protein in the chromosome, composed mainly of many small molecules of electropositively charged *histones.* The histones are organized into vast numbers of small, bobbinlike cores. Small segments of each DNA helix are coiled sequentially around one core after another. \nThe histone cores play an important role in regulation of DNA activity because as long as the DNA is packaged tightly, it cannot function as a template for formation of RNA or replication of new DNA. Furthermore, some of the regulatory proteins *decondense* the histone packaging of the DNA and allow small segments at a time to form RNA. \nSeveral nonhistone proteins are also major components of chromosomes, functioning as chromosomal structural proteins and, in connection with the genetic regulatory machinery, as activators, inhibitors, and enzymes. \nReplication of the chromosomes in their entirety occurs during the next few minutes after replication of the DNA helixes has been completed; the new DNA helixes collect new protein molecules as needed. The two newly formed chromosomes remain attached to each other (until time for mitosis) at a point called the *centromere* located near their center. These duplicated but still attached chromosomes are called *chromatids.* \n#### **CELL MITOSIS** \nThe actual process whereby the cell splits into two new cells is called *mitosis.* Once each chromosome has been replicated to form the two chromatids, mitos[is follows](#page-43-0) automatically within 1 or 2 hours in many cells. \n**Mitotic Apparatus: Function of the Centrioles.** One of the first events of mitosis takes place in the cytoplasm in or around the small structures called *centrioles* during the latter part of interphase*.* As shown in **Figure 3-**14, two pairs of centrioles lie close to each other near one pole of the nucleus. These centrioles, like the DNA and chromosomes, are also replicated during interphase, usually shortly before replication of the DNA. Each centriole is a small cylindrical body about 0.4 micrometer long and about 0.15 micrometer in diameter, consisting mainly of nine parallel tubular structures arranged in the form of a cylinder. The two centrioles of each pair lie at right angles to each other. Each pair of centrioles, along with attached *pericentriolar material,* is called a *centrosome.* \nShortly before mitosis takes place, the two pairs of centrioles begin to move apart from each other. This movement is caused by polymerization of protein microtubules growing between the respective centriole pairs and actually pushing them apart. At the same time, other microtubules grow radially away from each of the centriole pairs, forming a spiny star called the *aster,* in each end of the cell. Some of the spines of the aster penetrate the nuclear membrane and h[elp separate t](#page-43-0)he two sets of chromatids during mitosis. The complex of microtubules extending between the two new centriole pairs is called the *spindle,* and the entire set of microtubules plus the two pairs of centrioles is called the *mitotic apparatus.* \n**Prophase.** [The fi](#page-43-0)rst stage of mitosis, called *prophase,* is shown in **Figure 3-14***A, B,* and *C.* While the spindle is forming, the chromosomes of the nucleus (which in interphase consist of loosely coiled strands) become condensed into well-defined chromosomes. \n**Prometaphase.** During the prometaphase stage (see **Figure 3-14***D*), the growing microtubular spines of the aster fragment the nuclear envelope. At the same time, multiple mic[rotubu](#page-43-0)les from the aster attach to the chromatids at the centromeres, where the paired chromatids are still bound to each other. The tubules then pull one chromatid of each pair toward one cellular pole and its partner toward the opposite pole. \n**Metaphase.** During the metaphase stage (see **Figure 3-14***E*), the two asters of the mitotic apparatus are pushed farther apart. This pushing is believed to occur because the microtubular spines from the two asters, where they interdigitate with each other to form the mitotic spindle, push each other away. Minute contractile protein molecules called *\"molecular motors,\"* which may be composed of the muscle protein *actin,* extend between the respective spines a[nd, us](#page-43-0)ing a stepping action as in muscle, actively slide the spines in a reverse direction along each other. Simultaneously, the chromatids are pulled tightly by their attached microtubules to the very center of the cell, lining up to form the *equatorial plate* of the mitotic spindle. \n**Anaphase.** During the anaphase stage (see **Figure 3-14***F*), the two chromatids of each chromosome are pulled apart at the centromere. All 46 pairs of c[hromatids](#page-43-0) are sepa[rat](#page-43-0)ed, forming two separate sets of 46 *daughter chromosomes.* One of these sets is pulled toward one mitotic aster, and the other is pulled toward the other aster, as the two respective poles of the dividing cell are pushed still farther apart. \n**Telophase.** In the telophase stage (see **Figure 3-14***G* and *H*), the two sets of daughter chromosomes are pushed completely apart. Then, the mitotic apparatus dissipates, and a new nuclear membrane develops around each set of chromosomes. This membrane is formed from portions of the endoplasmic reticulum that are already present in the cytoplasm. Shortly thereafter, the cell pinches in two, midway between the two nuclei. This pinching is caused by the formation of a contractile ring of *microfilaments* composed of *actin* and probably *myosin* (the two contractile proteins of muscle) at the juncture of the newly developing cells that pinches them off from each other. \n#### **CONTROL OF CELL GROWTH AND CELL REPRODUCTION** \nSome cells grow and reproduce all the time, such as the blood-forming cells of the bone marrow, the germinal layers of the skin, and the epithelium of the gut. Many other cells, however, such as smooth muscle cells, may not reproduce for many years. A few cells, such as the neurons and most striated muscle cells, do not reproduce during the entire life of a person, except during the original period of fetal life. \nIn certain tissues, an insufficiency of some types of cells causes them to grow and reproduce rapidly until appropriate numbers of these cells are again available. For example, in some young animals, seven-eighths of the liver can be removed surgically, and the cells of the remaining one-eighth will grow and divide until the liver mass returns to almost normal. The same phenomenon occurs for many glandular cells and most cells of the bone marrow, subcutaneous tissue, intestinal epithelium, and almost any other tissue except highly differentiated cells such as nerve and muscle cells. \nThe mechanisms that maintain proper numbers of the different types of cells in the body are still poorly understood. However, experiments have shown at least three ways in which growth can be controlled. First, growth often is controlled by *growth factors* that come from other parts of the body. Some of these growth factors circulate in the blood, but others originate in adjacent tissues. For [exam](#page-43-0)ple, the epithelial cells of some glands, such as the pancreas, fail to grow without a growth factor from the underlying connective tissue of the gland. Second, most normal cells stop growing when they have run out of space for growth. This phenomenon occurs when cells are grown in tissue culture; the cells grow until they contact a solid object, and then growth stops. Third, cells grown in tissue culture often stop growing when minute am[ounts](#page-46-0) of their own secretions are allowed to collect in the culture medium. This mechanism, too, could provide a means for negative feedback control of growth. \n**Telomeres Prevent the Degradation of Chromosomes.** A *telomere* is a region of repetitive nucleotide sequences located at each end of a chromatid (**Figure 3-16**). Telomeres serve as protective caps that prevent the chromosome from deterioration during cell division. During cell division, a short piece of \"primer\" RNA attaches to the DNA strand to start the replication. However, because the primer does not attach at the very end of the DNA strand, the copy is missing a small section of the DNA. With each cell division, the copied DNA loses additional nucleotides from the telomere region. The nucleotide sequences provided by the telomeres therefore prevent the degradation of genes near the ends of chromosomes. Without telomeres, the genomes would progressively lose information and be truncated after each cell division. Thus, the telomeres can be considered to be disposable chromosomal buffers that help maintain stability of the genes but are gradually consumed during repeated cell divisions. \n \n**Figure 3-16.** Control of cell replication by telomeres and telomerase. The cells' chromosomes are capped by telomeres, which, in the absence of telomerase activity, shorten with each cell division until the cell stops replicating. Therefore, most cells of the body cannot replicate indefinitely. In cancer cells, telomerase is activated, and telomere length is maintained so that the cells continue to replicate themselves uncontrollably. \nEach time a cell divides, an average person loses 30 to 200 base pairs from the ends of that cell's telomeres. In human blood cells, the length of telomeres ranges from 8000 base pairs at birth to as low as 1500 in older people. Eventually, when the telomeres shorten to a critical length, the chromosomes become unstable, and the cells die. This process of telomere shortening is believed to be an important reason for some of the physiological changes associated with aging. Telomere erosion can also occur as a result of diseases, especially those associated with oxidative stress and inflammation. \nIn some cells, such as stem cells of the bone marrow or skin that must be replenished throughout life, or germ cells in the ovaries and testes, the enzyme *telomerase* adds bases to the ends of the telomeres so that many more generations of cells can be produced. However, telomerase activity is usually low in most cells of the body, and after many generations the descendent cells will inherit defective chromoso[mes, become](#page-46-0) *senescent,* and cease dividing. This process of telomere shortening is important in regulating cell proliferation and maintaining gene stability. In cancer cells, telomerase activity is abnormally activated so that telomere length is maintained, making it possible for the cells to replicate over and over again uncontrollably (see **Figure 3-16**). Some scientists have therefore proposed that telomere shortening protects us from cancer and other proliferative diseases. \n**Regulation of Cell Size.** Cell size is determined almost entirely by the amount of functioning DNA in the nucleus. If replication of the DNA does not occur, the cell grows to a certain size and thereafter remains at that size. Conversely, use of the chemical *colchicine* makes it possible to prevent formation of the mitotic spindle and therefore prevent mitosis, even though replication of the DNA continues. In this event, the nucleus contains far greater quantities of DNA than it normally does, and the cell grows proportionately larger. It is assumed that this cell growth results from increased production of RNA and cell proteins, which, in turn, cause the cell to grow larger. \n#### CELL DIFFERENTIATION \nA special characteristic of cell growth and cell division is *cell differentiation,* which refers to changes in the physical and functional properties of cells as they proliferate in the embryo to form the different body structures and organs. The following description of an especially interesting experiment helps explain these processes. \nWhen the nucleus from an intestinal mucosal cell of a frog is surgically implanted into a frog ovum from which the original ovum nucleus was removed, the result is often the formation of a normal frog. This experiment demonstrates that even the intestinal mucosal cell, which is a well-differentiated cell, carries all the necessary genetic information for development of all structures required in the frog's body. \nTherefore, it has become clear that differentiation results not from loss of genes but from selective repression of different gene promoters. In fact, electron micrographs suggest that some segments of DNA helixes that are wound around histone cores become so condensed that they no longer uncoil to form RNA molecules. One explanation for this is as follows. It has been supposed that the cellular genome begins at a certain stage of cell differentiation to produce a regulatory *protein* that forever after represses a select group of genes. Therefore, the repressed genes never function again. Regardless of the mechanism, mature human cells each produce a maximum of about 8000 to 10,000 proteins rather than the potential 20,000 to 25,000 or more that would be produced if all genes were active. \nEmbryological experiments have shown that certain cells in an embryo control differentiation of adjacent cells. For example, the *primordial chordamesoderm* is called the *primary organizer* of the embryo because it forms a focus around which the remainder of the embryo develops. It differentiates into a *mesodermal axis* that contains segmentally arranged *somites* and, as a result of *inductions* in the surrounding tissues, causes the formation of essentially all the organs of the body. \nAnother instance of induction occurs when the developing eye vesicles come into contact with the ectoderm of the head and cause the ectoderm to thicken into a lens plate that folds inward to form the lens of the eye. Therefore, a large share of the embryo develops as a result of such inductions, with one part of the body affecting another part, and this part affecting still other parts. \nThus, although our understanding of cell differentiation is still hazy, we are aware of many control mechanisms whereby differentiation *could* occur. \n#### APOPTOSIS—PROGRAMMED CELL DEATH \nThe many trillions of the body's cells are members of a highly organized community in which the total number of cells is regulated not only by controlling the rate of cell division, but also by controlling the rate of cell death. When cells are no longer needed or become a threat to the organism, they undergo a suicidal *programmed cell death,* or *apoptosis.* This process involves a specific proteolytic cascade that causes the cell to shrink and condense, disassemble its cytoskeleton, and alter its cell surface so that a neighboring phagocytic cell, such as a macrophage, can attach to the cell membrane and digest the cell. \nIn contrast to programmed death, cells that die as a result of an acute injury usually swell and burst due to loss of cell membrane integrity, a process called cell *necrosis.* Necrotic cells may spill their contents, causing inflammation and injury to neighboring cells. Apoptosis, however, is an orderly cell death that results in disassembly and phagocytosis of the cell before any leakage of its contents occurs, and neighboring cells usually remain healthy. \nApoptosis is initiated by activation of a family of proteases called *caspases*, which are enzymes that are synthesized and stored in the cell as inactive *procaspases*. The mechanisms of activation of caspases are complex but, once activated, the enzymes cleave and activate other procaspases, triggering a cascade that rapidly breaks down proteins within the cell. The cell thus dismantles itself, and its remains are rapidly digested by neighboring phagocytic cells. \nA tremendous amount of apoptosis occurs in tissues that are being remodeled during development. Even in adult humans, billions of cells die each hour in tissues such as the intestine and bone marrow and are replaced by new cells. Programmed cell death, however, is normally balanced by formation of new cells in healthy adults. Otherwise, the body's tissues would shrink or grow excessively. Abnormalities of apoptosis may play a key role in neurodegenerative diseases such as Alzheimer disease, as well as in cancer and autoimmune disorders. Some drugs that have been used successfully for chemotherapy appear to induce apoptosis in cancer cells. \n#### CANCER \nCancer may be caused by *mutation* or by some other *abnormal activation* of cellular genes that control cell growth and cell mitosis. *Proto-oncogenes* are normal genes that code for various proteins that control cell adhesion, growth and division. If mutated or excessively activated, proto-oncogenes can become abnormally functioning *oncogenes* capable of causing cancer*.* As many as 100 different oncogenes have been discovered in human cancers. \nAlso present in all cells are *antioncogenes,* also called *tumor suppressor genes*, which suppress the activation of specific oncogenes. Therefore, loss or inactivation of antioncogenes can allow activation of oncogenes that lead to cancer. \nFor several reasons, only a minute fraction of the cells that mutate in the body ever lead to cancer: \n- • First, most mutated cells have less survival capability than normal cells, and they simply die.\n- • Second, only a few of the mutated cells that survive become cancerous because most mutated cells still have normal feedback controls that prevent excessive growth.\n- • Third, cells that are potentially cancerous are often destroyed by the body's immune system before they grow into a cancer. \nMost mutated cells form abnormal proteins within their cell bodies because of their altered genes, and these proteins activate the body's immune system, causing it to form antibodies or sensitized lymphocytes that react against the cancerous cells, destroying them. In people whose immune systems have been suppressed, such as in persons taking immunosuppressant drugs after kidney or heart transplantation, the probability that a cancer will develop is multiplied as much as fivefold. \n• Fourth, the simultaneous presence of several different activated oncogenes is usually required to cause a cancer. For example, one such gene might promote rapid reproduction of a cell line, but no cancer occurs because another mutant gene is not present simultaneously to form the needed blood vessels. \nWhat is it that causes the altered genes? Considering that many trillions of new cells are formed each year in humans, a better question might be to ask why all of us do not develop millions or billions of mutant cancerous cells. The answer is the incredible precision with which DNA chromosomal strands are replicated in each cell before mitosis can take place, along with the proofreading process that cuts and repairs any abnormal DNA strand before the mitotic process is allowed to proceed. Yet, despite these inherited cellular precautions, probably one newly formed cell in every few million still has significant mutant characteristics. \nThus, chance alone is all that is required for mutations to take place, so we can suppose that a large number of cancers are merely the result of an unlucky occurrence. However, the probability of mutations can be greatly increased when a person is exposed to certain chemical, physical, or biological factors, including the following: \n- 1. *Ionizing radiation,* such as x-rays, gamma rays, particle radiation from radioactive substances, and even ultraviolet light, can predispose individuals to cancer. Ions formed in tissue cells under the influence of such radiation are highly reactive and can rupture DNA strands, causing many mutations.\n- 2. *Chemical substances* of certain types may also cause mutations. It was discovered long ago that various aniline dye derivatives are likely to cause cancer, and thus workers in chemical plants producing such substances, if unprotected, have a special predisposition to cancer. Chemical substances that can cause mutation are called *carcinogens.* The carcinogens that currently cause the greatest number of deaths are those in cigarette smoke. These carcinogens cause over 30% of all cancer deaths and at least 85% of lung cancer deaths.\n- 3. *Physical irritants* can also lead to cancer, such as continued abrasion of the linings of the intestinal tract by some types of food. The damage to the tissues leads to rapid mitotic replacement of the cells; the more rapid the mitosis, the greater the chance for mutation.\n- 4. *Hereditary tendency* to cancer occurs in some families. This hereditary tendency results from the fact that most cancers require not one mutation but two or more mutations before cancer occurs. In families that are particularly predisposed to cancer, it is presumed that one or more cancerous genes are already \n- mutated in the inherited genome. Therefore, far fewer additional mutations must take place in such family members before a cancer begins to grow.\n- 5. *Certain types of oncoviruses* can cause various types of cancer. Some examples of viruses associated with cancers in humans include *human papilloma virus* (HPV), *hepatitis B and hepatitis C virus,* Epstein-Barr virus, human immunodeficiency virus (HIV), human T-cell leukemia virus, Kaposi sarcoma–associated herpes virus (KSHV), and Merkel cell polyomavirus. Although the mechanisms whereby oncoviruses cause cancer are not fully understood, there are at least two potential ways. In the case of DNA viruses, the DNA strand of the virus can insert itself directly into one of the chromosomes, thereby causing a mutation that leads to cancer. In the case of RNA viruses, some of these viruses carry with them an enzyme called *reverse transcriptase* that causes DNA to be transcribed from the RNA. The transcribed DNA then inserts itself into the animal cell genome, leading to cancer. \n**Invasive Characteristic of the Cancer Cell.** The major differences between a cancer cell and a normal cell are as follows: \n- 1. The cancer cell does not respect usual cellular growth limits because these cells presumably do not require all the same growth factors that are necessary to cause growth of normal cells.\n- 2. Cancer cells are often far less adhesive to one another than are normal cells. Therefore, they tend to wander through the tissues, enter the blood stream, and be transported all through the body, where they form nidi for numerous new cancerous growths.\n- 3. Some cancers also produce *angiogenic factors* that cause many new blood vessels to grow into the cancer, thus supplying the nutrients required for cancer growth. \n**Why Do Cancer Cells Kill?** Cancer tissue competes with normal tissues for nutrients. Because cancer cells continue to proliferate indefinitely, with their numbers multiplying every day, cancer cells soon demand essentially all the nutrition available to the body or to an essential part of the body. As a result, normal tissues gradually sustain nutritive death. \nSome cancers cause disruption of vital organ functions. For example, a lung cancer might replace healthy tissue to the extent that the lungs cannot absorb enough oxygen to maintain tissues in the rest of the body. \n#### Bibliography \nAlberts B, Johnson A, Lewis J, et al: Molecular Biology of the Cell, 6th ed. New York: Garland Science 2014. \n- Armanios M: Telomeres and age-related disease: how telomere biology informs clinical paradigms. J Clin Invest 123:996, 2013.\n- Bickmore WA, van Steensel B: Genome architecture: domain organization of interphase chromosomes. Cell 152:1270, 2013.\n- Calcinotto A, Kohli J, Zagato E, Pellegrini L, Demaria M, Alimonti A: Cellular senescence: aging, cancer, and injury. Physiol Rev 99:1047-1078, 2019.\n- Clift D, Schuh M: Restarting life: fertilization and the transition from meiosis to mitosis. Nat Rev Mol Cell Biol 14:549, 2013.\n- Coppola CJ, C Ramaker R, Mendenhall EM: Identification and function of enhancers in the human genome. Hum Mol Genet 25(R2):R190-R197, 2016.\n- Feinberg AP: The key role of epigenetics in human disease prevention and mitigation. N Engl J Med 378:1323-1334, 2018.\n- Fyodorov DV, Zhou BR, Skoultchi AI, Bai Y: Emerging roles of linker histones in regulating chromatin structure and function. Nat Rev Mol Cell Biol 19:192-206, 2018.\n- Haberle V, Stark A: Eukaryotic core promoters and the functional basis of transcription initiation. Nat Rev Mol Cell Biol 19:621-637, 2018.\n- Kaushik S, Cuervo AM: The coming of age of chaperone-mediated autophagy. Nat Rev Mol Cell Biol 19:365-381, 2018.\n- Krump NA, You J: Molecular mechanisms of viral oncogenesis in humans. Nat Rev Microbiol 16:684-698, 2018.\n- Leidal AM, Levine B, Debnath J: Autophagy and the cell biology of age-related disease. Nat Cell Biol 20:1338-1348, 2018.\n- Maciejowski J, de Lange T: Telomeres in cancer: tumour suppression and genome instability. Nat Rev Mol Cell Biol 18:175-186, 2017.\n- McKinley KL, Cheeseman IM: The molecular basis for centromere identity and function. Nat Rev Mol Cell Biol 17:16-29, 2016.\n- Monk D, Mackay DJG, Eggermann T, Maher ER, Riccio A: Genomic imprinting disorders: lessons on how genome, epigenome and environment interact. Nat Rev Genet 10:235, 2019.\n- Müller S, Almouzni G: Chromatin dynamics during the cell cycle at centromeres. Nat Rev Genet 18:192-208, 2017.\n- Nigg EA, Holland AJ: Once and only once: mechanisms of centriole duplication and their deregulation in disease. Nat Rev Mol Cell Biol 19:297-312, 2018.\n- Palozola KC, Lerner J, Zaret KS: A changing paradigm of transcriptional memory propagation through mitosis. Nat Rev Mol Cell Biol 20:55-64, 2019.\n- Perez MF, Lehner B: Intergenerational and transgenerational epigenetic inheritance in animals. Nat Cell Biol 21:143, 2019.\n- Prosser SL, Pelletier L: Mitotic spindle assembly in animal cells: a fine balancing act. Nat Rev Mol Cell Biol 18:187-201, 2017.\n- Schmid M, Jensen TH. Controlling nuclear RNA levels. Nat Rev Genet 19:518-529, 2018.\n- Treiber T, Treiber N, Meister G: Regulation of microRNA biogenesis and its crosstalk with other cellular pathways. Nat Rev Mol Cell Biol 20:5-20, 2019. \n\n\nFigure 4-1 lists the approximate concentrations of important electrolytes and other substances in the *extracellular fluid* and *intracellular fluid*. Note that the extracellular fluid contains a large amount of *sodium* but only a small amount of *potassium*. The opposite is true of the intracellular fluid. Also, the extracellular fluid contains a large amount of *chloride* ions, whereas the intracellular fluid contains very little of these ions. However, the concentrations of *phosphates* and *proteins* in the intracellular fluid are considerably greater than those in the extracellular fluid. These differences are extremely important to the life of the cell. The purpose of this chapter is to explain how the differences are brought about by the cell membrane transport mechanisms. \n \n**Figure 4-1.** Chemical compositions of extracellular and intracellular fluids. The question marks indicate that the precise values for intracellular fluid are unknown. The *red line* indicates the cell membrane.\n\nThe structure of the membrane covering the outside of every cell of the body is discussed in Chapter 2 and illustrated in Figure 2-3 and Figure 4-2. This membrane consists almost entirely of a *lipid bilayer* with large numbers of protein molecules in the lipid, many of which penetrate all the way through the membrane. \nThe lipid bilayer is not miscible with the extracellular fluid or the intracellular fluid. Therefore, it constitutes a barrier against movement of water molecules and water-soluble substances between the extracellular and intracellular fluid compartments. However, as shown in **Figure 4-2** by the leftmost arrow, lipid-soluble substances can diffuse directly through the lipid substance. \nThe membrane protein molecules interrupt the continuity of the lipid bilayer, constituting an alternative pathway through the cell membrane. Many of these penetrating proteins can function as *transport proteins*. Some proteins have watery spaces all the way through the molecule and allow free movement of water, as well as selected ions or molecules; these proteins are called *channel proteins*. Other proteins, called *carrier proteins*, bind with molecules or ions that are to be transported, and conformational changes in the protein molecules then move the substances through the interstices of the protein to the \n \n**Figure 4-2.** Transport pathways through the cell membrane and the basic mechanisms of transport. \n \n**Figure 4-3.** Diffusion of a fluid molecule during one thousandth of a second. \nother side of the membrane. Channel proteins and carrier proteins are usually selective for the types of molecules or ions that are allowed to cross the membrane. \n**\"Diffusion\" Versus \"Active Transport.\"** Transport through the cell membrane, either directly through the lipid bilayer or through the proteins, occurs via one of two basic processes, *diffusion* or *active transport.* \nAlthough many variations of these basic mechanisms exist, *diffusion* means random molecular movement of substances molecule by molecule, either through intermolecular spaces in the membrane or in combination with a carrier protein. The energy that causes diffusion is the energy of the normal kinetic motion of matter. \nIn contrast, *active transport* means movement of ions or other substances across the membrane in combination with a carrier protein in such a way that the carrier protein causes the substance to move against an energy gradient, such as from a low-concentration state to a highconcentration state. This movement requires an additional source of energy besides kinetic energy. A more detailed explanation of the basic physics and physical chemistry of these two processes is provided later in this chapter. \n#### DIFFUSION \nAll molecules and ions in the body fluids, including water molecules and dissolved substances, are in constant motion, with each particle moving in its separate way. The motion of these particles is what physicists call \"heat\" the greater the motion, the higher the temperature—and the motion never ceases, except at absolute zero temperature. When a moving molecule, [A, approac](#page-51-0)hes a stationary molecule, B, the electrostatic and other nuclear forces of molecule A repel molecule B, transferring some of the energy of motion of molecule A to molecule B. Consequently, molecule B gains kinetic energy of motion, whereas molecule A slows down, losing some of its kinetic energy. As shown in **Figure 4-3**, a single molecule in a solution bounces among the other molecules—first in one direction, then another, then another, and so forth randomly bouncing thousands of times each second. This continual movement of molecules among one another in liquids or gases is called *diffusion.* \nIons diffuse in the same manner as whole molecules, and even suspended colloid particles diffuse in a similar manner, except that the colloids diffuse far less rapidly than molecular substances because of their large size. \n#### **DIFFUSION THROUGH THE CELL MEMBRANE** \nDiffusion through the cell membrane is divided into two subtypes, called *simple diffusion* and *facilitated diffusion.* Simple diffusion means that kinetic movement of molecules or ions occurs through a membrane opening or through intermolecular spaces without interaction with carrier proteins in the membrane. The rate of diffusion is determined by the amount of substance available, the velocity of kinetic motion, and the number and sizes of openings in the membrane through which the molecules or ions can move. \nFacilitated diffusion requires interaction of a carrier protein. The carrier protein aids passage of molecules or ions through the membrane by binding chemically with them and shuttlin[g them thro](#page-50-0)ugh the membrane in this form. \nSimple diffusion can occur through the cell membrane by two pathways: (1) through the interstices of the lipid bilayer if the diffusing substance is lipid-soluble; and (2) through watery channels that penetrate all the way through some of the large transport proteins, as shown to the left in **Figure 4-2**. \n**Diffusion of Lipid-Soluble Substances Through the Lipid Bilayer.** The *lipid solubility* of a substance is an important factor for determining how rapidly it diffuses through the lipid bilayer. For example, the lipid solubilities of oxygen, nitrogen, carbon dioxide, and alcohols are high, and all these substances can dissolve directly in the lipid bilayer and diffuse through the cell membrane in the same manner that diffusion of water solutes occurs in a watery solution. The rate of diffusion of each of these substances through the membrane is directly proportional to its lipid solubility. Especially large amounts of oxygen can be transported in this way; therefore, oxygen can be delivered to the interior of the cell almost as though the cell membrane did not exist. \n**Diffusion of Water and Other Lipid-Insoluble Molecules Through Protein Channels.** Even though water is highly insoluble in the membrane lipids, it readily passes through channels in protein molecules that penetrate all the way through the membrane. Many of the body's cell membranes contain protein \"pores\" called *aquaporins* that selectively permit rapid passage of water through the membrane. The aquaporins are highly specialized, and there are at least 13 different types in various cells of mammals. \nThe rapidity with which water molecules can diffuse through most cell membranes is astounding. For example, the total amount of water that diffuses in each direction through the red blood cell membrane during each second is about 100 times as great as the volume of the red blood cell. \nOther lipid-insoluble molecules can pass through the protein pore channels in the same way as water molecules if they are water-soluble and small enough. However, as they become larger, their penetration falls off rapidly. For example, the diameter of the urea molecule is only 20% greater than that of water, yet its penetration through the cell membrane pores is about 1000 times less than that of water. Even so, given the astonishing rate of water penetration, this amount of urea penetration still allows rapid transport of urea through the membrane within minutes. \n#### **DIFFUSION THROUGH PROTEIN PORES AND CHANNELS—SELECTIVE PERMEABILITY AND \"GATING\" OF CHANNELS** \nComputerized three-dimensional reconstructions of protein pores and channels have demonstrated tubular pathways all the way from the extracellular to the intracellular fluid. Therefore, substances can move by simple diffusion directly along these pores and channels from one side of the membrane to the other. \nPores are composed of integral cell membrane proteins that form open tubes through the membrane and are always open. However, the diameter of a pore and its electrical charges provide selectivity that permits only certain molecules to pass through. For example, *aquaporins* permit rapid passage of water through cell membranes but exclude other molecules. Aquaporins have a narrow pore that permits water molecules to diffuse through the membrane in single file. The pore is too narrow to permit passage of any hydrated ions. As discussed in Chapters 28 and 76, the density of some aquaporins (e.g., aquaporin-2) in cell membranes is not static but is altered in different physiological conditions. \nThe protein channels are distinguished by two important characteristics: (1) they are often *selectively permeable* to certain substances; and (2) many of the channels can be opened or closed by *gates* that are regulated by electrical signals *(voltage-gated channels)* or chemicals that bind to the channel proteins *(ligand-gated channels).* Thus, ion channels are flexible dynamic structures, and subtle conformational changes influence gating and ion selectivity. \n**Selective Permeability of Protein Channels.** Many protein channels are highly selective for transport of one or more specific ions or molecules. This selectivity results from specific characteristics of the channel, such as its diameter, shape, and the nature of the electrical charges and chemical bonds along its inside surfaces. \n*Potassium channels* permit passage of potassium ions across the cell membrane about 1000 times more readily than they permit passage of sodium ions. This high degree of selectivity cannot be explained entirely by the \n \n**Figure 4-4.** The structure of a potassium channel. The channel is composed of four subunits (only two of which are shown), each with two transmembrane helices. A narrow selectivity filter is formed from the pore loops, and carbonyl oxygens line the walls of the selectivity filter, forming sites for transiently binding dehydrated potassium ions. The interaction of the potassium ions with carbonyl oxygens causes the potassium ions to shed their bound water molecules, permitting the dehydrated potassium ions to pass through the pore. \nmolecular diameters of the ions because potassium ions are slightly larger than sodium ions. Using x-ray crystallography, potassium channels were found to have a *tetrameric structure* consisting of four identical protein subunits surrounding a central pore (**Figure 4-4**). At the top of the channel pore are *pore loops* that form a narrow *selectivity filter*. Lining the selectivity filter are *carbonyl oxygens.* When hydrated potassium ions enter the selectivity filter, they interact with the carbonyl oxygens and shed most of their bound water molecules, permitting the dehydrated potassium ions to pass through the channel. The carbonyl oxygens are too far apart, however, to enable them to interact closely with the smaller sodium ions, which are therefore effectively excluded by the selectivity filter from passing through the pore. \nDifferent selectivity filters for the various ion channels are believed to determine, in large part, the specificity of various channels for cations or anions or for particular ions, such as sodium (Na+), potassium (K+), and calcium (Ca2+), that gain access to the channels. \nOne of the most important of the protein channels, the *sodium channel,* is only 0.3 to 0.5 nanometer in diameter, but the ability of sodium channels to discriminate sodium ions among other competing ions in the surrounding fluids is crucial for proper cellular function. \n \n**Figure 4-5.** Transport of sodium and potassium ions through protein channels. Also shown are conformational changes in the protein molecules to open or close the \"gates\" guarding the channels. \nThe narrowest part of the sodium channel's open pore, the *selectivity filter*, is lined with *strongly negatively charged* amino acid residues, as shown in the top panel of **Figure 4-5**. These strong negative charges can pull small *dehydrated* sodium ions away from their hydrating water molecules into these channels, although the ions do not need to be fully dehydrated to pass through the channels. Once in the channel, the sodium ions diffuse in either direction according to the usual laws of diffusion. Thus, the sodium channel is highly selective for passage of sodium ions. \n**Gating of Protein Channels.** Gating of protein channels provides a means of controlling ion permeability of the channels. This mechanism is shown in both panels of **Figure 4-5** for selective gating of sodium and potassium ions. Some of the gates are thought to be gatelike extensions of the transport protein molecule, which can close the opening of the channel or can be lifted away from the opening by a conformational change in the shape of the protein molecule. \nThe opening and closing of gates are controlled in two principal ways: \n1. Voltage gating. In the case of voltage gating, the molecular conformation of the gate or its chemical bonds responds to the electrical potential across the cell membrane. For example, in the top panel of Figure 4-5, a strong negative charge on the inside of the cell membrane may cause the outside sodium gates to remain tightly closed. Conversely, when the inside of the membrane loses its negative charge, these gates open suddenly and allow sodium to pass inward through the sodium pores. This process is the basic mechanism for eliciting action potentials in nerves that are responsible for nerve signals. In \n- the bottom panel of **Figure 4-5**, the potassium gates are on the intracellular ends of the potassium channels, and they open when the inside of the cell membrane becomes positively charged. The opening of these gates is partly responsible for terminating the action potential, a process discussed in Chapter 5.\n- 2. Chemical (ligand) gating. Some protein channel gates are opened by the binding of a chemical substance (a ligand) with the protein, which causes a conformational or chemical bonding change in the protein molecule that opens or closes the gate. One of the most important instances of chemical gating is the effect of the neurotransmitter acetylcholine on the acetylcholine receptor which serves as a ligand-gated ion channel. Acetylcholine opens the gate of this channel, providing a negatively charged pore about 0.65 nanometer in diameter that allows uncharged molecules or positive ions smaller than this diameter to pass through. This gate is exceedingly important for the transmission of nerve signals from one nerve cell to another (see Chapter 46) and from nerve cells to muscle cells to cause muscle contraction (see Chapter 7). \n#### Open-State Versus Closed-State of Gated Channels. \nFigure 4-6A shows two recordings of electrical current flowing through a single sodium channel when there was an approximately 25-millivolt potential gradient across the membrane. Note that the channel conducts current in an all-or-none fashion. That is, the gate of the channel snaps open and then snaps closed, with each open state lasting for only a fraction of a millisecond, up to several milliseconds, demonstrating the rapidity with which changes can occur during the opening and closing of the protein gates. At one voltage potential, the channel may remain closed all the time or almost all the time, whereas at another voltage, it may remain open either all or most of the time. At in-between voltages, as shown in the figure, the gates tend to snap open and closed intermittently, resulting in an average current flow somewhere between the minimum and maximum. \nPatch Clamp Method for Recording Ion Current Flow Through Single Channels. The patch clamp method for recording ion current flow through single protein channels is illustrated in Figure 4-6B. A micropipette with a tip diameter of only 1 or 2 micrometers is abutted against the outside of a cell membrane. Suction is then applied inside the pipette to pull the membrane against the tip of the pipette, which creates a seal where the edges of the pipette touch the cell membrane. The result is a minute membrane \"patch\" at the tip of the pipette through which electrical current flow can be recorded. \nAlternatively, as shown at the bottom right in **Figure 4-6B**, the small cell membrane patch at the end of the pipette can be torn away from the cell. The pipette with its sealed patch is then inserted into a free solution, which \n \n**Figure 4-6. A**, Recording of current flow through a single voltagegated sodium channel, demonstrating the all or none principle for opening and closing of the channel. **B**, Patch clamp method for recording current flow through a single protein channel. To the left, the recording is performed from a \"patch\" of a living cell membrane. To the right, the recording is from a membrane patch that has been torn away from the cell. \nallows the concentrations of ions both inside the micropipette and in the outside solution to be altered as desired. Also, the voltage between the two sides of the membrane can be set, or \"clamped,\" to a given voltage. \nIt has been possible to make such patches small enough so that only a single channel protein is found in the membrane patch being studied. By varying the concentrations of different ions, as well as the voltage across the membrane, one can determine the transport characteristics of the single channel, along with its gating properties. \n \n**Figure 4-7.** Effect of concentration of a substance on the rate of diffusion through a membrane by simple diffusion and facilitated diffusion. This graph shows that facilitated diffusion approaches a maximum rate, called the *Vmax.* \n#### **FACILITATED DIFFUSION REQUIRES MEMBRANE CARRIER PROTEINS** \nFacilitated diffusion is also called *carrier-mediated diffusion* because a substance transported in this manner diffuses through the membrane with the help of a specific carrier protein. That is, the carrier *facilitates* diffusion of the substance to the other side. \nFacilitated diffusion differs from simple diffusion in the following important way. Although the rat[e of simple](#page-54-1) diffusion through an open channel increases proportionately with the concentration of the diffusing substance, in facilitated diffusion the rate of diffusion approaches a maximum, called Vmax, as the concentration of the diffusing substance increases. This difference between simple diffusion and facilitated diffusion is demonstrated in **Figure 4-7**. The figure shows t[hat a](#page-55-0)s the concentration of the diffusing substance increases, the rate of simple diffusion continues to increase proportionately but, in the case of facilitated diffusion, the rate of diffusion cannot rise higher than the Vmax level. \nWhat is it that limits the rate of facilitated diffusion? A probable answer is the mechanism illustrated in **Figure 4-8**. This Figure shows a carrier protein with a pore large enough to transport a specific molecule partway through. It also shows a binding receptor on the inside of the protein carrier. The molecule to be transported enters the pore and becomes bound. Then, in a fraction of a second, a conformational or chemical change occurs in the carrier protein, so that the pore now opens to the opposite side of the membrane. Because the binding force of the receptor is weak, the thermal motion of the attached molecule causes it to break away and be released on the opposite side of the membrane. The rate at which molecules can be transported by this mechanism can never be greater than the rate at which the carrier protein molecule can undergo change back and forth between its two states. Note specifically, though, that this mechanism allows the transported molecule to move—that is, diffuse—in either direction through the membrane. \n \nFigure 4-8. Postulated mechanism for facilitated diffusion. \nAmong the many substances that cross cell membranes by facilitated diffusion are *glucose* and most of the *amino acids*. In the case of glucose, at least 14 members of a family of membrane proteins (called *GLUT*) that transport glucose molecules have been discovered in various tissues. Some of these GLUT proteins transport other monosaccharides that have structures similar to that of glucose, including galactose and fructose. One of these, glucose transporter 4 (GLUT4), is activated by insulin, which can increase the rate of facilitated diffusion of glucose as much as 10- to 20-fold in insulin-sensitive tissues. This is the principal mechanism whereby insulin controls glucose use in the body, as discussed in Chapter 79.\n\nBy now, it is evident that many substances can diffuse through the cell membrane. What is usually important is the *net* rate of diffusion of a substance in the desired direction. This net rate is determined by several factors. \n**Net Diffusion Rate Is Proportional to the Concentration Difference Across a Membrane. Figure 4-9.4** shows a cell membrane with a high concentration of a substance on the outside and a low concentration of a substance on the inside. The rate at which the substance diffuses *inward* is proportional to the concentration of molecules on the *outside* because this concentration determines how many molecules strike the outside of the membrane each second. Conversely, the rate at which molecules diffuse *outward* is proportional to their concentration *inside* the membrane. Therefore, the rate of net diffusion into the cell is proportional to the concentration on the outside *minus* the concentration on the inside: \nNet diffusion $\\propto (C_o - C_i)$ \n \n**Figure 4-9.** Effect of concentration difference (**A**), electrical potential difference affecting negative ions (**B**), and pressure difference (**C**) to cause diffusion of molecules and ions through a cell membrane. $C_o$ , concentration outside the cell; $C_i$ , concentration inside the cell; $P_1$ pressure 1; $P_2$ pressure 2. \nin which $C_0$ is the concentration outside and $C_i$ is the concentration inside the cell. \nMembrane Electrical Potential and Diffusion of lons-The \"Nernst Potential.\" If an electrical potential is applied across the membrane, as shown in Figure **4-9B**, the electrical charges of the ions cause them to move through the membrane even though no concentration difference exists to cause movement. Thus, in the left panel of Figure 4-9B, the concentration of negative ions is the same on both sides of the membrane, but a positive charge has been applied to the right side of the membrane, and a negative charge has been applied to the left, creating an electrical gradient across the membrane. The positive charge attracts the negative ions, whereas the negative charge repels them. Therefore, net diffusion occurs from left to right. After some time, large quantities of negative ions have moved to the right, creating the condition shown in the right panel of Figure 4-9B, in which a concentration difference of the ions has developed in the direction opposite to the electrical potential difference. The concentration difference now tends to move the ions to the left, whereas the electrical difference tends to move them to the right. When the concentration difference rises high enough, the two effects balance each other. At normal body temperature (98.6°F; 37°C), the electrical difference that will balance a given concentration difference \nof *univalent* ions—such as Na+ ions—can be determined from the following formula, called the *Nernst equation*: \nEMF (in millivolts) =\n$$\\pm 61\\log \\frac{C_1}{C_2}$$ \nin which EMF is the electromotive force (voltage) between side 1 and side 2 of the membrane, $C_1$ is the concentration on side 1, and $C_2$ is the concentration on side 2. This equation is extremely important in understanding the transmission of nerve impulses and is discussed in Chapter 5. \n#### Effect of a Pressure Difference Across the Membrane. \nAt times, a considerable pressure difference develops between the two sides of a diffusible membrane. This pressure difference occurs, for example, at the blood capillary membranes in all tissues of the body. The pressure in many capillaries is about 20 mm Hg greater inside than outside. \nPressure actually means the sum of all the forces of the different molecules striking a unit surface area at a given instant. Therefore, having a higher pressure on one side of a membrane than on the other side means that the sum of all the forces of the molecules striking the channels on that side of the membrane is greater than on the other side. In most cases, this situation is caused by greater numbers of molecules striking the membrane per second on one side than on the other side. The result is that increased amounts of energy are available to cause a net movement of molecules from the high-pressure side toward the low-pressure side. This effect is demonstrated in **Figure 4-9C**, which shows a piston developing high pressure on one side of a pore, thereby causing more molecules to strike the pore on this side and, therefore, more molecules to diffuse to the other side.\n\nBy far, the most abundant substance that diffuses through the cell membrane is water. Enough water ordinarily diffuses in each direction through the red blood cell membrane per second to equal about 100 times the volume of the cell itself. Yet, the amount that normally diffuses in the two directions is balanced so precisely that zero net movement of water occurs. Therefore, the volume of the cell remains constant. However, under certain conditions, a concentration difference for water can develop across a membrane. When this concentration difference for water develops, net movement of water does occur across the cell membrane, causing the cell to swell or shrink, depending on the direction of the water movement. This process of net movement of water caused by a concentration difference of water is called osmosis. \nTo illustrate osmosis, let us assume the conditions shown in Figure 4-10, with pure water on one side of the cell membrane and a solution of sodium chloride on the \n \n**Figure 4-10.** Osmosis at a cell membrane when a sodium chloride solution is placed on one side of the membrane and water is placed on the other side. \nother side. Water molecules pass through the cell membrane with ease, whereas sodium and chloride ions pass through only with difficulty. Therefore, sodium chloride solution is actually a mixture of permeant water molecules and nonpermeant sodium and chloride ions, and the membrane is said to be *selectively permeable* to water but much less so to sodium and chloride ions. Yet, the presence of the sodium and chloride has displaced some of the water molecules on the side of the membrane where these ions are present and, therefore, has reduced the concentration of water molecules to less than that of pure water. As a result, in the example shown in Figure 4-10, more water molecules strike the channels on the left side, where there is pure water, than on the right side, where the water concentration has been reduced. Thus, net movement of water occurs from left to right-that is, osmosis occurs from the pure water into the sodium chloride solution. \n#### **Osmotic Pressure** \nIf in **Figure 4-10** pressure were applied to the sodium chloride solution, osmosis of water into this solution would be slowed, stopped, or even reversed. The amount of pressure required to stop osmosis is called the *osmotic pressure* of the sodium chloride solution. \nThe principle of a pressure difference opposing osmosis is demonstrated in **Figure 4-11**, which shows a selectively permeable membrane separating two columns of fluid, one containing pure water and the other containing a solution of water and any solute that will not penetrate the membrane. Osmosis of water from chamber B into chamber A causes the levels of the fluid columns to become farther and farther apart, until eventually a pressure difference develops between the two sides of the membrane that is great enough to oppose the osmotic effect. The pressure difference across the membrane at this point is equal to the osmotic pressure of the solution that contains the nondiffusible solute. \n \n**Figure 4-11.** Demonstration of osmotic pressure caused by osmosis at a semipermeable membrane. \n#### **Importance of Number of Osmotic Particles (Molar Concentration) in Determining Osmotic Pressure.** \nThe osmotic pressure exerted by particles in a solution, whether they are molecules or ions, is determined by the number of particles per unit volume of fluid, not by the mass of the particles. The reason for this is that each particle in a solution, regardless of its mass, exerts, on average, the same amount of pressure against the membrane. That is, large particles, which have greater mass (m) than small particles, move at a slower velocity (v). The small particles move at higher velocities in such a way that their average kinetic energies (k), as determined by the following equation, \n$$k = \\frac{mv^2}{2}$$ \nare the same for each small particle as for each large particle. Consequently, the factor that determines the osmotic pressure of a solution is the concentration of the solution in terms of the number of particles (which is the same as its *molar concentration* if it is a nondissociated molecule), not in terms of mass of the solute. \n**Osmolality—The Osmole.** To express the concentration of a solution in terms of numbers of particles, a unit called the *osmole* is used in place of grams. \nOne osmole is 1 gram molecular weight of osmotically active solute. Thus, 180 grams of glucose, which is 1 gram molecular weight of glucose, is equal to 1 osmole of glucose because glucose does not dissociate into ions. If a solute dissociates into two ions, 1 gram molecular weight of the solute will become 2 osmoles because the number of osmotically active particles is now twice as great as for the nondissociated solute. Therefore, when fully dissociated, 1 gram molecular weight of sodium chloride, 58.5 grams, is equal to 2 osmoles. \nThus, a solution that has *1 osmole of solute dissolved in each kilogram of water* is said to have an *osmolality of 1 osmole per kilogram,* and a solution that has 1/1000 osmole dissolved per kilogram has an osmolality of 1 milliosmole per kilogram. The normal osmolality of the extracellular and intracellular fluids is about *300 milliosmoles per kilogram of water.* \n**Relationship of Osmolality to Osmotic Pressure.** At normal body temperature, 37°C (98.6°F), a concentration of 1 osmole per liter will cause 19,300 mm Hg osmotic pressure in the solution. Likewise, *1 milliosmole* per liter concentration is equivalent to *19.3 mm Hg* osmotic pressure. Multiplying this value by the 300-milliosmolar concentration of the body fluids gives a total calculated osmotic pressure of the body fluids of 5790 mm Hg. The measured value for this, however, averages only about 5500 mm Hg. The reason for this difference is that many ions in the body fluids, such as sodium and chloride ions, are highly attracted to one another; consequently, they cannot move entirely unrestrained in the fluids and create their full osmotic pressure potential. Therefore, on average, the actual osmotic pressure of the body fluids is about 0.93 times the calculated value. \n**The Term** *Osmolarity***.** *Osmolarity* is the osmolar concentration expressed as *osmoles per liter of solution* rather than osmoles per kilogram of water. Although, strictly speaking, it is osmoles per kilogram of water (osmolality) that determines osmotic pressure, the quantitative differences between osmolarity and osmolality are less than 1% for dilute solutions such as those in the body. Because it is far more practical to measure osmolarity than osmolality, measuring osmolarity is the usual practice in physiological studies. \n#### ACTIVE TRANSPORT OF SUBSTANCES THROUGH MEMBRANES \nAt times, a large concentration of a substance is required in the intracellular fluid, even though the extracellular fluid contains only a small concentration. This situation is true, for example, for potassium ions. Conversely, it is important to keep the concentrations of other ions very low inside the cell, even though their concentrations in the extracellular fluid are high. This situation is especially true for sodium ions. Neither of these two effects could occur by simple diffusion because simple diffusion eventually equilibrates concentrations on the two sides of the membrane. Instead, some energy source must cause excess movement of potassium ions to the inside of cells and excess movement of sodium ions to the outside of cells. When a cell membrane moves molecules or ions uphill against a concentration gradient (or uphill against an electrical or pressure gradient), the process is called *active transport.* \nSome examples of substances that are actively transported through at least some cell membranes include sodium, potassium, calcium, iron, hydrogen, chloride, iodide, and urate ions, several different sugars, and most of the amino acids. \n**Primary Active Transport and Secondary Active Transport.** Active transport is divided into two types according to the source of the energy used to facilitate the transport, *primary active transport* and *secondary active transport.* In primary active transport, the energy is derived directly from the breakdown of adenosine triphosphate (ATP) or some other high-energy phosphate compound. In secondary active transport, the energy is derived secondarily from energy that has been stored in the form of ionic concentration differences of secondary molecular or ionic substances between the two sides of a cell membrane, created originally by primary active transport. In both cases, transport depends on *carrier proteins* that penetrate through the cell membrane, as is true for facilitated diffusion. However, in active transport, the carrier protein functions differently from the carrier in facilitated diffusion because it is capable of imparting energy to the transported substance to move it against the electrochemical gradient. The following sections provide some examples of primary active transport and secondary active transport, with more detailed explanations of their principles of function. \n#### **PRIMARY ACTIVE TRANSPORT** \n#### **Sodium-Potassium Pump Transports Sodium Ions Out of Cells and Potassium Ions into Cells** \nAmong the substances that are transported by primary active transport are sodium, potassium, calcium, hydrogen, chloride, and a few other ions. The active transport mechanism that has been studied in greatest detail is the *sodium-potassium* (Na+-K+) pump, a transporter that pumps sodium ions outward through the cell membrane of all cells and, at the same time, pumps potassium ions from the outside to the inside. This pump is responsible for mainta[ining the so](#page-58-0)dium and potassium concentration differences across the cell membrane, as well as for establishing a negative electrical voltage inside the cells. Indeed, Chapter 5 shows that this pump is also the basis of nerve function, transmitting nerve signals throughout the nervous system. \n**Figure 4-12** shows the basic physical components of the Na+-K+ pump. The *carrier protein* is a complex of two separate globular proteins—a larger one called the α subunit, with a molecular weight of about 100,000, and a smaller one called the β subunit, with a molecular weight of about 55,000. Although the function of the smaller protein is not known (except that it might anchor the protein complex in the lipid membrane), the larger protein has three specific features that are important for the functioning of the pump: \n1. It has three *binding sites for sodium ions* on the portion of the protein that protrudes to the inside of the cell. \n \n**Figure 4-12.** Postulated mechanism of the sodium-potassium pump. ADP, Adenosine diphosphate; ATP, adenosine triphosphate; Pi, phosphate ion. \n- 2. It has two *binding sites for potassium ions* on the outside.\n- 3. The inside portion of this protein near the sodium binding sites has adenosine triphosphatase (AT-Pase) activity. \nWhen two potassium ions bind on the outside of the carrier protein and three sodium ions bind on the inside, the ATPase function of the protein becomes activated. Activation of the ATPase function leads to cleavage of one molecule of ATP, splitting it to adenosine diphosphate (ADP) and liberating a high-energy phosphate bond of energy. This liberated energy is believed to cause a chemical and conformational change in the protein carrier molecule, extruding three sodium ions to the outside and two potassium ions to the inside. \nAs with other enzymes, the Na+-K+ ATPase pump can run in reverse. If the electrochemical gradients for Na+ and K+ are experimentally increased to the degree that the energy stored in their gradients is greater than the chemical energy of ATP hydrolysis, these ions will move down their concentration gradients, and the Na+-K+ pump will synthesize ATP from ADP and phosphate. The phosphorylated form of the Na+-K+ pump, therefore, can either donate its phosphate to ADP to produce ATP or use the energy to change its conformation and pump Na+ out of the cell and K+ into the cell. The relative concentrations of ATP, ADP, and phosphate, as well as the electrochemical gradients for Na+ and K+, determine the direction of the enzyme reaction. For some cells, such as electrically active nerve cells, 60% to 70% of the cell's energy requirement may be devoted to pumping Na+ out of the cell and K+ into the cell. \n**The Na+-K+ Pump Is Important for Controlling Cell Volume.** One of the most important functions of the Na+-K+ pump is to control the cell volume. Without function of this pump, most cells of the body would swell until they burst. \nThe mechanism for controlling the volume is as follows. Inside the cell are large numbers of proteins and other organic molecules that cannot escape from the cell. Most of these proteins and other organic molecules are negatively charged and, therefore, attract large numbers of potassium, sodium, and other positive ions. All these molecules and ions then cause osmosis of water to the interior of the cell. Unless this process is checked, the cell will swell indefinitely until it bursts. The normal mechanism for preventing this outcome is the Na+-K+ pump. Note again that this mechanism pumps three Na+ ions to the outside of the cell for every two K+ ions pumped to the interior. Also, the membrane is far less permeable to sodium ions than to potassium ions and, once the sodium ions are on the outside, they have a strong tendency to stay there. This process thus represents a net loss of ions out the cell, which also initiates osmosis of water out of the cell. \nIf a cell begins to swell for any reason, the Na+-K+ pump is automatically activated, moving still more ions to the exterior and carrying water with them. Therefore, the Na+-K+ pump performs a continual surveillance role in maintaining normal cell volume. \n**Electrogenic Nature of the Na+-K+ Pump.** The fact that the Na+-K+ pump moves three Na+ ions to the exterior for every two K+ ions that are moved to the interior means that a net of one positive charge is moved from the interior of the cell to the exterior of the cell for each cycle of the pump. This action creates positivity outside the cell but results in a deficit of positive ions inside the cell; that is, it causes negativity on the inside. Therefore, the Na+-K+ pump is said to be *electrogenic* because it creates an electrical potential across the cell membrane. As discussed in Chapter 5, this electrical potential is a basic requirement in nerve and muscle fibers for transmitting nerve and muscle signals. \n#### **Primary Active Transport of Calcium Ions** \nAnother important primary active transport mechanism is the *calcium pump*. Calcium ions are normally maintained at an extremely low concentration in the intracellular cytosol of virtually all cells in the body, at a concentration about 10,000 times less than that in the extracellular fluid. This level of maintenance is achieved mainly by two primary active transport calcium pumps. One, which is in the cell membrane, pumps calcium to the outside of the cell. The other pumps calcium ions into one or more of the intracellular vesicular organelles of the cell, such as the sarcoplasmic reticulum of muscle cells and the mitochondria in all cells. In each of these cases, the carrier protein penetrates the membrane and functions as an enzyme ATPase, with the same capability to cleave ATP as the ATPase of the sodium carrier protein. The difference is that this protein has a highly specific binding site for calcium instead of for sodium.\n\nPrimary active transport of hydrogen ions is especially important at two places in the body: (1) in the gastric glands of the stomach; and (2) in the late distal tubules and cortical collecting ducts of the kidneys. \nIn the gastric glands, the deep-lying *parietal cells* have the most potent primary active mechanism for transporting hydrogen ions of any part of the body. This mechanism is the basis for secreting hydrochloric acid in stomach digestive secretions. At the secretory ends of the gastric gland parietal cells, the hydrogen ion concentration is increased as much as a million-fold and then is released into the stomach, along with chloride ions, to form hydrochloric acid. \nIn the renal tubules, special *intercalated cells* found in the late distal tubules and cortical collecting ducts also transport hydrogen ions by primary active transport. In this case, large amounts of hydrogen ions are secreted from the blood into the renal tubular fluid for the purpose of eliminating excess hydrogen ions from the body fluids. The hydrogen ions can be secreted into the renal tubular fluid against a concentration gradient of about 900-fold. Yet, as discussed in Chapter 31, most of these hydrogen ions combine with tubular fluid buffers before they are eliminated in the urine \n#### **Energetics of Primary Active Transport** \nThe amount of energy required to transport a substance actively through a membrane is determined by how much the substance is concentrated during transport. Compared with the energy required to concentrate a substance 10-fold, concentrating it 100-fold requires twice as much energy, and concentrating it 1000-fold requires three times as much energy. In other words, the energy required is proportional to the *logarithm* of the degree that the substance is concentrated, as expressed by the following formula: \nEnergy (in calories per osmole) = 1400 log\n$$\\frac{C_1}{C_2}$$ \nThus, in terms of calories, the amount of energy required to concentrate 1 osmole of a substance 10-fold is about 1400 calories, whereas to concentrate it 100-fold, 2800 calories are required. One can see that the energy expenditure for concentrating substances in cells or for removing substances from cells against a concentration gradient can be tremendous. Some cells, such as those lining the renal tubules and many glandular cells, expend as much as 90% of their energy for this purpose alone.\n\nWhen sodium ions are transported out of cells by primary active transport, a large concentration gradient of \nsodium ions across the cell membrane usually develops, with a high concentration outside the cell and a low concentration inside. This gradient represents a storehouse of energy, because the excess sodium outside the cell membrane is always attempting to diffuse to the interior. Under appropriate conditions, this diffusion energy of sodium can pull other substances along with the sodium through the cell membrane. This phenomenon, called *cotransport,* is one form of *secondary active transport.* \nFor sodium to pull another substance along with it, a coupling mechanism is required; this is achieved by means of still another carrier protein in the cell membrane. The carrier in this case serves as an attachment point for both the sodium ion and the substance to be co-transported. Once they are both attached, the energy gradient of the sodium ion causes the sodium ion and the other substance to be transported together to the interior of the cell. \nIn *counter-transport,* sodium ions again attempt to diffuse to the interior of the cell because of their large concentration gradient. However, this time, the substance to be transported is on the inside of the cell and is transported to the outside. Therefore, the sodium ion binds to the carrier protein, where it projects to the exterior surface of the membrane, and the substance to be countertransported binds to the interior projection of the carrier protein. Once both have become bound, a conformational change occurs, and energy released by the action of the sodium ion moving to the interior causes the other substance to move to the exterior. \n#### **Co-Transport o[f Glucose a](#page-60-0)nd Amino Acids Along with Sodium Ions** \nGlucose and many amino acids are transported into most cells against large concentration gradients; the mechanism of this action is entirely by co-transport, as shown in **Figure 4-13**. Note that the transport carrier protein has two binding sites on its exterior side, one for sodium and one for glucose. Also, the concentration of sodium ions is high on the outside and low on the inside, which provides energy for the transport. A special property of the transport protein is that a conformational change to allow sodium movement to the interior will not occur until a glucose molecule also attaches. When they both become attached, the conformational change takes place, and the sodium and glucose are transported to the inside of the cell at the same time. Hence, this is a *sodium-glucose co-transporter*. Sodium-glucose cotransporters are especially important for transporting glucose across renal and intestinal epithelial cells, as discussed in Chapters 28 and 66. \n*Sodium co-transport of amino acids* occurs in the same manner as for glucose, except that it uses a different set of transport proteins. At least five *amino acid transport proteins* have been identified, each of which is responsible for transporting one subset of amino acids with specific molecular characteristics. \n \n**Figure 4-13** Postulated mechanism for sodium co-transport of glucose. \n \n**Figure 4-14.** Sodium counter-transport of calcium and hydrogen ions. \nSodium co-transport of glucose and amino acids occurs especially through the epithelial cells of the intestinal tract and the renal tubules of the kidneys to promote absorption of these substances into the blood. This process will be discussed in later chapters. \nOther important co-transport mechanisms in at least some cells include co-transport of potassium, chloride, bicarbonate, phosphate, iodine, iron, and urate ions. \n#### **Sodium Counter-Tran[sport of Ca](#page-60-1)lcium and Hydrogen Ions** \nTwo especially important counter-transporters (i.e., transport in a direction opposite to the primary ion) are *sodium-calcium counter-transport* and *sodium-hydrogen counter-transport* (**Figure 4-14**). \nSodium-calcium counter-transport occurs through all or almost all cell membranes, with sodium ions moving to the interior and calcium ions to the exterior; both are bound to the same transport protein in a countertransport mode. This mechanism is in addition to the primary active transport of calcium that occurs in some cells. \nSodium-hydrogen counter-transport occurs in several tissues. An especially important example is in the *proximal tubules* of the kidneys, where sodium ions move from the lumen of the tubule to the interior of the tubular cell and hydrogen ions are counter-transported into the tubule lumen. As a mechanism for concentrating hydrogen ions, counter-transport is not nearly as powerful as the primary active transport of hydrogen ions that occurs in the more distal renal tubules, but it can transport extremely *large numbers of hydrogen ions,* thus making it a key to hydrogen ion control in the body fluids, as discussed in detail in Chapter 31. \n \n**Figure 4-15.** Basic mechanism of active transport across a layer of cells. \n#### **ACTIVE TRANSPORT THROUGH CELLULAR SHEETS** \nAt many places in the body, substances must be transported all the way through a cellular sheet instead of simply through the cell membrane. Transport of this type occurs through the following: (1) intestinal epithelium; (2) epithelium of the renal tubules; (3) epithelium of all exocrine glands; (4) epithelium of the gallbladder; and (5) membrane of the choroid plexus of the brain, along with other membranes. \nThe bas[ic mechanism](#page-61-0) for transport of a substance through a cellular sheet is as follows: (1) *active transport* through the cell membrane *on one side* of the transporting cells in the sheet; and then (2) either *simple diffusion* or *facilitated diffusion* through the membrane *on the opposite side* of the cell. \n**Figure 4-15** shows a mechanism for the transport of sodium ions through the epithelial sheet of the intestines, gallbladder, and renal tubules. This figure shows that the epithelial cells are connected together tightly at the luminal pole by means of junctions. The brush border on the luminal surfaces of the cells is permeable to both sodium ions and water. Therefore, sodium and water diffuse readily from the lumen into the interior of the cell. Then, at the basal and lateral membranes of the cells, sodium ions are actively transported into the extracellular fluid of the surrounding connective tissue and blood vessels. This action creates a high sodium ion concentration gradient across these membranes, which in turn causes osmosis of water. Thus, active transport of sodium ions at the basolateral sides of the epithelial cells results in the transport not only of sodium ions but also of water. \nIt is through these mechanisms that almost all nutrients, ions, and other substances are absorbed into the blood from the intestine. These mechanisms are also how the same substances are reabsorbed from the glomerular filtrate by the renal tubules. \nNumerous examples of the different types of transport discussed in this chapter are provided throughout this text. \n#### Bibliography \nAgre P, Kozono D: Aquaporin water channels: molecular mechanisms for human diseases. FEBS Lett 555:72, 2003. \nBröer S: Amino acid transport across mammalian intestinal and renal epithelia. Physiol Rev 88:249, 2008. \nDeCoursey TE: Voltage-gated proton channels: molecular biology, physiology, and pathophysiology of the H(V) family. Physiol Rev 93:599, 2013. \nDiPolo R, Beaugé L: Sodium/calcium exchanger: influence of metabolic regulation on ion carrier interactions. Physiol Rev 86:155, 2006. \nDrummond HA, Jernigan NL, Grifoni SC: Sensing tension: epithelial sodium channel/acid-sensing ion channel proteins in cardiovascular homeostasis. Hypertension 51:1265, 2008. \nEastwood AL, Goodman MB: Insight into DEG/ENaC channel gating from genetics and structure. Physiology (Bethesda) 27:282, 2012. \nFischbarg J: Fluid transport across leaky epithelia: central role of the tight junction and supporting role of aquaporins. Physiol Rev 90:1271, 2010. \nGadsby DC: Ion channels versus ion pumps: the principal difference, in principle. Nat Rev Mol Cell Biol 10:344, 2009. \nGhezzi C, Loo DDF, Wright EM. Physiology of renal glucose handling via SGLT1, SGLT2 and GLUT2. Diabetologia 61:2087-2097, 2018. \nHilge M: Ca2+ regulation of ion transport in the Na+/Ca2+ exchanger. J Biol Chem 287:31641, 2012. \nJentsch TJ, Pusch M. CLC Chloride channels and transporters: structure, function, physiology, and disease. Physiol Rev 2018 98:1493- 1590, 2018. \nKaksonen M, Roux A. Mechanisms of clathrin-mediated endocytosis. Nat Rev Mol Cell Biol 19:313-326, 2018. \nKandasamy P, Gyimesi G, Kanai Y, Hediger MA. Amino acid transporters revisited: new views in health and disease. Trends Biochem Sci 43:752-789, 2018. \nPapadopoulos MC, Verkman AS: Aquaporin water channels in the nervous system. Nat Rev Neurosci 14:265, 2013. \nRieg T, Vallon V. Development of SGLT1 and SGLT2 inhibitors. Diabetologia 61:2079-2086, 2018. \nSachs F: Stretch-activated ion channels: what are they? Physiology 25:50, 2010. \nSchwab A, Fabian A, Hanley PJ, Stock C: Role of ion channels and transporters in cell migration. Physiol Rev 92:1865, 2012. \nStransky L, Cotter K, Forgac M. The function of V-ATPases in cancer. Physiol Rev 96:1071-1091, 2016 \nTian J, Xie ZJ: The Na-K-ATPase and calcium-signaling microdomains. Physiology (Bethesda) 23:205, 2008. \nVerkman AS, Anderson MO, Papadopoulos MC. Aquaporins: important but elusive drug targets. Nat Rev Drug Discov 13:259-277, 2014. \nWright EM, Loo DD, Hirayama BA: Biology of human sodium glucose transporters. Physiol Rev 91:733, 2011. \n\n\nElectrical potentials exist across the membranes of virtually all cells of the body. Some cells, such as nerve and muscle cells, generate rapidly changing electrochemical impulses at their membranes, and these impulses are used to transmit signals along the nerve or muscle membranes. In other types of cells, such as glandular cells, macrophages, and ciliated cells, local changes in membrane potentials also activate many of the cell's functions. This chapter reviews the basic mechanisms whereby membrane potentials are generated at rest and during action by nerve and muscle cells. See Video 5-1. \n\n\nIn Figure 5-1A, the potassium concentration is great inside a nerve fiber membrane but very low outside the membrane. Let us assume that the membrane in this case is permeable to the potassium ions but not to any other ions. Because of the large potassium concentration gradient from the inside toward the outside, there is a strong tendency for potassium ions to diffuse outward through the membrane. As they do so, they carry positive electrical charges to the outside, thus creating electropositivity outside the membrane and electronegativity inside the membrane because of negative anions that remain behind and do not diffuse outward with the potassium. Within about 1 millisecond, the potential difference between the inside and outside, called the diffusion potential, becomes great enough to block further net potassium diffusion to the exterior, despite the high potassium ion concentration gradient. In the normal mammalian nerve fiber, the potential difference is about 94 millivolts, with negativity inside the fiber membrane. \n**Figure 5-1***B* shows the same phenomenon as in **Figure 5-1***A*, but this time with a high concentration of sodium ions *outside* the membrane and a low concentration of sodium ions *inside*. These ions are also positively charged. This time, the membrane is highly permeable to the sodium ions but is impermeable to all other ions. Diffusion of the positively charged sodium ions to the inside \ncreates a membrane potential of opposite polarity to that in **Figure 5-1***A*, with negativity outside and positivity inside. Again, the membrane potential rises high enough within milliseconds to block further net diffusion of sodium ions to the inside; however, this time, in the mammalian nerve fiber, *the potential is about 61 millivolts positive inside the fiber.* \nThus, in both parts of **Figure 5-1**, we see that a concentration difference of ions across a selectively permeable membrane can, under appropriate conditions, create a membrane potential. Later in this chapter, we show that many of the rapid changes in membrane potentials observed during nerve and muscle impulse transmission result from such rapidly changing diffusion potentials. \nThe Nernst Equation Describes the Relationship of Diffusion Potential to the lon Concentration Difference Across a Membrane. The diffusion potential across a membrane that exactly opposes the net diffusion of a particular ion through the membrane is called the *Nernst potential* for that ion, a term that was introduced in Chapter 4. The magnitude of the Nernst potential is determined by the *ratio* of the concentrations of that specific ion on the two sides of the membrane. The greater this ratio, the greater the tendency for the ion to diffuse in one direction and therefore the greater the Nernst potential required to prevent additional net diffusion. The following equation, called the *Nernst equation*, can be used to calculate the Nernst potential for any univalent ion at the normal body temperature of 98.6°F (37°C): \nEMF (millivolts) =\n$$\\pm \\frac{61}{z} \\times log \\frac{Concentration\\ inside}{Concentration\\ outside}$$ \nwhere EMF is the electromotive force and z is the electrical charge of the ion (e.g., +1 for $K^+$ ). \nWhen using this formula, it is usually assumed that the potential in the extracellular fluid outside the membrane remains at zero potential, and the Nernst potential is the potential inside the membrane. Also, the sign of the potential is positive (+) if the ion diffusing from inside to outside is a negative ion, and it is negative (–) if the ion is positive. Thus, when the concentration of positive potassium ions on the inside is 10 times that on the outside, the log of 10 is 1, so the Nernst potential calculates to be -61 millivolts inside the membrane.\n\n**Figure 5-1 A**, Establishment of a diffusion potential across a nerve fiber membrane, caused by diffusion of potassium ions from inside the cell to outside the cell through a membrane that is selectively permeable only to potassium. **B**, Establishment of a diffusion potential when the nerve fiber membrane is permeable only to sodium ions. Note that the internal membrane potential is negative when potassium ions diffuse and positive when sodium ions diffuse because of opposite concentration gradients of these two ions. \nThe Goldman Equation Is Used to Calculate the Diffusion Potential When the Membrane Is Permeable to Several Different Ions. When a membrane is permeable to several different ions, the diffusion potential that develops depends on three factors: (1) the polarity of the electrical charge of each ion; (2) the permeability of the membrane (P) to each ion; and (3) the concentration (C) of the respective ions on the inside (i) and outside (o) of the membrane. Thus, the following formula, called the *Goldman equation* or the *Goldman-Hodgkin-Katz equation*, gives the calculated membrane potential on the *inside* of the membrane when two univalent positive ions, sodium (Na+) and potassium (K+), and one univalent negative ion, chloride (Cl-), are involved: \n$$EMF \\ (millivolts) = -61 \\times log \\frac{C_{Na_{i}^{+}}P_{Na^{+}} + C_{K_{i}^{+}}P_{K^{+}} + C_{Cl_{0}}P_{Cl^{-}}}{C_{Na_{0}^{+}}P_{Na^{+}} + C_{K_{0}^{+}}P_{K^{+}} + C_{Cl_{i}^{-}}P_{Cl^{-}}}$$ \nSeveral key points become evident from the Goldman equation. First, sodium, potassium, and chloride ions are the most important ions involved in the development of membrane potentials in nerve and muscle fibers, as well as in the neuronal cells. The concentration gradient of each of these ions across the membrane helps determine the voltage of the membrane potential. \nSecond, the quantitative importance of each of the ions in determining the voltage is proportional to the membrane permeability for that particular ion. If the membrane has zero permeability to sodium and chloride ions, the membrane potential becomes entirely dominated by the concentration gradient of potassium ions alone, and the resulting potential will be equal to the Nernst potential for potassium. The same holds true for each of the other two ions if the membrane should become selectively permeable for either one of them alone. \nThird, a positive ion concentration gradient from *inside* the membrane to the *outside* causes electronegativity \n**Table 5-1** Resting Membrane Potential in Different Cell Types \n| Cell Type | Resting Potential (mV) |\n|-----------------|---------------------------|\n| Neurons | −60 to −70 |\n| Skeletal muscle | −85 to −95 |\n| Smooth muscle | −50 to −60 |\n| Cardiac muscle | −80 to −90 |\n| Hair (cochlea) | –15 to –40 |\n| Astrocyte | -80 to -90 |\n| Erythrocyte | −8 to −12 |\n| Photoreceptor | –40 (dark) to –70 (light) | \ninside the membrane. The reason for this phenomenon is that excess positive ions diffuse to the outside when their concentration is higher inside than outside the membrane. This diffusion carries positive charges to the outside but leaves the nondiffusible negative anions on the inside, thus creating electronegativity on the inside. The opposite effect occurs when there is a gradient for a negative ion. That is, a chloride ion gradient from the outside to the inside causes negativity inside the cell because excess negatively charged chloride ions diffuse to the inside while leaving the nondiffusible positive ions on the outside. \nFourth, as explained later, the permeability of the sodium and potassium channels undergoes rapid changes during transmission of a nerve impulse, whereas the permeability of the chloride channels does not change greatly during this process. Therefore, rapid changes in sodium and potassium permeability are primarily responsible for signal transmission in neurons, which is the subject of most of the remainder of this chapter. \nResting Membrane Potential of Different Cell Types. In some cells, such as the cardiac pacemaker cells discussed in Chapter 10, the membrane potential is continuously changing, and the cells are never \"resting\". In many other cells, even excitable cells, there is a quiescent period in which a resting membrane potential can be measured. Table 5-1 shows the approximate resting membrane potentials of some different types of cells. The membrane potential is obviously very dynamic in excitable cells such as neurons, in which action potentials occur. However, even in nonexcitable cells, the membrane potential (voltage) also changes in response to various stimuli, which alter activities for the various ion transporters, ion channels, and membrane permeability for sodium, potassium, calcium, and chloride ions. The resting membrane potential is, therefore, only a brief transient state for many cells. \n**Electrochemical Driving Force.** When multiple ions contribute to the membrane potential, the equilibrium potential for any of the contributing ions will differ from the membrane potential, and there will be an *electrochemical driving force* ( $V_{df}$ ) for each ion that tends to cause net \n \n**Figure 5-2** Measurement of the membrane potential of the nerve fiber using a microelectrode. \nmovement of the ion across the membrane. This driving force is equal to the difference between the membrane potential $(V_m)$ and the equilibrium potential of the ion $(V_{eq})$ Thus, $V_{df} = V_m - V_{eq}.$ \nThe arithmetic sign of $V_{df}$ (positive or negative) and the valence of the ion (cation or anion) can be used to predict the direction of ion flow across the membrane, into or out of the cell. For cations such as $Na^+$ and $K^+$ , a positive $V_{df}$ predicts ion movement out of the cell down its electrochemical gradient, and a negative $V_{df}$ predicts ion movement into the cell. For anions, such as $Cl^-$ , a positive $V_{df}$ predicts ion movement into the cell, and a negative $V_{df}$ predicts ion movement out of the cell. When $V_m = V_{eq}$ , there is no net movement of the ion into or out of the cell. Also, the direction of ion flux through the membrane reverses as $V_m$ becomes greater than or less than $V_{eq}$ ; hence, the equilibrium potential ( $V_{eq}$ ) is also called the *reversal potential*. \n#### Measuring the Membrane Potential \nThe method for measuring the membrane potential is simple in theory but often difficult in practice because of the small size of most of the cells and fibers. Figure 5-2 shows a small micropipette filled with an electrolyte solution. The micropipette is impaled through the cell membrane to the interior of the fiber. Another electrode, called the indifferent electrode, is then placed in the extracellular fluid, and the potential difference between the inside and outside of the fiber is measured using an appropriate voltmeter. This voltmeter is a highly sophisticated electronic apparatus that is capable of measuring small voltages despite extremely high resistance to electrical flow through the tip of the micropipette, which has a lumen diameter usually less than 1 micrometer and a resistance of more than 1 million ohms. For recording rapid changes in the membrane potential during transmission of nerve impulses, the microelectrode is connected to an oscilloscope, as explained later in the chapter. \nThe lower part of **Figure 5-3** shows the electrical potential that is measured at each point in or near the nerve fiber membrane, beginning at the left side of the figure and passing to the right. As long as the electrode is outside the neuronal membrane, the recorded potential \n \n**Figure 5-3** Distribution of positively and negatively charged ions in the extracellular fluid surrounding a nerve fiber and in the fluid inside the fiber. Note the alignment of negative charges along the inside surface of the membrane and positive charges along the outside surface. The *lower panel* displays the abrupt changes in membrane potential that occur at the membranes on the two sides of the fiber. \nis zero, which is the potential of the extracellular fluid. Then, as the recording electrode passes through the voltage change area at the cell membrane (called the *electrical dipole layer*), the potential decreases abruptly to –70 millivolts. Moving across the center of the fiber, the potential remains at a steady –70-millivolt level but reverses back to zero the instant it passes through the membrane on the opposite side of the fiber. \nTo create a negative potential inside the membrane, only enough positive ions to develop the electrical dipole layer at the membrane itself must be transported outward. The remaining ions inside the nerve fiber can be both positive and negative, as shown in the upper panel of Figure 5-3. Therefore, transfer of an incredibly small number of ions through the membrane can establish the normal resting potential of -70 millivolts inside the nerve fiber, which means that only about 1/3,000,000 to 1/100,000,000 of the total positive charges inside the fiber must be transferred. Also, an equally small number of positive ions moving from outside to inside the fiber can reverse the potential from -70 millivolts to as much as +35 millivolts within as little as 1/10,000 of a second. Rapid shifting of ions in this manner causes the nerve signals discussed in subsequent sections of this chapter.\n\nThe resting membrane potential of large nerve fibers when they are not transmitting nerve signals is about -70 millivolts. That is, the potential *inside the fiber* is 70 millivolts more negative than the potential in the extracellular fluid on the outside of the fiber. In the next few paragraphs, the transport properties of the resting nerve membrane for \n \n**Figure 5-4** Functional characteristics of the Na+-K+ pump and the K+ \"leak\" channels. The K+ leak channels also leak Na+ ions into the cell slightly but are much more permeable to K+. ADP, Adenosine diphosphate; ATP, adenosine triphosphate. \nsodium and potassium and the factors that determine the level of this resting potential are explained. \nActive Transport of Sodium and Potassium lons Through the Membrane—the Sodium-Potassium (Na+-K+) Pump. Recall from Chapter 4 that all cell membranes of the body have a powerful Na+-K+ pump that continually transports sodium ions to the outside of the cell and potassium ions to the inside, as illustrated on the left side in Figure 5-4. Note that this is an *electrogenic pump* because three Na+ ions are pumped to the outside for each two K+ ions to the inside, leaving a net deficit of positive ions on the inside and causing a negative potential inside the cell membrane. \nThe $Na^+-K^+$ pump also causes large concentration gradients for sodium and potassium across the resting nerve membrane. These gradients are as follows: \nNa+ (outside): 142 mEq/L \nNa+ (inside): 14 mEq/L \nK+(outside): 4 mEq/L \nK+(inside): 140 mEq/L \nThe ratios of these two respective ions from the inside to the outside are as follows: \n$$Na_{inside}^{+}/Na_{outside}^{+}=0.1$$ \n$$K^{+}_{inside}/K^{+}_{outside} = 35.0$$ \n**Leakage of Potassium Through the Nerve Cell Membrane.** The right side of **Figure 5-4** shows a channel protein (sometimes called a *tandem pore domain, potassium channel*, or *potassium* $[K^+]$ *\"leak\" channel*) in the nerve membrane through which potassium ions can leak, even in a resting cell. The basic structure of potassium channels was described in Chapter 4 (**Figure 4-4**). These $K^+$ leak \n \n**Figure 5-5** Establishment of resting membrane potentials under three conditions. **A**, When the membrane potential is caused entirely by potassium diffusion alone. **B**, When the membrane potential is caused by diffusion of both sodium and potassium ions. **C**, When the membrane potential is caused by diffusion of both sodium and potassium ions plus pumping of both these ions by the Na+-K+ pump. \nchannels may also leak sodium ions slightly but are far more permeable to potassium than to sodium, normally about 100 times as permeable. As discussed later, this differential in permeability is a key factor in determining the level of the normal resting membrane potential.\n\n**Figure 5-5** shows the important factors in the establishment of the normal resting membrane potential. They are as follows. \n#### Contribution of the Potassium Diffusion Potential. \nIn **Figure 5-5A**, we assume that the only movement of ions through the membrane is diffusion of potassium ions, as demonstrated by the open channels between the potassium symbol $(K^+)$ inside and outside the mem- \nbrane. Because of the high ratio of potassium ions inside to outside, 35:1, the Nernst potential corresponding to this ratio is –94 millivolts because the logarithm of 35 is 1.54, and this, multiplied by –61 millivolts, is –94 millivolts. Therefore, if potassium ions were the only factor causing the resting potential, the resting potential *inside the fiber* would be equal to –94 millivolts, as shown in the figure. \nContribution of Sodium Diffusion Through the **Nerve Membrane. Figure 5-5***B* shows the addition of slight permeability of the nerve membrane to sodium ions, caused by the minute diffusion of sodium ions through the K+-Na+ leak channels. The ratio of sodium ions from inside to outside the membrane is 0.1, which gives a calculated Nernst potential for the inside of the membrane of +61 millivolts. Also shown in **Figure 5-5***B* is the Nernst potential for potassium diffusion of -94 millivolts. How do these interact with each other, and what will be the summated potential? This question can be answered by using the Goldman equation described previously. Intuitively, one can see that if the membrane is highly permeable to potassium but only slightly permeable to sodium, the diffusion of potassium contributes far more to the membrane potential than the diffusion of sodium. In the normal nerve fiber, the permeability of the membrane to potassium is about 100 times as great as its permeability to sodium. Using this value in the Goldman equation, and considering only sodium and potassium, gives a potential inside the membrane of -86 millivolts, which is near the potassium potential shown in the figure. \n**Contribution of the Na**+-**K**+ **Pump.** In **Figure 5-5***C*, the Na+-K+ pump is shown to provide an additional contribution to the resting potential. This figure shows that continuous pumping of three sodium ions to the outside occurs for each two potassium ions pumped to the inside of the membrane. The pumping of more sodium ions to the outside than the potassium ions being pumped to the inside causes a continual loss of positive charges from inside the membrane, creating an additional degree of negativity (about –4 millivolts additional) on the inside, beyond that which can be accounted for by diffusion alone. \nTherefore, as shown in **Figure 5-5***C*, the net membrane potential when all these factors are operative at the same time is about –90 millivolts. However, additional ions, such as chloride, must also be considered in calculating the membrane potential. \nIn summary, the diffusion potentials alone caused by potassium and sodium diffusion would give a membrane potential of about -86 millivolts, with almost all of this being determined by potassium diffusion. An additional -4 millivolts is then contributed to the membrane potential by the continuously acting electrogenic Na+-K+ pump, and there is a contribution of chloride ions. As mentioned previously, the resting membrane potential \n \n \n**Figure 5-6** Typical action potential recorded by the method shown in the *upper panel*. \nvaries in different cells from as low as around -10 millivolts in erythrocytes to as high as -90 millivolts in skeletal muscle cells. \n#### **NEURON ACTION POTENTIAL** \nNerve signals are transmitted by *action potentials*, which are rapid changes in the membrane potential that spread rapidly along the nerve fiber membrane. Each action potential begins with a sudden change from the normal resting negative membrane potential to a positive potential and ends with an almost equally rapid change back to the negative potential. To conduct a nerve signal, the action potential moves along the nerve fiber until it comes to the fiber's end. \nThe upper panel of **Figure 5-6** shows the changes that occur at the membrane during the action potential, with the transfer of positive charges to the interior of the fiber at its onset and the return of positive charges to the exterior at its end. The lower panel shows graphically the successive changes in membrane potential over a few 10,000ths of a second, illustrating the explosive onset of the action potential and the almost equally rapid recovery. \nThe successive stages of the action potential are as follows. \n**Resting Stage.** The resting stage is the resting membrane potential before the action potential begins. The membrane is said to be \"polarized\" during this stage because of the –70 millivolts negative membrane potential that is present. \n**Depolarization Stage.** At this time, the membrane suddenly becomes permeable to sodium ions, allowing rapid diffusion of positively charged sodium ions to the interior of the axon. The normal polarized state of −70 millivolts is immediately neutralized by the inflowing, positively charged sodium ions, with the potential rising rapidly in the positive direction—a process called *depolarization.* In large nerve fibers, the great excess of positive sodium ions moving to the inside causes the membrane potential to actually overshoot beyond the zero level and to become somewhat positive. In some smaller fibers, as well as in many central nervous system neurons, the potential merely approaches the zero level and does not overshoot to the positive state. \n**Repolarization Stage.** Within a few 10,000ths of a second after the membrane becomes highly permeable to sodium ions, the sodium channels begin to close, and the potassium channels open to a greater degree than normal. Then, rapid diffusion of potassium ions to the exterior reestablishes the normal negative resting membrane potential, which is called *repolarization* of the membrane. \nTo explain more fully the factors that cause both depolarization and repolarization, we will describe the special characteristics of two other types of transport channels through the nerve membrane, the voltage-gated sodium and potassium channels. \n#### **VOLTAGE-GATED SODIUM AND POTASSIUM CHANNELS** \nThe necessary factor in causing both depolarization and repolarization of the nerve membrane during the action potential is the *voltage-gated sodium channel.* A *voltagegated potassium channel* also plays an important role in increasing the rapidity of repolarization of the membrane. *These two voltage-gated channels are in addition to the Na*+*-K*+ *pump and the K*+ *leak channels.* \n#### **Activation and Inactivation of the Voltage-Gated Sodium Channel** \nThe upper panel of **[Figure 5-7](#page-67-0)** shows the voltage-gated sodium channel in three separate states. This channel has two *gates*—one near the outside of the channel called the *activation gate,* and another near the inside called the *inactivation gate.* The upper left of the figure depicts the state of these two gates in the normal resting membrane when the membrane potential is −70 millivolts. In this state, the activation gate is closed, which prevents any entry of sodium ions to the interior of the fiber through these sodium channels. \n**Activation of the Sodium Channel.** When the membrane potential becomes less negative than during the resting state, rising from −70 millivolts toward zero, it finally reaches a voltage—usually somewhere around −55 millivolts—that causes a sudden conformational \n \n**Figure 5-7** Characteristics of the voltage-gated sodium *(top)* and potassium *(bottom)* channels, showing successive activation and inactivation of the sodium channels and delayed activation of the potassium channels when the membrane potential is changed from the normal resting negative value to a positive value. \nchange in the activation gate, flipping it all the way to the open position. During this *activated state,* sodium ions can pour inward through the channel, increasing the sodium permeability of the membrane as much as 500- to 5000-fold. \n**Inactivation of the Sodium Channel.** The upper right panel of **[Figure 5-7](#page-67-0)** shows a third state of the sodium channel. The same increase in voltage that opens the activation gate also closes the inactivation gate. The inactivation gate, however, closes a few 10,000ths of a second after the activation gate opens. That is, the conformational change that flips the inactivation gate to the closed state is a slower process than the conformational change that opens the activation gate. Therefore, after the sodium channel has remained open for a few 10,000ths of a second, the inactivation gate closes, and sodium ions no longer can pour to the inside of the membrane. At this point, the membrane potential begins to return toward the resting membrane state, which is the repolarization process. \nAnother important characteristic of the sodium channel inactivation process is that the inactivation gate will not reopen until the membrane potential returns to or near the original resting membrane potential level. Therefore, it is usually not possible for the sodium channels to open again without first repolarizing the nerve fiber. \n#### **Voltage-Gated Potassium Channel and Its Activation** \nThe lower panel of **[Figure 5-7](#page-67-0)** shows the voltage-gated potassium channel in two states—during the resting state \n \n**Figure 5-8** Voltage clamp method for studying flow of ions through specific channels. \n(left) and toward the end of the action potential (right). During the resting state, the gate of the potassium channel is closed, and potassium ions are prevented from passing through this channel to the exterior. When the membrane potential rises from −70 millivolts toward zero, this voltage change causes a conformational opening of the gate and allows increased potassium diffusion outward through the channel. However, because of the slight delay in opening of the potassium channels, they open, for the most part, at about the same time that the sodium channels are beginning to close because of inactivation. Thus, the decrease in sodium entry to the cell and the simultaneous increase in potassium exit from the cell combine to speed the repolarization process, leading to full recovery of the resting membrane potential within another few 10,000ths of a second. \n**The Voltage Clamp Method for Measuring the Effect of Voltage on Opening and Closing of Voltage-Gated Channels.** The original research that led to quantitative understanding of the sodium and potassium channels was so ingenious that it led to Nobel Prizes for the scientists responsible, Hodgkin and Huxley, in 1963. The essence of these studies is shown in **[Figures. 5-8 and 5-9](#page-68-0)**. \n**[Figure 5-8](#page-68-0)** shows the *voltage clamp method,* which is used to measure the flow of ions through the different channels. In using this apparatus, two electrodes are inserted into the nerve fiber. One of these electrodes is used to measure the voltage of the membrane potential, and the other is used to conduct electrical current into or out of the nerve fiber. \nThis apparatus is used in the following way. The investigator decides which voltage to establish inside the nerve fiber. The electronic portion of the apparatus is then adjusted to the desired voltage, automatically injecting either positive or negative electricity through the current electrode at whatever rate is required to hold the voltage, as measured \n \n**Figure 5-9** Typical changes in conductance of sodium and potassium ion channels when the membrane potential is suddenly increased from the normal resting value of −70 millivolts to a positive value of +10 millivolts for 2 milliseconds. This figure shows that the sodium channels open (activate) and then close (inactivate) before the end of the 2 milliseconds, whereas the potassium channels only open (activate), and the rate of opening is much slower than that of the sodium channels. \nby the voltage electrode, at the level set by the operator. When the membrane potential is suddenly increased by this voltage clamp from −70 millivolts to zero, the voltagegated sodium and potassium channels open, and sodium and potassium ions begin to pour through the channels. To counterbalance the effect of these ion movements on the desired setting of the intracellular voltage, electrical current is injected automatically through the current electrode of the voltage clamp to maintain the intracellular voltage at the required steady zero level. To achieve this level, the current injected must be equal to but of opposite polarity to the net current flow through the membrane channels. To measure how much current flow is occurring at each instant, the current electrode is connected to an ampere meter that records the current flow, as demonstrated in **[Figure 5-8](#page-68-0)**. \nFinally, the investigator adjusts the concentrations of the ions to other than normal levels both inside and outside the nerve fiber and repeats the study. This experiment can be performed easily when using large nerve fibers removed from some invertebrates, especially the giant squid axon, which in some cases is as large as 1 millimeter in diameter. When sodium is the only permeant ion in the solutions inside and outside the squid axon, the voltage clamp measures current flow only through the sodium channels. When potassium is the only permeant ion, current flow only through the potassium channels is measured. \nAnother means for studying the flow of ions through an individual type of channel is to block one type of channel at a time. For example, the sodium channels can be blocked by a toxin called tetrodotoxin when it is applied to the outside of the cell membrane where the sodium activation gates are located. Conversely, tetraethylammonium ion blocks the potassium channels when it is applied to the interior of the nerve fiber. \n**[Figure 5-9](#page-68-1)** shows typical changes in conductance of the voltage-gated sodium and potassium channels when the membrane potential is suddenly changed through use of the voltage clamp, from −70 millivolts to +10 millivolts and then, 2 milliseconds later, back to −70 millivolts. Note the sudden opening of the sodium channels (the activation stage) within a small fraction of a millisecond after the membrane potential is increased to the positive value. However, during the next millisecond or so, the sodium channels automatically close (the inactivation stage). \nNote the opening (activation) of the potassium channels, which open less rapidly and reach their full open state only after the sodium channels have almost completely closed. Furthermore, once the potassium channels open, they remain open for the entire duration of the positive membrane potential and do not close again until after the membrane potential is decreased back to a negative value. \n#### **SUMMARY OF EVENTS THAT CAUSE THE ACTION POTENTIAL** \n**[Figure 5-10](#page-69-0)** summarizes the sequential events that occur during and shortly after the action potential. The bottom of the figure shows the changes in membrane conductance for sodium and potassium ions. During the resting state, before the action potential begins, the conductance for potassium ions is 50 to 100 times as great as the conductance for sodium ions. This disparity is caused by much greater leakage of potassium ions than sodium ions through the leak channels. However, at the onset of the action potential, the sodium channels almost instantaneously become activated and allow up to a 5000-fold increase in sodium conductance. The inactivation process then closes the sodium channels \n \n**Figure 5-10** Changes in sodium and potassium conductance during the course of the action potential. Sodium conductance increases several thousand–fold during the early stages of the action potential, whereas potassium conductance increases only about 30-fold during the latter stages of the action potential and for a short period thereafter. (These curves were constructed from theory presented in papers by Hodgkin and Huxley but transposed from a squid axon to apply to the membrane potentials of large mammalian nerve fibers.) \nwithin another fraction of a millisecond. The onset of the action potential also initiates voltage gating of the potassium channels, causing them to begin opening more slowly, a fraction of a millisecond after the sodium channels open. At the end of the action potential, the return of the membrane potential to the negative state causes the potassium channels to close back to their original status but, again, only after an additional millisecond or more delay. \nThe middle portion of **[Figure 5-10](#page-69-0)** shows the ratio of sodium to potassium conductance at each instant during the action potential, and above this depiction is the action potential itself. During the early portion of the action potential, the ratio of sodium to potassium conductance increases more than 1000-fold. Therefore, far more sodium ions flow to the interior of the fiber than potassium ions to the exterior. This is what causes the membrane potential to become positive at the action potential onset. Then, the sodium channels begin to close, and the potassium channels begin to open; thus, the ratio of conductance shifts far in favor of high potassium conductance but low sodium conductance. This shift allows for a very rapid loss of potassium ions to the exterior but virtually zero flow of sodium ions to the interior. Consequently, the action potential quickly returns to its baseline level. \n#### **Roles of Other Ions During the Action Potential** \nThus far, we have considered only the roles of sodium and potassium ions in generating the action potential. At least two other types of ions must be considered, negative anions and calcium ions. \n**Impermeant Negatively Charged Ions (Anions) Inside the Nerve Axon.** Inside the axon are many negatively charged ions that cannot go through the membrane channels. They include the anions of protein molecules and of many organic phosphate compounds and sulfate compounds, among others. Because these ions cannot leave the interior of the axon, any deficit of positive ions inside the membrane leaves an excess of these impermeant negative anions. Therefore, these impermeant negative ions are responsible for the negative charge inside the fiber when there is a net deficit of positively charged potassium ions and other positive ions. \n**Calcium Ions.** The membranes of almost all cells of the body have a calcium pump similar to the sodium pump, and calcium serves along with (or instead of) sodium in some cells to cause most of the action potential. Like the sodium pump, the calcium pump transports calcium ions from the interior to the exterior of the cell membrane (or into the endoplasmic reticulum of the cell), creating a calcium ion gradient of about 10,000-fold. This process leaves an internal cell concentration of calcium ions of about 10−7 molar, in contrast to an external concentration of about 10−3 molar. \nIn addition, there are *voltage-gated calcium channels*. Because the calcium ion concentration is more than 10,000 times greater in the extracellular fluid than in the intracellular fluid, there is a tremendous diffusion gradient and electrochemical driving force for the passive flow of calcium ions into the cells. These channels are slightly permeable to sodium ions and calcium ions, but their permeability to calcium is about 1000-fold greater than to sodium under normal physiological conditions. When the channels open in response to a stimulus that depolarizes the cell membrane, calcium ions flow to the interior of the cell. \nA major function of the voltage-gated calcium ion channels is to contribute to the depolarizing phase on the action potential in some cells. The gating of calcium channels, however, is relatively slow, requiring 10 to 20 times as long for activation as for the sodium channels. For this reason, they are often called *slow channels,* in contrast to the sodium channels, which are called *fast channels.* Therefore, the opening of calcium channels provides a more sustained depolarization, whereas the sodium channels play a key role in initiating action potentials. \nCalcium channels are numerous in cardiac muscle and smooth muscle. In fact, in some types of smooth muscle, the fast sodium channels are hardly present; therefore, the action potentials are caused almost entirely by the activation of slow calcium channels. \n**Increased Permeability of the Sodium Channels When There Is a Deficit of Calcium Ions.** The concentration of calcium ions in the extracellular fluid also has a profound effect on the voltage level at which the sodium channels become activated. When there is a deficit of calcium ions, the sodium channels become activated (opened) by a small increase of the membrane potential from its normal, very negative level. Therefore, the nerve fiber becomes highly excitable, sometimes discharging repetitively without provocation, rather than remaining in the resting state. In fact, the calcium ion concentration needs to fall only 50% below normal before spontaneous discharge occurs in some peripheral nerves, often causing *muscle \"tetany*.*\"* Muscle tetany is sometimes lethal because of tetanic contraction of the respiratory muscles. \nThe probable way in which calcium ions affect the sodium channels is as follows. These ions appear to bind to the exterior surfaces of the sodium channel protein. The positive charges of these calcium ions, in turn, alter the electrical state of the sodium channel protein, thus altering the voltage level required to open the sodium gate. \n#### **INITIATION OF THE ACTION POTENTIAL** \nThus far, we have explained the changing sodium and potassium permeability of the membrane, as well as the development of the action potential, but we have not explained what initiates the action potential. \n**A Positive-Feedback Cycle Opens the Sodium Channels.** As long as the membrane of the nerve fiber remains undisturbed, no action potential occurs in the normal nerve. However, if any event causes enough initial rise in the membrane potential from −70 millivolts toward the zero level, the rising voltage will cause many voltage-gated sodium channels to begin opening. This occurrence allows for the rapid inflow of sodium ions, which causes a further rise in the membrane potential, thus opening still more voltage-gated sodium channels and allowing more streaming of sodium ions to the interior of the fiber. This process is a positive feedback cycle that, once the feedback is strong enough, continues until all the voltage-gated sodium channels have become activated (opened). Then, within another fraction of a millisecond, the rising membrane potential causes closure of the sodium channels and opening of potassium channels, and the action potential soon terminates. \n**Initiation of the Action Potential Occurs Only After the Threshold Potential is Reached.** An action potential will not occur until the initial rise in membrane potential is great enough to create the positive feedback described in the preceding paragraph. This occurs when the number of sodium ions entering the fiber is greater than the number of potassium ions leaving the fiber. A sudden rise in membrane potential of 15 to 30 millivolts is usually required. Therefore, a sudden increase in the membrane potential in a large nerve fiber, from −70 millivolts up to about −55 millivolts, usually causes the explosive development of an action potential. This level of −55 millivolts is said to be the *threshold* for stimulation. \n#### PROPAGATION OF THE ACTION POTENTIAL \nIn the preceding paragraphs, we discussed the action potential as though it occurs at one spot on the membrane. However, an action potential elicited at any one point on an excitable membrane usually excites adjacent portions of the membrane, resulting in propagation of the action potential along the membrane. This mechanism is demonstrated in **[Figure 5-11.](#page-71-0)** \n**[Figure 5-11](#page-71-0)***A* shows a normal resting nerve fiber, and **[Figure 5-11](#page-71-0)***B* shows a nerve fiber that has been excited in its midportion, which suddenly develops increased permeability to sodium. The *arrows* show a local circuit of current flow from the depolarized areas of the membrane to the adjacent resting membrane areas. That is, positive electrical charges are carried by the inward-diffusing sodium ions through the depolarized membrane and then for several millimeters in both directions along the core of the axon. These positive charges increase the voltage for a distance of 1 to 3 millimeters inside the large myelinated fiber to above the threshold voltage value for initiating an action potential. Therefore, the sodium channels in these new areas immediately open, as shown in **[Figure 5-11](#page-71-0)***C* [and](#page-71-0) *D*, and the explosive action potential spreads. These newly depolarized areas produce still more local circuits of current flow farther along the membrane, causing progressively more and more depolarization. Thus, the depolarization process travels along the entire length of the fiber. This transmission of the depolarization process along a nerve or muscle fiber is called a *nerve* or *muscle impulse.* \n**Direction of Propagation.** As demonstrated in **[Figure 5-](#page-71-0) [11](#page-71-0)**, an excitable membrane has no single direction of propagation, but the action potential travels in all directions away from the stimulus—even along all branches of a nerve fiber—until the entire membrane has become depolarized. \n**All-or-Nothing Principle.** Once an action potential has been elicited at any point on the membrane of a normal fiber, the depolarization process travels over the entire membrane if conditions are right, but it does not travel at all if conditions are not right. This principle is called the *allor-nothing principle,* and it applies to all normal excitable tissues. Occasionally, the action potential reaches a point on the membrane at which it does not generate sufficient voltage to stimulate the next area of the membrane. When this situation occurs, the spread of depolarization stops. Therefore, for continued propagation of an impulse to occur, the ratio of action potential to threshold for excitation must at all times be greater than 1. This \"greater than 1\" requirement is called the *safety factor* for propagation. \n#### RE-ESTABLISHING SODIUM AND POTASSIUM IONIC GRADIENTS AFTER ACTION POTENTIALS ARE COMPLETED—IMPORTANCE OF ENERGY METABOLISM \nTransmission of each action potential along a nerve fiber slightly reduces the concentration differences of sodium and potassium inside and outside the membrane because sodium ions diffuse to the inside during depolarization, and potassium ions diffuse to the outside during \n+ + + + + + + + + + + + – – + + + + + + + + + + + + + + + + + + + + + – – + + + + + + + + + + + + + + + + + + + – – – – + + + + + + + + + + + + + + + + + + – – – – + + + + + + + + – – + + + + + + + + + + + + + + + + + + – – – – + + + + + + + + + + + + + + + + + + – – + + – – – – – – – – – – – – – – – – – – + + + + – – – – – – – – – – – – – – – – – – + + – – – – – – – – – – – – + + – – – – – – – – – + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – + + – – – – – – – – – – – – – – – – – – – + + + + – – – – – – – – – – – – – – – – – – + + + + – – – – – – – – A B C D \n**Figure 5-11** A–D, Propagation of action potentials in both directions along a conductive fiber. \nrepolarization. For a single action potential, this effect is so minute that it cannot be measured. Indeed, 100,000 to 50 million impulses can be transmitted by large nerve fibers before the concentration differences reach the point that action potential conduction ceases. With time, however, it becomes necessary to re-establish the sodium and potassium membrane concentration differences, which is achieved by action of the Na+-K+ pump in the same way as described previously for the original establishment of the resting potential. That is, sodium ions that have diffused to the interior of the cell during the action potentials and potassium ions that have diffused to the exterior must be returned to their original state by the Na+-K+ pump. Because this pump requires energy for operation, this \"recharging\" of the nerve fiber is an active metabolic process, using energy derived from the adenosine triphosphate (ATP) energy system of the cell. **[Figure 5-12](#page-71-1)** shows that the nerve fiber produces increased heat during recharging, which is a measure of energy expenditure when the nerve impulse frequency increases. \nA special feature of the Na+-K+ ATP pump is that its degree of activity is strongly stimulated when excess sodium ions accumulate inside the cell membrane. In fact, the pumping activity increases approximately in proportion to the third power of this intracellular sodium concentration. As the internal sodium concentration rises from 10 to 20 mEq/L, the activity of the pump does not merely double but increases about eightfold. Therefore, it is easy to understand how the recharging process of the nerve fiber can be set rapidly into motion whenever the concentration differences of sodium and potassium ions across the membrane begin to run down. \n#### PLATEAU IN SOME ACTION POTENTIALS \nIn some cases, the excited membrane does not repolarize immediately after depolarization; instead, the potential remains on a plateau near the peak of the spike potential for many milliseconds before repolarization begin. Such a plateau is shown in **[Figure 5-13](#page-72-0)**; one can readily see that \n \n**Figure 5-12** Heat production in a nerve fiber at rest and at progressively increasing rates of stimulation. \nthe plateau greatly prolongs the period of depolarization. This type of action potential occurs in heart muscle fibers, where the plateau lasts for as long as 0.2 to 0.3 second and causes contraction of heart muscle to last for this same long period. \nThe cause of the plateau is a combination of several factors. First, in heart muscle, two types of channels contribute to the depolarization process: (1) the usual voltage-activated sodium channels, called *fast channels;* and (2) voltageactivated calcium-sodium channels *(L-type calcium channels)*, which are slow to open and therefore are called *slow channels.* Opening of fast channels causes the spike portion of the action potential, whereas the prolonged opening of the slow calcium-sodium channels mainly allows calcium ions to enter the fiber, which is largely responsible for the plateau portion of the action potential. \nAnother factor that may be partly responsible for the plateau is that the voltage-gated potassium channels are slower to open than usual, often not opening much until the end of the plateau. This factor delays the return of the membrane potential toward its normal negative value of −70 millivolts. The plateau ends when the calciumsodium channels close, and permeability to potassium ions increases. \n#### RHYTHMICITY OF SOME EXCITABLE TISSUES—REPETITIVE DISCHARGE \nRepetitive self-induced discharges occur normally in the heart, in most smooth muscle, and in many of the neurons of the central nervous system. These rhythmical discharges cause the following: (1) rhythmical beat of the heart; (2) rhythmical peristalsis of the intestines; and (3) neuronal events such as the rhythmical control of breathing. \nIn addition, almost all other excitable tissues can discharge repetitively if the threshold for stimulation of the tissue cells is reduced to a low enough level. For example, even large nerve fibers and skeletal muscle fibers, which normally are highly stable, discharge repetitively when they \n \n**Figure 5-13** Action potential (in millivolts) from a Purkinje fiber of the heart, showing a plateau. \nare placed in a solution that contains the drug *veratridine*, which activates sodium ion channels, or when the calcium ion concentration decreases below a critical value, which increases the sodium permeability of the membrane. \n**Re-Excitation Process Necessary for Spontaneous Rhythmicity.** For spontaneous rhythmicity to occur, the membrane—even in its natural state—must be permeable enough to sodium ions (or to calcium and sodium ions through the slow calcium-sodium channels) to allow automatic membrane depolarization. Thus, **[Figure 5-14](#page-72-1)** shows that the resting membrane potential in the rhythmical control center of the heart is only −60 to −70 millivolts, which is not enough negative voltage to keep the sodium and calcium channels totally closed. Therefore, the following sequence occurs: (1) some sodium and calcium ions flow inward; (2) this activity increases the membrane voltage in the positive direction, which further increases membrane permeability; (3) still more ions flow inward; and (4) the permeability increases more, and so on, until an action potential is generated. Then, at the end of the action potential, the membrane repolarizes. After another delay of milliseconds or seconds, spontaneous excitability causes depolarization again, and a new action potential occurs spontaneously. This cycle continues over and over and causes self-induced rhythmical excitation of the excitable tissue. \nWhy does the membrane of the heart control center not depolarize immediately after it has become repolarized, rather than delaying for nearly 1 second before the onset of the next action potential? The answer can be found by observing the curve labeled \"potassium conductance\" in **[Figure 5-14](#page-72-1)**. This curve shows that toward the end of each action potential, and continuing for a short period thereafter, the membrane becomes more permeable to potassium ions. The increased outflow of potassium ions carries tremendous numbers of positive charges to the outside of the membrane, leaving considerably more negativity inside the fiber than would otherwise occur. This continues for nearly 1 second after the preceding action potential is over, thus drawing the membrane potential \n \n**Figure 5-14** Rhythmical action potentials (in millivolts) similar to those recorded in the rhythmical control center of the heart. Note their relationship to potassium conductance and to the state of hyperpolarization. \nnearer to the potassium Nernst potential. This state, called *hyperpolarization,* is also shown in **[Figure 5-14](#page-72-1)**. As long as this state exists, self–re-excitation will not occur. However, the increased potassium conductance (and the state of hyperpolarization) gradually disappears, as shown after each action potential is completed in the figure, thereby again allowing the membrane potential to increase up to the *threshold* for excitation. Then, suddenly, a new action potential results and the process occurs again and again. \n#### SPECIAL CHARACTERISTICS OF SIGNAL TRANSMISSION IN NERVE TRUNKS \n**Myelinated and Unmyelinated Nerve Fibers. [Figure](#page-73-0) [5-15](#page-73-0)** shows a cross section of a typical small nerve, revealing many large nerve fibers that constitute most of the cross-sectional area. However, a more careful look reveals many more small fibers lying between the large ones. The large fibers are *myelinated,* and the small ones are *unmyelinated.* The average nerve trunk contains about twice as many unmyelinated fibers as myelinated fibers. \n**[Figure 5-16](#page-73-1)** illustrates schematically the features of a typical myelinated fiber. The central core of the fiber is the *axon,* and the membrane of the axon is the membrane that actually conducts the action potential. The axon is filled in its center with *axoplasm,* which is a viscid intracellular fluid. Surrounding the axon is a *myelin sheath* that is often much thicker than the axon itself. About once every 1 to 3 millimeters along the length of the myelin sheath is a *node of Ranvier.* \nThe myelin sheath is deposited around the axon by *Schwann cells* in the following manner. The membrane of a Schwann cell first envelops the axon. The Schwann cell then rotates around the axon many times, laying down multiple layers of Schwann cell membrane containing the lipid substance *sphingomyelin.* This substance is an excellent \n \n**Figure 5-15** Cross section of a small nerve trunk containing both myelinated and unmyelinated fibers. \nelectrical insulator that decreases ion flow through the membrane about 5000-fold. At the juncture between each two successive Schwann cells along the axon, a small uninsulated area only 2 to 3 micrometers in length remains where ions still can flow with ease through the axon membrane between the extracellular fluid and intracellular fluid inside the axon. This area is called the *node of Ranvier.* \n#### **Saltatory Conduction in Myelinated Fibers from Node** \n**to Node.** Even though almost no ions can flow through the thick myelin sheaths of myelinated nerves, they can flow with ease through the nodes of Ranvier. Therefore, action potentials occur *only at the nodes.* Yet, the action potentials are conducted from node to node by *saltatory conduction*, as shown in **[Figure 5-17](#page-74-0)**. That is, electrical current flows through the surrounding extracellular fluid outside the myelin sheath, as well as through the axoplasm inside the axon from node to node, exciting successive nodes one after another. Thus, the nerve impulse jumps along the fiber, which is the origin of the term *saltatory.* \nSaltatory conduction is of value for two reasons: \n1. First, by causing the depolarization process to jump long intervals along the axis of the nerve fiber, this mechanism increases the velocity of nerve transmission in myelinated fibers as much as 5- to 50-fold. \n \n**Figure 5-16** Function of the Schwann cell to insulate nerve fibers. **A**, Wrapping of a Schwann cell membrane around a large axon to form the myelin sheath of the myelinated nerve fiber. **B**, Partial wrapping of the membrane and cytoplasm of a Schwann cell around multiple unmyelinated nerve fibers (shown in cross section). *(A, Modified from Leeson TS, Leeson R: Histology. Philadelphia: WB Saunders, 1979.)* \n2. Second, saltatory conduction conserves energy for the axon because only the nodes depolarize, allowing perhaps 100 times less loss of ions than would otherwise be necessary, and therefore requiring much less energy expenditure for re-establishing the sodium and potassium concentration differences across the membrane after a series of nerve impulses. \nThe excellent insulation afforded by the myelin membrane and the 50-fold decrease in membrane capacitance also allow repolarization to occur with little transfer of ions. \n**Velocity of Conduction in Nerve Fibers.** The velocity of action potential conduction in nerve fibers varies from as little as 0.25 m/sec in small unmyelinated fibers to as much as 100 m/sec—more than the length of a football field in 1 second—in large myelinated fibers. \n#### EXCITATION—THE PROCESS OF ELICITING THE ACTION POTENTIAL \nBasically, any factor that causes sodium ions to begin to diffuse inward through the membrane in sufficient numbers can set off automatic regenerative opening of the sodium channels. This automatic regenerative opening can result from *mechanical* disturbance of the membrane, *chemical* effects on the membrane, or passage of *electricity* through the membrane. All these approaches are used at different points in the body to elicit nerve or muscle action potentials: mechanical pressure to excite sensory nerve endings in the skin, chemical neurotransmitters to transmit signals from one neuron to the next in the brain, and electrical current to transmit signals between successive muscle cells in the heart and intestine. \n**Excitation of a Nerve Fiber by a Negatively Charged Metal Electrode.** The usual means for exciting a nerve or muscle in the experimental laboratory is to apply electricity to the nerve or muscle surface through two small electrodes, one of which is negatively charged and the other positively charged. When electricity is applied in this manner, the excitable membrane becomes stimulated at the negative electrode. \nRemember that the action potential is initiated by the opening of voltage-gated sodium channels. Furthermore, these channels are opened by a decrease in the normal resting electrical voltage across the membrane—that is, negative current from the electrode decreases the voltage on the outside of the membrane to a negative value nearer to the voltage of the negative potential inside the fiber. This effect decreases the electrical voltage across the membrane and allows the sodium channels to open, resulting in an action potential. Conversely, at the positive electrode, the injection of positive charges on the outside of the nerve membrane heightens the voltage difference across the membrane, rather than lessening it. This effect causes a state of hyperpolarization, which actually decreases the excitability of the fiber rather than causing an action potential. \n \n**Figure 5-17** Saltatory conduction along a myelinated axon. The flow of electrical current from node to node is illustrated by the *arrows.* \n#### Threshold for Excitation and Acute Local Potentials. \nA weak negative electrical stimulus may not be able to excite a fiber. However, when the voltage of the stimulus is increased, there comes a point at which excitation does take place. Figure 5-18 shows the effects of successively applied stimuli of progressing strength. A weak stimulus at point A causes the membrane potential to change from -70 to -65 millivolts, but this change is not sufficient for the automatic regenerative processes of the action potential to develop. At point B, the stimulus is greater, but the intensity is still not enough. The stimulus does, however, disturb the membrane potential locally for as long as 1 millisecond or more after both of these weak stimuli. These local potential changes are called acute local potentials and, when they fail to elicit an action potential, they are called acute subthreshold potentials. \nAt point C in **Figure 5-18**, the stimulus is even stronger. Now, the local potential has barely reached the *threshold level* required to elicit an action potential, but this occurs only after a short \"latent period.\" At point D, the stimulus is still stronger, the acute local potential is also stronger, and the action potential occurs after less of a latent period. \nThus, this figure shows that even a weak stimulus causes a local potential change at the membrane, but the intensity of the local potential must rise to a threshold level before the action potential is set off.",
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"page_content": "The structure of the membrane covering the outside of every cell of the body is discussed in Chapter 2 and illustrated in Figure 2-3 and Figure 4-2. This membrane consists almost entirely of a *lipid bilayer* with large numbers of protein molecules in the lipid, many of which penetrate all the way through the membrane. \nThe lipid bilayer is not miscible with the extracellular fluid or the intracellular fluid. Therefore, it constitutes a barrier against movement of water molecules and water-soluble substances between the extracellular and intracellular fluid compartments. However, as shown in **Figure 4-2** by the leftmost arrow, lipid-soluble substances can diffuse directly through the lipid substance. \nThe membrane protein molecules interrupt the continuity of the lipid bilayer, constituting an alternative pathway through the cell membrane. Many of these penetrating proteins can function as *transport proteins*. Some proteins have watery spaces all the way through the molecule and allow free movement of water, as well as selected ions or molecules; these proteins are called *channel proteins*. Other proteins, called *carrier proteins*, bind with molecules or ions that are to be transported, and conformational changes in the protein molecules then move the substances through the interstices of the protein to the \n \n**Figure 4-2.** Transport pathways through the cell membrane and the basic mechanisms of transport. \n \n**Figure 4-3.** Diffusion of a fluid molecule during one thousandth of a second. \nother side of the membrane. Channel proteins and carrier proteins are usually selective for the types of molecules or ions that are allowed to cross the membrane. \n**\"Diffusion\" Versus \"Active Transport.\"** Transport through the cell membrane, either directly through the lipid bilayer or through the proteins, occurs via one of two basic processes, *diffusion* or *active transport.* \nAlthough many variations of these basic mechanisms exist, *diffusion* means random molecular movement of substances molecule by molecule, either through intermolecular spaces in the membrane or in combination with a carrier protein. The energy that causes diffusion is the energy of the normal kinetic motion of matter. \nIn contrast, *active transport* means movement of ions or other substances across the membrane in combination with a carrier protein in such a way that the carrier protein causes the substance to move against an energy gradient, such as from a low-concentration state to a highconcentration state. This movement requires an additional source of energy besides kinetic energy. A more detailed explanation of the basic physics and physical chemistry of these two processes is provided later in this chapter. \n#### DIFFUSION \nAll molecules and ions in the body fluids, including water molecules and dissolved substances, are in constant motion, with each particle moving in its separate way. The motion of these particles is what physicists call \"heat\" the greater the motion, the higher the temperature—and the motion never ceases, except at absolute zero temperature. When a moving molecule, [A, approac](#page-51-0)hes a stationary molecule, B, the electrostatic and other nuclear forces of molecule A repel molecule B, transferring some of the energy of motion of molecule A to molecule B. Consequently, molecule B gains kinetic energy of motion, whereas molecule A slows down, losing some of its kinetic energy. As shown in **Figure 4-3**, a single molecule in a solution bounces among the other molecules—first in one direction, then another, then another, and so forth randomly bouncing thousands of times each second. This continual movement of molecules among one another in liquids or gases is called *diffusion.* \nIons diffuse in the same manner as whole molecules, and even suspended colloid particles diffuse in a similar manner, except that the colloids diffuse far less rapidly than molecular substances because of their large size. \n#### **DIFFUSION THROUGH THE CELL MEMBRANE** \nDiffusion through the cell membrane is divided into two subtypes, called *simple diffusion* and *facilitated diffusion.* Simple diffusion means that kinetic movement of molecules or ions occurs through a membrane opening or through intermolecular spaces without interaction with carrier proteins in the membrane. The rate of diffusion is determined by the amount of substance available, the velocity of kinetic motion, and the number and sizes of openings in the membrane through which the molecules or ions can move. \nFacilitated diffusion requires interaction of a carrier protein. The carrier protein aids passage of molecules or ions through the membrane by binding chemically with them and shuttlin[g them thro](#page-50-0)ugh the membrane in this form. \nSimple diffusion can occur through the cell membrane by two pathways: (1) through the interstices of the lipid bilayer if the diffusing substance is lipid-soluble; and (2) through watery channels that penetrate all the way through some of the large transport proteins, as shown to the left in **Figure 4-2**. \n**Diffusion of Lipid-Soluble Substances Through the Lipid Bilayer.** The *lipid solubility* of a substance is an important factor for determining how rapidly it diffuses through the lipid bilayer. For example, the lipid solubilities of oxygen, nitrogen, carbon dioxide, and alcohols are high, and all these substances can dissolve directly in the lipid bilayer and diffuse through the cell membrane in the same manner that diffusion of water solutes occurs in a watery solution. The rate of diffusion of each of these substances through the membrane is directly proportional to its lipid solubility. Especially large amounts of oxygen can be transported in this way; therefore, oxygen can be delivered to the interior of the cell almost as though the cell membrane did not exist. \n**Diffusion of Water and Other Lipid-Insoluble Molecules Through Protein Channels.** Even though water is highly insoluble in the membrane lipids, it readily passes through channels in protein molecules that penetrate all the way through the membrane. Many of the body's cell membranes contain protein \"pores\" called *aquaporins* that selectively permit rapid passage of water through the membrane. The aquaporins are highly specialized, and there are at least 13 different types in various cells of mammals. \nThe rapidity with which water molecules can diffuse through most cell membranes is astounding. For example, the total amount of water that diffuses in each direction through the red blood cell membrane during each second is about 100 times as great as the volume of the red blood cell. \nOther lipid-insoluble molecules can pass through the protein pore channels in the same way as water molecules if they are water-soluble and small enough. However, as they become larger, their penetration falls off rapidly. For example, the diameter of the urea molecule is only 20% greater than that of water, yet its penetration through the cell membrane pores is about 1000 times less than that of water. Even so, given the astonishing rate of water penetration, this amount of urea penetration still allows rapid transport of urea through the membrane within minutes. \n#### **DIFFUSION THROUGH PROTEIN PORES AND CHANNELS—SELECTIVE PERMEABILITY AND \"GATING\" OF CHANNELS** \nComputerized three-dimensional reconstructions of protein pores and channels have demonstrated tubular pathways all the way from the extracellular to the intracellular fluid. Therefore, substances can move by simple diffusion directly along these pores and channels from one side of the membrane to the other. \nPores are composed of integral cell membrane proteins that form open tubes through the membrane and are always open. However, the diameter of a pore and its electrical charges provide selectivity that permits only certain molecules to pass through. For example, *aquaporins* permit rapid passage of water through cell membranes but exclude other molecules. Aquaporins have a narrow pore that permits water molecules to diffuse through the membrane in single file. The pore is too narrow to permit passage of any hydrated ions. As discussed in Chapters 28 and 76, the density of some aquaporins (e.g., aquaporin-2) in cell membranes is not static but is altered in different physiological conditions. \nThe protein channels are distinguished by two important characteristics: (1) they are often *selectively permeable* to certain substances; and (2) many of the channels can be opened or closed by *gates* that are regulated by electrical signals *(voltage-gated channels)* or chemicals that bind to the channel proteins *(ligand-gated channels).* Thus, ion channels are flexible dynamic structures, and subtle conformational changes influence gating and ion selectivity. \n**Selective Permeability of Protein Channels.** Many protein channels are highly selective for transport of one or more specific ions or molecules. This selectivity results from specific characteristics of the channel, such as its diameter, shape, and the nature of the electrical charges and chemical bonds along its inside surfaces. \n*Potassium channels* permit passage of potassium ions across the cell membrane about 1000 times more readily than they permit passage of sodium ions. This high degree of selectivity cannot be explained entirely by the \n \n**Figure 4-4.** The structure of a potassium channel. The channel is composed of four subunits (only two of which are shown), each with two transmembrane helices. A narrow selectivity filter is formed from the pore loops, and carbonyl oxygens line the walls of the selectivity filter, forming sites for transiently binding dehydrated potassium ions. The interaction of the potassium ions with carbonyl oxygens causes the potassium ions to shed their bound water molecules, permitting the dehydrated potassium ions to pass through the pore. \nmolecular diameters of the ions because potassium ions are slightly larger than sodium ions. Using x-ray crystallography, potassium channels were found to have a *tetrameric structure* consisting of four identical protein subunits surrounding a central pore (**Figure 4-4**). At the top of the channel pore are *pore loops* that form a narrow *selectivity filter*. Lining the selectivity filter are *carbonyl oxygens.* When hydrated potassium ions enter the selectivity filter, they interact with the carbonyl oxygens and shed most of their bound water molecules, permitting the dehydrated potassium ions to pass through the channel. The carbonyl oxygens are too far apart, however, to enable them to interact closely with the smaller sodium ions, which are therefore effectively excluded by the selectivity filter from passing through the pore. \nDifferent selectivity filters for the various ion channels are believed to determine, in large part, the specificity of various channels for cations or anions or for particular ions, such as sodium (Na+), potassium (K+), and calcium (Ca2+), that gain access to the channels. \nOne of the most important of the protein channels, the *sodium channel,* is only 0.3 to 0.5 nanometer in diameter, but the ability of sodium channels to discriminate sodium ions among other competing ions in the surrounding fluids is crucial for proper cellular function. \n \n**Figure 4-5.** Transport of sodium and potassium ions through protein channels. Also shown are conformational changes in the protein molecules to open or close the \"gates\" guarding the channels. \nThe narrowest part of the sodium channel's open pore, the *selectivity filter*, is lined with *strongly negatively charged* amino acid residues, as shown in the top panel of **Figure 4-5**. These strong negative charges can pull small *dehydrated* sodium ions away from their hydrating water molecules into these channels, although the ions do not need to be fully dehydrated to pass through the channels. Once in the channel, the sodium ions diffuse in either direction according to the usual laws of diffusion. Thus, the sodium channel is highly selective for passage of sodium ions. \n**Gating of Protein Channels.** Gating of protein channels provides a means of controlling ion permeability of the channels. This mechanism is shown in both panels of **Figure 4-5** for selective gating of sodium and potassium ions. Some of the gates are thought to be gatelike extensions of the transport protein molecule, which can close the opening of the channel or can be lifted away from the opening by a conformational change in the shape of the protein molecule. \nThe opening and closing of gates are controlled in two principal ways: \n1. Voltage gating. In the case of voltage gating, the molecular conformation of the gate or its chemical bonds responds to the electrical potential across the cell membrane. For example, in the top panel of Figure 4-5, a strong negative charge on the inside of the cell membrane may cause the outside sodium gates to remain tightly closed. Conversely, when the inside of the membrane loses its negative charge, these gates open suddenly and allow sodium to pass inward through the sodium pores. This process is the basic mechanism for eliciting action potentials in nerves that are responsible for nerve signals. In \n- the bottom panel of **Figure 4-5**, the potassium gates are on the intracellular ends of the potassium channels, and they open when the inside of the cell membrane becomes positively charged. The opening of these gates is partly responsible for terminating the action potential, a process discussed in Chapter 5.\n- 2. Chemical (ligand) gating. Some protein channel gates are opened by the binding of a chemical substance (a ligand) with the protein, which causes a conformational or chemical bonding change in the protein molecule that opens or closes the gate. One of the most important instances of chemical gating is the effect of the neurotransmitter acetylcholine on the acetylcholine receptor which serves as a ligand-gated ion channel. Acetylcholine opens the gate of this channel, providing a negatively charged pore about 0.65 nanometer in diameter that allows uncharged molecules or positive ions smaller than this diameter to pass through. This gate is exceedingly important for the transmission of nerve signals from one nerve cell to another (see Chapter 46) and from nerve cells to muscle cells to cause muscle contraction (see Chapter 7). \n#### Open-State Versus Closed-State of Gated Channels. \nFigure 4-6A shows two recordings of electrical current flowing through a single sodium channel when there was an approximately 25-millivolt potential gradient across the membrane. Note that the channel conducts current in an all-or-none fashion. That is, the gate of the channel snaps open and then snaps closed, with each open state lasting for only a fraction of a millisecond, up to several milliseconds, demonstrating the rapidity with which changes can occur during the opening and closing of the protein gates. At one voltage potential, the channel may remain closed all the time or almost all the time, whereas at another voltage, it may remain open either all or most of the time. At in-between voltages, as shown in the figure, the gates tend to snap open and closed intermittently, resulting in an average current flow somewhere between the minimum and maximum. \nPatch Clamp Method for Recording Ion Current Flow Through Single Channels. The patch clamp method for recording ion current flow through single protein channels is illustrated in Figure 4-6B. A micropipette with a tip diameter of only 1 or 2 micrometers is abutted against the outside of a cell membrane. Suction is then applied inside the pipette to pull the membrane against the tip of the pipette, which creates a seal where the edges of the pipette touch the cell membrane. The result is a minute membrane \"patch\" at the tip of the pipette through which electrical current flow can be recorded. \nAlternatively, as shown at the bottom right in **Figure 4-6B**, the small cell membrane patch at the end of the pipette can be torn away from the cell. The pipette with its sealed patch is then inserted into a free solution, which \n \n**Figure 4-6. A**, Recording of current flow through a single voltagegated sodium channel, demonstrating the all or none principle for opening and closing of the channel. **B**, Patch clamp method for recording current flow through a single protein channel. To the left, the recording is performed from a \"patch\" of a living cell membrane. To the right, the recording is from a membrane patch that has been torn away from the cell. \nallows the concentrations of ions both inside the micropipette and in the outside solution to be altered as desired. Also, the voltage between the two sides of the membrane can be set, or \"clamped,\" to a given voltage. \nIt has been possible to make such patches small enough so that only a single channel protein is found in the membrane patch being studied. By varying the concentrations of different ions, as well as the voltage across the membrane, one can determine the transport characteristics of the single channel, along with its gating properties. \n \n**Figure 4-7.** Effect of concentration of a substance on the rate of diffusion through a membrane by simple diffusion and facilitated diffusion. This graph shows that facilitated diffusion approaches a maximum rate, called the *Vmax.* \n#### **FACILITATED DIFFUSION REQUIRES MEMBRANE CARRIER PROTEINS** \nFacilitated diffusion is also called *carrier-mediated diffusion* because a substance transported in this manner diffuses through the membrane with the help of a specific carrier protein. That is, the carrier *facilitates* diffusion of the substance to the other side. \nFacilitated diffusion differs from simple diffusion in the following important way. Although the rat[e of simple](#page-54-1) diffusion through an open channel increases proportionately with the concentration of the diffusing substance, in facilitated diffusion the rate of diffusion approaches a maximum, called Vmax, as the concentration of the diffusing substance increases. This difference between simple diffusion and facilitated diffusion is demonstrated in **Figure 4-7**. The figure shows t[hat a](#page-55-0)s the concentration of the diffusing substance increases, the rate of simple diffusion continues to increase proportionately but, in the case of facilitated diffusion, the rate of diffusion cannot rise higher than the Vmax level. \nWhat is it that limits the rate of facilitated diffusion? A probable answer is the mechanism illustrated in **Figure 4-8**. This Figure shows a carrier protein with a pore large enough to transport a specific molecule partway through. It also shows a binding receptor on the inside of the protein carrier. The molecule to be transported enters the pore and becomes bound. Then, in a fraction of a second, a conformational or chemical change occurs in the carrier protein, so that the pore now opens to the opposite side of the membrane. Because the binding force of the receptor is weak, the thermal motion of the attached molecule causes it to break away and be released on the opposite side of the membrane. The rate at which molecules can be transported by this mechanism can never be greater than the rate at which the carrier protein molecule can undergo change back and forth between its two states. Note specifically, though, that this mechanism allows the transported molecule to move—that is, diffuse—in either direction through the membrane. \n \nFigure 4-8. Postulated mechanism for facilitated diffusion. \nAmong the many substances that cross cell membranes by facilitated diffusion are *glucose* and most of the *amino acids*. In the case of glucose, at least 14 members of a family of membrane proteins (called *GLUT*) that transport glucose molecules have been discovered in various tissues. Some of these GLUT proteins transport other monosaccharides that have structures similar to that of glucose, including galactose and fructose. One of these, glucose transporter 4 (GLUT4), is activated by insulin, which can increase the rate of facilitated diffusion of glucose as much as 10- to 20-fold in insulin-sensitive tissues. This is the principal mechanism whereby insulin controls glucose use in the body, as discussed in Chapter 79.",
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"page_content": "By now, it is evident that many substances can diffuse through the cell membrane. What is usually important is the *net* rate of diffusion of a substance in the desired direction. This net rate is determined by several factors. \n**Net Diffusion Rate Is Proportional to the Concentration Difference Across a Membrane. Figure 4-9.4** shows a cell membrane with a high concentration of a substance on the outside and a low concentration of a substance on the inside. The rate at which the substance diffuses *inward* is proportional to the concentration of molecules on the *outside* because this concentration determines how many molecules strike the outside of the membrane each second. Conversely, the rate at which molecules diffuse *outward* is proportional to their concentration *inside* the membrane. Therefore, the rate of net diffusion into the cell is proportional to the concentration on the outside *minus* the concentration on the inside: \nNet diffusion $\\propto (C_o - C_i)$ \n \n**Figure 4-9.** Effect of concentration difference (**A**), electrical potential difference affecting negative ions (**B**), and pressure difference (**C**) to cause diffusion of molecules and ions through a cell membrane. $C_o$ , concentration outside the cell; $C_i$ , concentration inside the cell; $P_1$ pressure 1; $P_2$ pressure 2. \nin which $C_0$ is the concentration outside and $C_i$ is the concentration inside the cell. \nMembrane Electrical Potential and Diffusion of lons-The \"Nernst Potential.\" If an electrical potential is applied across the membrane, as shown in Figure **4-9B**, the electrical charges of the ions cause them to move through the membrane even though no concentration difference exists to cause movement. Thus, in the left panel of Figure 4-9B, the concentration of negative ions is the same on both sides of the membrane, but a positive charge has been applied to the right side of the membrane, and a negative charge has been applied to the left, creating an electrical gradient across the membrane. The positive charge attracts the negative ions, whereas the negative charge repels them. Therefore, net diffusion occurs from left to right. After some time, large quantities of negative ions have moved to the right, creating the condition shown in the right panel of Figure 4-9B, in which a concentration difference of the ions has developed in the direction opposite to the electrical potential difference. The concentration difference now tends to move the ions to the left, whereas the electrical difference tends to move them to the right. When the concentration difference rises high enough, the two effects balance each other. At normal body temperature (98.6°F; 37°C), the electrical difference that will balance a given concentration difference \nof *univalent* ions—such as Na+ ions—can be determined from the following formula, called the *Nernst equation*: \nEMF (in millivolts) =\n$$\\pm 61\\log \\frac{C_1}{C_2}$$ \nin which EMF is the electromotive force (voltage) between side 1 and side 2 of the membrane, $C_1$ is the concentration on side 1, and $C_2$ is the concentration on side 2. This equation is extremely important in understanding the transmission of nerve impulses and is discussed in Chapter 5. \n#### Effect of a Pressure Difference Across the Membrane. \nAt times, a considerable pressure difference develops between the two sides of a diffusible membrane. This pressure difference occurs, for example, at the blood capillary membranes in all tissues of the body. The pressure in many capillaries is about 20 mm Hg greater inside than outside. \nPressure actually means the sum of all the forces of the different molecules striking a unit surface area at a given instant. Therefore, having a higher pressure on one side of a membrane than on the other side means that the sum of all the forces of the molecules striking the channels on that side of the membrane is greater than on the other side. In most cases, this situation is caused by greater numbers of molecules striking the membrane per second on one side than on the other side. The result is that increased amounts of energy are available to cause a net movement of molecules from the high-pressure side toward the low-pressure side. This effect is demonstrated in **Figure 4-9C**, which shows a piston developing high pressure on one side of a pore, thereby causing more molecules to strike the pore on this side and, therefore, more molecules to diffuse to the other side.",
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"page_content": "By far, the most abundant substance that diffuses through the cell membrane is water. Enough water ordinarily diffuses in each direction through the red blood cell membrane per second to equal about 100 times the volume of the cell itself. Yet, the amount that normally diffuses in the two directions is balanced so precisely that zero net movement of water occurs. Therefore, the volume of the cell remains constant. However, under certain conditions, a concentration difference for water can develop across a membrane. When this concentration difference for water develops, net movement of water does occur across the cell membrane, causing the cell to swell or shrink, depending on the direction of the water movement. This process of net movement of water caused by a concentration difference of water is called osmosis. \nTo illustrate osmosis, let us assume the conditions shown in Figure 4-10, with pure water on one side of the cell membrane and a solution of sodium chloride on the \n \n**Figure 4-10.** Osmosis at a cell membrane when a sodium chloride solution is placed on one side of the membrane and water is placed on the other side. \nother side. Water molecules pass through the cell membrane with ease, whereas sodium and chloride ions pass through only with difficulty. Therefore, sodium chloride solution is actually a mixture of permeant water molecules and nonpermeant sodium and chloride ions, and the membrane is said to be *selectively permeable* to water but much less so to sodium and chloride ions. Yet, the presence of the sodium and chloride has displaced some of the water molecules on the side of the membrane where these ions are present and, therefore, has reduced the concentration of water molecules to less than that of pure water. As a result, in the example shown in Figure 4-10, more water molecules strike the channels on the left side, where there is pure water, than on the right side, where the water concentration has been reduced. Thus, net movement of water occurs from left to right-that is, osmosis occurs from the pure water into the sodium chloride solution. \n#### **Osmotic Pressure** \nIf in **Figure 4-10** pressure were applied to the sodium chloride solution, osmosis of water into this solution would be slowed, stopped, or even reversed. The amount of pressure required to stop osmosis is called the *osmotic pressure* of the sodium chloride solution. \nThe principle of a pressure difference opposing osmosis is demonstrated in **Figure 4-11**, which shows a selectively permeable membrane separating two columns of fluid, one containing pure water and the other containing a solution of water and any solute that will not penetrate the membrane. Osmosis of water from chamber B into chamber A causes the levels of the fluid columns to become farther and farther apart, until eventually a pressure difference develops between the two sides of the membrane that is great enough to oppose the osmotic effect. The pressure difference across the membrane at this point is equal to the osmotic pressure of the solution that contains the nondiffusible solute. \n \n**Figure 4-11.** Demonstration of osmotic pressure caused by osmosis at a semipermeable membrane. \n#### **Importance of Number of Osmotic Particles (Molar Concentration) in Determining Osmotic Pressure.** \nThe osmotic pressure exerted by particles in a solution, whether they are molecules or ions, is determined by the number of particles per unit volume of fluid, not by the mass of the particles. The reason for this is that each particle in a solution, regardless of its mass, exerts, on average, the same amount of pressure against the membrane. That is, large particles, which have greater mass (m) than small particles, move at a slower velocity (v). The small particles move at higher velocities in such a way that their average kinetic energies (k), as determined by the following equation, \n$$k = \\frac{mv^2}{2}$$ \nare the same for each small particle as for each large particle. Consequently, the factor that determines the osmotic pressure of a solution is the concentration of the solution in terms of the number of particles (which is the same as its *molar concentration* if it is a nondissociated molecule), not in terms of mass of the solute. \n**Osmolality—The Osmole.** To express the concentration of a solution in terms of numbers of particles, a unit called the *osmole* is used in place of grams. \nOne osmole is 1 gram molecular weight of osmotically active solute. Thus, 180 grams of glucose, which is 1 gram molecular weight of glucose, is equal to 1 osmole of glucose because glucose does not dissociate into ions. If a solute dissociates into two ions, 1 gram molecular weight of the solute will become 2 osmoles because the number of osmotically active particles is now twice as great as for the nondissociated solute. Therefore, when fully dissociated, 1 gram molecular weight of sodium chloride, 58.5 grams, is equal to 2 osmoles. \nThus, a solution that has *1 osmole of solute dissolved in each kilogram of water* is said to have an *osmolality of 1 osmole per kilogram,* and a solution that has 1/1000 osmole dissolved per kilogram has an osmolality of 1 milliosmole per kilogram. The normal osmolality of the extracellular and intracellular fluids is about *300 milliosmoles per kilogram of water.* \n**Relationship of Osmolality to Osmotic Pressure.** At normal body temperature, 37°C (98.6°F), a concentration of 1 osmole per liter will cause 19,300 mm Hg osmotic pressure in the solution. Likewise, *1 milliosmole* per liter concentration is equivalent to *19.3 mm Hg* osmotic pressure. Multiplying this value by the 300-milliosmolar concentration of the body fluids gives a total calculated osmotic pressure of the body fluids of 5790 mm Hg. The measured value for this, however, averages only about 5500 mm Hg. The reason for this difference is that many ions in the body fluids, such as sodium and chloride ions, are highly attracted to one another; consequently, they cannot move entirely unrestrained in the fluids and create their full osmotic pressure potential. Therefore, on average, the actual osmotic pressure of the body fluids is about 0.93 times the calculated value. \n**The Term** *Osmolarity***.** *Osmolarity* is the osmolar concentration expressed as *osmoles per liter of solution* rather than osmoles per kilogram of water. Although, strictly speaking, it is osmoles per kilogram of water (osmolality) that determines osmotic pressure, the quantitative differences between osmolarity and osmolality are less than 1% for dilute solutions such as those in the body. Because it is far more practical to measure osmolarity than osmolality, measuring osmolarity is the usual practice in physiological studies. \n#### ACTIVE TRANSPORT OF SUBSTANCES THROUGH MEMBRANES \nAt times, a large concentration of a substance is required in the intracellular fluid, even though the extracellular fluid contains only a small concentration. This situation is true, for example, for potassium ions. Conversely, it is important to keep the concentrations of other ions very low inside the cell, even though their concentrations in the extracellular fluid are high. This situation is especially true for sodium ions. Neither of these two effects could occur by simple diffusion because simple diffusion eventually equilibrates concentrations on the two sides of the membrane. Instead, some energy source must cause excess movement of potassium ions to the inside of cells and excess movement of sodium ions to the outside of cells. When a cell membrane moves molecules or ions uphill against a concentration gradient (or uphill against an electrical or pressure gradient), the process is called *active transport.* \nSome examples of substances that are actively transported through at least some cell membranes include sodium, potassium, calcium, iron, hydrogen, chloride, iodide, and urate ions, several different sugars, and most of the amino acids. \n**Primary Active Transport and Secondary Active Transport.** Active transport is divided into two types according to the source of the energy used to facilitate the transport, *primary active transport* and *secondary active transport.* In primary active transport, the energy is derived directly from the breakdown of adenosine triphosphate (ATP) or some other high-energy phosphate compound. In secondary active transport, the energy is derived secondarily from energy that has been stored in the form of ionic concentration differences of secondary molecular or ionic substances between the two sides of a cell membrane, created originally by primary active transport. In both cases, transport depends on *carrier proteins* that penetrate through the cell membrane, as is true for facilitated diffusion. However, in active transport, the carrier protein functions differently from the carrier in facilitated diffusion because it is capable of imparting energy to the transported substance to move it against the electrochemical gradient. The following sections provide some examples of primary active transport and secondary active transport, with more detailed explanations of their principles of function. \n#### **PRIMARY ACTIVE TRANSPORT** \n#### **Sodium-Potassium Pump Transports Sodium Ions Out of Cells and Potassium Ions into Cells** \nAmong the substances that are transported by primary active transport are sodium, potassium, calcium, hydrogen, chloride, and a few other ions. The active transport mechanism that has been studied in greatest detail is the *sodium-potassium* (Na+-K+) pump, a transporter that pumps sodium ions outward through the cell membrane of all cells and, at the same time, pumps potassium ions from the outside to the inside. This pump is responsible for mainta[ining the so](#page-58-0)dium and potassium concentration differences across the cell membrane, as well as for establishing a negative electrical voltage inside the cells. Indeed, Chapter 5 shows that this pump is also the basis of nerve function, transmitting nerve signals throughout the nervous system. \n**Figure 4-12** shows the basic physical components of the Na+-K+ pump. The *carrier protein* is a complex of two separate globular proteins—a larger one called the α subunit, with a molecular weight of about 100,000, and a smaller one called the β subunit, with a molecular weight of about 55,000. Although the function of the smaller protein is not known (except that it might anchor the protein complex in the lipid membrane), the larger protein has three specific features that are important for the functioning of the pump: \n1. It has three *binding sites for sodium ions* on the portion of the protein that protrudes to the inside of the cell. \n \n**Figure 4-12.** Postulated mechanism of the sodium-potassium pump. ADP, Adenosine diphosphate; ATP, adenosine triphosphate; Pi, phosphate ion. \n- 2. It has two *binding sites for potassium ions* on the outside.\n- 3. The inside portion of this protein near the sodium binding sites has adenosine triphosphatase (AT-Pase) activity. \nWhen two potassium ions bind on the outside of the carrier protein and three sodium ions bind on the inside, the ATPase function of the protein becomes activated. Activation of the ATPase function leads to cleavage of one molecule of ATP, splitting it to adenosine diphosphate (ADP) and liberating a high-energy phosphate bond of energy. This liberated energy is believed to cause a chemical and conformational change in the protein carrier molecule, extruding three sodium ions to the outside and two potassium ions to the inside. \nAs with other enzymes, the Na+-K+ ATPase pump can run in reverse. If the electrochemical gradients for Na+ and K+ are experimentally increased to the degree that the energy stored in their gradients is greater than the chemical energy of ATP hydrolysis, these ions will move down their concentration gradients, and the Na+-K+ pump will synthesize ATP from ADP and phosphate. The phosphorylated form of the Na+-K+ pump, therefore, can either donate its phosphate to ADP to produce ATP or use the energy to change its conformation and pump Na+ out of the cell and K+ into the cell. The relative concentrations of ATP, ADP, and phosphate, as well as the electrochemical gradients for Na+ and K+, determine the direction of the enzyme reaction. For some cells, such as electrically active nerve cells, 60% to 70% of the cell's energy requirement may be devoted to pumping Na+ out of the cell and K+ into the cell. \n**The Na+-K+ Pump Is Important for Controlling Cell Volume.** One of the most important functions of the Na+-K+ pump is to control the cell volume. Without function of this pump, most cells of the body would swell until they burst. \nThe mechanism for controlling the volume is as follows. Inside the cell are large numbers of proteins and other organic molecules that cannot escape from the cell. Most of these proteins and other organic molecules are negatively charged and, therefore, attract large numbers of potassium, sodium, and other positive ions. All these molecules and ions then cause osmosis of water to the interior of the cell. Unless this process is checked, the cell will swell indefinitely until it bursts. The normal mechanism for preventing this outcome is the Na+-K+ pump. Note again that this mechanism pumps three Na+ ions to the outside of the cell for every two K+ ions pumped to the interior. Also, the membrane is far less permeable to sodium ions than to potassium ions and, once the sodium ions are on the outside, they have a strong tendency to stay there. This process thus represents a net loss of ions out the cell, which also initiates osmosis of water out of the cell. \nIf a cell begins to swell for any reason, the Na+-K+ pump is automatically activated, moving still more ions to the exterior and carrying water with them. Therefore, the Na+-K+ pump performs a continual surveillance role in maintaining normal cell volume. \n**Electrogenic Nature of the Na+-K+ Pump.** The fact that the Na+-K+ pump moves three Na+ ions to the exterior for every two K+ ions that are moved to the interior means that a net of one positive charge is moved from the interior of the cell to the exterior of the cell for each cycle of the pump. This action creates positivity outside the cell but results in a deficit of positive ions inside the cell; that is, it causes negativity on the inside. Therefore, the Na+-K+ pump is said to be *electrogenic* because it creates an electrical potential across the cell membrane. As discussed in Chapter 5, this electrical potential is a basic requirement in nerve and muscle fibers for transmitting nerve and muscle signals. \n#### **Primary Active Transport of Calcium Ions** \nAnother important primary active transport mechanism is the *calcium pump*. Calcium ions are normally maintained at an extremely low concentration in the intracellular cytosol of virtually all cells in the body, at a concentration about 10,000 times less than that in the extracellular fluid. This level of maintenance is achieved mainly by two primary active transport calcium pumps. One, which is in the cell membrane, pumps calcium to the outside of the cell. The other pumps calcium ions into one or more of the intracellular vesicular organelles of the cell, such as the sarcoplasmic reticulum of muscle cells and the mitochondria in all cells. In each of these cases, the carrier protein penetrates the membrane and functions as an enzyme ATPase, with the same capability to cleave ATP as the ATPase of the sodium carrier protein. The difference is that this protein has a highly specific binding site for calcium instead of for sodium.",
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"page_content": "Primary active transport of hydrogen ions is especially important at two places in the body: (1) in the gastric glands of the stomach; and (2) in the late distal tubules and cortical collecting ducts of the kidneys. \nIn the gastric glands, the deep-lying *parietal cells* have the most potent primary active mechanism for transporting hydrogen ions of any part of the body. This mechanism is the basis for secreting hydrochloric acid in stomach digestive secretions. At the secretory ends of the gastric gland parietal cells, the hydrogen ion concentration is increased as much as a million-fold and then is released into the stomach, along with chloride ions, to form hydrochloric acid. \nIn the renal tubules, special *intercalated cells* found in the late distal tubules and cortical collecting ducts also transport hydrogen ions by primary active transport. In this case, large amounts of hydrogen ions are secreted from the blood into the renal tubular fluid for the purpose of eliminating excess hydrogen ions from the body fluids. The hydrogen ions can be secreted into the renal tubular fluid against a concentration gradient of about 900-fold. Yet, as discussed in Chapter 31, most of these hydrogen ions combine with tubular fluid buffers before they are eliminated in the urine \n#### **Energetics of Primary Active Transport** \nThe amount of energy required to transport a substance actively through a membrane is determined by how much the substance is concentrated during transport. Compared with the energy required to concentrate a substance 10-fold, concentrating it 100-fold requires twice as much energy, and concentrating it 1000-fold requires three times as much energy. In other words, the energy required is proportional to the *logarithm* of the degree that the substance is concentrated, as expressed by the following formula: \nEnergy (in calories per osmole) = 1400 log\n$$\\frac{C_1}{C_2}$$ \nThus, in terms of calories, the amount of energy required to concentrate 1 osmole of a substance 10-fold is about 1400 calories, whereas to concentrate it 100-fold, 2800 calories are required. One can see that the energy expenditure for concentrating substances in cells or for removing substances from cells against a concentration gradient can be tremendous. Some cells, such as those lining the renal tubules and many glandular cells, expend as much as 90% of their energy for this purpose alone.\n\nWhen sodium ions are transported out of cells by primary active transport, a large concentration gradient of \nsodium ions across the cell membrane usually develops, with a high concentration outside the cell and a low concentration inside. This gradient represents a storehouse of energy, because the excess sodium outside the cell membrane is always attempting to diffuse to the interior. Under appropriate conditions, this diffusion energy of sodium can pull other substances along with the sodium through the cell membrane. This phenomenon, called *cotransport,* is one form of *secondary active transport.* \nFor sodium to pull another substance along with it, a coupling mechanism is required; this is achieved by means of still another carrier protein in the cell membrane. The carrier in this case serves as an attachment point for both the sodium ion and the substance to be co-transported. Once they are both attached, the energy gradient of the sodium ion causes the sodium ion and the other substance to be transported together to the interior of the cell. \nIn *counter-transport,* sodium ions again attempt to diffuse to the interior of the cell because of their large concentration gradient. However, this time, the substance to be transported is on the inside of the cell and is transported to the outside. Therefore, the sodium ion binds to the carrier protein, where it projects to the exterior surface of the membrane, and the substance to be countertransported binds to the interior projection of the carrier protein. Once both have become bound, a conformational change occurs, and energy released by the action of the sodium ion moving to the interior causes the other substance to move to the exterior. \n#### **Co-Transport o[f Glucose a](#page-60-0)nd Amino Acids Along with Sodium Ions** \nGlucose and many amino acids are transported into most cells against large concentration gradients; the mechanism of this action is entirely by co-transport, as shown in **Figure 4-13**. Note that the transport carrier protein has two binding sites on its exterior side, one for sodium and one for glucose. Also, the concentration of sodium ions is high on the outside and low on the inside, which provides energy for the transport. A special property of the transport protein is that a conformational change to allow sodium movement to the interior will not occur until a glucose molecule also attaches. When they both become attached, the conformational change takes place, and the sodium and glucose are transported to the inside of the cell at the same time. Hence, this is a *sodium-glucose co-transporter*. Sodium-glucose cotransporters are especially important for transporting glucose across renal and intestinal epithelial cells, as discussed in Chapters 28 and 66. \n*Sodium co-transport of amino acids* occurs in the same manner as for glucose, except that it uses a different set of transport proteins. At least five *amino acid transport proteins* have been identified, each of which is responsible for transporting one subset of amino acids with specific molecular characteristics. \n \n**Figure 4-13** Postulated mechanism for sodium co-transport of glucose. \n \n**Figure 4-14.** Sodium counter-transport of calcium and hydrogen ions. \nSodium co-transport of glucose and amino acids occurs especially through the epithelial cells of the intestinal tract and the renal tubules of the kidneys to promote absorption of these substances into the blood. This process will be discussed in later chapters. \nOther important co-transport mechanisms in at least some cells include co-transport of potassium, chloride, bicarbonate, phosphate, iodine, iron, and urate ions. \n#### **Sodium Counter-Tran[sport of Ca](#page-60-1)lcium and Hydrogen Ions** \nTwo especially important counter-transporters (i.e., transport in a direction opposite to the primary ion) are *sodium-calcium counter-transport* and *sodium-hydrogen counter-transport* (**Figure 4-14**). \nSodium-calcium counter-transport occurs through all or almost all cell membranes, with sodium ions moving to the interior and calcium ions to the exterior; both are bound to the same transport protein in a countertransport mode. This mechanism is in addition to the primary active transport of calcium that occurs in some cells. \nSodium-hydrogen counter-transport occurs in several tissues. An especially important example is in the *proximal tubules* of the kidneys, where sodium ions move from the lumen of the tubule to the interior of the tubular cell and hydrogen ions are counter-transported into the tubule lumen. As a mechanism for concentrating hydrogen ions, counter-transport is not nearly as powerful as the primary active transport of hydrogen ions that occurs in the more distal renal tubules, but it can transport extremely *large numbers of hydrogen ions,* thus making it a key to hydrogen ion control in the body fluids, as discussed in detail in Chapter 31. \n \n**Figure 4-15.** Basic mechanism of active transport across a layer of cells. \n#### **ACTIVE TRANSPORT THROUGH CELLULAR SHEETS** \nAt many places in the body, substances must be transported all the way through a cellular sheet instead of simply through the cell membrane. Transport of this type occurs through the following: (1) intestinal epithelium; (2) epithelium of the renal tubules; (3) epithelium of all exocrine glands; (4) epithelium of the gallbladder; and (5) membrane of the choroid plexus of the brain, along with other membranes. \nThe bas[ic mechanism](#page-61-0) for transport of a substance through a cellular sheet is as follows: (1) *active transport* through the cell membrane *on one side* of the transporting cells in the sheet; and then (2) either *simple diffusion* or *facilitated diffusion* through the membrane *on the opposite side* of the cell. \n**Figure 4-15** shows a mechanism for the transport of sodium ions through the epithelial sheet of the intestines, gallbladder, and renal tubules. This figure shows that the epithelial cells are connected together tightly at the luminal pole by means of junctions. The brush border on the luminal surfaces of the cells is permeable to both sodium ions and water. Therefore, sodium and water diffuse readily from the lumen into the interior of the cell. Then, at the basal and lateral membranes of the cells, sodium ions are actively transported into the extracellular fluid of the surrounding connective tissue and blood vessels. This action creates a high sodium ion concentration gradient across these membranes, which in turn causes osmosis of water. Thus, active transport of sodium ions at the basolateral sides of the epithelial cells results in the transport not only of sodium ions but also of water. \nIt is through these mechanisms that almost all nutrients, ions, and other substances are absorbed into the blood from the intestine. These mechanisms are also how the same substances are reabsorbed from the glomerular filtrate by the renal tubules. \nNumerous examples of the different types of transport discussed in this chapter are provided throughout this text. \n#### Bibliography \nAgre P, Kozono D: Aquaporin water channels: molecular mechanisms for human diseases. FEBS Lett 555:72, 2003. \nBröer S: Amino acid transport across mammalian intestinal and renal epithelia. Physiol Rev 88:249, 2008. \nDeCoursey TE: Voltage-gated proton channels: molecular biology, physiology, and pathophysiology of the H(V) family. Physiol Rev 93:599, 2013. \nDiPolo R, Beaugé L: Sodium/calcium exchanger: influence of metabolic regulation on ion carrier interactions. Physiol Rev 86:155, 2006. \nDrummond HA, Jernigan NL, Grifoni SC: Sensing tension: epithelial sodium channel/acid-sensing ion channel proteins in cardiovascular homeostasis. Hypertension 51:1265, 2008. \nEastwood AL, Goodman MB: Insight into DEG/ENaC channel gating from genetics and structure. Physiology (Bethesda) 27:282, 2012. \nFischbarg J: Fluid transport across leaky epithelia: central role of the tight junction and supporting role of aquaporins. Physiol Rev 90:1271, 2010. \nGadsby DC: Ion channels versus ion pumps: the principal difference, in principle. Nat Rev Mol Cell Biol 10:344, 2009. \nGhezzi C, Loo DDF, Wright EM. Physiology of renal glucose handling via SGLT1, SGLT2 and GLUT2. Diabetologia 61:2087-2097, 2018. \nHilge M: Ca2+ regulation of ion transport in the Na+/Ca2+ exchanger. J Biol Chem 287:31641, 2012. \nJentsch TJ, Pusch M. CLC Chloride channels and transporters: structure, function, physiology, and disease. Physiol Rev 2018 98:1493- 1590, 2018. \nKaksonen M, Roux A. Mechanisms of clathrin-mediated endocytosis. Nat Rev Mol Cell Biol 19:313-326, 2018. \nKandasamy P, Gyimesi G, Kanai Y, Hediger MA. Amino acid transporters revisited: new views in health and disease. Trends Biochem Sci 43:752-789, 2018. \nPapadopoulos MC, Verkman AS: Aquaporin water channels in the nervous system. Nat Rev Neurosci 14:265, 2013. \nRieg T, Vallon V. Development of SGLT1 and SGLT2 inhibitors. Diabetologia 61:2079-2086, 2018. \nSachs F: Stretch-activated ion channels: what are they? Physiology 25:50, 2010. \nSchwab A, Fabian A, Hanley PJ, Stock C: Role of ion channels and transporters in cell migration. Physiol Rev 92:1865, 2012. \nStransky L, Cotter K, Forgac M. The function of V-ATPases in cancer. Physiol Rev 96:1071-1091, 2016 \nTian J, Xie ZJ: The Na-K-ATPase and calcium-signaling microdomains. Physiology (Bethesda) 23:205, 2008. \nVerkman AS, Anderson MO, Papadopoulos MC. Aquaporins: important but elusive drug targets. Nat Rev Drug Discov 13:259-277, 2014. \nWright EM, Loo DD, Hirayama BA: Biology of human sodium glucose transporters. Physiol Rev 91:733, 2011. \n",
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"page_content": "Electrical potentials exist across the membranes of virtually all cells of the body. Some cells, such as nerve and muscle cells, generate rapidly changing electrochemical impulses at their membranes, and these impulses are used to transmit signals along the nerve or muscle membranes. In other types of cells, such as glandular cells, macrophages, and ciliated cells, local changes in membrane potentials also activate many of the cell's functions. This chapter reviews the basic mechanisms whereby membrane potentials are generated at rest and during action by nerve and muscle cells. See Video 5-1. \n",
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"page_content": "In Figure 5-1A, the potassium concentration is great inside a nerve fiber membrane but very low outside the membrane. Let us assume that the membrane in this case is permeable to the potassium ions but not to any other ions. Because of the large potassium concentration gradient from the inside toward the outside, there is a strong tendency for potassium ions to diffuse outward through the membrane. As they do so, they carry positive electrical charges to the outside, thus creating electropositivity outside the membrane and electronegativity inside the membrane because of negative anions that remain behind and do not diffuse outward with the potassium. Within about 1 millisecond, the potential difference between the inside and outside, called the diffusion potential, becomes great enough to block further net potassium diffusion to the exterior, despite the high potassium ion concentration gradient. In the normal mammalian nerve fiber, the potential difference is about 94 millivolts, with negativity inside the fiber membrane. \n**Figure 5-1***B* shows the same phenomenon as in **Figure 5-1***A*, but this time with a high concentration of sodium ions *outside* the membrane and a low concentration of sodium ions *inside*. These ions are also positively charged. This time, the membrane is highly permeable to the sodium ions but is impermeable to all other ions. Diffusion of the positively charged sodium ions to the inside \ncreates a membrane potential of opposite polarity to that in **Figure 5-1***A*, with negativity outside and positivity inside. Again, the membrane potential rises high enough within milliseconds to block further net diffusion of sodium ions to the inside; however, this time, in the mammalian nerve fiber, *the potential is about 61 millivolts positive inside the fiber.* \nThus, in both parts of **Figure 5-1**, we see that a concentration difference of ions across a selectively permeable membrane can, under appropriate conditions, create a membrane potential. Later in this chapter, we show that many of the rapid changes in membrane potentials observed during nerve and muscle impulse transmission result from such rapidly changing diffusion potentials. \nThe Nernst Equation Describes the Relationship of Diffusion Potential to the lon Concentration Difference Across a Membrane. The diffusion potential across a membrane that exactly opposes the net diffusion of a particular ion through the membrane is called the *Nernst potential* for that ion, a term that was introduced in Chapter 4. The magnitude of the Nernst potential is determined by the *ratio* of the concentrations of that specific ion on the two sides of the membrane. The greater this ratio, the greater the tendency for the ion to diffuse in one direction and therefore the greater the Nernst potential required to prevent additional net diffusion. The following equation, called the *Nernst equation*, can be used to calculate the Nernst potential for any univalent ion at the normal body temperature of 98.6°F (37°C): \nEMF (millivolts) =\n$$\\pm \\frac{61}{z} \\times log \\frac{Concentration\\ inside}{Concentration\\ outside}$$ \nwhere EMF is the electromotive force and z is the electrical charge of the ion (e.g., +1 for $K^+$ ). \nWhen using this formula, it is usually assumed that the potential in the extracellular fluid outside the membrane remains at zero potential, and the Nernst potential is the potential inside the membrane. Also, the sign of the potential is positive (+) if the ion diffusing from inside to outside is a negative ion, and it is negative (–) if the ion is positive. Thus, when the concentration of positive potassium ions on the inside is 10 times that on the outside, the log of 10 is 1, so the Nernst potential calculates to be -61 millivolts inside the membrane.",
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"page_content": "Physiology is the science that seeks to explain the physical and chemical mechanisms that are responsible for the origin, development, and progression of life. Each type of life, from the simplest virus to the largest tree or the complicated human being, has its own functional characteristics. Therefore, the vast field of physiology can be divided into viral physiology, bacterial physiology, cellular physiology, plant physiology, invertebrate physiology, vertebrate physiology, mammalian physiology, human physiology, and many more subdivisions. \n**Human Physiology.** The science of human physiology attempts to explain the specific characteristics and mechanisms of the human body that make it a living being. The fact that we remain alive is the result of complex control systems. Hunger makes us seek food, and fear makes us seek refuge. Sensations of cold make us look for warmth. Other forces cause us to seek fellowship and to reproduce. The fact that we are sensing, feeling, and knowledgeable beings is part of this automatic sequence of life; these special attributes allow us to exist under widely varying conditions that otherwise would make life impossible. \nHuman physiology links the basic sciences with medicine and integrates multiple functions of the cells, tissues, and organs into the functions of the living human being. This integration requires communication and coordination by a vast array of control systems that operate at every level—from the genes that program synthesis of molecules to the complex nervous and hormonal systems that coordinate functions of cells, tissues, and organs throughout the body. Thus, the coordinated functions of the human body are much more than the sum of its parts, and life in health, as well as in disease states, relies on this total function. Although the main focus of this book is on normal human physiology, we will also discuss, to some extent, *pathophysiology,* which is the study of disordered body function and the basis for clinical medicine. \n#### CELLS ARE THE LIVING UNITS OF THE BODY \nThe basic living unit of the body is the cell. Each tissue or organ is an aggregate of many different cells held together by intercellular supporting structures. \nEach type of cell is specially adapted to perform one or a few particular functions. For example, the red blood cells, numbering about 25 trillion in each person, transport oxygen from the lungs to the tissues. Although the red blood cells are the most abundant of any single type of cell in the body, there are also trillions of additional cells of other types that perform functions different from those of the red blood cell. The entire body, then, contains about 35 to 40 trillion human cells. \nThe many cells of the body often differ markedly from one another but all have certain basic characteristics that are alike. For example, oxygen reacts with carbohydrate, fat, and protein to release the energy required for all cells to function. Furthermore, the general chemical mechanisms for changing nutrients into energy are basically the same in all cells, and all cells deliver products of their chemical reactions into the surrounding fluids. \nAlmost all cells also have the ability to reproduce additional cells of their own type. Fortunately, when cells of a particular type are destroyed, the remaining cells of this type usually generate new cells until the supply is replenished. \n**Microorganisms Living in the Body Outnumber Human Cells.** In addition to human cells, trillions of microbes inhabit the body, living on the skin and in the mouth, gut, and nose. The gastrointestinal tract, for example, normally contains a complex and dynamic population of 400 to 1000 species of microorganisms that outnumber our human cells. Communities of microorganisms that inhabit the body, often called *microbiota,* can cause diseases, but most of the time they live in harmony with their human hosts and provide vital functions that are essential for survival of their hosts. Although the importance of gut microbiota in the digestion of foodstuffs is widely recognized, additional roles for the body's microbes in nutrition, immunity, and other functions are just beginning to be appreciated and represent an intensive area of biomedical research. \n#### EXTRACELLULAR FLUID—THE \"INTERNAL ENVIRONMENT\" \nAbout 50% to 70% of the adult human body is fluid, mainly a water solution of ions and other substances. Although most of this fluid is inside the cells and is called *intracellular fluid,* about one-third is in the spaces outside the cells and is called *extracellular fluid.* This extracellular fluid is in constant motion throughout the body. It is transported rapidly in the circulating blood and then mixed between the blood and tissue fluids by diffusion through the capillary walls. \nIn the extracellular fluid are the ions and nutrients needed by the cells to maintain life. Thus, all cells live in essentially the same environment—the extracellular fluid. For this reason, the extracellular fluid is also called the *internal environment* of the body, or the *milieu intérieur,* a term introduced by the great 19th-century French physiologist Claude Bernard (1813–1878). \nCells are capable of living and performing their special functions as long as the proper concentrations of oxygen, glucose, different ions, amino acids, fatty substances, and other constituents are available in this internal environment. \n#### **Differences in Extracellular and Intracellular Fluids.** \nThe extracellular fluid contains large amounts of sodium, chloride, and bicarbonate ions plus nutrients for the cells, such as oxygen, glucose, fatty acids, and amino acids. It also contains carbon dioxide that is being transported from the cells to the lungs to be excreted, plus other cellular waste products that are being transported to the kidneys for excretion. \nThe intracellular fluid contains large amounts of potassium, magnesium, and phosphate ions instead of the sodium and chloride ions found in the extracellular fluid. Special mechanisms for transporting ions through the cell membranes maintain the ion concentration differences between the extracellular and intracellular fluids. These transport processes are discussed in Chapter 4. \n#### HOMEOSTASIS—MAINTENANCE OF A NEARLY CONSTANT INTERNAL ENVIRONMENT \nIn 1929, the American physiologist Walter Cannon (1871–1945) coined the term *homeostasis* to describe the *maintenance of nearly constant conditions in the internal environment*. Essentially, all organs and tissues of the body perform functions that help maintain these relatively constant conditions. For example, the lungs provide oxygen to the extracellular fluid to replenish the oxygen used by the cells, the kidneys maintain constant ion concentrations, and the gastrointestinal system provides nutrients while eliminating waste from the body. \nThe various ions, nutrients, waste products, and other constituents of the body are normally regulated within a range of values, rather than at fixed values. For some of the body's constituents, this range is extremely small. Variations in the blood hydrogen ion concentration, for example, are normally less than 5 *nanomoles/L* (0.000000005 moles/L). The blood sodium concentration is also tightly regulated, normally varying only a few *millimoles* per liter, even with large changes in sodium intake, but these variations of sodium concentration are at least 1 million times greater than for hydrogen ions. \nPowerful control systems exist for maintaining concentrations of sodium and hydrogen ions, as well as for most of the other ions, nutrients, and substances in the body at levels that permit the cells, tissues, and organs to perform their normal functions, despite wide environmental variations and challenges from injury and diseases. \nMuch of this text is concerned with how each organ or tissue contributes to homeostasis. Normal body functions require integrated actions of cells, tissues, organs, and multiple nervous, hormonal, and local control systems that together contribute to homeostasis and good health. \n**Homeostatic Compensations in Diseases.** *Disease* is often considered to be a state of disrupted homeostasis. However, even in the presence of disease, homeostatic mechanisms continue to operate and maintain vital functions through multiple compensations. In some cases, these compensations may lead to major deviations of the body's functions from the normal range, making it difficult to distinguish the primary cause of the disease from the compensatory responses. For example, diseases that impair the kidneys' ability to excrete salt and water may lead to high blood pressure, which initially helps return excretion to normal so that a balance between intake and renal excretion can be maintained. This balance is needed to maintain life, but, over long periods of time, the high blood pressure can damage various organs, including the kidneys, causing even greater increases in blood pressure and more renal damage. Thus, homeostatic compensations that ensue after injury, disease, or major environmental challenges to the body may represent trade-offs that are necessary to maintain vital body functions but, in the long term, contribute to additional abnormalities of body function. The discipline of *pathophysiology* seeks to explain how the various physiological processes are altered in diseases or injury. \nThis chapter outlines the different functional systems of the body and their contributions to homeostasis. We then briefly discuss the basic theory of the body's control systems that allow the functional systems to operate in support of one another. \n#### **EXTRACELLULAR FLUID TRANSPORT AND MIXING SYSTEM—THE BLOOD CIRCULATORY SYSTEM** \nExtracellular fluid is transported through the body in two stages. The first stage is movement of blood through the body in the blood vessels. The second is movement of fluid between the blood capillaries and the *intercellular spaces* between the tissue cells. \n**[Figure 1-1](#page-9-0)** shows the overall circulation of blood. All the blood in the circulation traverses the entire circuit an average \n \n**Figure 1-1.** General organization of the circulatory system. \nof once each minute when the body is at rest and as many as six times each minute when a person is extremely active. \nAs blood passes through blood capillaries, continual exchange of extracellular fluid occurs between the plasma portion of the blood and the interstitial fluid that fills the intercellular spaces. This process is shown in **Figure 1-2**. The capillary walls are permeable to most molecules in the blood plasma, with the exception of plasma proteins, which are too large to pass through capillaries readily. Therefore, large amounts of fluid and its dissolved constituents *diffuse* back and forth between the blood and the tissue spaces, as shown by the arrows in **Figure 1-2**. \nThis process of diffusion is caused by kinetic motion of the molecules in the plasma and the interstitial fluid. \n \n**Figure 1-2.** Diffusion of fluid and dissolved constituents through the capillary walls and interstitial spaces. \nThat is, the fluid and dissolved molecules are continually moving and bouncing in all directions in the plasma and fluid in the intercellular spaces, as well as through capillary pores. Few cells are located more than 50 micrometers from a capillary, which ensures diffusion of almost any substance from the capillary to the cell within a few seconds. Thus, the extracellular fluid everywhere in the body—both that of the plasma and that of the interstitial fluid—is continually being mixed, thereby maintaining homogeneity of extracellular fluid throughout the body.\n\n**Respiratory System. Figure 1-1** shows that each time blood passes through the body, it also flows through the lungs. The blood picks up *oxygen* in alveoli, thus acquiring the oxygen needed by cells. The membrane between the alveoli and the lumen of the pulmonary capillaries, the *alveolar membrane*, is only 0.4 to 2.0 micrometers thick, and oxygen rapidly diffuses by molecular motion through this membrane into the blood. \n**Gastrointestinal Tract.** A large portion of the blood pumped by the heart also passes through the walls of the gastrointestinal tract. Here different dissolved nutrients, including *carbohydrates*, *fatty acids*, and *amino acids*, are absorbed from ingested food into the extracellular fluid of the blood. \nLiver and Other Organs That Perform Primarily Metabolic Functions. Not all substances absorbed from the gastrointestinal tract can be used in their absorbed form by the cells. The liver changes the chemical compositions of many of these substances to more usable forms, and other tissues of the body—fat cells, gastrointestinal mucosa, kidneys, and endocrine glands—help modify the absorbed substances or store them until they are needed. The liver also eliminates certain waste products produced in the body and toxic substances that are ingested. \n**Musculoskeletal System.** How does the musculoskeletal system contribute to homeostasis? The answer is obvious and simple. Were it not for the muscles, the body could not move to obtain the foods required for nutrition. The musculoskeletal system also provides motility for protection against adverse surroundings, without which the entire body, along with its homeostatic mechanisms, could be destroyed. \n#### **REMOVAL OF METABOLIC END PRODUCTS** \n**Removal of Carbon Dioxide by the Lungs.** At the same time that blood picks up oxygen in the lungs, *carbon dioxide* is released from the blood into lung alveoli; the respiratory movement of air into and out of the lungs carries carbon dioxide to the atmosphere. Carbon dioxide is the most abundant of all the metabolism products. \n**Kidneys.** Passage of blood through the kidneys removes most of the other substances from the plasma besides carbon dioxide that are not needed by cells. These substances include different end products of cellular metabolism, such as urea and uric acid; they also include excesses of ions and water from the food that accumulate in the extracellular fluid. \nThe kidneys perform their function first by filtering large quantities of plasma through the glomerular capillaries into the tubules and then reabsorbing into the blood substances needed by the body, such as glucose, amino acids, appropriate amounts of water, and many of the ions. Most of the other substances that are not needed by the body, especially metabolic waste products such as urea and creatinine, are reabsorbed poorly and pass through the renal tubules into the urine. \n**Gastrointestinal Tract.** Undigested material that enters the gastrointestinal tract and some waste products of metabolism are eliminated in the feces. \n**Liver.** Among the many functions of the liver is detoxification or removal of ingested drugs and chemicals. The liver secretes many of these wastes into the bile to be eventually eliminated in the feces. \n#### **REGULATION OF BODY FUNCTIONS** \n**Nervous System.** The nervous system is composed of three major parts—the *sensory input portion,* the *central nervous system* (or *integrative portion*), and the *motor output portion.* Sensory receptors detect the state of the body and its surroundings. For example, receptors in the skin alert us whenever an object touches the skin. The eyes are sensory organs that give us a visual image of the surrounding area. The ears are also sensory organs. The central nervous system is composed of the brain and spinal cord. The brain stores information, generates thoughts, creates ambition, and determines reactions that the body performs in response to the sensations. Appropriate signals are then transmitted through the motor output portion of the nervous system to carry out one's desires. \nAn important segment of the nervous system is called the *autonomic system.* It operates at a subconscious level and controls many functions of internal organs, including the level of pumping activity by the heart, movements of the gastrointestinal tract, and secretion by many of the body's glands. \n**Hormone Systems.** Located in the body are *endocrine glands,* organs and tissues that secrete chemical substances called *hormones.* Hormones are transported in the extracellular fluid to other parts of the body to help regulate cellular function. For example, *thyroid hormone* increases the rates of most chemical reactions in all cells, thus helping set the tempo of bodily activity. *Insulin* controls glucose metabolism, *adrenocortical hormones* control sodium and potassium ions and protein metabolism, and *parathyroid hormone* controls bone calcium and phosphate. Thus, the hormones provide a regulatory system that complements the nervous system. The nervous system controls many muscular and secretory activities of the body, whereas the hormonal system regulates many metabolic functions. The nervous and hormonal systems normally work together in a coordinated manner to control essentially all the organ systems of the body. \n#### **PROTECTION OF THE BODY** \n**Immune System.** The immune system includes white blood cells, tissue cells derived from white blood cells, the thymus, lymph nodes, and lymph vessels that protect the body from pathogens such as bacteria, viruses, parasites, and fungi. The immune system provides a mechanism for the body to carry out the following: (1) distinguish its own cells from harmful foreign cells and substances; and (2) destroy the invader by *phagocytosis* or by producing *sensitized lymphocytes* or specialized proteins (e.g., *antibodies*) that destroy or neutralize the invader. \n**Integumentary System.** The skin and its various appendages (including the hair, nails, glands, and other structures) cover, cushion, and protect the deeper tissues and organs of the body and generally provide a boundary between the body's internal environment and the outside world. The integumentary system is also important for temperature regulation and excretion of wastes, and it provides a sensory interface between the body and the external environment. The skin generally comprises about 12% to 15% of body weight. \n#### **REPRODUCTION** \nAlthough reproduction is sometimes not considered a homeostatic function, it helps maintain homeostasis by generating new beings to take the place of those that are dying. This may sound like a permissive usage of the term *homeostasis,* but it illustrates that in the final analysis, essentially all body structures are organized to help maintain the automaticity and continuity of life. \n#### CONTROL SYSTEMS OF THE BODY \nThe human body has thousands of control systems. Some of the most intricate of these systems are genetic control systems that operate in all cells to help regulate intracellular and extracellular functions. This subject is discussed in Chapter 3. \nMany other control systems operate *within the organs* to regulate functions of the individual parts of the organs; others operate throughout the entire body *to control the interrelationships between the organs.* For example, the respiratory system, operating in association with the nervous system, regulates the concentration of carbon dioxide in the extracellular fluid. The liver and pancreas control glucose concentration in the extracellular fluid, and the kidneys regulate concentrations of hydrogen, sodium, potassium, phosphate, and other ions in the extracellular fluid. \n#### **EXAMPLES OF CONTROL MECHANISMS** \n**Regulation of Oxygen and Carbon Dioxide Concentrations in the Extracellular Fluid.** Because oxygen is one of the major substances required for chemical reactions in cells, the body has a special control mechanism to maintain an almost exact and constant oxygen concentration in the extracellular fluid. This mechanism depends principally on the chemical characteristics of *hemoglobin,* which is present in red blood cells. Hemoglobin combines with oxygen as the blood passes through the lungs. Then, as the blood passes through the tissue capillaries, hemoglobin, because of its own strong chemical affinity for oxygen, does not release oxygen into the tissue fluid if too much oxygen is already there. However, if oxygen concentration in the tissue fluid is too low, sufficient oxygen is released to re-establish an adequate concentration. Thus, regulation of oxygen concentration in the tissues relies to a great extent on the chemical characteristics of hemoglobin. This regulation is called the *oxygen-buffering function of hemoglobin.* \nCarbon dioxide concentration in the extracellular fluid is regulated in a much different way. Carbon dioxide is a major end product of oxidative reactions in cells. If all the carbon dioxide formed in the cells continued to accumulate in the tissue fluids, all energy-giving reactions of the cells would cease. Fortunately, a higher than normal carbon dioxide concentration in the blood *excites the respiratory center,* causing a person to breathe rapidly and deeply. This deep rapid breathing increases expiration of carbon dioxide and, therefore, removes excess carbon dioxide from the blood and tissue fluids. This process continues until the concentration returns to normal. \n \n**Figure 1-3.** Negative feedback control of arterial pressure by the arterial baroreceptors. Signals from the sensor (baroreceptors) are sent to the medulla of the brain, where they are compared with a reference set point. When arterial pressure increases above normal, this abnormal pressure increases nerve impulses from the baroreceptors to the medulla of the brain, where the input signals are compared with the set point, generating an error signal that leads to decreased sympathetic nervous system activity. Decreased sympathetic activity causes dilation of blood vessels and reduced pumping activity of the heart, which return arterial pressure toward normal. \n**Regulation of Arterial Blood Pressure.** Several systems contribute to arterial blood pressure regulation. One of these, the *baroreceptor system,* is an excellent example of a rapidly acting control mechanism (**[Figure 1-3](#page-11-0)**). In the walls of the bifurcation region of the carotid arteries in the neck, and also in the arch of the aorta in the thorax, are many nerve receptors called *baroreceptors* that are stimulated by stretch of the arterial wall. When arterial pressure rises too high, the baroreceptors send barrages of nerve impulses to the medulla of the brain. Here, these impulses inhibit the *vasomotor center,* which in turn decreases the number of impulses transmitted from the vasomotor center through the sympathetic nervous system to the heart and blood vessels. Lack of these impulses causes diminished pumping activity by the heart and dilation of peripheral blood vessels, allowing increased blood flow through the vessels. Both these effects decrease the arterial pressure, moving it back toward normal. \nConversely, a decrease in arterial pressure below normal relaxes the stretch receptors, allowing the vasomotor center to become more active than usual, thereby causing vasoconstriction and increased heart pumping. The initial decrease in arterial pressure thus initiates negative feedback mechanisms that raise arterial pressure back toward normal. \n#### **Normal Ranges and Physical Characteristics of Important Extracellular Fluid Constituents** \n**[Table 1-1](#page-12-0)** lists some important constituents and physical characteristics of extracellular fluid, along with their normal values, normal ranges, and maximum limits without causing death. Note the narrowness of the normal range for each one. Values outside these ranges are often caused by illness, injury, or major environmental challenges. \n| Constituent | Normal Value | Normal Range | Approximate Short-Term Nonlethal Limit | Unit |\n|-------------------------|--------------|----------------|----------------------------------------|---------|\n| Oxygen (venous) | 40 | 25–40 | 10–1000 | mm Hg |\n| Carbon dioxide (venous) | 45 | 41–51 | 5–80 | mm Hg |\n| Sodium ion | 142 | 135–145 | 115–175 | mmol/L |\n| Potassium ion | 4.2 | 3.5-5.3 | 1.5–9.0 | mmol/L |\n| Calcium ion | 1.2 | 1.0-1.4 | 0.5–2.0 | mmol/L |\n| Chloride ion | 106 | 98–108 | 70–130 | mmol/L |\n| Bicarbonate ion | 24 | 22–29 | 8–45 | mmol/L |\n| Glucose | 90 | 70–115 | 20–1500 | mg/dl |\n| Body temperature | 98.4 (37.0) | 98-98.8 (37.0) | 65–110 (18.3–43.3) | °F (°C) |\n| Acid-base (venous) | 7.4 | 7.3–7.5 | 6.9–8.0 | рН | \nTable 1-1 Important Constituents and Physical Characteristics of Extracellular Fluid \nMost important are the limits beyond which abnormalities can cause death. For example, an increase in the body temperature of only 11°F (7°C) above normal can lead to a vicious cycle of increasing cellular metabolism that destroys the cells. Note also the narrow range for acid-base balance in the body, with a normal pH value of 7.4 and lethal values only about 0.5 on either side of normal. Whenever the potassium ion concentration decreases to less than one-third normal, paralysis may result from the inability of the nerves to carry signals. Alternatively, if potassium ion concentration increases to two or more times normal, the heart muscle is likely to be severely depressed. Also, when the calcium ion concentration falls below about one-half normal, a person is likely to experience tetanic contraction of muscles throughout the body because of the spontaneous generation of excess nerve impulses in peripheral nerves. When the glucose concentration falls below one-half normal, a person frequently exhibits extreme mental irritability and sometimes even has convulsions. \nThese examples should give one an appreciation for the necessity of the vast numbers of control systems that keep the body operating in health. In the absence of any one of these controls, serious body malfunction or death can result. \n#### **CHARACTERISTICS OF CONTROL SYSTEMS** \nThe aforementioned examples of homeostatic control mechanisms are only a few of the many thousands in the body, all of which have some common characteristics, as explained in this section.\n\nMost control systems of the body act by *negative feed-back*, which can be explained by reviewing some of the homeostatic control systems mentioned previously. In the regulation of carbon dioxide concentration, a high concentration of carbon dioxide in the extracellular fluid increases pulmonary ventilation. This, in turn, decreases \nthe extracellular fluid carbon dioxide concentration because the lungs expire greater amounts of carbon dioxide from the body. Thus, the high concentration of carbon dioxide initiates events that decrease the concentration toward normal, which is *negative* to the initiating stimulus. Conversely, a carbon dioxide concentration that falls too low results in feedback to increase the concentration. This response is also negative to the initiating stimulus. \nIn the arterial pressure—regulating mechanisms, a high pressure causes a series of reactions that promote reduced pressure, or a low pressure causes a series of reactions that promote increased pressure. In both cases, these effects are negative with respect to the initiating stimulus. \nTherefore, in general, if some factor becomes excessive or deficient, a control system initiates *negative feedback*, which consists of a series of changes that return the factor toward a certain mean value, thus maintaining homeostasis. \nGain of a Control System. The degree of effectiveness with which a control system maintains constant conditions is determined by the gain of negative feedback. For example, let us assume that a large volume of blood is transfused into a person whose baroreceptor pressure control system is not functioning, and the arterial pressure rises from the normal level of 100 mm Hg up to 175 mm Hg. Then, let us assume that the same volume of blood is injected into the same person when the baroreceptor system is functioning, and this time the pressure increases by only 25 mm Hg. Thus, the feedback control system has caused a \"correction\" of -50 mm Hg, from 175 mm Hg to 125 mm Hg. There remains an increase in pressure of +25 mm Hg, called the \"error,\" which means that the control system is not 100% effective in preventing change. The gain of the system is then calculated by using the following formula: \n$$Gain = \\frac{Correction}{Frror}$$ \nThus, in the baroreceptor system example, the correction is -50 mm Hg, and the error persisting is +25 mm Hg. Therefore, the gain of the person's baroreceptor system \n \n**Figure 1-4.** Recovery of heart pumping caused by negative feedback after 1 liter of blood is removed from the circulation. Death is caused by positive feedback when 2 liters or more blood is removed. \nfor control of arterial pressure is −50 divided by +25, or −2. That is, a disturbance that increases or decreases the arterial pressure does so only one-third as much as would occur if this control system were not present. \nThe gains of some other physiological control systems are much greater than that of the baroreceptor system. For example, the gain of the system controlling internal body temperature when a person is exposed to moderately cold weather is about −33. Therefore, one can see that the temperature control system is much more effective than the baroreceptor pressure control system. \n#### **Positive Feedback May Cause Vicious Cycles and Death** \nWhy do most control systems of the body operate by negative feedback rather than by positive feedback? If one considers the nature of positive feedback, it is obvious that positive feedback leads to instability rather than stability and, in some cases, can cause death. \n**[Figure 1-4](#page-13-0)** shows an example in which death can ensue from positive feedback. This figure depicts the pumping effectiveness of the heart, showing the heart of a healthy human pumping about 5 liters of blood per minute. If the person suddenly bleeds a total of 2 liters, the amount of blood in the body is decreased to such a low level that not enough blood is available for the heart to pump effectively. As a result, the arterial pressure falls, and the flow of blood to the heart muscle through the coronary vessels diminishes. This scenario results in weakening of the heart, further diminished pumping, a further decrease in coronary blood flow, and still more weakness of the heart; the cycle repeats itself again and again until death occurs. Note that each cycle in the feedback results in further weakening of the heart. In other words, the initiating stimulus causes more of the same, which is *positive feedback.* \nPositive feedback is sometimes known as a \"vicious cycle,\" but a mild degree of positive feedback can be overcome by the negative feedback control mechanisms of the body, and the vicious cycle then fails to develop. For example, if the person in the aforementioned example bleeds only 1 liter instead of 2 liters, the normal negative feedback mechanisms for controlling cardiac output and arterial pressure can counterbalance the positive feedback and the person can recover, as shown by the dashed curve of **[Figure 1-4](#page-13-0)**. \n**Positive Feedback Can Sometimes Be Useful.** The body sometimes uses positive feedback to its advantage. Blood clotting is an example of a valuable use of positive feedback. When a blood vessel is ruptured, and a clot begins to form, multiple enzymes called *clotting factors* are activated within the clot. Some of these enzymes act on other inactivated enzymes of the immediately adjacent blood, thus causing more blood clotting. This process continues until the hole in the vessel is plugged and bleeding no longer occurs. On occasion, this mechanism can get out of hand and cause formation of unwanted clots. In fact, this is what initiates most acute heart attacks, which can be caused by a clot beginning on the inside surface of an atherosclerotic plaque in a coronary artery and then growing until the artery is blocked. \nChildbirth is another situation in which positive feedback is valuable. When uterine contractions become strong enough for the baby's head to begin pushing through the cervix, stretching of the cervix sends signals through the uterine muscle back to the body of the uterus, causing even more powerful contractions. Thus, the uterine contractions stretch the cervix, and cervical stretch causes stronger contractions. When this process becomes powerful enough, the baby is born. If they are not powerful enough, the contractions usually die out, and a few days pass before they begin again. \nAnother important use of positive feedback is for the generation of nerve signals. Stimulation of the membrane of a nerve fiber causes slight leakage of sodium ions through sodium channels in the nerve membrane to the fiber's interior. The sodium ions entering the fiber then change the membrane potential, which, in turn, causes more opening of channels, more change of potential, still more opening of channels, and so forth. Thus, a slight leak becomes an explosion of sodium entering the interior of the nerve fiber, which creates the nerve action potential. This action potential, in turn, causes electrical current to flow along the outside and inside of the fiber and initiates additional action potentials. This process continues until the nerve signal goes all the way to the end of the fiber. \nIn each case in which positive feedback is useful, the positive feedback is part of an overall negative feedback process. For example, in the case of blood clotting, the positive feedback clotting process is a negative feedback process for the maintenance of normal blood volume. Also, the positive feedback that causes nerve signals allows the nerves to participate in thousands of negative feedback nervous control systems. \n#### **More Complex Types of Control Systems—Feed-Forward and Adaptive Control** \nLater in this text, when we study the nervous system, we shall see that this system contains great numbers of interconnected control mechanisms. Some are simple feedback systems similar to those already discussed. Many are not. For example, some movements of the body occur so rapidly that there is not enough time for nerve signals to travel from the peripheral parts of the body all the way to the brain and then back to the periphery again to control the movement. Therefore, the brain uses a mechanism called *feed-forward control* to cause required muscle contractions. Sensory nerve signals from the moving parts apprise the brain about whether the movement is performed correctly. If not, the brain corrects the feed-forward signals that it sends to the muscles the *next* time the movement is required. Then, if still further correction is necessary, this process will be performed again for subsequent movements. This process is called *adaptive control.* Adaptive control, in a sense, is delayed negative feedback. \nThus, one can see how complex the feedback control systems of the body can be. A person's life depends on all of them. Therefore, much of this text is devoted to discussing these life-giving mechanisms. \n#### **PHYSIOLOGICAL VARIABILITY** \nAlthough some physiological variables, such as plasma concentrations of potassium, calcium, and hydrogen ions, are tightly regulated, others, such as body weight and adiposity, show wide variation among different individuals and even in the same individual at different stages of life. Blood pressure, cardiac pumping, metabolic rate, nervous system activity, hormones, and other physiological variables change throughout the day as we move about and engage in normal daily activities. Therefore, when we discuss \"normal\" values, it is with the understanding that many of the body's control systems are constantly reacting to perturbations, and that variability may exist among different individuals, depending on body weight and height, diet, age, sex, environment, genetics, and other factors. \nFor simplicity, discussion of physiological functions often focuses on the \"average\" 70-kg young, lean male. However, the American male no longer weighs an average of 70 kg; he now weighs over 88 kg, and the average American female weighs over 76 kg, more than the average man in the 1960s. Body weight has also increased substantially in most other industrialized countries during the past 40 to 50 years. \nExcept for reproductive and hormonal functions, many other physiological functions and normal values are often discussed in terms of male physiology. However, there are clearly differences in male and female physiology beyond the obvious differences that relate to reproduction. These differences can have important consequences for understanding normal physiology as well as for treatment of diseases. \nAge-related and ethnic or racial differences in physiology also have important influences on body composition, physiological control systems, and pathophysiology of diseases. For example, in a lean young male the total body water is about 60% of body weight. As a person grows and ages, this percentage gradually decreases, partly because aging is usually associated with declining skeletal muscle mass and increasing fat mass. Aging may also cause a decline in the function and effectiveness of some organs and physiological control systems. \nThese sources of physiological variability—sex differences, aging, ethnic, and racial—are complex but important considerations when discussing normal physiology and the pathophysiology of diseases. \n#### SUMMARY—AUTOMATICITY OF THE BODY \nThe main purpose of this chapter has been to discuss briefly the overall organization of the body and the means whereby the different parts of the body operate in harmony. To summarize, the body is actually a *social order of about 35 to 40 trillion cells* organized into different functional structures, some of which are called *organs.* Each functional structure contributes its share to the maintenance of homeostasis in the extracellular fluid, which is called the *internal environment.* As long as normal conditions are maintained in this internal environment, the cells of the body continue to live and function properly. Each cell benefits from homeostasis and, in turn, each cell contributes its share toward the maintenance of homeostasis. This reciprocal interplay provides continuous automaticity of the body until one or more functional systems lose their ability to contribute their share of function. When this happens, all the cells of the body suffer. Extreme dysfunction leads to death; moderate dysfunction leads to sickness. \n#### Bibliography \nAdolph EF: Physiological adaptations: hypertrophies and superfunctions. Am Sci 60:608, 1972. \nBentsen MA, Mirzadeh Z, Schwartz MW: Revisiting how the brain senses glucose-and why. Cell Metab 29:11, 2019. \nBernard C: Lectures on the Phenomena of Life Common to Animals and Plants. Springfield, IL: Charles C Thomas, 1974. \nCannon WB: Organization for physiological homeostasis. Physiol Rev 9:399, 1929. \nChien S: Mechanotransduction and endothelial cell homeostasis: the wisdom of the cell. Am J Physiol Heart Circ Physiol 292:H1209, 2007. \nDiBona GF: Physiology in perspective: the wisdom of the body. Neural control of the kidney. Am J Physiol Regul Integr Comp Physiol 289:R633, 2005. \nDickinson MH, Farley CT, Full RJ, et al: How animals move: an integrative view. Science 288:100, 2000. \nEckel-Mahan K, Sassone-Corsi P: Metabolism and the circadian clock converge. Physiol Rev 93:107, 2013. \n- Guyton AC: Arterial Pressure and Hypertension. Philadelphia: WB Saunders, 1980.\n- Herman MA, Kahn BB: Glucose transport and sensing in the maintenance of glucose homeostasis and metabolic harmony. J Clin Invest 116:1767, 2006.\n- Kabashima K, Honda T, Ginhoux F, Egawa G: The immunological anatomy of the skin. Nat Rev Immunol 19:19, 2019.\n- Khramtsova EA, Davis LK, Stranger BE: The role of sex in the genomics of human complex traits. Nat Rev Genet 20: 173, 2019.\n- Kim KS, Seeley RJ, Sandoval DA: Signalling from the periphery to the brain that regulates energy homeostasis. Nat Rev Neurosci 19:185, 2018.\n- Nishida AH, Ochman H: A great-ape view of the gut microbiome. Nat Rev Genet 20:185, 2019.\n- Orgel LE: The origin of life on the earth. Sci Am 271:76,1994.\n- Reardon C, Murray K, Lomax AE: Neuroimmune communication in health and disease. Physiol Rev 98:2287-2316, 2018.\n- Sender R, Fuchs S, Milo R: Revised estimates for the number of human and bacteria cells in the body. PLoS Biol 14(8):e1002533, 2016.\n- Smith HW: From Fish to Philosopher. New York: Doubleday, 1961. \n\n\nEach of the trillions of cells in a human being is a living structure that can survive for months or years, provided its surrounding fluids contain appropriate nutrients. Cells are the building blocks of the body, providing structure for the body's tissues and organs, ingesting nutrients and converting them to energy, and performing specialized functions. Cells also contain the body's hereditary code, which controls the substances synthesized by the cells and permits them to make copies of themselves. \n#### ORGANIZATION OF THE CELL \nA schematic drawing of a typical cell, as seen by the light microscope, is shown in **[Figure 2-1](#page-16-0)**. Its two major parts are the *nucleus* and the *cytoplasm.* The nucleus is separated from the cytoplasm by a *nuclear membrane,* and the cytoplasm is separated from the surrounding fluids by a *cell membrane,* also called the *plasma membrane.* \nThe different substances that make up the cell are collectively called *protoplasm.* Protoplasm is composed mainly of five basic substances—water, electrolytes, proteins, lipids, and carbohydrates. \n**Water.** Most cells, except for fat cells, are comprised mainly of water in a concentration of 70% to 85%. Many cellular chemicals are dissolved in the water. Others are suspended in the water as solid particulates. Chemical reactions take place among the dissolved chemicals or at the surfaces of the suspended particles or membranes. \n**Ions.** Important ions in the cell include *potassium, magnesium, phosphate, sulfate, bicarbonate,* and smaller quantities of *sodium, chloride,* and *calcium.* These ions are all discussed in Chapter 4, which considers the interrelations between the intracellular and extracellular fluids. \nThe ions provide inorganic chemicals for cellular reactions and are necessary for the operation of some cellular control mechanisms. For example, ions acting at the cell membrane are required for the transmission of electrochemical impulses in nerve and muscle fibers. \n**Proteins.** After water, the most abundant substances in most cells are proteins, which normally constitute 10% to 20% of the cell mass. These proteins can be divided into two types, *structural proteins* and *functional proteins.* \nStructural proteins are present in the cell mainly in the form of long filaments that are polymers of many individual protein molecules. A prominent use of such intracellular filaments is to form *microtubules,* which provide the cytoskeletons of cellular organelles such as cilia, nerve axons, the mitotic spindles of cells undergoing mitosis, and a tangled mass of thin filamentous tubules that hold the parts of the cytoplasm and nucleoplasm together in their respective compartments. Fibrillar proteins are found outside the cell, especially in the collagen and elastin fibers of connective tissue, and elsewhere, such as in blood vessel walls, tendons, and ligaments. \nThe *functional proteins* are usually composed of combinations of a few molecules in tubular-globular form. These proteins are mainly the *enzymes* of the cell and, in contrast to the fibrillar proteins, are often mobile in the cell fluid. Also, many of them are adherent to membranous structures inside the cell and catalyze specific intracellular chemical reactions. For example, the chemical reactions that split glucose into its component parts and then combine these with oxygen to form carbon dioxide and water while simultaneously providing energy for cellular function are all catalyzed by a series of protein enzymes. \n**Lipids.** Lipids are several types of substances that are grouped together because of their common property of being soluble in fat solvents. Especially important lipids \n \n**Figure 2-1.** Illustration of cell structures visible with a light microscope. \n \n**Figure 2-2.** Reconstruction of a typical cell, showing the internal organelles in the cytoplasm and nucleus. \nare *phospholipids* and *cholesterol,* which together constitute only about 2% of the total cell mass. Phospholipids and cholesterol are mainly insoluble in water and therefore are used to form the cell membrane and intracellular membrane barriers that separate the different cell compartments. \nIn addition to phospholipids and cholesterol, some cells contain large quantities of *triglycerides,* also called *neutral fats.* In *fat cells (adipocytes),* triglycerides often account for as much as 95% of the cell mass. The fat stored in these cells represents the body's main storehouse of energy-giving nutrients that can later be used to provide energy wherever it is needed in the body. \n**Carbohydrates.** Carbohydrates play a major role in cell nutrition and, as parts of glycoprotein molecules, have structural functions. Most human cells do not maintain large stores of carbohydrates; the amount usually averages only about 1% of their total mass but increases to as much as 3% in muscle cells and, occasionally, to 6% in liver cells. However, carbohydrate in the form of dissolved glucose is always present in the surrounding extracellular fluid so that it is readily available to the cell. Also, a small amount of carbohydrate is stored in cells as *glycogen,* an insoluble polymer of glucose that can be depolymerized and used rapidly to supply the cell's energy needs. \n#### CELL STRUCTURE \nThe cell contains highly organized physical structures called *intracellular organelles,* which are critical for cell function. For example, without one of the organelles, the *mitochondria,* more than 95% of the cell's energy release from nutrients would cease immediately. The most important organelles and other structures of the cell are shown in **[Figure 2-2](#page-17-0)**. \n#### **MEMBRANOUS STRUCTURES OF THE CELL** \nMost organelles of the cell are covered by membranes composed primarily of lipids and proteins. These membranes include the *cell membrane, nuclear membrane, membrane of the endoplasmic reticulum,* and *membranes of the mitochondria, lysosomes,* and *Golgi apparatus.* \n \n**Figure 2-3.** Structure of the cell membrane showing that it is composed mainly of a lipid bilayer of phospholipid molecules, but with large numbers of protein molecules protruding through the layer. Also, carbohydrate moieties are attached to the protein molecules on the outside of the membrane and to additional protein molecules on the inside. \nThe lipids in membranes provide a barrier that impedes movement of water and water-soluble substances from one cell compartment to another because water is not soluble in lipids. However, protein molecules often penetrate all the way through membranes, thus providing specialized pathways, often organized into actual *pores,* for passage of specific substances through membranes. Also, many other membrane proteins are *enzymes,* which catalyze a multitude of different chemical reactions, discussed here and in subsequent chapters. \n#### **Cell Membrane** \nThe cell membrane (also called the *plasma membrane*) envelops the cell and is a thin, pliable, elastic structure only 7.5 to 10 nanometers thick. It is composed almost entirely of proteins and lipids. The approximate composition is 55% proteins, 25% phospholipids, 13% cholesterol, 4% other lipids, and 3% carbohydrates. \n**The Cell Membrane Lipid Barrier Impedes Penetration by Water-Soluble Substances. [Figure 2-3](#page-18-0)** shows the structure of the cell membrane. Its basic structure is a *lipid bilayer,* which is a thin, double-layered film of lipids—each layer only one molecule thick—that is continuous over the entire cell surface. Interspersed in this lipid film are large globular proteins. \nThe basic lipid bilayer is composed of three main types of lipids—*phospholipids, sphingolipids,* and *cholesterol*. Phospholipids are the most abundant cell membrane lipids. One end of each phospholipid molecule is *hydrophilic* and soluble in water*.* The other end is *hydrophobic* and soluble only in fats. The phosphate end of the phospholipid is hydrophilic, and the fatty acid portion is hydrophobic. \nBecause the hydrophobic portions of the phospholipid molecules are repelled by water but are mutually attracted to one another, they have a natural tendency to attach to one another in the middle of the membrane, as shown in **[Figure 2-3](#page-18-0)**. The hydrophilic phosphate portions then constitute the two surfaces of the complete cell membrane, in contact with *intracellular* water on the inside of the membrane and *extracellular* water on the outside surface. \nThe lipid layer in the middle of the membrane is impermeable to the usual water-soluble substances, such as ions, glucose, and urea. Conversely, fat-soluble substances, such as oxygen, carbon dioxide, and alcohol, can penetrate this portion of the membrane with ease. \nSphingolipids, derived from the amino alcohol *sphingosine*, also have hydrophobic and hydrophilic groups and are present in small amounts in the cell membranes, especially nerve cells. Complex sphingolipids in cell membranes are thought to serve several functions, including protection from harmful environmental factors, signal transmission, and adhesion sites for extracellular proteins. \nCholesterol molecules in membranes are also lipids because their steroid nuclei are highly fat-soluble. These molecules, in a sense, are dissolved in the bilayer of the membrane. They mainly help determine the degree of permeability (or impermeability) of the bilayer to watersoluble constituents of body fluids. Cholesterol controls much of the fluidity of the membrane as well. \n#### **Integral and Peripheral Cell Membrane Proteins.** \n**[Figure 2-3](#page-18-0)** also shows globular masses floating in the lipid bilayer. These membrane proteins are mainly *glycoproteins.* There are two types of cell membrane proteins, *integral proteins,* which protrude all the way through the membrane, and *peripheral proteins,* which are attached only to one surface of the membrane and do not penetrate all the way through. \nMany of the integral proteins provide structural *channels* (or *pores*) through which water molecules and watersoluble substances, especially ions, can diffuse between extracellular and intracellular fluids. These protein channels also have selective properties that allow preferential diffusion of some substances over others. \nOther integral proteins act as *carrier proteins* for transporting substances that otherwise could not penetrate the lipid bilayer. Sometimes, these carrier proteins even transport substances in the direction opposite to their electrochemical gradients for diffusion, which is called *active transport.* Still others act as *enzymes.* \nIntegral membrane proteins can also serve as *receptors* for water-soluble chemicals, such as peptide hormones, that do not easily penetrate the cell membrane. Interaction of cell membrane receptors with specific *ligands* that bind to the receptor causes conformational changes in the receptor protein. This process, in turn, enzymatically activates the intracellular part of the protein or induces interactions between the receptor and proteins in the cytoplasm that act as *second messengers,* relaying the signal from the extracellular part of the receptor to the interior of the cell. In this way, integral proteins spanning the cell membrane provide a means of conveying information about the environment to the cell interior. \nPeripheral protein molecules are often attached to integral proteins. These peripheral proteins function almost entirely as enzymes or as controllers of transport of substances through cell membrane *pores.* \n#### **Membrane Carbohydrates—The Cell \"Glycocalyx.\"** \nMembrane carbohydrates occur almost invariably in combination with proteins or lipids in the form of *glycoproteins* or *glycolipids.* In fact, most of the integral proteins are glycoproteins, and about one-tenth of the membrane lipid molecules are glycolipids. The *glyco-* portions of these molecules almost invariably protrude to the outside of the cell, dangling outward from the cell surface. Many other carbohydrate compounds, called *proteoglycans* which are mainly carbohydrates bound to small protein cores—are loosely attached to the outer surface of the cell as well. Thus, the entire outside surface of the cell often has a loose carbohydrate coat called the *glycocalyx.* \nThe carbohydrate moieties attached to the outer surface of the cell have several important functions: \n- 1. Many of them have a negative electrical charge, which gives most cells an overall negative surface charge that repels other negatively charged objects.\n- 2. The glycocalyx of some cells attaches to the glycocalyx of other cells, thus attaching cells to one another.\n- 3. Many of the carbohydrates act as *receptors* for binding hormones, such as insulin. When bound, this combination activates attached internal proteins that in turn activate a cascade of intracellular enzymes.\n- 4. Some carbohydrate moieties enter into immune reactions, as discussed in Chapter 35. \n#### **CYTOPLASM AND ITS ORGANELLES** \nThe cytoplasm is filled with minute and large dispersed particles and organelles. The jelly-like fluid portion of the cytoplasm in which the particles are dispersed is called *cytosol* and contains mainly dissolved proteins, electrolytes, and glucose. \nDispersed in the cytoplasm are neutral fat globules, glycogen granules, ribosomes, secretory vesicles, and five especially important organelles—the *endoplasmic reticulum,* the *Golgi apparatus, mitochondria, lysosomes,* and *peroxisomes.* \n#### **Endoplasmic Reticulum** \n**[Figure 2-2](#page-17-0)** shows the *endoplasmic reticulum,* a network of tubular structures called *cisternae* and flat vesicular structures in the cytoplasm. This organelle helps process molecules made by the cell and transports them to their specific destinations inside or outside the cell. The tubules and vesicles interconnect. Also, their walls are constructed of lipid bilayer membranes that contain large amounts of proteins, similar to the cell membrane. The total surface area of this structure in some cells—the liver cells, for example—can be as much as 30 to 40 times the cell membrane area. \nThe detailed structure of a small portion of endoplasmic reticulum is shown in **[Figure 2-4](#page-20-0)**. The space inside the tubules and vesicles is filled with *endoplasmic matrix,* a watery medium that is different from fluid in the cytosol outside the endoplasmic reticulum. Electron micrographs show that the space inside the endoplasmic reticulum is connected with the space between the two membrane surfaces of the nuclear membrane. \nSubstances formed in some parts of the cell enter the space of the endoplasmic reticulum and are then directed to other parts of the cell. Also, the vast surface area of this \n \n**Figure 2-4.** Structure of the endoplasmic reticulum. \nreticulum and the multiple enzyme systems attached to its membranes provide the mechanisms for a major share of the cell's metabolic functions. \n**Ribosomes and the Rough (Granular) Endoplasmic Reticulum.** Attached to the outer surfaces of many parts of the endoplasmic reticulum are large numbers of minute granular particles called *ribosomes.* Where these particles are present, the reticulum is called the *rough (granular) endoplasmic reticulum.* The ribosomes are composed of a mixture of RNA and proteins; they function to synthesize new protein molecules in the cell, as discussed later in this chapter and in Chapter 3. \n**Smooth (Agranular) Endoplasmic Reticulum.** Part of the endoplasmic reticulum has no attached ribosomes. This part is called the *smooth,* or *agranular, endoplasmic reticulum.* The smooth reticulum functions for the synthesis of lipid substances and for other processes of the cells promoted by intrareticular enzymes. \n#### **Golgi Apparatus** \nThe Golgi apparatus, shown in **[Figure 2-5](#page-20-1)**, is closely related to the endoplasmic reticulum. It has membranes similar to those of the smooth endoplasmic reticulum. The Golgi apparatus is usually composed of four or more stacked layers of thin, flat, enclosed vesicles lying near one side of the nucleus. This apparatus is prominent in secretory cells, where it is located on the side of the cell from which secretory substances are extruded. \nThe Golgi apparatus functions in association with the endoplasmic reticulum. As shown in **[Figure 2-5](#page-20-1)**, small *transport vesicles* (also called *endoplasmic reticulum vesicles* [*ER vesicles*]) continually pinch off from the endoplasmic reticulum and shortly thereafter fuse with the Golgi apparatus. In this way, substances entrapped in ER \n \n**Figure 2-5.** A typical Golgi apparatus and its relationship to the endoplasmic reticulum (ER) and the nucleus. \nvesicles are transported from the endoplasmic reticulum to the Golgi apparatus. The transported substances are then processed in the Golgi apparatus to form lysosomes, secretory vesicles, and other cytoplasmic components (discussed later in this chapter). \n#### **Lysosomes** \nLysosomes, shown in **[Figure 2-2](#page-17-0)**, are vesicular organelles that form by breaking off from the Golgi apparatus; they then disperse throughout the cytoplasm. The lysosomes provide an *intracellular digestive system* that allows the cell to digest the following: (1) damaged cellular structures; (2) food particles that have been ingested by the cell; and (3) unwanted matter such as bacteria. Lysosome are different in various cell types but are usually 250 to 750 nanometers in diameter. They are surrounded by typical lipid bilayer membranes and are filled with large numbers of small granules, 5 to 8 nanometers in diameter, which are protein aggregates of as many as 40 different *hydrolase (digestive) enzymes.* A hydrolytic enzyme is capable of splitting an organic compound into two or more parts by combining hydrogen from a water molecule with one part of the compound and combining the hydroxyl portion of the water molecule with the other part of the compound. For example, protein is hydrolyzed to form amino acids, glycogen is hydrolyzed to form glucose, and lipids are hydrolyzed to form fatty acids and glycerol. \nHydrolytic enzymes are highly concentrated in lysosomes. Ordinarily, the membrane surrounding the lysosome prevents the enclosed hydrolytic enzymes from coming into contact with other substances in the cell and therefore prevents their digestive actions. However, some conditions of the cell break the membranes of lysosomes, allowing release of the digestive enzymes. These enzymes then split the organic substances with which they come in contact into small, highly diffusible substances such as \n \n**Figure 2-6.** Secretory granules (secretory vesicles) in acinar cells of the pancreas. \namino acids and glucose. Some of the specific functions of lysosomes are discussed later in this chapter. \n#### **Peroxisomes** \nPeroxisomes are physically similar to lysosomes, but they are different in two important ways. First, they are believed to be formed by self-replication (or perhaps by budding off from the smooth endoplasmic reticulum) rather than from the Golgi apparatus. Second, they contain *oxidases* rather than hydrolases. Several of the oxidases are capable of combining oxygen with hydrogen ions derived from different intracellular chemicals to form hydrogen peroxide (H2O2). Hydrogen peroxide is a highly oxidizing substance and is used in association with *catalase,* another oxidase enzyme present in large quantities in peroxisomes, to oxidize many substances that might otherwise be poisonous to the cell. For example, about half the alcohol that a person drinks is detoxified into acetaldehyde by the peroxisomes of the liver cells in this manner. A major function of peroxisomes is to catabolize long-chain fatty acids. \n#### **Secretory Vesicles** \nOne of the important functions of many cells is secretion of special chemical substances. Almost all such secretory substances are formed by the endoplasmic reticulum– Golgi apparatus system and are then released from the Golgi apparatus into the cytoplasm in the form of storage vesicles called *secretory vesicles* or *secretory granules.* **[Figure 2-6](#page-21-0)** shows typical secretory vesicles inside pancreatic acinar cells; these vesicles store protein proenzymes (enzymes that are not yet activated). The proenzymes are secreted later through the outer cell membrane into the pancreatic duct and then into the duodenum, where they become activated and perform digestive functions on the food in the intestinal tract. \n#### **Mitochondria** \nThe mitochondria, shown in **[Figure 2-2](#page-17-0)** and **[Figure 2-7](#page-21-1)**, are called the *powerhouses* of the cell. Without them, cells would be unable to extract enough energy from the nutrients, and essentially all cellular functions would cease. \n \n**Figure 2-7.** Structure of a mitochondrion. \nMitochondria are present in all areas of each cell's cytoplasm, but the total number per cell varies from less than 100 up to several thousand, depending on the energy requirements of the cell. Cardiac muscle cells (cardiomyocytes), for example, use large amounts of energy and have far more mitochondria than fat cells (adipocytes), which are much less active and use less energy. Furthermore, the mitochondria are concentrated in those portions of the cell responsible for the major share of its energy metabolism. They are also variable in size and shape. Some mitochondria are only a few hundred nanometers in diameter and are globular in shape, whereas others are elongated and are as large as 1 micrometer in diameter and 7 micrometers long. Still others are branching and filamentous. \nThe basic structure of the mitochondrion, shown in **[Figure 2-7](#page-21-1)**, is composed mainly of two lipid bilayerprotein membranes, an *outer membrane* and an *inner membrane.* Many infoldings of the inner membrane form shelves or tubules called *cristae* onto which oxidative enzymes are attached. The cristae provide a large surface area for chemical reactions to occur. In addition, the inner cavity of the mitochondrion is filled with a *matrix* that contains large quantities of dissolved enzymes necessary for extracting energy from nutrients. These enzymes operate in association with oxidative enzymes on the cristae to cause oxidation of nutrients, thereby forming carbon dioxide and water and, at the same time, releasing energy. The liberated energy is used to synthesize a high-energy substance called *adenosine triphosphate* (ATP). ATP is then transported out of the mitochondrion and diffuses throughout the cell to release its own energy wherever it is needed for performing cellular functions. The chemical details of ATP formation by the mitochondrion are provided in Chapter 68, but some basic functions of ATP in the cell are introduced later in this chapter. \nMitochondria are self-replicative, which means that one mitochondrion can form a second one, a third one, and so on whenever the cell needs increased amounts of ATP. Indeed, the mitochondria contain DNA similar to that found in the cell nucleus. In Chapter 3, we will see that DNA is the basic constituent of the nucleus that \n \n**Figure 2-8.** Cell cytoskeleton composed of protein fibers called microfilaments, intermediate filaments, and microtubules. \ncontrols replication of the cell. The DNA of the mitochondrion plays a similar role, controlling replication of the mitochondrion. Cells that are faced with increased energy demands—for example, in skeletal muscles subjected to chronic exercise training—may increase the density of mitochondria to supply the additional energy required. \n#### **Cell Cytoskeleton—Filament and Tubular Structures** \nThe cell cytoskeleton is a network of fibrillar proteins organized into filaments or tubules. These originate as precursor proteins synthesized by ribosomes in the cytoplasm. The precursor molecules then polymerize to form *filaments* (**[Figure 2-8](#page-22-0)**). As an example, large numbers of actin *microfilaments* frequently occur in the outer zone of the cytoplasm, called the *ectoplasm,* to form an elastic support for the cell membrane. Also, in muscle cells, actin and myosin filaments are organized into a special contractile machine that is the basis for muscle contraction, as discussed in Chapter 6. \n*Intermediate filaments* are generally strong ropelike filaments that often work together with microtubules, providing strength and support for the fragile tubulin structures. They are called *intermediate* because their average diameter is between that of narrower actin microfilaments and wider myosin filaments found in muscle cells. Their functions are mainly mechanical, and they are less dynamic than actin microfilaments or microtubules. All cells have intermediate filaments, although the protein subunits of these structures vary, depending on the cell type. Specific intermediate filaments found in various cells include desmin filaments in muscle cells, neurofilaments in neurons, and keratins in epithelial cells. \nA special type of stiff filament composed of polymerized *tubulin* molecules is used in all cells to construct strong tubular structures, the *microtubules.* **[Figure 2-8](#page-22-0)** shows typical microtubules of a cell. \nAnother example of microtubules is the tubular skeletal structure in the center of each cilium that radiates upward from the cell cytoplasm to the tip of the cilium. This structure is discussed later in the chapter (see **[Figure 2-18](#page-31-0)**). Also, both the *centrioles* and *mitotic spindles* of cells undergoing mitosis are composed of stiff microtubules. \nA major function of microtubules is to act as a *cytoskeleton,* providing rigid physical structures for certain parts of cells. The cell cytoskeleton not only determines cell shape but also participates in cell division, allows cells to move, and provides a tracklike system that directs the movement of organelles in the cells. Microtubules serve as the conveyor belts for the intracellular transport of vesicles, granules, and organelles such as mitochondria. \n#### **Nucleus** \nThe nucleus is the control center of the cell and sends messages to the cell to grow and mature, replicate, or die. Briefly, the nucleus contains large quantities of DNA, \n \n**Figure 2-9.** Structure of the nucleus. \nwhich comprise the *genes.* The genes determine the characteristics of the cell's proteins, including the structural proteins, as well as the intracellular enzymes that control cytoplasmic and nuclear activities. \nThe genes also control and promote cell reproduction. The genes first reproduce to create two identical sets of genes; then the cell splits by a special process called *mitosis* to form two daughter cells, each of which receives one of the two sets of DNA genes. All these activities of the nucleus are discussed in Chapter 3. \nUnfortunately, the appearance of the nucleus under the microscope does not provide many clues to the mechanisms whereby the nucleus performs its control activities. **[Figure 2-9](#page-23-0)** shows the light microscopic appearance of the *interphase* nucleus (during the period between mitoses), revealing darkly staining *chromatin material* throughout the nucleoplasm. During mitosis, the chromatin material organizes in the form of highly structured *chromosomes,* which can then be easily identified using the light microscope, as illustrated in Chapter 3. \n**Nuclear Membrane.** The *nuclear membrane,* also called the *nuclear envelope,* is actually two separate bilayer membranes, one inside the other. The outer membrane is continuous with the endoplasmic reticulum of the cell cytoplasm, and the space between the two nuclear membranes is also continuous with the space inside the endoplasmic reticulum, as shown in **[Figure 2-9](#page-23-0)**. \nThe nuclear membrane is penetrated by several thousand *nuclear pores.* Large complexes of proteins are attached at the edges of the pores so that the central area of each pore is only about 9 nanometers in diameter. Even this size is large enough to allow molecules up to a molecular weight of 44,000 to pass through with reasonable ease. \n**Nucleoli and Formation of Ribosomes.** The nuclei of most cells contain one or more highly staining structures called *nucleoli.* The nucleolus, unlike most other organelles discussed here, does not have a limiting membrane. Instead, it is simply an accumulation of large amounts of \n \n**Figure 2-10.** Comparison of sizes of precellular organisms with that of the average cell in the human body. \nRNA and proteins of the types found in ribosomes. The nucleolus enlarges considerably when the cell is actively synthesizing proteins. \nFormation of the nucleoli (and of the ribosomes in the cytoplasm outside the nucleus) begins in the nucleus. First, specific DNA genes in the chromosomes cause RNA to be synthesized. Some of this synthesized RNA is stored in the nucleoli, but most of it is transported outward through the nuclear pores into the cytoplasm. Here it is used in conjunction with specific proteins to assemble \"mature\" ribosomes that play an essential role in forming cytoplasmic proteins, as discussed in Chapter 3. \n#### COMPARISON OF THE ANIMAL CELL WITH PRECELLULAR FORMS OF LIFE \nThe cell is a complicated organism that required many hundreds of millions of years to develop after the earliest forms of life, microorganisms that may have been similar to present-day *viruses,* first appeared on earth. **[Figure 2-10](#page-23-1)** shows the relative sizes of the following: (1) the smallest known virus; (2) a large virus; (3) a *Rickettsia;* (4) a *bacterium;* and (5) a *nucleated cell,* This demonstrates that the cell has a diameter about 1000 times that of the smallest virus and therefore a volume about 1 billion times that of the smallest virus. Correspondingly, the functions and anatomical organization of the cell are also far more complex than those of the virus. \nThe essential life-giving constituent of the small virus is a *nucleic acid* embedded in a coat of protein. This nucleic acid is composed of the same basic nucleic acid constituents (DNA or RNA) found in mammalian cells and is capable of reproducing itself under appropriate conditions. Thus, the virus propagates its lineage from generation to generation and is therefore a living structure in the same way that cells and humans are living structures. \nAs life evolved, other chemicals in addition to nucleic acid and simple proteins became integral parts of the organism, and specialized functions began to develop in different parts of the virus. A membrane formed around the virus and, inside the membrane, a fluid matrix appeared. Specialized chemicals then developed inside the fluid to perform special functions; many protein enzymes appeared that were capable of catalyzing chemical reactions, thus determining the organism's activities. \nIn still later stages of life, particularly in the rickettsial and bacterial stages, *organelles* developed inside the organism. These represent physical structures of chemical aggregates that perform functions in a more efficient manner than what can be achieved by dispersed chemicals throughout the fluid matrix. \nFinally, in the nucleated cell, still more complex organelles developed, the most important of which is the *nucleus*. The nucleus distinguishes this type of cell from all lower forms of life; it provides a control center for all cellular activities and for reproduction of new cells generation after generation, with each new cell having almost exactly the same structure as its progenitor. \n#### FUNCTIONAL SYSTEMS OF THE CELL \nIn the remainder of this chapter, we discuss some functional systems of the cell that make it a living organism. \n#### **ENDOCYTOSIS—INGESTION BY THE CELL** \nIf a cell is to live and grow and reproduce, it must obtain nutrients and other substances from the surrounding fluids. Most substances pass through the cell membrane by the processes of diffusion and *active transport.* \nDiffusion involves simple movement through the membrane caused by the random motion of the molecules of the substance. Substances move through cell membrane pores or, in the case of lipid-soluble substances, through the lipid matrix of the membrane. \nActive transport involves actually carrying a substance through the membrane by a physical protein structure that penetrates all the way through the membrane. These active transport mechanisms are so important to cell function that they are presented in detail in Chapter 4. \nLarge particles enter the cell by a specialized function of the cell membrane called *endocytosis* (Video 2-1). The principal forms of endocytosis are *pinocytosis* and *phagocytosis.* Pinocytosis means the ingestion of minute particles that form vesicles of extracellular fluid and particulate constituents inside the cell cytoplasm. Phagocytosis means the ingestion of large particles, such as bacteria, whole cells, or portions of degenerating tissue. \n**Pinocytosis.** Pinocytosis occurs continually in the cell membranes of most cells, but is especially rapid in some cells. For example, it occurs so rapidly in macrophages that about 3% of the total macrophage membrane is engulfed in the form of vesicles each minute. Even so, the pinocytotic vesicles are so small—usually only 100 to 200 nanometers in diameter—that most of them can be seen only with an electron microscope. \n \n**Figure 2-11.** Mechanism of pinocytosis. \nPinocytosis is the only means whereby most large macromolecules, such as most proteins, can enter cells. In fact, the rate at which pinocytotic vesicles form is usually enhanced when such macromolecules attach to the cell membrane. \n**[Figure 2-11](#page-24-0)** demonstrates the successive steps of pinocytosis *(A–D),* showing three molecules of protein attaching to the membrane. These molecules usually attach to specialized protein *receptors* on the surface of the membrane that are specific for the type of protein that is to be absorbed. The receptors generally are concentrated in small pits on the outer surface of the cell membrane, called *coated pits.* On the inside of the cell membrane beneath these pits is a latticework of fibrillar protein called *clathrin,* as well as other proteins, perhaps including contractile filaments of *actin* and *myosin.* Once the protein molecules have bound with the receptors, the surface properties of the local membrane change in such a way that the entire pit invaginates inward, and fibrillar proteins surrounding the invaginating pit cause its borders to close over the attached proteins, as well as over a small amount of extracellular fluid. Immediately thereafter, the invaginated portion of the membrane breaks away from the surface of the cell, forming a *pinocytotic vesicle* inside the cytoplasm of the cell. \nWhat causes the cell membrane to go through the necessary contortions to form pinocytotic vesicles is still unclear. This process requires energy from within the cell, which is supplied by ATP, a high-energy substance discussed later in this chapter. This process also requires the presence of calcium ions in the extracellular fluid, which probably react with contractile protein filaments beneath the coated pits to provide the force for pinching the vesicles away from the cell membrane. \n**Phagocytosis.** Phagocytosis occurs in much the same way as pinocytosis, except that it involves large particles rather than molecules. Only certain cells have the capability of phagocytosis—notably, tissue macrophages and some white blood cells. \n \n**Figure 2-12.** Digestion of substances in pinocytotic or phagocytic vesicles by enzymes derived from lysosomes. \nPhagocytosis is initiated when a particle such as a bacterium, dead cell, or tissue debris binds with receptors on the surface of the phagocyte. In the case of bacteria, each bacterium is usually already attached to a specific antibody; it is the antibody that attaches to the phagocyte receptors, dragging the bacterium along with it. This intermediation of antibodies is called *opsonization,* which is discussed in Chapters 34 and 35. \nPhagocytosis occurs in the following steps: \n- 1. The cell membrane receptors attach to the surface ligands of the particle.\n- 2. The edges of the membrane around the points of attachment evaginate outward within a fraction of a second to surround the entire particle; then, progressively more and more membrane receptors attach to the particle ligands. All this occurs suddenly in a zipper-like manner to form a closed *phagocytic vesicle.*\n- 3. Actin and other contractile fibrils in the cytoplasm surround the phagocytic vesicle and contract around its outer edge, pushing the vesicle to the interior.\n- 4. The contractile proteins then pinch the stem of the vesicle so completely that the vesicle separates from the cell membrane, leaving the vesicle in the cell interior in the same way that pinocytotic vesicles are formed. \n#### **LYSOSOMES DIGEST PINOCYTOTIC AND PHAGOCYTIC FOREIGN SUBSTANCES INSIDE THE CELL** \nAlmost immediately after a pinocytotic or phagocytic vesicle appears inside a cell, one or more *lysosomes* become attached to the vesicle and empty their *acid hydrolases* to the inside of the vesicle, as shown in **[Figure 2-12](#page-25-0)**. Thus, a *digestive vesicle* is formed inside the cell cytoplasm in which the vesicular hydrolases begin hydrolyzing the proteins, carbohydrates, lipids, and other substances in the vesicle. The products of digestion are small molecules of substances such as amino acids, glucose, and phosphates that can diffuse through the membrane of the vesicle into the cytoplasm. What is left of the digestive vesicle, called the *residual body,* represents indigestible substances. In most cases, the residual body is finally excreted through the cell membrane by a process called *exocytosis,* which is essentially the opposite of endocytosis. Thus, the pinocytotic and phagocytic vesicles containing lysosomes can be called the *digestive organs* of the cells. \n**Lysosomes and Regression of Tissues and Autolysis of Damaged Cells.** Tissues of the body often regress to a smaller size. For example, this regression occurs in the uterus after pregnancy, in muscles during long periods of inactivity, and in mammary glands at the end of lactation. Lysosomes are responsible for much of this regression. \nAnother special role of the lysosomes is the removal of damaged cells or damaged portions of cells from tissues. Damage to the cell—caused by heat, cold, trauma, chemicals, or any other factor—induces lysosomes to rupture. The released hydrolases immediately begin to digest the surrounding organic substances. If the damage is slight, only a portion of the cell is removed, and the cell is then repaired. If the damage is severe, the entire cell is digested, a process called *autolysis.* In this way, the cell is completely removed, and a new cell of the same type is formed, ordinarily by mitotic reproduction of an adjacent cell to take the place of the old one. \nThe lysosomes also contain bactericidal agents that can kill phagocytized bacteria before they cause cellular damage. These agents include the following: (1) *lysozyme,* which dissolves the bacterial cell wall; (2) *lysoferrin,* which binds iron and other substances before they can promote bacterial growth; and (3) acid at a pH of about 5.0, which activates the hydrolases and inactivates bacterial metabolic systems. \n#### **Autophagy and Recycling of Cell Organelles.** \nLysosomes play a key role in the process of *autophagy,* which literally means \"to eat oneself.\" Autophagy is a housekeeping process whereby obsolete organelles and large protein aggregates are degraded and recycled (**[Figure 2-13](#page-26-0)**). Worn-out cell organelles are transferred to lysosomes by double-membrane structures called *autophagosomes,* which are formed in the cytosol. Invagination of the lysosomal membrane and the formation of vesicles provides another pathway for cytosolic structures to be transported into the lumen of lysosomes. Once inside the lysosomes, the organelles are digested, and the nutrients are reused by the cell. Autophagy contributes to the routine turnover of cytoplasmic components; it is a key mechanism for tissue development, cell survival when nutrients are scarce, and maintenance of homeostasis. In liver cells, for example, the average mitochondrion normally has a life span of only about 10 days before it is destroyed. \n \n**Figure 2-13.** Schematic diagram of autophagy steps. \n#### **SYNTHESIS OF CELLULAR STRUCTURES BY ENDOPLASMIC RETICULUM AND GOLGI APPARATUS** \n#### **Endoplasmic Reticulum Functions** \nThe extensiveness of the endoplasmic reticulum and Golgi apparatus in secretory cells has already been emphasized. These structures are formed primarily of lipid bilayer membranes, similar to the cell membrane, and their walls are loaded with protein enzymes that catalyze the synthesis of many substances required by the cell. \nMost synthesis begins in the endoplasmic reticulum. The products formed there are then passed on to the Golgi apparatus, where they are further processed before being released into the cytoplasm. First, however, let us note the specific products that are synthesized in specific portions of the endoplasmic reticulum and Golgi apparatus. \n#### **Proteins Synthesis by the Rough Endoplasmic Reticu-** \n**lum.** The rough endoplasmic reticulum is characterized by large numbers of ribosomes attached to the outer surfaces of the endoplasmic reticulum membrane. As discussed in Chapter 3, protein molecules are synthesized within the structures of the ribosomes. The ribosomes extrude some of the synthesized protein molecules directly into the cytosol, but they also extrude many more through the wall of the endoplasmic reticulum to the interior of the endoplasmic vesicles and tubules into the *endoplasmic matrix.* \n#### **Lipid Synthesis by the Smooth Endoplasmic Reticu-** \n**lum.** The endoplasmic reticulum also synthesizes lipids, especially phospholipids and cholesterol. These lipids are rapidly incorporated into the lipid bilayer of the endoplasmic reticulum, thus causing the endoplasmic reticulum to grow more extensive. This process occurs mainly in the smooth portion of the endoplasmic reticulum. \nTo keep the endoplasmic reticulum from growing beyond the needs of the cell, small vesicles called *ER vesicles* or *transport vesicles* continually break away from the smooth reticulum; most of these vesicles then migrate rapidly to the Golgi apparatus. \n#### **Other Functions of the Endoplasmic Reticulum.** \nOther significant functions of the endoplasmic reticulum, especially the smooth reticulum, include the following: \n- 1. It provides the enzymes that control glycogen breakdown when glycogen is to be used for energy.\n- 2. It provides a vast number of enzymes that are capable of detoxifying substances, such as drugs, that might damage the cell. It achieves detoxification by processes such as coagulation, oxidation, hydrolysis, and conjugation with glycuronic acid. \n#### **Golgi Apparatus Functions** \n**Synthetic Functions of the Golgi Apparatus.** Although a major function of the Golgi apparatus is to provide additional processing of substances already formed in the endoplasmic reticulum, it can also synthesize certain carbohydrates that cannot be formed in the endoplasmic reticulum. This is especially true for the formation of large saccharide polymers bound with small amounts of protein; important examples include *hyaluronic acid* and *chondroitin sulfate.* \nA few of the many functions of hyaluronic acid and chondroitin sulfate in the body are as follows: (1) they are the major components of proteoglycans secreted in mucus and other glandular secretions; (2) they are the major components of the *ground substance*, or nonfibrous components of the extracellular matrix, outside the cells in the interstitial spaces, which act as fillers between collagen fibers and cells; (3) they are principal components of the organic matrix in both cartilage and bone; and (4) they are important in many cell activities, including migration and proliferation. \n \n**Figure 2-14.** Formation of proteins, lipids, and cellular vesicles by the endoplasmic reticulum and Golgi apparatus. \nProcessing of Endoplasmic Secretions by the Golgi Apparatus—Formation of Vesicles. Figure 2-14 summarizes the major functions of the endoplasmic reticulum and Golgi apparatus. As substances are formed in the endoplasmic reticulum, especially proteins, they are transported through the tubules toward portions of the smooth endoplasmic reticulum that lie nearest to the Golgi apparatus. At this point, transport vesicles composed of small envelopes of smooth endoplasmic reticulum continually break away and diffuse to the deepest layer of the Golgi apparatus. Inside these vesicles are synthesized proteins and other products from the endoplasmic reticulum. \nThe transport vesicles instantly fuse with the Golgi apparatus and empty their contained substances into the vesicular spaces of the Golgi apparatus. Here, additional carbohydrate moieties are added to the secretions. Also, an important function of the Golgi apparatus is to compact the endoplasmic reticular secretions into highly concentrated packets. As the secretions pass toward the outermost layers of the Golgi apparatus, the compaction and processing proceed. Finally, both small and large vesicles continually break away from the Golgi apparatus, carrying with them the compacted secretory substances and diffusing throughout the cell. \nThe following example provides an idea of the timing of these processes. When a glandular cell is bathed in amino acids, newly formed protein molecules can be detected in the granular endoplasmic reticulum within 3 to 5 minutes. Within 20 minutes, newly formed proteins are already present in the Golgi apparatus and, within 1 to 2 hours, the proteins are secreted from the surface of the cell. \nTypes of Vesicles Formed by the Golgi Apparatus—Secretory Vesicles and Lysosomes. In a highly secretory cell, the vesicles formed by the Golgi apparatus are mainly secretory vesicles containing proteins that are secreted through the surface of the cell membrane. These secretory vesicles first diffuse to the cell membrane and then fuse with it and empty their substances to the exterior by the mechanism called exocytosis. Exocytosis, in most cases, is stimulated by entry of calcium ions into the cell. Calcium ions interact with the vesicular membrane and cause its fusion with the cell membrane, followed by exocytosis—opening of the membrane's outer surface and extrusion of its contents outside the cell. Some vesicles, however, are destined for intracellular use. \n**Use of Intracellular Vesicles to Replenish Cellular Membranes.** Some intracellular vesicles formed by the Golgi apparatus fuse with the cell membrane or with the membranes of intracellular structures such as the mitochondria and even the endoplasmic reticulum. This fusion increases the expanse of these membranes and replenishes the membranes as they are used up. For example, the cell membrane loses much of its substance every time it forms a phagocytic or pinocytotic vesicle, and the vesicular membranes of the Golgi apparatus continually replenish the cell membrane. \nIn summary, the membranous system of the endoplasmic reticulum and Golgi apparatus are highly metabolic and capable of forming new intracellular structures and secretory substances to be extruded from the cell.\n\nThe principal substances from which cells extract energy are foods that react chemically with oxygen—carbohydrates, fats, and proteins. In the human body, essentially all carbohydrates are converted into *glucose* by the digestive tract and liver before they reach the other cells of the body. Similarly, proteins are converted into *amino acids*, and fats are converted into *fatty acids*. **Figure 2-15** shows oxygen and the foodstuffs—glucose, fatty acids, and amino acids—all entering the cell. Inside the cell, they react chemically with oxygen under the influence of enzymes that control the reactions and channel the energy released in the proper direction. The details of all these digestive and metabolic functions are provided in Chapters 63 through 73. \nBriefly, almost all these oxidative reactions occur inside the mitochondria, and the energy that is released is used to form the high-energy compound ATP. Then, ATP, not the original food, is used throughout the cell to energize almost all the subsequent intracellular metabolic reactions. \n \n**Figure 2-15.** Formation of adenosine triphosphate (ATP) in the cell showing that most of the ATP is formed in the mitochondria. (ADP, Adenosine diphosphate; CoA, coenzyme A.) \n#### **Functional Characteristics of Adenosine Triphosphate** \n$$\\begin{array}{c|ccccccccccccccccccccccccccccccccccc$$ \n**Adenosine triphosphate** \nATP is a nucleotide composed of the following: (1) the nitrogenous base *adenine;* (2) the pentose sugar *ribose;* and (3) three *phosphate radicals.* The last two phosphate radicals are connected with the remainder of the molecule by *high-energy phosphate bonds,* which are represented in the formula shown by the symbol ∼. *Under the physical and chemical conditions of the body,* each of these high-energy bonds contains about 12,000 calories of energy per mole of ATP, which is many times greater than the energy stored in the average chemical bond, thus giving rise to the term *high-energy bond.* Furthermore, the high-energy phosphate bond is very labile, so that it can be split instantly on demand whenever energy is required to promote other intracellular reactions. \nWhen ATP releases its energy, a phosphoric acid radical is split away, and *adenosine diphosphate* (ADP) is formed. This released energy is used to energize many of the cell's other functions, such as syntheses of substances and muscular contraction. \nTo reconstitute the cellular ATP as it is used up, energy derived from the cellular nutrients causes ADP and phosphoric acid to recombine to form new ATP, and the entire process is repeated over and over. For these reasons, ATP has been called the *energy currency* of the cell because it can be spent and reformed continually, having a turnover time of only a few minutes. \n**Chemical Processes in the Formation of ATP—Role of the Mitochondria.** On entry into the cells, glucose is converted by enzymes in the *cytoplasm* into *pyruvic acid* (a process called *glycolysis*). A small amount of ADP is changed into ATP by the energy released during this conversion, but this amount accounts for less than 5% of the overall energy metabolism of the cell. \nAbout 95% of the cell's ATP formation occurs in the mitochondria. The pyruvic acid derived from carbohydrates, fatty acids from lipids, and amino acids from proteins is eventually converted into the compound *acetyl-coenzyme A* (CoA) in the matrix of mitochondria. This substance, in turn, is further dissolved (for the purpose of extracting its energy) by another series of enzymes in the mitochondrion matrix, undergoing dissolution in a sequence of chemical reactions called the *citric acid cycle,* or *Krebs cycle.* These chemical reactions are so important that they are explained in detail in Chapter 68. \nIn this citric acid cycle, acetyl-CoA is split into its component parts, *hydrogen atoms* and *carbon dioxide.* The carbon dioxide diffuses out of the mitochondria and eventually out of the cell; finally, it is excreted from the body through the lungs. \nThe hydrogen atoms, conversely, are highly reactive; they combine with oxygen that has also diffused into the mitochondria. This combination releases a tremendous amount of energy, which is used by mitochondria to convert large amounts of ADP to ATP. The processes of these reactions are complex, requiring the participation of many protein enzymes that are integral parts of mitochondrial *membranous shelves* that protrude into the mitochondrial matrix. The initial event is the removal of an electron from the hydrogen atom, thus converting it to a hydrogen ion. The terminal event is the combination of hydrogen ions with oxygen to form water and the release of large amounts of energy to globular proteins that protrude like knobs from the membranes of the mitochondrial shelves; these proteins are called *ATP synthetase*. Finally, the enzyme ATP synthetase uses the energy from the hydrogen ions to convert ADP to ATP. The newly formed ATP is transported out of the mitochondria into all parts of the cell cytoplasm and nucleoplasm, where it energizes multiple cell functions. \nThis overall process for formation of ATP is called the *chemiosmotic mechanism* of ATP formation. The chemical and physical details of this mechanism are presented \n \n**Figure 2-16.** Use of adenosine triphosphate (ATP; formed in the mitochondrion) to provide energy for three major cellular functions—membrane transport, protein synthesis, and muscle contraction. (ADP, Adenosine diphosphate.) \nin Chapter 68, and many of the detailed metabolic functions of ATP in the body are discussed in Chapters 68 through 72. \n**Uses of ATP for Cellular Function.** Energy from ATP is used to promote three major categories of cellular functions: (1) *transport* of substances through multiple cell membranes; (2) *synthesis of chemical compounds* throughout the cell; and (3) *mechanical work.* These uses of ATP are illustrated by the examples in **Figure 2-16**: (1) to supply energy for the transport of sodium through the cell membrane; (2) to promote protein synthesis by the ribosomes; and (3) to supply the energy needed during muscle contraction. \nIn addition to the membrane transport of sodium, energy from ATP is required for the membrane transport of potassium, calcium, magnesium, phosphate, chloride, urate, and hydrogen ions and many other ions, as well as various organic substances. Membrane transport is so important to cell function that some cells—the renal tubular cells, for example—use as much as 80% of the ATP that they form for this purpose alone. \nIn addition to synthesizing proteins, cells make phospholipids, cholesterol, purines, pyrimidines, and many other substances. Synthesis of almost any chemical compound requires energy. For example, a single protein molecule might be composed of as many as several thousand amino acids attached to one another by peptide linkages. The formation of each of these linkages requires energy derived from the breakdown of four high-energy bonds; thus, many thousand ATP molecules must release their energy as each protein molecule is formed. Indeed, some cells use as much as 75% of all the ATP formed in the cell \n \nFigure 2-17. Ameboid motion by a cell. \nsimply to synthesize new chemical compounds, especially protein molecules; this is particularly true during the growth phase of cells. \nAnother use of ATP is to supply energy for special cells to perform mechanical work. We discuss in Chapter 6 that each contraction of a muscle fiber requires the expenditure of large quantities of ATP energy. Other cells perform mechanical work in other ways, especially by *ciliary* and *ameboid motion*, described later in this chapter. The source of energy for all these types of mechanical work is ATP. \nIn summary, ATP is readily available to release its energy rapidly wherever it is needed in the cell. To replace ATP used by the cell, much slower chemical reactions break down carbohydrates, fats, and proteins and use the energy derived from these processes to form new ATP. More than 95% of this ATP is formed in the mitochondria, which is why the mitochondria are called the *powerhouses* of the cell. \n#### **LOCOMOTION OF CELLS** \nThe most obvious type of movement in the body is that which occurs in skeletal, cardiac, and smooth muscle cells, which constitute almost 50% of the entire body mass. The specialized functions of these cells are discussed in Chapters 6 through 9. Two other types of movement—ameboid locomotion and ciliary movement—occur in other cells. \n#### AMEBOID MOVEMENT \nAmeboid movement is a crawling-like movement of an entire cell in relation to its surroundings, such as movement of white blood cells through tissues. This type of movement gets its name from the fact that amebae move in this manner, and amebae have provided an excellent tool for studying the phenomenon. \nTypically, ameboid locomotion begins with the protrusion of a *pseudopodium* from one end of the cell. The pseudopodium projects away from the cell body and partially secures itself in a new tissue area; then the remainder of the cell is pulled toward the pseudopodium. **Figure 2-17** \ndemonstrates this process, showing an elongated cell, the right-hand end of which is a protruding pseudopodium. The membrane of this end of the cell is continually moving forward, and the membrane at the left-hand end of the cell is continually following along as the cell moves. \n**Mechanism of Ameboid Locomotion. [Figure 2-17](#page-29-1)** shows the general principle of ameboid motion. Basically, this results from the continual formation of new cell membrane at the leading edge of the pseudopodium and continual absorption of the membrane in the mid and rear portions of the cell. Two other effects are also essential for forward movement of the cell. The first is attachment of the pseudopodium to surrounding tissues so that it becomes fixed in its leading position while the remainder of the cell body is being pulled forward toward the point of attachment. This attachment is caused by *receptor proteins* that line the insides of exocytotic vesicles. When the vesicles become part of the pseudopodial membrane, they open so that their insides evert to the outside, and the receptors now protrude to the outside and attach to ligands in the surrounding tissues. \nAt the opposite end of the cell, the receptors pull away from their ligands and form new endocytotic vesicles. Then, inside the cell, these vesicles stream toward the pseudopodial end of the cell, where they are used to form new membrane for the pseudopodium. \nThe second essential effect for locomotion is to provide the energy required to pull the cell body in the direction of the pseudopodium. A moderate to large amount of the protein *actin* is in the cytoplasm of all cells*.* Much of the actin is in the form of single molecules that do not provide any motive power; however, these molecules polymerize to form a filamentous network, and the network contracts when it binds with an actin-binding protein such as *myosin.* The entire process is energized by the high-energy compound ATP. This is what occurs in the pseudopodium of a moving cell, where such a network of actin filaments forms anew inside the enlarging pseudopodium. Contraction also occurs in the ectoplasm of the cell body, where a preexisting actin network is already present beneath the cell membrane. \n#### **Types of Cells That Exhibit Ameboid Locomotion.** \nThe most common cells to exhibit ameboid locomotion in the human body are the *white blood cells* when they move out of the blood into the tissues to form *tissue macrophages.* Other types of cells can also move by ameboid locomotion under certain circumstances. For example, fibroblasts move into a damaged area to help repair the damage, and even the germinal cells of the skin, although ordinarily completely sessile cells, move toward a cut area to repair the opening. Cell locomotion is also especially important in the development of the embryo and fetus after fertilization of an ovum. For example, embryonic cells often must migrate long distances from their sites of origin to new areas during the development of special structures. \nSome types of cancer cells, such as sarcomas, which arise from connective tissue cells, are especially proficient at ameboid movement. This partially accounts for their relatively rapid spreading from one part of the body to another, known as *metastasis.* \n**Control of Ameboid Locomotion—Chemotaxis.** An important initiator of ameboid locomotion is the process called *chemotaxis,* which results from the appearance of certain chemical substances in the tissues. Any chemical substance that causes chemotaxis to occur is called a *chemotactic substance.* Most cells that exhibit ameboid locomotion move toward the source of a chemotactic substance—that is, from an area of lower concentration toward an area of higher concentration. This is called *positive chemotaxis.* Some cells move away from the source, which is called *negative chemotaxis.* \nHow does chemotaxis control the direction of ameboid locomotion? Although the answer is not certain, it is known that the side of the cell most exposed to the chemotactic substance develops membrane changes that cause pseudopodial protrusion. \n#### **CILIA AND CILIARY MOVEMENTS** \nThere are two types of cilia, *motile* and *nonmotile*, or *primary*, cilia. Motile cilia can undergo a whiplike movement on the surfaces of cells. This movement occurs mainly in two places in the human body, on the surfaces of the respiratory airways and on the inside surfaces of the uterine tubes (fallopian tubes) of the reproductive tract. In the nasal cavity and lower respiratory airways, the whiplike motion of motile cilia causes a layer of mucus to move at a rate of about 1 cm/min toward the pharynx, in this way continually clearing these passageways of mucus and particles that have become trapped in the mucus. In the uterine tubes, cilia cause slow movement of fluid from the ostium of the uterine tube toward the uterus cavity; this movement of fluid transports the ovum from the ovary to the uterus. \nAs shown in **[Figure 2-18](#page-31-0)**, a cilium has the appearance of a sharp-pointed straight or curved hair that projects 2 to 4 micrometers from the surface of the cell. Often, many motile cilia project from a single cell—for example, as many as 200 cilia on the surface of each epithelial cell inside the respiratory passageways. The cilium is covered by an outcropping of the cell membrane, and it is supported by 11 microtubules—nine double tubules located around the periphery of the cilium and two single tubules down the center, as demonstrated in the cross section shown in **[Figure 2-18](#page-31-0)**. Each cilium is an outgrowth of a structure that lies immediately beneath the cell membrane, called the *basal body* of the cilium. \nThe *flagellum of a sperm* is similar to a motile cilium; in fact, it has much the same type of structure and same type of contractile mechanism. The flagellum, however, is much longer and moves in quasisinusoidal waves instead of whiplike movements. \n \n**Figure 2-18.** Structure and function of the cilium. *(Modified from Satir P: Cilia. Sci Am 204:108, 1961.)* \nIn the inset of **[Figure 2-18](#page-31-0)**, movement of the motile cilium is shown. The cilium moves forward with a sudden, rapid whiplike stroke 10 to 20 times per second, bending sharply where it projects from the surface of the cell. Then it moves backward slowly to its initial position. The rapid, forward-thrusting, whiplike movement pushes the fluid lying adjacent to the cell in the direction that the cilium moves; the slow dragging movement in the backward direction has almost no effect on fluid movement. As a result, the fluid is continually propelled in the direction of the fast-forward stroke. Because most motile ciliated cells have large numbers of cilia on their surfaces, and because all the cilia are oriented in the same direction, this is an effective means for moving fluids from one part of the surface to another. \n**Mechanism of Ciliary Movement.** Although not all aspects of ciliary movement are known, we are aware of the following elements. First, the nine double tubules and two single tubules are all linked to one another by a complex of protein cross-linkages; this total complex of tubules and cross-linkages is called the *axoneme.* Second, even after removal of the membrane and destruction of other elements of the cilium in addition to the axoneme, the cilium can still beat under appropriate conditions. Third, two conditions are necessary for continued beating of the axoneme after removal of the other structures of the cilium: (1) the availability of ATP; and (2) appropriate ionic conditions, especially appropriate concentrations of magnesium and calcium. Fourth, during forward motion of the cilium, the double tubules on the front edge of the cilium slide outward toward the tip of the cilium, whereas those on the back edge remain in place. Fifth, multiple protein arms composed of the protein *dynein,* which has adenosine triphosphatase (ATPase) enzymatic activity, project from each double tubule toward an adjacent double tubule. \nGiven this basic information, it has been determined that the release of energy from ATP in contact with the ATPase dynein arms causes the heads of these arms to \"crawl\" rapidly along the surface of the adjacent double tubule. If the front tubules crawl outward while the back tubules remain stationary, bending occurs. \nThe way in which cilia contraction is controlled is not well understood. The cilia of some genetically abnormal cells do not have the two central single tubules, and these cilia fail to beat. Therefore, it is presumed that some signal, perhaps an electrochemical signal, is transmitted along these two central tubules to activate the dynein arms. \n**Nonmotile Primary Cilia Serve as Cell Sensory \"Antennae.\"** *Primary cilia* are nonmotile and generally occur only as a single cilium on each cell. Although the physiological functions of primary cilia are not fully understood, current evidence indicates that they function as cellular ''sensory antennae,\" which coordinate cellular signaling pathways involved in chemical and mechanical sensation, signal transduction, and cell growth. In the kidneys, for example, primary cilia are found in most epithelial cells of the tubules, projecting into the tubule lumen and acting as a flow sensor. In response to fluid flow over the tubular epithelial cells, the primary cilia bend and cause flow-induced changes in intracellular calcium signaling. These signals, in turn, initiate multiple effects on the cells. Defects in signaling by primary cilia in renal tubular epithelial cells are thought to contribute to various disorders, including the development of large fluid-filled cysts, a condition called *polycystic kidney disease*. \n#### Bibliography \nAlberts B, Johnson A, Lewis J, et al: Molecular Biology of the Cell, 6th ed. New York: Garland Science, 2014. \nBrandizzi F, Barlowe C: Organization of the ER-Golgi interface for membrane traffic control. Nat Rev Mol Cell Biol 14:382, 2013. \nDikic I, Elazar Z. Mechanism and medical implications of mammalian autophagy. Nat Rev Mol Cell Biol 19:349, 2018. \nEisner V, Picard M, Hajnóczky G. Mitochondrial dynamics in adaptive and maladaptive cellular stress responses. Nat Cell Biol 20:755, 2018. \nGalluzzi L, Yamazaki T, Kroemer G. Linking cellular stress responses to systemic homeostasis. Nat Rev Mol Cell Biol 19:731, 2018. \n- Guerriero CJ, Brodsky JL: The delicate balance between secreted protein folding and endoplasmic reticulum-associated degradation in human physiology. Physiol Rev 92:537, 2012.\n- Harayama T, Riezman H. Understanding the diversity of membrane lipid composition. Nat Rev Mol Cell Biol 19:281, 2018.\n- Insall R: The interaction between pseudopods and extracellular signalling during chemotaxis and directed migration. Curr Opin Cell Biol 25:526, 2013.\n- Kaksonen M, Roux A. Mechanisms of clathrin-mediated endocytosis. Nat Rev Mol Cell Biol 19:313, 2018.\n- Lawrence RE, Zoncu R. The lysosome as a cellular centre for signalling, metabolism and quality control. Nat Cell Biol 21: 133, 2019.\n- Nakamura N, Wei JH, Seemann J: Modular organization of the mammalian Golgi apparatus. Curr Opin Cell Biol 24:467, 2012. \n- Palikaras K, Lionaki E, Tavernarakis N. Mechanisms of mitophagy in cellular homeostasis, physiology and pathology. Nat Cell Biol 20:1013, 2018.\n- Sezgin E, Levental I, Mayor S, Eggeling C. The mystery of membrane organization: composition, regulation and roles of lipid rafts. Nat Rev Mol Cell Biol 18:361, 2017.\n- Spinelli JB, Haigis MC. The multifaceted contributions of mitochondria to cellular metabolism. Nat Cell Biol. 20:745, 2018.\n- Walker CL, Pomatto LCD, Tripathi DN, Davies KJA. Redox regulation of homeostasis and proteostasis in peroxisomes. Physiol Rev 98:89, 2018.\n- Zhou K, Gaullier G, Luger K. Nucleosome structure and dynamics are coming of age. Nat Struct Mol Biol 26:3, 2019. \n\n\nGenes, which are located in the nuclei of all cells of the body, control heredity from parents to children, as well as the daily functioning of all the body's cells. The genes control cell function by determining which structures, enzymes, and chemicals are synthesized within the cell. \n**Figure 3-1** shows the general schema of genetic control. Each gene, which is composed of *deoxyribonucleic acid* (DNA), controls the formation of another nucleic acid, *ribonucleic acid* (RNA); this RNA then spreads throughout the cell to control formation of a specific protein. The entire process, from *transcription* of the genetic code in the nucleus to *translation* of the RNA code and the formation of proteins in the cell cytoplasm, is often referred to as *gene expression.* \nBecause the human body has approximately 20,000 to 25,000 different genes that code for proteins in each cell, it is possible to form a large number of different cellular proteins. In fact, RNA molecules transcribed from the same segment of DNA—the same gene—can be processed in more than one way by the cell, giving rise to alternate versions of the protein. The total number of different proteins produced by the various cell types in humans is estimated to be at least 100,000. \nSome of the cellular proteins are *structural proteins,* which, in association with various lipids and carbohydrates, form structures of the various intracellular organelles discussed in Chapter 2. However, most of the proteins are *enzymes* that catalyze different chemical reactions in the cells. For example, enzymes promote all the oxidative reactions that supply energy to the cell, along with synthesis of all the cell chemicals, such as lipids, glycogen, and adenosine triphosphate (ATP). \n#### CELL NUCLEUS GENES CONTROL PROT[EIN](#page-34-0) SYNTHESIS \nIn the cell nucleus, large numbers of genes are attached end on end in extremely long, double-stranded helical molecules of DNA having molecular weights measured in the billions. A very short segment of such a molecule is shown in **Figure 3-2**. This molecule is composed of several simple chemical compounds bound together in a regular pattern, the details of which are explained in the next few paragraphs. \n#### **Building Blocks of DNA** \n**Figure 3-3** shows the basic chemical compounds involved in the formation of DNA. These compounds include the following: (1) *phosphoric acid;* (2) a sugar [called](#page-34-0) *deoxyribose;* and (3) four nitrogenous *bases* (two purines, *adenine* and *guanine,* and two pyrimidines, *thymine* and *cytosine*). The phosphoric acid and deoxyribose form the two helical strands that are the backbone of the DNA molecule, and the nitrogenous bases lie between the two strands and connect them, as illustrated in **Figure 3-2**. \n#### **Nucleotides** \nThe first stage of DNA formation is to combine one molecule of phosphoric acid, one molecule of deoxyribose, and one of the four bases to form an acidic nucleotide. Four separate nucleotides are thus formed, one for each of the four bases: *deoxyadenylic, deoxythymidylic, deoxyguanylic,* and *deoxycytidylic acids*. **Figure 3-4** shows the chemical \n \n**Figure 3-1** The general schema whereby genes control cell function. *mRNA,* Messenger RNA. \n \n**Figure 3-2** The helical double-stranded structure of the gene. The outside strands are composed of phosphoric acid and the sugar deoxyribose. The internal molecules connecting the two strands of the helix are purine and pyrimidine bases, which determine the \"code\" of the gene. \n \nFigure 3-3 The basic building blocks of DNA. \nstructure of deoxyadenylic acid, and **Figure 3-5** shows simple symbols for the four nucleotides that form DNA. \n#### Nucleotides Are Organized to Form Two Strands of DNA Loosely Bound to Each Other \n**Figure 3-2** shows the manner in which multiple nucleotides are bound together to form two strands of DNA. The two strands are, in turn, loosely bonded with each other by weak cross-linkages, as illustrated in **Figure 3-6** by the central dashed lines. Note that the backbone of each DNA strand is composed of alternating phosphoric acid and deoxyribose molecules. In turn, purine and pyrimidine bases are attached to the sides of the deoxyribose molecules. Then, by means of loose *hydrogen bonds* (dashed \n**Figure 3-4.** Deoxyadenylic acid, one of the nucleotides that make up DNA. \n \n**Figure 3-5.** Symbols for the four nucleotides that combine to form DNA. Each nucleotide contains phosphoric acid (P), deoxyribose (D), and one of the four nucleotide bases: adenine (A); thymine (T); guanine (G); or cytosine (C). \nlines) between the purine and pyrimidine bases, the two respective DNA strands are held together. Note the following caveats, however: \n- 1. Each purine base *adenine* of one strand always bonds with a pyrimidine base *thymine* of the other strand.\n- 2. Each purine base *guanine* always bonds with a pyrimidine base *cytosine*. \nThus, in **Figure 3**-6, the sequence of complementary pairs of bases is CG, CG, GC, TA, CG, TA, GC, AT, and AT. Because of the looseness of the hydrogen bonds, the two strands can pull apart with ease, and they do so many times during the course of their function in the cell. \nTo put the DNA of **Figure 3**-6 into its proper physical perspective, one could merely pick up the two ends and twist them into a helix. Ten pairs of nucleotides are present in each full turn of the helix in the DNA molecule. \n#### **GENETIC CODE** \nThe importance of DNA lies in its ability to control the formation of proteins in the cell, which it achieves by means of a *genetic code*. That is, when the two strands of a DNA molecule are split apart, the purine and pyrimidine bases projecting to the side of each DNA strand are exposed, as shown by the top strand in **Figure 3-7**. It is these projecting bases that form the genetic code. \nThe genetic code consists of successive \"triplets\" of bases—that is, each three successive bases is a *code word*. The successive triplets eventually control the sequence of amino acids in a protein molecule that is to be synthesized in the cell. Note in **Figure 3**-6 that the top strand of DNA, reading from left to right, has the genetic code GGC, AGA, CTT, with the triplets being separated from one another by the arrows. As we follow this genetic code through **Figure 3**-7 and **Figure 3**-8, we see that these three respective triplets are responsible for successive placement of the three amino acids, *proline, serine,* and *glutamic acid,* in a newly formed molecule of protein.\n\nBecause DNA is located in the cell nucleus, yet most of the cell functions are carried out in the cytoplasm, there must be some means for DNA genes of the nucleus to control chemical reactions of the cytoplasm. This control \n \n**Figure 3-6.** Arrangement of deoxyribose nucleotides in a double strand of DNA. \n \nR C \nP \nP R C P R G P R U \n**Figure 3-7.** Combination of ribose nucleotides with a strand of DNA to form a molecule of RNA that carries the genetic code from the gene to the cytoplasm. The *RNA polymerase* enzyme moves along the DNA strand and builds the RNA molecule. \n**Figure 3-8.** A portion of an RNA molecule showing thre[e](#page-36-0) RNA codons—CCG, UCU, and GAA—that control attachment of the three amino acids, proline, serine, and glutamic acid, respectively, to the growing RNA chain. **Proline Serine Glutamic acid** \nis achieved through the intermediary of another type of nucleic acid, RNA, the formation of which is controlled by DNA of the nucleus. Thus, as shown in **Figure 3-7**, the code is transferred to RNA in a process called *transcription.* The RNA, in turn, diffuses from the nucleus through nuclear pores into the cytoplasmic compartment, where it controls protein synthesis. \n#### **RNA IS SYNTHESIZED IN THE NUCLEUS FROM A DNA TEMPLATE** \nDuring RNA synthesis, the two strands of DNA separate temporarily; one of these strands is used as a template for synthesis of an RNA molecule. The code triplets in the DNA result in the formation of *complementary* code triplets (called *codons*) in the RNA. These codons, in turn, will control the sequence of amino acids in a protein to be synthesized in the cell cytoplasm. \n**Building Blocks of RNA.** The basic building blocks of RNA are almost the same as those of DNA, except for two differences. First, the sugar deoxyribose is not used in RNA formation. In its place is another sugar of slightly different composition, *ribose,* which contains an extra hydroxyl ion appended to the ribose ring structure. Second, thymine is replaced by another pyrimidine, *uracil.* \n**Formation of RNA Nucleotides.** The basic building blocks of RNA form *RNA nucleotides,* exactly as described previously for DNA synthesis. Here again, four separate nucleotides are used to form RNA. These nucleotides contain the bases *adenine, guanine, cytosine,* and *uracil.* Note that these bases are the same as in DNA, except that uracil in RNA replaces thymine in DNA. \n**\"Activation\" of RNA Nucleotides.** The next step in the synthesis of RNA is \"activation\" of RNA nucleotides by an enzyme, *RNA polymerase.* This activation occurs by adding two extra phosphate radicals to each nucleotide to form \ntriphosphates (shown in **Figure 3-7** by the two RNA nucleotides to the far right during RNA chain formation). These last two phosphates are combined with the nucleotide by \nP R U P R G P R A P R A \nP R C \n*high-energy phosphate bonds* derived from ATP in the cell. The result of this activation process is that large quantities of ATP energy are made available to each of the nucleotides. This energy is used to promote chemical reactions that add each new RNA nucleotide at the end of the developing RNA [chain.](#page-36-0) \n#### **RNA CHAIN ASSEMBLY FROM ACTIVATED NUCLEOTIDES USING THE DNA STRAND AS A TEMPLATE** \nAs shown in **Figure 3-7,** assembly of RNA is accomplished under the influence of an enzyme, *RNA polymerase.* This large protein enzyme has many functional properties necessary for formation of RNA, as follows: \n- 1. In the DNA strand immediately ahead of the gene to be transcribed is a sequence of nucleotides called the *promoter.* The RNA polymerase has an appropriate complementary structure that recognizes this promoter and becomes attached to it, which is the essential step for initiating the formation of RNA.\n- 2. After the RNA polymerase attaches to the promoter, the polymerase causes unwinding of about two turns of the DNA helix and separation of the unwound portions of the two strands.\n- 3. The polymerase then moves along the DNA strand, temporarily unwinding and separating the two DNA strands at each stage of its movement. As it moves along, at each stage it adds a new activated RNA nucleotide to the end of the newly forming RNA chain through the following steps:\n- a. First, it causes a hydrogen bond to form between the end base of the DNA strand and the base of an RNA nucleotide in the nucleoplasm. \n- b. Then, one at a time, the RNA polymerase breaks two of the three phosphate radicals away from each of these RNA nucleotides, liberating large amounts of energy from the broken high-energy phosphate bonds. This energy is used to cause covalent linkage of the remaining phosphate on the nucleotide with the ribose on the end of the growing RNA chain.\n- c. When the RNA polymerase reaches the end of the DNA gene, it encounters a new sequence of DNA nucleotides called the *chain-terminating sequence,* which causes the polymerase and the newly formed RNA chain to break away from the DNA strand. The polymerase then can be used again and again to form more new RNA chains.\n- d. As the new RNA strand is formed, its weak hydrogen bonds with the DNA template break away because the DNA has a high affinity for rebonding with its own complementary DNA strand. Thus, the RNA chain is forced away from the DNA and is released into the nucleoplasm. \nTherefore, the code that is present in the DNA strand is eventually transmitted in *complementary* form to the RNA chain. The ribose nucleotide bases always combine with the deoxyribose bases in the following combinations: \n| DNA Base | RNA Base |\n|----------|----------|\n| guanine | Cytosine |\n| cytosine | Guanine |\n| adenine | Uracil |\n| thymine | adenine | \n**There Are Several Different Types of RNA.** As research on RNA has continued to advance, many different types of RNA have been discovered. Some types of RNA are involved in protein synthesis, whereas other types serve gene regulatory functions or are involved in posttranscriptional modification of RNA. The functions of some types of RNA, especially those that do not appear to code for proteins, are still mysterious. The following six types of RNA play independent and different roles in protein synthesis: \n- 1. *Precursor messenger RNA* (pre-mRNA) is a large, immature, single strand of RNA that is processed in the nucleus to form mature messenger RNA (mRNA). The pre-RNA includes two different types of segments, called *introns,* which are removed by a process called splicing, and *exons,* which are retained in the final mRNA.\n- 2. *Small nuclear RNA* (snRNA) directs the splicing of pre-mRNA to form mRNA.\n- 3. *Messenger RNA* (mRNA) carries the genetic code to the cytoplasm for controlling the type of protein formed.\n- 4. *Transfer RNA* (tRNA) transports activated amino acids to the ribosomes to be used in assembling the protein molecule.\n- 5. *Ribosomal RNA,* along with about 75 different proteins, forms *ribosomes,* the physical and chemical \n- structures on which protein molecules are actually assembled.\n- 6. *MicroRNAs* (miRNAs) are single-stranded RNA molecules of 21 to 23 nucleotides that can regulate gene transcription and translation. \n#### **MESSENGER RN[A—THE C](#page-36-1)ODONS** \n*Messenger RNA* molecules are long single RNA strands that are suspended in the cytoplasm. These molecules are composed of several hundred to several thousand RNA nucleotides in unpaired strands, and they contain *c[odons](#page-36-0)* that are exactly complementary to the code triplets of t[he](#page-37-0) DNA genes. **Figure 3-8** shows a small segment of mRNA. Its codons are CCG, UCU, and GAA, which are the codons for the amino acids proline, serine, and glutamic acid. The transcription of these codons from the DNA molecule to the RNA molecule is shown in **Figure 3-7**. \n**RNA Codons for the Different Amino Acids. [Table 3](#page-37-0)-1** lists the RNA codons for the 20 common amino acids found in protein molecules. Note that most of the amino acids are represented by more than one codon; also, one codon represents the signal \"start manufacturing the protein molecule,\" and three codons represent \"stop manufacturing the protein molecule.\" In **Table 3-1**, these two \n**Table 3-1** RNA Codons for Amino Acids and for Start and Stop \n| Amino Acid | | | RNA Codons | | | |\n|---------------|-----|-----|------------|-----|-----|-----|\n| Alanine | GCU | GCC | GCA | GCG | | |\n| Arginine | CGU | CGC | CGA | CGG | AGA | AGG |\n| Asparagine | AAU | AAC | | | | |\n| Aspartic acid | GAU | GAC | | | | |\n| Cysteine | UGU | UGC | | | | |\n| Glutamic acid | GAA | GAG | | | | |\n| Glutamine | CAA | CAG | | | | |\n| Glycine | GGU | GGC | GGA | GGG | | |\n| Histidine | CAU | CAC | | | | |\n| Isoleucine | AUU | AUC | AUA | | | |\n| Leucine | CUU | CUC | CUA | CUG | UUA | UUG |\n| Lysine | AAA | AAG | | | | |\n| Methionine | AUG | | | | | |\n| Phenylalanine | UUU | UUC | | | | |\n| Proline | CCU | CCC | CCA | CCG | | |\n| Serine | UCU | UCC | UCA | UCG | AGC | AGU |\n| Threonine | ACU | ACC | ACA | ACG | | |\n| Tryptophan | UGG | | | | | |\n| Tyrosine | UAU | UAC | | | | |\n| Valine | GUU | GUC | GUA | GUG | | |\n| Start (CI) | AUG | | | | | |\n| Stop (CT) | UAA | UAG | UGA | | | | \n*CI,* Chain-initiating; *CT,* chain-terminating. \ntypes of codons are designated CI for \"chain-initiating\" or \"start\" codon and CT for \"chain-terminating\" or \"stop\" codon. \n#### **TRANSFER RNA—THE ANTICODONS** \nAnother type of RNA that is essential for protein synthesis is called transfer RNA (tRNA) because it transfers amino acids to protein molecules as the protein is being synthesized. Each type of tRNA combines specifically with 1 of the 20 amino acids that are to be incorporated into proteins. The tRNA then acts as a *carrier* to transport its specific type of amino acid to the ribosomes, where protein molecules are forming. In the ribosomes, each specific type of tRNA recognizes a particular codon on the mRNA (described later) and thereby delivers [the appro](#page-38-0)priate amino acid to the appropriate place in the chain of the newly forming protein molecule. \nTransfer RNA, which contains only about 80 nucleotides, is a relatively small molecule in comparison with mRNA. It is a folded chain of nucleotides with a cloverleaf appearance similar to that shown in **Figure 3-9**. At one end of the molecule there is always an adenylic acid to which the transported amino acid attaches at a hydroxyl group of the ribose in the adenylic acid. \nBecause the function of tRNA is to cause attachment of a specific amino acid to a forming protein chain, it is essential that each type of t[RNA also ha](#page-38-0)ve specificity for a particular codon in the mRNA. The specific code in the tRNA that allows it to recognize a specific codon is again a triplet of nucleotide bases and is called an *anticodon.* This anticodon is located approximately in the middle of the tRNA molecule (at the bottom of the cloverleaf configuration shown in **Figure 3-9**). During formation of the protein molecule, the anticodon bases combine loosely by hydrogen bonding with the codon bases of the mRNA. In this way, the respective amino acids are lined up one after another along the mRNA chain, thus establishing the \n \n**Figure 3-9.** A messenger RNA strand is moving through two ribosomes. As each codon passes through, an amino acid is added to the growing protein chain, which is shown in the right-hand ribosome. The transfer RNA molecule transports each specific amino acid to the newly forming protein. \nappropriate sequence of amino acids in the newly forming protein molecule. \n#### **RIBOSOMAL RNA** \nThe third type of RNA in the cell is ribosomal RNA, which constitutes about 60% of the *ribosome.* The remainder of the ribosome is protein, including about 75 types of proteins that are both structural proteins and enzymes needed to manufacture proteins. \nThe ribosome is the physical structure in the cytoplasm on which proteins are actually synthesized. However, it always functions in association with the other two types of RNA; *tRNA* transports amino acids to the ribosome for incorporation into the developing protein, whereas *mRNA* provides the information necessary for sequencing the amino acids in proper order for each specific type of protein to be manufactured. Thus, the ribosome acts as a manufacturing plant in which the protein molecules are formed. \n**Formation of Ribosomes in the Nucleolus.** The DNA genes for the formation of ribosomal RNA are located in five pairs of chromosomes in the nucleus. Each of these chromosomes contains many duplicates of these particular genes because of the large amounts of ribosomal RNA required for cellular function. \nAs the ribosomal RNA forms, it collects in the *nucleolus,* a specialized structure lying adjacent to the chromosomes. When large amounts of ribosomal RNA are being synthesized, as occurs in cells that manufacture large amounts of protein, the nucleolus is a large structure, whereas in cells that synthesize little protein, the nucleolus may not even be seen. Ribosomal RNA is specially processed in the nucleolus, where it binds with ribosomal proteins to form granular condensation products that are primordial subunits of ribosomes. These subunits are then released from the nucleolus and transported through the large pores of the nuclear envelope to almost all parts of the cytoplasm. After the subunits enter the cytoplasm, they are assembled to form mature functional ribosomes. Therefore, proteins are formed in the cytoplasm of the cell, but not in the cell nucleus, because the nucleus does not cont[ain m](#page-39-0)ature ribosomes. \n#### **miRNA AND SMALL INTERFERING RNA** \nA fourth type of RNA in the cell is *microRNA* (miRNA); miRNA are short (21 to 23 nucleotides) single-stranded RNA fragments that regulate gene expression **(Figure 3-10).** The miRNAs are encoded from the transcribed DNA of genes, but they are not translated into proteins and are therefore often called *noncoding RNA*. The miRNAs are processed by the cell into molecules that are complementary to mRNA and act to decrease gene expression. The generation of miRNAs involves special processing of longer primary precursor RNAs called *primiRNAs,* which are the primary transcripts of the gene. \n \n**Figure 3-10.** Regulation of gene expression by microRNA (miRNA). Primary miRNA (pri-miRNA), the primary transcripts of a gene processed in the cell nucleus by the microprocessor complex, are converted to pre-miRNAs. These pre-miRNAs are then further processed in the cytoplasm by *dicer,* an enzyme that helps assemble an RNAinduced silencing complex (RISC) and generates miRNAs. The miRNAs regulate gene expression by binding to the complementary region of the RNA and repressing translation or promoting degradation of the messenger RNA (mRNA) before it can be translated by the ribosome. \nThe pri-miRNAs are then processed in the cell nucleus by the *microprocessor complex* to pre-miRNAs, which are 70-nucleotide, stem loop structures. These pre-miRNAs are then further processed in the cytoplasm by a specific *dicer enzyme* that helps assemble an *RNA-induced silencing complex* (RISC) and generates miRNAs. \nThe miRNAs regulate gene expression by binding to the complementary region of the RNA and promoting repression of translation or degradation of the mRNA before it can be translated by the ribosome. miRNAs are believed to play an important role in normal regulation of cell function, and alterations in miRNA function have been associated with diseases such as cancer and heart disease. \nAnother type of miRNA is *small interfering RNA* (siRNA), also called *silencing RNA* or *short interfering RNA.* The siRNAs are short, double-stranded RNA molecules, comprised of 20 to 25 nucleotides, that interfere with expression of specific genes. siRNAs generally refer to synthetic miRNAs and can be administered to silence expression of specific genes. They are designed to avoid nuclear processing by the microprocessor complex and, after the siRNA enters the cytoplasm, it activates the RISC silencing complex, blocking the translation of mRNA. Because siRNAs can be tailored for any specific sequence in the gene, they can be used to block translation of any mRNA and therefore expression by any gene for which the nucleotide sequence is known. Researchers have proposed that siRNAs may become useful therapeutic tools to silence genes that contribute to the pathophysiology of diseases. \n#### TRANSLATION—FORMATION O[F](#page-38-0) PROTEINS ON THE RIBOSOMES \nWhen a molecule of mRNA comes in contact with a ribosome, it travels through the ribosome, beginning at a predetermined end of the RNA molecule specified by an appropriate sequence of RNA bases called the *chaininitiating codon.* Then, as shown in **Figure 3-9**, while the mRNA travels through the ribosome, a protein molecule is formed, a process called *translation.* Thus, the ribosome reads the codons of the mRNA in much the same way that a tape is read as it passes through the playback head of a tape recorder. Then, when a \"stop\" (or \"chainterminating\") codon slips past the ribosome, the end of a protein molecule is sig[naled, and t](#page-38-0)he p[rotein molecu](#page-40-0)le is freed into the cytoplasm. \n**Polyribosomes.** A single mRNA molecule can form protein molecules in several ribosomes at the same time because the initial end of the RNA strand can pass to a successive ribosome as it leaves the first, as shown at the bottom left in **Figure 3-9** and **Figure 3-11**. The protein molecules are in different stages of development in each ribosome. As a result, clusters of ribosomes frequently occur, with 3 to 10 ribosomes being attached to a single mRNA at the same time. These clusters are called *polyribosomes.* \nAn mRNA can cause formation of a protein molecule in any ribosome; there is no specificity of ribosomes for given types of protein. The ribosome is simply the physical manufacturing plant in which the chemical reactions take place. \n**Many Ribosomes Attach to the Endoplasmic Reticulum.** In Chapter 2, we noted that many ribosomes become attached to the endoplasmic reticulum. This attachment occurs because the initial ends of many forming protein molecules have amino acid sequences that immediately attach to specific receptor sites on the endoplasmic reticulum, causing these molecules to penetrate the \n \n**Figure 3-11.** The physical structure of the ribosomes, as well as their functional relationship to messenger RNA, transfer RNA, and the endoplasmic reticulum during the formation of protein molecules. \nreticulum wall and enter the endoplasmic reticulum matrix. This process gives a granular appearance to the portions of the reticulum where proteins are being formed and are entering the matrix of the reticulum. \n**Figure 3-11** shows the functional relationship of mRNA to the ribosomes and the manner in which the ribosomes attach to the membrane of the endoplasmic reticulum. Note the process of translation occurring in several ribosomes at the same time in response to the same strand of mRNA. Note also the newly forming polypeptide (protein) chains passing through the endoplasmic reticulum membrane into the endoplasmic matrix. \nIt should be noted that except in glandular cells, in which large amounts of protein-containing secretory vesicles are formed, most proteins synthesized by the ribosomes are released directly into the cytosol instead of into the endoplasmic reticulum. These proteins are enzymes and internal structural proteins of the cell. \n**Chemical Steps in Protein Synthesis.** Some of the chemical events that occur in the synthesis of a protein molecule are shown in **Figure 3-12**. This Fig. shows representative reactions for three separate amino acids, $AA_1$ , $AA_2$ , and $AA_{20}$ . The stages of the reactions are as follows: \n- Each amino acid is activated by a chemical process in which ATP combines with the amino acid to form an adenosine monophosphate complex with the amino acid, giving up two high-energy phosphate bonds in the process.\n- 2. The activated amino acid, having an excess of energy, then *combines with its specific tRNA to form an amino acid–tRNA complex* and, at the same time, releases the adenosine monophosphate.\n- 3. The tRNA carrying the amino acid complex then comes in contact with the mRNA molecule in the ribosome, where the anticodon of the tRNA attaches temporarily to its specific codon of the mRNA, thus lining up the amino acid in the appropriate sequence to form a protein molecule. \nThen, under the influence of the enzyme *peptidyl transferase* (one of the proteins in the ribosome), *peptide bonds* are formed between the successive amino acids, thus adding progressively to the protein chain. These \nchemical events require energy from two additional high-energy phosphate bonds, making a total of four high-energy bonds used for each amino acid added to the protein chain. Thus, the synthesis of proteins is one of the most energy-consuming processes of the cell. \n**Peptide Linkage—Combination of Amino Acids.** The successive amino acids in the protein chain combine with one another according to the typical reaction. \n$$\\begin{array}{cccccccccccccccccccccccccccccccccccc$$ \nIn this chemical reaction, a hydroxyl radical (OH-) is removed from the COOH portion of the first amino acid, and a hydrogen (H+) of the NH2 portion of the other amino acid is removed. These combine to form water, and the two reactive sites left on the two successive amino acids bond with each other, resulting in a single molecule. This process is called *peptide linkage*. As each additional amino acid is added, an additional peptide linkage is formed.\n\nMany thousand protein enzymes formed in the manner just described control essentially all the other chemical reactions that take place in cells. These enzymes promote synthesis of lipids, glycogen, purines, pyrimidines, and hundreds of other substances. We discuss many of these synthetic processes in relation to carbohydrate, lipid, and protein metabolism in Chapters 68 through 70. These substances each contribute to the various functions of the cells.\n\nFrom our discussion thus far, it is clear that the genes control both the physical and chemical functions of the cells. However, the degree of activation of respective genes must also be \n \n**Figure 3-12.** Chemical events in the formation of a protein molecule. AMP, Adenosine monophosphate; ATP, adenosine triphosphate; tRNA, transfer RNA. \ncontrolled; otherwise, some parts of the cell might overgrow or some chemical reactions might overact until they kill the cell. Each cell has powerful internal feedback control mechanisms that keep the various functional operations of the cell in step with one another. For each gene ( $\\approx 20,000-25,000$ genes in all), at least one such feedback mechanism exists. \nThere are basically two methods whereby the biochemical activities in the cell are controlled: (1) *genetic regulation,* in which the degree of activation of the genes and the formation of gene products are themselves controlled, and (2) *enzyme regulation,* in which the activity levels of already formed enzymes in the cell are controlled. \n#### **GENETIC REGULATION** \nGenetic regulation, or regulation of gene expression, covers the entire process from transcription of the genetic code in the nucleus to the formation of proteins in the cytoplasm. Regulation of gene expression provides all living organisms with the ability to respond to changes in their environment. In animals that have many different types of cells, tissues, and organs, differential regulation of gene expression also permits the different cell types in the body to each perform their specialized functions. Although a cardiac myocyte contains the same genetic code as a renal tubular epithelial cell, many genes are expressed in cardiac cells that are not expressed in renal tubular cells. The ultimate measure of gene \"expression\" is whether (and how much) of the gene products (proteins) are produced because proteins carry out cell functions specified by the genes. Regulation of gene expression can occur at any point in the pathways of transcription, RNA processing, and translation. \n**The Promoter Controls Gene Expression.** Synthesis of cellular proteins is a complex process that starts with transcription of DNA into RNA. Transcription of DNA is \n \n**Figure 3-13.** Gene transcription in eukaryotic cells. A complex arrangement of multiple clustered enhancer modules is interspersed with insulator elements, which can be located upstream or downstream of a basal promoter containing TATA box (TATA), proximal promoter elements (response elements, RE), and initiator sequences (INR). \ncontrolled by regulatory elements found in the promoter of a gene (Figure 3-13). In eukaryotes, which includes all mammals, the basal promoter consists of a sequence of bases (TATAAA) called the TATA box, the binding site for the TATA-binding protein and several other important transcription factors that are collectively referred to as the transcription factor IID complex. In addition to the transcription factor IID complex, this region is where transcription factor IIB binds to both the DNA and RNA polymerase 2 to facilitate transcription of the DNA into RNA. This basal promoter is found in all protein-coding genes, and the polymerase must bind with this basal promoter before it can begin traveling along the DNA strand to synthesize RNA. The upstream promoter is located farther upstream from the transcription start site and contains several binding sites for positive or negative transcription factors that can affect transcription through interactions with proteins bound to the basal promoter. The structure and transcription factor binding sites in the upstream promoter vary from gene to gene to give rise to the different expression patterns of genes in different tissues. \nTranscription of genes in eukaryotes is also influenced by *enhancers,* which are regions of DNA that can bind transcription factors. Enhancers can be located a great distance from the gene they act on or even on a different chromosome. They can also be located upstream or downstream of the gene that they regulate. Although enhancers may be located far from their target gene, they may be relatively close when DNA is coiled in the nucleus. It is estimated that there are more than 100,000 gene enhancer sequences in the human genome. \nIn the organization of the chromosome, it is important to separate active genes that are being transcribed from genes that are repressed. This separation can be challenging because multiple genes may be located close together on the chromosome. The separation is achieved by chromosomal *insulators.* These insulators are gene sequences that provide a barrier so that a specific gene is isolated against transcriptional influences from surrounding genes. Insulators can vary greatly in their DNA sequence and the proteins that bind to them. One way an insulator activity can be modulated is by *DNA methylation*, which is the case for the mammalian insulin-like growth factor 2 (IGF-2) gene. The mother's allele has an insulator between the enhancer and promoter of the gene that allows for the binding of a transcriptional repressor. However, the paternal DNA sequence is methylated such that the transcriptional repressor cannot bind to the insulator, and the IGF-2 gene is expressed from the paternal copy of the gene. \n#### **Other Mechanisms for Control of Transcription by the Promoter.** Variations in the basic mechanism for control of the promoter have been discovered in the past three decades. Without giving details, let us list some of them: \n- 1. A promoter is frequently controlled by transcription factors located elsewhere in the genome. That is, the regulatory gene causes the formation of a regulatory protein that in turn acts as an activator or repressor of transcription.\n- 2. Occasionally, many different promoters are controlled at the same time by the same regulatory protein. In some cases, the same regulatory protein functions as an activator for one promoter and as a repressor for another promoter.\n- 3. Some proteins are controlled not at the starting point of transcription on the DNA strand but farther along the strand. Sometimes, the control is not even at the DNA strand itself but occurs during the processing of the RNA molecules in the nucleus before they are released into the cytoplasm. Control may also occur at the level of protein formation in the cytoplasm during RNA translation by the ribosomes.\n- 4. In nucleated cells, the nuclear DNA is packaged in specific structural units, the *chromosomes.* Within \neach chromosome, the DNA is wound around small proteins called *histones,* which in turn are held tightly together in a compacted state by still other proteins. As long as the DNA is in this compacted state, it cannot function to form RNA. However, multiple control mechanisms are being discovered that can cause selected areas of chromosomes to become decompacted one part at a time, so that partial RNA transcription can occur. Even then, specific *transcriptor factor*s control the actual rate of transcription by the promoter in the chromosome. Thus, still higher orders of control are used to establish proper cell function. In addition, signals from outside the cell, such as some of the body's hormones, can activate specific chromosomal areas and specific transcription factors, therefore controlling the chemical machinery for function of the cell. \nBecause there are many thousands of different genes in each human cell, the large number of ways in which genetic activity can be controlled is not surprising. The gene control systems are especially important for controlling intracellular concentrations of amino acids, amino acid derivatives, and intermediate substrates and products of carbohydrate, lipid, and protein metabolism. \n#### **CONTROL OF INTRACELLULAR FUNCTION BY ENZYME REGULATION** \nIn addition to control of cell function by genetic regulation, cell activities are also controlled by intracellular inhibitors or activators that act directly on specific intracellular enzymes. Thus, enzyme regulation represents a second category of mechanisms whereby cellular biochemical functions can be controlled. \n**Enzyme Inhibition.** Some chemical substances formed in the cell have direct feedback effects to inhibit the specific enzyme systems that synthesize them. Almost always, the synthesized product acts on the first enzyme in a sequence, rather than on the subsequent enzymes, usually binding directly with the enzyme and causing an allosteric conformational change that inactivates it. One can readily recognize the importance of inactivating the first enzyme because this prevents buildup of intermediary products that are not used. \nEnzyme inhibition is another example of negative feedback control. It is responsible for controlling intracellular concentrations of multiple amino acids, purines, pyrimidines, vitamins, and other substances. \n**Enzyme Activation.** Enzymes that are normally inactive often can be activated when needed. An example of this phenomenon occurs when most of the ATP has been depleted in a cell. In this case, a considerable amount of cyclic adenosine monophosphate (cAMP) begins to be formed as a breakdown product of ATP. The presence of this cAMP, in turn, immediately activates the glycogensplitting enzyme phosphorylase, liberating glucose molecules that are rapidly metabolized, with their energy used for replenishment of the ATP stores. Thus, cAMP acts as an enzyme activator for the enzyme phosphorylase and thereby helps control intracellular ATP concentration. \nAnother interesting example of both enzyme inhibition and enzyme activation occurs in the formation of the purines and pyrimidines. These substances are needed by the cell in approximately equal quantities for the formation of DNA and RNA. When purines are formed, they *inhibit* the enzymes that are required for formation of additional purines. However, they *activate* the enzymes for formation of pyrimidines. Conversely, the pyrimidines inhibit their own enzymes but activate the purine enzymes. In this way, there is continual cross-talk between the synthesizing systems for these two substances, resulting in almost exactly equal amounts of the two substances in the cells at all times. \n**Summary.** There are two principal mechanisms whereby cells control proper proportions and quantities of different cellular constituents: (1) genetic regulation; and (2) enzyme regulation. The genes can be activated or inhibited, and likewise, the enzyme systems can be activated or inhibited. These regulatory mechanisms usually function as feedback control systems that continually monitor the cell's biochemical composition and make corrections as needed. However, on occasion, substances from outside the cell (especially some of the hormones discussed in this text) also control the intracellular biochemical reactions by activating or inhibiting one or more of the intracellular control systems. \n#### THE DNA–GENETIC SYSTEM CONTROLS CELL REPRODUCTION \nCell reproduction is another example of the ubiquitous role that the DNA–genetic system plays in all life processes. The genes and their regulatory mechanisms determine cell growth characteristics and when or whether cells will divide to form new cells. In this way, the allimportant genetic system controls each stage in the development of the human, from the single-cell fertilized ovum to the whole functioning body. Thus, if there is any central theme to life, it is the DNA–genetic system. \n#### **Life Cycle of the Cell** \nThe life cycle of a cell is the period from cell repr[oduction](#page-43-0) to the next cell reproduction. When mammalian cells *are not inhibited and are reproducing as rapidly as they can,* this life cycle may be as little as 10 to 30 hours. It is terminated by a series of distinct physical events called *mitosis* that cause division of the cell into two new daughter cells. The events of mitosis are shown in **Figure 3-14** and described later. The actual stage of mitosis, however, lasts for only about 30 minutes, and thus more than 95% of the life cycle of even rapidly reproducing cells is represented by the interval between mitosis, called *interphase.* \nExcept in special conditions of rapid cellular reproduction, inhibitory factors almost always slow or stop the \n \n**Figure 3-14.** Stages of cell reproduction. *A, B, C,* Prophase. *D,* Prometaphase. *E,* Metaphase. *F,* Anaphase. *G, H,* Telophase. \nuninhibited life cycle of the cell. Therefore, different cells of the body actually have life cycle periods that vary from as little as 10 hours for highly stimulated bone marrow cells to an entire lifetime of the human body for many nerve cells. \n#### **Cell Reproduction Begins with Replication of DNA** \nThe first step of cell reproduction is *replication (duplication) of all DNA in the chromosomes.* It is only after this replication has occurred that mitosis can take place. \nThe DNA begins to be duplicated 5 to 10 hours before mitosis, and the duplication is completed in 4 to 8 hours. The net result is two exact *replicas* of all DNA. These replicas become the DNA in the two new daughter cells that will be formed at mitosis. After replication of the DNA, there is another period of 1 to 2 hours before mitosis begins abruptly. Even during this period, preliminary changes that will lead to the mitotic process are beginning to take place. \n**DNA Replication.** DNA is replicated in much the same way that RNA is transcribed from DNA, except for a few important differences: \n1. Both strands of the DNA in each chromosome are replicated, not just one of them. \n \nFigure 3-15. DNA replication, showing the replication fork and leading and lagging strands of DNA. \n- 2. Both entire strands of the DNA helix are replicated from end to end, rather than small portions of them, as occurs in the transcription of RNA.\n- 3. Multiple enzymes called *DNA polymerase*, which is comparable to RNA polymerase, are essential for replicating DNA. DNA polymerase attaches to and moves along the DNA template strand, adding nucleotides in the 5′ to 3′ direction. Another enzyme, *DNA ligase*, causes bonding of successive DNA nucleotides to one another, using high-energy phosphate bonds to energize these attachments.\n- 4. Replication fork formation. Before DNA can be replicated, the double-stranded molecule must be \"unzipped\" into two single strands (Figure 3-15). Because the DNA helixes in each chromosome are approximately 6 centimeters in length and have millions of helical turns, it would be impossible for the two newly formed DNA helixes to uncoil from each other were it not for some special mechanism. This uncoiling is achieved by DNA helicase enzymes that break the hydrogen bonding between the base pairs of the DNA, permitting the two strands to separate into a Y shape known as the replication fork, the area that will be the template for replication to begin. \nDNA is directional in both strands, signified by a 5′ and 3′ end (see **Figure 3-15**). Replication progresses only in the 5′ to 3′ direction. At the replication fork one strand, the *leading strand*, is oriented in the 3′ to 5′ direction, toward the replication fork, while the *lagging strand* is oriented 5′ to 3′, away from the replication fork. Because of their different orientations, the two strands are replicated differently. \n5. *Primer binding*. Once the DNA strands have been separated, a short piece of RNA called an *RNA primer* binds to the 3' end of the leading strand. Primers are generated by the enzyme *DNA primase*. \n- Primers always bind as the starting point for DNA replication.\n- 6. Elongation. DNA polymerases are responsible for creating the new strand by a process called *elongation*. Because replication proceeds in the 5′ to 3′ direction on the leading strand, the newly formed strand is continuous. The lagging strand begins replication by binding with multiple primers that are only several bases apart. DNA polymerase then adds pieces of DNA, called *Okazaki fragments*, to the strand between primers. This process of replication is discontinuous because the newly created Okazaki fragments are not yet connected. An enzyme, *DNA ligase*, joins the Okazaki fragments to form a single unified strand.\n- 7. Termination. After the continuous and discontinuous strands are both formed, the enzyme exonuclease removes the RNA primers from the original strands, and the primers are replaced with appropriate bases. Another exonuclease \"proofreads\" the newly formed DNA, checking and clipping off any mismatched or unpaired residues. \nAnother enzyme, *topoisomerase*, can transiently break the phosphodiester bond in the backbone of the DNA strand to prevent the DNA in front of the replication fork from being overwound. This reaction is reversible, and the phosphodiester bond reforms as the topoisomerase leaves. \nOnce completed, the parent strand and its complementary DNA strand coils into the double helix shape. The process of replication therefore produces two DNA molecules, each with one strand from the parent DNA and one new strand. For this reason, DNA replication is often described as *semiconservative*; half of the chain is part of the original DNA molecule and half is brand new. \n**DNA Repair, DNA \"Proofreading,\" and \"Mutation.\"**During the hour or so between DNA replication and \nthe beginning of mitosis, there is a period of active repair and \"proofreading\" of the DNA strands. Wherever inappropriate DNA nucleotides have been matched up with the nucleotides of the original template strand, special enzymes cut out the defective areas and replace them with appropriate complementary nucleotides. This repair process, which is achieved by the same DNA polymerases and DNA ligases that are used in replication, is referred to as *DNA proofreading.* \nBecause of repair and proofreading, mistakes are rarely made in the DNA replication process. When a mistake is made, it is called a *mutation.* The mutation may cause formation of some abnormal protein in the cell rather than a needed protein, which may lead to abnormal cellular function and sometimes even cell death. Given that many thousands of genes exist in the human genome, and that the period from one human generation to another is about 30 years, one would expect as many as 10 or many more mutations in the passage of the genome from parent to offspring. As a further protection, however, each human genome is represented by two separate sets of chromosomes, one derived from each parent, with almost identical genes. Therefore, one functional gene of each pair is almost always available to the child, despite mutations. \n#### **CHROMOSOMES AND THEIR REPLICATION** \nThe DNA helixes of the nucleus are packaged in chromosomes. The human cell contains 46 chromosomes arranged in 23 pairs. Most of the genes in the two chromosomes of each pair are identical or almost identical to each other, so it is usually stated that the different genes also exist in pairs, although occasionally this is not the case. \nIn addition to DNA, there is a large amount of protein in the chromosome, composed mainly of many small molecules of electropositively charged *histones.* The histones are organized into vast numbers of small, bobbinlike cores. Small segments of each DNA helix are coiled sequentially around one core after another. \nThe histone cores play an important role in regulation of DNA activity because as long as the DNA is packaged tightly, it cannot function as a template for formation of RNA or replication of new DNA. Furthermore, some of the regulatory proteins *decondense* the histone packaging of the DNA and allow small segments at a time to form RNA. \nSeveral nonhistone proteins are also major components of chromosomes, functioning as chromosomal structural proteins and, in connection with the genetic regulatory machinery, as activators, inhibitors, and enzymes. \nReplication of the chromosomes in their entirety occurs during the next few minutes after replication of the DNA helixes has been completed; the new DNA helixes collect new protein molecules as needed. The two newly formed chromosomes remain attached to each other (until time for mitosis) at a point called the *centromere* located near their center. These duplicated but still attached chromosomes are called *chromatids.* \n#### **CELL MITOSIS** \nThe actual process whereby the cell splits into two new cells is called *mitosis.* Once each chromosome has been replicated to form the two chromatids, mitos[is follows](#page-43-0) automatically within 1 or 2 hours in many cells. \n**Mitotic Apparatus: Function of the Centrioles.** One of the first events of mitosis takes place in the cytoplasm in or around the small structures called *centrioles* during the latter part of interphase*.* As shown in **Figure 3-**14, two pairs of centrioles lie close to each other near one pole of the nucleus. These centrioles, like the DNA and chromosomes, are also replicated during interphase, usually shortly before replication of the DNA. Each centriole is a small cylindrical body about 0.4 micrometer long and about 0.15 micrometer in diameter, consisting mainly of nine parallel tubular structures arranged in the form of a cylinder. The two centrioles of each pair lie at right angles to each other. Each pair of centrioles, along with attached *pericentriolar material,* is called a *centrosome.* \nShortly before mitosis takes place, the two pairs of centrioles begin to move apart from each other. This movement is caused by polymerization of protein microtubules growing between the respective centriole pairs and actually pushing them apart. At the same time, other microtubules grow radially away from each of the centriole pairs, forming a spiny star called the *aster,* in each end of the cell. Some of the spines of the aster penetrate the nuclear membrane and h[elp separate t](#page-43-0)he two sets of chromatids during mitosis. The complex of microtubules extending between the two new centriole pairs is called the *spindle,* and the entire set of microtubules plus the two pairs of centrioles is called the *mitotic apparatus.* \n**Prophase.** [The fi](#page-43-0)rst stage of mitosis, called *prophase,* is shown in **Figure 3-14***A, B,* and *C.* While the spindle is forming, the chromosomes of the nucleus (which in interphase consist of loosely coiled strands) become condensed into well-defined chromosomes. \n**Prometaphase.** During the prometaphase stage (see **Figure 3-14***D*), the growing microtubular spines of the aster fragment the nuclear envelope. At the same time, multiple mic[rotubu](#page-43-0)les from the aster attach to the chromatids at the centromeres, where the paired chromatids are still bound to each other. The tubules then pull one chromatid of each pair toward one cellular pole and its partner toward the opposite pole. \n**Metaphase.** During the metaphase stage (see **Figure 3-14***E*), the two asters of the mitotic apparatus are pushed farther apart. This pushing is believed to occur because the microtubular spines from the two asters, where they interdigitate with each other to form the mitotic spindle, push each other away. Minute contractile protein molecules called *\"molecular motors,\"* which may be composed of the muscle protein *actin,* extend between the respective spines a[nd, us](#page-43-0)ing a stepping action as in muscle, actively slide the spines in a reverse direction along each other. Simultaneously, the chromatids are pulled tightly by their attached microtubules to the very center of the cell, lining up to form the *equatorial plate* of the mitotic spindle. \n**Anaphase.** During the anaphase stage (see **Figure 3-14***F*), the two chromatids of each chromosome are pulled apart at the centromere. All 46 pairs of c[hromatids](#page-43-0) are sepa[rat](#page-43-0)ed, forming two separate sets of 46 *daughter chromosomes.* One of these sets is pulled toward one mitotic aster, and the other is pulled toward the other aster, as the two respective poles of the dividing cell are pushed still farther apart. \n**Telophase.** In the telophase stage (see **Figure 3-14***G* and *H*), the two sets of daughter chromosomes are pushed completely apart. Then, the mitotic apparatus dissipates, and a new nuclear membrane develops around each set of chromosomes. This membrane is formed from portions of the endoplasmic reticulum that are already present in the cytoplasm. Shortly thereafter, the cell pinches in two, midway between the two nuclei. This pinching is caused by the formation of a contractile ring of *microfilaments* composed of *actin* and probably *myosin* (the two contractile proteins of muscle) at the juncture of the newly developing cells that pinches them off from each other. \n#### **CONTROL OF CELL GROWTH AND CELL REPRODUCTION** \nSome cells grow and reproduce all the time, such as the blood-forming cells of the bone marrow, the germinal layers of the skin, and the epithelium of the gut. Many other cells, however, such as smooth muscle cells, may not reproduce for many years. A few cells, such as the neurons and most striated muscle cells, do not reproduce during the entire life of a person, except during the original period of fetal life. \nIn certain tissues, an insufficiency of some types of cells causes them to grow and reproduce rapidly until appropriate numbers of these cells are again available. For example, in some young animals, seven-eighths of the liver can be removed surgically, and the cells of the remaining one-eighth will grow and divide until the liver mass returns to almost normal. The same phenomenon occurs for many glandular cells and most cells of the bone marrow, subcutaneous tissue, intestinal epithelium, and almost any other tissue except highly differentiated cells such as nerve and muscle cells. \nThe mechanisms that maintain proper numbers of the different types of cells in the body are still poorly understood. However, experiments have shown at least three ways in which growth can be controlled. First, growth often is controlled by *growth factors* that come from other parts of the body. Some of these growth factors circulate in the blood, but others originate in adjacent tissues. For [exam](#page-43-0)ple, the epithelial cells of some glands, such as the pancreas, fail to grow without a growth factor from the underlying connective tissue of the gland. Second, most normal cells stop growing when they have run out of space for growth. This phenomenon occurs when cells are grown in tissue culture; the cells grow until they contact a solid object, and then growth stops. Third, cells grown in tissue culture often stop growing when minute am[ounts](#page-46-0) of their own secretions are allowed to collect in the culture medium. This mechanism, too, could provide a means for negative feedback control of growth. \n**Telomeres Prevent the Degradation of Chromosomes.** A *telomere* is a region of repetitive nucleotide sequences located at each end of a chromatid (**Figure 3-16**). Telomeres serve as protective caps that prevent the chromosome from deterioration during cell division. During cell division, a short piece of \"primer\" RNA attaches to the DNA strand to start the replication. However, because the primer does not attach at the very end of the DNA strand, the copy is missing a small section of the DNA. With each cell division, the copied DNA loses additional nucleotides from the telomere region. The nucleotide sequences provided by the telomeres therefore prevent the degradation of genes near the ends of chromosomes. Without telomeres, the genomes would progressively lose information and be truncated after each cell division. Thus, the telomeres can be considered to be disposable chromosomal buffers that help maintain stability of the genes but are gradually consumed during repeated cell divisions. \n \n**Figure 3-16.** Control of cell replication by telomeres and telomerase. The cells' chromosomes are capped by telomeres, which, in the absence of telomerase activity, shorten with each cell division until the cell stops replicating. Therefore, most cells of the body cannot replicate indefinitely. In cancer cells, telomerase is activated, and telomere length is maintained so that the cells continue to replicate themselves uncontrollably. \nEach time a cell divides, an average person loses 30 to 200 base pairs from the ends of that cell's telomeres. In human blood cells, the length of telomeres ranges from 8000 base pairs at birth to as low as 1500 in older people. Eventually, when the telomeres shorten to a critical length, the chromosomes become unstable, and the cells die. This process of telomere shortening is believed to be an important reason for some of the physiological changes associated with aging. Telomere erosion can also occur as a result of diseases, especially those associated with oxidative stress and inflammation. \nIn some cells, such as stem cells of the bone marrow or skin that must be replenished throughout life, or germ cells in the ovaries and testes, the enzyme *telomerase* adds bases to the ends of the telomeres so that many more generations of cells can be produced. However, telomerase activity is usually low in most cells of the body, and after many generations the descendent cells will inherit defective chromoso[mes, become](#page-46-0) *senescent,* and cease dividing. This process of telomere shortening is important in regulating cell proliferation and maintaining gene stability. In cancer cells, telomerase activity is abnormally activated so that telomere length is maintained, making it possible for the cells to replicate over and over again uncontrollably (see **Figure 3-16**). Some scientists have therefore proposed that telomere shortening protects us from cancer and other proliferative diseases. \n**Regulation of Cell Size.** Cell size is determined almost entirely by the amount of functioning DNA in the nucleus. If replication of the DNA does not occur, the cell grows to a certain size and thereafter remains at that size. Conversely, use of the chemical *colchicine* makes it possible to prevent formation of the mitotic spindle and therefore prevent mitosis, even though replication of the DNA continues. In this event, the nucleus contains far greater quantities of DNA than it normally does, and the cell grows proportionately larger. It is assumed that this cell growth results from increased production of RNA and cell proteins, which, in turn, cause the cell to grow larger. \n#### CELL DIFFERENTIATION \nA special characteristic of cell growth and cell division is *cell differentiation,* which refers to changes in the physical and functional properties of cells as they proliferate in the embryo to form the different body structures and organs. The following description of an especially interesting experiment helps explain these processes. \nWhen the nucleus from an intestinal mucosal cell of a frog is surgically implanted into a frog ovum from which the original ovum nucleus was removed, the result is often the formation of a normal frog. This experiment demonstrates that even the intestinal mucosal cell, which is a well-differentiated cell, carries all the necessary genetic information for development of all structures required in the frog's body. \nTherefore, it has become clear that differentiation results not from loss of genes but from selective repression of different gene promoters. In fact, electron micrographs suggest that some segments of DNA helixes that are wound around histone cores become so condensed that they no longer uncoil to form RNA molecules. One explanation for this is as follows. It has been supposed that the cellular genome begins at a certain stage of cell differentiation to produce a regulatory *protein* that forever after represses a select group of genes. Therefore, the repressed genes never function again. Regardless of the mechanism, mature human cells each produce a maximum of about 8000 to 10,000 proteins rather than the potential 20,000 to 25,000 or more that would be produced if all genes were active. \nEmbryological experiments have shown that certain cells in an embryo control differentiation of adjacent cells. For example, the *primordial chordamesoderm* is called the *primary organizer* of the embryo because it forms a focus around which the remainder of the embryo develops. It differentiates into a *mesodermal axis* that contains segmentally arranged *somites* and, as a result of *inductions* in the surrounding tissues, causes the formation of essentially all the organs of the body. \nAnother instance of induction occurs when the developing eye vesicles come into contact with the ectoderm of the head and cause the ectoderm to thicken into a lens plate that folds inward to form the lens of the eye. Therefore, a large share of the embryo develops as a result of such inductions, with one part of the body affecting another part, and this part affecting still other parts. \nThus, although our understanding of cell differentiation is still hazy, we are aware of many control mechanisms whereby differentiation *could* occur. \n#### APOPTOSIS—PROGRAMMED CELL DEATH \nThe many trillions of the body's cells are members of a highly organized community in which the total number of cells is regulated not only by controlling the rate of cell division, but also by controlling the rate of cell death. When cells are no longer needed or become a threat to the organism, they undergo a suicidal *programmed cell death,* or *apoptosis.* This process involves a specific proteolytic cascade that causes the cell to shrink and condense, disassemble its cytoskeleton, and alter its cell surface so that a neighboring phagocytic cell, such as a macrophage, can attach to the cell membrane and digest the cell. \nIn contrast to programmed death, cells that die as a result of an acute injury usually swell and burst due to loss of cell membrane integrity, a process called cell *necrosis.* Necrotic cells may spill their contents, causing inflammation and injury to neighboring cells. Apoptosis, however, is an orderly cell death that results in disassembly and phagocytosis of the cell before any leakage of its contents occurs, and neighboring cells usually remain healthy. \nApoptosis is initiated by activation of a family of proteases called *caspases*, which are enzymes that are synthesized and stored in the cell as inactive *procaspases*. The mechanisms of activation of caspases are complex but, once activated, the enzymes cleave and activate other procaspases, triggering a cascade that rapidly breaks down proteins within the cell. The cell thus dismantles itself, and its remains are rapidly digested by neighboring phagocytic cells. \nA tremendous amount of apoptosis occurs in tissues that are being remodeled during development. Even in adult humans, billions of cells die each hour in tissues such as the intestine and bone marrow and are replaced by new cells. Programmed cell death, however, is normally balanced by formation of new cells in healthy adults. Otherwise, the body's tissues would shrink or grow excessively. Abnormalities of apoptosis may play a key role in neurodegenerative diseases such as Alzheimer disease, as well as in cancer and autoimmune disorders. Some drugs that have been used successfully for chemotherapy appear to induce apoptosis in cancer cells. \n#### CANCER \nCancer may be caused by *mutation* or by some other *abnormal activation* of cellular genes that control cell growth and cell mitosis. *Proto-oncogenes* are normal genes that code for various proteins that control cell adhesion, growth and division. If mutated or excessively activated, proto-oncogenes can become abnormally functioning *oncogenes* capable of causing cancer*.* As many as 100 different oncogenes have been discovered in human cancers. \nAlso present in all cells are *antioncogenes,* also called *tumor suppressor genes*, which suppress the activation of specific oncogenes. Therefore, loss or inactivation of antioncogenes can allow activation of oncogenes that lead to cancer. \nFor several reasons, only a minute fraction of the cells that mutate in the body ever lead to cancer: \n- • First, most mutated cells have less survival capability than normal cells, and they simply die.\n- • Second, only a few of the mutated cells that survive become cancerous because most mutated cells still have normal feedback controls that prevent excessive growth.\n- • Third, cells that are potentially cancerous are often destroyed by the body's immune system before they grow into a cancer. \nMost mutated cells form abnormal proteins within their cell bodies because of their altered genes, and these proteins activate the body's immune system, causing it to form antibodies or sensitized lymphocytes that react against the cancerous cells, destroying them. In people whose immune systems have been suppressed, such as in persons taking immunosuppressant drugs after kidney or heart transplantation, the probability that a cancer will develop is multiplied as much as fivefold. \n• Fourth, the simultaneous presence of several different activated oncogenes is usually required to cause a cancer. For example, one such gene might promote rapid reproduction of a cell line, but no cancer occurs because another mutant gene is not present simultaneously to form the needed blood vessels. \nWhat is it that causes the altered genes? Considering that many trillions of new cells are formed each year in humans, a better question might be to ask why all of us do not develop millions or billions of mutant cancerous cells. The answer is the incredible precision with which DNA chromosomal strands are replicated in each cell before mitosis can take place, along with the proofreading process that cuts and repairs any abnormal DNA strand before the mitotic process is allowed to proceed. Yet, despite these inherited cellular precautions, probably one newly formed cell in every few million still has significant mutant characteristics. \nThus, chance alone is all that is required for mutations to take place, so we can suppose that a large number of cancers are merely the result of an unlucky occurrence. However, the probability of mutations can be greatly increased when a person is exposed to certain chemical, physical, or biological factors, including the following: \n- 1. *Ionizing radiation,* such as x-rays, gamma rays, particle radiation from radioactive substances, and even ultraviolet light, can predispose individuals to cancer. Ions formed in tissue cells under the influence of such radiation are highly reactive and can rupture DNA strands, causing many mutations.\n- 2. *Chemical substances* of certain types may also cause mutations. It was discovered long ago that various aniline dye derivatives are likely to cause cancer, and thus workers in chemical plants producing such substances, if unprotected, have a special predisposition to cancer. Chemical substances that can cause mutation are called *carcinogens.* The carcinogens that currently cause the greatest number of deaths are those in cigarette smoke. These carcinogens cause over 30% of all cancer deaths and at least 85% of lung cancer deaths.\n- 3. *Physical irritants* can also lead to cancer, such as continued abrasion of the linings of the intestinal tract by some types of food. The damage to the tissues leads to rapid mitotic replacement of the cells; the more rapid the mitosis, the greater the chance for mutation.\n- 4. *Hereditary tendency* to cancer occurs in some families. This hereditary tendency results from the fact that most cancers require not one mutation but two or more mutations before cancer occurs. In families that are particularly predisposed to cancer, it is presumed that one or more cancerous genes are already \n- mutated in the inherited genome. Therefore, far fewer additional mutations must take place in such family members before a cancer begins to grow.\n- 5. *Certain types of oncoviruses* can cause various types of cancer. Some examples of viruses associated with cancers in humans include *human papilloma virus* (HPV), *hepatitis B and hepatitis C virus,* Epstein-Barr virus, human immunodeficiency virus (HIV), human T-cell leukemia virus, Kaposi sarcoma–associated herpes virus (KSHV), and Merkel cell polyomavirus. Although the mechanisms whereby oncoviruses cause cancer are not fully understood, there are at least two potential ways. In the case of DNA viruses, the DNA strand of the virus can insert itself directly into one of the chromosomes, thereby causing a mutation that leads to cancer. In the case of RNA viruses, some of these viruses carry with them an enzyme called *reverse transcriptase* that causes DNA to be transcribed from the RNA. The transcribed DNA then inserts itself into the animal cell genome, leading to cancer. \n**Invasive Characteristic of the Cancer Cell.** The major differences between a cancer cell and a normal cell are as follows: \n- 1. The cancer cell does not respect usual cellular growth limits because these cells presumably do not require all the same growth factors that are necessary to cause growth of normal cells.\n- 2. Cancer cells are often far less adhesive to one another than are normal cells. Therefore, they tend to wander through the tissues, enter the blood stream, and be transported all through the body, where they form nidi for numerous new cancerous growths.\n- 3. Some cancers also produce *angiogenic factors* that cause many new blood vessels to grow into the cancer, thus supplying the nutrients required for cancer growth. \n**Why Do Cancer Cells Kill?** Cancer tissue competes with normal tissues for nutrients. Because cancer cells continue to proliferate indefinitely, with their numbers multiplying every day, cancer cells soon demand essentially all the nutrition available to the body or to an essential part of the body. As a result, normal tissues gradually sustain nutritive death. \nSome cancers cause disruption of vital organ functions. For example, a lung cancer might replace healthy tissue to the extent that the lungs cannot absorb enough oxygen to maintain tissues in the rest of the body. \n#### Bibliography \nAlberts B, Johnson A, Lewis J, et al: Molecular Biology of the Cell, 6th ed. New York: Garland Science 2014. \n- Armanios M: Telomeres and age-related disease: how telomere biology informs clinical paradigms. J Clin Invest 123:996, 2013.\n- Bickmore WA, van Steensel B: Genome architecture: domain organization of interphase chromosomes. Cell 152:1270, 2013.\n- Calcinotto A, Kohli J, Zagato E, Pellegrini L, Demaria M, Alimonti A: Cellular senescence: aging, cancer, and injury. Physiol Rev 99:1047-1078, 2019.\n- Clift D, Schuh M: Restarting life: fertilization and the transition from meiosis to mitosis. Nat Rev Mol Cell Biol 14:549, 2013.\n- Coppola CJ, C Ramaker R, Mendenhall EM: Identification and function of enhancers in the human genome. Hum Mol Genet 25(R2):R190-R197, 2016.\n- Feinberg AP: The key role of epigenetics in human disease prevention and mitigation. N Engl J Med 378:1323-1334, 2018.\n- Fyodorov DV, Zhou BR, Skoultchi AI, Bai Y: Emerging roles of linker histones in regulating chromatin structure and function. Nat Rev Mol Cell Biol 19:192-206, 2018.\n- Haberle V, Stark A: Eukaryotic core promoters and the functional basis of transcription initiation. Nat Rev Mol Cell Biol 19:621-637, 2018.\n- Kaushik S, Cuervo AM: The coming of age of chaperone-mediated autophagy. Nat Rev Mol Cell Biol 19:365-381, 2018.\n- Krump NA, You J: Molecular mechanisms of viral oncogenesis in humans. Nat Rev Microbiol 16:684-698, 2018.\n- Leidal AM, Levine B, Debnath J: Autophagy and the cell biology of age-related disease. Nat Cell Biol 20:1338-1348, 2018.\n- Maciejowski J, de Lange T: Telomeres in cancer: tumour suppression and genome instability. Nat Rev Mol Cell Biol 18:175-186, 2017.\n- McKinley KL, Cheeseman IM: The molecular basis for centromere identity and function. Nat Rev Mol Cell Biol 17:16-29, 2016.\n- Monk D, Mackay DJG, Eggermann T, Maher ER, Riccio A: Genomic imprinting disorders: lessons on how genome, epigenome and environment interact. Nat Rev Genet 10:235, 2019.\n- Müller S, Almouzni G: Chromatin dynamics during the cell cycle at centromeres. Nat Rev Genet 18:192-208, 2017.\n- Nigg EA, Holland AJ: Once and only once: mechanisms of centriole duplication and their deregulation in disease. Nat Rev Mol Cell Biol 19:297-312, 2018.\n- Palozola KC, Lerner J, Zaret KS: A changing paradigm of transcriptional memory propagation through mitosis. Nat Rev Mol Cell Biol 20:55-64, 2019.\n- Perez MF, Lehner B: Intergenerational and transgenerational epigenetic inheritance in animals. Nat Cell Biol 21:143, 2019.\n- Prosser SL, Pelletier L: Mitotic spindle assembly in animal cells: a fine balancing act. Nat Rev Mol Cell Biol 18:187-201, 2017.\n- Schmid M, Jensen TH. Controlling nuclear RNA levels. Nat Rev Genet 19:518-529, 2018.\n- Treiber T, Treiber N, Meister G: Regulation of microRNA biogenesis and its crosstalk with other cellular pathways. Nat Rev Mol Cell Biol 20:5-20, 2019. \n\n\nFigure 4-1 lists the approximate concentrations of important electrolytes and other substances in the *extracellular fluid* and *intracellular fluid*. Note that the extracellular fluid contains a large amount of *sodium* but only a small amount of *potassium*. The opposite is true of the intracellular fluid. Also, the extracellular fluid contains a large amount of *chloride* ions, whereas the intracellular fluid contains very little of these ions. However, the concentrations of *phosphates* and *proteins* in the intracellular fluid are considerably greater than those in the extracellular fluid. These differences are extremely important to the life of the cell. The purpose of this chapter is to explain how the differences are brought about by the cell membrane transport mechanisms. \n \n**Figure 4-1.** Chemical compositions of extracellular and intracellular fluids. The question marks indicate that the precise values for intracellular fluid are unknown. The *red line* indicates the cell membrane.\n\nThe structure of the membrane covering the outside of every cell of the body is discussed in Chapter 2 and illustrated in Figure 2-3 and Figure 4-2. This membrane consists almost entirely of a *lipid bilayer* with large numbers of protein molecules in the lipid, many of which penetrate all the way through the membrane. \nThe lipid bilayer is not miscible with the extracellular fluid or the intracellular fluid. Therefore, it constitutes a barrier against movement of water molecules and water-soluble substances between the extracellular and intracellular fluid compartments. However, as shown in **Figure 4-2** by the leftmost arrow, lipid-soluble substances can diffuse directly through the lipid substance. \nThe membrane protein molecules interrupt the continuity of the lipid bilayer, constituting an alternative pathway through the cell membrane. Many of these penetrating proteins can function as *transport proteins*. Some proteins have watery spaces all the way through the molecule and allow free movement of water, as well as selected ions or molecules; these proteins are called *channel proteins*. Other proteins, called *carrier proteins*, bind with molecules or ions that are to be transported, and conformational changes in the protein molecules then move the substances through the interstices of the protein to the \n \n**Figure 4-2.** Transport pathways through the cell membrane and the basic mechanisms of transport. \n \n**Figure 4-3.** Diffusion of a fluid molecule during one thousandth of a second. \nother side of the membrane. Channel proteins and carrier proteins are usually selective for the types of molecules or ions that are allowed to cross the membrane. \n**\"Diffusion\" Versus \"Active Transport.\"** Transport through the cell membrane, either directly through the lipid bilayer or through the proteins, occurs via one of two basic processes, *diffusion* or *active transport.* \nAlthough many variations of these basic mechanisms exist, *diffusion* means random molecular movement of substances molecule by molecule, either through intermolecular spaces in the membrane or in combination with a carrier protein. The energy that causes diffusion is the energy of the normal kinetic motion of matter. \nIn contrast, *active transport* means movement of ions or other substances across the membrane in combination with a carrier protein in such a way that the carrier protein causes the substance to move against an energy gradient, such as from a low-concentration state to a highconcentration state. This movement requires an additional source of energy besides kinetic energy. A more detailed explanation of the basic physics and physical chemistry of these two processes is provided later in this chapter. \n#### DIFFUSION \nAll molecules and ions in the body fluids, including water molecules and dissolved substances, are in constant motion, with each particle moving in its separate way. The motion of these particles is what physicists call \"heat\" the greater the motion, the higher the temperature—and the motion never ceases, except at absolute zero temperature. When a moving molecule, [A, approac](#page-51-0)hes a stationary molecule, B, the electrostatic and other nuclear forces of molecule A repel molecule B, transferring some of the energy of motion of molecule A to molecule B. Consequently, molecule B gains kinetic energy of motion, whereas molecule A slows down, losing some of its kinetic energy. As shown in **Figure 4-3**, a single molecule in a solution bounces among the other molecules—first in one direction, then another, then another, and so forth randomly bouncing thousands of times each second. This continual movement of molecules among one another in liquids or gases is called *diffusion.* \nIons diffuse in the same manner as whole molecules, and even suspended colloid particles diffuse in a similar manner, except that the colloids diffuse far less rapidly than molecular substances because of their large size. \n#### **DIFFUSION THROUGH THE CELL MEMBRANE** \nDiffusion through the cell membrane is divided into two subtypes, called *simple diffusion* and *facilitated diffusion.* Simple diffusion means that kinetic movement of molecules or ions occurs through a membrane opening or through intermolecular spaces without interaction with carrier proteins in the membrane. The rate of diffusion is determined by the amount of substance available, the velocity of kinetic motion, and the number and sizes of openings in the membrane through which the molecules or ions can move. \nFacilitated diffusion requires interaction of a carrier protein. The carrier protein aids passage of molecules or ions through the membrane by binding chemically with them and shuttlin[g them thro](#page-50-0)ugh the membrane in this form. \nSimple diffusion can occur through the cell membrane by two pathways: (1) through the interstices of the lipid bilayer if the diffusing substance is lipid-soluble; and (2) through watery channels that penetrate all the way through some of the large transport proteins, as shown to the left in **Figure 4-2**. \n**Diffusion of Lipid-Soluble Substances Through the Lipid Bilayer.** The *lipid solubility* of a substance is an important factor for determining how rapidly it diffuses through the lipid bilayer. For example, the lipid solubilities of oxygen, nitrogen, carbon dioxide, and alcohols are high, and all these substances can dissolve directly in the lipid bilayer and diffuse through the cell membrane in the same manner that diffusion of water solutes occurs in a watery solution. The rate of diffusion of each of these substances through the membrane is directly proportional to its lipid solubility. Especially large amounts of oxygen can be transported in this way; therefore, oxygen can be delivered to the interior of the cell almost as though the cell membrane did not exist. \n**Diffusion of Water and Other Lipid-Insoluble Molecules Through Protein Channels.** Even though water is highly insoluble in the membrane lipids, it readily passes through channels in protein molecules that penetrate all the way through the membrane. Many of the body's cell membranes contain protein \"pores\" called *aquaporins* that selectively permit rapid passage of water through the membrane. The aquaporins are highly specialized, and there are at least 13 different types in various cells of mammals. \nThe rapidity with which water molecules can diffuse through most cell membranes is astounding. For example, the total amount of water that diffuses in each direction through the red blood cell membrane during each second is about 100 times as great as the volume of the red blood cell. \nOther lipid-insoluble molecules can pass through the protein pore channels in the same way as water molecules if they are water-soluble and small enough. However, as they become larger, their penetration falls off rapidly. For example, the diameter of the urea molecule is only 20% greater than that of water, yet its penetration through the cell membrane pores is about 1000 times less than that of water. Even so, given the astonishing rate of water penetration, this amount of urea penetration still allows rapid transport of urea through the membrane within minutes. \n#### **DIFFUSION THROUGH PROTEIN PORES AND CHANNELS—SELECTIVE PERMEABILITY AND \"GATING\" OF CHANNELS** \nComputerized three-dimensional reconstructions of protein pores and channels have demonstrated tubular pathways all the way from the extracellular to the intracellular fluid. Therefore, substances can move by simple diffusion directly along these pores and channels from one side of the membrane to the other. \nPores are composed of integral cell membrane proteins that form open tubes through the membrane and are always open. However, the diameter of a pore and its electrical charges provide selectivity that permits only certain molecules to pass through. For example, *aquaporins* permit rapid passage of water through cell membranes but exclude other molecules. Aquaporins have a narrow pore that permits water molecules to diffuse through the membrane in single file. The pore is too narrow to permit passage of any hydrated ions. As discussed in Chapters 28 and 76, the density of some aquaporins (e.g., aquaporin-2) in cell membranes is not static but is altered in different physiological conditions. \nThe protein channels are distinguished by two important characteristics: (1) they are often *selectively permeable* to certain substances; and (2) many of the channels can be opened or closed by *gates* that are regulated by electrical signals *(voltage-gated channels)* or chemicals that bind to the channel proteins *(ligand-gated channels).* Thus, ion channels are flexible dynamic structures, and subtle conformational changes influence gating and ion selectivity. \n**Selective Permeability of Protein Channels.** Many protein channels are highly selective for transport of one or more specific ions or molecules. This selectivity results from specific characteristics of the channel, such as its diameter, shape, and the nature of the electrical charges and chemical bonds along its inside surfaces. \n*Potassium channels* permit passage of potassium ions across the cell membrane about 1000 times more readily than they permit passage of sodium ions. This high degree of selectivity cannot be explained entirely by the \n \n**Figure 4-4.** The structure of a potassium channel. The channel is composed of four subunits (only two of which are shown), each with two transmembrane helices. A narrow selectivity filter is formed from the pore loops, and carbonyl oxygens line the walls of the selectivity filter, forming sites for transiently binding dehydrated potassium ions. The interaction of the potassium ions with carbonyl oxygens causes the potassium ions to shed their bound water molecules, permitting the dehydrated potassium ions to pass through the pore. \nmolecular diameters of the ions because potassium ions are slightly larger than sodium ions. Using x-ray crystallography, potassium channels were found to have a *tetrameric structure* consisting of four identical protein subunits surrounding a central pore (**Figure 4-4**). At the top of the channel pore are *pore loops* that form a narrow *selectivity filter*. Lining the selectivity filter are *carbonyl oxygens.* When hydrated potassium ions enter the selectivity filter, they interact with the carbonyl oxygens and shed most of their bound water molecules, permitting the dehydrated potassium ions to pass through the channel. The carbonyl oxygens are too far apart, however, to enable them to interact closely with the smaller sodium ions, which are therefore effectively excluded by the selectivity filter from passing through the pore. \nDifferent selectivity filters for the various ion channels are believed to determine, in large part, the specificity of various channels for cations or anions or for particular ions, such as sodium (Na+), potassium (K+), and calcium (Ca2+), that gain access to the channels. \nOne of the most important of the protein channels, the *sodium channel,* is only 0.3 to 0.5 nanometer in diameter, but the ability of sodium channels to discriminate sodium ions among other competing ions in the surrounding fluids is crucial for proper cellular function. \n \n**Figure 4-5.** Transport of sodium and potassium ions through protein channels. Also shown are conformational changes in the protein molecules to open or close the \"gates\" guarding the channels. \nThe narrowest part of the sodium channel's open pore, the *selectivity filter*, is lined with *strongly negatively charged* amino acid residues, as shown in the top panel of **Figure 4-5**. These strong negative charges can pull small *dehydrated* sodium ions away from their hydrating water molecules into these channels, although the ions do not need to be fully dehydrated to pass through the channels. Once in the channel, the sodium ions diffuse in either direction according to the usual laws of diffusion. Thus, the sodium channel is highly selective for passage of sodium ions. \n**Gating of Protein Channels.** Gating of protein channels provides a means of controlling ion permeability of the channels. This mechanism is shown in both panels of **Figure 4-5** for selective gating of sodium and potassium ions. Some of the gates are thought to be gatelike extensions of the transport protein molecule, which can close the opening of the channel or can be lifted away from the opening by a conformational change in the shape of the protein molecule. \nThe opening and closing of gates are controlled in two principal ways: \n1. Voltage gating. In the case of voltage gating, the molecular conformation of the gate or its chemical bonds responds to the electrical potential across the cell membrane. For example, in the top panel of Figure 4-5, a strong negative charge on the inside of the cell membrane may cause the outside sodium gates to remain tightly closed. Conversely, when the inside of the membrane loses its negative charge, these gates open suddenly and allow sodium to pass inward through the sodium pores. This process is the basic mechanism for eliciting action potentials in nerves that are responsible for nerve signals. In \n- the bottom panel of **Figure 4-5**, the potassium gates are on the intracellular ends of the potassium channels, and they open when the inside of the cell membrane becomes positively charged. The opening of these gates is partly responsible for terminating the action potential, a process discussed in Chapter 5.\n- 2. Chemical (ligand) gating. Some protein channel gates are opened by the binding of a chemical substance (a ligand) with the protein, which causes a conformational or chemical bonding change in the protein molecule that opens or closes the gate. One of the most important instances of chemical gating is the effect of the neurotransmitter acetylcholine on the acetylcholine receptor which serves as a ligand-gated ion channel. Acetylcholine opens the gate of this channel, providing a negatively charged pore about 0.65 nanometer in diameter that allows uncharged molecules or positive ions smaller than this diameter to pass through. This gate is exceedingly important for the transmission of nerve signals from one nerve cell to another (see Chapter 46) and from nerve cells to muscle cells to cause muscle contraction (see Chapter 7). \n#### Open-State Versus Closed-State of Gated Channels. \nFigure 4-6A shows two recordings of electrical current flowing through a single sodium channel when there was an approximately 25-millivolt potential gradient across the membrane. Note that the channel conducts current in an all-or-none fashion. That is, the gate of the channel snaps open and then snaps closed, with each open state lasting for only a fraction of a millisecond, up to several milliseconds, demonstrating the rapidity with which changes can occur during the opening and closing of the protein gates. At one voltage potential, the channel may remain closed all the time or almost all the time, whereas at another voltage, it may remain open either all or most of the time. At in-between voltages, as shown in the figure, the gates tend to snap open and closed intermittently, resulting in an average current flow somewhere between the minimum and maximum. \nPatch Clamp Method for Recording Ion Current Flow Through Single Channels. The patch clamp method for recording ion current flow through single protein channels is illustrated in Figure 4-6B. A micropipette with a tip diameter of only 1 or 2 micrometers is abutted against the outside of a cell membrane. Suction is then applied inside the pipette to pull the membrane against the tip of the pipette, which creates a seal where the edges of the pipette touch the cell membrane. The result is a minute membrane \"patch\" at the tip of the pipette through which electrical current flow can be recorded. \nAlternatively, as shown at the bottom right in **Figure 4-6B**, the small cell membrane patch at the end of the pipette can be torn away from the cell. The pipette with its sealed patch is then inserted into a free solution, which \n \n**Figure 4-6. A**, Recording of current flow through a single voltagegated sodium channel, demonstrating the all or none principle for opening and closing of the channel. **B**, Patch clamp method for recording current flow through a single protein channel. To the left, the recording is performed from a \"patch\" of a living cell membrane. To the right, the recording is from a membrane patch that has been torn away from the cell. \nallows the concentrations of ions both inside the micropipette and in the outside solution to be altered as desired. Also, the voltage between the two sides of the membrane can be set, or \"clamped,\" to a given voltage. \nIt has been possible to make such patches small enough so that only a single channel protein is found in the membrane patch being studied. By varying the concentrations of different ions, as well as the voltage across the membrane, one can determine the transport characteristics of the single channel, along with its gating properties. \n \n**Figure 4-7.** Effect of concentration of a substance on the rate of diffusion through a membrane by simple diffusion and facilitated diffusion. This graph shows that facilitated diffusion approaches a maximum rate, called the *Vmax.* \n#### **FACILITATED DIFFUSION REQUIRES MEMBRANE CARRIER PROTEINS** \nFacilitated diffusion is also called *carrier-mediated diffusion* because a substance transported in this manner diffuses through the membrane with the help of a specific carrier protein. That is, the carrier *facilitates* diffusion of the substance to the other side. \nFacilitated diffusion differs from simple diffusion in the following important way. Although the rat[e of simple](#page-54-1) diffusion through an open channel increases proportionately with the concentration of the diffusing substance, in facilitated diffusion the rate of diffusion approaches a maximum, called Vmax, as the concentration of the diffusing substance increases. This difference between simple diffusion and facilitated diffusion is demonstrated in **Figure 4-7**. The figure shows t[hat a](#page-55-0)s the concentration of the diffusing substance increases, the rate of simple diffusion continues to increase proportionately but, in the case of facilitated diffusion, the rate of diffusion cannot rise higher than the Vmax level. \nWhat is it that limits the rate of facilitated diffusion? A probable answer is the mechanism illustrated in **Figure 4-8**. This Figure shows a carrier protein with a pore large enough to transport a specific molecule partway through. It also shows a binding receptor on the inside of the protein carrier. The molecule to be transported enters the pore and becomes bound. Then, in a fraction of a second, a conformational or chemical change occurs in the carrier protein, so that the pore now opens to the opposite side of the membrane. Because the binding force of the receptor is weak, the thermal motion of the attached molecule causes it to break away and be released on the opposite side of the membrane. The rate at which molecules can be transported by this mechanism can never be greater than the rate at which the carrier protein molecule can undergo change back and forth between its two states. Note specifically, though, that this mechanism allows the transported molecule to move—that is, diffuse—in either direction through the membrane. \n \nFigure 4-8. Postulated mechanism for facilitated diffusion. \nAmong the many substances that cross cell membranes by facilitated diffusion are *glucose* and most of the *amino acids*. In the case of glucose, at least 14 members of a family of membrane proteins (called *GLUT*) that transport glucose molecules have been discovered in various tissues. Some of these GLUT proteins transport other monosaccharides that have structures similar to that of glucose, including galactose and fructose. One of these, glucose transporter 4 (GLUT4), is activated by insulin, which can increase the rate of facilitated diffusion of glucose as much as 10- to 20-fold in insulin-sensitive tissues. This is the principal mechanism whereby insulin controls glucose use in the body, as discussed in Chapter 79.\n\nBy now, it is evident that many substances can diffuse through the cell membrane. What is usually important is the *net* rate of diffusion of a substance in the desired direction. This net rate is determined by several factors. \n**Net Diffusion Rate Is Proportional to the Concentration Difference Across a Membrane. Figure 4-9.4** shows a cell membrane with a high concentration of a substance on the outside and a low concentration of a substance on the inside. The rate at which the substance diffuses *inward* is proportional to the concentration of molecules on the *outside* because this concentration determines how many molecules strike the outside of the membrane each second. Conversely, the rate at which molecules diffuse *outward* is proportional to their concentration *inside* the membrane. Therefore, the rate of net diffusion into the cell is proportional to the concentration on the outside *minus* the concentration on the inside: \nNet diffusion $\\propto (C_o - C_i)$ \n \n**Figure 4-9.** Effect of concentration difference (**A**), electrical potential difference affecting negative ions (**B**), and pressure difference (**C**) to cause diffusion of molecules and ions through a cell membrane. $C_o$ , concentration outside the cell; $C_i$ , concentration inside the cell; $P_1$ pressure 1; $P_2$ pressure 2. \nin which $C_0$ is the concentration outside and $C_i$ is the concentration inside the cell. \nMembrane Electrical Potential and Diffusion of lons-The \"Nernst Potential.\" If an electrical potential is applied across the membrane, as shown in Figure **4-9B**, the electrical charges of the ions cause them to move through the membrane even though no concentration difference exists to cause movement. Thus, in the left panel of Figure 4-9B, the concentration of negative ions is the same on both sides of the membrane, but a positive charge has been applied to the right side of the membrane, and a negative charge has been applied to the left, creating an electrical gradient across the membrane. The positive charge attracts the negative ions, whereas the negative charge repels them. Therefore, net diffusion occurs from left to right. After some time, large quantities of negative ions have moved to the right, creating the condition shown in the right panel of Figure 4-9B, in which a concentration difference of the ions has developed in the direction opposite to the electrical potential difference. The concentration difference now tends to move the ions to the left, whereas the electrical difference tends to move them to the right. When the concentration difference rises high enough, the two effects balance each other. At normal body temperature (98.6°F; 37°C), the electrical difference that will balance a given concentration difference \nof *univalent* ions—such as Na+ ions—can be determined from the following formula, called the *Nernst equation*: \nEMF (in millivolts) =\n$$\\pm 61\\log \\frac{C_1}{C_2}$$ \nin which EMF is the electromotive force (voltage) between side 1 and side 2 of the membrane, $C_1$ is the concentration on side 1, and $C_2$ is the concentration on side 2. This equation is extremely important in understanding the transmission of nerve impulses and is discussed in Chapter 5. \n#### Effect of a Pressure Difference Across the Membrane. \nAt times, a considerable pressure difference develops between the two sides of a diffusible membrane. This pressure difference occurs, for example, at the blood capillary membranes in all tissues of the body. The pressure in many capillaries is about 20 mm Hg greater inside than outside. \nPressure actually means the sum of all the forces of the different molecules striking a unit surface area at a given instant. Therefore, having a higher pressure on one side of a membrane than on the other side means that the sum of all the forces of the molecules striking the channels on that side of the membrane is greater than on the other side. In most cases, this situation is caused by greater numbers of molecules striking the membrane per second on one side than on the other side. The result is that increased amounts of energy are available to cause a net movement of molecules from the high-pressure side toward the low-pressure side. This effect is demonstrated in **Figure 4-9C**, which shows a piston developing high pressure on one side of a pore, thereby causing more molecules to strike the pore on this side and, therefore, more molecules to diffuse to the other side.\n\nBy far, the most abundant substance that diffuses through the cell membrane is water. Enough water ordinarily diffuses in each direction through the red blood cell membrane per second to equal about 100 times the volume of the cell itself. Yet, the amount that normally diffuses in the two directions is balanced so precisely that zero net movement of water occurs. Therefore, the volume of the cell remains constant. However, under certain conditions, a concentration difference for water can develop across a membrane. When this concentration difference for water develops, net movement of water does occur across the cell membrane, causing the cell to swell or shrink, depending on the direction of the water movement. This process of net movement of water caused by a concentration difference of water is called osmosis. \nTo illustrate osmosis, let us assume the conditions shown in Figure 4-10, with pure water on one side of the cell membrane and a solution of sodium chloride on the \n \n**Figure 4-10.** Osmosis at a cell membrane when a sodium chloride solution is placed on one side of the membrane and water is placed on the other side. \nother side. Water molecules pass through the cell membrane with ease, whereas sodium and chloride ions pass through only with difficulty. Therefore, sodium chloride solution is actually a mixture of permeant water molecules and nonpermeant sodium and chloride ions, and the membrane is said to be *selectively permeable* to water but much less so to sodium and chloride ions. Yet, the presence of the sodium and chloride has displaced some of the water molecules on the side of the membrane where these ions are present and, therefore, has reduced the concentration of water molecules to less than that of pure water. As a result, in the example shown in Figure 4-10, more water molecules strike the channels on the left side, where there is pure water, than on the right side, where the water concentration has been reduced. Thus, net movement of water occurs from left to right-that is, osmosis occurs from the pure water into the sodium chloride solution. \n#### **Osmotic Pressure** \nIf in **Figure 4-10** pressure were applied to the sodium chloride solution, osmosis of water into this solution would be slowed, stopped, or even reversed. The amount of pressure required to stop osmosis is called the *osmotic pressure* of the sodium chloride solution. \nThe principle of a pressure difference opposing osmosis is demonstrated in **Figure 4-11**, which shows a selectively permeable membrane separating two columns of fluid, one containing pure water and the other containing a solution of water and any solute that will not penetrate the membrane. Osmosis of water from chamber B into chamber A causes the levels of the fluid columns to become farther and farther apart, until eventually a pressure difference develops between the two sides of the membrane that is great enough to oppose the osmotic effect. The pressure difference across the membrane at this point is equal to the osmotic pressure of the solution that contains the nondiffusible solute. \n \n**Figure 4-11.** Demonstration of osmotic pressure caused by osmosis at a semipermeable membrane. \n#### **Importance of Number of Osmotic Particles (Molar Concentration) in Determining Osmotic Pressure.** \nThe osmotic pressure exerted by particles in a solution, whether they are molecules or ions, is determined by the number of particles per unit volume of fluid, not by the mass of the particles. The reason for this is that each particle in a solution, regardless of its mass, exerts, on average, the same amount of pressure against the membrane. That is, large particles, which have greater mass (m) than small particles, move at a slower velocity (v). The small particles move at higher velocities in such a way that their average kinetic energies (k), as determined by the following equation, \n$$k = \\frac{mv^2}{2}$$ \nare the same for each small particle as for each large particle. Consequently, the factor that determines the osmotic pressure of a solution is the concentration of the solution in terms of the number of particles (which is the same as its *molar concentration* if it is a nondissociated molecule), not in terms of mass of the solute. \n**Osmolality—The Osmole.** To express the concentration of a solution in terms of numbers of particles, a unit called the *osmole* is used in place of grams. \nOne osmole is 1 gram molecular weight of osmotically active solute. Thus, 180 grams of glucose, which is 1 gram molecular weight of glucose, is equal to 1 osmole of glucose because glucose does not dissociate into ions. If a solute dissociates into two ions, 1 gram molecular weight of the solute will become 2 osmoles because the number of osmotically active particles is now twice as great as for the nondissociated solute. Therefore, when fully dissociated, 1 gram molecular weight of sodium chloride, 58.5 grams, is equal to 2 osmoles. \nThus, a solution that has *1 osmole of solute dissolved in each kilogram of water* is said to have an *osmolality of 1 osmole per kilogram,* and a solution that has 1/1000 osmole dissolved per kilogram has an osmolality of 1 milliosmole per kilogram. The normal osmolality of the extracellular and intracellular fluids is about *300 milliosmoles per kilogram of water.* \n**Relationship of Osmolality to Osmotic Pressure.** At normal body temperature, 37°C (98.6°F), a concentration of 1 osmole per liter will cause 19,300 mm Hg osmotic pressure in the solution. Likewise, *1 milliosmole* per liter concentration is equivalent to *19.3 mm Hg* osmotic pressure. Multiplying this value by the 300-milliosmolar concentration of the body fluids gives a total calculated osmotic pressure of the body fluids of 5790 mm Hg. The measured value for this, however, averages only about 5500 mm Hg. The reason for this difference is that many ions in the body fluids, such as sodium and chloride ions, are highly attracted to one another; consequently, they cannot move entirely unrestrained in the fluids and create their full osmotic pressure potential. Therefore, on average, the actual osmotic pressure of the body fluids is about 0.93 times the calculated value. \n**The Term** *Osmolarity***.** *Osmolarity* is the osmolar concentration expressed as *osmoles per liter of solution* rather than osmoles per kilogram of water. Although, strictly speaking, it is osmoles per kilogram of water (osmolality) that determines osmotic pressure, the quantitative differences between osmolarity and osmolality are less than 1% for dilute solutions such as those in the body. Because it is far more practical to measure osmolarity than osmolality, measuring osmolarity is the usual practice in physiological studies. \n#### ACTIVE TRANSPORT OF SUBSTANCES THROUGH MEMBRANES \nAt times, a large concentration of a substance is required in the intracellular fluid, even though the extracellular fluid contains only a small concentration. This situation is true, for example, for potassium ions. Conversely, it is important to keep the concentrations of other ions very low inside the cell, even though their concentrations in the extracellular fluid are high. This situation is especially true for sodium ions. Neither of these two effects could occur by simple diffusion because simple diffusion eventually equilibrates concentrations on the two sides of the membrane. Instead, some energy source must cause excess movement of potassium ions to the inside of cells and excess movement of sodium ions to the outside of cells. When a cell membrane moves molecules or ions uphill against a concentration gradient (or uphill against an electrical or pressure gradient), the process is called *active transport.* \nSome examples of substances that are actively transported through at least some cell membranes include sodium, potassium, calcium, iron, hydrogen, chloride, iodide, and urate ions, several different sugars, and most of the amino acids. \n**Primary Active Transport and Secondary Active Transport.** Active transport is divided into two types according to the source of the energy used to facilitate the transport, *primary active transport* and *secondary active transport.* In primary active transport, the energy is derived directly from the breakdown of adenosine triphosphate (ATP) or some other high-energy phosphate compound. In secondary active transport, the energy is derived secondarily from energy that has been stored in the form of ionic concentration differences of secondary molecular or ionic substances between the two sides of a cell membrane, created originally by primary active transport. In both cases, transport depends on *carrier proteins* that penetrate through the cell membrane, as is true for facilitated diffusion. However, in active transport, the carrier protein functions differently from the carrier in facilitated diffusion because it is capable of imparting energy to the transported substance to move it against the electrochemical gradient. The following sections provide some examples of primary active transport and secondary active transport, with more detailed explanations of their principles of function. \n#### **PRIMARY ACTIVE TRANSPORT** \n#### **Sodium-Potassium Pump Transports Sodium Ions Out of Cells and Potassium Ions into Cells** \nAmong the substances that are transported by primary active transport are sodium, potassium, calcium, hydrogen, chloride, and a few other ions. The active transport mechanism that has been studied in greatest detail is the *sodium-potassium* (Na+-K+) pump, a transporter that pumps sodium ions outward through the cell membrane of all cells and, at the same time, pumps potassium ions from the outside to the inside. This pump is responsible for mainta[ining the so](#page-58-0)dium and potassium concentration differences across the cell membrane, as well as for establishing a negative electrical voltage inside the cells. Indeed, Chapter 5 shows that this pump is also the basis of nerve function, transmitting nerve signals throughout the nervous system. \n**Figure 4-12** shows the basic physical components of the Na+-K+ pump. The *carrier protein* is a complex of two separate globular proteins—a larger one called the α subunit, with a molecular weight of about 100,000, and a smaller one called the β subunit, with a molecular weight of about 55,000. Although the function of the smaller protein is not known (except that it might anchor the protein complex in the lipid membrane), the larger protein has three specific features that are important for the functioning of the pump: \n1. It has three *binding sites for sodium ions* on the portion of the protein that protrudes to the inside of the cell. \n \n**Figure 4-12.** Postulated mechanism of the sodium-potassium pump. ADP, Adenosine diphosphate; ATP, adenosine triphosphate; Pi, phosphate ion. \n- 2. It has two *binding sites for potassium ions* on the outside.\n- 3. The inside portion of this protein near the sodium binding sites has adenosine triphosphatase (AT-Pase) activity. \nWhen two potassium ions bind on the outside of the carrier protein and three sodium ions bind on the inside, the ATPase function of the protein becomes activated. Activation of the ATPase function leads to cleavage of one molecule of ATP, splitting it to adenosine diphosphate (ADP) and liberating a high-energy phosphate bond of energy. This liberated energy is believed to cause a chemical and conformational change in the protein carrier molecule, extruding three sodium ions to the outside and two potassium ions to the inside. \nAs with other enzymes, the Na+-K+ ATPase pump can run in reverse. If the electrochemical gradients for Na+ and K+ are experimentally increased to the degree that the energy stored in their gradients is greater than the chemical energy of ATP hydrolysis, these ions will move down their concentration gradients, and the Na+-K+ pump will synthesize ATP from ADP and phosphate. The phosphorylated form of the Na+-K+ pump, therefore, can either donate its phosphate to ADP to produce ATP or use the energy to change its conformation and pump Na+ out of the cell and K+ into the cell. The relative concentrations of ATP, ADP, and phosphate, as well as the electrochemical gradients for Na+ and K+, determine the direction of the enzyme reaction. For some cells, such as electrically active nerve cells, 60% to 70% of the cell's energy requirement may be devoted to pumping Na+ out of the cell and K+ into the cell. \n**The Na+-K+ Pump Is Important for Controlling Cell Volume.** One of the most important functions of the Na+-K+ pump is to control the cell volume. Without function of this pump, most cells of the body would swell until they burst. \nThe mechanism for controlling the volume is as follows. Inside the cell are large numbers of proteins and other organic molecules that cannot escape from the cell. Most of these proteins and other organic molecules are negatively charged and, therefore, attract large numbers of potassium, sodium, and other positive ions. All these molecules and ions then cause osmosis of water to the interior of the cell. Unless this process is checked, the cell will swell indefinitely until it bursts. The normal mechanism for preventing this outcome is the Na+-K+ pump. Note again that this mechanism pumps three Na+ ions to the outside of the cell for every two K+ ions pumped to the interior. Also, the membrane is far less permeable to sodium ions than to potassium ions and, once the sodium ions are on the outside, they have a strong tendency to stay there. This process thus represents a net loss of ions out the cell, which also initiates osmosis of water out of the cell. \nIf a cell begins to swell for any reason, the Na+-K+ pump is automatically activated, moving still more ions to the exterior and carrying water with them. Therefore, the Na+-K+ pump performs a continual surveillance role in maintaining normal cell volume. \n**Electrogenic Nature of the Na+-K+ Pump.** The fact that the Na+-K+ pump moves three Na+ ions to the exterior for every two K+ ions that are moved to the interior means that a net of one positive charge is moved from the interior of the cell to the exterior of the cell for each cycle of the pump. This action creates positivity outside the cell but results in a deficit of positive ions inside the cell; that is, it causes negativity on the inside. Therefore, the Na+-K+ pump is said to be *electrogenic* because it creates an electrical potential across the cell membrane. As discussed in Chapter 5, this electrical potential is a basic requirement in nerve and muscle fibers for transmitting nerve and muscle signals. \n#### **Primary Active Transport of Calcium Ions** \nAnother important primary active transport mechanism is the *calcium pump*. Calcium ions are normally maintained at an extremely low concentration in the intracellular cytosol of virtually all cells in the body, at a concentration about 10,000 times less than that in the extracellular fluid. This level of maintenance is achieved mainly by two primary active transport calcium pumps. One, which is in the cell membrane, pumps calcium to the outside of the cell. The other pumps calcium ions into one or more of the intracellular vesicular organelles of the cell, such as the sarcoplasmic reticulum of muscle cells and the mitochondria in all cells. In each of these cases, the carrier protein penetrates the membrane and functions as an enzyme ATPase, with the same capability to cleave ATP as the ATPase of the sodium carrier protein. The difference is that this protein has a highly specific binding site for calcium instead of for sodium.\n\nPrimary active transport of hydrogen ions is especially important at two places in the body: (1) in the gastric glands of the stomach; and (2) in the late distal tubules and cortical collecting ducts of the kidneys. \nIn the gastric glands, the deep-lying *parietal cells* have the most potent primary active mechanism for transporting hydrogen ions of any part of the body. This mechanism is the basis for secreting hydrochloric acid in stomach digestive secretions. At the secretory ends of the gastric gland parietal cells, the hydrogen ion concentration is increased as much as a million-fold and then is released into the stomach, along with chloride ions, to form hydrochloric acid. \nIn the renal tubules, special *intercalated cells* found in the late distal tubules and cortical collecting ducts also transport hydrogen ions by primary active transport. In this case, large amounts of hydrogen ions are secreted from the blood into the renal tubular fluid for the purpose of eliminating excess hydrogen ions from the body fluids. The hydrogen ions can be secreted into the renal tubular fluid against a concentration gradient of about 900-fold. Yet, as discussed in Chapter 31, most of these hydrogen ions combine with tubular fluid buffers before they are eliminated in the urine \n#### **Energetics of Primary Active Transport** \nThe amount of energy required to transport a substance actively through a membrane is determined by how much the substance is concentrated during transport. Compared with the energy required to concentrate a substance 10-fold, concentrating it 100-fold requires twice as much energy, and concentrating it 1000-fold requires three times as much energy. In other words, the energy required is proportional to the *logarithm* of the degree that the substance is concentrated, as expressed by the following formula: \nEnergy (in calories per osmole) = 1400 log\n$$\\frac{C_1}{C_2}$$ \nThus, in terms of calories, the amount of energy required to concentrate 1 osmole of a substance 10-fold is about 1400 calories, whereas to concentrate it 100-fold, 2800 calories are required. One can see that the energy expenditure for concentrating substances in cells or for removing substances from cells against a concentration gradient can be tremendous. Some cells, such as those lining the renal tubules and many glandular cells, expend as much as 90% of their energy for this purpose alone.\n\nWhen sodium ions are transported out of cells by primary active transport, a large concentration gradient of \nsodium ions across the cell membrane usually develops, with a high concentration outside the cell and a low concentration inside. This gradient represents a storehouse of energy, because the excess sodium outside the cell membrane is always attempting to diffuse to the interior. Under appropriate conditions, this diffusion energy of sodium can pull other substances along with the sodium through the cell membrane. This phenomenon, called *cotransport,* is one form of *secondary active transport.* \nFor sodium to pull another substance along with it, a coupling mechanism is required; this is achieved by means of still another carrier protein in the cell membrane. The carrier in this case serves as an attachment point for both the sodium ion and the substance to be co-transported. Once they are both attached, the energy gradient of the sodium ion causes the sodium ion and the other substance to be transported together to the interior of the cell. \nIn *counter-transport,* sodium ions again attempt to diffuse to the interior of the cell because of their large concentration gradient. However, this time, the substance to be transported is on the inside of the cell and is transported to the outside. Therefore, the sodium ion binds to the carrier protein, where it projects to the exterior surface of the membrane, and the substance to be countertransported binds to the interior projection of the carrier protein. Once both have become bound, a conformational change occurs, and energy released by the action of the sodium ion moving to the interior causes the other substance to move to the exterior. \n#### **Co-Transport o[f Glucose a](#page-60-0)nd Amino Acids Along with Sodium Ions** \nGlucose and many amino acids are transported into most cells against large concentration gradients; the mechanism of this action is entirely by co-transport, as shown in **Figure 4-13**. Note that the transport carrier protein has two binding sites on its exterior side, one for sodium and one for glucose. Also, the concentration of sodium ions is high on the outside and low on the inside, which provides energy for the transport. A special property of the transport protein is that a conformational change to allow sodium movement to the interior will not occur until a glucose molecule also attaches. When they both become attached, the conformational change takes place, and the sodium and glucose are transported to the inside of the cell at the same time. Hence, this is a *sodium-glucose co-transporter*. Sodium-glucose cotransporters are especially important for transporting glucose across renal and intestinal epithelial cells, as discussed in Chapters 28 and 66. \n*Sodium co-transport of amino acids* occurs in the same manner as for glucose, except that it uses a different set of transport proteins. At least five *amino acid transport proteins* have been identified, each of which is responsible for transporting one subset of amino acids with specific molecular characteristics. \n \n**Figure 4-13** Postulated mechanism for sodium co-transport of glucose. \n \n**Figure 4-14.** Sodium counter-transport of calcium and hydrogen ions. \nSodium co-transport of glucose and amino acids occurs especially through the epithelial cells of the intestinal tract and the renal tubules of the kidneys to promote absorption of these substances into the blood. This process will be discussed in later chapters. \nOther important co-transport mechanisms in at least some cells include co-transport of potassium, chloride, bicarbonate, phosphate, iodine, iron, and urate ions. \n#### **Sodium Counter-Tran[sport of Ca](#page-60-1)lcium and Hydrogen Ions** \nTwo especially important counter-transporters (i.e., transport in a direction opposite to the primary ion) are *sodium-calcium counter-transport* and *sodium-hydrogen counter-transport* (**Figure 4-14**). \nSodium-calcium counter-transport occurs through all or almost all cell membranes, with sodium ions moving to the interior and calcium ions to the exterior; both are bound to the same transport protein in a countertransport mode. This mechanism is in addition to the primary active transport of calcium that occurs in some cells. \nSodium-hydrogen counter-transport occurs in several tissues. An especially important example is in the *proximal tubules* of the kidneys, where sodium ions move from the lumen of the tubule to the interior of the tubular cell and hydrogen ions are counter-transported into the tubule lumen. As a mechanism for concentrating hydrogen ions, counter-transport is not nearly as powerful as the primary active transport of hydrogen ions that occurs in the more distal renal tubules, but it can transport extremely *large numbers of hydrogen ions,* thus making it a key to hydrogen ion control in the body fluids, as discussed in detail in Chapter 31. \n \n**Figure 4-15.** Basic mechanism of active transport across a layer of cells. \n#### **ACTIVE TRANSPORT THROUGH CELLULAR SHEETS** \nAt many places in the body, substances must be transported all the way through a cellular sheet instead of simply through the cell membrane. Transport of this type occurs through the following: (1) intestinal epithelium; (2) epithelium of the renal tubules; (3) epithelium of all exocrine glands; (4) epithelium of the gallbladder; and (5) membrane of the choroid plexus of the brain, along with other membranes. \nThe bas[ic mechanism](#page-61-0) for transport of a substance through a cellular sheet is as follows: (1) *active transport* through the cell membrane *on one side* of the transporting cells in the sheet; and then (2) either *simple diffusion* or *facilitated diffusion* through the membrane *on the opposite side* of the cell. \n**Figure 4-15** shows a mechanism for the transport of sodium ions through the epithelial sheet of the intestines, gallbladder, and renal tubules. This figure shows that the epithelial cells are connected together tightly at the luminal pole by means of junctions. The brush border on the luminal surfaces of the cells is permeable to both sodium ions and water. Therefore, sodium and water diffuse readily from the lumen into the interior of the cell. Then, at the basal and lateral membranes of the cells, sodium ions are actively transported into the extracellular fluid of the surrounding connective tissue and blood vessels. This action creates a high sodium ion concentration gradient across these membranes, which in turn causes osmosis of water. Thus, active transport of sodium ions at the basolateral sides of the epithelial cells results in the transport not only of sodium ions but also of water. \nIt is through these mechanisms that almost all nutrients, ions, and other substances are absorbed into the blood from the intestine. These mechanisms are also how the same substances are reabsorbed from the glomerular filtrate by the renal tubules. \nNumerous examples of the different types of transport discussed in this chapter are provided throughout this text. \n#### Bibliography \nAgre P, Kozono D: Aquaporin water channels: molecular mechanisms for human diseases. FEBS Lett 555:72, 2003. \nBröer S: Amino acid transport across mammalian intestinal and renal epithelia. Physiol Rev 88:249, 2008. \nDeCoursey TE: Voltage-gated proton channels: molecular biology, physiology, and pathophysiology of the H(V) family. Physiol Rev 93:599, 2013. \nDiPolo R, Beaugé L: Sodium/calcium exchanger: influence of metabolic regulation on ion carrier interactions. Physiol Rev 86:155, 2006. \nDrummond HA, Jernigan NL, Grifoni SC: Sensing tension: epithelial sodium channel/acid-sensing ion channel proteins in cardiovascular homeostasis. Hypertension 51:1265, 2008. \nEastwood AL, Goodman MB: Insight into DEG/ENaC channel gating from genetics and structure. Physiology (Bethesda) 27:282, 2012. \nFischbarg J: Fluid transport across leaky epithelia: central role of the tight junction and supporting role of aquaporins. Physiol Rev 90:1271, 2010. \nGadsby DC: Ion channels versus ion pumps: the principal difference, in principle. Nat Rev Mol Cell Biol 10:344, 2009. \nGhezzi C, Loo DDF, Wright EM. Physiology of renal glucose handling via SGLT1, SGLT2 and GLUT2. Diabetologia 61:2087-2097, 2018. \nHilge M: Ca2+ regulation of ion transport in the Na+/Ca2+ exchanger. J Biol Chem 287:31641, 2012. \nJentsch TJ, Pusch M. CLC Chloride channels and transporters: structure, function, physiology, and disease. Physiol Rev 2018 98:1493- 1590, 2018. \nKaksonen M, Roux A. Mechanisms of clathrin-mediated endocytosis. Nat Rev Mol Cell Biol 19:313-326, 2018. \nKandasamy P, Gyimesi G, Kanai Y, Hediger MA. Amino acid transporters revisited: new views in health and disease. Trends Biochem Sci 43:752-789, 2018. \nPapadopoulos MC, Verkman AS: Aquaporin water channels in the nervous system. Nat Rev Neurosci 14:265, 2013. \nRieg T, Vallon V. Development of SGLT1 and SGLT2 inhibitors. Diabetologia 61:2079-2086, 2018. \nSachs F: Stretch-activated ion channels: what are they? Physiology 25:50, 2010. \nSchwab A, Fabian A, Hanley PJ, Stock C: Role of ion channels and transporters in cell migration. Physiol Rev 92:1865, 2012. \nStransky L, Cotter K, Forgac M. The function of V-ATPases in cancer. Physiol Rev 96:1071-1091, 2016 \nTian J, Xie ZJ: The Na-K-ATPase and calcium-signaling microdomains. Physiology (Bethesda) 23:205, 2008. \nVerkman AS, Anderson MO, Papadopoulos MC. Aquaporins: important but elusive drug targets. Nat Rev Drug Discov 13:259-277, 2014. \nWright EM, Loo DD, Hirayama BA: Biology of human sodium glucose transporters. Physiol Rev 91:733, 2011. \n\n\nElectrical potentials exist across the membranes of virtually all cells of the body. Some cells, such as nerve and muscle cells, generate rapidly changing electrochemical impulses at their membranes, and these impulses are used to transmit signals along the nerve or muscle membranes. In other types of cells, such as glandular cells, macrophages, and ciliated cells, local changes in membrane potentials also activate many of the cell's functions. This chapter reviews the basic mechanisms whereby membrane potentials are generated at rest and during action by nerve and muscle cells. See Video 5-1. \n\n\nIn Figure 5-1A, the potassium concentration is great inside a nerve fiber membrane but very low outside the membrane. Let us assume that the membrane in this case is permeable to the potassium ions but not to any other ions. Because of the large potassium concentration gradient from the inside toward the outside, there is a strong tendency for potassium ions to diffuse outward through the membrane. As they do so, they carry positive electrical charges to the outside, thus creating electropositivity outside the membrane and electronegativity inside the membrane because of negative anions that remain behind and do not diffuse outward with the potassium. Within about 1 millisecond, the potential difference between the inside and outside, called the diffusion potential, becomes great enough to block further net potassium diffusion to the exterior, despite the high potassium ion concentration gradient. In the normal mammalian nerve fiber, the potential difference is about 94 millivolts, with negativity inside the fiber membrane. \n**Figure 5-1***B* shows the same phenomenon as in **Figure 5-1***A*, but this time with a high concentration of sodium ions *outside* the membrane and a low concentration of sodium ions *inside*. These ions are also positively charged. This time, the membrane is highly permeable to the sodium ions but is impermeable to all other ions. Diffusion of the positively charged sodium ions to the inside \ncreates a membrane potential of opposite polarity to that in **Figure 5-1***A*, with negativity outside and positivity inside. Again, the membrane potential rises high enough within milliseconds to block further net diffusion of sodium ions to the inside; however, this time, in the mammalian nerve fiber, *the potential is about 61 millivolts positive inside the fiber.* \nThus, in both parts of **Figure 5-1**, we see that a concentration difference of ions across a selectively permeable membrane can, under appropriate conditions, create a membrane potential. Later in this chapter, we show that many of the rapid changes in membrane potentials observed during nerve and muscle impulse transmission result from such rapidly changing diffusion potentials. \nThe Nernst Equation Describes the Relationship of Diffusion Potential to the lon Concentration Difference Across a Membrane. The diffusion potential across a membrane that exactly opposes the net diffusion of a particular ion through the membrane is called the *Nernst potential* for that ion, a term that was introduced in Chapter 4. The magnitude of the Nernst potential is determined by the *ratio* of the concentrations of that specific ion on the two sides of the membrane. The greater this ratio, the greater the tendency for the ion to diffuse in one direction and therefore the greater the Nernst potential required to prevent additional net diffusion. The following equation, called the *Nernst equation*, can be used to calculate the Nernst potential for any univalent ion at the normal body temperature of 98.6°F (37°C): \nEMF (millivolts) =\n$$\\pm \\frac{61}{z} \\times log \\frac{Concentration\\ inside}{Concentration\\ outside}$$ \nwhere EMF is the electromotive force and z is the electrical charge of the ion (e.g., +1 for $K^+$ ). \nWhen using this formula, it is usually assumed that the potential in the extracellular fluid outside the membrane remains at zero potential, and the Nernst potential is the potential inside the membrane. Also, the sign of the potential is positive (+) if the ion diffusing from inside to outside is a negative ion, and it is negative (–) if the ion is positive. Thus, when the concentration of positive potassium ions on the inside is 10 times that on the outside, the log of 10 is 1, so the Nernst potential calculates to be -61 millivolts inside the membrane.\n\n**Figure 5-1 A**, Establishment of a diffusion potential across a nerve fiber membrane, caused by diffusion of potassium ions from inside the cell to outside the cell through a membrane that is selectively permeable only to potassium. **B**, Establishment of a diffusion potential when the nerve fiber membrane is permeable only to sodium ions. Note that the internal membrane potential is negative when potassium ions diffuse and positive when sodium ions diffuse because of opposite concentration gradients of these two ions. \nThe Goldman Equation Is Used to Calculate the Diffusion Potential When the Membrane Is Permeable to Several Different Ions. When a membrane is permeable to several different ions, the diffusion potential that develops depends on three factors: (1) the polarity of the electrical charge of each ion; (2) the permeability of the membrane (P) to each ion; and (3) the concentration (C) of the respective ions on the inside (i) and outside (o) of the membrane. Thus, the following formula, called the *Goldman equation* or the *Goldman-Hodgkin-Katz equation*, gives the calculated membrane potential on the *inside* of the membrane when two univalent positive ions, sodium (Na+) and potassium (K+), and one univalent negative ion, chloride (Cl-), are involved: \n$$EMF \\ (millivolts) = -61 \\times log \\frac{C_{Na_{i}^{+}}P_{Na^{+}} + C_{K_{i}^{+}}P_{K^{+}} + C_{Cl_{0}}P_{Cl^{-}}}{C_{Na_{0}^{+}}P_{Na^{+}} + C_{K_{0}^{+}}P_{K^{+}} + C_{Cl_{i}^{-}}P_{Cl^{-}}}$$ \nSeveral key points become evident from the Goldman equation. First, sodium, potassium, and chloride ions are the most important ions involved in the development of membrane potentials in nerve and muscle fibers, as well as in the neuronal cells. The concentration gradient of each of these ions across the membrane helps determine the voltage of the membrane potential. \nSecond, the quantitative importance of each of the ions in determining the voltage is proportional to the membrane permeability for that particular ion. If the membrane has zero permeability to sodium and chloride ions, the membrane potential becomes entirely dominated by the concentration gradient of potassium ions alone, and the resulting potential will be equal to the Nernst potential for potassium. The same holds true for each of the other two ions if the membrane should become selectively permeable for either one of them alone. \nThird, a positive ion concentration gradient from *inside* the membrane to the *outside* causes electronegativity \n**Table 5-1** Resting Membrane Potential in Different Cell Types \n| Cell Type | Resting Potential (mV) |\n|-----------------|---------------------------|\n| Neurons | −60 to −70 |\n| Skeletal muscle | −85 to −95 |\n| Smooth muscle | −50 to −60 |\n| Cardiac muscle | −80 to −90 |\n| Hair (cochlea) | –15 to –40 |\n| Astrocyte | -80 to -90 |\n| Erythrocyte | −8 to −12 |\n| Photoreceptor | –40 (dark) to –70 (light) | \ninside the membrane. The reason for this phenomenon is that excess positive ions diffuse to the outside when their concentration is higher inside than outside the membrane. This diffusion carries positive charges to the outside but leaves the nondiffusible negative anions on the inside, thus creating electronegativity on the inside. The opposite effect occurs when there is a gradient for a negative ion. That is, a chloride ion gradient from the outside to the inside causes negativity inside the cell because excess negatively charged chloride ions diffuse to the inside while leaving the nondiffusible positive ions on the outside. \nFourth, as explained later, the permeability of the sodium and potassium channels undergoes rapid changes during transmission of a nerve impulse, whereas the permeability of the chloride channels does not change greatly during this process. Therefore, rapid changes in sodium and potassium permeability are primarily responsible for signal transmission in neurons, which is the subject of most of the remainder of this chapter. \nResting Membrane Potential of Different Cell Types. In some cells, such as the cardiac pacemaker cells discussed in Chapter 10, the membrane potential is continuously changing, and the cells are never \"resting\". In many other cells, even excitable cells, there is a quiescent period in which a resting membrane potential can be measured. Table 5-1 shows the approximate resting membrane potentials of some different types of cells. The membrane potential is obviously very dynamic in excitable cells such as neurons, in which action potentials occur. However, even in nonexcitable cells, the membrane potential (voltage) also changes in response to various stimuli, which alter activities for the various ion transporters, ion channels, and membrane permeability for sodium, potassium, calcium, and chloride ions. The resting membrane potential is, therefore, only a brief transient state for many cells. \n**Electrochemical Driving Force.** When multiple ions contribute to the membrane potential, the equilibrium potential for any of the contributing ions will differ from the membrane potential, and there will be an *electrochemical driving force* ( $V_{df}$ ) for each ion that tends to cause net \n \n**Figure 5-2** Measurement of the membrane potential of the nerve fiber using a microelectrode. \nmovement of the ion across the membrane. This driving force is equal to the difference between the membrane potential $(V_m)$ and the equilibrium potential of the ion $(V_{eq})$ Thus, $V_{df} = V_m - V_{eq}.$ \nThe arithmetic sign of $V_{df}$ (positive or negative) and the valence of the ion (cation or anion) can be used to predict the direction of ion flow across the membrane, into or out of the cell. For cations such as $Na^+$ and $K^+$ , a positive $V_{df}$ predicts ion movement out of the cell down its electrochemical gradient, and a negative $V_{df}$ predicts ion movement into the cell. For anions, such as $Cl^-$ , a positive $V_{df}$ predicts ion movement into the cell, and a negative $V_{df}$ predicts ion movement out of the cell. When $V_m = V_{eq}$ , there is no net movement of the ion into or out of the cell. Also, the direction of ion flux through the membrane reverses as $V_m$ becomes greater than or less than $V_{eq}$ ; hence, the equilibrium potential ( $V_{eq}$ ) is also called the *reversal potential*. \n#### Measuring the Membrane Potential \nThe method for measuring the membrane potential is simple in theory but often difficult in practice because of the small size of most of the cells and fibers. Figure 5-2 shows a small micropipette filled with an electrolyte solution. The micropipette is impaled through the cell membrane to the interior of the fiber. Another electrode, called the indifferent electrode, is then placed in the extracellular fluid, and the potential difference between the inside and outside of the fiber is measured using an appropriate voltmeter. This voltmeter is a highly sophisticated electronic apparatus that is capable of measuring small voltages despite extremely high resistance to electrical flow through the tip of the micropipette, which has a lumen diameter usually less than 1 micrometer and a resistance of more than 1 million ohms. For recording rapid changes in the membrane potential during transmission of nerve impulses, the microelectrode is connected to an oscilloscope, as explained later in the chapter. \nThe lower part of **Figure 5-3** shows the electrical potential that is measured at each point in or near the nerve fiber membrane, beginning at the left side of the figure and passing to the right. As long as the electrode is outside the neuronal membrane, the recorded potential \n \n**Figure 5-3** Distribution of positively and negatively charged ions in the extracellular fluid surrounding a nerve fiber and in the fluid inside the fiber. Note the alignment of negative charges along the inside surface of the membrane and positive charges along the outside surface. The *lower panel* displays the abrupt changes in membrane potential that occur at the membranes on the two sides of the fiber. \nis zero, which is the potential of the extracellular fluid. Then, as the recording electrode passes through the voltage change area at the cell membrane (called the *electrical dipole layer*), the potential decreases abruptly to –70 millivolts. Moving across the center of the fiber, the potential remains at a steady –70-millivolt level but reverses back to zero the instant it passes through the membrane on the opposite side of the fiber. \nTo create a negative potential inside the membrane, only enough positive ions to develop the electrical dipole layer at the membrane itself must be transported outward. The remaining ions inside the nerve fiber can be both positive and negative, as shown in the upper panel of Figure 5-3. Therefore, transfer of an incredibly small number of ions through the membrane can establish the normal resting potential of -70 millivolts inside the nerve fiber, which means that only about 1/3,000,000 to 1/100,000,000 of the total positive charges inside the fiber must be transferred. Also, an equally small number of positive ions moving from outside to inside the fiber can reverse the potential from -70 millivolts to as much as +35 millivolts within as little as 1/10,000 of a second. Rapid shifting of ions in this manner causes the nerve signals discussed in subsequent sections of this chapter.\n\nThe resting membrane potential of large nerve fibers when they are not transmitting nerve signals is about -70 millivolts. That is, the potential *inside the fiber* is 70 millivolts more negative than the potential in the extracellular fluid on the outside of the fiber. In the next few paragraphs, the transport properties of the resting nerve membrane for \n \n**Figure 5-4** Functional characteristics of the Na+-K+ pump and the K+ \"leak\" channels. The K+ leak channels also leak Na+ ions into the cell slightly but are much more permeable to K+. ADP, Adenosine diphosphate; ATP, adenosine triphosphate. \nsodium and potassium and the factors that determine the level of this resting potential are explained. \nActive Transport of Sodium and Potassium lons Through the Membrane—the Sodium-Potassium (Na+-K+) Pump. Recall from Chapter 4 that all cell membranes of the body have a powerful Na+-K+ pump that continually transports sodium ions to the outside of the cell and potassium ions to the inside, as illustrated on the left side in Figure 5-4. Note that this is an *electrogenic pump* because three Na+ ions are pumped to the outside for each two K+ ions to the inside, leaving a net deficit of positive ions on the inside and causing a negative potential inside the cell membrane. \nThe $Na^+-K^+$ pump also causes large concentration gradients for sodium and potassium across the resting nerve membrane. These gradients are as follows: \nNa+ (outside): 142 mEq/L \nNa+ (inside): 14 mEq/L \nK+(outside): 4 mEq/L \nK+(inside): 140 mEq/L \nThe ratios of these two respective ions from the inside to the outside are as follows: \n$$Na_{inside}^{+}/Na_{outside}^{+}=0.1$$ \n$$K^{+}_{inside}/K^{+}_{outside} = 35.0$$ \n**Leakage of Potassium Through the Nerve Cell Membrane.** The right side of **Figure 5-4** shows a channel protein (sometimes called a *tandem pore domain, potassium channel*, or *potassium* $[K^+]$ *\"leak\" channel*) in the nerve membrane through which potassium ions can leak, even in a resting cell. The basic structure of potassium channels was described in Chapter 4 (**Figure 4-4**). These $K^+$ leak \n \n**Figure 5-5** Establishment of resting membrane potentials under three conditions. **A**, When the membrane potential is caused entirely by potassium diffusion alone. **B**, When the membrane potential is caused by diffusion of both sodium and potassium ions. **C**, When the membrane potential is caused by diffusion of both sodium and potassium ions plus pumping of both these ions by the Na+-K+ pump. \nchannels may also leak sodium ions slightly but are far more permeable to potassium than to sodium, normally about 100 times as permeable. As discussed later, this differential in permeability is a key factor in determining the level of the normal resting membrane potential.\n\n**Figure 5-5** shows the important factors in the establishment of the normal resting membrane potential. They are as follows. \n#### Contribution of the Potassium Diffusion Potential. \nIn **Figure 5-5A**, we assume that the only movement of ions through the membrane is diffusion of potassium ions, as demonstrated by the open channels between the potassium symbol $(K^+)$ inside and outside the mem- \nbrane. Because of the high ratio of potassium ions inside to outside, 35:1, the Nernst potential corresponding to this ratio is –94 millivolts because the logarithm of 35 is 1.54, and this, multiplied by –61 millivolts, is –94 millivolts. Therefore, if potassium ions were the only factor causing the resting potential, the resting potential *inside the fiber* would be equal to –94 millivolts, as shown in the figure. \nContribution of Sodium Diffusion Through the **Nerve Membrane. Figure 5-5***B* shows the addition of slight permeability of the nerve membrane to sodium ions, caused by the minute diffusion of sodium ions through the K+-Na+ leak channels. The ratio of sodium ions from inside to outside the membrane is 0.1, which gives a calculated Nernst potential for the inside of the membrane of +61 millivolts. Also shown in **Figure 5-5***B* is the Nernst potential for potassium diffusion of -94 millivolts. How do these interact with each other, and what will be the summated potential? This question can be answered by using the Goldman equation described previously. Intuitively, one can see that if the membrane is highly permeable to potassium but only slightly permeable to sodium, the diffusion of potassium contributes far more to the membrane potential than the diffusion of sodium. In the normal nerve fiber, the permeability of the membrane to potassium is about 100 times as great as its permeability to sodium. Using this value in the Goldman equation, and considering only sodium and potassium, gives a potential inside the membrane of -86 millivolts, which is near the potassium potential shown in the figure. \n**Contribution of the Na**+-**K**+ **Pump.** In **Figure 5-5***C*, the Na+-K+ pump is shown to provide an additional contribution to the resting potential. This figure shows that continuous pumping of three sodium ions to the outside occurs for each two potassium ions pumped to the inside of the membrane. The pumping of more sodium ions to the outside than the potassium ions being pumped to the inside causes a continual loss of positive charges from inside the membrane, creating an additional degree of negativity (about –4 millivolts additional) on the inside, beyond that which can be accounted for by diffusion alone. \nTherefore, as shown in **Figure 5-5***C*, the net membrane potential when all these factors are operative at the same time is about –90 millivolts. However, additional ions, such as chloride, must also be considered in calculating the membrane potential. \nIn summary, the diffusion potentials alone caused by potassium and sodium diffusion would give a membrane potential of about -86 millivolts, with almost all of this being determined by potassium diffusion. An additional -4 millivolts is then contributed to the membrane potential by the continuously acting electrogenic Na+-K+ pump, and there is a contribution of chloride ions. As mentioned previously, the resting membrane potential \n \n \n**Figure 5-6** Typical action potential recorded by the method shown in the *upper panel*. \nvaries in different cells from as low as around -10 millivolts in erythrocytes to as high as -90 millivolts in skeletal muscle cells. \n#### **NEURON ACTION POTENTIAL** \nNerve signals are transmitted by *action potentials*, which are rapid changes in the membrane potential that spread rapidly along the nerve fiber membrane. Each action potential begins with a sudden change from the normal resting negative membrane potential to a positive potential and ends with an almost equally rapid change back to the negative potential. To conduct a nerve signal, the action potential moves along the nerve fiber until it comes to the fiber's end. \nThe upper panel of **Figure 5-6** shows the changes that occur at the membrane during the action potential, with the transfer of positive charges to the interior of the fiber at its onset and the return of positive charges to the exterior at its end. The lower panel shows graphically the successive changes in membrane potential over a few 10,000ths of a second, illustrating the explosive onset of the action potential and the almost equally rapid recovery. \nThe successive stages of the action potential are as follows. \n**Resting Stage.** The resting stage is the resting membrane potential before the action potential begins. The membrane is said to be \"polarized\" during this stage because of the –70 millivolts negative membrane potential that is present. \n**Depolarization Stage.** At this time, the membrane suddenly becomes permeable to sodium ions, allowing rapid diffusion of positively charged sodium ions to the interior of the axon. The normal polarized state of −70 millivolts is immediately neutralized by the inflowing, positively charged sodium ions, with the potential rising rapidly in the positive direction—a process called *depolarization.* In large nerve fibers, the great excess of positive sodium ions moving to the inside causes the membrane potential to actually overshoot beyond the zero level and to become somewhat positive. In some smaller fibers, as well as in many central nervous system neurons, the potential merely approaches the zero level and does not overshoot to the positive state. \n**Repolarization Stage.** Within a few 10,000ths of a second after the membrane becomes highly permeable to sodium ions, the sodium channels begin to close, and the potassium channels open to a greater degree than normal. Then, rapid diffusion of potassium ions to the exterior reestablishes the normal negative resting membrane potential, which is called *repolarization* of the membrane. \nTo explain more fully the factors that cause both depolarization and repolarization, we will describe the special characteristics of two other types of transport channels through the nerve membrane, the voltage-gated sodium and potassium channels. \n#### **VOLTAGE-GATED SODIUM AND POTASSIUM CHANNELS** \nThe necessary factor in causing both depolarization and repolarization of the nerve membrane during the action potential is the *voltage-gated sodium channel.* A *voltagegated potassium channel* also plays an important role in increasing the rapidity of repolarization of the membrane. *These two voltage-gated channels are in addition to the Na*+*-K*+ *pump and the K*+ *leak channels.* \n#### **Activation and Inactivation of the Voltage-Gated Sodium Channel** \nThe upper panel of **[Figure 5-7](#page-67-0)** shows the voltage-gated sodium channel in three separate states. This channel has two *gates*—one near the outside of the channel called the *activation gate,* and another near the inside called the *inactivation gate.* The upper left of the figure depicts the state of these two gates in the normal resting membrane when the membrane potential is −70 millivolts. In this state, the activation gate is closed, which prevents any entry of sodium ions to the interior of the fiber through these sodium channels. \n**Activation of the Sodium Channel.** When the membrane potential becomes less negative than during the resting state, rising from −70 millivolts toward zero, it finally reaches a voltage—usually somewhere around −55 millivolts—that causes a sudden conformational \n \n**Figure 5-7** Characteristics of the voltage-gated sodium *(top)* and potassium *(bottom)* channels, showing successive activation and inactivation of the sodium channels and delayed activation of the potassium channels when the membrane potential is changed from the normal resting negative value to a positive value. \nchange in the activation gate, flipping it all the way to the open position. During this *activated state,* sodium ions can pour inward through the channel, increasing the sodium permeability of the membrane as much as 500- to 5000-fold. \n**Inactivation of the Sodium Channel.** The upper right panel of **[Figure 5-7](#page-67-0)** shows a third state of the sodium channel. The same increase in voltage that opens the activation gate also closes the inactivation gate. The inactivation gate, however, closes a few 10,000ths of a second after the activation gate opens. That is, the conformational change that flips the inactivation gate to the closed state is a slower process than the conformational change that opens the activation gate. Therefore, after the sodium channel has remained open for a few 10,000ths of a second, the inactivation gate closes, and sodium ions no longer can pour to the inside of the membrane. At this point, the membrane potential begins to return toward the resting membrane state, which is the repolarization process. \nAnother important characteristic of the sodium channel inactivation process is that the inactivation gate will not reopen until the membrane potential returns to or near the original resting membrane potential level. Therefore, it is usually not possible for the sodium channels to open again without first repolarizing the nerve fiber. \n#### **Voltage-Gated Potassium Channel and Its Activation** \nThe lower panel of **[Figure 5-7](#page-67-0)** shows the voltage-gated potassium channel in two states—during the resting state \n \n**Figure 5-8** Voltage clamp method for studying flow of ions through specific channels. \n(left) and toward the end of the action potential (right). During the resting state, the gate of the potassium channel is closed, and potassium ions are prevented from passing through this channel to the exterior. When the membrane potential rises from −70 millivolts toward zero, this voltage change causes a conformational opening of the gate and allows increased potassium diffusion outward through the channel. However, because of the slight delay in opening of the potassium channels, they open, for the most part, at about the same time that the sodium channels are beginning to close because of inactivation. Thus, the decrease in sodium entry to the cell and the simultaneous increase in potassium exit from the cell combine to speed the repolarization process, leading to full recovery of the resting membrane potential within another few 10,000ths of a second. \n**The Voltage Clamp Method for Measuring the Effect of Voltage on Opening and Closing of Voltage-Gated Channels.** The original research that led to quantitative understanding of the sodium and potassium channels was so ingenious that it led to Nobel Prizes for the scientists responsible, Hodgkin and Huxley, in 1963. The essence of these studies is shown in **[Figures. 5-8 and 5-9](#page-68-0)**. \n**[Figure 5-8](#page-68-0)** shows the *voltage clamp method,* which is used to measure the flow of ions through the different channels. In using this apparatus, two electrodes are inserted into the nerve fiber. One of these electrodes is used to measure the voltage of the membrane potential, and the other is used to conduct electrical current into or out of the nerve fiber. \nThis apparatus is used in the following way. The investigator decides which voltage to establish inside the nerve fiber. The electronic portion of the apparatus is then adjusted to the desired voltage, automatically injecting either positive or negative electricity through the current electrode at whatever rate is required to hold the voltage, as measured \n \n**Figure 5-9** Typical changes in conductance of sodium and potassium ion channels when the membrane potential is suddenly increased from the normal resting value of −70 millivolts to a positive value of +10 millivolts for 2 milliseconds. This figure shows that the sodium channels open (activate) and then close (inactivate) before the end of the 2 milliseconds, whereas the potassium channels only open (activate), and the rate of opening is much slower than that of the sodium channels. \nby the voltage electrode, at the level set by the operator. When the membrane potential is suddenly increased by this voltage clamp from −70 millivolts to zero, the voltagegated sodium and potassium channels open, and sodium and potassium ions begin to pour through the channels. To counterbalance the effect of these ion movements on the desired setting of the intracellular voltage, electrical current is injected automatically through the current electrode of the voltage clamp to maintain the intracellular voltage at the required steady zero level. To achieve this level, the current injected must be equal to but of opposite polarity to the net current flow through the membrane channels. To measure how much current flow is occurring at each instant, the current electrode is connected to an ampere meter that records the current flow, as demonstrated in **[Figure 5-8](#page-68-0)**. \nFinally, the investigator adjusts the concentrations of the ions to other than normal levels both inside and outside the nerve fiber and repeats the study. This experiment can be performed easily when using large nerve fibers removed from some invertebrates, especially the giant squid axon, which in some cases is as large as 1 millimeter in diameter. When sodium is the only permeant ion in the solutions inside and outside the squid axon, the voltage clamp measures current flow only through the sodium channels. When potassium is the only permeant ion, current flow only through the potassium channels is measured. \nAnother means for studying the flow of ions through an individual type of channel is to block one type of channel at a time. For example, the sodium channels can be blocked by a toxin called tetrodotoxin when it is applied to the outside of the cell membrane where the sodium activation gates are located. Conversely, tetraethylammonium ion blocks the potassium channels when it is applied to the interior of the nerve fiber. \n**[Figure 5-9](#page-68-1)** shows typical changes in conductance of the voltage-gated sodium and potassium channels when the membrane potential is suddenly changed through use of the voltage clamp, from −70 millivolts to +10 millivolts and then, 2 milliseconds later, back to −70 millivolts. Note the sudden opening of the sodium channels (the activation stage) within a small fraction of a millisecond after the membrane potential is increased to the positive value. However, during the next millisecond or so, the sodium channels automatically close (the inactivation stage). \nNote the opening (activation) of the potassium channels, which open less rapidly and reach their full open state only after the sodium channels have almost completely closed. Furthermore, once the potassium channels open, they remain open for the entire duration of the positive membrane potential and do not close again until after the membrane potential is decreased back to a negative value. \n#### **SUMMARY OF EVENTS THAT CAUSE THE ACTION POTENTIAL** \n**[Figure 5-10](#page-69-0)** summarizes the sequential events that occur during and shortly after the action potential. The bottom of the figure shows the changes in membrane conductance for sodium and potassium ions. During the resting state, before the action potential begins, the conductance for potassium ions is 50 to 100 times as great as the conductance for sodium ions. This disparity is caused by much greater leakage of potassium ions than sodium ions through the leak channels. However, at the onset of the action potential, the sodium channels almost instantaneously become activated and allow up to a 5000-fold increase in sodium conductance. The inactivation process then closes the sodium channels \n \n**Figure 5-10** Changes in sodium and potassium conductance during the course of the action potential. Sodium conductance increases several thousand–fold during the early stages of the action potential, whereas potassium conductance increases only about 30-fold during the latter stages of the action potential and for a short period thereafter. (These curves were constructed from theory presented in papers by Hodgkin and Huxley but transposed from a squid axon to apply to the membrane potentials of large mammalian nerve fibers.) \nwithin another fraction of a millisecond. The onset of the action potential also initiates voltage gating of the potassium channels, causing them to begin opening more slowly, a fraction of a millisecond after the sodium channels open. At the end of the action potential, the return of the membrane potential to the negative state causes the potassium channels to close back to their original status but, again, only after an additional millisecond or more delay. \nThe middle portion of **[Figure 5-10](#page-69-0)** shows the ratio of sodium to potassium conductance at each instant during the action potential, and above this depiction is the action potential itself. During the early portion of the action potential, the ratio of sodium to potassium conductance increases more than 1000-fold. Therefore, far more sodium ions flow to the interior of the fiber than potassium ions to the exterior. This is what causes the membrane potential to become positive at the action potential onset. Then, the sodium channels begin to close, and the potassium channels begin to open; thus, the ratio of conductance shifts far in favor of high potassium conductance but low sodium conductance. This shift allows for a very rapid loss of potassium ions to the exterior but virtually zero flow of sodium ions to the interior. Consequently, the action potential quickly returns to its baseline level. \n#### **Roles of Other Ions During the Action Potential** \nThus far, we have considered only the roles of sodium and potassium ions in generating the action potential. At least two other types of ions must be considered, negative anions and calcium ions. \n**Impermeant Negatively Charged Ions (Anions) Inside the Nerve Axon.** Inside the axon are many negatively charged ions that cannot go through the membrane channels. They include the anions of protein molecules and of many organic phosphate compounds and sulfate compounds, among others. Because these ions cannot leave the interior of the axon, any deficit of positive ions inside the membrane leaves an excess of these impermeant negative anions. Therefore, these impermeant negative ions are responsible for the negative charge inside the fiber when there is a net deficit of positively charged potassium ions and other positive ions. \n**Calcium Ions.** The membranes of almost all cells of the body have a calcium pump similar to the sodium pump, and calcium serves along with (or instead of) sodium in some cells to cause most of the action potential. Like the sodium pump, the calcium pump transports calcium ions from the interior to the exterior of the cell membrane (or into the endoplasmic reticulum of the cell), creating a calcium ion gradient of about 10,000-fold. This process leaves an internal cell concentration of calcium ions of about 10−7 molar, in contrast to an external concentration of about 10−3 molar. \nIn addition, there are *voltage-gated calcium channels*. Because the calcium ion concentration is more than 10,000 times greater in the extracellular fluid than in the intracellular fluid, there is a tremendous diffusion gradient and electrochemical driving force for the passive flow of calcium ions into the cells. These channels are slightly permeable to sodium ions and calcium ions, but their permeability to calcium is about 1000-fold greater than to sodium under normal physiological conditions. When the channels open in response to a stimulus that depolarizes the cell membrane, calcium ions flow to the interior of the cell. \nA major function of the voltage-gated calcium ion channels is to contribute to the depolarizing phase on the action potential in some cells. The gating of calcium channels, however, is relatively slow, requiring 10 to 20 times as long for activation as for the sodium channels. For this reason, they are often called *slow channels,* in contrast to the sodium channels, which are called *fast channels.* Therefore, the opening of calcium channels provides a more sustained depolarization, whereas the sodium channels play a key role in initiating action potentials. \nCalcium channels are numerous in cardiac muscle and smooth muscle. In fact, in some types of smooth muscle, the fast sodium channels are hardly present; therefore, the action potentials are caused almost entirely by the activation of slow calcium channels. \n**Increased Permeability of the Sodium Channels When There Is a Deficit of Calcium Ions.** The concentration of calcium ions in the extracellular fluid also has a profound effect on the voltage level at which the sodium channels become activated. When there is a deficit of calcium ions, the sodium channels become activated (opened) by a small increase of the membrane potential from its normal, very negative level. Therefore, the nerve fiber becomes highly excitable, sometimes discharging repetitively without provocation, rather than remaining in the resting state. In fact, the calcium ion concentration needs to fall only 50% below normal before spontaneous discharge occurs in some peripheral nerves, often causing *muscle \"tetany*.*\"* Muscle tetany is sometimes lethal because of tetanic contraction of the respiratory muscles. \nThe probable way in which calcium ions affect the sodium channels is as follows. These ions appear to bind to the exterior surfaces of the sodium channel protein. The positive charges of these calcium ions, in turn, alter the electrical state of the sodium channel protein, thus altering the voltage level required to open the sodium gate. \n#### **INITIATION OF THE ACTION POTENTIAL** \nThus far, we have explained the changing sodium and potassium permeability of the membrane, as well as the development of the action potential, but we have not explained what initiates the action potential. \n**A Positive-Feedback Cycle Opens the Sodium Channels.** As long as the membrane of the nerve fiber remains undisturbed, no action potential occurs in the normal nerve. However, if any event causes enough initial rise in the membrane potential from −70 millivolts toward the zero level, the rising voltage will cause many voltage-gated sodium channels to begin opening. This occurrence allows for the rapid inflow of sodium ions, which causes a further rise in the membrane potential, thus opening still more voltage-gated sodium channels and allowing more streaming of sodium ions to the interior of the fiber. This process is a positive feedback cycle that, once the feedback is strong enough, continues until all the voltage-gated sodium channels have become activated (opened). Then, within another fraction of a millisecond, the rising membrane potential causes closure of the sodium channels and opening of potassium channels, and the action potential soon terminates. \n**Initiation of the Action Potential Occurs Only After the Threshold Potential is Reached.** An action potential will not occur until the initial rise in membrane potential is great enough to create the positive feedback described in the preceding paragraph. This occurs when the number of sodium ions entering the fiber is greater than the number of potassium ions leaving the fiber. A sudden rise in membrane potential of 15 to 30 millivolts is usually required. Therefore, a sudden increase in the membrane potential in a large nerve fiber, from −70 millivolts up to about −55 millivolts, usually causes the explosive development of an action potential. This level of −55 millivolts is said to be the *threshold* for stimulation. \n#### PROPAGATION OF THE ACTION POTENTIAL \nIn the preceding paragraphs, we discussed the action potential as though it occurs at one spot on the membrane. However, an action potential elicited at any one point on an excitable membrane usually excites adjacent portions of the membrane, resulting in propagation of the action potential along the membrane. This mechanism is demonstrated in **[Figure 5-11.](#page-71-0)** \n**[Figure 5-11](#page-71-0)***A* shows a normal resting nerve fiber, and **[Figure 5-11](#page-71-0)***B* shows a nerve fiber that has been excited in its midportion, which suddenly develops increased permeability to sodium. The *arrows* show a local circuit of current flow from the depolarized areas of the membrane to the adjacent resting membrane areas. That is, positive electrical charges are carried by the inward-diffusing sodium ions through the depolarized membrane and then for several millimeters in both directions along the core of the axon. These positive charges increase the voltage for a distance of 1 to 3 millimeters inside the large myelinated fiber to above the threshold voltage value for initiating an action potential. Therefore, the sodium channels in these new areas immediately open, as shown in **[Figure 5-11](#page-71-0)***C* [and](#page-71-0) *D*, and the explosive action potential spreads. These newly depolarized areas produce still more local circuits of current flow farther along the membrane, causing progressively more and more depolarization. Thus, the depolarization process travels along the entire length of the fiber. This transmission of the depolarization process along a nerve or muscle fiber is called a *nerve* or *muscle impulse.* \n**Direction of Propagation.** As demonstrated in **[Figure 5-](#page-71-0) [11](#page-71-0)**, an excitable membrane has no single direction of propagation, but the action potential travels in all directions away from the stimulus—even along all branches of a nerve fiber—until the entire membrane has become depolarized. \n**All-or-Nothing Principle.** Once an action potential has been elicited at any point on the membrane of a normal fiber, the depolarization process travels over the entire membrane if conditions are right, but it does not travel at all if conditions are not right. This principle is called the *allor-nothing principle,* and it applies to all normal excitable tissues. Occasionally, the action potential reaches a point on the membrane at which it does not generate sufficient voltage to stimulate the next area of the membrane. When this situation occurs, the spread of depolarization stops. Therefore, for continued propagation of an impulse to occur, the ratio of action potential to threshold for excitation must at all times be greater than 1. This \"greater than 1\" requirement is called the *safety factor* for propagation. \n#### RE-ESTABLISHING SODIUM AND POTASSIUM IONIC GRADIENTS AFTER ACTION POTENTIALS ARE COMPLETED—IMPORTANCE OF ENERGY METABOLISM \nTransmission of each action potential along a nerve fiber slightly reduces the concentration differences of sodium and potassium inside and outside the membrane because sodium ions diffuse to the inside during depolarization, and potassium ions diffuse to the outside during \n+ + + + + + + + + + + + – – + + + + + + + + + + + + + + + + + + + + + – – + + + + + + + + + + + + + + + + + + + – – – – + + + + + + + + + + + + + + + + + + – – – – + + + + + + + + – – + + + + + + + + + + + + + + + + + + – – – – + + + + + + + + + + + + + + + + + + – – + + – – – – – – – – – – – – – – – – – – + + + + – – – – – – – – – – – – – – – – – – + + – – – – – – – – – – – – + + – – – – – – – – – + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – + + – – – – – – – – – – – – – – – – – – – + + + + – – – – – – – – – – – – – – – – – – + + + + – – – – – – – – A B C D \n**Figure 5-11** A–D, Propagation of action potentials in both directions along a conductive fiber. \nrepolarization. For a single action potential, this effect is so minute that it cannot be measured. Indeed, 100,000 to 50 million impulses can be transmitted by large nerve fibers before the concentration differences reach the point that action potential conduction ceases. With time, however, it becomes necessary to re-establish the sodium and potassium membrane concentration differences, which is achieved by action of the Na+-K+ pump in the same way as described previously for the original establishment of the resting potential. That is, sodium ions that have diffused to the interior of the cell during the action potentials and potassium ions that have diffused to the exterior must be returned to their original state by the Na+-K+ pump. Because this pump requires energy for operation, this \"recharging\" of the nerve fiber is an active metabolic process, using energy derived from the adenosine triphosphate (ATP) energy system of the cell. **[Figure 5-12](#page-71-1)** shows that the nerve fiber produces increased heat during recharging, which is a measure of energy expenditure when the nerve impulse frequency increases. \nA special feature of the Na+-K+ ATP pump is that its degree of activity is strongly stimulated when excess sodium ions accumulate inside the cell membrane. In fact, the pumping activity increases approximately in proportion to the third power of this intracellular sodium concentration. As the internal sodium concentration rises from 10 to 20 mEq/L, the activity of the pump does not merely double but increases about eightfold. Therefore, it is easy to understand how the recharging process of the nerve fiber can be set rapidly into motion whenever the concentration differences of sodium and potassium ions across the membrane begin to run down. \n#### PLATEAU IN SOME ACTION POTENTIALS \nIn some cases, the excited membrane does not repolarize immediately after depolarization; instead, the potential remains on a plateau near the peak of the spike potential for many milliseconds before repolarization begin. Such a plateau is shown in **[Figure 5-13](#page-72-0)**; one can readily see that \n \n**Figure 5-12** Heat production in a nerve fiber at rest and at progressively increasing rates of stimulation. \nthe plateau greatly prolongs the period of depolarization. This type of action potential occurs in heart muscle fibers, where the plateau lasts for as long as 0.2 to 0.3 second and causes contraction of heart muscle to last for this same long period. \nThe cause of the plateau is a combination of several factors. First, in heart muscle, two types of channels contribute to the depolarization process: (1) the usual voltage-activated sodium channels, called *fast channels;* and (2) voltageactivated calcium-sodium channels *(L-type calcium channels)*, which are slow to open and therefore are called *slow channels.* Opening of fast channels causes the spike portion of the action potential, whereas the prolonged opening of the slow calcium-sodium channels mainly allows calcium ions to enter the fiber, which is largely responsible for the plateau portion of the action potential. \nAnother factor that may be partly responsible for the plateau is that the voltage-gated potassium channels are slower to open than usual, often not opening much until the end of the plateau. This factor delays the return of the membrane potential toward its normal negative value of −70 millivolts. The plateau ends when the calciumsodium channels close, and permeability to potassium ions increases. \n#### RHYTHMICITY OF SOME EXCITABLE TISSUES—REPETITIVE DISCHARGE \nRepetitive self-induced discharges occur normally in the heart, in most smooth muscle, and in many of the neurons of the central nervous system. These rhythmical discharges cause the following: (1) rhythmical beat of the heart; (2) rhythmical peristalsis of the intestines; and (3) neuronal events such as the rhythmical control of breathing. \nIn addition, almost all other excitable tissues can discharge repetitively if the threshold for stimulation of the tissue cells is reduced to a low enough level. For example, even large nerve fibers and skeletal muscle fibers, which normally are highly stable, discharge repetitively when they \n \n**Figure 5-13** Action potential (in millivolts) from a Purkinje fiber of the heart, showing a plateau. \nare placed in a solution that contains the drug *veratridine*, which activates sodium ion channels, or when the calcium ion concentration decreases below a critical value, which increases the sodium permeability of the membrane. \n**Re-Excitation Process Necessary for Spontaneous Rhythmicity.** For spontaneous rhythmicity to occur, the membrane—even in its natural state—must be permeable enough to sodium ions (or to calcium and sodium ions through the slow calcium-sodium channels) to allow automatic membrane depolarization. Thus, **[Figure 5-14](#page-72-1)** shows that the resting membrane potential in the rhythmical control center of the heart is only −60 to −70 millivolts, which is not enough negative voltage to keep the sodium and calcium channels totally closed. Therefore, the following sequence occurs: (1) some sodium and calcium ions flow inward; (2) this activity increases the membrane voltage in the positive direction, which further increases membrane permeability; (3) still more ions flow inward; and (4) the permeability increases more, and so on, until an action potential is generated. Then, at the end of the action potential, the membrane repolarizes. After another delay of milliseconds or seconds, spontaneous excitability causes depolarization again, and a new action potential occurs spontaneously. This cycle continues over and over and causes self-induced rhythmical excitation of the excitable tissue. \nWhy does the membrane of the heart control center not depolarize immediately after it has become repolarized, rather than delaying for nearly 1 second before the onset of the next action potential? The answer can be found by observing the curve labeled \"potassium conductance\" in **[Figure 5-14](#page-72-1)**. This curve shows that toward the end of each action potential, and continuing for a short period thereafter, the membrane becomes more permeable to potassium ions. The increased outflow of potassium ions carries tremendous numbers of positive charges to the outside of the membrane, leaving considerably more negativity inside the fiber than would otherwise occur. This continues for nearly 1 second after the preceding action potential is over, thus drawing the membrane potential \n \n**Figure 5-14** Rhythmical action potentials (in millivolts) similar to those recorded in the rhythmical control center of the heart. Note their relationship to potassium conductance and to the state of hyperpolarization. \nnearer to the potassium Nernst potential. This state, called *hyperpolarization,* is also shown in **[Figure 5-14](#page-72-1)**. As long as this state exists, self–re-excitation will not occur. However, the increased potassium conductance (and the state of hyperpolarization) gradually disappears, as shown after each action potential is completed in the figure, thereby again allowing the membrane potential to increase up to the *threshold* for excitation. Then, suddenly, a new action potential results and the process occurs again and again. \n#### SPECIAL CHARACTERISTICS OF SIGNAL TRANSMISSION IN NERVE TRUNKS \n**Myelinated and Unmyelinated Nerve Fibers. [Figure](#page-73-0) [5-15](#page-73-0)** shows a cross section of a typical small nerve, revealing many large nerve fibers that constitute most of the cross-sectional area. However, a more careful look reveals many more small fibers lying between the large ones. The large fibers are *myelinated,* and the small ones are *unmyelinated.* The average nerve trunk contains about twice as many unmyelinated fibers as myelinated fibers. \n**[Figure 5-16](#page-73-1)** illustrates schematically the features of a typical myelinated fiber. The central core of the fiber is the *axon,* and the membrane of the axon is the membrane that actually conducts the action potential. The axon is filled in its center with *axoplasm,* which is a viscid intracellular fluid. Surrounding the axon is a *myelin sheath* that is often much thicker than the axon itself. About once every 1 to 3 millimeters along the length of the myelin sheath is a *node of Ranvier.* \nThe myelin sheath is deposited around the axon by *Schwann cells* in the following manner. The membrane of a Schwann cell first envelops the axon. The Schwann cell then rotates around the axon many times, laying down multiple layers of Schwann cell membrane containing the lipid substance *sphingomyelin.* This substance is an excellent \n \n**Figure 5-15** Cross section of a small nerve trunk containing both myelinated and unmyelinated fibers. \nelectrical insulator that decreases ion flow through the membrane about 5000-fold. At the juncture between each two successive Schwann cells along the axon, a small uninsulated area only 2 to 3 micrometers in length remains where ions still can flow with ease through the axon membrane between the extracellular fluid and intracellular fluid inside the axon. This area is called the *node of Ranvier.* \n#### **Saltatory Conduction in Myelinated Fibers from Node** \n**to Node.** Even though almost no ions can flow through the thick myelin sheaths of myelinated nerves, they can flow with ease through the nodes of Ranvier. Therefore, action potentials occur *only at the nodes.* Yet, the action potentials are conducted from node to node by *saltatory conduction*, as shown in **[Figure 5-17](#page-74-0)**. That is, electrical current flows through the surrounding extracellular fluid outside the myelin sheath, as well as through the axoplasm inside the axon from node to node, exciting successive nodes one after another. Thus, the nerve impulse jumps along the fiber, which is the origin of the term *saltatory.* \nSaltatory conduction is of value for two reasons: \n1. First, by causing the depolarization process to jump long intervals along the axis of the nerve fiber, this mechanism increases the velocity of nerve transmission in myelinated fibers as much as 5- to 50-fold. \n \n**Figure 5-16** Function of the Schwann cell to insulate nerve fibers. **A**, Wrapping of a Schwann cell membrane around a large axon to form the myelin sheath of the myelinated nerve fiber. **B**, Partial wrapping of the membrane and cytoplasm of a Schwann cell around multiple unmyelinated nerve fibers (shown in cross section). *(A, Modified from Leeson TS, Leeson R: Histology. Philadelphia: WB Saunders, 1979.)* \n2. Second, saltatory conduction conserves energy for the axon because only the nodes depolarize, allowing perhaps 100 times less loss of ions than would otherwise be necessary, and therefore requiring much less energy expenditure for re-establishing the sodium and potassium concentration differences across the membrane after a series of nerve impulses. \nThe excellent insulation afforded by the myelin membrane and the 50-fold decrease in membrane capacitance also allow repolarization to occur with little transfer of ions. \n**Velocity of Conduction in Nerve Fibers.** The velocity of action potential conduction in nerve fibers varies from as little as 0.25 m/sec in small unmyelinated fibers to as much as 100 m/sec—more than the length of a football field in 1 second—in large myelinated fibers. \n#### EXCITATION—THE PROCESS OF ELICITING THE ACTION POTENTIAL \nBasically, any factor that causes sodium ions to begin to diffuse inward through the membrane in sufficient numbers can set off automatic regenerative opening of the sodium channels. This automatic regenerative opening can result from *mechanical* disturbance of the membrane, *chemical* effects on the membrane, or passage of *electricity* through the membrane. All these approaches are used at different points in the body to elicit nerve or muscle action potentials: mechanical pressure to excite sensory nerve endings in the skin, chemical neurotransmitters to transmit signals from one neuron to the next in the brain, and electrical current to transmit signals between successive muscle cells in the heart and intestine. \n**Excitation of a Nerve Fiber by a Negatively Charged Metal Electrode.** The usual means for exciting a nerve or muscle in the experimental laboratory is to apply electricity to the nerve or muscle surface through two small electrodes, one of which is negatively charged and the other positively charged. When electricity is applied in this manner, the excitable membrane becomes stimulated at the negative electrode. \nRemember that the action potential is initiated by the opening of voltage-gated sodium channels. Furthermore, these channels are opened by a decrease in the normal resting electrical voltage across the membrane—that is, negative current from the electrode decreases the voltage on the outside of the membrane to a negative value nearer to the voltage of the negative potential inside the fiber. This effect decreases the electrical voltage across the membrane and allows the sodium channels to open, resulting in an action potential. Conversely, at the positive electrode, the injection of positive charges on the outside of the nerve membrane heightens the voltage difference across the membrane, rather than lessening it. This effect causes a state of hyperpolarization, which actually decreases the excitability of the fiber rather than causing an action potential. \n \n**Figure 5-17** Saltatory conduction along a myelinated axon. The flow of electrical current from node to node is illustrated by the *arrows.* \n#### Threshold for Excitation and Acute Local Potentials. \nA weak negative electrical stimulus may not be able to excite a fiber. However, when the voltage of the stimulus is increased, there comes a point at which excitation does take place. Figure 5-18 shows the effects of successively applied stimuli of progressing strength. A weak stimulus at point A causes the membrane potential to change from -70 to -65 millivolts, but this change is not sufficient for the automatic regenerative processes of the action potential to develop. At point B, the stimulus is greater, but the intensity is still not enough. The stimulus does, however, disturb the membrane potential locally for as long as 1 millisecond or more after both of these weak stimuli. These local potential changes are called acute local potentials and, when they fail to elicit an action potential, they are called acute subthreshold potentials. \nAt point C in **Figure 5-18**, the stimulus is even stronger. Now, the local potential has barely reached the *threshold level* required to elicit an action potential, but this occurs only after a short \"latent period.\" At point D, the stimulus is still stronger, the acute local potential is also stronger, and the action potential occurs after less of a latent period. \nThus, this figure shows that even a weak stimulus causes a local potential change at the membrane, but the intensity of the local potential must rise to a threshold level before the action potential is set off.",
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"page_content": "Physiology is the science that seeks to explain the physical and chemical mechanisms that are responsible for the origin, development, and progression of life. Each type of life, from the simplest virus to the largest tree or the complicated human being, has its own functional characteristics. Therefore, the vast field of physiology can be divided into viral physiology, bacterial physiology, cellular physiology, plant physiology, invertebrate physiology, vertebrate physiology, mammalian physiology, human physiology, and many more subdivisions. \n**Human Physiology.** The science of human physiology attempts to explain the specific characteristics and mechanisms of the human body that make it a living being. The fact that we remain alive is the result of complex control systems. Hunger makes us seek food, and fear makes us seek refuge. Sensations of cold make us look for warmth. Other forces cause us to seek fellowship and to reproduce. The fact that we are sensing, feeling, and knowledgeable beings is part of this automatic sequence of life; these special attributes allow us to exist under widely varying conditions that otherwise would make life impossible. \nHuman physiology links the basic sciences with medicine and integrates multiple functions of the cells, tissues, and organs into the functions of the living human being. This integration requires communication and coordination by a vast array of control systems that operate at every level—from the genes that program synthesis of molecules to the complex nervous and hormonal systems that coordinate functions of cells, tissues, and organs throughout the body. Thus, the coordinated functions of the human body are much more than the sum of its parts, and life in health, as well as in disease states, relies on this total function. Although the main focus of this book is on normal human physiology, we will also discuss, to some extent, *pathophysiology,* which is the study of disordered body function and the basis for clinical medicine. \n#### CELLS ARE THE LIVING UNITS OF THE BODY \nThe basic living unit of the body is the cell. Each tissue or organ is an aggregate of many different cells held together by intercellular supporting structures. \nEach type of cell is specially adapted to perform one or a few particular functions. For example, the red blood cells, numbering about 25 trillion in each person, transport oxygen from the lungs to the tissues. Although the red blood cells are the most abundant of any single type of cell in the body, there are also trillions of additional cells of other types that perform functions different from those of the red blood cell. The entire body, then, contains about 35 to 40 trillion human cells. \nThe many cells of the body often differ markedly from one another but all have certain basic characteristics that are alike. For example, oxygen reacts with carbohydrate, fat, and protein to release the energy required for all cells to function. Furthermore, the general chemical mechanisms for changing nutrients into energy are basically the same in all cells, and all cells deliver products of their chemical reactions into the surrounding fluids. \nAlmost all cells also have the ability to reproduce additional cells of their own type. Fortunately, when cells of a particular type are destroyed, the remaining cells of this type usually generate new cells until the supply is replenished. \n**Microorganisms Living in the Body Outnumber Human Cells.** In addition to human cells, trillions of microbes inhabit the body, living on the skin and in the mouth, gut, and nose. The gastrointestinal tract, for example, normally contains a complex and dynamic population of 400 to 1000 species of microorganisms that outnumber our human cells. Communities of microorganisms that inhabit the body, often called *microbiota,* can cause diseases, but most of the time they live in harmony with their human hosts and provide vital functions that are essential for survival of their hosts. Although the importance of gut microbiota in the digestion of foodstuffs is widely recognized, additional roles for the body's microbes in nutrition, immunity, and other functions are just beginning to be appreciated and represent an intensive area of biomedical research. \n#### EXTRACELLULAR FLUID—THE \"INTERNAL ENVIRONMENT\" \nAbout 50% to 70% of the adult human body is fluid, mainly a water solution of ions and other substances. Although most of this fluid is inside the cells and is called *intracellular fluid,* about one-third is in the spaces outside the cells and is called *extracellular fluid.* This extracellular fluid is in constant motion throughout the body. It is transported rapidly in the circulating blood and then mixed between the blood and tissue fluids by diffusion through the capillary walls. \nIn the extracellular fluid are the ions and nutrients needed by the cells to maintain life. Thus, all cells live in essentially the same environment—the extracellular fluid. For this reason, the extracellular fluid is also called the *internal environment* of the body, or the *milieu intérieur,* a term introduced by the great 19th-century French physiologist Claude Bernard (1813–1878). \nCells are capable of living and performing their special functions as long as the proper concentrations of oxygen, glucose, different ions, amino acids, fatty substances, and other constituents are available in this internal environment. \n#### **Differences in Extracellular and Intracellular Fluids.** \nThe extracellular fluid contains large amounts of sodium, chloride, and bicarbonate ions plus nutrients for the cells, such as oxygen, glucose, fatty acids, and amino acids. It also contains carbon dioxide that is being transported from the cells to the lungs to be excreted, plus other cellular waste products that are being transported to the kidneys for excretion. \nThe intracellular fluid contains large amounts of potassium, magnesium, and phosphate ions instead of the sodium and chloride ions found in the extracellular fluid. Special mechanisms for transporting ions through the cell membranes maintain the ion concentration differences between the extracellular and intracellular fluids. These transport processes are discussed in Chapter 4. \n#### HOMEOSTASIS—MAINTENANCE OF A NEARLY CONSTANT INTERNAL ENVIRONMENT \nIn 1929, the American physiologist Walter Cannon (1871–1945) coined the term *homeostasis* to describe the *maintenance of nearly constant conditions in the internal environment*. Essentially, all organs and tissues of the body perform functions that help maintain these relatively constant conditions. For example, the lungs provide oxygen to the extracellular fluid to replenish the oxygen used by the cells, the kidneys maintain constant ion concentrations, and the gastrointestinal system provides nutrients while eliminating waste from the body. \nThe various ions, nutrients, waste products, and other constituents of the body are normally regulated within a range of values, rather than at fixed values. For some of the body's constituents, this range is extremely small. Variations in the blood hydrogen ion concentration, for example, are normally less than 5 *nanomoles/L* (0.000000005 moles/L). The blood sodium concentration is also tightly regulated, normally varying only a few *millimoles* per liter, even with large changes in sodium intake, but these variations of sodium concentration are at least 1 million times greater than for hydrogen ions. \nPowerful control systems exist for maintaining concentrations of sodium and hydrogen ions, as well as for most of the other ions, nutrients, and substances in the body at levels that permit the cells, tissues, and organs to perform their normal functions, despite wide environmental variations and challenges from injury and diseases. \nMuch of this text is concerned with how each organ or tissue contributes to homeostasis. Normal body functions require integrated actions of cells, tissues, organs, and multiple nervous, hormonal, and local control systems that together contribute to homeostasis and good health. \n**Homeostatic Compensations in Diseases.** *Disease* is often considered to be a state of disrupted homeostasis. However, even in the presence of disease, homeostatic mechanisms continue to operate and maintain vital functions through multiple compensations. In some cases, these compensations may lead to major deviations of the body's functions from the normal range, making it difficult to distinguish the primary cause of the disease from the compensatory responses. For example, diseases that impair the kidneys' ability to excrete salt and water may lead to high blood pressure, which initially helps return excretion to normal so that a balance between intake and renal excretion can be maintained. This balance is needed to maintain life, but, over long periods of time, the high blood pressure can damage various organs, including the kidneys, causing even greater increases in blood pressure and more renal damage. Thus, homeostatic compensations that ensue after injury, disease, or major environmental challenges to the body may represent trade-offs that are necessary to maintain vital body functions but, in the long term, contribute to additional abnormalities of body function. The discipline of *pathophysiology* seeks to explain how the various physiological processes are altered in diseases or injury. \nThis chapter outlines the different functional systems of the body and their contributions to homeostasis. We then briefly discuss the basic theory of the body's control systems that allow the functional systems to operate in support of one another. \n#### **EXTRACELLULAR FLUID TRANSPORT AND MIXING SYSTEM—THE BLOOD CIRCULATORY SYSTEM** \nExtracellular fluid is transported through the body in two stages. The first stage is movement of blood through the body in the blood vessels. The second is movement of fluid between the blood capillaries and the *intercellular spaces* between the tissue cells. \n**[Figure 1-1](#page-9-0)** shows the overall circulation of blood. All the blood in the circulation traverses the entire circuit an average \n \n**Figure 1-1.** General organization of the circulatory system. \nof once each minute when the body is at rest and as many as six times each minute when a person is extremely active. \nAs blood passes through blood capillaries, continual exchange of extracellular fluid occurs between the plasma portion of the blood and the interstitial fluid that fills the intercellular spaces. This process is shown in **Figure 1-2**. The capillary walls are permeable to most molecules in the blood plasma, with the exception of plasma proteins, which are too large to pass through capillaries readily. Therefore, large amounts of fluid and its dissolved constituents *diffuse* back and forth between the blood and the tissue spaces, as shown by the arrows in **Figure 1-2**. \nThis process of diffusion is caused by kinetic motion of the molecules in the plasma and the interstitial fluid. \n \n**Figure 1-2.** Diffusion of fluid and dissolved constituents through the capillary walls and interstitial spaces. \nThat is, the fluid and dissolved molecules are continually moving and bouncing in all directions in the plasma and fluid in the intercellular spaces, as well as through capillary pores. Few cells are located more than 50 micrometers from a capillary, which ensures diffusion of almost any substance from the capillary to the cell within a few seconds. Thus, the extracellular fluid everywhere in the body—both that of the plasma and that of the interstitial fluid—is continually being mixed, thereby maintaining homogeneity of extracellular fluid throughout the body.\n\n**Respiratory System. Figure 1-1** shows that each time blood passes through the body, it also flows through the lungs. The blood picks up *oxygen* in alveoli, thus acquiring the oxygen needed by cells. The membrane between the alveoli and the lumen of the pulmonary capillaries, the *alveolar membrane*, is only 0.4 to 2.0 micrometers thick, and oxygen rapidly diffuses by molecular motion through this membrane into the blood. \n**Gastrointestinal Tract.** A large portion of the blood pumped by the heart also passes through the walls of the gastrointestinal tract. Here different dissolved nutrients, including *carbohydrates*, *fatty acids*, and *amino acids*, are absorbed from ingested food into the extracellular fluid of the blood. \nLiver and Other Organs That Perform Primarily Metabolic Functions. Not all substances absorbed from the gastrointestinal tract can be used in their absorbed form by the cells. The liver changes the chemical compositions of many of these substances to more usable forms, and other tissues of the body—fat cells, gastrointestinal mucosa, kidneys, and endocrine glands—help modify the absorbed substances or store them until they are needed. The liver also eliminates certain waste products produced in the body and toxic substances that are ingested. \n**Musculoskeletal System.** How does the musculoskeletal system contribute to homeostasis? The answer is obvious and simple. Were it not for the muscles, the body could not move to obtain the foods required for nutrition. The musculoskeletal system also provides motility for protection against adverse surroundings, without which the entire body, along with its homeostatic mechanisms, could be destroyed. \n#### **REMOVAL OF METABOLIC END PRODUCTS** \n**Removal of Carbon Dioxide by the Lungs.** At the same time that blood picks up oxygen in the lungs, *carbon dioxide* is released from the blood into lung alveoli; the respiratory movement of air into and out of the lungs carries carbon dioxide to the atmosphere. Carbon dioxide is the most abundant of all the metabolism products. \n**Kidneys.** Passage of blood through the kidneys removes most of the other substances from the plasma besides carbon dioxide that are not needed by cells. These substances include different end products of cellular metabolism, such as urea and uric acid; they also include excesses of ions and water from the food that accumulate in the extracellular fluid. \nThe kidneys perform their function first by filtering large quantities of plasma through the glomerular capillaries into the tubules and then reabsorbing into the blood substances needed by the body, such as glucose, amino acids, appropriate amounts of water, and many of the ions. Most of the other substances that are not needed by the body, especially metabolic waste products such as urea and creatinine, are reabsorbed poorly and pass through the renal tubules into the urine. \n**Gastrointestinal Tract.** Undigested material that enters the gastrointestinal tract and some waste products of metabolism are eliminated in the feces. \n**Liver.** Among the many functions of the liver is detoxification or removal of ingested drugs and chemicals. The liver secretes many of these wastes into the bile to be eventually eliminated in the feces. \n#### **REGULATION OF BODY FUNCTIONS** \n**Nervous System.** The nervous system is composed of three major parts—the *sensory input portion,* the *central nervous system* (or *integrative portion*), and the *motor output portion.* Sensory receptors detect the state of the body and its surroundings. For example, receptors in the skin alert us whenever an object touches the skin. The eyes are sensory organs that give us a visual image of the surrounding area. The ears are also sensory organs. The central nervous system is composed of the brain and spinal cord. The brain stores information, generates thoughts, creates ambition, and determines reactions that the body performs in response to the sensations. Appropriate signals are then transmitted through the motor output portion of the nervous system to carry out one's desires. \nAn important segment of the nervous system is called the *autonomic system.* It operates at a subconscious level and controls many functions of internal organs, including the level of pumping activity by the heart, movements of the gastrointestinal tract, and secretion by many of the body's glands. \n**Hormone Systems.** Located in the body are *endocrine glands,* organs and tissues that secrete chemical substances called *hormones.* Hormones are transported in the extracellular fluid to other parts of the body to help regulate cellular function. For example, *thyroid hormone* increases the rates of most chemical reactions in all cells, thus helping set the tempo of bodily activity. *Insulin* controls glucose metabolism, *adrenocortical hormones* control sodium and potassium ions and protein metabolism, and *parathyroid hormone* controls bone calcium and phosphate. Thus, the hormones provide a regulatory system that complements the nervous system. The nervous system controls many muscular and secretory activities of the body, whereas the hormonal system regulates many metabolic functions. The nervous and hormonal systems normally work together in a coordinated manner to control essentially all the organ systems of the body. \n#### **PROTECTION OF THE BODY** \n**Immune System.** The immune system includes white blood cells, tissue cells derived from white blood cells, the thymus, lymph nodes, and lymph vessels that protect the body from pathogens such as bacteria, viruses, parasites, and fungi. The immune system provides a mechanism for the body to carry out the following: (1) distinguish its own cells from harmful foreign cells and substances; and (2) destroy the invader by *phagocytosis* or by producing *sensitized lymphocytes* or specialized proteins (e.g., *antibodies*) that destroy or neutralize the invader. \n**Integumentary System.** The skin and its various appendages (including the hair, nails, glands, and other structures) cover, cushion, and protect the deeper tissues and organs of the body and generally provide a boundary between the body's internal environment and the outside world. The integumentary system is also important for temperature regulation and excretion of wastes, and it provides a sensory interface between the body and the external environment. The skin generally comprises about 12% to 15% of body weight. \n#### **REPRODUCTION** \nAlthough reproduction is sometimes not considered a homeostatic function, it helps maintain homeostasis by generating new beings to take the place of those that are dying. This may sound like a permissive usage of the term *homeostasis,* but it illustrates that in the final analysis, essentially all body structures are organized to help maintain the automaticity and continuity of life. \n#### CONTROL SYSTEMS OF THE BODY \nThe human body has thousands of control systems. Some of the most intricate of these systems are genetic control systems that operate in all cells to help regulate intracellular and extracellular functions. This subject is discussed in Chapter 3. \nMany other control systems operate *within the organs* to regulate functions of the individual parts of the organs; others operate throughout the entire body *to control the interrelationships between the organs.* For example, the respiratory system, operating in association with the nervous system, regulates the concentration of carbon dioxide in the extracellular fluid. The liver and pancreas control glucose concentration in the extracellular fluid, and the kidneys regulate concentrations of hydrogen, sodium, potassium, phosphate, and other ions in the extracellular fluid. \n#### **EXAMPLES OF CONTROL MECHANISMS** \n**Regulation of Oxygen and Carbon Dioxide Concentrations in the Extracellular Fluid.** Because oxygen is one of the major substances required for chemical reactions in cells, the body has a special control mechanism to maintain an almost exact and constant oxygen concentration in the extracellular fluid. This mechanism depends principally on the chemical characteristics of *hemoglobin,* which is present in red blood cells. Hemoglobin combines with oxygen as the blood passes through the lungs. Then, as the blood passes through the tissue capillaries, hemoglobin, because of its own strong chemical affinity for oxygen, does not release oxygen into the tissue fluid if too much oxygen is already there. However, if oxygen concentration in the tissue fluid is too low, sufficient oxygen is released to re-establish an adequate concentration. Thus, regulation of oxygen concentration in the tissues relies to a great extent on the chemical characteristics of hemoglobin. This regulation is called the *oxygen-buffering function of hemoglobin.* \nCarbon dioxide concentration in the extracellular fluid is regulated in a much different way. Carbon dioxide is a major end product of oxidative reactions in cells. If all the carbon dioxide formed in the cells continued to accumulate in the tissue fluids, all energy-giving reactions of the cells would cease. Fortunately, a higher than normal carbon dioxide concentration in the blood *excites the respiratory center,* causing a person to breathe rapidly and deeply. This deep rapid breathing increases expiration of carbon dioxide and, therefore, removes excess carbon dioxide from the blood and tissue fluids. This process continues until the concentration returns to normal. \n \n**Figure 1-3.** Negative feedback control of arterial pressure by the arterial baroreceptors. Signals from the sensor (baroreceptors) are sent to the medulla of the brain, where they are compared with a reference set point. When arterial pressure increases above normal, this abnormal pressure increases nerve impulses from the baroreceptors to the medulla of the brain, where the input signals are compared with the set point, generating an error signal that leads to decreased sympathetic nervous system activity. Decreased sympathetic activity causes dilation of blood vessels and reduced pumping activity of the heart, which return arterial pressure toward normal. \n**Regulation of Arterial Blood Pressure.** Several systems contribute to arterial blood pressure regulation. One of these, the *baroreceptor system,* is an excellent example of a rapidly acting control mechanism (**[Figure 1-3](#page-11-0)**). In the walls of the bifurcation region of the carotid arteries in the neck, and also in the arch of the aorta in the thorax, are many nerve receptors called *baroreceptors* that are stimulated by stretch of the arterial wall. When arterial pressure rises too high, the baroreceptors send barrages of nerve impulses to the medulla of the brain. Here, these impulses inhibit the *vasomotor center,* which in turn decreases the number of impulses transmitted from the vasomotor center through the sympathetic nervous system to the heart and blood vessels. Lack of these impulses causes diminished pumping activity by the heart and dilation of peripheral blood vessels, allowing increased blood flow through the vessels. Both these effects decrease the arterial pressure, moving it back toward normal. \nConversely, a decrease in arterial pressure below normal relaxes the stretch receptors, allowing the vasomotor center to become more active than usual, thereby causing vasoconstriction and increased heart pumping. The initial decrease in arterial pressure thus initiates negative feedback mechanisms that raise arterial pressure back toward normal. \n#### **Normal Ranges and Physical Characteristics of Important Extracellular Fluid Constituents** \n**[Table 1-1](#page-12-0)** lists some important constituents and physical characteristics of extracellular fluid, along with their normal values, normal ranges, and maximum limits without causing death. Note the narrowness of the normal range for each one. Values outside these ranges are often caused by illness, injury, or major environmental challenges. \n| Constituent | Normal Value | Normal Range | Approximate Short-Term Nonlethal Limit | Unit |\n|-------------------------|--------------|----------------|----------------------------------------|---------|\n| Oxygen (venous) | 40 | 25–40 | 10–1000 | mm Hg |\n| Carbon dioxide (venous) | 45 | 41–51 | 5–80 | mm Hg |\n| Sodium ion | 142 | 135–145 | 115–175 | mmol/L |\n| Potassium ion | 4.2 | 3.5-5.3 | 1.5–9.0 | mmol/L |\n| Calcium ion | 1.2 | 1.0-1.4 | 0.5–2.0 | mmol/L |\n| Chloride ion | 106 | 98–108 | 70–130 | mmol/L |\n| Bicarbonate ion | 24 | 22–29 | 8–45 | mmol/L |\n| Glucose | 90 | 70–115 | 20–1500 | mg/dl |\n| Body temperature | 98.4 (37.0) | 98-98.8 (37.0) | 65–110 (18.3–43.3) | °F (°C) |\n| Acid-base (venous) | 7.4 | 7.3–7.5 | 6.9–8.0 | рН | \nTable 1-1 Important Constituents and Physical Characteristics of Extracellular Fluid \nMost important are the limits beyond which abnormalities can cause death. For example, an increase in the body temperature of only 11°F (7°C) above normal can lead to a vicious cycle of increasing cellular metabolism that destroys the cells. Note also the narrow range for acid-base balance in the body, with a normal pH value of 7.4 and lethal values only about 0.5 on either side of normal. Whenever the potassium ion concentration decreases to less than one-third normal, paralysis may result from the inability of the nerves to carry signals. Alternatively, if potassium ion concentration increases to two or more times normal, the heart muscle is likely to be severely depressed. Also, when the calcium ion concentration falls below about one-half normal, a person is likely to experience tetanic contraction of muscles throughout the body because of the spontaneous generation of excess nerve impulses in peripheral nerves. When the glucose concentration falls below one-half normal, a person frequently exhibits extreme mental irritability and sometimes even has convulsions. \nThese examples should give one an appreciation for the necessity of the vast numbers of control systems that keep the body operating in health. In the absence of any one of these controls, serious body malfunction or death can result. \n#### **CHARACTERISTICS OF CONTROL SYSTEMS** \nThe aforementioned examples of homeostatic control mechanisms are only a few of the many thousands in the body, all of which have some common characteristics, as explained in this section.\n\nMost control systems of the body act by *negative feed-back*, which can be explained by reviewing some of the homeostatic control systems mentioned previously. In the regulation of carbon dioxide concentration, a high concentration of carbon dioxide in the extracellular fluid increases pulmonary ventilation. This, in turn, decreases \nthe extracellular fluid carbon dioxide concentration because the lungs expire greater amounts of carbon dioxide from the body. Thus, the high concentration of carbon dioxide initiates events that decrease the concentration toward normal, which is *negative* to the initiating stimulus. Conversely, a carbon dioxide concentration that falls too low results in feedback to increase the concentration. This response is also negative to the initiating stimulus. \nIn the arterial pressure—regulating mechanisms, a high pressure causes a series of reactions that promote reduced pressure, or a low pressure causes a series of reactions that promote increased pressure. In both cases, these effects are negative with respect to the initiating stimulus. \nTherefore, in general, if some factor becomes excessive or deficient, a control system initiates *negative feedback*, which consists of a series of changes that return the factor toward a certain mean value, thus maintaining homeostasis. \nGain of a Control System. The degree of effectiveness with which a control system maintains constant conditions is determined by the gain of negative feedback. For example, let us assume that a large volume of blood is transfused into a person whose baroreceptor pressure control system is not functioning, and the arterial pressure rises from the normal level of 100 mm Hg up to 175 mm Hg. Then, let us assume that the same volume of blood is injected into the same person when the baroreceptor system is functioning, and this time the pressure increases by only 25 mm Hg. Thus, the feedback control system has caused a \"correction\" of -50 mm Hg, from 175 mm Hg to 125 mm Hg. There remains an increase in pressure of +25 mm Hg, called the \"error,\" which means that the control system is not 100% effective in preventing change. The gain of the system is then calculated by using the following formula: \n$$Gain = \\frac{Correction}{Frror}$$ \nThus, in the baroreceptor system example, the correction is -50 mm Hg, and the error persisting is +25 mm Hg. Therefore, the gain of the person's baroreceptor system \n \n**Figure 1-4.** Recovery of heart pumping caused by negative feedback after 1 liter of blood is removed from the circulation. Death is caused by positive feedback when 2 liters or more blood is removed. \nfor control of arterial pressure is −50 divided by +25, or −2. That is, a disturbance that increases or decreases the arterial pressure does so only one-third as much as would occur if this control system were not present. \nThe gains of some other physiological control systems are much greater than that of the baroreceptor system. For example, the gain of the system controlling internal body temperature when a person is exposed to moderately cold weather is about −33. Therefore, one can see that the temperature control system is much more effective than the baroreceptor pressure control system. \n#### **Positive Feedback May Cause Vicious Cycles and Death** \nWhy do most control systems of the body operate by negative feedback rather than by positive feedback? If one considers the nature of positive feedback, it is obvious that positive feedback leads to instability rather than stability and, in some cases, can cause death. \n**[Figure 1-4](#page-13-0)** shows an example in which death can ensue from positive feedback. This figure depicts the pumping effectiveness of the heart, showing the heart of a healthy human pumping about 5 liters of blood per minute. If the person suddenly bleeds a total of 2 liters, the amount of blood in the body is decreased to such a low level that not enough blood is available for the heart to pump effectively. As a result, the arterial pressure falls, and the flow of blood to the heart muscle through the coronary vessels diminishes. This scenario results in weakening of the heart, further diminished pumping, a further decrease in coronary blood flow, and still more weakness of the heart; the cycle repeats itself again and again until death occurs. Note that each cycle in the feedback results in further weakening of the heart. In other words, the initiating stimulus causes more of the same, which is *positive feedback.* \nPositive feedback is sometimes known as a \"vicious cycle,\" but a mild degree of positive feedback can be overcome by the negative feedback control mechanisms of the body, and the vicious cycle then fails to develop. For example, if the person in the aforementioned example bleeds only 1 liter instead of 2 liters, the normal negative feedback mechanisms for controlling cardiac output and arterial pressure can counterbalance the positive feedback and the person can recover, as shown by the dashed curve of **[Figure 1-4](#page-13-0)**. \n**Positive Feedback Can Sometimes Be Useful.** The body sometimes uses positive feedback to its advantage. Blood clotting is an example of a valuable use of positive feedback. When a blood vessel is ruptured, and a clot begins to form, multiple enzymes called *clotting factors* are activated within the clot. Some of these enzymes act on other inactivated enzymes of the immediately adjacent blood, thus causing more blood clotting. This process continues until the hole in the vessel is plugged and bleeding no longer occurs. On occasion, this mechanism can get out of hand and cause formation of unwanted clots. In fact, this is what initiates most acute heart attacks, which can be caused by a clot beginning on the inside surface of an atherosclerotic plaque in a coronary artery and then growing until the artery is blocked. \nChildbirth is another situation in which positive feedback is valuable. When uterine contractions become strong enough for the baby's head to begin pushing through the cervix, stretching of the cervix sends signals through the uterine muscle back to the body of the uterus, causing even more powerful contractions. Thus, the uterine contractions stretch the cervix, and cervical stretch causes stronger contractions. When this process becomes powerful enough, the baby is born. If they are not powerful enough, the contractions usually die out, and a few days pass before they begin again. \nAnother important use of positive feedback is for the generation of nerve signals. Stimulation of the membrane of a nerve fiber causes slight leakage of sodium ions through sodium channels in the nerve membrane to the fiber's interior. The sodium ions entering the fiber then change the membrane potential, which, in turn, causes more opening of channels, more change of potential, still more opening of channels, and so forth. Thus, a slight leak becomes an explosion of sodium entering the interior of the nerve fiber, which creates the nerve action potential. This action potential, in turn, causes electrical current to flow along the outside and inside of the fiber and initiates additional action potentials. This process continues until the nerve signal goes all the way to the end of the fiber. \nIn each case in which positive feedback is useful, the positive feedback is part of an overall negative feedback process. For example, in the case of blood clotting, the positive feedback clotting process is a negative feedback process for the maintenance of normal blood volume. Also, the positive feedback that causes nerve signals allows the nerves to participate in thousands of negative feedback nervous control systems. \n#### **More Complex Types of Control Systems—Feed-Forward and Adaptive Control** \nLater in this text, when we study the nervous system, we shall see that this system contains great numbers of interconnected control mechanisms. Some are simple feedback systems similar to those already discussed. Many are not. For example, some movements of the body occur so rapidly that there is not enough time for nerve signals to travel from the peripheral parts of the body all the way to the brain and then back to the periphery again to control the movement. Therefore, the brain uses a mechanism called *feed-forward control* to cause required muscle contractions. Sensory nerve signals from the moving parts apprise the brain about whether the movement is performed correctly. If not, the brain corrects the feed-forward signals that it sends to the muscles the *next* time the movement is required. Then, if still further correction is necessary, this process will be performed again for subsequent movements. This process is called *adaptive control.* Adaptive control, in a sense, is delayed negative feedback. \nThus, one can see how complex the feedback control systems of the body can be. A person's life depends on all of them. Therefore, much of this text is devoted to discussing these life-giving mechanisms. \n#### **PHYSIOLOGICAL VARIABILITY** \nAlthough some physiological variables, such as plasma concentrations of potassium, calcium, and hydrogen ions, are tightly regulated, others, such as body weight and adiposity, show wide variation among different individuals and even in the same individual at different stages of life. Blood pressure, cardiac pumping, metabolic rate, nervous system activity, hormones, and other physiological variables change throughout the day as we move about and engage in normal daily activities. Therefore, when we discuss \"normal\" values, it is with the understanding that many of the body's control systems are constantly reacting to perturbations, and that variability may exist among different individuals, depending on body weight and height, diet, age, sex, environment, genetics, and other factors. \nFor simplicity, discussion of physiological functions often focuses on the \"average\" 70-kg young, lean male. However, the American male no longer weighs an average of 70 kg; he now weighs over 88 kg, and the average American female weighs over 76 kg, more than the average man in the 1960s. Body weight has also increased substantially in most other industrialized countries during the past 40 to 50 years. \nExcept for reproductive and hormonal functions, many other physiological functions and normal values are often discussed in terms of male physiology. However, there are clearly differences in male and female physiology beyond the obvious differences that relate to reproduction. These differences can have important consequences for understanding normal physiology as well as for treatment of diseases. \nAge-related and ethnic or racial differences in physiology also have important influences on body composition, physiological control systems, and pathophysiology of diseases. For example, in a lean young male the total body water is about 60% of body weight. As a person grows and ages, this percentage gradually decreases, partly because aging is usually associated with declining skeletal muscle mass and increasing fat mass. Aging may also cause a decline in the function and effectiveness of some organs and physiological control systems. \nThese sources of physiological variability—sex differences, aging, ethnic, and racial—are complex but important considerations when discussing normal physiology and the pathophysiology of diseases. \n#### SUMMARY—AUTOMATICITY OF THE BODY \nThe main purpose of this chapter has been to discuss briefly the overall organization of the body and the means whereby the different parts of the body operate in harmony. To summarize, the body is actually a *social order of about 35 to 40 trillion cells* organized into different functional structures, some of which are called *organs.* Each functional structure contributes its share to the maintenance of homeostasis in the extracellular fluid, which is called the *internal environment.* As long as normal conditions are maintained in this internal environment, the cells of the body continue to live and function properly. Each cell benefits from homeostasis and, in turn, each cell contributes its share toward the maintenance of homeostasis. This reciprocal interplay provides continuous automaticity of the body until one or more functional systems lose their ability to contribute their share of function. When this happens, all the cells of the body suffer. Extreme dysfunction leads to death; moderate dysfunction leads to sickness. \n#### Bibliography \nAdolph EF: Physiological adaptations: hypertrophies and superfunctions. Am Sci 60:608, 1972. \nBentsen MA, Mirzadeh Z, Schwartz MW: Revisiting how the brain senses glucose-and why. Cell Metab 29:11, 2019. \nBernard C: Lectures on the Phenomena of Life Common to Animals and Plants. Springfield, IL: Charles C Thomas, 1974. \nCannon WB: Organization for physiological homeostasis. Physiol Rev 9:399, 1929. \nChien S: Mechanotransduction and endothelial cell homeostasis: the wisdom of the cell. Am J Physiol Heart Circ Physiol 292:H1209, 2007. \nDiBona GF: Physiology in perspective: the wisdom of the body. Neural control of the kidney. Am J Physiol Regul Integr Comp Physiol 289:R633, 2005. \nDickinson MH, Farley CT, Full RJ, et al: How animals move: an integrative view. Science 288:100, 2000. \nEckel-Mahan K, Sassone-Corsi P: Metabolism and the circadian clock converge. Physiol Rev 93:107, 2013. \n- Guyton AC: Arterial Pressure and Hypertension. Philadelphia: WB Saunders, 1980.\n- Herman MA, Kahn BB: Glucose transport and sensing in the maintenance of glucose homeostasis and metabolic harmony. J Clin Invest 116:1767, 2006.\n- Kabashima K, Honda T, Ginhoux F, Egawa G: The immunological anatomy of the skin. Nat Rev Immunol 19:19, 2019.\n- Khramtsova EA, Davis LK, Stranger BE: The role of sex in the genomics of human complex traits. Nat Rev Genet 20: 173, 2019.\n- Kim KS, Seeley RJ, Sandoval DA: Signalling from the periphery to the brain that regulates energy homeostasis. Nat Rev Neurosci 19:185, 2018.\n- Nishida AH, Ochman H: A great-ape view of the gut microbiome. Nat Rev Genet 20:185, 2019.\n- Orgel LE: The origin of life on the earth. Sci Am 271:76,1994.\n- Reardon C, Murray K, Lomax AE: Neuroimmune communication in health and disease. Physiol Rev 98:2287-2316, 2018.\n- Sender R, Fuchs S, Milo R: Revised estimates for the number of human and bacteria cells in the body. PLoS Biol 14(8):e1002533, 2016.\n- Smith HW: From Fish to Philosopher. New York: Doubleday, 1961. \n\n\nEach of the trillions of cells in a human being is a living structure that can survive for months or years, provided its surrounding fluids contain appropriate nutrients. Cells are the building blocks of the body, providing structure for the body's tissues and organs, ingesting nutrients and converting them to energy, and performing specialized functions. Cells also contain the body's hereditary code, which controls the substances synthesized by the cells and permits them to make copies of themselves. \n#### ORGANIZATION OF THE CELL \nA schematic drawing of a typical cell, as seen by the light microscope, is shown in **[Figure 2-1](#page-16-0)**. Its two major parts are the *nucleus* and the *cytoplasm.* The nucleus is separated from the cytoplasm by a *nuclear membrane,* and the cytoplasm is separated from the surrounding fluids by a *cell membrane,* also called the *plasma membrane.* \nThe different substances that make up the cell are collectively called *protoplasm.* Protoplasm is composed mainly of five basic substances—water, electrolytes, proteins, lipids, and carbohydrates. \n**Water.** Most cells, except for fat cells, are comprised mainly of water in a concentration of 70% to 85%. Many cellular chemicals are dissolved in the water. Others are suspended in the water as solid particulates. Chemical reactions take place among the dissolved chemicals or at the surfaces of the suspended particles or membranes. \n**Ions.** Important ions in the cell include *potassium, magnesium, phosphate, sulfate, bicarbonate,* and smaller quantities of *sodium, chloride,* and *calcium.* These ions are all discussed in Chapter 4, which considers the interrelations between the intracellular and extracellular fluids. \nThe ions provide inorganic chemicals for cellular reactions and are necessary for the operation of some cellular control mechanisms. For example, ions acting at the cell membrane are required for the transmission of electrochemical impulses in nerve and muscle fibers. \n**Proteins.** After water, the most abundant substances in most cells are proteins, which normally constitute 10% to 20% of the cell mass. These proteins can be divided into two types, *structural proteins* and *functional proteins.* \nStructural proteins are present in the cell mainly in the form of long filaments that are polymers of many individual protein molecules. A prominent use of such intracellular filaments is to form *microtubules,* which provide the cytoskeletons of cellular organelles such as cilia, nerve axons, the mitotic spindles of cells undergoing mitosis, and a tangled mass of thin filamentous tubules that hold the parts of the cytoplasm and nucleoplasm together in their respective compartments. Fibrillar proteins are found outside the cell, especially in the collagen and elastin fibers of connective tissue, and elsewhere, such as in blood vessel walls, tendons, and ligaments. \nThe *functional proteins* are usually composed of combinations of a few molecules in tubular-globular form. These proteins are mainly the *enzymes* of the cell and, in contrast to the fibrillar proteins, are often mobile in the cell fluid. Also, many of them are adherent to membranous structures inside the cell and catalyze specific intracellular chemical reactions. For example, the chemical reactions that split glucose into its component parts and then combine these with oxygen to form carbon dioxide and water while simultaneously providing energy for cellular function are all catalyzed by a series of protein enzymes. \n**Lipids.** Lipids are several types of substances that are grouped together because of their common property of being soluble in fat solvents. Especially important lipids \n \n**Figure 2-1.** Illustration of cell structures visible with a light microscope. \n \n**Figure 2-2.** Reconstruction of a typical cell, showing the internal organelles in the cytoplasm and nucleus. \nare *phospholipids* and *cholesterol,* which together constitute only about 2% of the total cell mass. Phospholipids and cholesterol are mainly insoluble in water and therefore are used to form the cell membrane and intracellular membrane barriers that separate the different cell compartments. \nIn addition to phospholipids and cholesterol, some cells contain large quantities of *triglycerides,* also called *neutral fats.* In *fat cells (adipocytes),* triglycerides often account for as much as 95% of the cell mass. The fat stored in these cells represents the body's main storehouse of energy-giving nutrients that can later be used to provide energy wherever it is needed in the body. \n**Carbohydrates.** Carbohydrates play a major role in cell nutrition and, as parts of glycoprotein molecules, have structural functions. Most human cells do not maintain large stores of carbohydrates; the amount usually averages only about 1% of their total mass but increases to as much as 3% in muscle cells and, occasionally, to 6% in liver cells. However, carbohydrate in the form of dissolved glucose is always present in the surrounding extracellular fluid so that it is readily available to the cell. Also, a small amount of carbohydrate is stored in cells as *glycogen,* an insoluble polymer of glucose that can be depolymerized and used rapidly to supply the cell's energy needs. \n#### CELL STRUCTURE \nThe cell contains highly organized physical structures called *intracellular organelles,* which are critical for cell function. For example, without one of the organelles, the *mitochondria,* more than 95% of the cell's energy release from nutrients would cease immediately. The most important organelles and other structures of the cell are shown in **[Figure 2-2](#page-17-0)**. \n#### **MEMBRANOUS STRUCTURES OF THE CELL** \nMost organelles of the cell are covered by membranes composed primarily of lipids and proteins. These membranes include the *cell membrane, nuclear membrane, membrane of the endoplasmic reticulum,* and *membranes of the mitochondria, lysosomes,* and *Golgi apparatus.* \n \n**Figure 2-3.** Structure of the cell membrane showing that it is composed mainly of a lipid bilayer of phospholipid molecules, but with large numbers of protein molecules protruding through the layer. Also, carbohydrate moieties are attached to the protein molecules on the outside of the membrane and to additional protein molecules on the inside. \nThe lipids in membranes provide a barrier that impedes movement of water and water-soluble substances from one cell compartment to another because water is not soluble in lipids. However, protein molecules often penetrate all the way through membranes, thus providing specialized pathways, often organized into actual *pores,* for passage of specific substances through membranes. Also, many other membrane proteins are *enzymes,* which catalyze a multitude of different chemical reactions, discussed here and in subsequent chapters. \n#### **Cell Membrane** \nThe cell membrane (also called the *plasma membrane*) envelops the cell and is a thin, pliable, elastic structure only 7.5 to 10 nanometers thick. It is composed almost entirely of proteins and lipids. The approximate composition is 55% proteins, 25% phospholipids, 13% cholesterol, 4% other lipids, and 3% carbohydrates. \n**The Cell Membrane Lipid Barrier Impedes Penetration by Water-Soluble Substances. [Figure 2-3](#page-18-0)** shows the structure of the cell membrane. Its basic structure is a *lipid bilayer,* which is a thin, double-layered film of lipids—each layer only one molecule thick—that is continuous over the entire cell surface. Interspersed in this lipid film are large globular proteins. \nThe basic lipid bilayer is composed of three main types of lipids—*phospholipids, sphingolipids,* and *cholesterol*. Phospholipids are the most abundant cell membrane lipids. One end of each phospholipid molecule is *hydrophilic* and soluble in water*.* The other end is *hydrophobic* and soluble only in fats. The phosphate end of the phospholipid is hydrophilic, and the fatty acid portion is hydrophobic. \nBecause the hydrophobic portions of the phospholipid molecules are repelled by water but are mutually attracted to one another, they have a natural tendency to attach to one another in the middle of the membrane, as shown in **[Figure 2-3](#page-18-0)**. The hydrophilic phosphate portions then constitute the two surfaces of the complete cell membrane, in contact with *intracellular* water on the inside of the membrane and *extracellular* water on the outside surface. \nThe lipid layer in the middle of the membrane is impermeable to the usual water-soluble substances, such as ions, glucose, and urea. Conversely, fat-soluble substances, such as oxygen, carbon dioxide, and alcohol, can penetrate this portion of the membrane with ease. \nSphingolipids, derived from the amino alcohol *sphingosine*, also have hydrophobic and hydrophilic groups and are present in small amounts in the cell membranes, especially nerve cells. Complex sphingolipids in cell membranes are thought to serve several functions, including protection from harmful environmental factors, signal transmission, and adhesion sites for extracellular proteins. \nCholesterol molecules in membranes are also lipids because their steroid nuclei are highly fat-soluble. These molecules, in a sense, are dissolved in the bilayer of the membrane. They mainly help determine the degree of permeability (or impermeability) of the bilayer to watersoluble constituents of body fluids. Cholesterol controls much of the fluidity of the membrane as well. \n#### **Integral and Peripheral Cell Membrane Proteins.** \n**[Figure 2-3](#page-18-0)** also shows globular masses floating in the lipid bilayer. These membrane proteins are mainly *glycoproteins.* There are two types of cell membrane proteins, *integral proteins,* which protrude all the way through the membrane, and *peripheral proteins,* which are attached only to one surface of the membrane and do not penetrate all the way through. \nMany of the integral proteins provide structural *channels* (or *pores*) through which water molecules and watersoluble substances, especially ions, can diffuse between extracellular and intracellular fluids. These protein channels also have selective properties that allow preferential diffusion of some substances over others. \nOther integral proteins act as *carrier proteins* for transporting substances that otherwise could not penetrate the lipid bilayer. Sometimes, these carrier proteins even transport substances in the direction opposite to their electrochemical gradients for diffusion, which is called *active transport.* Still others act as *enzymes.* \nIntegral membrane proteins can also serve as *receptors* for water-soluble chemicals, such as peptide hormones, that do not easily penetrate the cell membrane. Interaction of cell membrane receptors with specific *ligands* that bind to the receptor causes conformational changes in the receptor protein. This process, in turn, enzymatically activates the intracellular part of the protein or induces interactions between the receptor and proteins in the cytoplasm that act as *second messengers,* relaying the signal from the extracellular part of the receptor to the interior of the cell. In this way, integral proteins spanning the cell membrane provide a means of conveying information about the environment to the cell interior. \nPeripheral protein molecules are often attached to integral proteins. These peripheral proteins function almost entirely as enzymes or as controllers of transport of substances through cell membrane *pores.* \n#### **Membrane Carbohydrates—The Cell \"Glycocalyx.\"** \nMembrane carbohydrates occur almost invariably in combination with proteins or lipids in the form of *glycoproteins* or *glycolipids.* In fact, most of the integral proteins are glycoproteins, and about one-tenth of the membrane lipid molecules are glycolipids. The *glyco-* portions of these molecules almost invariably protrude to the outside of the cell, dangling outward from the cell surface. Many other carbohydrate compounds, called *proteoglycans* which are mainly carbohydrates bound to small protein cores—are loosely attached to the outer surface of the cell as well. Thus, the entire outside surface of the cell often has a loose carbohydrate coat called the *glycocalyx.* \nThe carbohydrate moieties attached to the outer surface of the cell have several important functions: \n- 1. Many of them have a negative electrical charge, which gives most cells an overall negative surface charge that repels other negatively charged objects.\n- 2. The glycocalyx of some cells attaches to the glycocalyx of other cells, thus attaching cells to one another.\n- 3. Many of the carbohydrates act as *receptors* for binding hormones, such as insulin. When bound, this combination activates attached internal proteins that in turn activate a cascade of intracellular enzymes.\n- 4. Some carbohydrate moieties enter into immune reactions, as discussed in Chapter 35. \n#### **CYTOPLASM AND ITS ORGANELLES** \nThe cytoplasm is filled with minute and large dispersed particles and organelles. The jelly-like fluid portion of the cytoplasm in which the particles are dispersed is called *cytosol* and contains mainly dissolved proteins, electrolytes, and glucose. \nDispersed in the cytoplasm are neutral fat globules, glycogen granules, ribosomes, secretory vesicles, and five especially important organelles—the *endoplasmic reticulum,* the *Golgi apparatus, mitochondria, lysosomes,* and *peroxisomes.* \n#### **Endoplasmic Reticulum** \n**[Figure 2-2](#page-17-0)** shows the *endoplasmic reticulum,* a network of tubular structures called *cisternae* and flat vesicular structures in the cytoplasm. This organelle helps process molecules made by the cell and transports them to their specific destinations inside or outside the cell. The tubules and vesicles interconnect. Also, their walls are constructed of lipid bilayer membranes that contain large amounts of proteins, similar to the cell membrane. The total surface area of this structure in some cells—the liver cells, for example—can be as much as 30 to 40 times the cell membrane area. \nThe detailed structure of a small portion of endoplasmic reticulum is shown in **[Figure 2-4](#page-20-0)**. The space inside the tubules and vesicles is filled with *endoplasmic matrix,* a watery medium that is different from fluid in the cytosol outside the endoplasmic reticulum. Electron micrographs show that the space inside the endoplasmic reticulum is connected with the space between the two membrane surfaces of the nuclear membrane. \nSubstances formed in some parts of the cell enter the space of the endoplasmic reticulum and are then directed to other parts of the cell. Also, the vast surface area of this \n \n**Figure 2-4.** Structure of the endoplasmic reticulum. \nreticulum and the multiple enzyme systems attached to its membranes provide the mechanisms for a major share of the cell's metabolic functions. \n**Ribosomes and the Rough (Granular) Endoplasmic Reticulum.** Attached to the outer surfaces of many parts of the endoplasmic reticulum are large numbers of minute granular particles called *ribosomes.* Where these particles are present, the reticulum is called the *rough (granular) endoplasmic reticulum.* The ribosomes are composed of a mixture of RNA and proteins; they function to synthesize new protein molecules in the cell, as discussed later in this chapter and in Chapter 3. \n**Smooth (Agranular) Endoplasmic Reticulum.** Part of the endoplasmic reticulum has no attached ribosomes. This part is called the *smooth,* or *agranular, endoplasmic reticulum.* The smooth reticulum functions for the synthesis of lipid substances and for other processes of the cells promoted by intrareticular enzymes. \n#### **Golgi Apparatus** \nThe Golgi apparatus, shown in **[Figure 2-5](#page-20-1)**, is closely related to the endoplasmic reticulum. It has membranes similar to those of the smooth endoplasmic reticulum. The Golgi apparatus is usually composed of four or more stacked layers of thin, flat, enclosed vesicles lying near one side of the nucleus. This apparatus is prominent in secretory cells, where it is located on the side of the cell from which secretory substances are extruded. \nThe Golgi apparatus functions in association with the endoplasmic reticulum. As shown in **[Figure 2-5](#page-20-1)**, small *transport vesicles* (also called *endoplasmic reticulum vesicles* [*ER vesicles*]) continually pinch off from the endoplasmic reticulum and shortly thereafter fuse with the Golgi apparatus. In this way, substances entrapped in ER \n \n**Figure 2-5.** A typical Golgi apparatus and its relationship to the endoplasmic reticulum (ER) and the nucleus. \nvesicles are transported from the endoplasmic reticulum to the Golgi apparatus. The transported substances are then processed in the Golgi apparatus to form lysosomes, secretory vesicles, and other cytoplasmic components (discussed later in this chapter). \n#### **Lysosomes** \nLysosomes, shown in **[Figure 2-2](#page-17-0)**, are vesicular organelles that form by breaking off from the Golgi apparatus; they then disperse throughout the cytoplasm. The lysosomes provide an *intracellular digestive system* that allows the cell to digest the following: (1) damaged cellular structures; (2) food particles that have been ingested by the cell; and (3) unwanted matter such as bacteria. Lysosome are different in various cell types but are usually 250 to 750 nanometers in diameter. They are surrounded by typical lipid bilayer membranes and are filled with large numbers of small granules, 5 to 8 nanometers in diameter, which are protein aggregates of as many as 40 different *hydrolase (digestive) enzymes.* A hydrolytic enzyme is capable of splitting an organic compound into two or more parts by combining hydrogen from a water molecule with one part of the compound and combining the hydroxyl portion of the water molecule with the other part of the compound. For example, protein is hydrolyzed to form amino acids, glycogen is hydrolyzed to form glucose, and lipids are hydrolyzed to form fatty acids and glycerol. \nHydrolytic enzymes are highly concentrated in lysosomes. Ordinarily, the membrane surrounding the lysosome prevents the enclosed hydrolytic enzymes from coming into contact with other substances in the cell and therefore prevents their digestive actions. However, some conditions of the cell break the membranes of lysosomes, allowing release of the digestive enzymes. These enzymes then split the organic substances with which they come in contact into small, highly diffusible substances such as \n \n**Figure 2-6.** Secretory granules (secretory vesicles) in acinar cells of the pancreas. \namino acids and glucose. Some of the specific functions of lysosomes are discussed later in this chapter. \n#### **Peroxisomes** \nPeroxisomes are physically similar to lysosomes, but they are different in two important ways. First, they are believed to be formed by self-replication (or perhaps by budding off from the smooth endoplasmic reticulum) rather than from the Golgi apparatus. Second, they contain *oxidases* rather than hydrolases. Several of the oxidases are capable of combining oxygen with hydrogen ions derived from different intracellular chemicals to form hydrogen peroxide (H2O2). Hydrogen peroxide is a highly oxidizing substance and is used in association with *catalase,* another oxidase enzyme present in large quantities in peroxisomes, to oxidize many substances that might otherwise be poisonous to the cell. For example, about half the alcohol that a person drinks is detoxified into acetaldehyde by the peroxisomes of the liver cells in this manner. A major function of peroxisomes is to catabolize long-chain fatty acids. \n#### **Secretory Vesicles** \nOne of the important functions of many cells is secretion of special chemical substances. Almost all such secretory substances are formed by the endoplasmic reticulum– Golgi apparatus system and are then released from the Golgi apparatus into the cytoplasm in the form of storage vesicles called *secretory vesicles* or *secretory granules.* **[Figure 2-6](#page-21-0)** shows typical secretory vesicles inside pancreatic acinar cells; these vesicles store protein proenzymes (enzymes that are not yet activated). The proenzymes are secreted later through the outer cell membrane into the pancreatic duct and then into the duodenum, where they become activated and perform digestive functions on the food in the intestinal tract. \n#### **Mitochondria** \nThe mitochondria, shown in **[Figure 2-2](#page-17-0)** and **[Figure 2-7](#page-21-1)**, are called the *powerhouses* of the cell. Without them, cells would be unable to extract enough energy from the nutrients, and essentially all cellular functions would cease. \n \n**Figure 2-7.** Structure of a mitochondrion. \nMitochondria are present in all areas of each cell's cytoplasm, but the total number per cell varies from less than 100 up to several thousand, depending on the energy requirements of the cell. Cardiac muscle cells (cardiomyocytes), for example, use large amounts of energy and have far more mitochondria than fat cells (adipocytes), which are much less active and use less energy. Furthermore, the mitochondria are concentrated in those portions of the cell responsible for the major share of its energy metabolism. They are also variable in size and shape. Some mitochondria are only a few hundred nanometers in diameter and are globular in shape, whereas others are elongated and are as large as 1 micrometer in diameter and 7 micrometers long. Still others are branching and filamentous. \nThe basic structure of the mitochondrion, shown in **[Figure 2-7](#page-21-1)**, is composed mainly of two lipid bilayerprotein membranes, an *outer membrane* and an *inner membrane.* Many infoldings of the inner membrane form shelves or tubules called *cristae* onto which oxidative enzymes are attached. The cristae provide a large surface area for chemical reactions to occur. In addition, the inner cavity of the mitochondrion is filled with a *matrix* that contains large quantities of dissolved enzymes necessary for extracting energy from nutrients. These enzymes operate in association with oxidative enzymes on the cristae to cause oxidation of nutrients, thereby forming carbon dioxide and water and, at the same time, releasing energy. The liberated energy is used to synthesize a high-energy substance called *adenosine triphosphate* (ATP). ATP is then transported out of the mitochondrion and diffuses throughout the cell to release its own energy wherever it is needed for performing cellular functions. The chemical details of ATP formation by the mitochondrion are provided in Chapter 68, but some basic functions of ATP in the cell are introduced later in this chapter. \nMitochondria are self-replicative, which means that one mitochondrion can form a second one, a third one, and so on whenever the cell needs increased amounts of ATP. Indeed, the mitochondria contain DNA similar to that found in the cell nucleus. In Chapter 3, we will see that DNA is the basic constituent of the nucleus that \n \n**Figure 2-8.** Cell cytoskeleton composed of protein fibers called microfilaments, intermediate filaments, and microtubules. \ncontrols replication of the cell. The DNA of the mitochondrion plays a similar role, controlling replication of the mitochondrion. Cells that are faced with increased energy demands—for example, in skeletal muscles subjected to chronic exercise training—may increase the density of mitochondria to supply the additional energy required. \n#### **Cell Cytoskeleton—Filament and Tubular Structures** \nThe cell cytoskeleton is a network of fibrillar proteins organized into filaments or tubules. These originate as precursor proteins synthesized by ribosomes in the cytoplasm. The precursor molecules then polymerize to form *filaments* (**[Figure 2-8](#page-22-0)**). As an example, large numbers of actin *microfilaments* frequently occur in the outer zone of the cytoplasm, called the *ectoplasm,* to form an elastic support for the cell membrane. Also, in muscle cells, actin and myosin filaments are organized into a special contractile machine that is the basis for muscle contraction, as discussed in Chapter 6. \n*Intermediate filaments* are generally strong ropelike filaments that often work together with microtubules, providing strength and support for the fragile tubulin structures. They are called *intermediate* because their average diameter is between that of narrower actin microfilaments and wider myosin filaments found in muscle cells. Their functions are mainly mechanical, and they are less dynamic than actin microfilaments or microtubules. All cells have intermediate filaments, although the protein subunits of these structures vary, depending on the cell type. Specific intermediate filaments found in various cells include desmin filaments in muscle cells, neurofilaments in neurons, and keratins in epithelial cells. \nA special type of stiff filament composed of polymerized *tubulin* molecules is used in all cells to construct strong tubular structures, the *microtubules.* **[Figure 2-8](#page-22-0)** shows typical microtubules of a cell. \nAnother example of microtubules is the tubular skeletal structure in the center of each cilium that radiates upward from the cell cytoplasm to the tip of the cilium. This structure is discussed later in the chapter (see **[Figure 2-18](#page-31-0)**). Also, both the *centrioles* and *mitotic spindles* of cells undergoing mitosis are composed of stiff microtubules. \nA major function of microtubules is to act as a *cytoskeleton,* providing rigid physical structures for certain parts of cells. The cell cytoskeleton not only determines cell shape but also participates in cell division, allows cells to move, and provides a tracklike system that directs the movement of organelles in the cells. Microtubules serve as the conveyor belts for the intracellular transport of vesicles, granules, and organelles such as mitochondria. \n#### **Nucleus** \nThe nucleus is the control center of the cell and sends messages to the cell to grow and mature, replicate, or die. Briefly, the nucleus contains large quantities of DNA, \n \n**Figure 2-9.** Structure of the nucleus. \nwhich comprise the *genes.* The genes determine the characteristics of the cell's proteins, including the structural proteins, as well as the intracellular enzymes that control cytoplasmic and nuclear activities. \nThe genes also control and promote cell reproduction. The genes first reproduce to create two identical sets of genes; then the cell splits by a special process called *mitosis* to form two daughter cells, each of which receives one of the two sets of DNA genes. All these activities of the nucleus are discussed in Chapter 3. \nUnfortunately, the appearance of the nucleus under the microscope does not provide many clues to the mechanisms whereby the nucleus performs its control activities. **[Figure 2-9](#page-23-0)** shows the light microscopic appearance of the *interphase* nucleus (during the period between mitoses), revealing darkly staining *chromatin material* throughout the nucleoplasm. During mitosis, the chromatin material organizes in the form of highly structured *chromosomes,* which can then be easily identified using the light microscope, as illustrated in Chapter 3. \n**Nuclear Membrane.** The *nuclear membrane,* also called the *nuclear envelope,* is actually two separate bilayer membranes, one inside the other. The outer membrane is continuous with the endoplasmic reticulum of the cell cytoplasm, and the space between the two nuclear membranes is also continuous with the space inside the endoplasmic reticulum, as shown in **[Figure 2-9](#page-23-0)**. \nThe nuclear membrane is penetrated by several thousand *nuclear pores.* Large complexes of proteins are attached at the edges of the pores so that the central area of each pore is only about 9 nanometers in diameter. Even this size is large enough to allow molecules up to a molecular weight of 44,000 to pass through with reasonable ease. \n**Nucleoli and Formation of Ribosomes.** The nuclei of most cells contain one or more highly staining structures called *nucleoli.* The nucleolus, unlike most other organelles discussed here, does not have a limiting membrane. Instead, it is simply an accumulation of large amounts of \n \n**Figure 2-10.** Comparison of sizes of precellular organisms with that of the average cell in the human body. \nRNA and proteins of the types found in ribosomes. The nucleolus enlarges considerably when the cell is actively synthesizing proteins. \nFormation of the nucleoli (and of the ribosomes in the cytoplasm outside the nucleus) begins in the nucleus. First, specific DNA genes in the chromosomes cause RNA to be synthesized. Some of this synthesized RNA is stored in the nucleoli, but most of it is transported outward through the nuclear pores into the cytoplasm. Here it is used in conjunction with specific proteins to assemble \"mature\" ribosomes that play an essential role in forming cytoplasmic proteins, as discussed in Chapter 3. \n#### COMPARISON OF THE ANIMAL CELL WITH PRECELLULAR FORMS OF LIFE \nThe cell is a complicated organism that required many hundreds of millions of years to develop after the earliest forms of life, microorganisms that may have been similar to present-day *viruses,* first appeared on earth. **[Figure 2-10](#page-23-1)** shows the relative sizes of the following: (1) the smallest known virus; (2) a large virus; (3) a *Rickettsia;* (4) a *bacterium;* and (5) a *nucleated cell,* This demonstrates that the cell has a diameter about 1000 times that of the smallest virus and therefore a volume about 1 billion times that of the smallest virus. Correspondingly, the functions and anatomical organization of the cell are also far more complex than those of the virus. \nThe essential life-giving constituent of the small virus is a *nucleic acid* embedded in a coat of protein. This nucleic acid is composed of the same basic nucleic acid constituents (DNA or RNA) found in mammalian cells and is capable of reproducing itself under appropriate conditions. Thus, the virus propagates its lineage from generation to generation and is therefore a living structure in the same way that cells and humans are living structures. \nAs life evolved, other chemicals in addition to nucleic acid and simple proteins became integral parts of the organism, and specialized functions began to develop in different parts of the virus. A membrane formed around the virus and, inside the membrane, a fluid matrix appeared. Specialized chemicals then developed inside the fluid to perform special functions; many protein enzymes appeared that were capable of catalyzing chemical reactions, thus determining the organism's activities. \nIn still later stages of life, particularly in the rickettsial and bacterial stages, *organelles* developed inside the organism. These represent physical structures of chemical aggregates that perform functions in a more efficient manner than what can be achieved by dispersed chemicals throughout the fluid matrix. \nFinally, in the nucleated cell, still more complex organelles developed, the most important of which is the *nucleus*. The nucleus distinguishes this type of cell from all lower forms of life; it provides a control center for all cellular activities and for reproduction of new cells generation after generation, with each new cell having almost exactly the same structure as its progenitor. \n#### FUNCTIONAL SYSTEMS OF THE CELL \nIn the remainder of this chapter, we discuss some functional systems of the cell that make it a living organism. \n#### **ENDOCYTOSIS—INGESTION BY THE CELL** \nIf a cell is to live and grow and reproduce, it must obtain nutrients and other substances from the surrounding fluids. Most substances pass through the cell membrane by the processes of diffusion and *active transport.* \nDiffusion involves simple movement through the membrane caused by the random motion of the molecules of the substance. Substances move through cell membrane pores or, in the case of lipid-soluble substances, through the lipid matrix of the membrane. \nActive transport involves actually carrying a substance through the membrane by a physical protein structure that penetrates all the way through the membrane. These active transport mechanisms are so important to cell function that they are presented in detail in Chapter 4. \nLarge particles enter the cell by a specialized function of the cell membrane called *endocytosis* (Video 2-1). The principal forms of endocytosis are *pinocytosis* and *phagocytosis.* Pinocytosis means the ingestion of minute particles that form vesicles of extracellular fluid and particulate constituents inside the cell cytoplasm. Phagocytosis means the ingestion of large particles, such as bacteria, whole cells, or portions of degenerating tissue. \n**Pinocytosis.** Pinocytosis occurs continually in the cell membranes of most cells, but is especially rapid in some cells. For example, it occurs so rapidly in macrophages that about 3% of the total macrophage membrane is engulfed in the form of vesicles each minute. Even so, the pinocytotic vesicles are so small—usually only 100 to 200 nanometers in diameter—that most of them can be seen only with an electron microscope. \n \n**Figure 2-11.** Mechanism of pinocytosis. \nPinocytosis is the only means whereby most large macromolecules, such as most proteins, can enter cells. In fact, the rate at which pinocytotic vesicles form is usually enhanced when such macromolecules attach to the cell membrane. \n**[Figure 2-11](#page-24-0)** demonstrates the successive steps of pinocytosis *(A–D),* showing three molecules of protein attaching to the membrane. These molecules usually attach to specialized protein *receptors* on the surface of the membrane that are specific for the type of protein that is to be absorbed. The receptors generally are concentrated in small pits on the outer surface of the cell membrane, called *coated pits.* On the inside of the cell membrane beneath these pits is a latticework of fibrillar protein called *clathrin,* as well as other proteins, perhaps including contractile filaments of *actin* and *myosin.* Once the protein molecules have bound with the receptors, the surface properties of the local membrane change in such a way that the entire pit invaginates inward, and fibrillar proteins surrounding the invaginating pit cause its borders to close over the attached proteins, as well as over a small amount of extracellular fluid. Immediately thereafter, the invaginated portion of the membrane breaks away from the surface of the cell, forming a *pinocytotic vesicle* inside the cytoplasm of the cell. \nWhat causes the cell membrane to go through the necessary contortions to form pinocytotic vesicles is still unclear. This process requires energy from within the cell, which is supplied by ATP, a high-energy substance discussed later in this chapter. This process also requires the presence of calcium ions in the extracellular fluid, which probably react with contractile protein filaments beneath the coated pits to provide the force for pinching the vesicles away from the cell membrane. \n**Phagocytosis.** Phagocytosis occurs in much the same way as pinocytosis, except that it involves large particles rather than molecules. Only certain cells have the capability of phagocytosis—notably, tissue macrophages and some white blood cells. \n \n**Figure 2-12.** Digestion of substances in pinocytotic or phagocytic vesicles by enzymes derived from lysosomes. \nPhagocytosis is initiated when a particle such as a bacterium, dead cell, or tissue debris binds with receptors on the surface of the phagocyte. In the case of bacteria, each bacterium is usually already attached to a specific antibody; it is the antibody that attaches to the phagocyte receptors, dragging the bacterium along with it. This intermediation of antibodies is called *opsonization,* which is discussed in Chapters 34 and 35. \nPhagocytosis occurs in the following steps: \n- 1. The cell membrane receptors attach to the surface ligands of the particle.\n- 2. The edges of the membrane around the points of attachment evaginate outward within a fraction of a second to surround the entire particle; then, progressively more and more membrane receptors attach to the particle ligands. All this occurs suddenly in a zipper-like manner to form a closed *phagocytic vesicle.*\n- 3. Actin and other contractile fibrils in the cytoplasm surround the phagocytic vesicle and contract around its outer edge, pushing the vesicle to the interior.\n- 4. The contractile proteins then pinch the stem of the vesicle so completely that the vesicle separates from the cell membrane, leaving the vesicle in the cell interior in the same way that pinocytotic vesicles are formed. \n#### **LYSOSOMES DIGEST PINOCYTOTIC AND PHAGOCYTIC FOREIGN SUBSTANCES INSIDE THE CELL** \nAlmost immediately after a pinocytotic or phagocytic vesicle appears inside a cell, one or more *lysosomes* become attached to the vesicle and empty their *acid hydrolases* to the inside of the vesicle, as shown in **[Figure 2-12](#page-25-0)**. Thus, a *digestive vesicle* is formed inside the cell cytoplasm in which the vesicular hydrolases begin hydrolyzing the proteins, carbohydrates, lipids, and other substances in the vesicle. The products of digestion are small molecules of substances such as amino acids, glucose, and phosphates that can diffuse through the membrane of the vesicle into the cytoplasm. What is left of the digestive vesicle, called the *residual body,* represents indigestible substances. In most cases, the residual body is finally excreted through the cell membrane by a process called *exocytosis,* which is essentially the opposite of endocytosis. Thus, the pinocytotic and phagocytic vesicles containing lysosomes can be called the *digestive organs* of the cells. \n**Lysosomes and Regression of Tissues and Autolysis of Damaged Cells.** Tissues of the body often regress to a smaller size. For example, this regression occurs in the uterus after pregnancy, in muscles during long periods of inactivity, and in mammary glands at the end of lactation. Lysosomes are responsible for much of this regression. \nAnother special role of the lysosomes is the removal of damaged cells or damaged portions of cells from tissues. Damage to the cell—caused by heat, cold, trauma, chemicals, or any other factor—induces lysosomes to rupture. The released hydrolases immediately begin to digest the surrounding organic substances. If the damage is slight, only a portion of the cell is removed, and the cell is then repaired. If the damage is severe, the entire cell is digested, a process called *autolysis.* In this way, the cell is completely removed, and a new cell of the same type is formed, ordinarily by mitotic reproduction of an adjacent cell to take the place of the old one. \nThe lysosomes also contain bactericidal agents that can kill phagocytized bacteria before they cause cellular damage. These agents include the following: (1) *lysozyme,* which dissolves the bacterial cell wall; (2) *lysoferrin,* which binds iron and other substances before they can promote bacterial growth; and (3) acid at a pH of about 5.0, which activates the hydrolases and inactivates bacterial metabolic systems. \n#### **Autophagy and Recycling of Cell Organelles.** \nLysosomes play a key role in the process of *autophagy,* which literally means \"to eat oneself.\" Autophagy is a housekeeping process whereby obsolete organelles and large protein aggregates are degraded and recycled (**[Figure 2-13](#page-26-0)**). Worn-out cell organelles are transferred to lysosomes by double-membrane structures called *autophagosomes,* which are formed in the cytosol. Invagination of the lysosomal membrane and the formation of vesicles provides another pathway for cytosolic structures to be transported into the lumen of lysosomes. Once inside the lysosomes, the organelles are digested, and the nutrients are reused by the cell. Autophagy contributes to the routine turnover of cytoplasmic components; it is a key mechanism for tissue development, cell survival when nutrients are scarce, and maintenance of homeostasis. In liver cells, for example, the average mitochondrion normally has a life span of only about 10 days before it is destroyed. \n \n**Figure 2-13.** Schematic diagram of autophagy steps. \n#### **SYNTHESIS OF CELLULAR STRUCTURES BY ENDOPLASMIC RETICULUM AND GOLGI APPARATUS** \n#### **Endoplasmic Reticulum Functions** \nThe extensiveness of the endoplasmic reticulum and Golgi apparatus in secretory cells has already been emphasized. These structures are formed primarily of lipid bilayer membranes, similar to the cell membrane, and their walls are loaded with protein enzymes that catalyze the synthesis of many substances required by the cell. \nMost synthesis begins in the endoplasmic reticulum. The products formed there are then passed on to the Golgi apparatus, where they are further processed before being released into the cytoplasm. First, however, let us note the specific products that are synthesized in specific portions of the endoplasmic reticulum and Golgi apparatus. \n#### **Proteins Synthesis by the Rough Endoplasmic Reticu-** \n**lum.** The rough endoplasmic reticulum is characterized by large numbers of ribosomes attached to the outer surfaces of the endoplasmic reticulum membrane. As discussed in Chapter 3, protein molecules are synthesized within the structures of the ribosomes. The ribosomes extrude some of the synthesized protein molecules directly into the cytosol, but they also extrude many more through the wall of the endoplasmic reticulum to the interior of the endoplasmic vesicles and tubules into the *endoplasmic matrix.* \n#### **Lipid Synthesis by the Smooth Endoplasmic Reticu-** \n**lum.** The endoplasmic reticulum also synthesizes lipids, especially phospholipids and cholesterol. These lipids are rapidly incorporated into the lipid bilayer of the endoplasmic reticulum, thus causing the endoplasmic reticulum to grow more extensive. This process occurs mainly in the smooth portion of the endoplasmic reticulum. \nTo keep the endoplasmic reticulum from growing beyond the needs of the cell, small vesicles called *ER vesicles* or *transport vesicles* continually break away from the smooth reticulum; most of these vesicles then migrate rapidly to the Golgi apparatus. \n#### **Other Functions of the Endoplasmic Reticulum.** \nOther significant functions of the endoplasmic reticulum, especially the smooth reticulum, include the following: \n- 1. It provides the enzymes that control glycogen breakdown when glycogen is to be used for energy.\n- 2. It provides a vast number of enzymes that are capable of detoxifying substances, such as drugs, that might damage the cell. It achieves detoxification by processes such as coagulation, oxidation, hydrolysis, and conjugation with glycuronic acid. \n#### **Golgi Apparatus Functions** \n**Synthetic Functions of the Golgi Apparatus.** Although a major function of the Golgi apparatus is to provide additional processing of substances already formed in the endoplasmic reticulum, it can also synthesize certain carbohydrates that cannot be formed in the endoplasmic reticulum. This is especially true for the formation of large saccharide polymers bound with small amounts of protein; important examples include *hyaluronic acid* and *chondroitin sulfate.* \nA few of the many functions of hyaluronic acid and chondroitin sulfate in the body are as follows: (1) they are the major components of proteoglycans secreted in mucus and other glandular secretions; (2) they are the major components of the *ground substance*, or nonfibrous components of the extracellular matrix, outside the cells in the interstitial spaces, which act as fillers between collagen fibers and cells; (3) they are principal components of the organic matrix in both cartilage and bone; and (4) they are important in many cell activities, including migration and proliferation. \n \n**Figure 2-14.** Formation of proteins, lipids, and cellular vesicles by the endoplasmic reticulum and Golgi apparatus. \nProcessing of Endoplasmic Secretions by the Golgi Apparatus—Formation of Vesicles. Figure 2-14 summarizes the major functions of the endoplasmic reticulum and Golgi apparatus. As substances are formed in the endoplasmic reticulum, especially proteins, they are transported through the tubules toward portions of the smooth endoplasmic reticulum that lie nearest to the Golgi apparatus. At this point, transport vesicles composed of small envelopes of smooth endoplasmic reticulum continually break away and diffuse to the deepest layer of the Golgi apparatus. Inside these vesicles are synthesized proteins and other products from the endoplasmic reticulum. \nThe transport vesicles instantly fuse with the Golgi apparatus and empty their contained substances into the vesicular spaces of the Golgi apparatus. Here, additional carbohydrate moieties are added to the secretions. Also, an important function of the Golgi apparatus is to compact the endoplasmic reticular secretions into highly concentrated packets. As the secretions pass toward the outermost layers of the Golgi apparatus, the compaction and processing proceed. Finally, both small and large vesicles continually break away from the Golgi apparatus, carrying with them the compacted secretory substances and diffusing throughout the cell. \nThe following example provides an idea of the timing of these processes. When a glandular cell is bathed in amino acids, newly formed protein molecules can be detected in the granular endoplasmic reticulum within 3 to 5 minutes. Within 20 minutes, newly formed proteins are already present in the Golgi apparatus and, within 1 to 2 hours, the proteins are secreted from the surface of the cell. \nTypes of Vesicles Formed by the Golgi Apparatus—Secretory Vesicles and Lysosomes. In a highly secretory cell, the vesicles formed by the Golgi apparatus are mainly secretory vesicles containing proteins that are secreted through the surface of the cell membrane. These secretory vesicles first diffuse to the cell membrane and then fuse with it and empty their substances to the exterior by the mechanism called exocytosis. Exocytosis, in most cases, is stimulated by entry of calcium ions into the cell. Calcium ions interact with the vesicular membrane and cause its fusion with the cell membrane, followed by exocytosis—opening of the membrane's outer surface and extrusion of its contents outside the cell. Some vesicles, however, are destined for intracellular use. \n**Use of Intracellular Vesicles to Replenish Cellular Membranes.** Some intracellular vesicles formed by the Golgi apparatus fuse with the cell membrane or with the membranes of intracellular structures such as the mitochondria and even the endoplasmic reticulum. This fusion increases the expanse of these membranes and replenishes the membranes as they are used up. For example, the cell membrane loses much of its substance every time it forms a phagocytic or pinocytotic vesicle, and the vesicular membranes of the Golgi apparatus continually replenish the cell membrane. \nIn summary, the membranous system of the endoplasmic reticulum and Golgi apparatus are highly metabolic and capable of forming new intracellular structures and secretory substances to be extruded from the cell.\n\nThe principal substances from which cells extract energy are foods that react chemically with oxygen—carbohydrates, fats, and proteins. In the human body, essentially all carbohydrates are converted into *glucose* by the digestive tract and liver before they reach the other cells of the body. Similarly, proteins are converted into *amino acids*, and fats are converted into *fatty acids*. **Figure 2-15** shows oxygen and the foodstuffs—glucose, fatty acids, and amino acids—all entering the cell. Inside the cell, they react chemically with oxygen under the influence of enzymes that control the reactions and channel the energy released in the proper direction. The details of all these digestive and metabolic functions are provided in Chapters 63 through 73. \nBriefly, almost all these oxidative reactions occur inside the mitochondria, and the energy that is released is used to form the high-energy compound ATP. Then, ATP, not the original food, is used throughout the cell to energize almost all the subsequent intracellular metabolic reactions. \n \n**Figure 2-15.** Formation of adenosine triphosphate (ATP) in the cell showing that most of the ATP is formed in the mitochondria. (ADP, Adenosine diphosphate; CoA, coenzyme A.) \n#### **Functional Characteristics of Adenosine Triphosphate** \n$$\\begin{array}{c|ccccccccccccccccccccccccccccccccccc$$ \n**Adenosine triphosphate** \nATP is a nucleotide composed of the following: (1) the nitrogenous base *adenine;* (2) the pentose sugar *ribose;* and (3) three *phosphate radicals.* The last two phosphate radicals are connected with the remainder of the molecule by *high-energy phosphate bonds,* which are represented in the formula shown by the symbol ∼. *Under the physical and chemical conditions of the body,* each of these high-energy bonds contains about 12,000 calories of energy per mole of ATP, which is many times greater than the energy stored in the average chemical bond, thus giving rise to the term *high-energy bond.* Furthermore, the high-energy phosphate bond is very labile, so that it can be split instantly on demand whenever energy is required to promote other intracellular reactions. \nWhen ATP releases its energy, a phosphoric acid radical is split away, and *adenosine diphosphate* (ADP) is formed. This released energy is used to energize many of the cell's other functions, such as syntheses of substances and muscular contraction. \nTo reconstitute the cellular ATP as it is used up, energy derived from the cellular nutrients causes ADP and phosphoric acid to recombine to form new ATP, and the entire process is repeated over and over. For these reasons, ATP has been called the *energy currency* of the cell because it can be spent and reformed continually, having a turnover time of only a few minutes. \n**Chemical Processes in the Formation of ATP—Role of the Mitochondria.** On entry into the cells, glucose is converted by enzymes in the *cytoplasm* into *pyruvic acid* (a process called *glycolysis*). A small amount of ADP is changed into ATP by the energy released during this conversion, but this amount accounts for less than 5% of the overall energy metabolism of the cell. \nAbout 95% of the cell's ATP formation occurs in the mitochondria. The pyruvic acid derived from carbohydrates, fatty acids from lipids, and amino acids from proteins is eventually converted into the compound *acetyl-coenzyme A* (CoA) in the matrix of mitochondria. This substance, in turn, is further dissolved (for the purpose of extracting its energy) by another series of enzymes in the mitochondrion matrix, undergoing dissolution in a sequence of chemical reactions called the *citric acid cycle,* or *Krebs cycle.* These chemical reactions are so important that they are explained in detail in Chapter 68. \nIn this citric acid cycle, acetyl-CoA is split into its component parts, *hydrogen atoms* and *carbon dioxide.* The carbon dioxide diffuses out of the mitochondria and eventually out of the cell; finally, it is excreted from the body through the lungs. \nThe hydrogen atoms, conversely, are highly reactive; they combine with oxygen that has also diffused into the mitochondria. This combination releases a tremendous amount of energy, which is used by mitochondria to convert large amounts of ADP to ATP. The processes of these reactions are complex, requiring the participation of many protein enzymes that are integral parts of mitochondrial *membranous shelves* that protrude into the mitochondrial matrix. The initial event is the removal of an electron from the hydrogen atom, thus converting it to a hydrogen ion. The terminal event is the combination of hydrogen ions with oxygen to form water and the release of large amounts of energy to globular proteins that protrude like knobs from the membranes of the mitochondrial shelves; these proteins are called *ATP synthetase*. Finally, the enzyme ATP synthetase uses the energy from the hydrogen ions to convert ADP to ATP. The newly formed ATP is transported out of the mitochondria into all parts of the cell cytoplasm and nucleoplasm, where it energizes multiple cell functions. \nThis overall process for formation of ATP is called the *chemiosmotic mechanism* of ATP formation. The chemical and physical details of this mechanism are presented \n \n**Figure 2-16.** Use of adenosine triphosphate (ATP; formed in the mitochondrion) to provide energy for three major cellular functions—membrane transport, protein synthesis, and muscle contraction. (ADP, Adenosine diphosphate.) \nin Chapter 68, and many of the detailed metabolic functions of ATP in the body are discussed in Chapters 68 through 72. \n**Uses of ATP for Cellular Function.** Energy from ATP is used to promote three major categories of cellular functions: (1) *transport* of substances through multiple cell membranes; (2) *synthesis of chemical compounds* throughout the cell; and (3) *mechanical work.* These uses of ATP are illustrated by the examples in **Figure 2-16**: (1) to supply energy for the transport of sodium through the cell membrane; (2) to promote protein synthesis by the ribosomes; and (3) to supply the energy needed during muscle contraction. \nIn addition to the membrane transport of sodium, energy from ATP is required for the membrane transport of potassium, calcium, magnesium, phosphate, chloride, urate, and hydrogen ions and many other ions, as well as various organic substances. Membrane transport is so important to cell function that some cells—the renal tubular cells, for example—use as much as 80% of the ATP that they form for this purpose alone. \nIn addition to synthesizing proteins, cells make phospholipids, cholesterol, purines, pyrimidines, and many other substances. Synthesis of almost any chemical compound requires energy. For example, a single protein molecule might be composed of as many as several thousand amino acids attached to one another by peptide linkages. The formation of each of these linkages requires energy derived from the breakdown of four high-energy bonds; thus, many thousand ATP molecules must release their energy as each protein molecule is formed. Indeed, some cells use as much as 75% of all the ATP formed in the cell \n \nFigure 2-17. Ameboid motion by a cell. \nsimply to synthesize new chemical compounds, especially protein molecules; this is particularly true during the growth phase of cells. \nAnother use of ATP is to supply energy for special cells to perform mechanical work. We discuss in Chapter 6 that each contraction of a muscle fiber requires the expenditure of large quantities of ATP energy. Other cells perform mechanical work in other ways, especially by *ciliary* and *ameboid motion*, described later in this chapter. The source of energy for all these types of mechanical work is ATP. \nIn summary, ATP is readily available to release its energy rapidly wherever it is needed in the cell. To replace ATP used by the cell, much slower chemical reactions break down carbohydrates, fats, and proteins and use the energy derived from these processes to form new ATP. More than 95% of this ATP is formed in the mitochondria, which is why the mitochondria are called the *powerhouses* of the cell. \n#### **LOCOMOTION OF CELLS** \nThe most obvious type of movement in the body is that which occurs in skeletal, cardiac, and smooth muscle cells, which constitute almost 50% of the entire body mass. The specialized functions of these cells are discussed in Chapters 6 through 9. Two other types of movement—ameboid locomotion and ciliary movement—occur in other cells. \n#### AMEBOID MOVEMENT \nAmeboid movement is a crawling-like movement of an entire cell in relation to its surroundings, such as movement of white blood cells through tissues. This type of movement gets its name from the fact that amebae move in this manner, and amebae have provided an excellent tool for studying the phenomenon. \nTypically, ameboid locomotion begins with the protrusion of a *pseudopodium* from one end of the cell. The pseudopodium projects away from the cell body and partially secures itself in a new tissue area; then the remainder of the cell is pulled toward the pseudopodium. **Figure 2-17** \ndemonstrates this process, showing an elongated cell, the right-hand end of which is a protruding pseudopodium. The membrane of this end of the cell is continually moving forward, and the membrane at the left-hand end of the cell is continually following along as the cell moves. \n**Mechanism of Ameboid Locomotion. [Figure 2-17](#page-29-1)** shows the general principle of ameboid motion. Basically, this results from the continual formation of new cell membrane at the leading edge of the pseudopodium and continual absorption of the membrane in the mid and rear portions of the cell. Two other effects are also essential for forward movement of the cell. The first is attachment of the pseudopodium to surrounding tissues so that it becomes fixed in its leading position while the remainder of the cell body is being pulled forward toward the point of attachment. This attachment is caused by *receptor proteins* that line the insides of exocytotic vesicles. When the vesicles become part of the pseudopodial membrane, they open so that their insides evert to the outside, and the receptors now protrude to the outside and attach to ligands in the surrounding tissues. \nAt the opposite end of the cell, the receptors pull away from their ligands and form new endocytotic vesicles. Then, inside the cell, these vesicles stream toward the pseudopodial end of the cell, where they are used to form new membrane for the pseudopodium. \nThe second essential effect for locomotion is to provide the energy required to pull the cell body in the direction of the pseudopodium. A moderate to large amount of the protein *actin* is in the cytoplasm of all cells*.* Much of the actin is in the form of single molecules that do not provide any motive power; however, these molecules polymerize to form a filamentous network, and the network contracts when it binds with an actin-binding protein such as *myosin.* The entire process is energized by the high-energy compound ATP. This is what occurs in the pseudopodium of a moving cell, where such a network of actin filaments forms anew inside the enlarging pseudopodium. Contraction also occurs in the ectoplasm of the cell body, where a preexisting actin network is already present beneath the cell membrane. \n#### **Types of Cells That Exhibit Ameboid Locomotion.** \nThe most common cells to exhibit ameboid locomotion in the human body are the *white blood cells* when they move out of the blood into the tissues to form *tissue macrophages.* Other types of cells can also move by ameboid locomotion under certain circumstances. For example, fibroblasts move into a damaged area to help repair the damage, and even the germinal cells of the skin, although ordinarily completely sessile cells, move toward a cut area to repair the opening. Cell locomotion is also especially important in the development of the embryo and fetus after fertilization of an ovum. For example, embryonic cells often must migrate long distances from their sites of origin to new areas during the development of special structures. \nSome types of cancer cells, such as sarcomas, which arise from connective tissue cells, are especially proficient at ameboid movement. This partially accounts for their relatively rapid spreading from one part of the body to another, known as *metastasis.* \n**Control of Ameboid Locomotion—Chemotaxis.** An important initiator of ameboid locomotion is the process called *chemotaxis,* which results from the appearance of certain chemical substances in the tissues. Any chemical substance that causes chemotaxis to occur is called a *chemotactic substance.* Most cells that exhibit ameboid locomotion move toward the source of a chemotactic substance—that is, from an area of lower concentration toward an area of higher concentration. This is called *positive chemotaxis.* Some cells move away from the source, which is called *negative chemotaxis.* \nHow does chemotaxis control the direction of ameboid locomotion? Although the answer is not certain, it is known that the side of the cell most exposed to the chemotactic substance develops membrane changes that cause pseudopodial protrusion. \n#### **CILIA AND CILIARY MOVEMENTS** \nThere are two types of cilia, *motile* and *nonmotile*, or *primary*, cilia. Motile cilia can undergo a whiplike movement on the surfaces of cells. This movement occurs mainly in two places in the human body, on the surfaces of the respiratory airways and on the inside surfaces of the uterine tubes (fallopian tubes) of the reproductive tract. In the nasal cavity and lower respiratory airways, the whiplike motion of motile cilia causes a layer of mucus to move at a rate of about 1 cm/min toward the pharynx, in this way continually clearing these passageways of mucus and particles that have become trapped in the mucus. In the uterine tubes, cilia cause slow movement of fluid from the ostium of the uterine tube toward the uterus cavity; this movement of fluid transports the ovum from the ovary to the uterus. \nAs shown in **[Figure 2-18](#page-31-0)**, a cilium has the appearance of a sharp-pointed straight or curved hair that projects 2 to 4 micrometers from the surface of the cell. Often, many motile cilia project from a single cell—for example, as many as 200 cilia on the surface of each epithelial cell inside the respiratory passageways. The cilium is covered by an outcropping of the cell membrane, and it is supported by 11 microtubules—nine double tubules located around the periphery of the cilium and two single tubules down the center, as demonstrated in the cross section shown in **[Figure 2-18](#page-31-0)**. Each cilium is an outgrowth of a structure that lies immediately beneath the cell membrane, called the *basal body* of the cilium. \nThe *flagellum of a sperm* is similar to a motile cilium; in fact, it has much the same type of structure and same type of contractile mechanism. The flagellum, however, is much longer and moves in quasisinusoidal waves instead of whiplike movements. \n \n**Figure 2-18.** Structure and function of the cilium. *(Modified from Satir P: Cilia. Sci Am 204:108, 1961.)* \nIn the inset of **[Figure 2-18](#page-31-0)**, movement of the motile cilium is shown. The cilium moves forward with a sudden, rapid whiplike stroke 10 to 20 times per second, bending sharply where it projects from the surface of the cell. Then it moves backward slowly to its initial position. The rapid, forward-thrusting, whiplike movement pushes the fluid lying adjacent to the cell in the direction that the cilium moves; the slow dragging movement in the backward direction has almost no effect on fluid movement. As a result, the fluid is continually propelled in the direction of the fast-forward stroke. Because most motile ciliated cells have large numbers of cilia on their surfaces, and because all the cilia are oriented in the same direction, this is an effective means for moving fluids from one part of the surface to another. \n**Mechanism of Ciliary Movement.** Although not all aspects of ciliary movement are known, we are aware of the following elements. First, the nine double tubules and two single tubules are all linked to one another by a complex of protein cross-linkages; this total complex of tubules and cross-linkages is called the *axoneme.* Second, even after removal of the membrane and destruction of other elements of the cilium in addition to the axoneme, the cilium can still beat under appropriate conditions. Third, two conditions are necessary for continued beating of the axoneme after removal of the other structures of the cilium: (1) the availability of ATP; and (2) appropriate ionic conditions, especially appropriate concentrations of magnesium and calcium. Fourth, during forward motion of the cilium, the double tubules on the front edge of the cilium slide outward toward the tip of the cilium, whereas those on the back edge remain in place. Fifth, multiple protein arms composed of the protein *dynein,* which has adenosine triphosphatase (ATPase) enzymatic activity, project from each double tubule toward an adjacent double tubule. \nGiven this basic information, it has been determined that the release of energy from ATP in contact with the ATPase dynein arms causes the heads of these arms to \"crawl\" rapidly along the surface of the adjacent double tubule. If the front tubules crawl outward while the back tubules remain stationary, bending occurs. \nThe way in which cilia contraction is controlled is not well understood. The cilia of some genetically abnormal cells do not have the two central single tubules, and these cilia fail to beat. Therefore, it is presumed that some signal, perhaps an electrochemical signal, is transmitted along these two central tubules to activate the dynein arms. \n**Nonmotile Primary Cilia Serve as Cell Sensory \"Antennae.\"** *Primary cilia* are nonmotile and generally occur only as a single cilium on each cell. Although the physiological functions of primary cilia are not fully understood, current evidence indicates that they function as cellular ''sensory antennae,\" which coordinate cellular signaling pathways involved in chemical and mechanical sensation, signal transduction, and cell growth. In the kidneys, for example, primary cilia are found in most epithelial cells of the tubules, projecting into the tubule lumen and acting as a flow sensor. In response to fluid flow over the tubular epithelial cells, the primary cilia bend and cause flow-induced changes in intracellular calcium signaling. These signals, in turn, initiate multiple effects on the cells. Defects in signaling by primary cilia in renal tubular epithelial cells are thought to contribute to various disorders, including the development of large fluid-filled cysts, a condition called *polycystic kidney disease*. \n#### Bibliography \nAlberts B, Johnson A, Lewis J, et al: Molecular Biology of the Cell, 6th ed. New York: Garland Science, 2014. \nBrandizzi F, Barlowe C: Organization of the ER-Golgi interface for membrane traffic control. Nat Rev Mol Cell Biol 14:382, 2013. \nDikic I, Elazar Z. Mechanism and medical implications of mammalian autophagy. Nat Rev Mol Cell Biol 19:349, 2018. \nEisner V, Picard M, Hajnóczky G. Mitochondrial dynamics in adaptive and maladaptive cellular stress responses. Nat Cell Biol 20:755, 2018. \nGalluzzi L, Yamazaki T, Kroemer G. Linking cellular stress responses to systemic homeostasis. Nat Rev Mol Cell Biol 19:731, 2018. \n- Guerriero CJ, Brodsky JL: The delicate balance between secreted protein folding and endoplasmic reticulum-associated degradation in human physiology. Physiol Rev 92:537, 2012.\n- Harayama T, Riezman H. Understanding the diversity of membrane lipid composition. Nat Rev Mol Cell Biol 19:281, 2018.\n- Insall R: The interaction between pseudopods and extracellular signalling during chemotaxis and directed migration. Curr Opin Cell Biol 25:526, 2013.\n- Kaksonen M, Roux A. Mechanisms of clathrin-mediated endocytosis. Nat Rev Mol Cell Biol 19:313, 2018.\n- Lawrence RE, Zoncu R. The lysosome as a cellular centre for signalling, metabolism and quality control. Nat Cell Biol 21: 133, 2019.\n- Nakamura N, Wei JH, Seemann J: Modular organization of the mammalian Golgi apparatus. Curr Opin Cell Biol 24:467, 2012. \n- Palikaras K, Lionaki E, Tavernarakis N. Mechanisms of mitophagy in cellular homeostasis, physiology and pathology. Nat Cell Biol 20:1013, 2018.\n- Sezgin E, Levental I, Mayor S, Eggeling C. The mystery of membrane organization: composition, regulation and roles of lipid rafts. Nat Rev Mol Cell Biol 18:361, 2017.\n- Spinelli JB, Haigis MC. The multifaceted contributions of mitochondria to cellular metabolism. Nat Cell Biol. 20:745, 2018.\n- Walker CL, Pomatto LCD, Tripathi DN, Davies KJA. Redox regulation of homeostasis and proteostasis in peroxisomes. Physiol Rev 98:89, 2018.\n- Zhou K, Gaullier G, Luger K. Nucleosome structure and dynamics are coming of age. Nat Struct Mol Biol 26:3, 2019. \n\n\nGenes, which are located in the nuclei of all cells of the body, control heredity from parents to children, as well as the daily functioning of all the body's cells. The genes control cell function by determining which structures, enzymes, and chemicals are synthesized within the cell. \n**Figure 3-1** shows the general schema of genetic control. Each gene, which is composed of *deoxyribonucleic acid* (DNA), controls the formation of another nucleic acid, *ribonucleic acid* (RNA); this RNA then spreads throughout the cell to control formation of a specific protein. The entire process, from *transcription* of the genetic code in the nucleus to *translation* of the RNA code and the formation of proteins in the cell cytoplasm, is often referred to as *gene expression.* \nBecause the human body has approximately 20,000 to 25,000 different genes that code for proteins in each cell, it is possible to form a large number of different cellular proteins. In fact, RNA molecules transcribed from the same segment of DNA—the same gene—can be processed in more than one way by the cell, giving rise to alternate versions of the protein. The total number of different proteins produced by the various cell types in humans is estimated to be at least 100,000. \nSome of the cellular proteins are *structural proteins,* which, in association with various lipids and carbohydrates, form structures of the various intracellular organelles discussed in Chapter 2. However, most of the proteins are *enzymes* that catalyze different chemical reactions in the cells. For example, enzymes promote all the oxidative reactions that supply energy to the cell, along with synthesis of all the cell chemicals, such as lipids, glycogen, and adenosine triphosphate (ATP). \n#### CELL NUCLEUS GENES CONTROL PROT[EIN](#page-34-0) SYNTHESIS \nIn the cell nucleus, large numbers of genes are attached end on end in extremely long, double-stranded helical molecules of DNA having molecular weights measured in the billions. A very short segment of such a molecule is shown in **Figure 3-2**. This molecule is composed of several simple chemical compounds bound together in a regular pattern, the details of which are explained in the next few paragraphs. \n#### **Building Blocks of DNA** \n**Figure 3-3** shows the basic chemical compounds involved in the formation of DNA. These compounds include the following: (1) *phosphoric acid;* (2) a sugar [called](#page-34-0) *deoxyribose;* and (3) four nitrogenous *bases* (two purines, *adenine* and *guanine,* and two pyrimidines, *thymine* and *cytosine*). The phosphoric acid and deoxyribose form the two helical strands that are the backbone of the DNA molecule, and the nitrogenous bases lie between the two strands and connect them, as illustrated in **Figure 3-2**. \n#### **Nucleotides** \nThe first stage of DNA formation is to combine one molecule of phosphoric acid, one molecule of deoxyribose, and one of the four bases to form an acidic nucleotide. Four separate nucleotides are thus formed, one for each of the four bases: *deoxyadenylic, deoxythymidylic, deoxyguanylic,* and *deoxycytidylic acids*. **Figure 3-4** shows the chemical \n \n**Figure 3-1** The general schema whereby genes control cell function. *mRNA,* Messenger RNA. \n \n**Figure 3-2** The helical double-stranded structure of the gene. The outside strands are composed of phosphoric acid and the sugar deoxyribose. The internal molecules connecting the two strands of the helix are purine and pyrimidine bases, which determine the \"code\" of the gene. \n \nFigure 3-3 The basic building blocks of DNA. \nstructure of deoxyadenylic acid, and **Figure 3-5** shows simple symbols for the four nucleotides that form DNA. \n#### Nucleotides Are Organized to Form Two Strands of DNA Loosely Bound to Each Other \n**Figure 3-2** shows the manner in which multiple nucleotides are bound together to form two strands of DNA. The two strands are, in turn, loosely bonded with each other by weak cross-linkages, as illustrated in **Figure 3-6** by the central dashed lines. Note that the backbone of each DNA strand is composed of alternating phosphoric acid and deoxyribose molecules. In turn, purine and pyrimidine bases are attached to the sides of the deoxyribose molecules. Then, by means of loose *hydrogen bonds* (dashed \n**Figure 3-4.** Deoxyadenylic acid, one of the nucleotides that make up DNA. \n \n**Figure 3-5.** Symbols for the four nucleotides that combine to form DNA. Each nucleotide contains phosphoric acid (P), deoxyribose (D), and one of the four nucleotide bases: adenine (A); thymine (T); guanine (G); or cytosine (C). \nlines) between the purine and pyrimidine bases, the two respective DNA strands are held together. Note the following caveats, however: \n- 1. Each purine base *adenine* of one strand always bonds with a pyrimidine base *thymine* of the other strand.\n- 2. Each purine base *guanine* always bonds with a pyrimidine base *cytosine*. \nThus, in **Figure 3**-6, the sequence of complementary pairs of bases is CG, CG, GC, TA, CG, TA, GC, AT, and AT. Because of the looseness of the hydrogen bonds, the two strands can pull apart with ease, and they do so many times during the course of their function in the cell. \nTo put the DNA of **Figure 3**-6 into its proper physical perspective, one could merely pick up the two ends and twist them into a helix. Ten pairs of nucleotides are present in each full turn of the helix in the DNA molecule. \n#### **GENETIC CODE** \nThe importance of DNA lies in its ability to control the formation of proteins in the cell, which it achieves by means of a *genetic code*. That is, when the two strands of a DNA molecule are split apart, the purine and pyrimidine bases projecting to the side of each DNA strand are exposed, as shown by the top strand in **Figure 3-7**. It is these projecting bases that form the genetic code. \nThe genetic code consists of successive \"triplets\" of bases—that is, each three successive bases is a *code word*. The successive triplets eventually control the sequence of amino acids in a protein molecule that is to be synthesized in the cell. Note in **Figure 3**-6 that the top strand of DNA, reading from left to right, has the genetic code GGC, AGA, CTT, with the triplets being separated from one another by the arrows. As we follow this genetic code through **Figure 3**-7 and **Figure 3**-8, we see that these three respective triplets are responsible for successive placement of the three amino acids, *proline, serine,* and *glutamic acid,* in a newly formed molecule of protein.\n\nBecause DNA is located in the cell nucleus, yet most of the cell functions are carried out in the cytoplasm, there must be some means for DNA genes of the nucleus to control chemical reactions of the cytoplasm. This control \n \n**Figure 3-6.** Arrangement of deoxyribose nucleotides in a double strand of DNA. \n \nR C \nP \nP R C P R G P R U \n**Figure 3-7.** Combination of ribose nucleotides with a strand of DNA to form a molecule of RNA that carries the genetic code from the gene to the cytoplasm. The *RNA polymerase* enzyme moves along the DNA strand and builds the RNA molecule. \n**Figure 3-8.** A portion of an RNA molecule showing thre[e](#page-36-0) RNA codons—CCG, UCU, and GAA—that control attachment of the three amino acids, proline, serine, and glutamic acid, respectively, to the growing RNA chain. **Proline Serine Glutamic acid** \nis achieved through the intermediary of another type of nucleic acid, RNA, the formation of which is controlled by DNA of the nucleus. Thus, as shown in **Figure 3-7**, the code is transferred to RNA in a process called *transcription.* The RNA, in turn, diffuses from the nucleus through nuclear pores into the cytoplasmic compartment, where it controls protein synthesis. \n#### **RNA IS SYNTHESIZED IN THE NUCLEUS FROM A DNA TEMPLATE** \nDuring RNA synthesis, the two strands of DNA separate temporarily; one of these strands is used as a template for synthesis of an RNA molecule. The code triplets in the DNA result in the formation of *complementary* code triplets (called *codons*) in the RNA. These codons, in turn, will control the sequence of amino acids in a protein to be synthesized in the cell cytoplasm. \n**Building Blocks of RNA.** The basic building blocks of RNA are almost the same as those of DNA, except for two differences. First, the sugar deoxyribose is not used in RNA formation. In its place is another sugar of slightly different composition, *ribose,* which contains an extra hydroxyl ion appended to the ribose ring structure. Second, thymine is replaced by another pyrimidine, *uracil.* \n**Formation of RNA Nucleotides.** The basic building blocks of RNA form *RNA nucleotides,* exactly as described previously for DNA synthesis. Here again, four separate nucleotides are used to form RNA. These nucleotides contain the bases *adenine, guanine, cytosine,* and *uracil.* Note that these bases are the same as in DNA, except that uracil in RNA replaces thymine in DNA. \n**\"Activation\" of RNA Nucleotides.** The next step in the synthesis of RNA is \"activation\" of RNA nucleotides by an enzyme, *RNA polymerase.* This activation occurs by adding two extra phosphate radicals to each nucleotide to form \ntriphosphates (shown in **Figure 3-7** by the two RNA nucleotides to the far right during RNA chain formation). These last two phosphates are combined with the nucleotide by \nP R U P R G P R A P R A \nP R C \n*high-energy phosphate bonds* derived from ATP in the cell. The result of this activation process is that large quantities of ATP energy are made available to each of the nucleotides. This energy is used to promote chemical reactions that add each new RNA nucleotide at the end of the developing RNA [chain.](#page-36-0) \n#### **RNA CHAIN ASSEMBLY FROM ACTIVATED NUCLEOTIDES USING THE DNA STRAND AS A TEMPLATE** \nAs shown in **Figure 3-7,** assembly of RNA is accomplished under the influence of an enzyme, *RNA polymerase.* This large protein enzyme has many functional properties necessary for formation of RNA, as follows: \n- 1. In the DNA strand immediately ahead of the gene to be transcribed is a sequence of nucleotides called the *promoter.* The RNA polymerase has an appropriate complementary structure that recognizes this promoter and becomes attached to it, which is the essential step for initiating the formation of RNA.\n- 2. After the RNA polymerase attaches to the promoter, the polymerase causes unwinding of about two turns of the DNA helix and separation of the unwound portions of the two strands.\n- 3. The polymerase then moves along the DNA strand, temporarily unwinding and separating the two DNA strands at each stage of its movement. As it moves along, at each stage it adds a new activated RNA nucleotide to the end of the newly forming RNA chain through the following steps:\n- a. First, it causes a hydrogen bond to form between the end base of the DNA strand and the base of an RNA nucleotide in the nucleoplasm. \n- b. Then, one at a time, the RNA polymerase breaks two of the three phosphate radicals away from each of these RNA nucleotides, liberating large amounts of energy from the broken high-energy phosphate bonds. This energy is used to cause covalent linkage of the remaining phosphate on the nucleotide with the ribose on the end of the growing RNA chain.\n- c. When the RNA polymerase reaches the end of the DNA gene, it encounters a new sequence of DNA nucleotides called the *chain-terminating sequence,* which causes the polymerase and the newly formed RNA chain to break away from the DNA strand. The polymerase then can be used again and again to form more new RNA chains.\n- d. As the new RNA strand is formed, its weak hydrogen bonds with the DNA template break away because the DNA has a high affinity for rebonding with its own complementary DNA strand. Thus, the RNA chain is forced away from the DNA and is released into the nucleoplasm. \nTherefore, the code that is present in the DNA strand is eventually transmitted in *complementary* form to the RNA chain. The ribose nucleotide bases always combine with the deoxyribose bases in the following combinations: \n| DNA Base | RNA Base |\n|----------|----------|\n| guanine | Cytosine |\n| cytosine | Guanine |\n| adenine | Uracil |\n| thymine | adenine | \n**There Are Several Different Types of RNA.** As research on RNA has continued to advance, many different types of RNA have been discovered. Some types of RNA are involved in protein synthesis, whereas other types serve gene regulatory functions or are involved in posttranscriptional modification of RNA. The functions of some types of RNA, especially those that do not appear to code for proteins, are still mysterious. The following six types of RNA play independent and different roles in protein synthesis: \n- 1. *Precursor messenger RNA* (pre-mRNA) is a large, immature, single strand of RNA that is processed in the nucleus to form mature messenger RNA (mRNA). The pre-RNA includes two different types of segments, called *introns,* which are removed by a process called splicing, and *exons,* which are retained in the final mRNA.\n- 2. *Small nuclear RNA* (snRNA) directs the splicing of pre-mRNA to form mRNA.\n- 3. *Messenger RNA* (mRNA) carries the genetic code to the cytoplasm for controlling the type of protein formed.\n- 4. *Transfer RNA* (tRNA) transports activated amino acids to the ribosomes to be used in assembling the protein molecule.\n- 5. *Ribosomal RNA,* along with about 75 different proteins, forms *ribosomes,* the physical and chemical \n- structures on which protein molecules are actually assembled.\n- 6. *MicroRNAs* (miRNAs) are single-stranded RNA molecules of 21 to 23 nucleotides that can regulate gene transcription and translation. \n#### **MESSENGER RN[A—THE C](#page-36-1)ODONS** \n*Messenger RNA* molecules are long single RNA strands that are suspended in the cytoplasm. These molecules are composed of several hundred to several thousand RNA nucleotides in unpaired strands, and they contain *c[odons](#page-36-0)* that are exactly complementary to the code triplets of t[he](#page-37-0) DNA genes. **Figure 3-8** shows a small segment of mRNA. Its codons are CCG, UCU, and GAA, which are the codons for the amino acids proline, serine, and glutamic acid. The transcription of these codons from the DNA molecule to the RNA molecule is shown in **Figure 3-7**. \n**RNA Codons for the Different Amino Acids. [Table 3](#page-37-0)-1** lists the RNA codons for the 20 common amino acids found in protein molecules. Note that most of the amino acids are represented by more than one codon; also, one codon represents the signal \"start manufacturing the protein molecule,\" and three codons represent \"stop manufacturing the protein molecule.\" In **Table 3-1**, these two \n**Table 3-1** RNA Codons for Amino Acids and for Start and Stop \n| Amino Acid | | | RNA Codons | | | |\n|---------------|-----|-----|------------|-----|-----|-----|\n| Alanine | GCU | GCC | GCA | GCG | | |\n| Arginine | CGU | CGC | CGA | CGG | AGA | AGG |\n| Asparagine | AAU | AAC | | | | |\n| Aspartic acid | GAU | GAC | | | | |\n| Cysteine | UGU | UGC | | | | |\n| Glutamic acid | GAA | GAG | | | | |\n| Glutamine | CAA | CAG | | | | |\n| Glycine | GGU | GGC | GGA | GGG | | |\n| Histidine | CAU | CAC | | | | |\n| Isoleucine | AUU | AUC | AUA | | | |\n| Leucine | CUU | CUC | CUA | CUG | UUA | UUG |\n| Lysine | AAA | AAG | | | | |\n| Methionine | AUG | | | | | |\n| Phenylalanine | UUU | UUC | | | | |\n| Proline | CCU | CCC | CCA | CCG | | |\n| Serine | UCU | UCC | UCA | UCG | AGC | AGU |\n| Threonine | ACU | ACC | ACA | ACG | | |\n| Tryptophan | UGG | | | | | |\n| Tyrosine | UAU | UAC | | | | |\n| Valine | GUU | GUC | GUA | GUG | | |\n| Start (CI) | AUG | | | | | |\n| Stop (CT) | UAA | UAG | UGA | | | | \n*CI,* Chain-initiating; *CT,* chain-terminating. \ntypes of codons are designated CI for \"chain-initiating\" or \"start\" codon and CT for \"chain-terminating\" or \"stop\" codon. \n#### **TRANSFER RNA—THE ANTICODONS** \nAnother type of RNA that is essential for protein synthesis is called transfer RNA (tRNA) because it transfers amino acids to protein molecules as the protein is being synthesized. Each type of tRNA combines specifically with 1 of the 20 amino acids that are to be incorporated into proteins. The tRNA then acts as a *carrier* to transport its specific type of amino acid to the ribosomes, where protein molecules are forming. In the ribosomes, each specific type of tRNA recognizes a particular codon on the mRNA (described later) and thereby delivers [the appro](#page-38-0)priate amino acid to the appropriate place in the chain of the newly forming protein molecule. \nTransfer RNA, which contains only about 80 nucleotides, is a relatively small molecule in comparison with mRNA. It is a folded chain of nucleotides with a cloverleaf appearance similar to that shown in **Figure 3-9**. At one end of the molecule there is always an adenylic acid to which the transported amino acid attaches at a hydroxyl group of the ribose in the adenylic acid. \nBecause the function of tRNA is to cause attachment of a specific amino acid to a forming protein chain, it is essential that each type of t[RNA also ha](#page-38-0)ve specificity for a particular codon in the mRNA. The specific code in the tRNA that allows it to recognize a specific codon is again a triplet of nucleotide bases and is called an *anticodon.* This anticodon is located approximately in the middle of the tRNA molecule (at the bottom of the cloverleaf configuration shown in **Figure 3-9**). During formation of the protein molecule, the anticodon bases combine loosely by hydrogen bonding with the codon bases of the mRNA. In this way, the respective amino acids are lined up one after another along the mRNA chain, thus establishing the \n \n**Figure 3-9.** A messenger RNA strand is moving through two ribosomes. As each codon passes through, an amino acid is added to the growing protein chain, which is shown in the right-hand ribosome. The transfer RNA molecule transports each specific amino acid to the newly forming protein. \nappropriate sequence of amino acids in the newly forming protein molecule. \n#### **RIBOSOMAL RNA** \nThe third type of RNA in the cell is ribosomal RNA, which constitutes about 60% of the *ribosome.* The remainder of the ribosome is protein, including about 75 types of proteins that are both structural proteins and enzymes needed to manufacture proteins. \nThe ribosome is the physical structure in the cytoplasm on which proteins are actually synthesized. However, it always functions in association with the other two types of RNA; *tRNA* transports amino acids to the ribosome for incorporation into the developing protein, whereas *mRNA* provides the information necessary for sequencing the amino acids in proper order for each specific type of protein to be manufactured. Thus, the ribosome acts as a manufacturing plant in which the protein molecules are formed. \n**Formation of Ribosomes in the Nucleolus.** The DNA genes for the formation of ribosomal RNA are located in five pairs of chromosomes in the nucleus. Each of these chromosomes contains many duplicates of these particular genes because of the large amounts of ribosomal RNA required for cellular function. \nAs the ribosomal RNA forms, it collects in the *nucleolus,* a specialized structure lying adjacent to the chromosomes. When large amounts of ribosomal RNA are being synthesized, as occurs in cells that manufacture large amounts of protein, the nucleolus is a large structure, whereas in cells that synthesize little protein, the nucleolus may not even be seen. Ribosomal RNA is specially processed in the nucleolus, where it binds with ribosomal proteins to form granular condensation products that are primordial subunits of ribosomes. These subunits are then released from the nucleolus and transported through the large pores of the nuclear envelope to almost all parts of the cytoplasm. After the subunits enter the cytoplasm, they are assembled to form mature functional ribosomes. Therefore, proteins are formed in the cytoplasm of the cell, but not in the cell nucleus, because the nucleus does not cont[ain m](#page-39-0)ature ribosomes. \n#### **miRNA AND SMALL INTERFERING RNA** \nA fourth type of RNA in the cell is *microRNA* (miRNA); miRNA are short (21 to 23 nucleotides) single-stranded RNA fragments that regulate gene expression **(Figure 3-10).** The miRNAs are encoded from the transcribed DNA of genes, but they are not translated into proteins and are therefore often called *noncoding RNA*. The miRNAs are processed by the cell into molecules that are complementary to mRNA and act to decrease gene expression. The generation of miRNAs involves special processing of longer primary precursor RNAs called *primiRNAs,* which are the primary transcripts of the gene. \n \n**Figure 3-10.** Regulation of gene expression by microRNA (miRNA). Primary miRNA (pri-miRNA), the primary transcripts of a gene processed in the cell nucleus by the microprocessor complex, are converted to pre-miRNAs. These pre-miRNAs are then further processed in the cytoplasm by *dicer,* an enzyme that helps assemble an RNAinduced silencing complex (RISC) and generates miRNAs. The miRNAs regulate gene expression by binding to the complementary region of the RNA and repressing translation or promoting degradation of the messenger RNA (mRNA) before it can be translated by the ribosome. \nThe pri-miRNAs are then processed in the cell nucleus by the *microprocessor complex* to pre-miRNAs, which are 70-nucleotide, stem loop structures. These pre-miRNAs are then further processed in the cytoplasm by a specific *dicer enzyme* that helps assemble an *RNA-induced silencing complex* (RISC) and generates miRNAs. \nThe miRNAs regulate gene expression by binding to the complementary region of the RNA and promoting repression of translation or degradation of the mRNA before it can be translated by the ribosome. miRNAs are believed to play an important role in normal regulation of cell function, and alterations in miRNA function have been associated with diseases such as cancer and heart disease. \nAnother type of miRNA is *small interfering RNA* (siRNA), also called *silencing RNA* or *short interfering RNA.* The siRNAs are short, double-stranded RNA molecules, comprised of 20 to 25 nucleotides, that interfere with expression of specific genes. siRNAs generally refer to synthetic miRNAs and can be administered to silence expression of specific genes. They are designed to avoid nuclear processing by the microprocessor complex and, after the siRNA enters the cytoplasm, it activates the RISC silencing complex, blocking the translation of mRNA. Because siRNAs can be tailored for any specific sequence in the gene, they can be used to block translation of any mRNA and therefore expression by any gene for which the nucleotide sequence is known. Researchers have proposed that siRNAs may become useful therapeutic tools to silence genes that contribute to the pathophysiology of diseases. \n#### TRANSLATION—FORMATION O[F](#page-38-0) PROTEINS ON THE RIBOSOMES \nWhen a molecule of mRNA comes in contact with a ribosome, it travels through the ribosome, beginning at a predetermined end of the RNA molecule specified by an appropriate sequence of RNA bases called the *chaininitiating codon.* Then, as shown in **Figure 3-9**, while the mRNA travels through the ribosome, a protein molecule is formed, a process called *translation.* Thus, the ribosome reads the codons of the mRNA in much the same way that a tape is read as it passes through the playback head of a tape recorder. Then, when a \"stop\" (or \"chainterminating\") codon slips past the ribosome, the end of a protein molecule is sig[naled, and t](#page-38-0)he p[rotein molecu](#page-40-0)le is freed into the cytoplasm. \n**Polyribosomes.** A single mRNA molecule can form protein molecules in several ribosomes at the same time because the initial end of the RNA strand can pass to a successive ribosome as it leaves the first, as shown at the bottom left in **Figure 3-9** and **Figure 3-11**. The protein molecules are in different stages of development in each ribosome. As a result, clusters of ribosomes frequently occur, with 3 to 10 ribosomes being attached to a single mRNA at the same time. These clusters are called *polyribosomes.* \nAn mRNA can cause formation of a protein molecule in any ribosome; there is no specificity of ribosomes for given types of protein. The ribosome is simply the physical manufacturing plant in which the chemical reactions take place. \n**Many Ribosomes Attach to the Endoplasmic Reticulum.** In Chapter 2, we noted that many ribosomes become attached to the endoplasmic reticulum. This attachment occurs because the initial ends of many forming protein molecules have amino acid sequences that immediately attach to specific receptor sites on the endoplasmic reticulum, causing these molecules to penetrate the \n \n**Figure 3-11.** The physical structure of the ribosomes, as well as their functional relationship to messenger RNA, transfer RNA, and the endoplasmic reticulum during the formation of protein molecules. \nreticulum wall and enter the endoplasmic reticulum matrix. This process gives a granular appearance to the portions of the reticulum where proteins are being formed and are entering the matrix of the reticulum. \n**Figure 3-11** shows the functional relationship of mRNA to the ribosomes and the manner in which the ribosomes attach to the membrane of the endoplasmic reticulum. Note the process of translation occurring in several ribosomes at the same time in response to the same strand of mRNA. Note also the newly forming polypeptide (protein) chains passing through the endoplasmic reticulum membrane into the endoplasmic matrix. \nIt should be noted that except in glandular cells, in which large amounts of protein-containing secretory vesicles are formed, most proteins synthesized by the ribosomes are released directly into the cytosol instead of into the endoplasmic reticulum. These proteins are enzymes and internal structural proteins of the cell. \n**Chemical Steps in Protein Synthesis.** Some of the chemical events that occur in the synthesis of a protein molecule are shown in **Figure 3-12**. This Fig. shows representative reactions for three separate amino acids, $AA_1$ , $AA_2$ , and $AA_{20}$ . The stages of the reactions are as follows: \n- Each amino acid is activated by a chemical process in which ATP combines with the amino acid to form an adenosine monophosphate complex with the amino acid, giving up two high-energy phosphate bonds in the process.\n- 2. The activated amino acid, having an excess of energy, then *combines with its specific tRNA to form an amino acid–tRNA complex* and, at the same time, releases the adenosine monophosphate.\n- 3. The tRNA carrying the amino acid complex then comes in contact with the mRNA molecule in the ribosome, where the anticodon of the tRNA attaches temporarily to its specific codon of the mRNA, thus lining up the amino acid in the appropriate sequence to form a protein molecule. \nThen, under the influence of the enzyme *peptidyl transferase* (one of the proteins in the ribosome), *peptide bonds* are formed between the successive amino acids, thus adding progressively to the protein chain. These \nchemical events require energy from two additional high-energy phosphate bonds, making a total of four high-energy bonds used for each amino acid added to the protein chain. Thus, the synthesis of proteins is one of the most energy-consuming processes of the cell. \n**Peptide Linkage—Combination of Amino Acids.** The successive amino acids in the protein chain combine with one another according to the typical reaction. \n$$\\begin{array}{cccccccccccccccccccccccccccccccccccc$$ \nIn this chemical reaction, a hydroxyl radical (OH-) is removed from the COOH portion of the first amino acid, and a hydrogen (H+) of the NH2 portion of the other amino acid is removed. These combine to form water, and the two reactive sites left on the two successive amino acids bond with each other, resulting in a single molecule. This process is called *peptide linkage*. As each additional amino acid is added, an additional peptide linkage is formed.\n\nMany thousand protein enzymes formed in the manner just described control essentially all the other chemical reactions that take place in cells. These enzymes promote synthesis of lipids, glycogen, purines, pyrimidines, and hundreds of other substances. We discuss many of these synthetic processes in relation to carbohydrate, lipid, and protein metabolism in Chapters 68 through 70. These substances each contribute to the various functions of the cells.\n\nFrom our discussion thus far, it is clear that the genes control both the physical and chemical functions of the cells. However, the degree of activation of respective genes must also be \n \n**Figure 3-12.** Chemical events in the formation of a protein molecule. AMP, Adenosine monophosphate; ATP, adenosine triphosphate; tRNA, transfer RNA. \ncontrolled; otherwise, some parts of the cell might overgrow or some chemical reactions might overact until they kill the cell. Each cell has powerful internal feedback control mechanisms that keep the various functional operations of the cell in step with one another. For each gene ( $\\approx 20,000-25,000$ genes in all), at least one such feedback mechanism exists. \nThere are basically two methods whereby the biochemical activities in the cell are controlled: (1) *genetic regulation,* in which the degree of activation of the genes and the formation of gene products are themselves controlled, and (2) *enzyme regulation,* in which the activity levels of already formed enzymes in the cell are controlled. \n#### **GENETIC REGULATION** \nGenetic regulation, or regulation of gene expression, covers the entire process from transcription of the genetic code in the nucleus to the formation of proteins in the cytoplasm. Regulation of gene expression provides all living organisms with the ability to respond to changes in their environment. In animals that have many different types of cells, tissues, and organs, differential regulation of gene expression also permits the different cell types in the body to each perform their specialized functions. Although a cardiac myocyte contains the same genetic code as a renal tubular epithelial cell, many genes are expressed in cardiac cells that are not expressed in renal tubular cells. The ultimate measure of gene \"expression\" is whether (and how much) of the gene products (proteins) are produced because proteins carry out cell functions specified by the genes. Regulation of gene expression can occur at any point in the pathways of transcription, RNA processing, and translation. \n**The Promoter Controls Gene Expression.** Synthesis of cellular proteins is a complex process that starts with transcription of DNA into RNA. Transcription of DNA is \n \n**Figure 3-13.** Gene transcription in eukaryotic cells. A complex arrangement of multiple clustered enhancer modules is interspersed with insulator elements, which can be located upstream or downstream of a basal promoter containing TATA box (TATA), proximal promoter elements (response elements, RE), and initiator sequences (INR). \ncontrolled by regulatory elements found in the promoter of a gene (Figure 3-13). In eukaryotes, which includes all mammals, the basal promoter consists of a sequence of bases (TATAAA) called the TATA box, the binding site for the TATA-binding protein and several other important transcription factors that are collectively referred to as the transcription factor IID complex. In addition to the transcription factor IID complex, this region is where transcription factor IIB binds to both the DNA and RNA polymerase 2 to facilitate transcription of the DNA into RNA. This basal promoter is found in all protein-coding genes, and the polymerase must bind with this basal promoter before it can begin traveling along the DNA strand to synthesize RNA. The upstream promoter is located farther upstream from the transcription start site and contains several binding sites for positive or negative transcription factors that can affect transcription through interactions with proteins bound to the basal promoter. The structure and transcription factor binding sites in the upstream promoter vary from gene to gene to give rise to the different expression patterns of genes in different tissues. \nTranscription of genes in eukaryotes is also influenced by *enhancers,* which are regions of DNA that can bind transcription factors. Enhancers can be located a great distance from the gene they act on or even on a different chromosome. They can also be located upstream or downstream of the gene that they regulate. Although enhancers may be located far from their target gene, they may be relatively close when DNA is coiled in the nucleus. It is estimated that there are more than 100,000 gene enhancer sequences in the human genome. \nIn the organization of the chromosome, it is important to separate active genes that are being transcribed from genes that are repressed. This separation can be challenging because multiple genes may be located close together on the chromosome. The separation is achieved by chromosomal *insulators.* These insulators are gene sequences that provide a barrier so that a specific gene is isolated against transcriptional influences from surrounding genes. Insulators can vary greatly in their DNA sequence and the proteins that bind to them. One way an insulator activity can be modulated is by *DNA methylation*, which is the case for the mammalian insulin-like growth factor 2 (IGF-2) gene. The mother's allele has an insulator between the enhancer and promoter of the gene that allows for the binding of a transcriptional repressor. However, the paternal DNA sequence is methylated such that the transcriptional repressor cannot bind to the insulator, and the IGF-2 gene is expressed from the paternal copy of the gene. \n#### **Other Mechanisms for Control of Transcription by the Promoter.** Variations in the basic mechanism for control of the promoter have been discovered in the past three decades. Without giving details, let us list some of them: \n- 1. A promoter is frequently controlled by transcription factors located elsewhere in the genome. That is, the regulatory gene causes the formation of a regulatory protein that in turn acts as an activator or repressor of transcription.\n- 2. Occasionally, many different promoters are controlled at the same time by the same regulatory protein. In some cases, the same regulatory protein functions as an activator for one promoter and as a repressor for another promoter.\n- 3. Some proteins are controlled not at the starting point of transcription on the DNA strand but farther along the strand. Sometimes, the control is not even at the DNA strand itself but occurs during the processing of the RNA molecules in the nucleus before they are released into the cytoplasm. Control may also occur at the level of protein formation in the cytoplasm during RNA translation by the ribosomes.\n- 4. In nucleated cells, the nuclear DNA is packaged in specific structural units, the *chromosomes.* Within \neach chromosome, the DNA is wound around small proteins called *histones,* which in turn are held tightly together in a compacted state by still other proteins. As long as the DNA is in this compacted state, it cannot function to form RNA. However, multiple control mechanisms are being discovered that can cause selected areas of chromosomes to become decompacted one part at a time, so that partial RNA transcription can occur. Even then, specific *transcriptor factor*s control the actual rate of transcription by the promoter in the chromosome. Thus, still higher orders of control are used to establish proper cell function. In addition, signals from outside the cell, such as some of the body's hormones, can activate specific chromosomal areas and specific transcription factors, therefore controlling the chemical machinery for function of the cell. \nBecause there are many thousands of different genes in each human cell, the large number of ways in which genetic activity can be controlled is not surprising. The gene control systems are especially important for controlling intracellular concentrations of amino acids, amino acid derivatives, and intermediate substrates and products of carbohydrate, lipid, and protein metabolism. \n#### **CONTROL OF INTRACELLULAR FUNCTION BY ENZYME REGULATION** \nIn addition to control of cell function by genetic regulation, cell activities are also controlled by intracellular inhibitors or activators that act directly on specific intracellular enzymes. Thus, enzyme regulation represents a second category of mechanisms whereby cellular biochemical functions can be controlled. \n**Enzyme Inhibition.** Some chemical substances formed in the cell have direct feedback effects to inhibit the specific enzyme systems that synthesize them. Almost always, the synthesized product acts on the first enzyme in a sequence, rather than on the subsequent enzymes, usually binding directly with the enzyme and causing an allosteric conformational change that inactivates it. One can readily recognize the importance of inactivating the first enzyme because this prevents buildup of intermediary products that are not used. \nEnzyme inhibition is another example of negative feedback control. It is responsible for controlling intracellular concentrations of multiple amino acids, purines, pyrimidines, vitamins, and other substances. \n**Enzyme Activation.** Enzymes that are normally inactive often can be activated when needed. An example of this phenomenon occurs when most of the ATP has been depleted in a cell. In this case, a considerable amount of cyclic adenosine monophosphate (cAMP) begins to be formed as a breakdown product of ATP. The presence of this cAMP, in turn, immediately activates the glycogensplitting enzyme phosphorylase, liberating glucose molecules that are rapidly metabolized, with their energy used for replenishment of the ATP stores. Thus, cAMP acts as an enzyme activator for the enzyme phosphorylase and thereby helps control intracellular ATP concentration. \nAnother interesting example of both enzyme inhibition and enzyme activation occurs in the formation of the purines and pyrimidines. These substances are needed by the cell in approximately equal quantities for the formation of DNA and RNA. When purines are formed, they *inhibit* the enzymes that are required for formation of additional purines. However, they *activate* the enzymes for formation of pyrimidines. Conversely, the pyrimidines inhibit their own enzymes but activate the purine enzymes. In this way, there is continual cross-talk between the synthesizing systems for these two substances, resulting in almost exactly equal amounts of the two substances in the cells at all times. \n**Summary.** There are two principal mechanisms whereby cells control proper proportions and quantities of different cellular constituents: (1) genetic regulation; and (2) enzyme regulation. The genes can be activated or inhibited, and likewise, the enzyme systems can be activated or inhibited. These regulatory mechanisms usually function as feedback control systems that continually monitor the cell's biochemical composition and make corrections as needed. However, on occasion, substances from outside the cell (especially some of the hormones discussed in this text) also control the intracellular biochemical reactions by activating or inhibiting one or more of the intracellular control systems. \n#### THE DNA–GENETIC SYSTEM CONTROLS CELL REPRODUCTION \nCell reproduction is another example of the ubiquitous role that the DNA–genetic system plays in all life processes. The genes and their regulatory mechanisms determine cell growth characteristics and when or whether cells will divide to form new cells. In this way, the allimportant genetic system controls each stage in the development of the human, from the single-cell fertilized ovum to the whole functioning body. Thus, if there is any central theme to life, it is the DNA–genetic system. \n#### **Life Cycle of the Cell** \nThe life cycle of a cell is the period from cell repr[oduction](#page-43-0) to the next cell reproduction. When mammalian cells *are not inhibited and are reproducing as rapidly as they can,* this life cycle may be as little as 10 to 30 hours. It is terminated by a series of distinct physical events called *mitosis* that cause division of the cell into two new daughter cells. The events of mitosis are shown in **Figure 3-14** and described later. The actual stage of mitosis, however, lasts for only about 30 minutes, and thus more than 95% of the life cycle of even rapidly reproducing cells is represented by the interval between mitosis, called *interphase.* \nExcept in special conditions of rapid cellular reproduction, inhibitory factors almost always slow or stop the \n \n**Figure 3-14.** Stages of cell reproduction. *A, B, C,* Prophase. *D,* Prometaphase. *E,* Metaphase. *F,* Anaphase. *G, H,* Telophase. \nuninhibited life cycle of the cell. Therefore, different cells of the body actually have life cycle periods that vary from as little as 10 hours for highly stimulated bone marrow cells to an entire lifetime of the human body for many nerve cells. \n#### **Cell Reproduction Begins with Replication of DNA** \nThe first step of cell reproduction is *replication (duplication) of all DNA in the chromosomes.* It is only after this replication has occurred that mitosis can take place. \nThe DNA begins to be duplicated 5 to 10 hours before mitosis, and the duplication is completed in 4 to 8 hours. The net result is two exact *replicas* of all DNA. These replicas become the DNA in the two new daughter cells that will be formed at mitosis. After replication of the DNA, there is another period of 1 to 2 hours before mitosis begins abruptly. Even during this period, preliminary changes that will lead to the mitotic process are beginning to take place. \n**DNA Replication.** DNA is replicated in much the same way that RNA is transcribed from DNA, except for a few important differences: \n1. Both strands of the DNA in each chromosome are replicated, not just one of them. \n \nFigure 3-15. DNA replication, showing the replication fork and leading and lagging strands of DNA. \n- 2. Both entire strands of the DNA helix are replicated from end to end, rather than small portions of them, as occurs in the transcription of RNA.\n- 3. Multiple enzymes called *DNA polymerase*, which is comparable to RNA polymerase, are essential for replicating DNA. DNA polymerase attaches to and moves along the DNA template strand, adding nucleotides in the 5′ to 3′ direction. Another enzyme, *DNA ligase*, causes bonding of successive DNA nucleotides to one another, using high-energy phosphate bonds to energize these attachments.\n- 4. Replication fork formation. Before DNA can be replicated, the double-stranded molecule must be \"unzipped\" into two single strands (Figure 3-15). Because the DNA helixes in each chromosome are approximately 6 centimeters in length and have millions of helical turns, it would be impossible for the two newly formed DNA helixes to uncoil from each other were it not for some special mechanism. This uncoiling is achieved by DNA helicase enzymes that break the hydrogen bonding between the base pairs of the DNA, permitting the two strands to separate into a Y shape known as the replication fork, the area that will be the template for replication to begin. \nDNA is directional in both strands, signified by a 5′ and 3′ end (see **Figure 3-15**). Replication progresses only in the 5′ to 3′ direction. At the replication fork one strand, the *leading strand*, is oriented in the 3′ to 5′ direction, toward the replication fork, while the *lagging strand* is oriented 5′ to 3′, away from the replication fork. Because of their different orientations, the two strands are replicated differently. \n5. *Primer binding*. Once the DNA strands have been separated, a short piece of RNA called an *RNA primer* binds to the 3' end of the leading strand. Primers are generated by the enzyme *DNA primase*. \n- Primers always bind as the starting point for DNA replication.\n- 6. Elongation. DNA polymerases are responsible for creating the new strand by a process called *elongation*. Because replication proceeds in the 5′ to 3′ direction on the leading strand, the newly formed strand is continuous. The lagging strand begins replication by binding with multiple primers that are only several bases apart. DNA polymerase then adds pieces of DNA, called *Okazaki fragments*, to the strand between primers. This process of replication is discontinuous because the newly created Okazaki fragments are not yet connected. An enzyme, *DNA ligase*, joins the Okazaki fragments to form a single unified strand.\n- 7. Termination. After the continuous and discontinuous strands are both formed, the enzyme exonuclease removes the RNA primers from the original strands, and the primers are replaced with appropriate bases. Another exonuclease \"proofreads\" the newly formed DNA, checking and clipping off any mismatched or unpaired residues. \nAnother enzyme, *topoisomerase*, can transiently break the phosphodiester bond in the backbone of the DNA strand to prevent the DNA in front of the replication fork from being overwound. This reaction is reversible, and the phosphodiester bond reforms as the topoisomerase leaves. \nOnce completed, the parent strand and its complementary DNA strand coils into the double helix shape. The process of replication therefore produces two DNA molecules, each with one strand from the parent DNA and one new strand. For this reason, DNA replication is often described as *semiconservative*; half of the chain is part of the original DNA molecule and half is brand new. \n**DNA Repair, DNA \"Proofreading,\" and \"Mutation.\"**During the hour or so between DNA replication and \nthe beginning of mitosis, there is a period of active repair and \"proofreading\" of the DNA strands. Wherever inappropriate DNA nucleotides have been matched up with the nucleotides of the original template strand, special enzymes cut out the defective areas and replace them with appropriate complementary nucleotides. This repair process, which is achieved by the same DNA polymerases and DNA ligases that are used in replication, is referred to as *DNA proofreading.* \nBecause of repair and proofreading, mistakes are rarely made in the DNA replication process. When a mistake is made, it is called a *mutation.* The mutation may cause formation of some abnormal protein in the cell rather than a needed protein, which may lead to abnormal cellular function and sometimes even cell death. Given that many thousands of genes exist in the human genome, and that the period from one human generation to another is about 30 years, one would expect as many as 10 or many more mutations in the passage of the genome from parent to offspring. As a further protection, however, each human genome is represented by two separate sets of chromosomes, one derived from each parent, with almost identical genes. Therefore, one functional gene of each pair is almost always available to the child, despite mutations. \n#### **CHROMOSOMES AND THEIR REPLICATION** \nThe DNA helixes of the nucleus are packaged in chromosomes. The human cell contains 46 chromosomes arranged in 23 pairs. Most of the genes in the two chromosomes of each pair are identical or almost identical to each other, so it is usually stated that the different genes also exist in pairs, although occasionally this is not the case. \nIn addition to DNA, there is a large amount of protein in the chromosome, composed mainly of many small molecules of electropositively charged *histones.* The histones are organized into vast numbers of small, bobbinlike cores. Small segments of each DNA helix are coiled sequentially around one core after another. \nThe histone cores play an important role in regulation of DNA activity because as long as the DNA is packaged tightly, it cannot function as a template for formation of RNA or replication of new DNA. Furthermore, some of the regulatory proteins *decondense* the histone packaging of the DNA and allow small segments at a time to form RNA. \nSeveral nonhistone proteins are also major components of chromosomes, functioning as chromosomal structural proteins and, in connection with the genetic regulatory machinery, as activators, inhibitors, and enzymes. \nReplication of the chromosomes in their entirety occurs during the next few minutes after replication of the DNA helixes has been completed; the new DNA helixes collect new protein molecules as needed. The two newly formed chromosomes remain attached to each other (until time for mitosis) at a point called the *centromere* located near their center. These duplicated but still attached chromosomes are called *chromatids.* \n#### **CELL MITOSIS** \nThe actual process whereby the cell splits into two new cells is called *mitosis.* Once each chromosome has been replicated to form the two chromatids, mitos[is follows](#page-43-0) automatically within 1 or 2 hours in many cells. \n**Mitotic Apparatus: Function of the Centrioles.** One of the first events of mitosis takes place in the cytoplasm in or around the small structures called *centrioles* during the latter part of interphase*.* As shown in **Figure 3-**14, two pairs of centrioles lie close to each other near one pole of the nucleus. These centrioles, like the DNA and chromosomes, are also replicated during interphase, usually shortly before replication of the DNA. Each centriole is a small cylindrical body about 0.4 micrometer long and about 0.15 micrometer in diameter, consisting mainly of nine parallel tubular structures arranged in the form of a cylinder. The two centrioles of each pair lie at right angles to each other. Each pair of centrioles, along with attached *pericentriolar material,* is called a *centrosome.* \nShortly before mitosis takes place, the two pairs of centrioles begin to move apart from each other. This movement is caused by polymerization of protein microtubules growing between the respective centriole pairs and actually pushing them apart. At the same time, other microtubules grow radially away from each of the centriole pairs, forming a spiny star called the *aster,* in each end of the cell. Some of the spines of the aster penetrate the nuclear membrane and h[elp separate t](#page-43-0)he two sets of chromatids during mitosis. The complex of microtubules extending between the two new centriole pairs is called the *spindle,* and the entire set of microtubules plus the two pairs of centrioles is called the *mitotic apparatus.* \n**Prophase.** [The fi](#page-43-0)rst stage of mitosis, called *prophase,* is shown in **Figure 3-14***A, B,* and *C.* While the spindle is forming, the chromosomes of the nucleus (which in interphase consist of loosely coiled strands) become condensed into well-defined chromosomes. \n**Prometaphase.** During the prometaphase stage (see **Figure 3-14***D*), the growing microtubular spines of the aster fragment the nuclear envelope. At the same time, multiple mic[rotubu](#page-43-0)les from the aster attach to the chromatids at the centromeres, where the paired chromatids are still bound to each other. The tubules then pull one chromatid of each pair toward one cellular pole and its partner toward the opposite pole. \n**Metaphase.** During the metaphase stage (see **Figure 3-14***E*), the two asters of the mitotic apparatus are pushed farther apart. This pushing is believed to occur because the microtubular spines from the two asters, where they interdigitate with each other to form the mitotic spindle, push each other away. Minute contractile protein molecules called *\"molecular motors,\"* which may be composed of the muscle protein *actin,* extend between the respective spines a[nd, us](#page-43-0)ing a stepping action as in muscle, actively slide the spines in a reverse direction along each other. Simultaneously, the chromatids are pulled tightly by their attached microtubules to the very center of the cell, lining up to form the *equatorial plate* of the mitotic spindle. \n**Anaphase.** During the anaphase stage (see **Figure 3-14***F*), the two chromatids of each chromosome are pulled apart at the centromere. All 46 pairs of c[hromatids](#page-43-0) are sepa[rat](#page-43-0)ed, forming two separate sets of 46 *daughter chromosomes.* One of these sets is pulled toward one mitotic aster, and the other is pulled toward the other aster, as the two respective poles of the dividing cell are pushed still farther apart. \n**Telophase.** In the telophase stage (see **Figure 3-14***G* and *H*), the two sets of daughter chromosomes are pushed completely apart. Then, the mitotic apparatus dissipates, and a new nuclear membrane develops around each set of chromosomes. This membrane is formed from portions of the endoplasmic reticulum that are already present in the cytoplasm. Shortly thereafter, the cell pinches in two, midway between the two nuclei. This pinching is caused by the formation of a contractile ring of *microfilaments* composed of *actin* and probably *myosin* (the two contractile proteins of muscle) at the juncture of the newly developing cells that pinches them off from each other. \n#### **CONTROL OF CELL GROWTH AND CELL REPRODUCTION** \nSome cells grow and reproduce all the time, such as the blood-forming cells of the bone marrow, the germinal layers of the skin, and the epithelium of the gut. Many other cells, however, such as smooth muscle cells, may not reproduce for many years. A few cells, such as the neurons and most striated muscle cells, do not reproduce during the entire life of a person, except during the original period of fetal life. \nIn certain tissues, an insufficiency of some types of cells causes them to grow and reproduce rapidly until appropriate numbers of these cells are again available. For example, in some young animals, seven-eighths of the liver can be removed surgically, and the cells of the remaining one-eighth will grow and divide until the liver mass returns to almost normal. The same phenomenon occurs for many glandular cells and most cells of the bone marrow, subcutaneous tissue, intestinal epithelium, and almost any other tissue except highly differentiated cells such as nerve and muscle cells. \nThe mechanisms that maintain proper numbers of the different types of cells in the body are still poorly understood. However, experiments have shown at least three ways in which growth can be controlled. First, growth often is controlled by *growth factors* that come from other parts of the body. Some of these growth factors circulate in the blood, but others originate in adjacent tissues. For [exam](#page-43-0)ple, the epithelial cells of some glands, such as the pancreas, fail to grow without a growth factor from the underlying connective tissue of the gland. Second, most normal cells stop growing when they have run out of space for growth. This phenomenon occurs when cells are grown in tissue culture; the cells grow until they contact a solid object, and then growth stops. Third, cells grown in tissue culture often stop growing when minute am[ounts](#page-46-0) of their own secretions are allowed to collect in the culture medium. This mechanism, too, could provide a means for negative feedback control of growth. \n**Telomeres Prevent the Degradation of Chromosomes.** A *telomere* is a region of repetitive nucleotide sequences located at each end of a chromatid (**Figure 3-16**). Telomeres serve as protective caps that prevent the chromosome from deterioration during cell division. During cell division, a short piece of \"primer\" RNA attaches to the DNA strand to start the replication. However, because the primer does not attach at the very end of the DNA strand, the copy is missing a small section of the DNA. With each cell division, the copied DNA loses additional nucleotides from the telomere region. The nucleotide sequences provided by the telomeres therefore prevent the degradation of genes near the ends of chromosomes. Without telomeres, the genomes would progressively lose information and be truncated after each cell division. Thus, the telomeres can be considered to be disposable chromosomal buffers that help maintain stability of the genes but are gradually consumed during repeated cell divisions. \n \n**Figure 3-16.** Control of cell replication by telomeres and telomerase. The cells' chromosomes are capped by telomeres, which, in the absence of telomerase activity, shorten with each cell division until the cell stops replicating. Therefore, most cells of the body cannot replicate indefinitely. In cancer cells, telomerase is activated, and telomere length is maintained so that the cells continue to replicate themselves uncontrollably. \nEach time a cell divides, an average person loses 30 to 200 base pairs from the ends of that cell's telomeres. In human blood cells, the length of telomeres ranges from 8000 base pairs at birth to as low as 1500 in older people. Eventually, when the telomeres shorten to a critical length, the chromosomes become unstable, and the cells die. This process of telomere shortening is believed to be an important reason for some of the physiological changes associated with aging. Telomere erosion can also occur as a result of diseases, especially those associated with oxidative stress and inflammation. \nIn some cells, such as stem cells of the bone marrow or skin that must be replenished throughout life, or germ cells in the ovaries and testes, the enzyme *telomerase* adds bases to the ends of the telomeres so that many more generations of cells can be produced. However, telomerase activity is usually low in most cells of the body, and after many generations the descendent cells will inherit defective chromoso[mes, become](#page-46-0) *senescent,* and cease dividing. This process of telomere shortening is important in regulating cell proliferation and maintaining gene stability. In cancer cells, telomerase activity is abnormally activated so that telomere length is maintained, making it possible for the cells to replicate over and over again uncontrollably (see **Figure 3-16**). Some scientists have therefore proposed that telomere shortening protects us from cancer and other proliferative diseases. \n**Regulation of Cell Size.** Cell size is determined almost entirely by the amount of functioning DNA in the nucleus. If replication of the DNA does not occur, the cell grows to a certain size and thereafter remains at that size. Conversely, use of the chemical *colchicine* makes it possible to prevent formation of the mitotic spindle and therefore prevent mitosis, even though replication of the DNA continues. In this event, the nucleus contains far greater quantities of DNA than it normally does, and the cell grows proportionately larger. It is assumed that this cell growth results from increased production of RNA and cell proteins, which, in turn, cause the cell to grow larger. \n#### CELL DIFFERENTIATION \nA special characteristic of cell growth and cell division is *cell differentiation,* which refers to changes in the physical and functional properties of cells as they proliferate in the embryo to form the different body structures and organs. The following description of an especially interesting experiment helps explain these processes. \nWhen the nucleus from an intestinal mucosal cell of a frog is surgically implanted into a frog ovum from which the original ovum nucleus was removed, the result is often the formation of a normal frog. This experiment demonstrates that even the intestinal mucosal cell, which is a well-differentiated cell, carries all the necessary genetic information for development of all structures required in the frog's body. \nTherefore, it has become clear that differentiation results not from loss of genes but from selective repression of different gene promoters. In fact, electron micrographs suggest that some segments of DNA helixes that are wound around histone cores become so condensed that they no longer uncoil to form RNA molecules. One explanation for this is as follows. It has been supposed that the cellular genome begins at a certain stage of cell differentiation to produce a regulatory *protein* that forever after represses a select group of genes. Therefore, the repressed genes never function again. Regardless of the mechanism, mature human cells each produce a maximum of about 8000 to 10,000 proteins rather than the potential 20,000 to 25,000 or more that would be produced if all genes were active. \nEmbryological experiments have shown that certain cells in an embryo control differentiation of adjacent cells. For example, the *primordial chordamesoderm* is called the *primary organizer* of the embryo because it forms a focus around which the remainder of the embryo develops. It differentiates into a *mesodermal axis* that contains segmentally arranged *somites* and, as a result of *inductions* in the surrounding tissues, causes the formation of essentially all the organs of the body. \nAnother instance of induction occurs when the developing eye vesicles come into contact with the ectoderm of the head and cause the ectoderm to thicken into a lens plate that folds inward to form the lens of the eye. Therefore, a large share of the embryo develops as a result of such inductions, with one part of the body affecting another part, and this part affecting still other parts. \nThus, although our understanding of cell differentiation is still hazy, we are aware of many control mechanisms whereby differentiation *could* occur. \n#### APOPTOSIS—PROGRAMMED CELL DEATH \nThe many trillions of the body's cells are members of a highly organized community in which the total number of cells is regulated not only by controlling the rate of cell division, but also by controlling the rate of cell death. When cells are no longer needed or become a threat to the organism, they undergo a suicidal *programmed cell death,* or *apoptosis.* This process involves a specific proteolytic cascade that causes the cell to shrink and condense, disassemble its cytoskeleton, and alter its cell surface so that a neighboring phagocytic cell, such as a macrophage, can attach to the cell membrane and digest the cell. \nIn contrast to programmed death, cells that die as a result of an acute injury usually swell and burst due to loss of cell membrane integrity, a process called cell *necrosis.* Necrotic cells may spill their contents, causing inflammation and injury to neighboring cells. Apoptosis, however, is an orderly cell death that results in disassembly and phagocytosis of the cell before any leakage of its contents occurs, and neighboring cells usually remain healthy. \nApoptosis is initiated by activation of a family of proteases called *caspases*, which are enzymes that are synthesized and stored in the cell as inactive *procaspases*. The mechanisms of activation of caspases are complex but, once activated, the enzymes cleave and activate other procaspases, triggering a cascade that rapidly breaks down proteins within the cell. The cell thus dismantles itself, and its remains are rapidly digested by neighboring phagocytic cells. \nA tremendous amount of apoptosis occurs in tissues that are being remodeled during development. Even in adult humans, billions of cells die each hour in tissues such as the intestine and bone marrow and are replaced by new cells. Programmed cell death, however, is normally balanced by formation of new cells in healthy adults. Otherwise, the body's tissues would shrink or grow excessively. Abnormalities of apoptosis may play a key role in neurodegenerative diseases such as Alzheimer disease, as well as in cancer and autoimmune disorders. Some drugs that have been used successfully for chemotherapy appear to induce apoptosis in cancer cells. \n#### CANCER \nCancer may be caused by *mutation* or by some other *abnormal activation* of cellular genes that control cell growth and cell mitosis. *Proto-oncogenes* are normal genes that code for various proteins that control cell adhesion, growth and division. If mutated or excessively activated, proto-oncogenes can become abnormally functioning *oncogenes* capable of causing cancer*.* As many as 100 different oncogenes have been discovered in human cancers. \nAlso present in all cells are *antioncogenes,* also called *tumor suppressor genes*, which suppress the activation of specific oncogenes. Therefore, loss or inactivation of antioncogenes can allow activation of oncogenes that lead to cancer. \nFor several reasons, only a minute fraction of the cells that mutate in the body ever lead to cancer: \n- • First, most mutated cells have less survival capability than normal cells, and they simply die.\n- • Second, only a few of the mutated cells that survive become cancerous because most mutated cells still have normal feedback controls that prevent excessive growth.\n- • Third, cells that are potentially cancerous are often destroyed by the body's immune system before they grow into a cancer. \nMost mutated cells form abnormal proteins within their cell bodies because of their altered genes, and these proteins activate the body's immune system, causing it to form antibodies or sensitized lymphocytes that react against the cancerous cells, destroying them. In people whose immune systems have been suppressed, such as in persons taking immunosuppressant drugs after kidney or heart transplantation, the probability that a cancer will develop is multiplied as much as fivefold. \n• Fourth, the simultaneous presence of several different activated oncogenes is usually required to cause a cancer. For example, one such gene might promote rapid reproduction of a cell line, but no cancer occurs because another mutant gene is not present simultaneously to form the needed blood vessels. \nWhat is it that causes the altered genes? Considering that many trillions of new cells are formed each year in humans, a better question might be to ask why all of us do not develop millions or billions of mutant cancerous cells. The answer is the incredible precision with which DNA chromosomal strands are replicated in each cell before mitosis can take place, along with the proofreading process that cuts and repairs any abnormal DNA strand before the mitotic process is allowed to proceed. Yet, despite these inherited cellular precautions, probably one newly formed cell in every few million still has significant mutant characteristics. \nThus, chance alone is all that is required for mutations to take place, so we can suppose that a large number of cancers are merely the result of an unlucky occurrence. However, the probability of mutations can be greatly increased when a person is exposed to certain chemical, physical, or biological factors, including the following: \n- 1. *Ionizing radiation,* such as x-rays, gamma rays, particle radiation from radioactive substances, and even ultraviolet light, can predispose individuals to cancer. Ions formed in tissue cells under the influence of such radiation are highly reactive and can rupture DNA strands, causing many mutations.\n- 2. *Chemical substances* of certain types may also cause mutations. It was discovered long ago that various aniline dye derivatives are likely to cause cancer, and thus workers in chemical plants producing such substances, if unprotected, have a special predisposition to cancer. Chemical substances that can cause mutation are called *carcinogens.* The carcinogens that currently cause the greatest number of deaths are those in cigarette smoke. These carcinogens cause over 30% of all cancer deaths and at least 85% of lung cancer deaths.\n- 3. *Physical irritants* can also lead to cancer, such as continued abrasion of the linings of the intestinal tract by some types of food. The damage to the tissues leads to rapid mitotic replacement of the cells; the more rapid the mitosis, the greater the chance for mutation.\n- 4. *Hereditary tendency* to cancer occurs in some families. This hereditary tendency results from the fact that most cancers require not one mutation but two or more mutations before cancer occurs. In families that are particularly predisposed to cancer, it is presumed that one or more cancerous genes are already \n- mutated in the inherited genome. Therefore, far fewer additional mutations must take place in such family members before a cancer begins to grow.\n- 5. *Certain types of oncoviruses* can cause various types of cancer. Some examples of viruses associated with cancers in humans include *human papilloma virus* (HPV), *hepatitis B and hepatitis C virus,* Epstein-Barr virus, human immunodeficiency virus (HIV), human T-cell leukemia virus, Kaposi sarcoma–associated herpes virus (KSHV), and Merkel cell polyomavirus. Although the mechanisms whereby oncoviruses cause cancer are not fully understood, there are at least two potential ways. In the case of DNA viruses, the DNA strand of the virus can insert itself directly into one of the chromosomes, thereby causing a mutation that leads to cancer. In the case of RNA viruses, some of these viruses carry with them an enzyme called *reverse transcriptase* that causes DNA to be transcribed from the RNA. The transcribed DNA then inserts itself into the animal cell genome, leading to cancer. \n**Invasive Characteristic of the Cancer Cell.** The major differences between a cancer cell and a normal cell are as follows: \n- 1. The cancer cell does not respect usual cellular growth limits because these cells presumably do not require all the same growth factors that are necessary to cause growth of normal cells.\n- 2. Cancer cells are often far less adhesive to one another than are normal cells. Therefore, they tend to wander through the tissues, enter the blood stream, and be transported all through the body, where they form nidi for numerous new cancerous growths.\n- 3. Some cancers also produce *angiogenic factors* that cause many new blood vessels to grow into the cancer, thus supplying the nutrients required for cancer growth. \n**Why Do Cancer Cells Kill?** Cancer tissue competes with normal tissues for nutrients. Because cancer cells continue to proliferate indefinitely, with their numbers multiplying every day, cancer cells soon demand essentially all the nutrition available to the body or to an essential part of the body. As a result, normal tissues gradually sustain nutritive death. \nSome cancers cause disruption of vital organ functions. For example, a lung cancer might replace healthy tissue to the extent that the lungs cannot absorb enough oxygen to maintain tissues in the rest of the body. \n#### Bibliography \nAlberts B, Johnson A, Lewis J, et al: Molecular Biology of the Cell, 6th ed. New York: Garland Science 2014. \n- Armanios M: Telomeres and age-related disease: how telomere biology informs clinical paradigms. J Clin Invest 123:996, 2013.\n- Bickmore WA, van Steensel B: Genome architecture: domain organization of interphase chromosomes. Cell 152:1270, 2013.\n- Calcinotto A, Kohli J, Zagato E, Pellegrini L, Demaria M, Alimonti A: Cellular senescence: aging, cancer, and injury. Physiol Rev 99:1047-1078, 2019.\n- Clift D, Schuh M: Restarting life: fertilization and the transition from meiosis to mitosis. Nat Rev Mol Cell Biol 14:549, 2013.\n- Coppola CJ, C Ramaker R, Mendenhall EM: Identification and function of enhancers in the human genome. Hum Mol Genet 25(R2):R190-R197, 2016.\n- Feinberg AP: The key role of epigenetics in human disease prevention and mitigation. N Engl J Med 378:1323-1334, 2018.\n- Fyodorov DV, Zhou BR, Skoultchi AI, Bai Y: Emerging roles of linker histones in regulating chromatin structure and function. Nat Rev Mol Cell Biol 19:192-206, 2018.\n- Haberle V, Stark A: Eukaryotic core promoters and the functional basis of transcription initiation. Nat Rev Mol Cell Biol 19:621-637, 2018.\n- Kaushik S, Cuervo AM: The coming of age of chaperone-mediated autophagy. Nat Rev Mol Cell Biol 19:365-381, 2018.\n- Krump NA, You J: Molecular mechanisms of viral oncogenesis in humans. Nat Rev Microbiol 16:684-698, 2018.\n- Leidal AM, Levine B, Debnath J: Autophagy and the cell biology of age-related disease. Nat Cell Biol 20:1338-1348, 2018.\n- Maciejowski J, de Lange T: Telomeres in cancer: tumour suppression and genome instability. Nat Rev Mol Cell Biol 18:175-186, 2017.\n- McKinley KL, Cheeseman IM: The molecular basis for centromere identity and function. Nat Rev Mol Cell Biol 17:16-29, 2016.\n- Monk D, Mackay DJG, Eggermann T, Maher ER, Riccio A: Genomic imprinting disorders: lessons on how genome, epigenome and environment interact. Nat Rev Genet 10:235, 2019.\n- Müller S, Almouzni G: Chromatin dynamics during the cell cycle at centromeres. Nat Rev Genet 18:192-208, 2017.\n- Nigg EA, Holland AJ: Once and only once: mechanisms of centriole duplication and their deregulation in disease. Nat Rev Mol Cell Biol 19:297-312, 2018.\n- Palozola KC, Lerner J, Zaret KS: A changing paradigm of transcriptional memory propagation through mitosis. Nat Rev Mol Cell Biol 20:55-64, 2019.\n- Perez MF, Lehner B: Intergenerational and transgenerational epigenetic inheritance in animals. Nat Cell Biol 21:143, 2019.\n- Prosser SL, Pelletier L: Mitotic spindle assembly in animal cells: a fine balancing act. Nat Rev Mol Cell Biol 18:187-201, 2017.\n- Schmid M, Jensen TH. Controlling nuclear RNA levels. Nat Rev Genet 19:518-529, 2018.\n- Treiber T, Treiber N, Meister G: Regulation of microRNA biogenesis and its crosstalk with other cellular pathways. Nat Rev Mol Cell Biol 20:5-20, 2019. \n\n\nFigure 4-1 lists the approximate concentrations of important electrolytes and other substances in the *extracellular fluid* and *intracellular fluid*. Note that the extracellular fluid contains a large amount of *sodium* but only a small amount of *potassium*. The opposite is true of the intracellular fluid. Also, the extracellular fluid contains a large amount of *chloride* ions, whereas the intracellular fluid contains very little of these ions. However, the concentrations of *phosphates* and *proteins* in the intracellular fluid are considerably greater than those in the extracellular fluid. These differences are extremely important to the life of the cell. The purpose of this chapter is to explain how the differences are brought about by the cell membrane transport mechanisms. \n \n**Figure 4-1.** Chemical compositions of extracellular and intracellular fluids. The question marks indicate that the precise values for intracellular fluid are unknown. The *red line* indicates the cell membrane.\n\nThe structure of the membrane covering the outside of every cell of the body is discussed in Chapter 2 and illustrated in Figure 2-3 and Figure 4-2. This membrane consists almost entirely of a *lipid bilayer* with large numbers of protein molecules in the lipid, many of which penetrate all the way through the membrane. \nThe lipid bilayer is not miscible with the extracellular fluid or the intracellular fluid. Therefore, it constitutes a barrier against movement of water molecules and water-soluble substances between the extracellular and intracellular fluid compartments. However, as shown in **Figure 4-2** by the leftmost arrow, lipid-soluble substances can diffuse directly through the lipid substance. \nThe membrane protein molecules interrupt the continuity of the lipid bilayer, constituting an alternative pathway through the cell membrane. Many of these penetrating proteins can function as *transport proteins*. Some proteins have watery spaces all the way through the molecule and allow free movement of water, as well as selected ions or molecules; these proteins are called *channel proteins*. Other proteins, called *carrier proteins*, bind with molecules or ions that are to be transported, and conformational changes in the protein molecules then move the substances through the interstices of the protein to the \n \n**Figure 4-2.** Transport pathways through the cell membrane and the basic mechanisms of transport. \n \n**Figure 4-3.** Diffusion of a fluid molecule during one thousandth of a second. \nother side of the membrane. Channel proteins and carrier proteins are usually selective for the types of molecules or ions that are allowed to cross the membrane. \n**\"Diffusion\" Versus \"Active Transport.\"** Transport through the cell membrane, either directly through the lipid bilayer or through the proteins, occurs via one of two basic processes, *diffusion* or *active transport.* \nAlthough many variations of these basic mechanisms exist, *diffusion* means random molecular movement of substances molecule by molecule, either through intermolecular spaces in the membrane or in combination with a carrier protein. The energy that causes diffusion is the energy of the normal kinetic motion of matter. \nIn contrast, *active transport* means movement of ions or other substances across the membrane in combination with a carrier protein in such a way that the carrier protein causes the substance to move against an energy gradient, such as from a low-concentration state to a highconcentration state. This movement requires an additional source of energy besides kinetic energy. A more detailed explanation of the basic physics and physical chemistry of these two processes is provided later in this chapter. \n#### DIFFUSION \nAll molecules and ions in the body fluids, including water molecules and dissolved substances, are in constant motion, with each particle moving in its separate way. The motion of these particles is what physicists call \"heat\" the greater the motion, the higher the temperature—and the motion never ceases, except at absolute zero temperature. When a moving molecule, [A, approac](#page-51-0)hes a stationary molecule, B, the electrostatic and other nuclear forces of molecule A repel molecule B, transferring some of the energy of motion of molecule A to molecule B. Consequently, molecule B gains kinetic energy of motion, whereas molecule A slows down, losing some of its kinetic energy. As shown in **Figure 4-3**, a single molecule in a solution bounces among the other molecules—first in one direction, then another, then another, and so forth randomly bouncing thousands of times each second. This continual movement of molecules among one another in liquids or gases is called *diffusion.* \nIons diffuse in the same manner as whole molecules, and even suspended colloid particles diffuse in a similar manner, except that the colloids diffuse far less rapidly than molecular substances because of their large size. \n#### **DIFFUSION THROUGH THE CELL MEMBRANE** \nDiffusion through the cell membrane is divided into two subtypes, called *simple diffusion* and *facilitated diffusion.* Simple diffusion means that kinetic movement of molecules or ions occurs through a membrane opening or through intermolecular spaces without interaction with carrier proteins in the membrane. The rate of diffusion is determined by the amount of substance available, the velocity of kinetic motion, and the number and sizes of openings in the membrane through which the molecules or ions can move. \nFacilitated diffusion requires interaction of a carrier protein. The carrier protein aids passage of molecules or ions through the membrane by binding chemically with them and shuttlin[g them thro](#page-50-0)ugh the membrane in this form. \nSimple diffusion can occur through the cell membrane by two pathways: (1) through the interstices of the lipid bilayer if the diffusing substance is lipid-soluble; and (2) through watery channels that penetrate all the way through some of the large transport proteins, as shown to the left in **Figure 4-2**. \n**Diffusion of Lipid-Soluble Substances Through the Lipid Bilayer.** The *lipid solubility* of a substance is an important factor for determining how rapidly it diffuses through the lipid bilayer. For example, the lipid solubilities of oxygen, nitrogen, carbon dioxide, and alcohols are high, and all these substances can dissolve directly in the lipid bilayer and diffuse through the cell membrane in the same manner that diffusion of water solutes occurs in a watery solution. The rate of diffusion of each of these substances through the membrane is directly proportional to its lipid solubility. Especially large amounts of oxygen can be transported in this way; therefore, oxygen can be delivered to the interior of the cell almost as though the cell membrane did not exist. \n**Diffusion of Water and Other Lipid-Insoluble Molecules Through Protein Channels.** Even though water is highly insoluble in the membrane lipids, it readily passes through channels in protein molecules that penetrate all the way through the membrane. Many of the body's cell membranes contain protein \"pores\" called *aquaporins* that selectively permit rapid passage of water through the membrane. The aquaporins are highly specialized, and there are at least 13 different types in various cells of mammals. \nThe rapidity with which water molecules can diffuse through most cell membranes is astounding. For example, the total amount of water that diffuses in each direction through the red blood cell membrane during each second is about 100 times as great as the volume of the red blood cell. \nOther lipid-insoluble molecules can pass through the protein pore channels in the same way as water molecules if they are water-soluble and small enough. However, as they become larger, their penetration falls off rapidly. For example, the diameter of the urea molecule is only 20% greater than that of water, yet its penetration through the cell membrane pores is about 1000 times less than that of water. Even so, given the astonishing rate of water penetration, this amount of urea penetration still allows rapid transport of urea through the membrane within minutes. \n#### **DIFFUSION THROUGH PROTEIN PORES AND CHANNELS—SELECTIVE PERMEABILITY AND \"GATING\" OF CHANNELS** \nComputerized three-dimensional reconstructions of protein pores and channels have demonstrated tubular pathways all the way from the extracellular to the intracellular fluid. Therefore, substances can move by simple diffusion directly along these pores and channels from one side of the membrane to the other. \nPores are composed of integral cell membrane proteins that form open tubes through the membrane and are always open. However, the diameter of a pore and its electrical charges provide selectivity that permits only certain molecules to pass through. For example, *aquaporins* permit rapid passage of water through cell membranes but exclude other molecules. Aquaporins have a narrow pore that permits water molecules to diffuse through the membrane in single file. The pore is too narrow to permit passage of any hydrated ions. As discussed in Chapters 28 and 76, the density of some aquaporins (e.g., aquaporin-2) in cell membranes is not static but is altered in different physiological conditions. \nThe protein channels are distinguished by two important characteristics: (1) they are often *selectively permeable* to certain substances; and (2) many of the channels can be opened or closed by *gates* that are regulated by electrical signals *(voltage-gated channels)* or chemicals that bind to the channel proteins *(ligand-gated channels).* Thus, ion channels are flexible dynamic structures, and subtle conformational changes influence gating and ion selectivity. \n**Selective Permeability of Protein Channels.** Many protein channels are highly selective for transport of one or more specific ions or molecules. This selectivity results from specific characteristics of the channel, such as its diameter, shape, and the nature of the electrical charges and chemical bonds along its inside surfaces. \n*Potassium channels* permit passage of potassium ions across the cell membrane about 1000 times more readily than they permit passage of sodium ions. This high degree of selectivity cannot be explained entirely by the \n \n**Figure 4-4.** The structure of a potassium channel. The channel is composed of four subunits (only two of which are shown), each with two transmembrane helices. A narrow selectivity filter is formed from the pore loops, and carbonyl oxygens line the walls of the selectivity filter, forming sites for transiently binding dehydrated potassium ions. The interaction of the potassium ions with carbonyl oxygens causes the potassium ions to shed their bound water molecules, permitting the dehydrated potassium ions to pass through the pore. \nmolecular diameters of the ions because potassium ions are slightly larger than sodium ions. Using x-ray crystallography, potassium channels were found to have a *tetrameric structure* consisting of four identical protein subunits surrounding a central pore (**Figure 4-4**). At the top of the channel pore are *pore loops* that form a narrow *selectivity filter*. Lining the selectivity filter are *carbonyl oxygens.* When hydrated potassium ions enter the selectivity filter, they interact with the carbonyl oxygens and shed most of their bound water molecules, permitting the dehydrated potassium ions to pass through the channel. The carbonyl oxygens are too far apart, however, to enable them to interact closely with the smaller sodium ions, which are therefore effectively excluded by the selectivity filter from passing through the pore. \nDifferent selectivity filters for the various ion channels are believed to determine, in large part, the specificity of various channels for cations or anions or for particular ions, such as sodium (Na+), potassium (K+), and calcium (Ca2+), that gain access to the channels. \nOne of the most important of the protein channels, the *sodium channel,* is only 0.3 to 0.5 nanometer in diameter, but the ability of sodium channels to discriminate sodium ions among other competing ions in the surrounding fluids is crucial for proper cellular function. \n \n**Figure 4-5.** Transport of sodium and potassium ions through protein channels. Also shown are conformational changes in the protein molecules to open or close the \"gates\" guarding the channels. \nThe narrowest part of the sodium channel's open pore, the *selectivity filter*, is lined with *strongly negatively charged* amino acid residues, as shown in the top panel of **Figure 4-5**. These strong negative charges can pull small *dehydrated* sodium ions away from their hydrating water molecules into these channels, although the ions do not need to be fully dehydrated to pass through the channels. Once in the channel, the sodium ions diffuse in either direction according to the usual laws of diffusion. Thus, the sodium channel is highly selective for passage of sodium ions. \n**Gating of Protein Channels.** Gating of protein channels provides a means of controlling ion permeability of the channels. This mechanism is shown in both panels of **Figure 4-5** for selective gating of sodium and potassium ions. Some of the gates are thought to be gatelike extensions of the transport protein molecule, which can close the opening of the channel or can be lifted away from the opening by a conformational change in the shape of the protein molecule. \nThe opening and closing of gates are controlled in two principal ways: \n1. Voltage gating. In the case of voltage gating, the molecular conformation of the gate or its chemical bonds responds to the electrical potential across the cell membrane. For example, in the top panel of Figure 4-5, a strong negative charge on the inside of the cell membrane may cause the outside sodium gates to remain tightly closed. Conversely, when the inside of the membrane loses its negative charge, these gates open suddenly and allow sodium to pass inward through the sodium pores. This process is the basic mechanism for eliciting action potentials in nerves that are responsible for nerve signals. In \n- the bottom panel of **Figure 4-5**, the potassium gates are on the intracellular ends of the potassium channels, and they open when the inside of the cell membrane becomes positively charged. The opening of these gates is partly responsible for terminating the action potential, a process discussed in Chapter 5.\n- 2. Chemical (ligand) gating. Some protein channel gates are opened by the binding of a chemical substance (a ligand) with the protein, which causes a conformational or chemical bonding change in the protein molecule that opens or closes the gate. One of the most important instances of chemical gating is the effect of the neurotransmitter acetylcholine on the acetylcholine receptor which serves as a ligand-gated ion channel. Acetylcholine opens the gate of this channel, providing a negatively charged pore about 0.65 nanometer in diameter that allows uncharged molecules or positive ions smaller than this diameter to pass through. This gate is exceedingly important for the transmission of nerve signals from one nerve cell to another (see Chapter 46) and from nerve cells to muscle cells to cause muscle contraction (see Chapter 7). \n#### Open-State Versus Closed-State of Gated Channels. \nFigure 4-6A shows two recordings of electrical current flowing through a single sodium channel when there was an approximately 25-millivolt potential gradient across the membrane. Note that the channel conducts current in an all-or-none fashion. That is, the gate of the channel snaps open and then snaps closed, with each open state lasting for only a fraction of a millisecond, up to several milliseconds, demonstrating the rapidity with which changes can occur during the opening and closing of the protein gates. At one voltage potential, the channel may remain closed all the time or almost all the time, whereas at another voltage, it may remain open either all or most of the time. At in-between voltages, as shown in the figure, the gates tend to snap open and closed intermittently, resulting in an average current flow somewhere between the minimum and maximum. \nPatch Clamp Method for Recording Ion Current Flow Through Single Channels. The patch clamp method for recording ion current flow through single protein channels is illustrated in Figure 4-6B. A micropipette with a tip diameter of only 1 or 2 micrometers is abutted against the outside of a cell membrane. Suction is then applied inside the pipette to pull the membrane against the tip of the pipette, which creates a seal where the edges of the pipette touch the cell membrane. The result is a minute membrane \"patch\" at the tip of the pipette through which electrical current flow can be recorded. \nAlternatively, as shown at the bottom right in **Figure 4-6B**, the small cell membrane patch at the end of the pipette can be torn away from the cell. The pipette with its sealed patch is then inserted into a free solution, which \n \n**Figure 4-6. A**, Recording of current flow through a single voltagegated sodium channel, demonstrating the all or none principle for opening and closing of the channel. **B**, Patch clamp method for recording current flow through a single protein channel. To the left, the recording is performed from a \"patch\" of a living cell membrane. To the right, the recording is from a membrane patch that has been torn away from the cell. \nallows the concentrations of ions both inside the micropipette and in the outside solution to be altered as desired. Also, the voltage between the two sides of the membrane can be set, or \"clamped,\" to a given voltage. \nIt has been possible to make such patches small enough so that only a single channel protein is found in the membrane patch being studied. By varying the concentrations of different ions, as well as the voltage across the membrane, one can determine the transport characteristics of the single channel, along with its gating properties. \n \n**Figure 4-7.** Effect of concentration of a substance on the rate of diffusion through a membrane by simple diffusion and facilitated diffusion. This graph shows that facilitated diffusion approaches a maximum rate, called the *Vmax.* \n#### **FACILITATED DIFFUSION REQUIRES MEMBRANE CARRIER PROTEINS** \nFacilitated diffusion is also called *carrier-mediated diffusion* because a substance transported in this manner diffuses through the membrane with the help of a specific carrier protein. That is, the carrier *facilitates* diffusion of the substance to the other side. \nFacilitated diffusion differs from simple diffusion in the following important way. Although the rat[e of simple](#page-54-1) diffusion through an open channel increases proportionately with the concentration of the diffusing substance, in facilitated diffusion the rate of diffusion approaches a maximum, called Vmax, as the concentration of the diffusing substance increases. This difference between simple diffusion and facilitated diffusion is demonstrated in **Figure 4-7**. The figure shows t[hat a](#page-55-0)s the concentration of the diffusing substance increases, the rate of simple diffusion continues to increase proportionately but, in the case of facilitated diffusion, the rate of diffusion cannot rise higher than the Vmax level. \nWhat is it that limits the rate of facilitated diffusion? A probable answer is the mechanism illustrated in **Figure 4-8**. This Figure shows a carrier protein with a pore large enough to transport a specific molecule partway through. It also shows a binding receptor on the inside of the protein carrier. The molecule to be transported enters the pore and becomes bound. Then, in a fraction of a second, a conformational or chemical change occurs in the carrier protein, so that the pore now opens to the opposite side of the membrane. Because the binding force of the receptor is weak, the thermal motion of the attached molecule causes it to break away and be released on the opposite side of the membrane. The rate at which molecules can be transported by this mechanism can never be greater than the rate at which the carrier protein molecule can undergo change back and forth between its two states. Note specifically, though, that this mechanism allows the transported molecule to move—that is, diffuse—in either direction through the membrane. \n \nFigure 4-8. Postulated mechanism for facilitated diffusion. \nAmong the many substances that cross cell membranes by facilitated diffusion are *glucose* and most of the *amino acids*. In the case of glucose, at least 14 members of a family of membrane proteins (called *GLUT*) that transport glucose molecules have been discovered in various tissues. Some of these GLUT proteins transport other monosaccharides that have structures similar to that of glucose, including galactose and fructose. One of these, glucose transporter 4 (GLUT4), is activated by insulin, which can increase the rate of facilitated diffusion of glucose as much as 10- to 20-fold in insulin-sensitive tissues. This is the principal mechanism whereby insulin controls glucose use in the body, as discussed in Chapter 79.\n\nBy now, it is evident that many substances can diffuse through the cell membrane. What is usually important is the *net* rate of diffusion of a substance in the desired direction. This net rate is determined by several factors. \n**Net Diffusion Rate Is Proportional to the Concentration Difference Across a Membrane. Figure 4-9.4** shows a cell membrane with a high concentration of a substance on the outside and a low concentration of a substance on the inside. The rate at which the substance diffuses *inward* is proportional to the concentration of molecules on the *outside* because this concentration determines how many molecules strike the outside of the membrane each second. Conversely, the rate at which molecules diffuse *outward* is proportional to their concentration *inside* the membrane. Therefore, the rate of net diffusion into the cell is proportional to the concentration on the outside *minus* the concentration on the inside: \nNet diffusion $\\propto (C_o - C_i)$ \n \n**Figure 4-9.** Effect of concentration difference (**A**), electrical potential difference affecting negative ions (**B**), and pressure difference (**C**) to cause diffusion of molecules and ions through a cell membrane. $C_o$ , concentration outside the cell; $C_i$ , concentration inside the cell; $P_1$ pressure 1; $P_2$ pressure 2. \nin which $C_0$ is the concentration outside and $C_i$ is the concentration inside the cell. \nMembrane Electrical Potential and Diffusion of lons-The \"Nernst Potential.\" If an electrical potential is applied across the membrane, as shown in Figure **4-9B**, the electrical charges of the ions cause them to move through the membrane even though no concentration difference exists to cause movement. Thus, in the left panel of Figure 4-9B, the concentration of negative ions is the same on both sides of the membrane, but a positive charge has been applied to the right side of the membrane, and a negative charge has been applied to the left, creating an electrical gradient across the membrane. The positive charge attracts the negative ions, whereas the negative charge repels them. Therefore, net diffusion occurs from left to right. After some time, large quantities of negative ions have moved to the right, creating the condition shown in the right panel of Figure 4-9B, in which a concentration difference of the ions has developed in the direction opposite to the electrical potential difference. The concentration difference now tends to move the ions to the left, whereas the electrical difference tends to move them to the right. When the concentration difference rises high enough, the two effects balance each other. At normal body temperature (98.6°F; 37°C), the electrical difference that will balance a given concentration difference \nof *univalent* ions—such as Na+ ions—can be determined from the following formula, called the *Nernst equation*: \nEMF (in millivolts) =\n$$\\pm 61\\log \\frac{C_1}{C_2}$$ \nin which EMF is the electromotive force (voltage) between side 1 and side 2 of the membrane, $C_1$ is the concentration on side 1, and $C_2$ is the concentration on side 2. This equation is extremely important in understanding the transmission of nerve impulses and is discussed in Chapter 5. \n#### Effect of a Pressure Difference Across the Membrane. \nAt times, a considerable pressure difference develops between the two sides of a diffusible membrane. This pressure difference occurs, for example, at the blood capillary membranes in all tissues of the body. The pressure in many capillaries is about 20 mm Hg greater inside than outside. \nPressure actually means the sum of all the forces of the different molecules striking a unit surface area at a given instant. Therefore, having a higher pressure on one side of a membrane than on the other side means that the sum of all the forces of the molecules striking the channels on that side of the membrane is greater than on the other side. In most cases, this situation is caused by greater numbers of molecules striking the membrane per second on one side than on the other side. The result is that increased amounts of energy are available to cause a net movement of molecules from the high-pressure side toward the low-pressure side. This effect is demonstrated in **Figure 4-9C**, which shows a piston developing high pressure on one side of a pore, thereby causing more molecules to strike the pore on this side and, therefore, more molecules to diffuse to the other side.\n\nBy far, the most abundant substance that diffuses through the cell membrane is water. Enough water ordinarily diffuses in each direction through the red blood cell membrane per second to equal about 100 times the volume of the cell itself. Yet, the amount that normally diffuses in the two directions is balanced so precisely that zero net movement of water occurs. Therefore, the volume of the cell remains constant. However, under certain conditions, a concentration difference for water can develop across a membrane. When this concentration difference for water develops, net movement of water does occur across the cell membrane, causing the cell to swell or shrink, depending on the direction of the water movement. This process of net movement of water caused by a concentration difference of water is called osmosis. \nTo illustrate osmosis, let us assume the conditions shown in Figure 4-10, with pure water on one side of the cell membrane and a solution of sodium chloride on the \n \n**Figure 4-10.** Osmosis at a cell membrane when a sodium chloride solution is placed on one side of the membrane and water is placed on the other side. \nother side. Water molecules pass through the cell membrane with ease, whereas sodium and chloride ions pass through only with difficulty. Therefore, sodium chloride solution is actually a mixture of permeant water molecules and nonpermeant sodium and chloride ions, and the membrane is said to be *selectively permeable* to water but much less so to sodium and chloride ions. Yet, the presence of the sodium and chloride has displaced some of the water molecules on the side of the membrane where these ions are present and, therefore, has reduced the concentration of water molecules to less than that of pure water. As a result, in the example shown in Figure 4-10, more water molecules strike the channels on the left side, where there is pure water, than on the right side, where the water concentration has been reduced. Thus, net movement of water occurs from left to right-that is, osmosis occurs from the pure water into the sodium chloride solution. \n#### **Osmotic Pressure** \nIf in **Figure 4-10** pressure were applied to the sodium chloride solution, osmosis of water into this solution would be slowed, stopped, or even reversed. The amount of pressure required to stop osmosis is called the *osmotic pressure* of the sodium chloride solution. \nThe principle of a pressure difference opposing osmosis is demonstrated in **Figure 4-11**, which shows a selectively permeable membrane separating two columns of fluid, one containing pure water and the other containing a solution of water and any solute that will not penetrate the membrane. Osmosis of water from chamber B into chamber A causes the levels of the fluid columns to become farther and farther apart, until eventually a pressure difference develops between the two sides of the membrane that is great enough to oppose the osmotic effect. The pressure difference across the membrane at this point is equal to the osmotic pressure of the solution that contains the nondiffusible solute. \n \n**Figure 4-11.** Demonstration of osmotic pressure caused by osmosis at a semipermeable membrane. \n#### **Importance of Number of Osmotic Particles (Molar Concentration) in Determining Osmotic Pressure.** \nThe osmotic pressure exerted by particles in a solution, whether they are molecules or ions, is determined by the number of particles per unit volume of fluid, not by the mass of the particles. The reason for this is that each particle in a solution, regardless of its mass, exerts, on average, the same amount of pressure against the membrane. That is, large particles, which have greater mass (m) than small particles, move at a slower velocity (v). The small particles move at higher velocities in such a way that their average kinetic energies (k), as determined by the following equation, \n$$k = \\frac{mv^2}{2}$$ \nare the same for each small particle as for each large particle. Consequently, the factor that determines the osmotic pressure of a solution is the concentration of the solution in terms of the number of particles (which is the same as its *molar concentration* if it is a nondissociated molecule), not in terms of mass of the solute. \n**Osmolality—The Osmole.** To express the concentration of a solution in terms of numbers of particles, a unit called the *osmole* is used in place of grams. \nOne osmole is 1 gram molecular weight of osmotically active solute. Thus, 180 grams of glucose, which is 1 gram molecular weight of glucose, is equal to 1 osmole of glucose because glucose does not dissociate into ions. If a solute dissociates into two ions, 1 gram molecular weight of the solute will become 2 osmoles because the number of osmotically active particles is now twice as great as for the nondissociated solute. Therefore, when fully dissociated, 1 gram molecular weight of sodium chloride, 58.5 grams, is equal to 2 osmoles. \nThus, a solution that has *1 osmole of solute dissolved in each kilogram of water* is said to have an *osmolality of 1 osmole per kilogram,* and a solution that has 1/1000 osmole dissolved per kilogram has an osmolality of 1 milliosmole per kilogram. The normal osmolality of the extracellular and intracellular fluids is about *300 milliosmoles per kilogram of water.* \n**Relationship of Osmolality to Osmotic Pressure.** At normal body temperature, 37°C (98.6°F), a concentration of 1 osmole per liter will cause 19,300 mm Hg osmotic pressure in the solution. Likewise, *1 milliosmole* per liter concentration is equivalent to *19.3 mm Hg* osmotic pressure. Multiplying this value by the 300-milliosmolar concentration of the body fluids gives a total calculated osmotic pressure of the body fluids of 5790 mm Hg. The measured value for this, however, averages only about 5500 mm Hg. The reason for this difference is that many ions in the body fluids, such as sodium and chloride ions, are highly attracted to one another; consequently, they cannot move entirely unrestrained in the fluids and create their full osmotic pressure potential. Therefore, on average, the actual osmotic pressure of the body fluids is about 0.93 times the calculated value. \n**The Term** *Osmolarity***.** *Osmolarity* is the osmolar concentration expressed as *osmoles per liter of solution* rather than osmoles per kilogram of water. Although, strictly speaking, it is osmoles per kilogram of water (osmolality) that determines osmotic pressure, the quantitative differences between osmolarity and osmolality are less than 1% for dilute solutions such as those in the body. Because it is far more practical to measure osmolarity than osmolality, measuring osmolarity is the usual practice in physiological studies. \n#### ACTIVE TRANSPORT OF SUBSTANCES THROUGH MEMBRANES \nAt times, a large concentration of a substance is required in the intracellular fluid, even though the extracellular fluid contains only a small concentration. This situation is true, for example, for potassium ions. Conversely, it is important to keep the concentrations of other ions very low inside the cell, even though their concentrations in the extracellular fluid are high. This situation is especially true for sodium ions. Neither of these two effects could occur by simple diffusion because simple diffusion eventually equilibrates concentrations on the two sides of the membrane. Instead, some energy source must cause excess movement of potassium ions to the inside of cells and excess movement of sodium ions to the outside of cells. When a cell membrane moves molecules or ions uphill against a concentration gradient (or uphill against an electrical or pressure gradient), the process is called *active transport.* \nSome examples of substances that are actively transported through at least some cell membranes include sodium, potassium, calcium, iron, hydrogen, chloride, iodide, and urate ions, several different sugars, and most of the amino acids. \n**Primary Active Transport and Secondary Active Transport.** Active transport is divided into two types according to the source of the energy used to facilitate the transport, *primary active transport* and *secondary active transport.* In primary active transport, the energy is derived directly from the breakdown of adenosine triphosphate (ATP) or some other high-energy phosphate compound. In secondary active transport, the energy is derived secondarily from energy that has been stored in the form of ionic concentration differences of secondary molecular or ionic substances between the two sides of a cell membrane, created originally by primary active transport. In both cases, transport depends on *carrier proteins* that penetrate through the cell membrane, as is true for facilitated diffusion. However, in active transport, the carrier protein functions differently from the carrier in facilitated diffusion because it is capable of imparting energy to the transported substance to move it against the electrochemical gradient. The following sections provide some examples of primary active transport and secondary active transport, with more detailed explanations of their principles of function. \n#### **PRIMARY ACTIVE TRANSPORT** \n#### **Sodium-Potassium Pump Transports Sodium Ions Out of Cells and Potassium Ions into Cells** \nAmong the substances that are transported by primary active transport are sodium, potassium, calcium, hydrogen, chloride, and a few other ions. The active transport mechanism that has been studied in greatest detail is the *sodium-potassium* (Na+-K+) pump, a transporter that pumps sodium ions outward through the cell membrane of all cells and, at the same time, pumps potassium ions from the outside to the inside. This pump is responsible for mainta[ining the so](#page-58-0)dium and potassium concentration differences across the cell membrane, as well as for establishing a negative electrical voltage inside the cells. Indeed, Chapter 5 shows that this pump is also the basis of nerve function, transmitting nerve signals throughout the nervous system. \n**Figure 4-12** shows the basic physical components of the Na+-K+ pump. The *carrier protein* is a complex of two separate globular proteins—a larger one called the α subunit, with a molecular weight of about 100,000, and a smaller one called the β subunit, with a molecular weight of about 55,000. Although the function of the smaller protein is not known (except that it might anchor the protein complex in the lipid membrane), the larger protein has three specific features that are important for the functioning of the pump: \n1. It has three *binding sites for sodium ions* on the portion of the protein that protrudes to the inside of the cell. \n \n**Figure 4-12.** Postulated mechanism of the sodium-potassium pump. ADP, Adenosine diphosphate; ATP, adenosine triphosphate; Pi, phosphate ion. \n- 2. It has two *binding sites for potassium ions* on the outside.\n- 3. The inside portion of this protein near the sodium binding sites has adenosine triphosphatase (AT-Pase) activity. \nWhen two potassium ions bind on the outside of the carrier protein and three sodium ions bind on the inside, the ATPase function of the protein becomes activated. Activation of the ATPase function leads to cleavage of one molecule of ATP, splitting it to adenosine diphosphate (ADP) and liberating a high-energy phosphate bond of energy. This liberated energy is believed to cause a chemical and conformational change in the protein carrier molecule, extruding three sodium ions to the outside and two potassium ions to the inside. \nAs with other enzymes, the Na+-K+ ATPase pump can run in reverse. If the electrochemical gradients for Na+ and K+ are experimentally increased to the degree that the energy stored in their gradients is greater than the chemical energy of ATP hydrolysis, these ions will move down their concentration gradients, and the Na+-K+ pump will synthesize ATP from ADP and phosphate. The phosphorylated form of the Na+-K+ pump, therefore, can either donate its phosphate to ADP to produce ATP or use the energy to change its conformation and pump Na+ out of the cell and K+ into the cell. The relative concentrations of ATP, ADP, and phosphate, as well as the electrochemical gradients for Na+ and K+, determine the direction of the enzyme reaction. For some cells, such as electrically active nerve cells, 60% to 70% of the cell's energy requirement may be devoted to pumping Na+ out of the cell and K+ into the cell. \n**The Na+-K+ Pump Is Important for Controlling Cell Volume.** One of the most important functions of the Na+-K+ pump is to control the cell volume. Without function of this pump, most cells of the body would swell until they burst. \nThe mechanism for controlling the volume is as follows. Inside the cell are large numbers of proteins and other organic molecules that cannot escape from the cell. Most of these proteins and other organic molecules are negatively charged and, therefore, attract large numbers of potassium, sodium, and other positive ions. All these molecules and ions then cause osmosis of water to the interior of the cell. Unless this process is checked, the cell will swell indefinitely until it bursts. The normal mechanism for preventing this outcome is the Na+-K+ pump. Note again that this mechanism pumps three Na+ ions to the outside of the cell for every two K+ ions pumped to the interior. Also, the membrane is far less permeable to sodium ions than to potassium ions and, once the sodium ions are on the outside, they have a strong tendency to stay there. This process thus represents a net loss of ions out the cell, which also initiates osmosis of water out of the cell. \nIf a cell begins to swell for any reason, the Na+-K+ pump is automatically activated, moving still more ions to the exterior and carrying water with them. Therefore, the Na+-K+ pump performs a continual surveillance role in maintaining normal cell volume. \n**Electrogenic Nature of the Na+-K+ Pump.** The fact that the Na+-K+ pump moves three Na+ ions to the exterior for every two K+ ions that are moved to the interior means that a net of one positive charge is moved from the interior of the cell to the exterior of the cell for each cycle of the pump. This action creates positivity outside the cell but results in a deficit of positive ions inside the cell; that is, it causes negativity on the inside. Therefore, the Na+-K+ pump is said to be *electrogenic* because it creates an electrical potential across the cell membrane. As discussed in Chapter 5, this electrical potential is a basic requirement in nerve and muscle fibers for transmitting nerve and muscle signals. \n#### **Primary Active Transport of Calcium Ions** \nAnother important primary active transport mechanism is the *calcium pump*. Calcium ions are normally maintained at an extremely low concentration in the intracellular cytosol of virtually all cells in the body, at a concentration about 10,000 times less than that in the extracellular fluid. This level of maintenance is achieved mainly by two primary active transport calcium pumps. One, which is in the cell membrane, pumps calcium to the outside of the cell. The other pumps calcium ions into one or more of the intracellular vesicular organelles of the cell, such as the sarcoplasmic reticulum of muscle cells and the mitochondria in all cells. In each of these cases, the carrier protein penetrates the membrane and functions as an enzyme ATPase, with the same capability to cleave ATP as the ATPase of the sodium carrier protein. The difference is that this protein has a highly specific binding site for calcium instead of for sodium.\n\nPrimary active transport of hydrogen ions is especially important at two places in the body: (1) in the gastric glands of the stomach; and (2) in the late distal tubules and cortical collecting ducts of the kidneys. \nIn the gastric glands, the deep-lying *parietal cells* have the most potent primary active mechanism for transporting hydrogen ions of any part of the body. This mechanism is the basis for secreting hydrochloric acid in stomach digestive secretions. At the secretory ends of the gastric gland parietal cells, the hydrogen ion concentration is increased as much as a million-fold and then is released into the stomach, along with chloride ions, to form hydrochloric acid. \nIn the renal tubules, special *intercalated cells* found in the late distal tubules and cortical collecting ducts also transport hydrogen ions by primary active transport. In this case, large amounts of hydrogen ions are secreted from the blood into the renal tubular fluid for the purpose of eliminating excess hydrogen ions from the body fluids. The hydrogen ions can be secreted into the renal tubular fluid against a concentration gradient of about 900-fold. Yet, as discussed in Chapter 31, most of these hydrogen ions combine with tubular fluid buffers before they are eliminated in the urine \n#### **Energetics of Primary Active Transport** \nThe amount of energy required to transport a substance actively through a membrane is determined by how much the substance is concentrated during transport. Compared with the energy required to concentrate a substance 10-fold, concentrating it 100-fold requires twice as much energy, and concentrating it 1000-fold requires three times as much energy. In other words, the energy required is proportional to the *logarithm* of the degree that the substance is concentrated, as expressed by the following formula: \nEnergy (in calories per osmole) = 1400 log\n$$\\frac{C_1}{C_2}$$ \nThus, in terms of calories, the amount of energy required to concentrate 1 osmole of a substance 10-fold is about 1400 calories, whereas to concentrate it 100-fold, 2800 calories are required. One can see that the energy expenditure for concentrating substances in cells or for removing substances from cells against a concentration gradient can be tremendous. Some cells, such as those lining the renal tubules and many glandular cells, expend as much as 90% of their energy for this purpose alone.\n\nWhen sodium ions are transported out of cells by primary active transport, a large concentration gradient of \nsodium ions across the cell membrane usually develops, with a high concentration outside the cell and a low concentration inside. This gradient represents a storehouse of energy, because the excess sodium outside the cell membrane is always attempting to diffuse to the interior. Under appropriate conditions, this diffusion energy of sodium can pull other substances along with the sodium through the cell membrane. This phenomenon, called *cotransport,* is one form of *secondary active transport.* \nFor sodium to pull another substance along with it, a coupling mechanism is required; this is achieved by means of still another carrier protein in the cell membrane. The carrier in this case serves as an attachment point for both the sodium ion and the substance to be co-transported. Once they are both attached, the energy gradient of the sodium ion causes the sodium ion and the other substance to be transported together to the interior of the cell. \nIn *counter-transport,* sodium ions again attempt to diffuse to the interior of the cell because of their large concentration gradient. However, this time, the substance to be transported is on the inside of the cell and is transported to the outside. Therefore, the sodium ion binds to the carrier protein, where it projects to the exterior surface of the membrane, and the substance to be countertransported binds to the interior projection of the carrier protein. Once both have become bound, a conformational change occurs, and energy released by the action of the sodium ion moving to the interior causes the other substance to move to the exterior. \n#### **Co-Transport o[f Glucose a](#page-60-0)nd Amino Acids Along with Sodium Ions** \nGlucose and many amino acids are transported into most cells against large concentration gradients; the mechanism of this action is entirely by co-transport, as shown in **Figure 4-13**. Note that the transport carrier protein has two binding sites on its exterior side, one for sodium and one for glucose. Also, the concentration of sodium ions is high on the outside and low on the inside, which provides energy for the transport. A special property of the transport protein is that a conformational change to allow sodium movement to the interior will not occur until a glucose molecule also attaches. When they both become attached, the conformational change takes place, and the sodium and glucose are transported to the inside of the cell at the same time. Hence, this is a *sodium-glucose co-transporter*. Sodium-glucose cotransporters are especially important for transporting glucose across renal and intestinal epithelial cells, as discussed in Chapters 28 and 66. \n*Sodium co-transport of amino acids* occurs in the same manner as for glucose, except that it uses a different set of transport proteins. At least five *amino acid transport proteins* have been identified, each of which is responsible for transporting one subset of amino acids with specific molecular characteristics. \n \n**Figure 4-13** Postulated mechanism for sodium co-transport of glucose. \n \n**Figure 4-14.** Sodium counter-transport of calcium and hydrogen ions. \nSodium co-transport of glucose and amino acids occurs especially through the epithelial cells of the intestinal tract and the renal tubules of the kidneys to promote absorption of these substances into the blood. This process will be discussed in later chapters. \nOther important co-transport mechanisms in at least some cells include co-transport of potassium, chloride, bicarbonate, phosphate, iodine, iron, and urate ions. \n#### **Sodium Counter-Tran[sport of Ca](#page-60-1)lcium and Hydrogen Ions** \nTwo especially important counter-transporters (i.e., transport in a direction opposite to the primary ion) are *sodium-calcium counter-transport* and *sodium-hydrogen counter-transport* (**Figure 4-14**). \nSodium-calcium counter-transport occurs through all or almost all cell membranes, with sodium ions moving to the interior and calcium ions to the exterior; both are bound to the same transport protein in a countertransport mode. This mechanism is in addition to the primary active transport of calcium that occurs in some cells. \nSodium-hydrogen counter-transport occurs in several tissues. An especially important example is in the *proximal tubules* of the kidneys, where sodium ions move from the lumen of the tubule to the interior of the tubular cell and hydrogen ions are counter-transported into the tubule lumen. As a mechanism for concentrating hydrogen ions, counter-transport is not nearly as powerful as the primary active transport of hydrogen ions that occurs in the more distal renal tubules, but it can transport extremely *large numbers of hydrogen ions,* thus making it a key to hydrogen ion control in the body fluids, as discussed in detail in Chapter 31. \n \n**Figure 4-15.** Basic mechanism of active transport across a layer of cells. \n#### **ACTIVE TRANSPORT THROUGH CELLULAR SHEETS** \nAt many places in the body, substances must be transported all the way through a cellular sheet instead of simply through the cell membrane. Transport of this type occurs through the following: (1) intestinal epithelium; (2) epithelium of the renal tubules; (3) epithelium of all exocrine glands; (4) epithelium of the gallbladder; and (5) membrane of the choroid plexus of the brain, along with other membranes. \nThe bas[ic mechanism](#page-61-0) for transport of a substance through a cellular sheet is as follows: (1) *active transport* through the cell membrane *on one side* of the transporting cells in the sheet; and then (2) either *simple diffusion* or *facilitated diffusion* through the membrane *on the opposite side* of the cell. \n**Figure 4-15** shows a mechanism for the transport of sodium ions through the epithelial sheet of the intestines, gallbladder, and renal tubules. This figure shows that the epithelial cells are connected together tightly at the luminal pole by means of junctions. The brush border on the luminal surfaces of the cells is permeable to both sodium ions and water. Therefore, sodium and water diffuse readily from the lumen into the interior of the cell. Then, at the basal and lateral membranes of the cells, sodium ions are actively transported into the extracellular fluid of the surrounding connective tissue and blood vessels. This action creates a high sodium ion concentration gradient across these membranes, which in turn causes osmosis of water. Thus, active transport of sodium ions at the basolateral sides of the epithelial cells results in the transport not only of sodium ions but also of water. \nIt is through these mechanisms that almost all nutrients, ions, and other substances are absorbed into the blood from the intestine. These mechanisms are also how the same substances are reabsorbed from the glomerular filtrate by the renal tubules. \nNumerous examples of the different types of transport discussed in this chapter are provided throughout this text. \n#### Bibliography \nAgre P, Kozono D: Aquaporin water channels: molecular mechanisms for human diseases. FEBS Lett 555:72, 2003. \nBröer S: Amino acid transport across mammalian intestinal and renal epithelia. Physiol Rev 88:249, 2008. \nDeCoursey TE: Voltage-gated proton channels: molecular biology, physiology, and pathophysiology of the H(V) family. Physiol Rev 93:599, 2013. \nDiPolo R, Beaugé L: Sodium/calcium exchanger: influence of metabolic regulation on ion carrier interactions. Physiol Rev 86:155, 2006. \nDrummond HA, Jernigan NL, Grifoni SC: Sensing tension: epithelial sodium channel/acid-sensing ion channel proteins in cardiovascular homeostasis. Hypertension 51:1265, 2008. \nEastwood AL, Goodman MB: Insight into DEG/ENaC channel gating from genetics and structure. Physiology (Bethesda) 27:282, 2012. \nFischbarg J: Fluid transport across leaky epithelia: central role of the tight junction and supporting role of aquaporins. Physiol Rev 90:1271, 2010. \nGadsby DC: Ion channels versus ion pumps: the principal difference, in principle. Nat Rev Mol Cell Biol 10:344, 2009. \nGhezzi C, Loo DDF, Wright EM. Physiology of renal glucose handling via SGLT1, SGLT2 and GLUT2. Diabetologia 61:2087-2097, 2018. \nHilge M: Ca2+ regulation of ion transport in the Na+/Ca2+ exchanger. J Biol Chem 287:31641, 2012. \nJentsch TJ, Pusch M. CLC Chloride channels and transporters: structure, function, physiology, and disease. Physiol Rev 2018 98:1493- 1590, 2018. \nKaksonen M, Roux A. Mechanisms of clathrin-mediated endocytosis. Nat Rev Mol Cell Biol 19:313-326, 2018. \nKandasamy P, Gyimesi G, Kanai Y, Hediger MA. Amino acid transporters revisited: new views in health and disease. Trends Biochem Sci 43:752-789, 2018. \nPapadopoulos MC, Verkman AS: Aquaporin water channels in the nervous system. Nat Rev Neurosci 14:265, 2013. \nRieg T, Vallon V. Development of SGLT1 and SGLT2 inhibitors. Diabetologia 61:2079-2086, 2018. \nSachs F: Stretch-activated ion channels: what are they? Physiology 25:50, 2010. \nSchwab A, Fabian A, Hanley PJ, Stock C: Role of ion channels and transporters in cell migration. Physiol Rev 92:1865, 2012. \nStransky L, Cotter K, Forgac M. The function of V-ATPases in cancer. Physiol Rev 96:1071-1091, 2016 \nTian J, Xie ZJ: The Na-K-ATPase and calcium-signaling microdomains. Physiology (Bethesda) 23:205, 2008. \nVerkman AS, Anderson MO, Papadopoulos MC. Aquaporins: important but elusive drug targets. Nat Rev Drug Discov 13:259-277, 2014. \nWright EM, Loo DD, Hirayama BA: Biology of human sodium glucose transporters. Physiol Rev 91:733, 2011. \n\n\nElectrical potentials exist across the membranes of virtually all cells of the body. Some cells, such as nerve and muscle cells, generate rapidly changing electrochemical impulses at their membranes, and these impulses are used to transmit signals along the nerve or muscle membranes. In other types of cells, such as glandular cells, macrophages, and ciliated cells, local changes in membrane potentials also activate many of the cell's functions. This chapter reviews the basic mechanisms whereby membrane potentials are generated at rest and during action by nerve and muscle cells. See Video 5-1. \n\n\nIn Figure 5-1A, the potassium concentration is great inside a nerve fiber membrane but very low outside the membrane. Let us assume that the membrane in this case is permeable to the potassium ions but not to any other ions. Because of the large potassium concentration gradient from the inside toward the outside, there is a strong tendency for potassium ions to diffuse outward through the membrane. As they do so, they carry positive electrical charges to the outside, thus creating electropositivity outside the membrane and electronegativity inside the membrane because of negative anions that remain behind and do not diffuse outward with the potassium. Within about 1 millisecond, the potential difference between the inside and outside, called the diffusion potential, becomes great enough to block further net potassium diffusion to the exterior, despite the high potassium ion concentration gradient. In the normal mammalian nerve fiber, the potential difference is about 94 millivolts, with negativity inside the fiber membrane. \n**Figure 5-1***B* shows the same phenomenon as in **Figure 5-1***A*, but this time with a high concentration of sodium ions *outside* the membrane and a low concentration of sodium ions *inside*. These ions are also positively charged. This time, the membrane is highly permeable to the sodium ions but is impermeable to all other ions. Diffusion of the positively charged sodium ions to the inside \ncreates a membrane potential of opposite polarity to that in **Figure 5-1***A*, with negativity outside and positivity inside. Again, the membrane potential rises high enough within milliseconds to block further net diffusion of sodium ions to the inside; however, this time, in the mammalian nerve fiber, *the potential is about 61 millivolts positive inside the fiber.* \nThus, in both parts of **Figure 5-1**, we see that a concentration difference of ions across a selectively permeable membrane can, under appropriate conditions, create a membrane potential. Later in this chapter, we show that many of the rapid changes in membrane potentials observed during nerve and muscle impulse transmission result from such rapidly changing diffusion potentials. \nThe Nernst Equation Describes the Relationship of Diffusion Potential to the lon Concentration Difference Across a Membrane. The diffusion potential across a membrane that exactly opposes the net diffusion of a particular ion through the membrane is called the *Nernst potential* for that ion, a term that was introduced in Chapter 4. The magnitude of the Nernst potential is determined by the *ratio* of the concentrations of that specific ion on the two sides of the membrane. The greater this ratio, the greater the tendency for the ion to diffuse in one direction and therefore the greater the Nernst potential required to prevent additional net diffusion. The following equation, called the *Nernst equation*, can be used to calculate the Nernst potential for any univalent ion at the normal body temperature of 98.6°F (37°C): \nEMF (millivolts) =\n$$\\pm \\frac{61}{z} \\times log \\frac{Concentration\\ inside}{Concentration\\ outside}$$ \nwhere EMF is the electromotive force and z is the electrical charge of the ion (e.g., +1 for $K^+$ ). \nWhen using this formula, it is usually assumed that the potential in the extracellular fluid outside the membrane remains at zero potential, and the Nernst potential is the potential inside the membrane. Also, the sign of the potential is positive (+) if the ion diffusing from inside to outside is a negative ion, and it is negative (–) if the ion is positive. Thus, when the concentration of positive potassium ions on the inside is 10 times that on the outside, the log of 10 is 1, so the Nernst potential calculates to be -61 millivolts inside the membrane.\n\n**Figure 5-1 A**, Establishment of a diffusion potential across a nerve fiber membrane, caused by diffusion of potassium ions from inside the cell to outside the cell through a membrane that is selectively permeable only to potassium. **B**, Establishment of a diffusion potential when the nerve fiber membrane is permeable only to sodium ions. Note that the internal membrane potential is negative when potassium ions diffuse and positive when sodium ions diffuse because of opposite concentration gradients of these two ions. \nThe Goldman Equation Is Used to Calculate the Diffusion Potential When the Membrane Is Permeable to Several Different Ions. When a membrane is permeable to several different ions, the diffusion potential that develops depends on three factors: (1) the polarity of the electrical charge of each ion; (2) the permeability of the membrane (P) to each ion; and (3) the concentration (C) of the respective ions on the inside (i) and outside (o) of the membrane. Thus, the following formula, called the *Goldman equation* or the *Goldman-Hodgkin-Katz equation*, gives the calculated membrane potential on the *inside* of the membrane when two univalent positive ions, sodium (Na+) and potassium (K+), and one univalent negative ion, chloride (Cl-), are involved: \n$$EMF \\ (millivolts) = -61 \\times log \\frac{C_{Na_{i}^{+}}P_{Na^{+}} + C_{K_{i}^{+}}P_{K^{+}} + C_{Cl_{0}}P_{Cl^{-}}}{C_{Na_{0}^{+}}P_{Na^{+}} + C_{K_{0}^{+}}P_{K^{+}} + C_{Cl_{i}^{-}}P_{Cl^{-}}}$$ \nSeveral key points become evident from the Goldman equation. First, sodium, potassium, and chloride ions are the most important ions involved in the development of membrane potentials in nerve and muscle fibers, as well as in the neuronal cells. The concentration gradient of each of these ions across the membrane helps determine the voltage of the membrane potential. \nSecond, the quantitative importance of each of the ions in determining the voltage is proportional to the membrane permeability for that particular ion. If the membrane has zero permeability to sodium and chloride ions, the membrane potential becomes entirely dominated by the concentration gradient of potassium ions alone, and the resulting potential will be equal to the Nernst potential for potassium. The same holds true for each of the other two ions if the membrane should become selectively permeable for either one of them alone. \nThird, a positive ion concentration gradient from *inside* the membrane to the *outside* causes electronegativity \n**Table 5-1** Resting Membrane Potential in Different Cell Types \n| Cell Type | Resting Potential (mV) |\n|-----------------|---------------------------|\n| Neurons | −60 to −70 |\n| Skeletal muscle | −85 to −95 |\n| Smooth muscle | −50 to −60 |\n| Cardiac muscle | −80 to −90 |\n| Hair (cochlea) | –15 to –40 |\n| Astrocyte | -80 to -90 |\n| Erythrocyte | −8 to −12 |\n| Photoreceptor | –40 (dark) to –70 (light) | \ninside the membrane. The reason for this phenomenon is that excess positive ions diffuse to the outside when their concentration is higher inside than outside the membrane. This diffusion carries positive charges to the outside but leaves the nondiffusible negative anions on the inside, thus creating electronegativity on the inside. The opposite effect occurs when there is a gradient for a negative ion. That is, a chloride ion gradient from the outside to the inside causes negativity inside the cell because excess negatively charged chloride ions diffuse to the inside while leaving the nondiffusible positive ions on the outside. \nFourth, as explained later, the permeability of the sodium and potassium channels undergoes rapid changes during transmission of a nerve impulse, whereas the permeability of the chloride channels does not change greatly during this process. Therefore, rapid changes in sodium and potassium permeability are primarily responsible for signal transmission in neurons, which is the subject of most of the remainder of this chapter. \nResting Membrane Potential of Different Cell Types. In some cells, such as the cardiac pacemaker cells discussed in Chapter 10, the membrane potential is continuously changing, and the cells are never \"resting\". In many other cells, even excitable cells, there is a quiescent period in which a resting membrane potential can be measured. Table 5-1 shows the approximate resting membrane potentials of some different types of cells. The membrane potential is obviously very dynamic in excitable cells such as neurons, in which action potentials occur. However, even in nonexcitable cells, the membrane potential (voltage) also changes in response to various stimuli, which alter activities for the various ion transporters, ion channels, and membrane permeability for sodium, potassium, calcium, and chloride ions. The resting membrane potential is, therefore, only a brief transient state for many cells. \n**Electrochemical Driving Force.** When multiple ions contribute to the membrane potential, the equilibrium potential for any of the contributing ions will differ from the membrane potential, and there will be an *electrochemical driving force* ( $V_{df}$ ) for each ion that tends to cause net \n \n**Figure 5-2** Measurement of the membrane potential of the nerve fiber using a microelectrode. \nmovement of the ion across the membrane. This driving force is equal to the difference between the membrane potential $(V_m)$ and the equilibrium potential of the ion $(V_{eq})$ Thus, $V_{df} = V_m - V_{eq}.$ \nThe arithmetic sign of $V_{df}$ (positive or negative) and the valence of the ion (cation or anion) can be used to predict the direction of ion flow across the membrane, into or out of the cell. For cations such as $Na^+$ and $K^+$ , a positive $V_{df}$ predicts ion movement out of the cell down its electrochemical gradient, and a negative $V_{df}$ predicts ion movement into the cell. For anions, such as $Cl^-$ , a positive $V_{df}$ predicts ion movement into the cell, and a negative $V_{df}$ predicts ion movement out of the cell. When $V_m = V_{eq}$ , there is no net movement of the ion into or out of the cell. Also, the direction of ion flux through the membrane reverses as $V_m$ becomes greater than or less than $V_{eq}$ ; hence, the equilibrium potential ( $V_{eq}$ ) is also called the *reversal potential*. \n#### Measuring the Membrane Potential \nThe method for measuring the membrane potential is simple in theory but often difficult in practice because of the small size of most of the cells and fibers. Figure 5-2 shows a small micropipette filled with an electrolyte solution. The micropipette is impaled through the cell membrane to the interior of the fiber. Another electrode, called the indifferent electrode, is then placed in the extracellular fluid, and the potential difference between the inside and outside of the fiber is measured using an appropriate voltmeter. This voltmeter is a highly sophisticated electronic apparatus that is capable of measuring small voltages despite extremely high resistance to electrical flow through the tip of the micropipette, which has a lumen diameter usually less than 1 micrometer and a resistance of more than 1 million ohms. For recording rapid changes in the membrane potential during transmission of nerve impulses, the microelectrode is connected to an oscilloscope, as explained later in the chapter. \nThe lower part of **Figure 5-3** shows the electrical potential that is measured at each point in or near the nerve fiber membrane, beginning at the left side of the figure and passing to the right. As long as the electrode is outside the neuronal membrane, the recorded potential \n \n**Figure 5-3** Distribution of positively and negatively charged ions in the extracellular fluid surrounding a nerve fiber and in the fluid inside the fiber. Note the alignment of negative charges along the inside surface of the membrane and positive charges along the outside surface. The *lower panel* displays the abrupt changes in membrane potential that occur at the membranes on the two sides of the fiber. \nis zero, which is the potential of the extracellular fluid. Then, as the recording electrode passes through the voltage change area at the cell membrane (called the *electrical dipole layer*), the potential decreases abruptly to –70 millivolts. Moving across the center of the fiber, the potential remains at a steady –70-millivolt level but reverses back to zero the instant it passes through the membrane on the opposite side of the fiber. \nTo create a negative potential inside the membrane, only enough positive ions to develop the electrical dipole layer at the membrane itself must be transported outward. The remaining ions inside the nerve fiber can be both positive and negative, as shown in the upper panel of Figure 5-3. Therefore, transfer of an incredibly small number of ions through the membrane can establish the normal resting potential of -70 millivolts inside the nerve fiber, which means that only about 1/3,000,000 to 1/100,000,000 of the total positive charges inside the fiber must be transferred. Also, an equally small number of positive ions moving from outside to inside the fiber can reverse the potential from -70 millivolts to as much as +35 millivolts within as little as 1/10,000 of a second. Rapid shifting of ions in this manner causes the nerve signals discussed in subsequent sections of this chapter.\n\nThe resting membrane potential of large nerve fibers when they are not transmitting nerve signals is about -70 millivolts. That is, the potential *inside the fiber* is 70 millivolts more negative than the potential in the extracellular fluid on the outside of the fiber. In the next few paragraphs, the transport properties of the resting nerve membrane for \n \n**Figure 5-4** Functional characteristics of the Na+-K+ pump and the K+ \"leak\" channels. The K+ leak channels also leak Na+ ions into the cell slightly but are much more permeable to K+. ADP, Adenosine diphosphate; ATP, adenosine triphosphate. \nsodium and potassium and the factors that determine the level of this resting potential are explained. \nActive Transport of Sodium and Potassium lons Through the Membrane—the Sodium-Potassium (Na+-K+) Pump. Recall from Chapter 4 that all cell membranes of the body have a powerful Na+-K+ pump that continually transports sodium ions to the outside of the cell and potassium ions to the inside, as illustrated on the left side in Figure 5-4. Note that this is an *electrogenic pump* because three Na+ ions are pumped to the outside for each two K+ ions to the inside, leaving a net deficit of positive ions on the inside and causing a negative potential inside the cell membrane. \nThe $Na^+-K^+$ pump also causes large concentration gradients for sodium and potassium across the resting nerve membrane. These gradients are as follows: \nNa+ (outside): 142 mEq/L \nNa+ (inside): 14 mEq/L \nK+(outside): 4 mEq/L \nK+(inside): 140 mEq/L \nThe ratios of these two respective ions from the inside to the outside are as follows: \n$$Na_{inside}^{+}/Na_{outside}^{+}=0.1$$ \n$$K^{+}_{inside}/K^{+}_{outside} = 35.0$$ \n**Leakage of Potassium Through the Nerve Cell Membrane.** The right side of **Figure 5-4** shows a channel protein (sometimes called a *tandem pore domain, potassium channel*, or *potassium* $[K^+]$ *\"leak\" channel*) in the nerve membrane through which potassium ions can leak, even in a resting cell. The basic structure of potassium channels was described in Chapter 4 (**Figure 4-4**). These $K^+$ leak \n \n**Figure 5-5** Establishment of resting membrane potentials under three conditions. **A**, When the membrane potential is caused entirely by potassium diffusion alone. **B**, When the membrane potential is caused by diffusion of both sodium and potassium ions. **C**, When the membrane potential is caused by diffusion of both sodium and potassium ions plus pumping of both these ions by the Na+-K+ pump. \nchannels may also leak sodium ions slightly but are far more permeable to potassium than to sodium, normally about 100 times as permeable. As discussed later, this differential in permeability is a key factor in determining the level of the normal resting membrane potential.\n\n**Figure 5-5** shows the important factors in the establishment of the normal resting membrane potential. They are as follows. \n#### Contribution of the Potassium Diffusion Potential. \nIn **Figure 5-5A**, we assume that the only movement of ions through the membrane is diffusion of potassium ions, as demonstrated by the open channels between the potassium symbol $(K^+)$ inside and outside the mem- \nbrane. Because of the high ratio of potassium ions inside to outside, 35:1, the Nernst potential corresponding to this ratio is –94 millivolts because the logarithm of 35 is 1.54, and this, multiplied by –61 millivolts, is –94 millivolts. Therefore, if potassium ions were the only factor causing the resting potential, the resting potential *inside the fiber* would be equal to –94 millivolts, as shown in the figure. \nContribution of Sodium Diffusion Through the **Nerve Membrane. Figure 5-5***B* shows the addition of slight permeability of the nerve membrane to sodium ions, caused by the minute diffusion of sodium ions through the K+-Na+ leak channels. The ratio of sodium ions from inside to outside the membrane is 0.1, which gives a calculated Nernst potential for the inside of the membrane of +61 millivolts. Also shown in **Figure 5-5***B* is the Nernst potential for potassium diffusion of -94 millivolts. How do these interact with each other, and what will be the summated potential? This question can be answered by using the Goldman equation described previously. Intuitively, one can see that if the membrane is highly permeable to potassium but only slightly permeable to sodium, the diffusion of potassium contributes far more to the membrane potential than the diffusion of sodium. In the normal nerve fiber, the permeability of the membrane to potassium is about 100 times as great as its permeability to sodium. Using this value in the Goldman equation, and considering only sodium and potassium, gives a potential inside the membrane of -86 millivolts, which is near the potassium potential shown in the figure. \n**Contribution of the Na**+-**K**+ **Pump.** In **Figure 5-5***C*, the Na+-K+ pump is shown to provide an additional contribution to the resting potential. This figure shows that continuous pumping of three sodium ions to the outside occurs for each two potassium ions pumped to the inside of the membrane. The pumping of more sodium ions to the outside than the potassium ions being pumped to the inside causes a continual loss of positive charges from inside the membrane, creating an additional degree of negativity (about –4 millivolts additional) on the inside, beyond that which can be accounted for by diffusion alone. \nTherefore, as shown in **Figure 5-5***C*, the net membrane potential when all these factors are operative at the same time is about –90 millivolts. However, additional ions, such as chloride, must also be considered in calculating the membrane potential. \nIn summary, the diffusion potentials alone caused by potassium and sodium diffusion would give a membrane potential of about -86 millivolts, with almost all of this being determined by potassium diffusion. An additional -4 millivolts is then contributed to the membrane potential by the continuously acting electrogenic Na+-K+ pump, and there is a contribution of chloride ions. As mentioned previously, the resting membrane potential \n \n \n**Figure 5-6** Typical action potential recorded by the method shown in the *upper panel*. \nvaries in different cells from as low as around -10 millivolts in erythrocytes to as high as -90 millivolts in skeletal muscle cells. \n#### **NEURON ACTION POTENTIAL** \nNerve signals are transmitted by *action potentials*, which are rapid changes in the membrane potential that spread rapidly along the nerve fiber membrane. Each action potential begins with a sudden change from the normal resting negative membrane potential to a positive potential and ends with an almost equally rapid change back to the negative potential. To conduct a nerve signal, the action potential moves along the nerve fiber until it comes to the fiber's end. \nThe upper panel of **Figure 5-6** shows the changes that occur at the membrane during the action potential, with the transfer of positive charges to the interior of the fiber at its onset and the return of positive charges to the exterior at its end. The lower panel shows graphically the successive changes in membrane potential over a few 10,000ths of a second, illustrating the explosive onset of the action potential and the almost equally rapid recovery. \nThe successive stages of the action potential are as follows. \n**Resting Stage.** The resting stage is the resting membrane potential before the action potential begins. The membrane is said to be \"polarized\" during this stage because of the –70 millivolts negative membrane potential that is present. \n**Depolarization Stage.** At this time, the membrane suddenly becomes permeable to sodium ions, allowing rapid diffusion of positively charged sodium ions to the interior of the axon. The normal polarized state of −70 millivolts is immediately neutralized by the inflowing, positively charged sodium ions, with the potential rising rapidly in the positive direction—a process called *depolarization.* In large nerve fibers, the great excess of positive sodium ions moving to the inside causes the membrane potential to actually overshoot beyond the zero level and to become somewhat positive. In some smaller fibers, as well as in many central nervous system neurons, the potential merely approaches the zero level and does not overshoot to the positive state. \n**Repolarization Stage.** Within a few 10,000ths of a second after the membrane becomes highly permeable to sodium ions, the sodium channels begin to close, and the potassium channels open to a greater degree than normal. Then, rapid diffusion of potassium ions to the exterior reestablishes the normal negative resting membrane potential, which is called *repolarization* of the membrane. \nTo explain more fully the factors that cause both depolarization and repolarization, we will describe the special characteristics of two other types of transport channels through the nerve membrane, the voltage-gated sodium and potassium channels. \n#### **VOLTAGE-GATED SODIUM AND POTASSIUM CHANNELS** \nThe necessary factor in causing both depolarization and repolarization of the nerve membrane during the action potential is the *voltage-gated sodium channel.* A *voltagegated potassium channel* also plays an important role in increasing the rapidity of repolarization of the membrane. *These two voltage-gated channels are in addition to the Na*+*-K*+ *pump and the K*+ *leak channels.* \n#### **Activation and Inactivation of the Voltage-Gated Sodium Channel** \nThe upper panel of **[Figure 5-7](#page-67-0)** shows the voltage-gated sodium channel in three separate states. This channel has two *gates*—one near the outside of the channel called the *activation gate,* and another near the inside called the *inactivation gate.* The upper left of the figure depicts the state of these two gates in the normal resting membrane when the membrane potential is −70 millivolts. In this state, the activation gate is closed, which prevents any entry of sodium ions to the interior of the fiber through these sodium channels. \n**Activation of the Sodium Channel.** When the membrane potential becomes less negative than during the resting state, rising from −70 millivolts toward zero, it finally reaches a voltage—usually somewhere around −55 millivolts—that causes a sudden conformational \n \n**Figure 5-7** Characteristics of the voltage-gated sodium *(top)* and potassium *(bottom)* channels, showing successive activation and inactivation of the sodium channels and delayed activation of the potassium channels when the membrane potential is changed from the normal resting negative value to a positive value. \nchange in the activation gate, flipping it all the way to the open position. During this *activated state,* sodium ions can pour inward through the channel, increasing the sodium permeability of the membrane as much as 500- to 5000-fold. \n**Inactivation of the Sodium Channel.** The upper right panel of **[Figure 5-7](#page-67-0)** shows a third state of the sodium channel. The same increase in voltage that opens the activation gate also closes the inactivation gate. The inactivation gate, however, closes a few 10,000ths of a second after the activation gate opens. That is, the conformational change that flips the inactivation gate to the closed state is a slower process than the conformational change that opens the activation gate. Therefore, after the sodium channel has remained open for a few 10,000ths of a second, the inactivation gate closes, and sodium ions no longer can pour to the inside of the membrane. At this point, the membrane potential begins to return toward the resting membrane state, which is the repolarization process. \nAnother important characteristic of the sodium channel inactivation process is that the inactivation gate will not reopen until the membrane potential returns to or near the original resting membrane potential level. Therefore, it is usually not possible for the sodium channels to open again without first repolarizing the nerve fiber. \n#### **Voltage-Gated Potassium Channel and Its Activation** \nThe lower panel of **[Figure 5-7](#page-67-0)** shows the voltage-gated potassium channel in two states—during the resting state \n \n**Figure 5-8** Voltage clamp method for studying flow of ions through specific channels. \n(left) and toward the end of the action potential (right). During the resting state, the gate of the potassium channel is closed, and potassium ions are prevented from passing through this channel to the exterior. When the membrane potential rises from −70 millivolts toward zero, this voltage change causes a conformational opening of the gate and allows increased potassium diffusion outward through the channel. However, because of the slight delay in opening of the potassium channels, they open, for the most part, at about the same time that the sodium channels are beginning to close because of inactivation. Thus, the decrease in sodium entry to the cell and the simultaneous increase in potassium exit from the cell combine to speed the repolarization process, leading to full recovery of the resting membrane potential within another few 10,000ths of a second. \n**The Voltage Clamp Method for Measuring the Effect of Voltage on Opening and Closing of Voltage-Gated Channels.** The original research that led to quantitative understanding of the sodium and potassium channels was so ingenious that it led to Nobel Prizes for the scientists responsible, Hodgkin and Huxley, in 1963. The essence of these studies is shown in **[Figures. 5-8 and 5-9](#page-68-0)**. \n**[Figure 5-8](#page-68-0)** shows the *voltage clamp method,* which is used to measure the flow of ions through the different channels. In using this apparatus, two electrodes are inserted into the nerve fiber. One of these electrodes is used to measure the voltage of the membrane potential, and the other is used to conduct electrical current into or out of the nerve fiber. \nThis apparatus is used in the following way. The investigator decides which voltage to establish inside the nerve fiber. The electronic portion of the apparatus is then adjusted to the desired voltage, automatically injecting either positive or negative electricity through the current electrode at whatever rate is required to hold the voltage, as measured \n \n**Figure 5-9** Typical changes in conductance of sodium and potassium ion channels when the membrane potential is suddenly increased from the normal resting value of −70 millivolts to a positive value of +10 millivolts for 2 milliseconds. This figure shows that the sodium channels open (activate) and then close (inactivate) before the end of the 2 milliseconds, whereas the potassium channels only open (activate), and the rate of opening is much slower than that of the sodium channels. \nby the voltage electrode, at the level set by the operator. When the membrane potential is suddenly increased by this voltage clamp from −70 millivolts to zero, the voltagegated sodium and potassium channels open, and sodium and potassium ions begin to pour through the channels. To counterbalance the effect of these ion movements on the desired setting of the intracellular voltage, electrical current is injected automatically through the current electrode of the voltage clamp to maintain the intracellular voltage at the required steady zero level. To achieve this level, the current injected must be equal to but of opposite polarity to the net current flow through the membrane channels. To measure how much current flow is occurring at each instant, the current electrode is connected to an ampere meter that records the current flow, as demonstrated in **[Figure 5-8](#page-68-0)**. \nFinally, the investigator adjusts the concentrations of the ions to other than normal levels both inside and outside the nerve fiber and repeats the study. This experiment can be performed easily when using large nerve fibers removed from some invertebrates, especially the giant squid axon, which in some cases is as large as 1 millimeter in diameter. When sodium is the only permeant ion in the solutions inside and outside the squid axon, the voltage clamp measures current flow only through the sodium channels. When potassium is the only permeant ion, current flow only through the potassium channels is measured. \nAnother means for studying the flow of ions through an individual type of channel is to block one type of channel at a time. For example, the sodium channels can be blocked by a toxin called tetrodotoxin when it is applied to the outside of the cell membrane where the sodium activation gates are located. Conversely, tetraethylammonium ion blocks the potassium channels when it is applied to the interior of the nerve fiber. \n**[Figure 5-9](#page-68-1)** shows typical changes in conductance of the voltage-gated sodium and potassium channels when the membrane potential is suddenly changed through use of the voltage clamp, from −70 millivolts to +10 millivolts and then, 2 milliseconds later, back to −70 millivolts. Note the sudden opening of the sodium channels (the activation stage) within a small fraction of a millisecond after the membrane potential is increased to the positive value. However, during the next millisecond or so, the sodium channels automatically close (the inactivation stage). \nNote the opening (activation) of the potassium channels, which open less rapidly and reach their full open state only after the sodium channels have almost completely closed. Furthermore, once the potassium channels open, they remain open for the entire duration of the positive membrane potential and do not close again until after the membrane potential is decreased back to a negative value. \n#### **SUMMARY OF EVENTS THAT CAUSE THE ACTION POTENTIAL** \n**[Figure 5-10](#page-69-0)** summarizes the sequential events that occur during and shortly after the action potential. The bottom of the figure shows the changes in membrane conductance for sodium and potassium ions. During the resting state, before the action potential begins, the conductance for potassium ions is 50 to 100 times as great as the conductance for sodium ions. This disparity is caused by much greater leakage of potassium ions than sodium ions through the leak channels. However, at the onset of the action potential, the sodium channels almost instantaneously become activated and allow up to a 5000-fold increase in sodium conductance. The inactivation process then closes the sodium channels \n \n**Figure 5-10** Changes in sodium and potassium conductance during the course of the action potential. Sodium conductance increases several thousand–fold during the early stages of the action potential, whereas potassium conductance increases only about 30-fold during the latter stages of the action potential and for a short period thereafter. (These curves were constructed from theory presented in papers by Hodgkin and Huxley but transposed from a squid axon to apply to the membrane potentials of large mammalian nerve fibers.) \nwithin another fraction of a millisecond. The onset of the action potential also initiates voltage gating of the potassium channels, causing them to begin opening more slowly, a fraction of a millisecond after the sodium channels open. At the end of the action potential, the return of the membrane potential to the negative state causes the potassium channels to close back to their original status but, again, only after an additional millisecond or more delay. \nThe middle portion of **[Figure 5-10](#page-69-0)** shows the ratio of sodium to potassium conductance at each instant during the action potential, and above this depiction is the action potential itself. During the early portion of the action potential, the ratio of sodium to potassium conductance increases more than 1000-fold. Therefore, far more sodium ions flow to the interior of the fiber than potassium ions to the exterior. This is what causes the membrane potential to become positive at the action potential onset. Then, the sodium channels begin to close, and the potassium channels begin to open; thus, the ratio of conductance shifts far in favor of high potassium conductance but low sodium conductance. This shift allows for a very rapid loss of potassium ions to the exterior but virtually zero flow of sodium ions to the interior. Consequently, the action potential quickly returns to its baseline level. \n#### **Roles of Other Ions During the Action Potential** \nThus far, we have considered only the roles of sodium and potassium ions in generating the action potential. At least two other types of ions must be considered, negative anions and calcium ions. \n**Impermeant Negatively Charged Ions (Anions) Inside the Nerve Axon.** Inside the axon are many negatively charged ions that cannot go through the membrane channels. They include the anions of protein molecules and of many organic phosphate compounds and sulfate compounds, among others. Because these ions cannot leave the interior of the axon, any deficit of positive ions inside the membrane leaves an excess of these impermeant negative anions. Therefore, these impermeant negative ions are responsible for the negative charge inside the fiber when there is a net deficit of positively charged potassium ions and other positive ions. \n**Calcium Ions.** The membranes of almost all cells of the body have a calcium pump similar to the sodium pump, and calcium serves along with (or instead of) sodium in some cells to cause most of the action potential. Like the sodium pump, the calcium pump transports calcium ions from the interior to the exterior of the cell membrane (or into the endoplasmic reticulum of the cell), creating a calcium ion gradient of about 10,000-fold. This process leaves an internal cell concentration of calcium ions of about 10−7 molar, in contrast to an external concentration of about 10−3 molar. \nIn addition, there are *voltage-gated calcium channels*. Because the calcium ion concentration is more than 10,000 times greater in the extracellular fluid than in the intracellular fluid, there is a tremendous diffusion gradient and electrochemical driving force for the passive flow of calcium ions into the cells. These channels are slightly permeable to sodium ions and calcium ions, but their permeability to calcium is about 1000-fold greater than to sodium under normal physiological conditions. When the channels open in response to a stimulus that depolarizes the cell membrane, calcium ions flow to the interior of the cell. \nA major function of the voltage-gated calcium ion channels is to contribute to the depolarizing phase on the action potential in some cells. The gating of calcium channels, however, is relatively slow, requiring 10 to 20 times as long for activation as for the sodium channels. For this reason, they are often called *slow channels,* in contrast to the sodium channels, which are called *fast channels.* Therefore, the opening of calcium channels provides a more sustained depolarization, whereas the sodium channels play a key role in initiating action potentials. \nCalcium channels are numerous in cardiac muscle and smooth muscle. In fact, in some types of smooth muscle, the fast sodium channels are hardly present; therefore, the action potentials are caused almost entirely by the activation of slow calcium channels. \n**Increased Permeability of the Sodium Channels When There Is a Deficit of Calcium Ions.** The concentration of calcium ions in the extracellular fluid also has a profound effect on the voltage level at which the sodium channels become activated. When there is a deficit of calcium ions, the sodium channels become activated (opened) by a small increase of the membrane potential from its normal, very negative level. Therefore, the nerve fiber becomes highly excitable, sometimes discharging repetitively without provocation, rather than remaining in the resting state. In fact, the calcium ion concentration needs to fall only 50% below normal before spontaneous discharge occurs in some peripheral nerves, often causing *muscle \"tetany*.*\"* Muscle tetany is sometimes lethal because of tetanic contraction of the respiratory muscles. \nThe probable way in which calcium ions affect the sodium channels is as follows. These ions appear to bind to the exterior surfaces of the sodium channel protein. The positive charges of these calcium ions, in turn, alter the electrical state of the sodium channel protein, thus altering the voltage level required to open the sodium gate. \n#### **INITIATION OF THE ACTION POTENTIAL** \nThus far, we have explained the changing sodium and potassium permeability of the membrane, as well as the development of the action potential, but we have not explained what initiates the action potential. \n**A Positive-Feedback Cycle Opens the Sodium Channels.** As long as the membrane of the nerve fiber remains undisturbed, no action potential occurs in the normal nerve. However, if any event causes enough initial rise in the membrane potential from −70 millivolts toward the zero level, the rising voltage will cause many voltage-gated sodium channels to begin opening. This occurrence allows for the rapid inflow of sodium ions, which causes a further rise in the membrane potential, thus opening still more voltage-gated sodium channels and allowing more streaming of sodium ions to the interior of the fiber. This process is a positive feedback cycle that, once the feedback is strong enough, continues until all the voltage-gated sodium channels have become activated (opened). Then, within another fraction of a millisecond, the rising membrane potential causes closure of the sodium channels and opening of potassium channels, and the action potential soon terminates. \n**Initiation of the Action Potential Occurs Only After the Threshold Potential is Reached.** An action potential will not occur until the initial rise in membrane potential is great enough to create the positive feedback described in the preceding paragraph. This occurs when the number of sodium ions entering the fiber is greater than the number of potassium ions leaving the fiber. A sudden rise in membrane potential of 15 to 30 millivolts is usually required. Therefore, a sudden increase in the membrane potential in a large nerve fiber, from −70 millivolts up to about −55 millivolts, usually causes the explosive development of an action potential. This level of −55 millivolts is said to be the *threshold* for stimulation. \n#### PROPAGATION OF THE ACTION POTENTIAL \nIn the preceding paragraphs, we discussed the action potential as though it occurs at one spot on the membrane. However, an action potential elicited at any one point on an excitable membrane usually excites adjacent portions of the membrane, resulting in propagation of the action potential along the membrane. This mechanism is demonstrated in **[Figure 5-11.](#page-71-0)** \n**[Figure 5-11](#page-71-0)***A* shows a normal resting nerve fiber, and **[Figure 5-11](#page-71-0)***B* shows a nerve fiber that has been excited in its midportion, which suddenly develops increased permeability to sodium. The *arrows* show a local circuit of current flow from the depolarized areas of the membrane to the adjacent resting membrane areas. That is, positive electrical charges are carried by the inward-diffusing sodium ions through the depolarized membrane and then for several millimeters in both directions along the core of the axon. These positive charges increase the voltage for a distance of 1 to 3 millimeters inside the large myelinated fiber to above the threshold voltage value for initiating an action potential. Therefore, the sodium channels in these new areas immediately open, as shown in **[Figure 5-11](#page-71-0)***C* [and](#page-71-0) *D*, and the explosive action potential spreads. These newly depolarized areas produce still more local circuits of current flow farther along the membrane, causing progressively more and more depolarization. Thus, the depolarization process travels along the entire length of the fiber. This transmission of the depolarization process along a nerve or muscle fiber is called a *nerve* or *muscle impulse.* \n**Direction of Propagation.** As demonstrated in **[Figure 5-](#page-71-0) [11](#page-71-0)**, an excitable membrane has no single direction of propagation, but the action potential travels in all directions away from the stimulus—even along all branches of a nerve fiber—until the entire membrane has become depolarized. \n**All-or-Nothing Principle.** Once an action potential has been elicited at any point on the membrane of a normal fiber, the depolarization process travels over the entire membrane if conditions are right, but it does not travel at all if conditions are not right. This principle is called the *allor-nothing principle,* and it applies to all normal excitable tissues. Occasionally, the action potential reaches a point on the membrane at which it does not generate sufficient voltage to stimulate the next area of the membrane. When this situation occurs, the spread of depolarization stops. Therefore, for continued propagation of an impulse to occur, the ratio of action potential to threshold for excitation must at all times be greater than 1. This \"greater than 1\" requirement is called the *safety factor* for propagation. \n#### RE-ESTABLISHING SODIUM AND POTASSIUM IONIC GRADIENTS AFTER ACTION POTENTIALS ARE COMPLETED—IMPORTANCE OF ENERGY METABOLISM \nTransmission of each action potential along a nerve fiber slightly reduces the concentration differences of sodium and potassium inside and outside the membrane because sodium ions diffuse to the inside during depolarization, and potassium ions diffuse to the outside during \n+ + + + + + + + + + + + – – + + + + + + + + + + + + + + + + + + + + + – – + + + + + + + + + + + + + + + + + + + – – – – + + + + + + + + + + + + + + + + + + – – – – + + + + + + + + – – + + + + + + + + + + + + + + + + + + – – – – + + + + + + + + + + + + + + + + + + – – + + – – – – – – – – – – – – – – – – – – + + + + – – – – – – – – – – – – – – – – – – + + – – – – – – – – – – – – + + – – – – – – – – – + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – + + – – – – – – – – – – – – – – – – – – – + + + + – – – – – – – – – – – – – – – – – – + + + + – – – – – – – – A B C D \n**Figure 5-11** A–D, Propagation of action potentials in both directions along a conductive fiber. \nrepolarization. For a single action potential, this effect is so minute that it cannot be measured. Indeed, 100,000 to 50 million impulses can be transmitted by large nerve fibers before the concentration differences reach the point that action potential conduction ceases. With time, however, it becomes necessary to re-establish the sodium and potassium membrane concentration differences, which is achieved by action of the Na+-K+ pump in the same way as described previously for the original establishment of the resting potential. That is, sodium ions that have diffused to the interior of the cell during the action potentials and potassium ions that have diffused to the exterior must be returned to their original state by the Na+-K+ pump. Because this pump requires energy for operation, this \"recharging\" of the nerve fiber is an active metabolic process, using energy derived from the adenosine triphosphate (ATP) energy system of the cell. **[Figure 5-12](#page-71-1)** shows that the nerve fiber produces increased heat during recharging, which is a measure of energy expenditure when the nerve impulse frequency increases. \nA special feature of the Na+-K+ ATP pump is that its degree of activity is strongly stimulated when excess sodium ions accumulate inside the cell membrane. In fact, the pumping activity increases approximately in proportion to the third power of this intracellular sodium concentration. As the internal sodium concentration rises from 10 to 20 mEq/L, the activity of the pump does not merely double but increases about eightfold. Therefore, it is easy to understand how the recharging process of the nerve fiber can be set rapidly into motion whenever the concentration differences of sodium and potassium ions across the membrane begin to run down. \n#### PLATEAU IN SOME ACTION POTENTIALS \nIn some cases, the excited membrane does not repolarize immediately after depolarization; instead, the potential remains on a plateau near the peak of the spike potential for many milliseconds before repolarization begin. Such a plateau is shown in **[Figure 5-13](#page-72-0)**; one can readily see that \n \n**Figure 5-12** Heat production in a nerve fiber at rest and at progressively increasing rates of stimulation. \nthe plateau greatly prolongs the period of depolarization. This type of action potential occurs in heart muscle fibers, where the plateau lasts for as long as 0.2 to 0.3 second and causes contraction of heart muscle to last for this same long period. \nThe cause of the plateau is a combination of several factors. First, in heart muscle, two types of channels contribute to the depolarization process: (1) the usual voltage-activated sodium channels, called *fast channels;* and (2) voltageactivated calcium-sodium channels *(L-type calcium channels)*, which are slow to open and therefore are called *slow channels.* Opening of fast channels causes the spike portion of the action potential, whereas the prolonged opening of the slow calcium-sodium channels mainly allows calcium ions to enter the fiber, which is largely responsible for the plateau portion of the action potential. \nAnother factor that may be partly responsible for the plateau is that the voltage-gated potassium channels are slower to open than usual, often not opening much until the end of the plateau. This factor delays the return of the membrane potential toward its normal negative value of −70 millivolts. The plateau ends when the calciumsodium channels close, and permeability to potassium ions increases. \n#### RHYTHMICITY OF SOME EXCITABLE TISSUES—REPETITIVE DISCHARGE \nRepetitive self-induced discharges occur normally in the heart, in most smooth muscle, and in many of the neurons of the central nervous system. These rhythmical discharges cause the following: (1) rhythmical beat of the heart; (2) rhythmical peristalsis of the intestines; and (3) neuronal events such as the rhythmical control of breathing. \nIn addition, almost all other excitable tissues can discharge repetitively if the threshold for stimulation of the tissue cells is reduced to a low enough level. For example, even large nerve fibers and skeletal muscle fibers, which normally are highly stable, discharge repetitively when they \n \n**Figure 5-13** Action potential (in millivolts) from a Purkinje fiber of the heart, showing a plateau. \nare placed in a solution that contains the drug *veratridine*, which activates sodium ion channels, or when the calcium ion concentration decreases below a critical value, which increases the sodium permeability of the membrane. \n**Re-Excitation Process Necessary for Spontaneous Rhythmicity.** For spontaneous rhythmicity to occur, the membrane—even in its natural state—must be permeable enough to sodium ions (or to calcium and sodium ions through the slow calcium-sodium channels) to allow automatic membrane depolarization. Thus, **[Figure 5-14](#page-72-1)** shows that the resting membrane potential in the rhythmical control center of the heart is only −60 to −70 millivolts, which is not enough negative voltage to keep the sodium and calcium channels totally closed. Therefore, the following sequence occurs: (1) some sodium and calcium ions flow inward; (2) this activity increases the membrane voltage in the positive direction, which further increases membrane permeability; (3) still more ions flow inward; and (4) the permeability increases more, and so on, until an action potential is generated. Then, at the end of the action potential, the membrane repolarizes. After another delay of milliseconds or seconds, spontaneous excitability causes depolarization again, and a new action potential occurs spontaneously. This cycle continues over and over and causes self-induced rhythmical excitation of the excitable tissue. \nWhy does the membrane of the heart control center not depolarize immediately after it has become repolarized, rather than delaying for nearly 1 second before the onset of the next action potential? The answer can be found by observing the curve labeled \"potassium conductance\" in **[Figure 5-14](#page-72-1)**. This curve shows that toward the end of each action potential, and continuing for a short period thereafter, the membrane becomes more permeable to potassium ions. The increased outflow of potassium ions carries tremendous numbers of positive charges to the outside of the membrane, leaving considerably more negativity inside the fiber than would otherwise occur. This continues for nearly 1 second after the preceding action potential is over, thus drawing the membrane potential \n \n**Figure 5-14** Rhythmical action potentials (in millivolts) similar to those recorded in the rhythmical control center of the heart. Note their relationship to potassium conductance and to the state of hyperpolarization. \nnearer to the potassium Nernst potential. This state, called *hyperpolarization,* is also shown in **[Figure 5-14](#page-72-1)**. As long as this state exists, self–re-excitation will not occur. However, the increased potassium conductance (and the state of hyperpolarization) gradually disappears, as shown after each action potential is completed in the figure, thereby again allowing the membrane potential to increase up to the *threshold* for excitation. Then, suddenly, a new action potential results and the process occurs again and again. \n#### SPECIAL CHARACTERISTICS OF SIGNAL TRANSMISSION IN NERVE TRUNKS \n**Myelinated and Unmyelinated Nerve Fibers. [Figure](#page-73-0) [5-15](#page-73-0)** shows a cross section of a typical small nerve, revealing many large nerve fibers that constitute most of the cross-sectional area. However, a more careful look reveals many more small fibers lying between the large ones. The large fibers are *myelinated,* and the small ones are *unmyelinated.* The average nerve trunk contains about twice as many unmyelinated fibers as myelinated fibers. \n**[Figure 5-16](#page-73-1)** illustrates schematically the features of a typical myelinated fiber. The central core of the fiber is the *axon,* and the membrane of the axon is the membrane that actually conducts the action potential. The axon is filled in its center with *axoplasm,* which is a viscid intracellular fluid. Surrounding the axon is a *myelin sheath* that is often much thicker than the axon itself. About once every 1 to 3 millimeters along the length of the myelin sheath is a *node of Ranvier.* \nThe myelin sheath is deposited around the axon by *Schwann cells* in the following manner. The membrane of a Schwann cell first envelops the axon. The Schwann cell then rotates around the axon many times, laying down multiple layers of Schwann cell membrane containing the lipid substance *sphingomyelin.* This substance is an excellent \n \n**Figure 5-15** Cross section of a small nerve trunk containing both myelinated and unmyelinated fibers. \nelectrical insulator that decreases ion flow through the membrane about 5000-fold. At the juncture between each two successive Schwann cells along the axon, a small uninsulated area only 2 to 3 micrometers in length remains where ions still can flow with ease through the axon membrane between the extracellular fluid and intracellular fluid inside the axon. This area is called the *node of Ranvier.* \n#### **Saltatory Conduction in Myelinated Fibers from Node** \n**to Node.** Even though almost no ions can flow through the thick myelin sheaths of myelinated nerves, they can flow with ease through the nodes of Ranvier. Therefore, action potentials occur *only at the nodes.* Yet, the action potentials are conducted from node to node by *saltatory conduction*, as shown in **[Figure 5-17](#page-74-0)**. That is, electrical current flows through the surrounding extracellular fluid outside the myelin sheath, as well as through the axoplasm inside the axon from node to node, exciting successive nodes one after another. Thus, the nerve impulse jumps along the fiber, which is the origin of the term *saltatory.* \nSaltatory conduction is of value for two reasons: \n1. First, by causing the depolarization process to jump long intervals along the axis of the nerve fiber, this mechanism increases the velocity of nerve transmission in myelinated fibers as much as 5- to 50-fold. \n \n**Figure 5-16** Function of the Schwann cell to insulate nerve fibers. **A**, Wrapping of a Schwann cell membrane around a large axon to form the myelin sheath of the myelinated nerve fiber. **B**, Partial wrapping of the membrane and cytoplasm of a Schwann cell around multiple unmyelinated nerve fibers (shown in cross section). *(A, Modified from Leeson TS, Leeson R: Histology. Philadelphia: WB Saunders, 1979.)* \n2. Second, saltatory conduction conserves energy for the axon because only the nodes depolarize, allowing perhaps 100 times less loss of ions than would otherwise be necessary, and therefore requiring much less energy expenditure for re-establishing the sodium and potassium concentration differences across the membrane after a series of nerve impulses. \nThe excellent insulation afforded by the myelin membrane and the 50-fold decrease in membrane capacitance also allow repolarization to occur with little transfer of ions. \n**Velocity of Conduction in Nerve Fibers.** The velocity of action potential conduction in nerve fibers varies from as little as 0.25 m/sec in small unmyelinated fibers to as much as 100 m/sec—more than the length of a football field in 1 second—in large myelinated fibers. \n#### EXCITATION—THE PROCESS OF ELICITING THE ACTION POTENTIAL \nBasically, any factor that causes sodium ions to begin to diffuse inward through the membrane in sufficient numbers can set off automatic regenerative opening of the sodium channels. This automatic regenerative opening can result from *mechanical* disturbance of the membrane, *chemical* effects on the membrane, or passage of *electricity* through the membrane. All these approaches are used at different points in the body to elicit nerve or muscle action potentials: mechanical pressure to excite sensory nerve endings in the skin, chemical neurotransmitters to transmit signals from one neuron to the next in the brain, and electrical current to transmit signals between successive muscle cells in the heart and intestine. \n**Excitation of a Nerve Fiber by a Negatively Charged Metal Electrode.** The usual means for exciting a nerve or muscle in the experimental laboratory is to apply electricity to the nerve or muscle surface through two small electrodes, one of which is negatively charged and the other positively charged. When electricity is applied in this manner, the excitable membrane becomes stimulated at the negative electrode. \nRemember that the action potential is initiated by the opening of voltage-gated sodium channels. Furthermore, these channels are opened by a decrease in the normal resting electrical voltage across the membrane—that is, negative current from the electrode decreases the voltage on the outside of the membrane to a negative value nearer to the voltage of the negative potential inside the fiber. This effect decreases the electrical voltage across the membrane and allows the sodium channels to open, resulting in an action potential. Conversely, at the positive electrode, the injection of positive charges on the outside of the nerve membrane heightens the voltage difference across the membrane, rather than lessening it. This effect causes a state of hyperpolarization, which actually decreases the excitability of the fiber rather than causing an action potential. \n \n**Figure 5-17** Saltatory conduction along a myelinated axon. The flow of electrical current from node to node is illustrated by the *arrows.* \n#### Threshold for Excitation and Acute Local Potentials. \nA weak negative electrical stimulus may not be able to excite a fiber. However, when the voltage of the stimulus is increased, there comes a point at which excitation does take place. Figure 5-18 shows the effects of successively applied stimuli of progressing strength. A weak stimulus at point A causes the membrane potential to change from -70 to -65 millivolts, but this change is not sufficient for the automatic regenerative processes of the action potential to develop. At point B, the stimulus is greater, but the intensity is still not enough. The stimulus does, however, disturb the membrane potential locally for as long as 1 millisecond or more after both of these weak stimuli. These local potential changes are called acute local potentials and, when they fail to elicit an action potential, they are called acute subthreshold potentials. \nAt point C in **Figure 5-18**, the stimulus is even stronger. Now, the local potential has barely reached the *threshold level* required to elicit an action potential, but this occurs only after a short \"latent period.\" At point D, the stimulus is still stronger, the acute local potential is also stronger, and the action potential occurs after less of a latent period. \nThus, this figure shows that even a weak stimulus causes a local potential change at the membrane, but the intensity of the local potential must rise to a threshold level before the action potential is set off.",
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