text stringlengths 1.02k 3.02k |
|---|
moves through the hydrophobic core of the membrane. Once it is reduced, (QH2), ubiquinone delivers its electrons to the next complex in the electron transport chain. Q receives the electrons derived from NADH from complex I, and the electrons derived from FADH2 from complex II. This enzyme and FADH2 form a small complex that delivers electrons directly to the electron transport chain, bypassing the first complex. Since these electrons bypass and thus do not energize the proton pump in the first complex, fewer ATP molecules are made from the FADH2 electrons. The number of ATP molecules ultimately obtained is directly proportional to the number of protons pumped across the inner mitochondrial membrane. 298 Complex III Chapter 7 | Cellular Respiration The third complex is composed of cytochrome b, another Fe-S protein, Rieske center (2Fe-2S center), and cytochrome c proteins; this complex is also called cytochrome oxidoreductase. Cytochrome proteins have a prosthetic group of heme. The heme molecule is similar to the heme in hemoglobin, but it carries electrons, not oxygen. As a result, the iron ion at its core is reduced and oxidized as it passes the electrons, fluctuating between different oxidation states: Fe++ (reduced) and Fe+++ (oxidized). The heme molecules in the cytochromes have slightly different characteristics due to the effects of the different proteins binding them, giving slightly different characteristics to each complex. Complex III pumps protons through the membrane and passes its electrons to cytochrome c for transport to the fourth complex of proteins and enzymes (cytochrome c is the acceptor of electrons from Q; however, whereas Q carries pairs of electrons, cytochrome c can accept only one at a time). Complex IV The fourth complex is composed of cytochrome proteins c, a, and a3. This complex contains two heme groups (one in each of the two cytochromes, a, and a3) and three copper ions (a pair of CuA and one CuB in cytochrome a3). The cytochromes hold an oxygen molecule very tightly between the iron and copper ions until the oxygen is completely reduced. The reduced oxygen then picks up two hydrogen ions from the surrounding medium to make water (H2O). The removal of the hydrogen ions from the system contributes to the ion gradient used in the process of chemiosmosis. |
Chemiosmosis In chemiosmosis, the free energy from the series of redox reactions just described is used to pump hydrogen ions (protons) across the membrane. The uneven distribution of H+ ions across the membrane establishes both concentration and electrical gradients (thus, an electrochemical gradient), owing to the hydrogen ions’ positive charge and their aggregation on one side of the membrane. If the membrane were open to diffusion by the hydrogen ions, the ions would tend to diffuse back across into the matrix, driven by their electrochemical gradient. Recall that many ions cannot diffuse through the nonpolar regions of phospholipid membranes without the aid of ion channels. Similarly, hydrogen ions in the matrix space can only pass through the inner mitochondrial membrane through an integral membrane protein called ATP synthase (Figure 7.12). This complex protein acts as a tiny generator, turned by the force of the hydrogen ions diffusing through it, down their electrochemical gradient. The turning of parts of this molecular machine facilitates the addition of a phosphate to ADP, forming ATP, using the potential energy of the hydrogen ion gradient. This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 7 | Cellular Respiration 299 Figure 7.12 ATP synthase is a complex, molecular machine that uses a proton (H+) gradient to form ATP from ADP and inorganic phosphate (Pi). (Credit: modification of work by Klaus Hoffmeier) Dinitrophenol (DNP) is an uncoupler that makes the inner mitochondrial membrane leak protons (H+ ). It was used until 1938 as a weight-loss drug. Why do you think this might be an effective weight-loss drug? a. DNP dissipates the proton gradient in the matrix, preventing the production of ATP. The body then increases its metabolic rate, leading to weight loss. b. DNP decreases the proton gradient in the inner mitochondrial space, leading to rapid consumption of acetyl- CoA, which causes weight loss. c. DNP blocks the movement of protons through the ATP synthase, halting ATP production. The stored energy dissipates as heat, causing weight loss. d. DNP uncouples the production of ATP by increasing the proton gradient in the matrix. The stored energy dissipates as heat, causing weight loss. Chemiosmosis (Figure 7.13) is used to generate 90 percent of the ATP made during aerobic |
glucose catabolism; it is also the method used in the light reactions of photosynthesis to harness the energy of sunlight in the process of photophosphorylation. Recall that the production of ATP using the process of chemiosmosis in mitochondria is called oxidative phosphorylation. The overall result of these reactions is the production of ATP from the energy of the electrons removed from hydrogen atoms. These atoms were originally part of a glucose molecule. At the end of the pathway, the electrons are used to reduce an oxygen molecule to oxygen ions. The extra electrons on the oxygen attract hydrogen ions (protons) from the surrounding medium, and water is formed. 300 Chapter 7 | Cellular Respiration Figure 7.13 In oxidative phosphorylation, the pH gradient formed by the electron transport chain is used by ATP synthase to form ATP. Cyanide inhibits cytochrome c oxidase, a component of the electron transport chain. If cyanide poisoning occurs, would you expect the pH of the intermembrane space to increase or decrease? What effect would cyanide have on ATP synthesis? a. The proton concentration of the intermembrane space would decrease, stopping the production of ATP. b. The proton concentration of the intermembrane space would increase, leading to ATP formation. c. The hydrogen ion concentration of the intermembrane space would decrease, causing a high production of ATP. d. The proton concentration of the intermembrane space would increase, causing production of ATP in large amounts. ATP Yield The number of ATP molecules generated from the catabolism of glucose varies. For example, the number of hydrogen ions that the electron transport chain complexes can pump through the membrane varies between species. Another source of variance stems from the shuttle of electrons across the membranes of the mitochondria. (The NADH generated from glycolysis cannot easily enter mitochondria.) Thus, electrons are picked up on the inside of mitochondria by either NAD+ or FAD+. As you have learned earlier, these FAD+ molecules can transport fewer ions; consequently, fewer ATP molecules are generated when FAD+ acts as a carrier. NAD+ is used as the electron transporter in the liver and FAD+ acts in the brain. Another factor that affects the yield of ATP molecules generated from glucose is the fact that intermediate compounds in these pathways are used for other purposes. Glucose catabolism connects with the pathways that build or break down all other biochemical compounds in cells, and the result is |
somewhat messier than the ideal situations described thus far. For example, sugars other than glucose are fed into the glycolytic pathway for energy extraction. Moreover, the five-carbon sugars that form nucleic acids are made from intermediates in glycolysis. Certain nonessential amino acids can be made from intermediates of both glycolysis and the citric acid cycle. Lipids, such as cholesterol and triglycerides, are also made from intermediates in these pathways, and both amino acids and triglycerides are broken down for energy through these pathways. Overall, in living systems, these pathways of glucose catabolism extract about 34 percent of the energy contained in glucose. This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 7 | Cellular Respiration 301 Activity Use construction paper and other art materials to create your own diagram of the electron transport chain (ETC). Be sure to include all parts of the electron transport chain, as well as the electrons themselves, NAD+ and NADH, and oxygen. On your diagram, label all parts of the ETC that transfers the free energy from electrons to another form. Then, use your model to make predictions about each of the following. Then, share your answers with the class. a. What would happen to free energy release if a cytochrome failed to undergo one of the redox reactions involved in the electron transport chain? b. What ultimately happens to the free energy in the electrons that travel down the ETC? c. Did you remember to have a pair of electrons travel down the ETC? What would happen if only one electron reached oxygen? Think About It • Dinitrophenol (DNP) is an uncoupler that makes the inner mitochondrial membrane leaky to protons. It was used until 1938 as a weight loss drug. What effect would DNP have on the change in pH across the inner mitochondrial membrane and the overall process of cellular respiration? Why do you think DNP might be an effective weightloss drug? Why is DNP no longer used? • Cyanide inhibits cytochrome c oxidase, a component of the electron transport chain. If cyanide poisoning occurs, would you expect the pH of the intermembrane space to increase or decrease? Explain the effect of cyanide on ATP synthesis. 7.5 | Metabolism without Oxygen In this section, you will explore the following question: • What is the fundamental difference between |
anaerobic cellular respiration and the different types of fermentation? Connection for AP® Courses As was previously stated, under aerobic conditions cellular respiration can yield 36-38 ATP molecules. If oxygen is not present, ATP is only produced by substrate-level phosphorylation. Without oxygen, organisms must use another electron acceptor. Most organisms will use some form of fermentation to accomplish the regeneration of NAD+ to ensure the continuation of glycolysis. In alcohol fermentation, pyruvate from glycolysis is converted to ethyl alcohol; during lactic acid fermentation, pyruvate is reduced to form lactate as an end-product. Without fermentation and anaerobic respiration, we wouldn’t have yogurt or soy sauce. Nor would our muscle cells cramp from the buildup of lactate when we exercise vigorously and oxygen is scarce. Information presented and the examples highlighted in the section support concepts and Learning Objectives outlined in Big Idea 2 of the AP® Biology Curriculum Framework, as shown in the table. The Learning Objectives listed in the Curriculum Framework provide a transparent foundation for the AP® Biology course, an inquiry-based laboratory experience, instructional activities, and AP® exam questions. A Learning Objective merges required content with one or more of the seven Science Practices. Big Idea 2 Biological systems utilize free energy and molecular building blocks to grow, to reproduce, and to maintain dynamic homeostasis. 302 Chapter 7 | Cellular Respiration Enduring Understanding 2.A Essential Knowledge Science Practice Science Practice Learning Objective Essential Knowledge Science Practice Learning Objective Growth, reproduction and maintenance of living systems require free energy and matter. 2.A.2 Organisms capture and store free energy for use in biological processes. 1.4 The student can use representations and models to analyze situations or solve problems qualitatively and quantitatively. 3.1 The student can pose scientific questions. 2.4 The student is able to use representations to pose scientific questions about what mechanisms and structural features allow organisms to capture, store, and use free energy. 2.A.2 Organisms capture and store free energy for use in biological processes. 6.2 The student can construct explanations of phenomena based on evidence produced through scientific practices. 2.5 The student is able to construct explanations of the mechanisms and structural features of cells that allow organisms to capture, store, or use free energy. The Science Practice Challenge Questions contain additional test questions for this section that will help you prepare for the AP exam. These questions address the following standards |
: [APLO 2.21][APLO 2.24][APLO 4.14][APLO 4.26] In aerobic respiration, the final electron acceptor is an oxygen molecule, O2. If aerobic respiration occurs, then ATP will be produced using the energy of high-energy electrons carried by NADH or FADH2 to the electron transport chain. If aerobic respiration does not occur, NADH must be reoxidized to NAD+ for reuse as an electron carrier for the glycolytic pathway to continue. How is this done? Some living systems use an organic molecule as the final electron acceptor. Processes that use an organic molecule to regenerate NAD+ from NADH are collectively referred to as fermentation. In contrast, some living systems use an inorganic molecule as a final electron acceptor. Both methods are called anaerobic cellular respiration in which organisms convert energy for their use in the absence of oxygen. Anaerobic Cellular Respiration Certain prokaryotes, including some species of bacteria and Archaea, use anaerobic respiration. For example, the group of Archaea called methanogens reduces carbon dioxide to methane to oxidize NADH. These microorganisms are found in soil and in the digestive tracts of ruminants, such as cows and sheep. Similarly, sulfate-reducing bacteria and Archaea, most of which are anaerobic ( Figure 7.14), reduce sulfate to hydrogen sulfide to regenerate NAD+ from NADH. This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 7 | Cellular Respiration 303 Figure 7.14 The green color seen in these coastal waters is from an eruption of hydrogen sulfide-producing bacteria. These anaerobic, sulfate-reducing bacteria release hydrogen sulfide gas as they decompose algae in the water. (credit: modification of work by NASA/Jeff Schmaltz, MODIS Land Rapid Response Team at NASA GSFC, Visible Earth Catalog of NASA images) Visit this site (http://openstaxcollege.org/l/fermentation) to see anaerobic cellular respiration in action. How does the formation of NAD+ differ between aerobic and anaerobic respiration? a. NAD+ is formed in aerobic respiration by a fermentation process and formed in anaerobic respiration by oxidation of NADH. b. NAD+ is formed by a fermentation process in |
anaerobic conditions by the conversion of pyruvate into lactate and by simple oxidation of NADH in aerobic respiration. c. Under aerobic conditions, the electron acceptor is a molecule other than oxygen for NAD+ production, whereas under anaerobic conditions the electron acceptor is oxygen. d. NAD+ is formed by a fermentation process in anaerobic conditions whereas in aerobic respiration it is formed by the breakdown of pyruvate into lactic acid or alcohol. Lactic Acid Fermentation The fermentation method used by animals and certain bacteria, like those in yogurt, is lactic acid fermentation (Figure 7.15). This type of fermentation is used routinely in mammalian red blood cells and in skeletal muscle that has an insufficient oxygen supply to allow aerobic respiration to continue (that is, in muscles used to the point of fatigue). In muscles, lactic acid accumulation must be removed by the blood circulation and the lactate brought to the liver for further metabolism. The chemical reactions of lactic acid fermentation are the following: 304 Chapter 7 | Cellular Respiration Pyruvic acid + NADH ↔ lactic acid + NAD+ The enzyme used in this reaction is lactate dehydrogenase (LDH). The reaction can proceed in either direction, but the reaction from left to right is inhibited by acidic conditions. Such lactic acid accumulation was once believed to cause muscle stiffness, fatigue, and soreness, although more recent research disputes this hypothesis. Once the lactic acid has been removed from the muscle and circulated to the liver, it can be reconverted into pyruvic acid and further catabolized for energy. This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 7 | Cellular Respiration 305 Figure 7.15 Lactic acid fermentation is common in muscle cells that have run out of oxygen. Tremetol, a metabolic poison found in the white snake root plant, prevents the metabolism of lactate. When cows eat this plant, it is concentrated in the milk they produce. Humans who consume the milk become ill. Symptoms of this disease, which include vomiting, abdominal pain, and tremors, become worse after exercise. Why do you think this is the case? Tremetol, a metabolic poison found in the white snake root plant, prevents the metabolism of lactate. When cows eat this plant, it is concentrated in the milk they produce. Humans who consume the milk become ill. Symptoms of this disease, which |
include vomiting, abdominal pain, and tremors, become worse after exercise. Why do you think this is the case? a. Tremetol inhibits enzymes that convert lactate into less harmful compounds. Exercise worsens this by producing more lactate. b. Tremetol increases the production of lactate dehydrogenase, causing lactic acid to accumulate in the body. c. Tremetol inhibits the production of NAD+ after exercise. The lack of oxygen causes lactic acid to accumulate in the body. d. Tremetol binds to lactic acid, inhibiting its breakdown into other compounds and causing it to accumulate after exercising. Alcohol Fermentation Another familiar fermentation process is alcohol fermentation (Figure 7.16) that produces ethanol, an alcohol. The first chemical reaction of alcohol fermentation is the following (CO2 does not participate in the second reaction): pyruvic acid + H+ → CO2 + acetaldehyde + NADH + H+ → ethanol + NAD+ 306 Chapter 7 | Cellular Respiration The first reaction is catalyzed by pyruvate decarboxylase, a cytoplasmic enzyme, with a coenzyme of thiamine pyrophosphate (TPP, derived from vitamin B1 and also called thiamine). A carboxyl group is removed from pyruvic acid, releasing carbon dioxide as a gas. The loss of carbon dioxide reduces the size of the molecule by one carbon, making acetaldehyde. The second reaction is catalyzed by alcohol dehydrogenase to oxidize NADH to NAD+ and reduce acetaldehyde to ethanol. The fermentation of pyruvic acid by yeast produces the ethanol. Ethanol tolerance of yeast is variable, ranging from about 5 percent to 21 percent, depending on the yeast strain and environmental conditions. Figure 7.16 Fermentation of grape juice produces CO2 as a byproduct. Fermentation tanks have valves so that the pressure inside the tanks created by the carbon dioxide produced can be released. Other Types of Fermentation Other fermentation methods occur in bacteria. Many prokaryotes are facultatively anaerobic. This means that they can switch between aerobic respiration and fermentation, depending on the availability of oxygen. Certain prokaryotes, like Clostridia, are obligate anaerobes. Obligate anaerobes live and grow in the absence of molecular oxygen. Oxygen is a poison to these microorganisms and kills them on exposure. It should be noted that all forms of fermentation, except lactic acid fermentation |
, produce gas. The production of particular types of gas is used as an indicator of the fermentation of specific carbohydrates, which plays a role in the laboratory identification of the bacteria. Various methods of fermentation are used by assorted organisms to ensure an adequate supply of NAD+ for the sixth step in glycolysis. Without these pathways, that step would not occur and no ATP would be harvested from the breakdown of glucose. Lab Investigation Lab Investigation: Respiration of Sugars by Yeast. You are given the opportunity to design and conduct experiments to investigate whether yeasts are able to metabolize a variety of sugars, using gas pressure sensors or other means to measure CO2 production. Think About It Tremetol, a metabolic poison found in the white snake plant root, prevents the metabolism of lactate. When female cows eat this plant, tremetol becomes concentrated in their milk. Humans who consume the milk become ill. Explain why the symptoms of this disease, which include vomiting, abdominal pain, and tremors, becomes worse after exercise. This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 7 | Cellular Respiration 307 7.6 | Connections of Carbohydrate, Protein, and Lipid Metabolic Pathways In this section, you will explore the following question: • How do carbohydrate metabolic pathways, glycolysis, and the citric acid cycle interrelate with protein and lipid metabolism pathways? Connection for AP® Courses The breakdown and synthesis of carbohydrates, proteins, lipids, and nucleic acids connect with the metabolic pathways of glycolysis and the citric acid cycle but enter the pathways at different points. Thus, these macromolecules can be used as sources of free energy. Information presented and the examples highlighted in the section support concepts and Learning Objectives outlined in Big Idea 2 of the AP® Biology Curriculum Framework, as shown in the table. The Learning Objectives listed in the Curriculum Framework provide a transparent foundation for the AP® Biology course, an inquiry-based laboratory experience, instructional activities, and AP® exam questions. A Learning Objective merges required content with one or more of the seven Science Practices. Essential Knowledge 2.A.2 Organisms capture and store free energy for use in biological processes. Science Practice 6.2 The student can construct explanations of phenomena based on evidence produced through scientific practices. Learning Objective 2.5 The student is able to construct explanations of the mechanisms |
and structural features of cells that allow organisms to capture, store, or use free energy. Essential Knowledge Science Practice Learning Objective 2.A.1 All living systems require constant input of free energy. 6.1 The student can justify claims with evidence. 2.2 The student is able to justify a scientific claim that free energy is required for living systems to maintain organization, to grow or to reproduce, but that multiple strategies exist in different living systems. The Science Practice Challenge Questions contain additional test questions for this section that will help you prepare for the AP exam. These questions address the following standards: [APLO 2.5][APLO 2.15][APLO 3.20][APLO 1.5][APLO 1.26][APLO 4.18] You have learned about the catabolism of glucose, which provides energy to living cells. But living things consume more than glucose for food. How does a turkey sandwich end up as ATP in your cells? This happens because all of the catabolic pathways for carbohydrates, proteins, and lipids eventually connect into glycolysis and the citric acid cycle pathways (see Figure 7.18). Metabolic pathways should be thought of as porous—that is, substances enter from other pathways, and intermediates leave for other pathways. These pathways are not closed systems. Many of the substrates, intermediates, and products in a particular pathway are reactants in other pathways. Connections of Other Sugars to Glucose Metabolism Glycogen, a polymer of glucose, is an energy storage molecule in animals. When there is adequate ATP present, excess glucose is shunted into glycogen for storage. Glycogen is made and stored in both liver and muscle. The glycogen will be hydrolyzed into glucose 1-phosphate monomers (G-1-P) if blood sugar levels drop. The presence of glycogen as a source of glucose allows ATP to be produced for a longer period of time during exercise. Glycogen is broken down into G-1-P and converted into G-6-P in both muscle and liver cells, and this product enters the glycolytic pathway. 308 Chapter 7 | Cellular Respiration Sucrose is a disaccharide with a molecule of glucose and a molecule of fructose bonded together with a glycosidic linkage. Fructose is one of the three dietary monosaccharides, along with glucose and galactose (which is part of the milk sugar, the dis |
accharide lactose), which are absorbed directly into the bloodstream during digestion. The catabolism of both fructose and galactose produces the same number of ATP molecules as glucose. Connections of Proteins to Glucose Metabolism Proteins are hydrolyzed by a variety of enzymes in cells. Most of the time, the amino acids are recycled into the synthesis of new proteins. If there are excess amino acids, however, or if the body is in a state of starvation, some amino acids will be shunted into the pathways of glucose catabolism (Figure 7.17). Each amino acid must have its amino group removed prior to entry into these pathways. The amino group is converted into ammonia. In mammals, the liver synthesizes urea from two ammonia molecules and a carbon dioxide molecule. Thus, urea is the principal waste product in mammals, produced from the nitrogen originating in amino acids, and it leaves the body in urine. Figure 7.17 The carbon skeletons of certain amino acids (indicated in boxes) derived from proteins can feed into the citric acid cycle. (credit: modification of work by Mikael Häggström) Connections of Lipid and Glucose Metabolisms The lipids that are connected to the glucose pathways are cholesterol and triglycerides. Cholesterol is a lipid that contributes to cell membrane flexibility and is a precursor of steroid hormones. The synthesis of cholesterol starts with acetyl groups and proceeds in only one direction. The process cannot be reversed. Triglycerides are a form of long-term energy storage in animals. Triglycerides are made of glycerol and three fatty acids. Animals can make most of the fatty acids they need. Triglycerides can be both made and broken down through parts of the glucose catabolism pathways. Glycerol can be phosphorylated to glycerol-3-phosphate, which continues through glycolysis. Fatty acids are catabolized in a process called beta-oxidation that takes place in the matrix of the mitochondria and converts their fatty acid chains into two carbon units of acetyl groups. The acetyl groups are picked up by CoA to form acetyl CoA that proceeds into the citric acid cycle. This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 7 | Cellular Respiration 309 Figure 7.18 Glycogen from the liver and muscles, |
hydrolyzed into glucose-1-phosphate, together with fats and proteins, can feed into the catabolic pathways for carbohydrates. 310 Chapter 7 | Cellular Respiration Pathways of Photosynthesis and Cellular Metabolism The processes of photosynthesis and cellular metabolism consist of several very complex pathways. It is generally thought that the first cells arose in an aqueous environment—a “soup” of nutrients—probably on the surface of some porous clays. If these cells reproduced successfully and their numbers climbed steadily, it follows that the cells would begin to deplete the nutrients from the medium in which they lived as they shifted the nutrients into the components of their own bodies. This hypothetical situation would have resulted in natural selection favoring those organisms that could exist by using the nutrients that remained in their environment and by manipulating these nutrients into materials upon which they could survive. Selection would favor those organisms that could extract maximal value from the nutrients to which they had access. An early form of photosynthesis developed that harnessed the sun’s energy using water as a source of hydrogen atoms, but this pathway did not produce free oxygen (anoxygenic photosynthesis). (Early photosynthesis did not produce free oxygen because it did not use water as the source of hydrogen ions; instead, it used materials like hydrogen sulfide and consequently produced sulfur). It is thought that glycolysis developed at this time and could take advantage of the simple sugars being produced, but these reactions were unable to fully extract the energy stored in the carbohydrates. The development of glycolysis probably predated the evolution of photosynthesis, as it was well suited to extract energy from materials spontaneously accumulating in the “primeval soup.” A later form of photosynthesis used water as a source of electrons and hydrogen, and generated free oxygen. Over time, the atmosphere became oxygenated, but not before the oxygen released oxidized metals in the ocean and created a “rust” layer in the sediment, permitting the dating of the rise of the first oxygenic photosynthesizers. Living things adapted to exploit this new atmosphere that allowed aerobic respiration as we know it to evolve. When the full process of oxygenic photosynthesis developed and the atmosphere became oxygenated, cells were finally able to use the oxygen expelled by photosynthesis to extract considerably more energy from the sugar molecules using the citric acid cycle and oxidative phosphorylation. According to the Evolution Connection passage, in what order did the metabolic pathways evolve? a. 1. anoxygenic |
photosynthesis 2. glycolysis 3. oxygenic photosynthesis 4. citric acid cycle and oxidative phosphorylation b. 1. glycolysis 2. citric acid cycle and oxidative phosphorylation 3. anoxygenic photosynthesis 4. oxygenic photosynthesis c. 1. anoxygenic photosynthesis 2. oxygenic photosynthesis 3. glycolysis 4. citric acid cycle and oxidative phosphorylation d. 1. glycolysis 2. anoxygenic photosynthesis 3. oxygenic photosynthesis 4. citric acid cycle and oxidative phosphorylation This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 7 | Cellular Respiration 311 Think About It Explain how free energy can be obtained from the metabolism of carbohydrates, proteins, lipids, and even nucleic acids. Which of these molecules provides the largest amount of free energy? Justify your answer. 7.7 | Regulation of Cellular Respiration In this section, you will explore the following question: • What mechanisms control cellular respiration? Connection for AP® Courses Cellular respiration is controlled by a variety of means. For example, the entry of glucose into a cell is controlled by the transport proteins that aid glucose passage through the cell membrane. However, most of the control of the respiration processes is accomplished through negative feedback inhibition of specific enzymes that respond to the intracellular concentrations of ATP, ADP, NAD+, and FAD, etc. Information presented and the examples highlighted in the section support concepts and learning objectives outlined in Big Idea 2 of the AP® Biology Curriculum Framework, as shown in the table. The Learning Objectives listed in the Curriculum Framework provide a transparent foundation for the AP® Biology course, an inquiry-based laboratory experience, instructional activities, and AP® exam questions. A Learning Objective merges required content with one or more of the seven Science Practices. Big Idea 2 Enduring Understanding 2.C Biological systems utilize free energy and molecular building blocks to grow, to reproduce, and to maintain dynamic homeostasis. Organisms use feedback mechanisms to regulate growth and reproduction, and to maintain dynamic homeostasis. Essential Knowledge 2.C.1 Organisms use feedback mechanisms to maintain their internal environments and respond to external environmental changes. Science Practice 7.2 The student can connect concepts in and across domain(s) to generalize or extrapolate in and/or across enduring understandings and/ |
or big ideas. Learning Objective Essential Knowledge 2.16 The student is able to connect how organisms use negative feedback to maintain their internal environments. 2.C.1 Organisms use feedback mechanisms to maintain their internal environments and respond to external environmental changes. Science Practice 5.3 The student can evaluate the evidence provided by data sets in relation to a particular scientific question. Learning Objective 2.17 The student is able to evaluate data that show the effect(s) of changes in concentration of key molecules on negative feedback mechanisms. Cellular respiration must be regulated in order to provide balanced amounts of energy in the form of ATP. The cell also must generate a number of intermediate compounds that are used in the anabolism and catabolism of macromolecules. Without controls, metabolic reactions would quickly come to a stand-still as the forward and backward reactions reached a state of 312 Chapter 7 | Cellular Respiration equilibrium. Resources would be used inappropriately. A cell does not need the maximum amount of ATP that it can make all the time: At times, the cell needs to shunt some of the intermediates to pathways for amino acid, protein, glycogen, lipid, and nucleic acid production. In short, the cell needs to control its metabolism. Regulatory Mechanisms A variety of mechanisms is used to control cellular respiration. Some type of control exists at each stage of glucose metabolism. Access of glucose to the cell can be regulated using the GLUT proteins that transport glucose (Figure 7.19). Different forms of the GLUT protein control passage of glucose into the cells of specific tissues. Figure 7.19 GLUT4 is a glucose transporter that is stored in vesicles. A cascade of events that occurs upon insulin binding to a receptor in the plasma membrane causes GLUT4-containing vesicles to fuse with the plasma membrane so that glucose may be transported into the cell. Some reactions are controlled by having two different enzymes—one each for the two directions of a reversible reaction. Reactions that are catalyzed by only one enzyme can go to equilibrium, stalling the reaction. In contrast, if two different enzymes (each specific for a given direction) are necessary for a reversible reaction, the opportunity to control the rate of the reaction increases, and equilibrium is not reached. A number of enzymes involved in each of the pathways—in particular, the enzyme catalyzing the first committed reaction of the pathway—are controlled by attachment of a molecule to an allosteric site on the protein. The molecules most commonly used in this capacity |
are the nucleotides ATP, ADP, AMP, NAD+, and NADH. These regulators, allosteric effectors, may increase or decrease enzyme activity, depending on the prevailing conditions. The allosteric effector alters the steric structure of the enzyme, usually affecting the configuration of the active site. This alteration of the protein’s (the enzyme’s) structure either increases or decreases its affinity for its substrate, with the effect of increasing or decreasing the rate of the reaction. The attachment signals to the enzyme. This binding can increase or decrease the enzyme’s activity, providing feedback. This feedback type of control is effective as long as the chemical affecting it is attached to the enzyme. Once the overall concentration of the chemical decreases, it will diffuse away from the protein, and the control is relaxed. Control of Catabolic Pathways Enzymes, proteins, electron carriers, and pumps that play roles in glycolysis, the citric acid cycle, and the electron transport chain tend to catalyze non-reversible reactions. In other words, if the initial reaction takes place, the pathway is committed to proceeding with the remaining reactions. Whether a particular enzyme activity is released depends upon the energy needs of the cell (as reflected by the levels of ATP, ADP, and AMP). Glycolysis The control of glycolysis begins with the first enzyme in the pathway, hexokinase (Figure 7.20). This enzyme catalyzes the phosphorylation of glucose, which helps to prepare the compound for cleavage in a later step. The presence of the negatively charged phosphate in the molecule also prevents the sugar from leaving the cell. When hexokinase is inhibited, glucose diffuses out of the cell and does not become a substrate for the respiration pathways in that tissue. The product of the hexokinase reaction is glucose-6-phosphate, which accumulates when a later enzyme, phosphofructokinase, is inhibited. This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 7 | Cellular Respiration 313 Figure 7.20 The glycolysis pathway is primarily regulated at the three key enzymatic steps (1, 2, and 7) as indicated. Note that the first two steps that are regulated occur early in the pathway and involve hydrolysis of ATP. Phosphofructokinase is the main enzyme controlled in glycolysis. |
High levels of ATP, citrate, or a lower, more acidic pH decrease the enzyme’s activity. An increase in citrate concentration can occur because of a blockage in the citric acid cycle. Fermentation, with its production of organic acids like lactic acid, frequently accounts for the increased acidity in a cell; however, the products of fermentation do not typically accumulate in cells. The last step in glycolysis is catalyzed by pyruvate kinase. The pyruvate produced can proceed to be catabolized or converted into the amino acid alanine. If no more energy is needed and alanine is in adequate supply, the enzyme is inhibited. The enzyme’s activity is increased when fructose-1,6-bisphosphate levels increase. that fructose-1,6-bisphosphate is an intermediate in the first half of glycolysis.) The regulation of pyruvate kinase involves phosphorylation by a kinase (pyruvate kinase kinase), resulting in a less-active enzyme. Dephosphorylation by a phosphatase reactivates it. Pyruvate kinase is also regulated by ATP (a negative allosteric effect). (Recall If more energy is needed, more pyruvate will be converted into acetyl CoA through the action of pyruvate dehydrogenase. If either acetyl groups or NADH accumulate, there is less need for the reaction and the rate decreases. Pyruvate dehydrogenase is also regulated by phosphorylation: A kinase phosphorylates it to form an inactive enzyme, and a phosphatase reactivates it. The kinase and the phosphatase are also regulated. Citric Acid Cycle The citric acid cycle is controlled through the enzymes that catalyze the reactions that make the first two molecules of NADH (Figure 7.10). These enzymes are isocitrate dehydrogenase and α‑ketoglutarate dehydrogenase. When adequate ATP and NADH levels are available, the rates of these reactions decrease. When more ATP is needed, as reflected in rising ADP levels, the rate increases. α-ketoglutarate dehydrogenase will also be affected by the levels of succinyl CoA—a subsequent intermediate in the cycle—causing a decrease in activity. A decrease in the rate of operation of the pathway at this point is not necessarily negative, as the |
increased levels of the α-ketoglutarate not used by the citric acid cycle can be used by the cell for amino acid (glutamate) synthesis. Electron Transport Chain Specific enzymes of the electron transport chain are unaffected by feedback inhibition, but the rate of electron transport through the pathway is affected by the levels of ADP and ATP. Greater ATP consumption by a cell is indicated by a buildup of ADP. As ATP usage decreases, the concentration of ADP decreases, and now, ATP begins to build up in the cell. This change in the relative concentration of ADP to ATP triggers the cell to slow down the electron transport chain. 314 Chapter 7 | Cellular Respiration Visit this site (http://openstaxcollege.org/l/electron_transp) to see an animation of the electron transport chain and ATP synthesis. Which statement best describes the formation and importance of the hydrogen ion gradient during the electron transport chain? a. A hydrogen ion gradient across the membrane establishes a concentration gradient and not an electrical gradient, thus assisting during the electron transport chain. b. A hydrogen ion gradient is established by pumping two hydrogen ions across the membrane from the matrix in the intermembrane space. Its uneven distribution across the membrane establishes both concentration and electrical gradients. c. A hydrogen ion gradient is established by pumping four hydrogen ions across the membrane from the matrix into the intermembrane space and its uneven distribution across the membrane establishes concentration and electrical gradients. d. Hydrogen ions are present in the intermembrane space from the beginning and results in the formation of gradients necessary for the function of ATP synthase. For a summary of feedback controls in cellular respiration, see Table 7.1. Summary of Feedback Controls in Cellular Respiration Pathway Enzyme affected Elevated levels of effector glycolysis hexokinase glucose-6-phosphate phosphofructokinase low-energy charge (ATP, AMP), fructose-6-phosphate via fructose-2,6-bisphosphate high-energy charge (ATP, AMP), citrate, acidic pH pyruvate kinase fructose-1,6-bisphosphate high-energy charge (ATP, AMP), alanine pyruvate to acetyl CoA conversion pyruvate dehydrogenase ADP, pyruvate acetyl CoA, ATP, NADH citric acid cycle isocitrate dehydrogenase ADP ATP |
, NADH Table 7.1 Effect on pathway activity decrease increase decrease increase decrease increase decrease increase decrease This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 7 | Cellular Respiration 315 Summary of Feedback Controls in Cellular Respiration Pathway Enzyme affected Elevated levels of effector α-ketoglutarate dehydrogenase Calcium ions, ADP ATP, NADH, succinyl CoA electron transport chain Table 7.1 ADP ATP Effect on pathway activity increase decrease increase decrease Think About It Phosphofructokinase is a key enzyme in glycolysis. High levels of ATP or citrate or low pH can decrease the enzyme’s activity. Explain why this is beneficial to the cell. 316 Chapter 7 | Cellular Respiration KEY TERMS acetyl CoA combination of an acetyl group derived from pyruvic acid and coenzyme A, which is made from pantothenic acid (a B-group vitamin) aerobic respiration process in which organisms convert energy in the presence of oxygen anaerobic process that does not use oxygen anaerobic cellular respiration process in which organisms convert energy for their use in the absence of oxygen ATP synthase (also, F1F0 ATP synthase) membrane-embedded protein complex that adds a phosphate to ADP with energy from protons diffusing through it chemiosmosis process in which there is a production of adenosine triphosphate (ATP) in cellular metabolism by the involvement of a proton gradient across a membrane citric acid cycle cells (also, Krebs cycle) series of enzyme-catalyzed chemical reactions of central importance in all living dephosphorylation removal of a phosphate group from a molecule fermentation process of regenerating NAD+ with either an inorganic or organic compound serving as the final electron acceptor; occurs in the absence of oxygen GLUT protein integral membrane protein that transports glucose glycolysis process of breaking glucose into two three-carbon molecules with the production of ATP and NADH isomerase enzyme that converts a molecule into its isomer Krebs cycle (also, citric acid cycle) alternate name for the citric acid cycle, named after Hans Krebs who first identified the steps in the pathway in the 1930s in pigeon flight muscles; see citric acid cycle oxidative phosphorylation production of ATP using the process of chemiosmosis and oxygen phosphorylation addition of a high-energy |
phosphate to a compound, usually a metabolic intermediate, a protein, or ADP prosthetic group (also, prosthetic cofactor) molecule bound to a protein that facilitates the function of the protein pyruvate three-carbon sugar that can be decarboxylated and oxidized to make acetyl CoA, which enters the citric acid cycle under aerobic conditions; the end product of glycolysis redox reaction chemical reaction that consists of the coupling of an oxidation reaction and a reduction reaction substrate-level phosphorylation production of ATP from ADP using the excess energy from a chemical reaction and a phosphate group from a reactant TCA cycle (also, citric acid cycle) alternate name for the citric acid cycle, named after the group name for citric acid, tricarboxylic acid (TCA); see citric acid cycle ubiquinone soluble electron transporter in the electron transport chain that connects the first or second complex to the third CHAPTER SUMMARY 7.1 Energy in Living Systems ATP functions as the energy currency for cells. It allows the cell to store energy briefly and transport it within the cell to support endergonic chemical reactions. The structure of ATP is that of an RNA nucleotide with three phosphates attached. As ATP is used for energy, a phosphate group or two are detached, and either ADP or AMP is produced. Energy derived This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 7 | Cellular Respiration 317 from glucose catabolism is used to convert ADP into ATP. When ATP is used in a reaction, the third phosphate is temporarily attached to a substrate in a process called phosphorylation. The two processes of ATP regeneration that are used in conjunction with glucose catabolism are substrate-level phosphorylation and oxidative phosphorylation through the process of chemiosmosis. 7.2 Glycolysis Glycolysis is the first pathway used in the breakdown of glucose to extract energy. It was probably one of the earliest metabolic pathways to evolve and is used by nearly all of the organisms on earth. Glycolysis consists of two parts: The first part prepares the six-carbon ring of glucose for cleavage into two three-carbon sugars. ATP is invested in the process during this half to energize the separation. The second half of glycolysis extracts ATP and high-energy electrons from hydrogen atoms and attaches them to NAD+. |
Two ATP molecules are invested in the first half and four ATP molecules are formed by substrate phosphorylation during the second half. This produces a net gain of two ATP and two NADH molecules for the cell. 7.3 Oxidation of Pyruvate and the Citric Acid Cycle In the presence of oxygen, pyruvate is transformed into an acetyl group attached to a carrier molecule of coenzyme A. The resulting acetyl CoA can enter several pathways, but most often, the acetyl group is delivered to the citric acid cycle for further catabolism. During the conversion of pyruvate into the acetyl group, a molecule of carbon dioxide and two highenergy electrons are removed. The carbon dioxide accounts for two (conversion of two pyruvate molecules) of the six carbons of the original glucose molecule. The electrons are picked up by NAD+, and the NADH carries the electrons to a later pathway for ATP production. At this point, the glucose molecule that originally entered cellular respiration has been completely oxidized. Chemical potential energy stored within the glucose molecule has been transferred to electron carriers or has been used to synthesize a few ATPs. The citric acid cycle is a series of redox and decarboxylation reactions that remove high-energy electrons and carbon dioxide. The electrons temporarily stored in molecules of NADH and FADH2 are used to generate ATP in a subsequent pathway. One molecule of either GTP or ATP is produced by substrate-level phosphorylation on each turn of the cycle. There is no comparison of the cyclic pathway with a linear one. 7.4 Oxidative Phosphorylation The electron transport chain is the portion of aerobic respiration that uses free oxygen as the final electron acceptor of the electrons removed from the intermediate compounds in glucose catabolism. The electron transport chain is composed of four large, multiprotein complexes embedded in the inner mitochondrial membrane and two small diffusible electron carriers shuttling electrons between them. The electrons are passed through a series of redox reactions, with a small amount of free energy used at three points to transport hydrogen ions across a membrane. This process contributes to the gradient used in chemiosmosis. The electrons passing through the electron transport chain gradually lose energy, Highenergy electrons donated to the chain by either NADH or FADH2 complete the chain, as low-energy electrons reduce oxygen molecules and form water. The level of free energy of the electrons drops from |
about 60 kcal/mol in NADH or 45 kcal/mol in FADH2 to about 0 kcal/mol in water. The end products of the electron transport chain are water and ATP. A number of intermediate compounds of the citric acid cycle can be diverted into the anabolism of other biochemical molecules, such as nonessential amino acids, sugars, and lipids. These same molecules can serve as energy sources for the glucose pathways. 7.5 Metabolism without Oxygen If NADH cannot be oxidized through aerobic respiration, another electron acceptor is used. Most organisms will use some form of fermentation to accomplish the regeneration of NAD+, ensuring the continuation of glycolysis. The regeneration of NAD+ in fermentation is not accompanied by ATP production; therefore, the potential of NADH to produce ATP using an electron transport chain is not utilized. 7.6 Connections of Carbohydrate, Protein, and Lipid Metabolic Pathways The breakdown and synthesis of carbohydrates, proteins, and lipids connect with the pathways of glucose catabolism. The simple sugars are galactose, fructose, glycogen, and pentose. These are catabolized during glycolysis. The amino acids from proteins connect with glucose catabolism through pyruvate, acetyl CoA, and components of the citric acid cycle. Cholesterol synthesis starts with acetyl groups, and the components of triglycerides come from glycerol-3-phosphate from glycolysis and acetyl groups produced in the mitochondria from pyruvate. 318 Chapter 7 | Cellular Respiration 7.7 Regulation of Cellular Respiration Cellular respiration is controlled by a variety of means. The entry of glucose into a cell is controlled by the transport proteins that aid glucose passage through the cell membrane. Most of the control of the respiration processes is accomplished through the control of specific enzymes in the pathways. This is a type of negative feedback, turning the enzymes off. The enzymes respond most often to the levels of the available nucleosides ATP, ADP, AMP, NAD+, and FAD. Other intermediates of the pathway also affect certain enzymes in the systems. REVIEW QUESTIONS 1. What is the most important energy currency used by cells? a. ATP b. ADP c. AMP d. adenosine 2. What happens when a chemical is reduced during a reaction? a. The compound is reduced to a simpler form. b. An electron is added to the chemical. |
c. A hydrogen atom is removed from the substrate. d. acts as a catabolic reaction 3. Which of the following molecules are oxidizing agents? a. FAD+ and NAD+ b. FADH2 and NADH c. FAD and FADH2 d. NAD+ and NADH 4. Which of the following reactions releases energy? a. AMP + phosphate → ADP + H2 O b. ADP + phosphate → ATP + H2 O c. ATP + H2 O → ADP + Phosphate d. AMP + H2 O → ATP + Phosphate 5. During the second half of glycolysis, what occurs? a. ATP is used up. b. Fructose is split in two. c. ATP is produced. d. Glucose becomes fructose. 6. GLUTs are integral membrane proteins that assist in the facilitated diffusion of glucose into and out of cells. What reaction in glycolysis prevents glucose from being transported back out of the cell? a. Hexokinase dephosphorylates glucose using ATP, creating a glucose molecules that can’t cross the hydrophilic portion of the plasma membrane. b. Hexokinase phosphorylates glucose using ADP, creating a glucose molecules that can’t cross the hydrophobic interior of the plasma membrane. c. Hexokinase dephosphorylates glucose using ADP, creating a glucose molecule that can’t cross the hydrophilic portion of the plasma membrane. d. Hexokinase phosphorylates glucose using ATP, creating a glucose molecule that can’t cross the hydrophobic interior of the plasma membrane. 7. How many ATP molecules are used and produced per molecule of glucose during glycolysis? a. The first half of glycolysis uses 2 ATPs, and the second half of glycolysis produces 4 ATPs. b. The first half of glycolysis produces 2 ATPs, and the second half of glycolysis uses 4 ATPs. c. The first half of glycolysis uses 4 ATPs, and the second half of glycolysis produces 2 ATPs. d. The first half of glycolysis produces 4 ATPS, and the second half of glycolysis uses 2 ATPs. 8. What is removed from pyruvate during its conversion into an acetyl group? a. oxygen b. |
ATP c. B vitamin d. carbon dioxide 9. What do the electrons added to NAD+ do in aerobic respiration? a. They become part of a fermentation pathway. b. They go to another pathway for ATP production. c. They energize the acetyl group in the citric acid cycle. d. They are converted to NADP. 10. GTP, which can be converted to ATP, is produced during which reaction of the citric acid cycle? This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 7 | Cellular Respiration 319 a. b. c. isocitrate into α -ketoglutarate succinyl-CoA into succinate fumarate into malate d. malate into oxaloacetate 11. How many NADH molecules are produced on each turn of the citric acid cycle? a. acetyl-CoA and NADH lactate, ATP, and CO2 b. c. glucose, ATP, and NAD+ d. pyruvate and NADH 18. What are the products of alcohol fermentation? a. one b. c. d. two three four 12. What compound receives electrons from NADH? a. FMN b. ubiquinone c. cytochrome c1 d. oxygen 13. Chemiosmosis involves the movement of what? Where does it occur? a. electrons across the cell membrane b. hydrogen atoms across a mitochondrial membrane c. hydrogen ions across a mitochondrial membrane d. glucose through the cell membrane 14. What is the function of an electron in the electron transport chain? a. b. c. d. to dephosphorylate ATP, producing ADP to power active transport pumps to reduce heme in complex III to oxidize oxygen 15. What would be the outcome if hydrogen ions were able to diffuse through the mitochondrial membrane into the mitochondria without the need for integral membrane proteins? a. ATP would not be produced. b. Pyruvate would not be produced. c. Citric acid would not be produced. a. methane and NADH b. lactic acid and FAD+ c. ethanol and NAD+ d. pyruvic acid and NADH 19. In the first step of glycolysis, what is glucose transformed into? a. glucose-6-phosphate b. fructose-1,6-bisphosphate c. dihydroxyacetone phosphate d |
. phosphoenolpyruvate 20. What is beta-oxidation? a. b. c. d. the main process used to break down glucose the main process used to assemble glucose the main process used to break down fatty acids the main process used to remove amino groups from amino acids 21. Which of the following statements about catabolic pathways is false? a. Carbohydrates can feed into oxidative phosphorylation. b. Glycerol can be broken down into glucose and feed into glycolysis. c. Amino acids can feed into pyruvate oxidation. d. Fatty acids can feed into the citric acid cycle. 22. What impact, if any, do high levels of ADP have on glycolysis? a. They increase the activity of enzymes involved with glycolysis. b. The high levels decrease the activity of enzymes d. Carbon dioxide would not be produced. involved with glycolysis. 16. Which of the following fermentation methods can occur in animal skeletal muscles? a. lactic acid fermentation b. alcohol fermentation c. mixed acid fermentation d. propionic fermentation 17. Which molecules are produced in glycolysis and used in fermentation? c. They have no effect on the activity of any enzymes involved with glycolysis. d. The high levels slow down all pathways involved with glycolysis. 23. The control of which enzyme exerts the greatest control of glycolysis? 320 Chapter 7 | Cellular Respiration a. hexokinase b. phosphofructokinase c. glucose-6-phosphatase d. aldolase concentration increases relative to ADP? a. decreased activity of phosphofructokinase b. increased activity of pyruvate kinase c. decreased activity of isocitrate dehydrogenase 24. Which of the following does not occur as ATP d. slowdown of the electron transport chain CRITICAL THINKING QUESTIONS 25. Why is it beneficial for cells to use ATP rather than directly using the energy stored in the bonds of carbohydrates to power cellular reactions? What are the greatest drawbacks to harnessing energy from the bonds of several different compounds? a. ATP is readily available in the form of a single unit that provides a consistent, appropriate amount of energy. The cell would need to tailor each reaction to each energy source if it harvested energy from different compounds. b. ATP energy cannot activate the ROS dependent stress response whereas food molecules are responsible for activating ROS. c |
. ATP is low in energy, but food molecules possess higher levels of energy that cells can use. d. ATP is readily available to cells, unlike compounds that have to first be phosphorylated in order to release their energy. 26. What role does NAD+ play in redox reactions? a. NAD+, an oxidizing agent, can accept electrons and protons from organic molecules and get reduced to NADH. b. NAD+, a reducing agent, can donate its electrons and protons to organic molecules. c. NAD+, an oxidizing agent, can accept electrons from organic molecules and get reduced to NADH2. d. NAD+, a reducing agent, can donate its electrons and protons to inorganic molecules. a. RH acts as a reducing agent and donates its electrons to the oxidizing agent NAD+, forming NADH and R. b. NAD+, the oxidizing agent, donates its electrons to the reducing agent RH, forming R and NADH. c. RH acts as an oxidizing agent and donates electrons to the reducing agent NAD+, producing NADH and R. d. NAD+, the reducing agent, accepts electrons from the oxidizing agent RH, producing NADH and R. 28. Nearly all organisms on earth carry out some form of glycolysis. How does this fact support or not support the assertion that glycolysis is one of the oldest metabolic pathways? a. To be present in so many different organisms, glycolysis was probably present in a common ancestor rather than evolving many separate times. b. Glycolysis is present in nearly all organisms because it is an advanced and recently evolved pathway that has been widely used as it is so beneficial. c. Glycolysis is absent in a few higher organisms. This contradicts the fact that it is one of the oldest metabolic pathways. d. Glycolysis is present in some organisms and absent in others. The mentioned fact may or may not support this assertion. 27. Which statement best explains how electrons are transferred and the role of each species. Remember that R represents a hydrocarbon molecule and RH represents the same molecule with a particular hydrogen identified. RH + NAD+ → NADH + R 29. Red blood cells (RBCs) do not perform aerobic respiration, but they do perform glycolysis. Why do all cells need an energy source and what would happen if glycolysis were blocked in a red blood cell? This OpenStax book |
is available for free at http://cnx.org/content/col12078/1.6 Chapter 7 | Cellular Respiration 321 a. Cells require energy to perform certain basic a. Pyruvate dehydrogenase removes a carboxyl functions. Blocking glycolysis in RBCs causes imbalance in the membrane potential, leading to cell death. b. Cells need energy to perform cell division. Blocking glycolysis in RBCs interrupts the process of mitosis leading to nondisjunction. c. Cells maintain the influx and efflux of organic substances using energy. Blocking glycolysis stops the binding of CO2 cell death. to the RBCs, causing d. Cells require energy to recognize attacking pathogens. Blocked glycolysis inhibits the process of recognition, causing invasion of the RBCs by a pathogen. 30. What is the primary difference between a circular pathway and a linear pathway? a. The reactant and the product are the same in a circular pathway but different in a linear pathway. b. The circular pathway components get exhausted whereas those of the linear pathway do not and are continually regenerated. c. Circular pathways are not suited for amphibolic pathways whereas linear pathways are. d. Circular pathways contain a single chemical reaction that is repeated while linear pathways have multiple events. 31. Cellular respiration breaks down glucose and releases carbon dioxide and water. Which steps in the oxidation of pyruvate produces carbon dioxide? a. Removal of a carboxyl group from pyruvate releases carbon dioxide. The pyruvate dehydrogenase complex comes into play. b. Removal of an acetyl group from pyruvate releases carbon dioxide. The pyruvate decarboxylase complex comes into play. c. Removal of a carbonyl group from pyruvate releases carbon dioxide. The pyruvate dehydrogenase complex comes into play. d. Removal of an acetyl group from pyruvate releases carbon dioxide. The pyruvate dehydrogenase complex comes into play. 32. What three steps are included in the breakdown of pyruvate? group from pyruvate producing carbon dioxide. Dihydrolipoyl transacetylase oxidizes a hydroxyethyl group to an acetyl group, producing NADH. Lastly, an enzyme-bound acetyl group is transferred to CoA, producing a molecule of acetyl-CoA. |
b. Pyruvate dehydrogenase oxidizes hydroxyethyl group to an acetyl group, producing NADH. It further removes a carboxyl group from pyruvate producing carbon dioxide. Lastly, dihydrolipoyl transacetylase transfers enzyme-bound acetyl group to CoA forming an acetyl-CoA molecule. c. Pyruvate dehydrogenase transfers enzyme- bound acetyl group to CoA forming an acetyl CoA molecule. It then oxidizes a hydroxyethyl group to an acetyl group, producing NADH. Dihydrolipoyl transacetylase removes a carboxyl group from pyruvate producing carbon dioxide. d. Pyruvate dehydrogenase removes carboxyl group from pyruvate producing carbon dioxide. Dihydrolipoyl dehydrogenase transfers enzymebound acetyl groups to CoA forming an acetylCoA molecule. Lastly, a hydroxyethyl group is oxidized to an acetyl group, producing NADH. 33. How do the roles of ubiquinone and cytochrome c differ from the other components of the electron transport chain? a. CoQ and cytochrome c are mobile electron carriers while NADH dehydrogenase and succinate dehydrogenase are bound to the inner mitochondrial membrane. b. CoQ and cytochrome covalently bind electrons while NADH dehydrogenase and succinate dehydrogenase are bound to the inner mitochondrial membrane. c. CoQ and cytochrome c are bound to the inner mitochondrial membrane while NADH dehydrogenase and succinate dehydrogenase are mobile electron carriers. d. CoQ and cytochrome c covalently bind electrons while NADH dehydrogenase and succinate dehydrogenase are mobile electron carriers. 34. What accounts for the different number of ATP molecules that are formed through cellular respiration? 322 Chapter 7 | Cellular Respiration a. Transport of NADH from cytosol to mitochondria is an active process that decreases the number of ATP produced. b. The ATPs produced are utilized in the anaplerotic reactions that are used for the replenishment of the intermediates. c. Most of the ATP’s produced are rapidly used for the phosphorylation of certain compounds found in plants. d. A large number of ATP molecules are used in the detoxification of xenobiotic compounds produced during cellular respiration. CO2 + H2 + NADH → CH4 + H |
2 O + NAD+ a. Anaerobic respiration, because the final electron acceptor is inorganic. b. Aerobic respiration, because oxygen is the final electron acceptor. c. Anaerobic respiration, because NADH donates its electrons to a methane molecule. d. Aerobic respiration, because water is being produced as a product. 38. Would you describe metabolic pathways as inherently wasteful or inherently economical, and why? 35. Which of the following best describes complex IV in the electron transport chain? a. Complex IV consists of an oxygen molecule held between the cytochrome and copper ions. The electrons flowing finally reach the oxygen, producing water. b. Complex IV contains a molecule of flavin mononucleotide and iron-sulfur clusters. The electrons from NADH are transported here to coenzyme Q. c. Complex IV contains cytochrome b, c, and Fe-S. Here, the proton motive Q cycle takes place. d. Complex IV contains a membrane-bound enzyme that accepts electrons from FADH2 to make FAD. This electron is then transferred to ubiquinone. 36. What is the primary difference between fermentation and anaerobic respiration? a. Fermentation uses only glycolysis and its final electron acceptor is an organic molecule, whereas anaerobic respiration uses glycolysis, TCA and the ETC but finally give electrons to an inorganic molecule. b. Fermentation uses glycolysis, TCA and ETC but finally gives electrons to an inorganic molecule, whereas anaerobic respiration uses only glycolysis and its final electron acceptor is an organic molecule. c. Fermentation uses glycolysis and its final electron acceptor is an inorganic molecule, whereas anaerobic respiration uses glycolysis, TCA and ETC but finally give electrons to an organic molecule. d. Fermentation uses glycolysis, TCA and ETC but finally gives electrons to an organic molecule, whereas anaerobic respiration uses only glycolysis and its final electron acceptor is an inorganic molecule. a. Metabolic pathways are economical due to feedback inhibition. Also, intermediates from one pathway can be utilized by other pathways. b. Metabolic pathways are wasteful as they perform uncoordinated catabolic and anabolic reactions that wastes some of the energy that is stored. c. Metabolic pathways are economical due to the presence of ana |
plerotic reactions that replenish the intermediates. d. Metabolic pathways are wasteful as most of the energy produced is utilized in maintaining the reduced environment of the cytosol. 39. What lipids are connected to glucose catabolism pathways and how are they connected? a. Cholesterol and triglycerides can be converted to glycerol-3-phosphate that continues through glycolysis. b. Glucagon and glycogen can be converted to 3-phosphoglyceraldehyde that is an intermediate of glycolysis. c. Chylomicrons and fatty acids get converted to 1,3-bisphosphoglycerate that continues in glycolysis, forming pyruvate. d. Sphingolipids and triglycerides form glucagon that can be fed into glycolysis. 40. How does citrate from the citric acid cycle affect glycolysis? a. Citrate and ATP are negative regulators of phosphofructokinase-1. b. Citrate and ATP are negative regulators of hexokinase. c. Citrate and ATP are positive regulators of phosphofructokinase-1. d. Citrate and ATP are positive regulators of hexokinase. 37. What type of cellular respiration is represented in the following equation, and why? 41. Why might negative feedback mechanisms be more common than positive feedback mechanisms in living cells? This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 7 | Cellular Respiration 323 a. Negative feedback mechanisms maintain homeostasis whereas positive feedback drives the system away from equilibrium. b. Positive feedback mechanisms maintain a balanced amount of substances whereas negative feedback restricts them. c. Negative feedback turns the system off, making it deficient of certain substances. Positive feedback balances out these deficits. d. Positive feedback brings substance amounts back to equilibrium while negative feedback produces excess amounts of the substance. TEST PREP FOR AP® COURSES 42. The table shows the amount of oxygen consumed (third column) by different animals (first column) at different temperatures. This type of apparatus measures the change in volume of air to detect the removal of oxygen. However, organisms produce carbon dioxide as they take in oxygen. To provide accurate measurements, what would you need to add to the setup? a. a substance that removes carbon dioxide gas b. a plant that will add oxygen to allow an animal to breathe c. a glucose |
reserve d. a substance that adds carbon dioxide gas 43. According to the data, the crickets at 25∘ C have greater oxygen consumption per gram of tissue than do the crickets at 10∘ C. This trend in oxygen consumption is the opposite of that in mice. The difference in trends in oxygen consumption among crickets and mice is due to what? a. b. c. d. their difference in size their mode of nutrition their difference in metabolic heat production their mode of ATP production 44. Where in a cell does glycolysis take place in both prokaryotes and eukaryotes? a. b. c. d. the cytosol the mitochondria the plasma membrane the nucleus 45. A new species of obligate anaerobe, a bacterium, has been found that lives in hot, acidic conditions. While other pathways may also be present, which metabolic pathway is the most likely to be present in this species? a. aerobic respiration b. the citric acid cycle c. oxidative phosphorylation d. glycolysis 46. What evidence provides the strongest support that glycolysis is an older and more conserved pathway than the citric acid cycle? 324 47. a. Glycolysis is the primitive pathway as it is found in all three domains. It also occurs in anaerobic conditions and in the cytosol. b. This pathway occurs in the cytosol, is found in all animals and plants, and does not require oxygen. c. Glycolysis takes place in anaerobic conditions, can metabolize cholesterol and fatty acids, and occurs even in methanogens. d. This pathway only occurs in the mitochondria. It is highly flexible because it is found in almost all organisms. 49. Chapter 7 | Cellular Respiration a. Cytochrome c would not pass electrons from complex III to complex IV. b. Ubiquinone would not pass electrons from complex III to complex IV. c. NADH would not be converted to NAD+ and the electron transport chain would stop. d. No protons would be pumped across the membrane. Where do the electrons moving along the membrane in the figure come from, and where do the electrons end up? a. The electrons are released by NADH and and finally accepted by oxygen to FADH 2 form water. b. The electrons are given off by water and finally accepted by NAD+ and FAD+ to produce the energy currencies NADH and FADH |
ed by ubiquinone that are, in turn, transferred from complex I to complex II. Water finally accepts the electrons. d. The electrons are given out by NADH and and are, in turn, finally accepted by FADH 2 H2 O. 50. Glucose catabolism pathways are sequential and lead to the production of ATP. What is the correct order of the pathways for the breakdown of a molecule of glucose as shown in the formula? C6 H12 O6 + O2 → CO2 + H2 O + energy What is Structure X in the graphic? a. b. the inner mitochondrial membrane the mitochondrial matrix c. a eukaryotic plasma membrane d. the cytosol 48. What would be the most direct result of blocking structure Z in the graphic? This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 7 | Cellular Respiration 325 a. oxidative phosphorylation → citric acid cycle → oxidation of pyruvate → glycolysis b. the oxidation of pyruvate → citric acid cycle → glycolysis → oxidative phosphorylation c. glycolysis → oxidation of pyruvate → citric acid cycle → oxidative phosphorylation d. citric acid cycle → glycolysis → oxidative phosphorylation → oxidation of pyruvate 51. Which of the following statements most directly supports the claim that different species of organisms use different metabolic strategies to meet their energy requirements for growth, reproduction, and homeostasis? a. During cold periods, pond-dwelling animals can increase the number of unsaturated fatty acids in their cell membranes while some plants make antifreeze proteins to prevent ice crystal formation in their tissues. b. Bacteria lack introns while many eukaryotic genes contain many of these intervening sequences. c. Carnivores have more teeth that are specialized for ripping food while herbivores have more teeth specialized for grinding food. d. Plants generally use starch molecules for storage while animals use glycogen and fats for storage. 52. Which of the following best describes how the citric acid cycle relates to glycolysis, oxidative phosphorylation, and chemiosmosis? a. Glycolysis produces pyruvate, which is, which donate electrons to the electron converted to acetyl-CoA and enters the citric acid cycle. This cycle produces NADH and FAD |
H 2 transport chain to pump protons and produce ATP through chemiosmosis. Production of ATP using an electron transport chain and chemiosmosis is called oxidative phosphorylation. b. The citric acid produces pyruvate, which converts to glucose to enter glycolysis. This pathway produces NADH and FADH 2 enter oxidative phosphorylation to produce ATP through chemiosmosis., which c. Citric acid produces NADH and FADH 2, which undergo oxidative phosphorylation. This produces ATP by pumping protons through chemiosmosis. The ATP produced is utilized in large amount in the process of glycolysis. d. Glycolysis produces pyruvate, which directly enters the citric acid cycle. This cycle produces the energy currency that undergoes the electron transport chain to produce water and ATP. SCIENCE PRACTICE CHALLENGE QUESTIONS 53. Combustion of carbohydrates, like in a fireplace, is a reduction-oxidation reaction in which the carbon atom is oxidized and the oxygen atom is reduced, producing water and carbon dioxide. Oxidative phosphorylation and glycolysis are also reduction-oxidation reactions that produce the same products. Explain the differences and similarities among these abiotic and biotic processes in terms of the changes in entropy and heat that contribute to the free energy extracted from chemical bonds, the spontaneity of each, and the role of catalysis. 54. A. [Extension] Living systems require free energy to carry out cellular functions, and employ various strategies to capture, use, and store free energy. Explain the advantage that the higher energy efficiency per kg of the Krebs cycle provides to you compared to a metabolism based on glycolysis alone. Your explanation should make use of all the following facts: • ΔG for glycolysis is -135kJ per mole of glucose • ΔG for aerobic respiration is -2880kJ per mole glucose • • the basal metabolic rate of mammals is often represented as -300kJ/day • m0.75 the molar mass of glucose is 180 g/mole B. Explain the bioenergetic difference between aerobic and anaerobic respiration in terms of the difference between free-energy production and power. Your explanation should make use of all the following facts: • power is the rate of free-energy production • cancer cells derive most of their free energy from glycolysis • enzymes of the cit |
ric acid (Kreb’s) cycle form coordinate complexes on the cytoskeleton within the mitochondria C. The life cycle of the human parasite Trypanosoma brucei is divided between the body of the tsetse fly and the human blood stream. The parasite causes “sleeping sickness” in Sub-Saharan Africa. Within the human bloodstream, the parasite depends on glycolysis, with enzymes compartmentalized in a membrane-bound organelle called the glycosome. In the insect host, the parasite utilizes glycolysis as well as substrate-level and oxidative phosphorylation. Explain the advantage of a life cycle in the human host that employs anaerobic respiration with a rate of free-energy production that is enhanced by compartmentalization in the glycosome and a life cycle in the insect host that is aerobic. D. Predict the advantages of a biological system that uses both glycolysis and oxidative phosphorylation. Your 326 Chapter 7 | Cellular Respiration prediction should make use of all the following facts: • • signaling can be used to detect low-oxygen environments and to regulate response some cells, such as muscle and blood cells, must function in both low- and high-oxygen environments • glycolysis is reversible • • the citric acid cycle is not reversible thermoregulation is needed for homeostasis 55. Dinitrophenol (DNP) was used in the manufacture of munitions in World War I. In the 1930s, it was used as a weight loss drug. Use in the U.S. cannot be regulated by the FDA because DNP is considered a dietary supplement. Attempts to ban the drug in the U.K. following the death of four users in 2015 failed in Parliament. DNP is a small molecule that is soluble in the mitochondrial inner membrane. The hydroxyl group reversibly dissociates a proton. Figure 7.21 A. Predict the effect of DNP on the electrochemical gradient across the inner mitochondrial membrane. B. Explain how DNP can be used to reduce weight. C. The effects of DNP can be reversed by administering glucose. However, treatment with a combination of glucose and 2-deoxyglucose, which is an inhibitor of glycolysis, does not reverse the effects of DNP. Explain, in terms of the products of glycolysis, why this reversal of the effects of DNP was unexpected. (Hint: It |
might be useful to review the reactants and products of glycolysis.) D. Obesity correlates with an epidemic of other health issues, such as elevated blood pressure, heart disease, and diabetes II. A slow-release form of DNP (CRMP) is patented. With slow-release technology, a drug can be delivered in small doses over time from a pill whose matrix limits solubility. A simple but nonscientific question that can be raised is: Will a slow-release drug retard progress toward behavioral changes that can reduce the magnitude of this epidemic? Scientific questions can be pursued by testing the outcomes predicted by possible answers. Refine this question for discussion in small groups. Be prepared to justify the merits of your question. 56. As shown in Figure 7.11, cyanide inhibits the electron transport chain by competing with O2 molecules for the cytochrome c oxidase heme group. Carbon monoxide (CO) has a similar effect. Both cyanide and carbon monoxide cause poisoning in victims of smoke inhalation. This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 A. Predict the effects of these poisons on the following properties of mitochondria just after exposure: the pH of the intermembrane space, the concentration of NADH, and the rate of production of ATP in the matrix. Justify your predictions. B. Rotenone is a poison that blocks the transfer of electrons from Complex I of the electron transport chain to ubiquinone. Methylene blue is a molecule with many uses involving its reduction-oxidation properties. Recent studies show the effectiveness of methylene blue in increasing the body’s metabolic rate and as a treatment for Alzheimer’s patients. The oxidized form of methylene blue is reduced by NADH, and its reduced form is oxidized by O2. Explain the use of methylene blue as an antidote for rotenone poisoning. 57. E. coli are enteric (gut-dwelling) facultative anaerobic bacteria. (Facultative anaerobes can grow either with or without free oxygen. Obligatory anaerobes grow only in the absence of free oxygen.) Researchers planned to grow cultures of E. coli under a range of conditions to model the transition from strictly anaerobic to aerobic respiration. The oxygen content of atmospheres at constant total pressure will be controlled by volumes of nitrogen and oxygen gases. Ratios |
of volume, r = VO2/VN2 between 0 and 0.25 of shaken growth flasks can be measured in terms of optical density, which is the percent of transmission of light through a sample of the growing E. coli culture. A rule of thumb is that the range of strict anaerobes is when r < 0.01, and the boundary for aerobic respiration is when r = 0.05. A large number of flasks that can be constantly shaken at fixed temperature, and from which samples can be taken without atmospheric contamination, are available for this study. These results of the experiment will be used to infer growth rates of E. coli along the entire 7.5 m length of the average human intestine (small intestine and large intestine), where the oxygen content varies from atmospheric to anaerobic conditions. The retention time of food in the small intestine, whose average length is 2.5 m, is approximately four hours. The retention time of food over the entire length of the intestine is between 24 and 72 hours. A. Describe and apply a mathematical model that can be used to represent the variation of oxygen environments of a bacterium that is being transported with the food along the length of the intestine. B. Design the experimental sampling times in terms of growth intervals of interest in this study: i) the time when the bacteria is passing the small-large intestine boundary; ii) the time when the bacteria reaches the end of the large intestine; and iii) the time when the bacterium reaches facultative anaerobic conditions, r < 0.05. C. Sketch a graph that predicts the distribution of aerobic, facultative anaerobic and obligatory anaerobic bacteria along the length of the entire intestine based on these parameters. Keep in mind that anaerobes have a lower Chapter 7 | Cellular Respiration 327 respiration rate. 58. White snakeroot is a plant that contains chemicals that deactivate the enzyme lactate dehydrogenase. Humans who consume milk from cows or goats that eat white snakeroot can become ill. Symptoms of milk poisoning include vomiting, abdominal pain, and tremors, which become worse after exercise. Beyond childhood, most people do not express the enzyme lactase that catalyzes the breakdown of lactose into glucose and galactose. Consumption of milk can produce symptoms similar to those of milk poisoning. After a period of consumption of dairy foods, though, prebiotic adaptation (changes in the microbes in the intestine) imparts lactose tolerance. |
Since dairy foods are a valuable source of calcium, proteins, and vitamin D, considerable research has been conducted to characterize adaptation. Explain the similarities and differences between the effect of milk poisoning by white snakeroot and lactose intolerance, and the possibility of prebiotic adaptation for each. 328 Chapter 7 | Cellular Respiration This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 8 | Photosynthesis 329 8 | PHOTOSYNTHESIS Figure 8.1 This world map shows Earth’s distribution of photosynthesis as seen via chlorophyll a concentrations. On land, this is evident via terrestrial plants, and in oceanic zones, via phytoplankton. (credit: modification of work by SeaWiFS Project, NASA/Goddard Space Flight Center and ORBIMAGE) Chapter Outline 8.1: Overview of Photosynthesis 8.2: The Light-Dependent Reaction of Photosynthesis 8.3: Using Light to Make Organic Molecules Introduction All biological processes require energy. To get this energy, many organisms access stored energy by eating, that is, by ingesting other organisms. But where does the stored energy in food originate? Almost all of this energy can be traced back to photosynthesis. Photosynthetic organisms are the basis for almost all of the food webs on the planet. For example, the Indian River Lagoon, a 156 mile mixture of fresh and salt water along the eastern coast of Florida, depends on its sea grass for the survival of its marine life. Unfortunately, when certain algal phytoplankton species grow in overabundance, it destroys the sea grass. Scientists conducted a 16 year study of algal blooms and found that extreme climate conditions, such as cold weather and low rainfall, change which particular species of phytoplankton is more likely to bloom, resulting in a die-off of sea grass, decrease in other marine life, and changes in salinity. The research study can be found here (http://openstaxcollege.org/l/ 32algae). 330 Chapter 8 | Photosynthesis 8.1 | Overview of Photosynthesis In this section, you will explore the following questions: • What is the relevance of photosynthesis to living organisms? • What are the main cellular structures involved in photosynthesis? • What are the substrates and products of photosynthesis? Connection for AP® Courses As we learned in Chapter 7 |
, all living organisms, from simple bacteria to complex plants and animals, require free energy to carry out cellular processes, such as growth and reproduction. Organisms use various strategies to capture, store, transform, and transfer free energy, including photosynthesis. Photosynthesis allows organisms to access enormous amounts of free energy from the sun and transform it to the chemical energy of sugars. Although all organisms carry out some form of cellular respiration, only certain organisms, called photoautotrophs, can perform photosynthesis. Examples of photoautotrophs include plants, algae, some unicellular eukaryotes, and cyanobacteria. They require the presence of chlorophyll, a specialized pigment that absorbs certain wavelengths of the visible light spectrum to harness free energy from the sun. Photosynthesis is a process where components of water and carbon dioxide are used to assemble carbohydrate molecules and where oxygen waste products are released into the atmosphere. In eukaryotes, the reactions of photosynthesis occur in chloroplasts; in prokaryotes, such as cyanobacteria, the reactions are less localized and occur within membranes and in the cytoplasm. (The structural features of the chloroplast that participate in photosynthesis will be explored in more detail later in The Light-Dependent Reactions of Photosynthesis and Using Light Energy to Make Organic Molecules.) Although photosynthesis and cellular respiration evolved as independent processes—with photosynthesis creating an oxidizing atmosphere early in Earth’s history—today they are interdependent. As we studied in Cellular Respiration, aerobic cellular respiration taps into the oxidizing ability of oxygen to synthesize the organic compounds that are used to power cellular processes. Information presented and the examples highlighted in the section support concepts and learning objectives outlined in Big Idea 1 and Big Idea 2 of the AP® Biology Curriculum Framework, as shown in the table. The learning objectives listed in the Curriculum Framework provide a transparent foundation for the AP® Biology course, an inquiry-based laboratory experience, instructional activities, and AP® exam questions. A learning objective merges required content with one or more of the seven science practices. Big Idea 1 The process of evolution drives the diversity and unity of life. Enduring Understanding 1.B Organisms are linked by lines of descent from common ancestry. Essential Knowledge 1.B.1 Structural and functional evidence supports the relatedness of all domains, with organisms shared many conserved core processes. Science Practice Learning Objective Big Idea 2 Enduring Understanding 2.A 6 |
.1 The student can justify claims with evidence. 1.15 The student is able to describe specific examples of conserved core biological processes and features shared by all domains s or within one domain of life, and how these shared, conserved core processes and features support the concept of common ancestry for all organisms. Biological systems utilize free energy and molecular building blocks to grow, to reproduce, and to maintain dynamic homeostasis. Growth, reproduction and maintenance of living systems require free energy and matter. This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 8 | Photosynthesis 331 Essential Knowledge 2.A.2 Organisms use various strategies to capture and store free energy for use in biological processes. Science Practice Science Practice Learning Objective 1.4 The student can use representations and models to analyze situations or solve problems qualitatively and quantitatively. 3.1 The student can pose scientific questions. 2.4 The student is able to use representations to pose scientific questions about what mechanisms and structural features allow organisms to capture, store, and use free energy. Essential Knowledge 2.A.2 Organisms use various strategies to capture and store free energy for use in biological processes. Science Practice Learning Objective 6.2 The student can construct explanations of phenomena based on evidence produced through scientific practices. 2.5 The student is able to construct explanations of the mechanisms and structural features of cells that allow organisms to capture, store, or use free energy. Importance of Photosynthesis Photosynthesis is essential to all life on earth; both plants and animals depend on it. It is the only biological process that can capture energy that originates in outer space (sunlight) and convert it into chemical compounds (carbohydrates) that every organism uses to power its metabolism. In brief, the energy of sunlight is captured and used to energize electrons, whose energy is then stored in the covalent bonds of sugar molecules. How long lasting and stable are those covalent bonds? The energy extracted today by the burning of coal and petroleum products represents sunlight energy captured and stored by photosynthesis almost 200 million years ago. Plants, algae, and a group of bacteria called cyanobacteria are the only organisms capable of performing photosynthesis (Figure 8.2). Because they use light to manufacture their own food, they are called photoautotrophs (literally, “self-feeders using light”). Other organisms, such as animals, fungi, and most other bacteria, are |
termed heterotrophs (“other feeders”), because they must rely on the sugars produced by photosynthetic organisms for their energy needs. A third very interesting group of bacteria synthesize sugars, not by using sunlight’s energy, but by extracting energy from inorganic chemical compounds; hence, they are referred to as chemoautotrophs. 332 Chapter 8 | Photosynthesis Figure 8.2 Photoautotrophs including (a) plants, (b) algae, and (c) cyanobacteria synthesize their organic compounds via photosynthesis using sunlight as an energy source. Cyanobacteria and planktonic algae can grow over enormous areas in water, at times completely covering the surface. In a (d) deep sea vent, chemoautotrophs, such as these (e) thermophilic bacteria, capture energy from inorganic compounds to produce organic compounds. The ecosystem surrounding the vents has a diverse array of animals, such as tubeworms, crustaceans, and octopi that derive energy from the bacteria. (credit a: modification of work by Steve Hillebrand, U.S. Fish and Wildlife Service; credit b: modification of work by "eutrophication&hypoxia"/Flickr; credit c: modification of work by NASA; credit d: University of Washington, NOAA; credit e: modification of work by Mark Amend, West Coast and Polar Regions Undersea Research Center, UAF, NOAA) The importance of photosynthesis is not just that it can capture sunlight’s energy. A lizard sunning itself on a cold day can use the sun’s energy to warm up. Photosynthesis is vital because it evolved as a way to store the energy in solar radiation (the “photo-” part) as energy in the carbon-carbon bonds of carbohydrate molecules (the “-synthesis” part). Those carbohydrates are the energy source that heterotrophs use to power the synthesis of ATP via respiration. Therefore, photosynthesis powers 99 percent of Earth’s ecosystems. When a top predator, such as a wolf, preys on a deer (Figure 8.3), the wolf is at the end of an energy path that went from nuclear reactions on the surface of the sun, to light, to photosynthesis, to vegetation, to deer, and finally to wolf. Figure 8.3 The energy stored in carbohydrate molecules from photosynthesis passes through the food chain. The predator |
that eats these deer receives a portion of the energy that originated in the photosynthetic vegetation that the deer consumed. (credit: modification of work by Steve VanRiper, U.S. Fish and Wildlife Service) This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 8 | Photosynthesis 333 Think About It • Why do scientists think that photosynthesis evolved before aerobic cellular respiration? • Why do carnivores, such as lions, depend on photosynthesis to survive? What evidence supports the claim that photosynthesis and cellular respiration are interdependent processes? Main Structures and Summary of Photosynthesis Photosynthesis is a multi-step process that requires sunlight, carbon dioxide (which is low in energy), and water as substrates (Figure 8.4). After the process is complete, it releases oxygen and produces glyceraldehyde-3-phosphate (GA3P), simple carbohydrate molecules (which are high in energy) that can subsequently be converted into glucose, sucrose, or any of dozens of other sugar molecules. These sugar molecules contain energy and the energized carbon that all living things need to survive. Figure 8.4 Photosynthesis uses solar energy, carbon dioxide, and water to produce energy-storing carbohydrates. Oxygen is generated as a waste product of photosynthesis. The following is the chemical equation for photosynthesis (Figure 8.5): Figure 8.5 The basic equation for photosynthesis is deceptively simple. In reality, the process takes place in many steps involving intermediate reactants and products. Glucose, the primary energy source in cells, is made from two three-carbon GA3Ps. Although the equation looks simple, the many steps that take place during photosynthesis are actually quite complex. Before 334 Chapter 8 | Photosynthesis learning the details of how photoautotrophs turn sunlight into food, it is important to become familiar with the structures involved. In plants, photosynthesis generally takes place in leaves, which consist of several layers of cells. The process of photosynthesis occurs in a middle layer called the mesophyll. The gas exchange of carbon dioxide and oxygen occurs through small, regulated openings called stomata (singular: stoma), which also play roles in the regulation of gas exchange and water balance. The stomata are typically located on the underside of the leaf, which helps to minimize water loss. Each stoma is flanked by guard cells that regulate the opening and closing of |
the stomata by swelling or shrinking in response to osmotic changes. In all autotrophic eukaryotes, photosynthesis takes place inside an organelle called a chloroplast. For plants, chloroplastcontaining cells exist in the mesophyll. Chloroplasts have a double membrane envelope (composed of an outer membrane and an inner membrane). Within the chloroplast are stacked, disc-shaped structures called thylakoids. Embedded in the thylakoid membrane is chlorophyll, a pigment (molecule that absorbs light) responsible for the initial interaction between light and plant material, and numerous proteins that make up the electron transport chain. The thylakoid membrane encloses an internal space called the thylakoid lumen. As shown in Figure 8.6, a stack of thylakoids is called a granum, and the liquid-filled space surrounding the granum is called stroma or “bed” (not to be confused with stoma or “mouth,” an opening on the leaf epidermis). Figure 8.6 Photosynthesis takes place in chloroplasts, which have an outer membrane and an inner membrane. Stacks of thylakoids called grana form a third membrane layer. On a hot, dry day, plants close their stomata to conserve water. What impact will this have on photosynthesis? a. Rate of photosynthesis will be inhibited as the level of carbon dioxide decreases. b. Rate of photosynthesis will be inhibited as the level of oxygen decreases. c. The rate of photosynthesis will increase as the level of carbon dioxide increases. d. Rate of photosynthesis will increase as the level of oxygen increases. The Two Parts of Photosynthesis Photosynthesis takes place in two sequential stages: the light-dependent reactions and the light independent-reactions. In the light-dependent reactions, energy from sunlight is absorbed by chlorophyll and that energy is converted into stored chemical energy. In the light-independent reactions, the chemical energy harvested during the light-dependent reactions drives the assembly of sugar molecules from carbon dioxide. Therefore, although the light-independent reactions do not use light as a reactant, they require the products of the light-dependent reactions to function. In addition, several enzymes of the light-independent reactions are activated by light. The light-dependent reactions utilize certain molecules to temporarily store the energy: These are referred to as energy carriers. The energy carriers that move energy from |
light-dependent reactions to light-independent reactions can be thought of as “full” because they are rich in energy. After the energy is This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 8 | Photosynthesis 335 released, the “empty” energy carriers return to the light-dependent reaction to obtain more energy. Figure 8.7 illustrates the components inside the chloroplast where the light-dependent and light-independent reactions take place. Figure 8.7 Photosynthesis takes place in two stages: light dependent reactions and the Calvin cycle. Light-dependent reactions, which take place in the thylakoid membrane, use light energy to make ATP and NADPH. The Calvin cycle, which takes place in the stroma, uses energy derived from these compounds to make GA3P from CO2. Click the link (http://openstaxcollege.org/l/photosynthesis) to learn more about photosynthesis. Explain how the light reactions and light independent reactions (Calvin cycle) of photosynthesis are interdependent on each other. a. The light reactions produces ATP and NADPH, which are then used in the Calvin cycle. b. The light reactions produces NADP+ and ADP, which are then used in the Calvin cycle. c. The light reactions uses NADPH and ATP, which are produced by the Calvin cycle. d. The light reactions produce only NADPH, which is produced by the Calvin cycle. 336 Chapter 8 | Photosynthesis Photosynthesis at the Grocery Store Figure 8.8 Foods that humans consume originate from photosynthesis. (credit: Associação Brasileira de Supermercados) Major grocery stores in the United States are organized into departments, such as dairy, meats, produce, bread, cereals, and so forth. Each aisle (Figure 8.8) contains hundreds, if not thousands, of different products for customers to buy and consume. Although there is a large variety, each item links back to photosynthesis. Meats and dairy link, because the animals were fed plant-based foods. The breads, cereals, and pastas come largely from starchy grains, which are the seeds of photosynthesis-dependent plants. What about desserts and drinks? All of these products contain sugar—sucrose is a plant product, a disaccharide, a carbohydrate molecule, which is built directly from photosynthesis. Moreover, many items are |
less obviously derived from plants: For instance, paper goods are generally plant products, and many plastics (abundant as products and packaging) are derived from algae. Virtually every spice and flavoring in the spice aisle was produced by a plant as a leaf, root, bark, flower, fruit, or stem. Ultimately, photosynthesis connects to every meal and every food a person consumes. Where would photosynthetic organisms likely be placed on a food web within most ecosystems? a. at the base b. near the top c. d. in the middle, but generally closer to the top in the middle, but generally closer to the base 8.2 | The Light-Dependent Reaction of Photosynthesis In this section, you will explore the following questions: • How do plants absorb energy from sunlight? • What are the differences between short and long wavelengths of light? What wavelengths are used in photosynthesis? • How and where does photosynthesis occur within a plant? This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 8 | Photosynthesis 337 Connection for AP® Courses Photosynthesis consists of two stages: the light-dependent reactions and the light-independent reactions or Calvin cycle. The light-dependent reactions occur when light is available. The overall equation for photosynthesis shows that is it a redox reaction; carbon dioxide is reduced and water is oxidized to produce oxygen: Energy + 6CO2 + H2 O → C6 H12 O6 + 6O2 The light-dependent reactions occur in the thylakoid membranes of chloroplasts, whereas the Calvin cycle occurs in the stroma of chloroplasts. Embedded in the thylakoid membranes are two photosystems (PS I and PS II), which are complexes of pigments that capture solar energy. Chlorophylls a and b absorb violet, blue, and red wavelengths from the visible light spectrum and reflect green. The carotenoid pigments absorb violet-blue-green light and reflect yellow-to-orange light. Environmental factors such as day length and temperature influence which pigments predominant at certain times of the year. Although the two photosystems run simultaneously, it is easier to explore them separately. Let’s begin with photosystem II. A photon of light strikes the antenna pigments of PS II to initiate photosynthesis. In the noncyclic pathway, PS II captures photons at a slightly higher energy level than PS |
I. (Remember that shorter wavelengths of light carry more energy.) The absorbed energy travels to the reaction center of the antenna pigment that contains chlorophyll a and boosts chlorophyll a electrons to a higher energy level. The electrons are accepted by a primary electron acceptor protein and then pass to the electron transport chain also embedded in the thylakoid membrane. The energy absorbed in PS II is enough to oxidize (split) water, releasing oxygen into the atmosphere; the electrons released from the oxidation of water replace the electrons that were boosted from the reaction center chlorophyll. As the electrons from the reaction center chlorophyll pass through the series of electron carrier proteins, hydrogen ions (H+) are pumped across the membrane via chemiosmosis into the interior of the thylakoid. (If this sounds familiar, it should. We studied chemiosmosis in our exploration of cellular respiration in Cellular Respiration.) This action builds up a high concentration of H+ ions, and as they flow through ATP synthase, molecules of ATP are formed. These molecules of ATP will be used to provide free energy for the synthesis of carbohydrate in the Calvin cycle, the second stage of photosynthesis. The electron transport chain connects PS II and PS I. Similar to the events occurring in PS II, this second photosystem absorbs a second photon of light, resulting in the formation of a molecule of NADPH from NADP+. The energy carried in NADPH also is used to power the chemical reactions of the Calvin cycle. Information presented and the examples highlighted in the section support concepts and learning objectives outlined in Big Idea 2 of the AP® Biology Curriculum Framework, as shown in the table. The learning objectives listed in the Curriculum Framework provide a transparent foundation for the AP® Biology course, an inquiry-based laboratory experience, instructional activities, and AP® exam questions. A learning objective merges required content with one or more of the seven science practices. Big Idea 2 Enduring Understanding 2.A Biological systems utilize free energy and molecular building blocks to grow, to reproduce, and to maintain dynamic homeostasis. Growth, reproduction and maintenance of living systems require free energy and matter. Essential Knowledge 2.A.2 The light-independent reactions of photosynthesis in eukaryotes involve a series of reactions that capture free energy present in light. Science Practice Science Practice Learning Objective 1.4 The student can use representations and models to analyze situations or solve problems qualitatively and quantitatively. 3.1 The student can pose scientific |
questions. 2.4 The student is able to use representations to pose scientific questions about what mechanisms and structural features allow organisms to capture, store, and use free energy. Essential Knowledge 2.A.2 The light-independent reactions of photosynthesis in eukaryotes involve a series of reactions that capture free energy present in light. Science Practice 6.2 The student can construct explanations of phenomena based on evidence produced through scientific practices. 338 Chapter 8 | Photosynthesis Learning Objective 2.5 The student is able to construct explanations of the mechanisms and structural features of cells that allow organisms to capture, store, or use free energy. Big Idea 4 Enduring Understanding 4.A Essential Knowledge Science Practice Learning Objective Essential Knowledge Science Practice Learning Objective Essential Knowledge Science Practice Learning Objective Biological systems interact, and these systems and their interactions possess complex properties. Interactions within biological systems lead to complex properties. 4.A.2 Chloroplasts are specialized organelles that capture energy through photosynthesis. 6.4 The student can make claims and predictions about natural phenomena based on scientific theories and models. 4.4 The student is able to make a prediction about the interactions of subcellular organelles. 4.A.2 Chloroplasts are specialized organelles that capture energy through photosynthesis. 6.2 The student can construct explanations of phenomena based on evidence produced through scientific practices. 4.5 The student is able to construct explanations based on scientific evidence as to how interactions of subcellular structures provide essential functions. 4.A.2 Chloroplasts are specialized organelles that capture energy through photosynthesis. 1.4 The student can use representations and models to analyze situations or solve problems qualitatively and quantitatively. 4.6 The student is able to use representations and models to analyze situations qualitatively to describe how interactions of subcellular structures, which possess specialized functions, provide essential functions. The Science Practice Challenge Questions contain additional test questions for this section that will help you prepare for the AP exam. These questions address the following standards: [APLO 2.5][APLO 2.16][APLO 2.18][APLO 1.9][APLO 1.32][APLO 4.14][APLO 2.2][APLO 2.3][APLO 2.23][APLO 1.15][APLO 1.29] How can light be used to make food? When a person turns on a lamp, electrical energy becomes light energy. Like all other forms of kinetic energy |
, light can travel, change form, and be harnessed to do work. In the case of photosynthesis, light energy is converted into chemical energy, which photoautotrophs use to build carbohydrate molecules (Figure 8.9). However, autotrophs only use a few specific components of sunlight. This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 8 | Photosynthesis 339 Figure 8.9 Photoautotrophs can capture light energy from the sun, converting it into the chemical energy used to build food molecules. (credit: Gerry Atwell) What Is Light Energy? The sun emits an enormous amount of electromagnetic radiation (solar energy). Humans can see only a fraction of this energy, which portion is therefore referred to as “visible light.” The manner in which solar energy travels is described as waves. Scientists can determine the amount of energy of a wave by measuring its wavelength, the distance between consecutive points of a wave. A single wave is measured from two consecutive points, such as from crest to crest or from trough to trough (Figure 8.10). Figure 8.10 The wavelength of a single wave is the distance between two consecutive points of similar position (two crests or two troughs) along the wave. Visible light constitutes only one of many types of electromagnetic radiation emitted from the sun and other stars. Scientists differentiate the various types of radiant energy from the sun within the electromagnetic spectrum. The electromagnetic spectrum is the range of all possible frequencies of radiation (Figure 8.11). The difference between wavelengths relates to the amount of energy carried by them. 340 Chapter 8 | Photosynthesis Figure 8.11 The sun emits energy in the form of electromagnetic radiation. This radiation exists at different wavelengths, each of which has its own characteristic energy. All electromagnetic radiation, including visible light, is characterized by its wavelength. Each type of electromagnetic radiation travels at a particular wavelength. The longer the wavelength (or the more stretched out it appears in the diagram), the less energy is carried. Short, tight waves carry the most energy. This may seem illogical, but think of it in terms of a piece of moving a heavy rope. It takes little effort by a person to move a rope in long, wide waves. To make a rope move in short, tight waves, a person would need to apply significantly more energy. The electromagnetic spectrum (Figure 8.11) shows several types of electromagnetic radiation originating from the sun, |
including X-rays and ultraviolet (UV) rays. The higher-energy waves can penetrate tissues and damage cells and DNA, explaining why both X-rays and UV rays can be harmful to living organisms. Absorption of Light Light energy initiates the process of photosynthesis when pigments absorb the light. Organic pigments, whether in the human retina or the chloroplast thylakoid, have a narrow range of energy levels that they can absorb. Energy levels lower than those represented by red light are insufficient to raise an orbital electron to a populatable, excited (quantum) state. Energy levels higher than those in blue light will physically tear the molecules apart, called bleaching. So retinal pigments can only “see” (absorb) 700 nm to 400 nm light, which is therefore called visible light. For the same reasons, plants pigment molecules absorb only light in the wavelength range of 700 nm to 400 nm; plant physiologists refer to this range for plants as photosynthetically active radiation. The visible light seen by humans as white light actually exists in a rainbow of colors. Certain objects, such as a prism or a drop of water, disperse white light to reveal the colors to the human eye. The visible light portion of the electromagnetic spectrum shows the rainbow of colors, with violet and blue having shorter wavelengths, and therefore higher energy. At the other end of the spectrum toward red, the wavelengths are longer and have lower energy (Figure 8.12). This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 8 | Photosynthesis 341 Figure 8.12 The colors of visible light do not carry the same amount of energy. Violet has the shortest wavelength and therefore carries the most energy, whereas red has the longest wavelength and carries the least amount of energy. (credit: modification of work by NASA) Understanding Pigments Different kinds of pigments exist, and each absorbs only certain wavelengths (colors) of visible light. Pigments reflect or transmit the wavelengths they cannot absorb, making them appear in the corresponding color. Chlorophylls and carotenoids are the two major classes of photosynthetic pigments found in plants and algae; each class has multiple types of pigment molecules. There are five major chlorophylls: a, b, c and d and a related molecule found in prokaryotes called bacteriochlorophyll. Chlorophyll a and chlorophyll b are |
found in higher plant chloroplasts and will be the focus of the following discussion. With dozens of different forms, carotenoids are a much larger group of pigments. The carotenoids found in fruit—such as the red of tomato (lycopene), the yellow of corn seeds (zeaxanthin), or the orange of an orange peel (β-carotene)—are used as advertisements to attract seed dispersers. In photosynthesis, carotenoids function as photosynthetic pigments that are very efficient molecules for the disposal of excess energy. When a leaf is exposed to full sun, the light-dependent reactions are required to process an enormous amount of energy; if that energy is not handled properly, it can do significant damage. Therefore, many carotenoids reside in the thylakoid membrane, absorb excess energy, and safely dissipate that energy as heat. Each type of pigment can be identified by the specific pattern of wavelengths it absorbs from visible light, which is the absorption spectrum. The graph in Figure 8.13 shows the absorption spectra for chlorophyll a, chlorophyll b, and a type of carotenoid pigment called β-carotene (which absorbs blue and green light). Notice how each pigment has a distinct set of peaks and troughs, revealing a highly specific pattern of absorption. Chlorophyll a absorbs wavelengths from either end of the visible spectrum (blue and red), but not green. Because green is reflected or transmitted, chlorophyll appears green. Carotenoids absorb in the short-wavelength blue region, and reflect the longer yellow, red, and orange wavelengths. 342 Chapter 8 | Photosynthesis Figure 8.13 (a) Chlorophyll a, (b) chlorophyll b, and (c) β-carotene are hydrophobic organic pigments found in the thylakoid membrane. Chlorophyll a and b, which are identical except for the part indicated in the red box, are responsible for the green color of leaves. β-carotene is responsible for the orange color in carrots. Each pigment has (d) a unique absorbance spectrum. Many photosynthetic organisms have a mixture of pigments; using them, the organism can absorb energy from a wider range of wavelengths. Not all photosynthetic organisms have full access to sunlight. Some organisms grow underwater where light intensity and quality decrease and change with depth. Other organisms grow in competition for light. Plants |
on the rainforest floor must be able to absorb any bit of light that comes through, because the taller trees absorb most of the sunlight and scatter the remaining solar radiation (Figure 8.14). Figure 8.14 Plants that commonly grow in the shade have adapted to low levels of light by changing the relative concentrations of their chlorophyll pigments. (credit: Jason Hollinger) When studying a photosynthetic organism, scientists can determine the types of pigments present by generating absorption spectra. An instrument called a spectrophotometer can differentiate which wavelengths of light a substance can absorb. Spectrophotometers measure transmitted light and compute from it the absorption. By extracting pigments from leaves and placing these samples into a spectrophotometer, scientists can identify which wavelengths of light an organism can absorb. Additional methods for the identification of plant pigments include various types of chromatography that separate the pigments by their relative affinities to solid and mobile phases. This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 8 | Photosynthesis 343 How Light-Dependent Reactions Work The overall function of light-dependent reactions is to convert solar energy into chemical energy in the form of NADPH and ATP. This chemical energy supports the light-independent reactions and fuels the assembly of sugar molecules. The light-dependent reactions are depicted in Figure 8.15. Protein complexes and pigment molecules work together to produce NADPH and ATP. Figure 8.15 A photosystem consists of a light-harvesting complex and a reaction center. Pigments in the lightharvesting complex pass light energy to two special chlorophyll a molecules in the reaction center. The light excites an electron from the chlorophyll a pair, which passes to the primary electron acceptor. The excited electron must then be replaced. In (a) photosystem II, the electron comes from the splitting of water, which releases oxygen as a waste product. In (b) photosystem I, the electron comes from the chloroplast electron transport chain discussed below. The actual step that converts light energy into chemical energy takes place in a multiprotein complex called a photosystem, two types of which are found embedded in the thylakoid membrane, photosystem II (PSII) and photosystem I (PSI) (Figure 8.16). The two complexes differ on the basis of what they oxidize (that is, the source of the low-energy electron |
supply) and what they reduce (the place to which they deliver their energized electrons). Both photosystems have the same basic structure; a number of antenna proteins to which the chlorophyll molecules are bound surround the reaction center where the photochemistry takes place. Each photosystem is serviced by the lightharvesting complex, which passes energy from sunlight to the reaction center; it consists of multiple antenna proteins that contain a mixture of 300–400 chlorophyll a and b molecules as well as other pigments like carotenoids. The absorption of a single photon or distinct quantity or “packet” of light by any of the chlorophylls pushes that molecule into an excited state. In short, the light energy has now been captured by biological molecules but is not stored in any useful form yet. The energy is transferred from chlorophyll to chlorophyll until eventually (after about a millionth of a second), it is delivered to the reaction center. Up to this point, only energy has been transferred between molecules, not electrons. 344 Chapter 8 | Photosynthesis Figure 8.16 In the photosystem II (PSII) reaction center, energy from sunlight is used to extract electrons from water. The electrons travel through the chloroplast electron transport chain to photosystem I (PSI), which reduces NADP+ to NADPH. The electron transport chain moves protons across the thylakoid membrane into the lumen. At the same time, splitting of water adds protons to the lumen, and reduction of NADPH removes protons from the stroma. The net result is a low pH in the thylakoid lumen, and a high pH in the stroma. ATP synthase uses this electrochemical gradient to make ATP. What is the external source of the electrons that ultimately pass through photosynthetic electron transport chains? a. carbon dioxide b. NADPH c. oxygen d. water The reaction center contains a pair of chlorophyll a molecules with a special property. Those two chlorophylls can undergo oxidation upon excitation; they can actually give up an electron in a process called a photoact. It is at this step in the reaction center, this step in photosynthesis, that light energy is converted into an excited electron. All of the subsequent steps involve getting that electron onto the energy carrier NADPH for delivery to the Calvin cycle where the electron is deposited onto carbon for long-term storage in the form of a carbohydrate.PSII and PSI |
are two major components of the photosynthetic electron transport chain, which also includes the cytochrome complex. The cytochrome complex, an enzyme composed of two protein complexes, transfers the electrons from the carrier molecule plastoquinone (Pq) to the protein plastocyanin (Pc), thus enabling both the transfer of protons across the thylakoid membrane and the transfer of electrons from PSII to PSI. The reaction center of PSII (called P680) delivers its high-energy electrons, one at the time, to the primary electron acceptor, and through the electron transport chain (Pq to cytochrome complex to plastocyanine) to PSI. P680’s missing electron is replaced by extracting a low-energy electron from water; thus, water is split and PSII is re-reduced after every photoact. Splitting one H2O molecule releases two electrons, two hydrogen atoms, and one atom of oxygen. Splitting two molecules is required to form one molecule of diatomic O2 gas. About 10 percent of the oxygen is used by mitochondria in the leaf to support oxidative phosphorylation. The remainder escapes to the atmosphere where it is used by aerobic organisms to support respiration. This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 8 | Photosynthesis 345 As electrons move through the proteins that reside between PSII and PSI, they lose energy. That energy is used to move hydrogen atoms from the stromal side of the membrane to the thylakoid lumen. Those hydrogen atoms, plus the ones produced by splitting water, accumulate in the thylakoid lumen and will be used to synthesize ATP in a later step. Because the electrons have lost energy prior to their arrival at PSI, they must be re-energized by PSI, hence, another photon is absorbed by the PSI antenna. That energy is relayed to the PSI reaction center (called P700). P700 is oxidized and sends a high-energy electron to NADP+ to form NADPH. Thus, PSII captures the energy to create proton gradients to make ATP, and PSI captures the energy to reduce NADP+ into NADPH. The two photosystems work in concert, in part, to guarantee that the production of NADPH will roughly equal the production of ATP. Other mechanisms exist to |
fine tune that ratio to exactly match the chloroplast’s constantly changing energy needs. Generating an Energy Carrier: ATP As in the intermembrane space of the mitochondria during cellular respiration, the buildup of hydrogen ions inside the thylakoid lumen creates a concentration gradient. The passive diffusion of hydrogen ions from high concentration (in the thylakoid lumen) to low concentration (in the stroma) is harnessed to create ATP, just as in the electron transport chain of cellular respiration. The ions build up energy because of diffusion and because they all have the same electrical charge, repelling each other. To release this energy, hydrogen ions will rush through any opening, similar to water jetting through a hole in a dam. In the thylakoid, that opening is a passage through a specialized protein channel called the ATP synthase. The energy released by the hydrogen ion stream allows ATP synthase to attach a third phosphate group to ADP, which forms a molecule of ATP (Figure 8.16). The flow of hydrogen ions through ATP synthase is called chemiosmosis because the ions move from an area of high to an area of low concentration through a semi-permeable structure. Visit this site (http://openstaxcollege.org/l/light_reactions) and click through the animation to view the process of photosynthesis within a leaf. What role do electrons play in the formation of NADPH? a. Electrons from PS I cause the reduction of NADPH to NADP+. b. Electrons from PSII cause the reduction of NADP+ to NADPH. c. Electrons from PS I cause the reduction of NADP+ to NADPH. d. Electrons are gained which causes the oxidation of NADP+. 346 Chapter 8 | Photosynthesis Figure 8.17 The anatomy of a leaf. The cuticle and epidermis are the outer layers of the leaf and protect it from drying out. Chloroplasts are found in the mesophyll cells and are where photosynthesis occurs. Gas is exchanged through pores called stomata, which are opened and closed by the guard cells. Legend: 1) cuticle 2) upper epidermis 3) palisade mesophyll 4) spongy mesophyll 5) lower epidermis 6) stoma 7) guard cells 8) xylem 9) phloem 10) vascular bundle. If the stomata were sealed, what |
would happen to oxygen (O2) and carbon dioxide (CO2) photosynthesizing leaf? levels in a a. O2 levels would increase and CO2 levels would decrease. b. CO2 levels would increase and O2 levels would decrease. This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 8 | Photosynthesis 347 c. O2 and CO2 levels would both decrease. d. O2 and CO2 levels would both increase. Think About It On a hot, dry day, plants close their stomata to conserve water. Predict the impact of this on photosynthesis and justify your prediction. 8.3 | Using Light to Make Organic Molecules In this section, you will explore the following questions: • What are the reactions in the Calvin cycle described as the light-independent reactions? • Why does the term “carbon fixation” describe the products of the Calvin cycle? • What is the role of photosynthesis in the energy cycle of all living organisms? Connection for AP® Courses The free energy stored in ATP and NADPH produced in the light-dependent reactions is used to power the chemical reactions of the light-independent reactions or Calvin cycle, which can occur during both the day and night. In the Calvin cycle, an enzyme called ribulose biphosphate carboxylase (RuBisCO), catalyzes a reaction with CO2 and another molecule called ribulose biphosphate (RuBP) that is regenerated from a previous Calvin cycle. After a series of chemical reactions, the carbon from carbon dioxide in the atmosphere is “fixed” into carbohydrates, specifically a three-carbon molecule called glyceraldehydes-3-phosphate (G3P). (Again, count the carbons as we explore the Calvin cycle.) After three turns of the cycle, a three-carbon molecule of G3P leaves the cycle to become part of a carbohydrate molecule. The remaining G3P molecules stay in the cycle to be regenerated into RuBP, which is then ready to react with more incoming CO2. In other words, the cell generates a stockpile of G3P to be assembled into organic molecules, including carbohydrates. Each step of the Calvin cycle is catalyzed by specific enzymes. (You do not have to memorize the reactions of the Calvin cycle; however, if provided with a diagram of the cycle, you should be able to interpret it.) |
Some plants evolved chemical modifications to more efficiently trap CO2 if environmental conditions limit its availability. For example, when it’s hot outside, plants tend to keep their stomata closed to prevent excessive water loss; when the outside temperature cools, stomata open and plants take in CO2 and use a more efficient system to feed it into the Calvin cycle. As we explored in Overview of Photosynthesis, photosynthesis forms an energy link with cellular respiration. Plants need both photosynthesis and respiration in order to conduct metabolic processes during both light and dark times. Therefore, plant cells contain both chloroplasts and mitochondria. Information presented and the examples highlighted in the section, support concepts and learning objectives outlined in Big Idea 2 of the AP® Biology Curriculum Framework, as shown in the table. The learning objectives listed in the Curriculum Framework provide a transparent foundation for the AP® Biology course, an inquiry-based laboratory experience, instructional activities, and AP® exam questions. A learning objective merges required content with one or more of the seven science practices. Big Idea 2 Biological systems utilize free energy and molecular building blocks to grow, to reproduce, and to maintain dynamic homeostasis. 348 Enduring Understanding 2.A Growth, reproduction and maintenance of living systems require free energy and matter. Chapter 8 | Photosynthesis Essential Knowledge 2.A.2 Light energy captured in photosynthesis is stored in carbohydrates produced during the Calvin cycle. Science Practice Learning Objective 1.4 The student can use representations and models to analyze situations or solve problems qualitatively and quantitatively. 2.4 The student is able to use representations to pose scientific questions about what mechanisms and structural features allow organisms to capture, store, and use free energy. Essential Knowledge 2.A.2 Light energy captured in photosynthesis is stored in carbohydrates produced during the Calvin cycle Science Practice Learning Objective 6.2 The student can construct explanations of phenomena based on evidence produced through scientific practices. 2.5 The student is able to construct explanations of the mechanisms and structural features of cells that allow organisms to capture, store, or use free energy. The Science Practice Challenge Questions contain additional test questions for this section that will help you prepare for the AP exam. These questions address the following standards: [APLO 2.5][APLO 2.11][APLO 4.17] The Calvin Cycle After the energy from the sun is converted into chemical energy and temporarily stored in ATP and NADPH molecules, the cell has the fuel needed to build carbohydrate molecules for long |
-term energy storage. The products of the light-dependent reactions, ATP and NADPH, have lifespans in the range of millionths of seconds, whereas the products of the lightindependent reactions (carbohydrates and other forms of reduced carbon) can survive for hundreds of millions of years. The carbohydrate molecules made will have a backbone of carbon atoms. Where does the carbon come from? It comes from carbon dioxide, the gas that is a waste product of respiration in microbes, fungi, plants, and animals. In plants, carbon dioxide (CO2) enters the leaves through stomata, where it diffuses over short distances through intercellular spaces until it reaches the mesophyll cells. Once in the mesophyll cells, CO2 diffuses into the stroma of the chloroplast—the site of light-independent reactions of photosynthesis. These reactions actually have several names associated with them. Another term, the Calvin cycle, is named for the man who discovered it, and because these reactions function as a cycle. Others call it the Calvin-Benson cycle to include the name of another scientist involved in its discovery. The most outdated name is dark reactions, because light is not directly required (Figure 8.18). However, the term dark reaction can be misleading because it implies incorrectly that the reaction only occurs at night or is independent of light, which is why most scientists and instructors no longer use it. This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 8 | Photosynthesis 349 Figure 8.18 Light reactions harness energy from the sun to produce chemical bonds, ATP, and NADPH. These energycarrying molecules are made in the stroma where carbon fixation takes place. The light-independent reactions of the Calvin cycle can be organized into three basic stages: fixation, reduction, and regeneration. Stage 1: Fixation In the stroma, in addition to CO2, two other components are present to initiate the light-independent reactions: an enzyme called ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO), and three molecules of ribulose bisphosphate (RuBP), as shown in Figure 8.19. RuBP has five atoms of carbon, flanked by two phosphates. 350 Chapter 8 | Photosynthesis Figure 8.19 The Calvin cycle has three stages. In stage 1, the enzyme RuBisCO incorporates carbon dioxide |
into an organic molecule, 3-PGA. In stage 2, the organic molecule is reduced using electrons supplied by NADPH. In stage 3, RuBP, the molecule that starts the cycle, is regenerated so that the cycle can continue. Only one carbon dioxide molecule is incorporated at a time, so the cycle must be completed three times to produce a single threecarbon GA3P molecule, and six times to produce a six-carbon glucose molecule. Which of the following statements is true? a. b. c. d. In photosynthesis, oxygen, carbon dioxide, ATP and NADPH are reactants. GA3P and water are products. In photosynthesis, chlorophyll, water and carbon dioxide are reactants. GA3P and oxygen are products. In photosynthesis, water, carbon dioxide, ATP and NADPH are reactants. RuBP and oxygen are products. In photosynthesis, water and carbon dioxide are reactants. GA3P and oxygen are products. RuBisCO catalyzes a reaction between CO2 and RuBP. For each CO2 molecule that reacts with one RuBP, two molecules of another compound (3-PGA) form. PGA has three carbons and one phosphate. Each turn of the cycle involves only one RuBP and one carbon dioxide and forms two molecules of 3-PGA. The number of carbon atoms remains the same, as the atoms move to form new bonds during the reactions (3 atoms from 3CO2 + 15 atoms from 3RuBP = 18 atoms in 3 atoms of 3-PGA). This process is called carbon fixation, because CO2 is “fixed” from an inorganic form into organic molecules. Stage 2: Reduction ATP and NADPH are used to convert the six molecules of 3-PGA into six molecules of a chemical called glyceraldehyde 3-phosphate (G3P). That is a reduction reaction because it involves the gain of electrons by 3-PGA. Recall that a reduction is the gain of an electron by an atom or molecule. Six molecules of both ATP and NADPH are used. For ATP, energy is released with the loss of the terminal phosphate atom, converting it into ADP; for NADPH, both energy and a hydrogen atom are lost, converting it into NADP+. Both of these molecules return to the nearby light-dependent reactions to be reused and reenergized. Stage 3: Regeneration Interestingly, at this point, only one of the G3P |
molecules leaves the Calvin cycle and is sent to the cytoplasm to contribute to the formation of other compounds needed by the plant. Because the G3P exported from the chloroplast has three carbon atoms, it takes three “turns” of the Calvin cycle to fix enough net carbon to export one G3P. But each turn makes two G3Ps, thus three turns make six G3Ps. One is exported while the remaining five G3P molecules remain in the cycle and are used This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 8 | Photosynthesis 351 to regenerate RuBP, which enables the system to prepare for more CO2 to be fixed. Three more molecules of ATP are used in these regeneration reactions. This link (http://openstaxcollege.org/l/calvin_cycle) leads to an animation of the Calvin cycle. Click stage 1, stage 2, and then stage 3 to see G3P and ATP regenerate to form RuBP. Explain why the process of producing glucose in plants is a cycle. a. Three RuBP molecules get converted to three G3P, and two G3P molecules with the help of three ATPs are converted back to three molecules of RuBP. b. Three RuBP molecules get converted to six G3P, and five G3P molecules with the help of three ATPs are converted back to three molecules of RuBP. c. Three RuBP molecules get converted to five G3P, and three G3P molecules with the help of three ATPs are converted back to three molecules of RuBP. d. Three RuBP molecules get converted to six G3P, and five G3P molecules with the help of five ATPs are converted back to three molecules of RuBP. 352 Chapter 8 | Photosynthesis Figure 8.20 The harsh conditions of the desert have led plants like these cacti to evolve variations of the lightindependent reactions of photosynthesis. These variations increase the efficiency of water usage, helping to conserve water and energy. (credit: Piotr Wojtkowski) Which of the following events is associated with the development of oxygenic photosynthesis? a. Photosynthetic organisms began to use NADPH and ATP as an energy source. b. Photosynthetic organisms evolved from single-celled bacteria into multicellular plants. c. Photosynthetic organisms began to use two photosystems instead |
of one. d. Photosynthetic organisms began to use light reactions as well as dark reactions. The Energy Cycle Whether the organism is a bacterium, plant, or animal, all living things access energy by breaking down carbohydrate molecules. But if plants make carbohydrate molecules, why would they need to break them down, especially when it has been shown that the gas organisms release as a “waste product” (CO2) acts as a substrate for the formation of more food in photosynthesis? Remember, living things need energy to perform life functions. In addition, an organism can either make its own food or eat another organism—either way, the food still needs to be broken down. Finally, in the process of breaking down food, called cellular respiration, heterotrophs release needed energy and produce “waste” in the form of CO2 gas. In nature, there is no such thing as waste. Every single atom of matter and energy is conserved, recycling over and over infinitely. Substances change form or move from one type of molecule to another, but their constituent atoms never disappear. (Figure 1.21 is an illustrative example of this process.) CO2 is no more a form of waste than oxygen is wasteful to photosynthesis. Both are byproducts of reactions that move on to other reactions. Photosynthesis absorbs light energy to build carbohydrates in chloroplasts, and aerobic cellular respiration releases energy by using oxygen to metabolize carbohydrates in the cytoplasm and mitochondria. Both processes use electron transport chains to capture the energy necessary to drive other reactions. These two powerhouse processes, photosynthesis and cellular respiration, function in biological, cyclical harmony to allow organisms to access life-sustaining energy that originates millions of miles away in a burning star humans call the sun. This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 8 | Photosynthesis 353 Figure 8.21 Photosynthesis consumes carbon dioxide and produces oxygen. Aerobic respiration consumes oxygen and produces carbon dioxide. These two processes play an important role in the carbon cycle. (credit: modification of work by Stuart Bassil) Photosynthesis and aerobic respiration are interrelated in important ways. During photosynthesis, plants take in carbon dioxide and water. The water molecule is split, the oxygen is released into the atmosphere, and the carbon dioxide is used to build carbohydrates. During aerobic respiration, organisms take in water and |
oxygen for respiration and produce carbon dioxide. The Earth did not contain oxygen in its atmosphere throughout much of its history, even after life on Earth had already began. It did, however, contain carbon dioxide. What does this suggest about when photosynthetic organisms evolved, relative to non-photosynthetic organisms, and why? a. Photosynthetic organisms evolved before non-photosynthetic organisms because no oxygen was present in the atmosphere when life began. b. Photosynthetic organisms evolved after non-photosynthetic organisms because no oxygen was present in the atmosphere when life began. c. Non-photosynthetic organisms evolved before photosynthetic organisms because no oxygen was present in the atmosphere when life began. d. Photosynthetic organisms evolved before non-photosynthetic organisms because no oxygen was present in the atmosphere when life began. 354 Chapter 8 | Photosynthesis Activity Create a model or diagram to show the links between photosynthesis and cellular respiration. Think About It What cellular features and processes are similar in both respiration and photosynthesis? This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 8 | Photosynthesis KEY TERMS 355 absorption spectrum range of wavelengths of electromagnetic radiation absorbed by a given substance antenna protein pigment molecule that directly absorbs light and transfers the energy absorbed to other pigment molecules Calvin cycle light-independent reactions of photosynthesis that convert carbon dioxide from the atmosphere into carbohydrates using the energy and reducing power of ATP and NADPH carbon fixation process of converting inorganic CO2 gas into organic compounds carotenoid photosynthetic pigment that functions to dispose of excess energy chemoautotroph sunlight organism that can build organic molecules using energy derived from inorganic chemicals instead of chlorophyll a form of chlorophyll that absorbs violet-blue and red light and consequently has a bluish-green color; the only pigment molecule that performs the photochemistry by getting excited and losing an electron to the electron transport chain chlorophyll b accessory pigment that absorbs blue and red-orange light and consequently has a yellowish-green tint chloroplast organelle in which photosynthesis takes place cytochrome complex group of reversibly oxidizable and reducible proteins that forms part of the electron transport chain between photosystem II and photosystem I electromagnetic spectrum range of all possible frequencies of radiation electron transport chain group of proteins between PSII and PSI that pass energized electrons and use the energy released by the electrons to move hydrogen ions against their concentration gradient into |
the thylakoid lumen granum stack of thylakoids located inside a chloroplast heterotroph organism that consumes organic substances or other organisms for food light harvesting complex complex that passes energy from sunlight to the reaction center in each photosystem; it consists of multiple antenna proteins that contain a mixture of 300–400 chlorophyll a and b molecules as well as other pigments like carotenoids light-dependent reaction first stage of photosynthesis where certain wavelengths of the visible light are absorbed to form two energy-carrying molecules (ATP and NADPH) light-independent reaction second stage of photosynthesis, though which carbon dioxide is used to build carbohydrate molecules using energy from ATP and NADPH mesophyll middle layer of chlorophyll-rich cells in a leaf P680 reaction center of photosystem II P700 reaction center of photosystem I photoact ejection of an electron from a reaction center using the energy of an absorbed photon photoautotroph organism capable of producing its own organic compounds from sunlight photon distinct quantity or “packet” of light energy photosystem group of proteins, chlorophyll, and other pigments that are used in the light-dependent reactions of photosynthesis to absorb light energy and convert it into chemical energy photosystem I integral pigment and protein complex in thylakoid membranes that uses light energy to transport electrons 356 Chapter 8 | Photosynthesis from plastocyanin to NADP+ (which becomes reduced to NADPH in the process) photosystem II integral protein and pigment complex in thylakoid membranes that transports electrons from water to the electron transport chain; oxygen is a product of PSII pigment molecule that is capable of absorbing certain wavelengths of light and reflecting others (which accounts for its color) primary electron acceptor from the reaction center pigment or other organic molecule in the reaction center that accepts an energized electron reaction center complex of chlorophyll molecules and other organic molecules that is assembled around a special pair of chlorophyll molecules and a primary electron acceptor; capable of undergoing oxidation and reduction reduction gain of electron(s) by an atom or molecule spectrophotometer instrument that can measure transmitted light and compute the absorption stoma opening that regulates gas exchange and water evaporation between leaves and the environment, typically situated on the underside of leaves stroma fluid-filled space surrounding the grana inside a chloroplast where the light-independent reactions of photosynthesis take place thylakoid disc-shaped, membrane-bound structure inside a chlorop |
last where the light-dependent reactions of photosynthesis take place; stacks of thylakoids are called grana thylakoid lumen transport aqueous space bound by a thylakoid membrane where protons accumulate during light-driven electron wavelength distance between consecutive points of equal position (two crests or two troughs) of a wave in a graphic representation; inversely proportional to the energy of the radiation CHAPTER SUMMARY 8.1 Overview of Photosynthesis The process of photosynthesis transformed life on Earth. By harnessing energy from the sun, the evolution of photosynthesis allowed living things access to enormous amounts of energy. Because of photosynthesis, living things gained access to sufficient energy that allowed them to build new structures and achieve the biodiversity evident today. Only certain organisms, called photoautotrophs, can perform photosynthesis; they require the presence of chlorophyll, a specialized pigment that absorbs certain portions of the visible spectrum and can capture energy from sunlight. Photosynthesis uses carbon dioxide and water to assemble carbohydrate molecules and release oxygen as a waste product into the atmosphere. Eukaryotic autotrophs, such as plants and algae, have organelles called chloroplasts in which photosynthesis takes place, and starch accumulates. In prokaryotes, such as cyanobacteria, the process is less localized and occurs within folded membranes, extensions of the plasma membrane, and in the cytoplasm. 8.2 The Light-Dependent Reaction of Photosynthesis The pigments of the first part of photosynthesis, the light-dependent reactions, absorb energy from sunlight. A photon strikes the antenna pigments of photosystem II to initiate photosynthesis. The energy travels to the reaction center that contains chlorophyll a and then to the electron transport chain, which pumps hydrogen ions into the thylakoid interior. This action builds up a high concentration of ions. The ions flow through ATP synthase via chemiosmosis to form molecules of ATP, which are used for the formation of sugar molecules in the second stage of photosynthesis. Photosystem I absorbs a second photon, which results in the formation of an NADPH molecule, another energy and reducing power carrier for the light-independent reactions. 8.3 Using Light to Make Organic Molecules Using the energy carriers formed in the first steps of photosynthesis, the light-independent reactions, or the Calvin cycle, take in CO2 from the environment. An enzyme, RuBisCO, catalyzes a reaction with CO2 and |
another molecule, RuBP. This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 8 | Photosynthesis 357 After three cycles, a three-carbon molecule of G3P leaves the cycle to become part of a carbohydrate molecule. The remaining G3P molecules stay in the cycle to be regenerated into RuBP, which is then ready to react with more CO2. Photosynthesis forms an energy cycle with the process of cellular respiration. Plants need both photosynthesis and respiration for their ability to function in both the light and dark, and to be able to interconvert essential metabolites. Therefore, plants contain both chloroplasts and mitochondria. REVIEW QUESTIONS 1. Which of the following components is not used by both plants and cyanobacteria to carry out photosynthesis? 7. Which portion of the electromagnetic radiation originating from the sun is harmful to living tissues? a. carbon dioxide b. chlorophyll c. chloroplasts d. water a. blue b. green c. infrared d. ultraviolet 2. Why are chemoautotrophs not considered the same as photoautotrophs if they both extract energy and make sugars? a. Chemoautotrophs use wavelengths of light not available to photoautotrophs. b. Chemoautotrophs extract energy from inorganic chemical compounds. c. Photoautotrophs prefer the blue side of the visible light spectrum. d. Photoautotrophs make glucose, while chemoautotrophs make galactose. 3. In which compartment of the plant cell do the lightindependent reactions of photosynthesis take place? a. mesophyll b. outer membrane c. d. stroma thylakoid 4. What is a part of grana? a. b. c. d. the Calvin cycle the inner membrane stroma thylakoids 5. What are two major products of photosynthesis? a. chlorophyll and oxygen b. oxygen and carbon dioxide c. d. sugars/carbohydrates and oxygen sugars/carbohydrates and carbon dioxide 6. What is the primary energy source for cells? a. glucose b. c. d. starch sucrose triglycerides 8. The amount of energy in a wave can be measured using what trait? a. color intensity b. distance from trough to crest c. the amount of sugar produced d. wavelength 9. What portion of the electromagnetic radiation emitted |
by the sun has the least energy? a. gamma b. c. infrared radio d. X-rays 10. What is the function of carotenoids in photosynthesis? a. They supplement chlorophyll absorption. b. They are visible in the fall during leaf color changes. c. They absorb excess energy and dissipate it as heat. d. They limit chlorophyll absorption. 11. Which of the following structures is not a component of a photosystem? a. antenna molecule b. ATP synthase c. primary electron acceptor d. reaction center 12. Which complex is not involved in producing the electromotive force of ATP synthesis? a. ATP synthase b. cytochrome complex c. Photosystem I d. Photosystem II 13. What can be calculated from a wavelength 358 Chapter 8 | Photosynthesis measurement of light? a. a specific portion of the visible spectrum b. color intensity the amount of energy of a wave of light c. d. a. 3-PGA b. NADPH c. RuBisCO d. RuBP the distance from trough to crest of the wave 18. Which pathway is used by both plants and animals? 14. Which molecule must enter the Calvin cycle continually for the light-independent reactions to take place? a. CO2 b. RuBisCO c. RuBP d. 3-PGA 15. Which order of molecular conversions is correct for the Calvin cycle? a. RuBP + G3P → 3-PGA → sugar b. RuBisCO → CO2 → RuBP → G3P c. RuBP + CO2 → [RuBisCO]3-PGA → G3P d. CO2 → 3-PGA → RuBP → G3P 16. Which statement correctly describes carbon fixation? a. b. c. d. the conversion of CO2 compound into an organic the use of RuBisCO to form 3-PGA the production of carbohydrate molecules from G3P the use of ATP and NADPH to reduce CO2 17. Which substance catalyzes carbon fixation? CRITICAL THINKING QUESTIONS 22. What are the roles of ATP and NADPH in photosynthesis? a. carbon fixation b. cellular respiration c. photosystem II d. photosynthesis 19. Which of the following organisms is a heterotroph? a. Cyanobacterium b. intestinal bacteria c. kelp d. pond algae 20. What is the role of rib |
ulose-1,5-bisphosphate, abreviated RuBisCO, in photosynthesis? a. b. c. d. It catalyzes the reaction between CO2 ribulose bisphosphate (RuBP). and It catalyzes the reaction that produces glyceraldehyde3-phosphate (G3P). It catalyzes the reaction that regenerates RuBP. It catalyzes the reaction utilizing ATP and NADPH. 21. What is the product of the Calvin cycle? a. Glucose b. Glyceraldehyde-3-Phosphate c. Phosphoglycerate (PGA) d. sucrose a. ATP and NADPH are forms of chemical energy produced from the light dependent reactions to be used in the light independent reactions that produce sugars. b. ATP and NADPH are forms of chemical energy produced from the light independent reactions, to be used in the light dependent reactions that produce sugars. c. ATP and NADPH are forms of chemical energy produced from the light dependent reactions to be used in the light independent reactions that produce proteins. d. ATP and NADPH are forms of chemical energy produced from the light dependent reactions to be used in the light independent reactions that use sugars as reactants. This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 8 | Photosynthesis 359 23. What is the overall outcome of the light reactions in photosynthesis? a. NADPH and ATP molecules are produced during the light reactions and are used to power the light independent reactions. b. NADPH and ATP molecules are produced during the light reactions, which are used to power the light dependent reactions. c. Sugar and ATP are produced during the light reactions, which are used to power the light independent reactions. d. Carbon dioxide and NADPH are produced during the light reactions, which are used to power the light dependent reactions. 24. How does the equation relate to both photosynthesis and cellular respiration? a. Photosynthesis utilizes energy to build carbohydrates while cellular respiration metabolizes carbohydrates. b. Photosynthesis utilizes energy to metabolize carbohydrates while cellular respiration builds carbohydrates. c. Photosynthesis and cellular respiration both utilize carbon dioxide and water to produce carbohydrates. d. Photosynthesis and cellular respiration both metabolize carbohydrates to produce carbon dioxide and water. 25. How is the energy from the sun transported within chloroplasts? a. When photons strike photos |
ystem (PS) II, pigments pass the light energy to chlorophyll a molecules that excite an electron, which is then passed to the electron transport chain. The cytochrome complex transfers protons across the thylakoid membrane and transfers electrons from PS-II to PS-I. The products of the light dependent reaction are used to power the Calvin cycle to produce glucose. b. When photons strike photosystem (PS) I, pigments pass the light energy to chlorophyll, molecules that excite electrons, which is then passed to the electron transport chain. The cytochrome complex then transfers protons across the thylakoid membrane and transfers electrons from PS-II to PS-I. The products of the light dependent reaction are used to power the Calvin cycle to produce glucose. c. When photons strike photosystem (PS) II, pigments pass the light energy to chlorophyll molecules that in turn excite electrons, which are then passed to the electron transport chain. The cytochrome complex transfers protons across the thylakoid membrane and transfers electrons from PS-I to PS-II. The products of the light dependent reaction are used to power the Calvin cycle to produce glucose. d. When photons strike photosystem (PS) II, pigments pass the light energy to chlorophyll molecules that excite electrons, which is then passed to the electron transport chain. The cytochrome complex transfers protons across the thylakoid membrane and transfers electrons from PS II to PS I. The products of the light independent reaction are used to power the Calvin cycle to produce glucose. 26. Explain why X-rays and ultraviolet light wavelengths are dangerous to living tissues. a. UV and X-rays are high energy waves that penetrate the tissues and damage cells. b. UV and X-rays are low energy waves that penetrate the tissues and damage cells. c. UV and X-rays cannot penetrate tissues and thus damage the cells. d. UV and X-rays can penetrate tissues and thus do not damage the cells. 27. If a plant were to be exposed to only red light, would photosynthesis be possible? a. Photosynthesis does not take place. b. The rate of photosynthesis increases sharply. c. The rate of photosynthesis decreases drastically. d. The rate of photosynthesis decreases and then increases. 360 Chapter 8 | Photosynthesis 28. Describe the electron transfer pathway from photosystem II to photosystem I in the light-dependent reactions. 31. |
How do desert plants prevent water loss from the heat, which would compromise photosynthesis? a. by using CAM photosynthesis and by closing a. After splitting water in PS-II, high energy stomatal pores during the night electrons are delivered through the chloroplast electron transport chain to PS-I. b. After splitting water in PS-I, high energy electrons are delivered through the chloroplast electron transport chain to PS-II. c. After the photosynthesis reaction, the released products like glucose help in the transfer of electrons from PS-II to PS-I. d. After the completion of the light dependent reactions, the electrons are transferred from PSII to PS-I. 29. What will happen to a plant leaf that loses CO2 quickly? too a. no effect on the rate of photosynthesis b. Photosynthesis will slow down or stop possibly. b. by using CAM photosynthesis and by opening of stomatal pores during the night c. by using CAM photosynthesis and by keeping stomatal pores closed at all times d. by bypassing CAM photosynthesis and by keeping stomatal pores closed at night 32. Why are carnivores, such as lions, dependent on photosynthesis to survive? a. because the prey of lions are generally herbivores which depend on heterotrophs b. because the prey of lions are generally smaller carnivorous animals which depend on nonphotosynthetic organisms c. because the prey of lions are generally herbivores which depend on autotrophs c. Photosynthesis will increase exponentially. d. because the prey of lions are generally omnivores that depend only on autotrophs. 33. Why does it take three turns of the Calvin cycle to produce G3P, the initial product of photosynthesis? a. To fix enough carbon to export one G3P molecule. b. To fix enough oxygen to export one G3P molecule. c. To produce RuBisCO as an end product. d. To produce ATP and NADPH for fixation of G3P. 35. What evidence exists that the evolution of photosynthesis and cellular respiration support the concept that there is a common ancestry for all organisms? d. Photosynthesis will decrease and then increase. 30. Carbon, in the form of CO2, must be taken from the atmosphere and attached to an existing organic molecule in the Calvin cycle. Therefore, the carbon is bound to the molecule. The products of the cycle only occur because of the added carbon. What are |
the products of the Calvin cycle and what is regenerated? a. The product of the Calvin cycle is glyceraldehyde-3 phosphate and RuBP is regenerated. b. The product of the Calvin cycle is glyceraldehyde-3 phosphate and RuBisCO is regenerated. c. The product of the Calvin cycle is a 3-PGA molecule and glyceraldehyde-3 phosphate is regenerated. d. The product of the Calvin cycle is glyceraldehyde-3 phosphate and oxygen is regenerated. TEST PREP FOR AP® COURSES 34. Photosynthesis and cellular respiration are found throughout the eukaryotic world. They are complementary to each other because they each use products of the other process. What do the two pathways share? a. chloroplasts and mitochondria b. Photosystems I and II c. d. the cytochrome complex thylakoids This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 8 | Photosynthesis 361 a. All organisms perform cellular respiration, using oxygen and glucose, which are produced by photosynthesis. b. All organisms perform cellular respiration using carbon dioxide and glucose, which are produced by photosynthesis. c. All organisms perform cellular respiration using oxygen and lipids, which are produced by photosynthesis. d. All organisms perform cellular respiration using carbon dioxide and lipids, which are produced by photosynthesis. 36. 38. Why do the light-dependent reactions of photosynthesis take place in the thylakoid? a. Photosystem I is anchored to the membrane, but not photosystem II. b. The cytochrome complex requires a membrane for chemiosmosis to occur. c. The light-dependent reactions depend on the presence of carbon dioxide. d. Light energy is absorbed by the thylakoid membrane. 39. Metabolic pathways both produce and use energy to perform their reactions. How does the Calvin cycle help to harness, store, and use energy in its pathway? a. The Calvin cycle harnesses energy in the form of 6 ATP and 6 NADPH that are used to produce Fructose-3- phosphate (F3P) molecules. These store the energy captured from photosynthesis. The cycle uses this energy to regenerate RuBP. b. The Calvin cycle harnesses energy in the form of 6 ATP and 6 NADPH that are used to produce Glyceraldehyde- |
3- phosphate (GA3P) molecules. These store the energy captured from photosynthesis. The cycle uses this energy to regenerate RuBP. c. The Calvin cycle harnesses energy in the form of 3 ATP and 3 NADPH that are used to produce Glyceraldehyde-3- phosphate (GA3P) molecules. These store the energy captured from photosynthesis. The cycle uses this energy to regenerate the RuBP. d. The Calvin cycle harnesses energy in the form of 6 ATP and 3 NADPH that are used to produce Glyceraldehyde-3- phosphate (GA3P) molecules. These store energy captured from photosynthesis. The cycle uses this energy to regenerate RuBP. 40. Based on Figure 8.18, which would most likely cause a plant to run out of NADP? a. missing the ATP synthase enzyme b. exposure to light c. A lack of water would prevent H+ and NADP+ from forming NADPH d. not enough CO2 41. As temperatures increase, gases such as CO2 faster. As a result, plant leaves will lose CO2 rate than normal. If the amount of light impacting on the leaf and the amount of water available is adequate, predict how this loss of gas will affect photosynthesis in the leaf. at a faster diffuse Correctly label the indicated parts of a chloroplast. a. A. stroma, B. outer membrane, C. granum, D. thylakoid, E. inner membrane b. A. outer membrane, B. stroma,C. granum, D. thylakoid, E. inner membrane c. A. outer membrane, B. stroma, C. granum, D. inner membrane, E. thylakoid d. A. stroma, B. outer membrane, C. inner membrane, D. granum, E. thylakoid 37. What cellular features and processes are similar in both photosynthesis and cellular respiration? a. Both processes are contained in organelles with single membranes, and both use a version of the cytochrome complex. b. Both processes are contained in organelles with double membranes, and neither use a version of the cytochrome complex. c. Both processes are contained in organelles with double membranes, and use a version of the cytochrome complex. d. Both processes are contained in organelles with single membranes, and neither use a version of the cytochrome |
complex. 362 Chapter 8 | Photosynthesis a. Loss of gases, mainly CO2, will not affect photosynthesis in the leaf, as adequate amounts of water and light are still present which will let the Calvin cycle run smoothly. b. Loss of gases, mainly CO2, will affect photosynthesis in the leaf, as the Calvin cycle will become faster to compensate for the loss. c. Loss of gases, mainly CO2, will not affect photosynthesis in the leaf, as stored reservoirs of CO2 in the leaf can be utilized in such times. d. Loss of gases, mainly CO2, will affect photosynthesis in the leaf, as the Calvin cycle will slow down and possibly stop because of inadequate carbon to fix in the system. 42. How do the cytochrome complex components involved in photosynthesis contribute to the electron transport chain? a. Photosystem I excites the electron as it moves down the electron transport chain into Photosystem II. b. Plastoquinone and plastocyanine perform redox reactions that allow the electron to move down the electron transport chain into Photosystem I. c. ATP synthase “de-excites” the electron as it moves down the electron transport chain into Photosystem I. d. RuBisCO excites the electron as it moves down the electron transport chain into Photosystem II. 43. Discuss how membranes in chloroplasts contribute to the organelles’ essential functions. a. The inner membrane contains the chemicals needed for the Calvin cycle and also components of the light dependent reactions. The thylakoid membrane contains photosystems I and II, as well as the enzyme NAD+ reductase. b. The inner membrane contains only the chemicals needed for the Calvin cycle. The thylakoid membrane contains components of the light dependent reactions, photosystems I and II, and the enzyme NAD+ reductase. c. The inner membrane contains components of the light dependent reactions as well as photosystems I and II. The thylakoid membrane contains the chemicals needed for the Calvin cycle and also the enzyme NAD+ reductase. d. The inner membrane contains the chemicals needed for the Calvin cycle, components of the light dependent reactions and photosystems I and II. The thylakoid membrane contains the enzyme NAD+ reductase. 44. If the absorption spectrum of photosynthetic pigments was restricted to the green portion of the spectrum, which pigment or pigments would be affected the least |
? a. carotenoids b. chlorophyll a c. chlorophyll b d. chlorophyll c 45. Describe the passage of energy from light until it is captured in the primary electron acceptor. a. Chlorophyll molecules in the photosystems are excited and pass the energy to the primary electron acceptor where the energy is used to excite electrons from the splitting of water. b. Chlorophyll a molecules in the photosystems are excited and pass the energy to the primary electron acceptor where the energy is used to excite electrons from the splitting of water. c. Chlorophyll b molecules in the photosystems are excited and pass the energy to the primary electron acceptor where the energy is used to excite electrons from the splitting of water. d. Chlorophyll molecules in the photosystems absorb light and get excited in the primary electron acceptor from where the energy is used to excite electrons from the splitting of water. SCIENCE PRACTICE CHALLENGE QUESTIONS 46. On a hot, dry day, plants close their stomata to conserve water. Explain the connection between the oxidation of water in photosystem II of the light-dependent reactions and the synthesis of glyceraldehyde-3-phosphate (G3PA) in the light-independent reactions. Predict the effect of closed stomata on the synthesis of G3PA and justify the prediction. This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 8 | Photosynthesis 363 47. The emergence of photosynthetic organisms is recorded in layers of sedimentary rock known as a banded iron formation. Dark-colored and iron-rich bands composed of hematite (Fe2O3) and magnetite (Fe3O4) only a few millimeters thick alternate with light-colored and iron-poor shale or chert. Hematite and magnetite can form precipitates from water that has a high concentration of dissolved oxygen. Shale and chert can form under conditions that have high concentrations of carbonates -2). These banded iron formations appeared 3.7 (CO3 billion years ago (and became less common 1.8 billion years ago). Justify the claim that these sedimentary rock formations reveal early Earth conditions. 48. The following diagram summarizes the light reactions of photosynthesis. Figure 8.22 The diagram shows light-dependent reactions of |
photosynthesis, including the reaction centers, electron transport chains, and the overall reactions within each of these. The free energy per electron is shown for the oxidation-reduction reactions. The free change of the captured radiant energy is shown. 2N ADP+ + 2H+ + 2H2 O + 3ADP + 3Pi → O2 + 4H+ + 2N ADPH + 3ATP A. In the overall mass balance equation for the light reactions shown above, identify the source of electrons for the synthesis of NADPH. B. Calculate the number of electrons transferred in this reaction. C. Using the free energies per electron displayed, calculate the free energy change of the lightdependent reactions. D. Given that the free energy change for the hydrolysis of ATP is -31.5 kJ/mole and the free energy change for the formation of NADPH from NADP+ is 18 kJ/mole, calculate the total production of free energy for the light reactions. E. Using this definition of energy efficiency, calculate the efficiency of the light reaction of photosynthesis: energy efficiency = free energy produced/energy input. 49. Algae can be used for food and fuel. To maximize profit from algae production under artificial light, researchers proposed an experiment to determine the dependence of the efficiency of the process used to grow the algae on light intensity (“brightness”) that will be purchased from the electric company. The algae will be grown on a flat sheet that will be continuously washed with dissolved carbon dioxide and nutrients. Light-emitting diodes (LEDs) will be used to illuminate the growth sheet. Photodiodes placed above and below the sheet will be used to detect light transmitted through and reflected from the algal mat. The intensity of light can be varied, and the algae can be removed, filtered, and dried. The amount of stored energy in the algal mats can be determined by calorimetry. A. Identify a useful definition of efficiency for this study and justify your choice. B. Frequencies of light emitted by the LEDs will not be variables but must be specified for the construction of the apparatus. Identify the frequencies of light that should be used in the experiment and justify your choice. C. Evaluate the claim that the experiment is based on the assumption that there is an upper limit on the intensity of light used to support growth of algae. Predict a possible effect on algal growth if light with too great an intensity is used |
and justify the prediction. D. Design an experiment by describing a procedure that can be used to determine the relationship between light intensity and efficiency. 50. The classical theory of evolution is based on a gradual transformation, the accumulation of many random mutations that are selected. The biological evidence for evolution is overwhelming, particularly when one considers what has not changed: core conserved characteristics. A. Describe three conserved characteristics common to both chloroplasts and mitochondria. 364 Chapter 8 | Photosynthesis small. Explain how the enzyme carbonate anhydrase can increase the availability of carbon dioxide to the cell. C. Larsson and Axelsson conducted experiments in which the growth medium was fixed at two different pH levels and determined the effects of AZ and DIDS on the rate of photosynthesis by measuring oxygen concentrations at various times. The results are shown in the two graphs below. The arrows indicate the time points during which -, AZ, and DIDS were added to each system. HCO3 Some hypotheses that have been proposed to account for biological diversity are saltatory, involving sudden changes, rather than gradualist. In defense of the classical gradualist theory of evolution, nearly all biologists in the late 1960s rejected the theory of endosymbiosis as presented by Lynn Margulis in 1967. B. Suppose that you want to disprove the theory of endosymbiosis. Explain how the following evidence could disprove the theory: i. a “transitional species” with cellular features that are intermediate cells with and without mitochondria ii. a “transitional organelle” with some features, such as compartmentalized metabolic processes, but not other features, such as DNA Explain how the following evidence supports the theory of endosymbiosis: iii. bacteria live within your intestines, but you still have a separate identity iv. no one has directly observed the fusion of two organisms in which a single organism results 51. Discovering the carbon-fixation reactions (or lightindependent reactions) of photosynthesis earned Melvin Calvin a Nobel Prize in 1961. The isolation and identification of the products of algae exposed to 14C revealed the path of carbon in photosynthesis. 14C was fed -). to the algal culture in the form of bicarbonate ion (HCO3 To agitate the culture, air, which contains CO2, was bubbled through the system, so there were two sources of carbon. Since Calvin’s experiment, research has focused on the way carbon from a solution containing |
bicarbonate ions is absorbed by algae. In aqueous solution, the bicarbonate -) is in equilibrium with dissolved CO2 as anion (HCO3 shown in the equation below: H+ + HCO3 H2 O + CO2 − ←⎯⎯→⎯⎯ In a later experiment, Larsson and Axelsson (1999) used acetazolamide (AZ), a carbonate anhydrase inhibitor, to inhibit enzymes that convert bicarbonate into carbon dioxide. They also used disulfonate (DIDS), an inhibitor of the transport of anions, such as the bicarbonate ion, through the plasma membrane. A. Pose a scientific question that can be pursued with AZ and DIDS in terms of the path of carbon in photosynthesis. B. The plasma membrane is permeable to the nonpolar, uncharged carbon dioxide molecule. However, the concentration of carbon dioxide in solution can be very Figure 8.23 This figure displays the effects of AZ and DIDS on the rate of photosynthesis of two systems, system A and system B, in a line graph. The line graph plots the oxygen concentration over time. In which system, A or B, is there a strong reliance on the bicarbonate ion as the mechanism of carbon uptake by the cell? Justify your answer using the data. D. If both systems are dosed with the same concentrations of bicarbonate ion, in which system, A or B, is the pH higher? Justify your answer using the data and the bicarbonate-carbon dioxide equilibrium equation. This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 9 | Cell Communication 365 9 | CELL COMMUNICATION Figure 9.1 Have you ever become separated from a friend while in a crowd? If so, you know the challenge of searching for someone when surrounded by thousands of other people. If you and your friend have cell phones, your chances of finding each other are good. A cell phone’s ability to send and receive messages makes it an ideal communication device. (credit: modification of work by Vincent and Bella Productions) Chapter Outline 9.1: Signaling Molecules and Cellular Receptors 9.2: Propagation of the Signal 9.3: Response to the Signal 9.4: Signaling in Single-C |
elled Organisms Introduction Imagine what life would be like if you and the people around you could not communicate. You would not be able to express your wishes, nor could you ask questions to find out more about your environment. Social organization is dependent on communication between the individuals; without communication, society would fall apart. As with people, it is vital for a cell to interact with its environment. This is true whether it is a unicellular organism or one of many cells forming a larger organism. In order to respond to external stimuli, cells have developed complex mechanisms of communication that can receive a message, transfer the information across the plasma membrane, and produce changes within the cell in response to the message. In multicellular organisms, cells send and receive chemical messages constantly to coordinate the actions of distant organs, tissues, and cells. While the necessity for cellular communication in larger organisms seems obvious, even single-celled organisms communicate with each other. Yeast cells signal each other to aid in mating. Some forms of bacteria coordinate their actions in order to form large complexes called biofilms (Figure 9.18) or to organize the production of toxins to remove competing organisms. The ability of cells to communicate through chemical signals originated in single cells and was essential for the evolution of multicellular organisms. 366 Chapter 9 | Cell Communication Cell signaling is vital to the survival of organisms. For example, chemical signals tell cells when to make hormones such as insulin. Cell division also depends on chemical signals. When the chemical signals do not function properly, cells can divide uncontrollably, forming cancerous tumors. Scientists recently discovered a cell signaling pathway that protects cancer cells from being killed by the body’s immune system. The hope is to use this knowledge to create treatments that target this cell signaling pathway so that the cancer cells self destruct. More about that can be found here (http://openstaxcollege.org/l/ 32cancerdefense) : “Scientists pinpoint a new line of defense used by cancer cells.” 9.1 | Signaling Molecules and Cellular Receptors In this section, you will explore the following questions: • What are the four types of signaling that are found in multicellular organisms? • What are the differences between internal receptors and cell-surface receptors? • What is the relationship between a ligand’s structure and its mechanism of action? Connection for AP® Courses Just like you communicate with your classmates face-to-face, using your phone, or via e-mail, cells communicate |
with each other by both inter’and intracellular signaling. Cells detect and respond to changes in the environment using signaling pathways. Signaling pathways enable organisms to coordinate cellular activities and metabolic processes. Errors in these pathways can cause disease. Signaling cells secrete molecules called ligands that bind to target cells and initiate a chain of events within the target cell. For example, when epinephrine is released, binding to target cells, those cells respond by converting glycogen to glucose. Cell communication can happen over short distances. For example, neurotransmitters are released across a synapse to transfer messages between neurons Figure 1.3. Gap junctions and plasmodesmata allow small molecules, including signaling molecules, to flow between neighboring cells. Cell communication can also happen over long distances using. For example, hormones released from endocrine cells travel to target cells in multiple body systems. How does a ligand such as a hormone traveling through the bloodstream “know” when it has reached its target organ to initiate a cellular response? Nearly all cell signaling pathways involve three stages: reception, signal transduction, and cellular response. Cell signaling pathways begin when the ligand binds to a receptor, a protein that is embedded in the plasma membrane of the target cell or found in the cell cytoplasm. The receptors are very specific, and each ligand is recognized by a different one. This stage of the pathway is called reception. Molecules that are nonpolar, such as steroids, diffuse across the cell membrane and bind to internal receptors. In turn, the receptor-ligand complex moves to the nucleus and interacts with cellular DNA. This changes how a gene is expressed. Polar ligands, on the other hand, interact with membrane receptor protein. Some membrane receptors work by changing conformation so that certain ions, such as Na+ and K+, can pass through the plasma membrane. Other membrane receptors interact with a G-protein on the cytoplasmic side of the plasma membrane, which causes a series of reactions inside the cell. Disruptions to this process are linked to several diseases, including cholera. It is important to keep in mind that each cell has a variety of receptors, allowing it to respond to a variety of stimuli. Some receptors can bind several different ligands; for example, odorant molecules/receptors associated with the sense of smell in animals. Once the signaling molecule and receptor interact, a cascade of events called signal transduction usually amplifies the signal inside the cell. |
The content presented in this section supports the Learning Objectives outlined in Big Idea 3 of the AP® Biology Curriculum Framework listed. The AP® Learning Objectives merge Essential knowledge content with one or more of the seven Science Practices. These objectives provide a transparent foundation for the AP® Biology course, along with inquirybased laboratory experiences, instructional activities, and AP® Exam questions. Big Idea 3 Enduring Understanding 3.D Living systems store, retrieve, transmit and respond to information essential to life processes. Cells communicate by generating, transmitting and receiving chemical signals. This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 9 | Cell Communication 367 Essential Knowledge Science Practice Learning Objective Essential Knowledge Science Practice Learning Objective 3.D.3 Signal transduction pathways link signal reception with cellular response. 6.2 The student can construct explanations of phenomena based on evidence produced through scientific practices. 3.34 The student is able to construct explanations of cell communication through cell-to-cell direct contact or through chemical signaling. 3.D.3 Signal transduction pathways link signal reception with cellular response. 1.1 The student can create representations and models of natural or man-made phenomena and systems in the domain. 3.35 The student is able to create representations that depict how cell-to-cell communication occurs by direct contact or from a distance through chemical signaling. The Science Practice Challenge Questions contain contains additional test questions for this section that will help you prepare for the AP exam. These questions address the following standards: [APLO 3.33][APLO 3.36] There are two kinds of communication in the world of living cells. Communication between cells is called intercellular signaling, and communication within a cell is called intracellular signaling. An easy way to remember the distinction is by understanding the Latin origin of the prefixes: inter- means "between" (for example, intersecting lines are those that cross each other) and intra- means "inside" (like intravenous). Chemical signals are released by signaling cells in the form of small, usually volatile or soluble molecules called ligands. A ligand is a molecule that binds another specific molecule, in some cases, delivering a signal in the process. Ligands can thus be thought of as signaling molecules. Ligands interact with proteins in target cells, which are cells that are affected by chemical signals; these proteins are also called receptors. Ligands and receptors exist in several varieties; however, |
a specific ligand will have a specific receptor that typically binds only that ligand. Forms of Signaling There are four categories of chemical signaling found in multicellular organisms: paracrine signaling, endocrine signaling, autocrine signaling, and direct signaling across gap junctions (Figure 9.2). The main difference between the different categories of signaling is the distance that the signal travels through the organism to reach the target cell. Not all cells are affected by the same signals. 368 Chapter 9 | Cell Communication Figure 9.2 In chemical signaling, a cell may target itself (autocrine signaling), a cell connected by gap junctions, a nearby cell (paracrine signaling), or a distant cell (endocrine signaling). Paracrine signaling acts on nearby cells, endocrine signaling uses the circulatory system to transport ligands, and autocrine signaling acts on the signaling cell. Signaling via gap junctions involves signaling molecules moving directly between adjacent cells. Paracrine Signaling Signals that act locally between cells that are close together are called paracrine signals. Paracrine signals move by diffusion through the extracellular matrix. These types of signals usually elicit quick responses that last only a short amount of time. In order to keep the response localized, paracrine ligand molecules are normally quickly degraded by enzymes or removed by neighboring cells. Removing the signals will reestablish the concentration gradient for the signal, allowing them to quickly diffuse through the intracellular space if released again. One example of paracrine signaling is the transfer of signals across synapses between nerve cells. A nerve cell consists of a cell body, several short, branched extensions called dendrites that receive stimuli, and a long extension called an axon, which transmits signals to other nerve cells or muscle cells. The junction between nerve cells where signal transmission occurs is called a synapse. A synaptic signal is a chemical signal that travels between nerve cells. Signals within the nerve cells are propagated by fast-moving electrical impulses. When these impulses reach the end of the axon, the signal continues on to a dendrite of the next cell by the release of chemical ligands called neurotransmitters by the presynaptic cell (the cell emitting the signal). The neurotransmitters are transported across the very small distances between nerve cells, which are called chemical synapses (Figure 9.3). The small distance between nerve cells allows the signal to travel quickly; this enables an immediate response, such as, Take your hand off |
the stove! When the neurotransmitter binds the receptor on the surface of the postsynaptic cell, the electrochemical potential of the target cell changes, and the next electrical impulse is launched. The neurotransmitters that are released into the chemical synapse are degraded quickly or get reabsorbed by the presynaptic cell so that the recipient nerve cell can recover quickly and be prepared to respond rapidly to the next synaptic signal. This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 9 | Cell Communication 369 Figure 9.3 The distance between the presynaptic cell and the postsynaptic cell—called the synaptic gap—is very small and allows for rapid diffusion of the neurotransmitter. Enzymes in the synapatic cleft degrade some types of neurotransmitters to terminate the signal. Endocrine Signaling Signals from distant cells are called endocrine signals, and they originate from endocrine cells. (In the body, many endocrine cells are located in endocrine glands, such as the thyroid gland, the hypothalamus, and the pituitary gland.) These types of signals usually produce a slower response but have a longer-lasting effect. The ligands released in endocrine signaling are called hormones, signaling molecules that are produced in one part of the body but affect other body regions some distance away. Hormones travel the large distances between endocrine cells and their target cells via the bloodstream, which is a relatively slow way to move throughout the body. Because of their form of transport, hormones get diluted and are present in low concentrations when they act on their target cells. This is different from paracrine signaling, in which local concentrations of ligands can be very high. Autocrine Signaling Autocrine signals are produced by signaling cells that can also bind to the ligand that is released. This means the signaling cell and the target cell can be the same or a similar cell (the prefix auto- means self, a reminder that the signaling cell sends a signal to itself). This type of signaling often occurs during the early development of an organism to ensure that cells develop into the correct tissues and take on the proper function. Autocrine signaling also regulates pain sensation and inflammatory responses. Further, if a cell is infected with a virus, the cell can signal itself to undergo programmed cell death, killing the virus in the process. In some cases, neighboring cells of the same type are also influenced by the released ligand. In embryological development |
, this process of stimulating a group of neighboring cells may help to direct the differentiation of identical cells into the same cell type, thus ensuring the proper developmental outcome. Direct Signaling Across Gap Junctions Gap junctions in animals and plasmodesmata in plants are connections between the plasma membranes of neighboring cells. These fluid-filled channels allow small signaling molecules, called intracellular mediators, to diffuse between the two cells. Small molecules, such as calcium ions (Ca2+), are able to move between cells, but large molecules like proteins and DNA cannot fit through the channels. The specificity of the channels ensures that the cells remain independent but can quickly and easily transmit signals. The transfer of signaling molecules communicates the current state of the cell that is directly next to the target cell; this allows a group of cells to coordinate their response to a signal that only one of them may have received. In plants, plasmodesmata are ubiquitous, making the entire plant into a giant communication network. 370 Chapter 9 | Cell Communication Types of Receptors Receptors are protein molecules in the target cell or on its surface that bind ligand. There are two types of receptors, internal receptors and cell-surface receptors. Internal receptors Internal receptors, also known as intracellular or cytoplasmic receptors, are found in the cytoplasm of the cell and respond to hydrophobic ligand molecules that are able to travel across the plasma membrane. Once inside the cell, many of these molecules bind to proteins that act as regulators of mRNA synthesis (transcription) to mediate gene expression. Gene expression is the cellular process of transforming the information in a cell's DNA into a sequence of amino acids, which ultimately forms a protein. When the ligand binds to the internal receptor, a conformational change is triggered that exposes a DNA-binding site on the protein. The ligand-receptor complex moves into the nucleus, then binds to specific regulatory regions of the chromosomal DNA and promotes the initiation of transcription (Figure 9.4). Transcription is the process of copying the information in a cells DNA into a special form of RNA called messenger RNA (mRNA); the cell uses information in the mRNA (which moves out into the cytoplasm and associates with ribosomes) to link specific amino acids in the correct order, producing a protein. Internal receptors can directly influence gene expression without having to pass the signal on to other receptors or messengers. Figure 9.4 Hydrophobic signaling molecules typically diffuse across |
the plasma membrane and interact with intracellular receptors in the cytoplasm. Many intracellular receptors are transcription factors that interact with DNA in the nucleus and regulate gene expression. Cell-Surface Receptors Cell-surface receptors, also known as transmembrane receptors, are cell surface, membrane-anchored (integral) proteins that bind to external ligand molecules. This type of receptor spans the plasma membrane and performs signal transduction, in which an extracellular signal is converted into an intracellular signal. Ligands that interact with cell-surface receptors do not have to enter the cell that they affect. Cell-surface receptors are also called cell-specific proteins or markers because they are specific to individual cell types. Because cell-surface receptor proteins are fundamental to normal cell functioning, it should come as no surprise that a malfunction in any one of these proteins could have severe consequences. Errors in the protein structures of certain receptor molecules have been shown to play a role in hypertension (high blood pressure), asthma, heart disease, and cancer. Each cell-surface receptor has three main components: an external ligand-binding domain, a hydrophobic membranespanning region, and an intracellular domain inside the cell. The ligand-binding domain is also called the extracellular domain. The size and extent of each of these domains vary widely, depending on the type of receptor. This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 9 | Cell Communication 371 How Viruses Recognize a Host Unlike living cells, many viruses do not have a plasma membrane or any of the structures necessary to sustain life. Some viruses are simply composed of an inert protein shell containing DNA or RNA. To reproduce, viruses must invade a living cell, which serves as a host, and then take over the hosts cellular apparatus. But how does a virus recognize its host? Viruses often bind to cell-surface receptors on the host cell. For example, the virus that causes human influenza (flu) binds specifically to receptors on membranes of cells of the respiratory system. Chemical differences in the cell-surface receptors among hosts mean that a virus that infects a specific species (for example, humans) cannot infect another species (for example, chickens). However, viruses have very small amounts of DNA or RNA compared to humans, and, as a result, viral reproduction can occur rapidly. Viral reproduction invariably produces errors that can lead |
to changes in newly produced viruses; these changes mean that the viral proteins that interact with cell-surface receptors may evolve in such a way that they can bind to receptors in a new host. Such changes happen randomly and quite often in the reproductive cycle of a virus, but the changes only matter if a virus with new binding properties comes into contact with a suitable host. In the case of influenza, this situation can occur in settings where animals and people are in close contact, such as poultry and swine farms. [1] Once a virus jumps to a new host, it can spread quickly. Scientists watch newly appearing viruses (called emerging viruses) closely in the hope that such monitoring can reduce the likelihood of global viral epidemics. What requirements must be met for a new virus to emerge and spread? a. The virus must infect at least two different animals before infecting humans. b. The virus must come into contact with a new host so mutations will occur which allow the virus to bind to that host. c. A mutation must occur in the host allowing the virus to bind to the host. d. A mutation must occur in the virus allowing the virus to infect a new host, and the virus must come into contact with this host. Cell-surface receptors are involved in most of the signaling in multicellular organisms. There are three general categories of cell-surface receptors: ion channel-linked receptors, G-protein-linked receptors, and enzyme-linked receptors. Ion channel-linked receptors bind a ligand and open a channel through the membrane that allows specific ions to pass through. To form a channel, this type of cell-surface receptor has an extensive membrane-spanning region. In order to interact with the phospholipid fatty acid tails that form the center of the plasma membrane, many of the amino acids in the membrane-spanning region are hydrophobic in nature. Conversely, the amino acids that line the inside of the channel are hydrophilic to allow for the passage of water or ions. When a ligand binds to the extracellular region of the channel, there is a conformational change in the proteins structure that allows ions such as sodium, calcium, magnesium, and hydrogen to pass through (Figure 9.5). 1. A. B. Sigalov, The School of Nature. IV. Learning from Viruses, Self/Nonself 1, no. 4 (2010): 282-298. Y. Cao, X. Koh, L. Dong, X. Du, A. |
Wu, X. Ding, H. Deng, Y. Shu, J. Chen, T. Jiang, Rapid Estimation of Binding Activity of Influenza Virus Hemagglutinin to Human and Avian Receptors, PLoS One 6, no. 4 (2011): e18664. 372 Chapter 9 | Cell Communication Figure 9.5 Gated ion channels form a pore through the plasma membrane that opens when the signaling molecule binds. The open pore then allows ions to flow into or out of the cell. G-protein-linked receptors bind a ligand and activate a membrane protein called a G-protein. The activated G-protein then interacts with either an ion channel or an enzyme in the membrane (Figure 9.6). All G-protein-linked receptors have seven transmembrane domains, but each receptor has its own specific extracellular domain and G-protein-binding site. Cell signaling using G-protein-linked receptors occurs as a cyclic series of events. Before the ligand binds, the inactive Gprotein can bind to a newly revealed site on the receptor specific for its binding. Once the G-protein binds to the receptor, the resultant shape change activates the G-protein, which releases GDP and picks up GTP. The subunits of the G-protein then split into the α subunit and the βγ subunit. One or both of these G-protein fragments may be able to activate other proteins as a result. After awhile, the GTP on the active α subunit of the G-protein is hydrolyzed to GDP and the βγ subunit is deactivated. The subunits reassociate to form the inactive G-protein and the cycle begins anew. Figure 9.6 Heterotrimeric G proteins have three subunits: α, β, and γ. When a signaling molecule binds to a G-proteincoupled receptor in the plasma membrane, a GDP molecule associated with the α subunit is exchanged for GTP. The β and γ subunits dissociate from the α subunit, and a cellular response is triggered either by the α subunit or the dissociated βγ pair. Hydrolysis of GTP to GDP terminates the signal. This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 9 | Cell Communication 373 G-protein-linked receptors have been extensively studied and much has been learned about their roles in maintaining |
health. Bacteria that are pathogenic to humans can release poisons that interrupt specific G-protein-linked receptor function, leading to illnesses such as pertussis, botulism, and cholera. In cholera (Figure 9.7), for example, the water-borne bacterium Vibrio cholerae produces a toxin, choleragen, that binds to cells lining the small intestine. The toxin then enters these intestinal cells, where it modifies a G-protein that controls the opening of a chloride channel and causes it to remain continuously active, resulting in large losses of fluids from the body and potentially fatal dehydration as a result. Figure 9.7 Transmitted primarily through contaminated drinking water, cholera is a major cause of death in the developing world and in areas where natural disasters interrupt the availability of clean water. The cholera bacterium, Vibrio cholerae, creates a toxin that modifies G-protein-mediated cell signaling pathways in the intestines. Modern sanitation eliminates the threat of cholera outbreaks, such as the one that swept through New York City in 1866. This poster from that era shows how, at that time, the way that the disease was transmitted was not understood. (credit: New York City Sanitary Commission) Enzyme-linked receptors are cell-surface receptors with intracellular domains that are associated with an enzyme. In some cases, the intracellular domain of the receptor itself is an enzyme. Other enzyme-linked receptors have a small intracellular domain that interacts directly with an enzyme. The enzyme-linked receptors normally have large extracellular and intracellular domains, but the membrane-spanning region consists of a single alpha-helical region of the peptide strand. When a ligand binds to the extracellular domain, a signal is transferred through the membrane, activating the enzyme. Activation of the enzyme sets off a chain of events within the cell that eventually leads to a response. One example of this type of enzyme-linked receptor is the tyrosine kinase receptor (Figure 9.8). A kinase is an enzyme that transfers phosphate groups from ATP to another protein. The tyrosine kinase receptor transfers phosphate groups to tyrosine molecules (tyrosine residues). First, signaling molecules bind to the extracellular domain of two nearby tyrosine kinase receptors. The two neighboring receptors then bond together, or dimerize. Phosphates are then added to tyrosine residues on |
the intracellular domain of the receptors (phosphorylation). The phosphorylated residues can then transmit the signal to the next messenger within the cytoplasm. 374 Chapter 9 | Cell Communication Figure 9.8 A receptor tyrosine kinase is an enzyme-linked receptor with a single transmembrane region, and extracellular and intracellular domains. Binding of a signaling molecule to the extracellular domain causes the receptor to dimerize. Tyrosine residues on the intracellular domain are then autophosphorylated, triggering a downstream cellular response. The signal is terminated by a phosphatase that removes the phosphates from the phosphotyrosine residues. HER2 is a receptor tyrosine kinase. In 20 percent of human breast cancer cases, HER2 is permanently activated, resulting in unregulated cell division. Lapatinib, a drug used to treat breast cancer, inhibits HER2 receptor tyrosine kinase autophosphorylation, the process by which the receptor adds phosphates onto itself. This reduces tumor growth by 50 percent. Besides autophosphorylation, which of the following steps would be inhibited by Lapatinib? a. dimerization and the downstream cellular response b. phosphatase activity, dimerization, and the downstream cellular response c. d. signaling molecule binding, dimerization, and the downstream cellular response the downstream cellular response This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 9 | Cell Communication 375 Signaling Molecules Produced by signaling cells and the subsequent binding to receptors in target cells, ligands act as chemical signals that travel to the target cells to coordinate responses. The types of molecules that serve as ligands are incredibly varied and range from small proteins to small ions like calcium (Ca2+). Small Hydrophobic Ligands Small hydrophobic ligands can directly diffuse through the plasma membrane and interact with internal receptors. Important members of this class of ligands are the steroid hormones. Steroids are lipids that have a hydrocarbon skeleton with four fused rings; different steroids have different functional groups attached to the carbon skeleton. Steroid hormones include the female sex hormone, estradiol, which is a type of estrogen; the male sex hormone, testosterone; and cholesterol, which is an important structural component of biological membranes and a precursor of steriod hormones (Figure 9.9). Other hydrophobic hormones include thyroid hormones |
and vitamin D. In order to be soluble in blood, hydrophobic ligands must bind to carrier proteins while they are being transported through the bloodstream. Figure 9.9 Steroid hormones have similar chemical structures to their precursor, cholesterol. Because these molecules are small and hy<|endoftext|>drophobic, they can diffuse directly across the plasma membrane into the cell, where they interact with internal receptors. Water-Soluble Ligands Water-soluble ligands are polar and therefore cannot pass through the plasma membrane unaided; sometimes, they are too large to pass through the membrane at all. Instead, most water-soluble ligands bind to the extracellular domain of cellsurface receptors. This group of ligands is quite diverse and includes small molecules, peptides, and proteins. Other Ligands Nitric oxide (NO) is a gas that also acts as a ligand. It is able to diffuse directly across the plasma membrane, and one of its roles is to interact with receptors in smooth muscle and induce relaxation of the tissue. NO has a very short half-life and therefore only functions over short distances. Nitroglycerin, a treatment for heart disease, acts by triggering the release of NO, which causes blood vessels to dilate (expand), thus restoring blood flow to the heart. 376 Chapter 9 | Cell Communication Think About It • Cells grown in the laboratory are placed in a solution containing a dye that is unable to pass through the plasma membrane. If a ligand is then added to the solution, observations show that the dye enters the cell. Describe the type of receptor the ligand most likely binds to and explain your reasoning. • HER2 is a receptor tyrosine kinase. In 30 percent of human breast cancers, HER2 is permanently activated, resulting in unregulated cell division. Lapatinib, a drug used to treat breast cancer, inhibits HER2 receptor tyrosine kinase autophosphorylation (the process by which the receptor adds phosphate onto itself), thus reducing tumor growth. Besides autophosphorylation, explain another feature of the cell signaling pathway that can be affected by Lapatinib. • In certain cancers, the GTPase activity of RAS G-protein in inhibited. This means that the RAS Gprotein can no longer hydrolyze GTP into GDP. Explain what effect this would have on downstream cellular events. 9.2 | Propagation of the Signal In this section, you will explore the following |
questions: • How does the binding of a ligand initiate signal transduction throughout a cell? • What is the role of second messengers in signal transduction? Connection for AP® Courses During signal transduction, a series of relay proteins inside the cytoplasm of the target cell activate target proteins, resulting in a cellular response. These cascades are complex because of the interplay between proteins. A significant contributor to cell signaling cascades is the phosphorylation of molecules by enzymes known as kinases. (Substrate–level phosphorylation was studied when you learned about glycolysis.) By adding a phosphate group, phosphorylation changes the shapes of proteins. This change in shape activates or inactivates them. Second messengers, e.g., cAMP and Ca2+, are often used to transmit signals within a cell. Information presented and the examples highlighted in the section support concepts and Learning Objectives outlined in Big Idea 3 of the AP® Biology Curriculum Framework. The Learning Objectives listed in the Curriculum Framework provide a transparent foundation for the AP® Biology course, an inquiry-based laboratory experience, instructional activities, and AP® Exam questions. A Learning Objective merges required content with one or more of the seven Science Practices. Big Idea 3 Enduring Understanding 3.D Essential Knowledge Science Practice Learning Objective Living systems store, retrieve, transmit and respond to information essential to life processes. Cells communicate by generating, transmitting and receiving chemical signals. 3.D.3 Signal transduction pathways link signal reception with cellular response. 1.5 The student can re-express key elements of natural phenomena across multiple representations in the domain. 3.36 The student is able to describe a model that expresses the key elements of signal transduction pathways by which a signal is converted to a cellular response. This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 9 | Cell Communication 377 The Science Practice Challenge Questions contain additional test questions for this section that will help you prepare for the AP exam. These questions address the following standards: [APLO 3.33][APLO 3.4][APLO 4.22][APLO 2.5][APLO 3.32][APLO 3.38] Once a ligand binds to a receptor, the signal is transmitted through the membrane and into the cytoplasm. Continuation of a signal in this manner is called signal transduction. Signal transduction only occurs |
with cell-surface receptors because internal receptors are able to interact directly with DNA in the nucleus to initiate protein synthesis. When a ligand binds to its receptor, conformational changes occur that affect the receptor’s intracellular domain. Conformational changes of the extracellular domain upon ligand binding can propagate through the membrane region of the receptor and lead to activation of the intracellular domain or its associated proteins. In some cases, binding of the ligand causes dimerization of the receptor, which means that two receptors bind to each other to form a stable complex called a dimer. A dimer is a chemical compound formed when two molecules (often identical) join together. The binding of the receptors in this manner enables their intracellular domains to come into close contact and activate each other. Binding Initiates a Signaling Pathway After the ligand binds to the cell-surface receptor, the activation of the receptor’s intracellular components sets off a chain of events that is called a signaling pathway or a signaling cascade. In a signaling pathway, second messengers, enzymes, and activated proteins interact with specific proteins, which are in turn activated in a chain reaction that eventually leads to a change in the cell’s environment (Figure 9.10). The events in the cascade occur in a series, much like a current flows in a river. Interactions that occur before a certain point are defined as upstream events, and events after that point are called downstream events. 378 Chapter 9 | Cell Communication Figure 9.10 The epidermal growth factor (EGF) receptor (EGFR) is a receptor tyrosine kinase involved in the regulation of cell growth, wound healing, and tissue repair. When EGF binds to the EGFR, a cascade of downstream events causes the cell to grow and divide. If EGFR is activated at inappropriate times, uncontrolled cell growth may occur. In certain cancers, the GTPase activity of the RAS G-protein is inhibited. This means that the RAS protein can no longer hydrolyze GTP into GDP. What effect would this have on downstream cellular events? a. Cells will not proliferate. b. ERK is permanently inactivated. c. Regulated cell division. d. Uncontrolled cell proliferation. Signaling pathways can get very complicated very quickly because most cellular proteins can affect different downstream events, depending on the conditions within the cell. A single pathway can branch off toward different endpoints based on the interplay between two or |
more signaling pathways, and the same ligands are often used to initiate different signals in This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 9 | Cell Communication 379 different cell types. This variation in response is due to differences in protein expression in different cell types. Another complicating element is signal integration of the pathways, in which signals from two or more different cell-surface receptors merge to activate the same response in the cell. This process can ensure that multiple external requirements are met before a cell commits to a specific response. The effects of extracellular signals can also be amplified by enzymatic cascades. At the initiation of the signal, a single ligand binds to a single receptor. However, activation of a receptor-linked enzyme can activate many copies of a component of the signaling cascade, which amplifies the signal. Methods of Intracellular Signaling The induction of a signaling pathway depends on the modification of a cellular component by an enzyme. There are numerous enzymatic modifications that can occur, and they are recognized in turn by the next component downstream. The following are some of the more common events in intracellular signaling. Observe an animation of cell signaling at this site (http://openstaxcollege.org/l/cell_signals). Hemophilia is a rare condition in which the blood lacks sufficient clotting factors. These factors are required for the platelets to bind together and form clots. How does this interfere with the cell signals during wound healing? a. delay and prevention of the cell signal required for wound healing b. activate the cell signal required for wound healing c. activate and enhance the cell signals for wound healing d. cell signal will remain unaffected Phosphorylation One of the most common chemical modifications that occurs in signaling pathways is the addition of a phosphate group –3) to a molecule such as a protein in a process called phosphorylation. The phosphate can be added to a nucleotide (PO4 such as GMP to form GDP or GTP. Phosphates are also often added to serine, threonine, and tyrosine residues of proteins, where they replace the hydroxyl group of the amino acid (Figure 9.11). The transfer of the phosphate is catalyzed by an enzyme called a kinase. Various kinases are named for the substrate they phosphorylate. Phosphorylation of serine and threonine residues often activates enzymes. |
Phosphorylation of tyrosine residues can either affect the activity of an enzyme or create a binding site that interacts with downstream components in the signaling cascade. Phosphorylation may activate or inactivate enzymes, and the reversal of phosphorylation, dephosphorylation by a phosphatase, will reverse the effect. 380 Chapter 9 | Cell Communication Figure 9.11 In protein phosphorylation, a phosphate group (PO4 threonine, and tyrosine. -3 ) is added to residues of the amino acids serine, Second Messengers Second messengers are small molecules that propagate a signal after it has been initiated by the binding of the signaling molecule to the receptor. These molecules help to spread a signal through the cytoplasm by altering the behavior of certain cellular proteins. Calcium ion is a widely used second messenger. The free concentration of calcium ions (Ca2+) within a cell is very low because ion pumps in the plasma membrane continuously use adenosine-5'-triphosphate (ATP) to remove it. For signaling purposes, Ca2+ is stored in cytoplasmic vesicles, such as the endoplasmic reticulum, or accessed from outside the cell. When signaling occurs, ligand-gated calcium ion channels allow the higher levels of Ca2+ that are present outside the cell (or in intracellular storage compartments) to flow into the cytoplasm, which raises the concentration of cytoplasmic Ca2+. The response to the increase in Ca2+ varies, depending on the cell type involved. For example, in the β-cells of the pancreas, Ca2+ signaling leads to the release of insulin, and in muscle cells, an increase in Ca2+ leads to muscle contractions. Another second messenger utilized in many different cell types is cyclic AMP (cAMP). Cyclic AMP is synthesized by the enzyme adenylyl cyclase from ATP (Figure 9.12). The main role of cAMP in cells is to bind to and activate an enzyme called cAMP-dependent kinase (A-kinase). A-kinase regulates many vital metabolic pathways: It phosphorylates serine and threonine residues of its target proteins, activating them in the process. A-kinase is found in many different types of cells, and the target proteins in each kind of cell are different. Differences give rise to the variation of the responses to c |
AMP in different cells. This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 9 | Cell Communication 381 Figure 9.12 This diagram shows the mechanism for the formation of cyclic AMP (cAMP). cAMP serves as a second messenger to activate or inactivate proteins within the cell. Termination of the signal occurs when an enzyme called phosphodiesterase converts cAMP into AMP. Present in small concentrations in the plasma membrane, inositol phospholipids are lipids that can also be converted into second messengers. Because these molecules are membrane components, they are located near membrane-bound receptors and can easily interact with them. Phosphatidylinositol (PI) is the main phospholipid that plays a role in cellular signaling. Enzymes known as kinases phosphorylate PI to form PI-phosphate (PIP) and PI-bisphosphate (PIP2). The enzyme phospholipase C cleaves PIP2 to form diacylglycerol (DAG) and inositol triphosphate (IP3) (Figure 9.13). These products of the cleavage of PIP2 serve as second messengers. Diacylglycerol (DAG) remains in the plasma membrane and activates protein kinase C (PKC), which then phosphorylates serine and threonine residues in its target proteins. IP3 diffuses into the cytoplasm and binds to ligand-gated calcium channels in the endoplasmic reticulum to release Ca2+ that continues the signal cascade. Figure 9.13 The enzyme phospholipase C breaks down PIP2 into IP3 and DAG, both of which serve as second messengers. 382 Chapter 9 | Cell Communication Think About It The same second messengers are used in many different cells, but the response to second messengers is different in each cell. How is this possible? 9.3 | Response to the Signal In this section you will explore the following questions: • How do signaling pathways direct protein expression, cellular metabolism, and cell growth? • What is the role of apoptosis in the development and maintenance of a healthy organism? Connection for AP® Courses The initiation of a signaling pathway results in a cellular response to changes in the external environment. This response can take many different forms, including protein synthesis, a |
change in cell metabolism, cell division and growth, or even cell death. As we will explore in more detail in later chapters, some pathways activate enzymes that interact within DNA transcription factors to promote gene expression, others can cause cells to store energy as glycogen as fat, or result in free energy availability in the form of glucose. Cell division and growth are almost always stimulated by external signals called growth factors; left unregulated, cell growth leads to cancer. Programmed cell death, or apoptosis, removes damaged or unnecessary cells and plays a vital role in development, including morphogenesis of fingers and toes. Termination of the cell signaling cascade is important to ensure that the response to a signal is appropriate in timing and intensity. Degradation of signaling molecules and dephosphorylation of intermediates of the pathway are two ways signals are terminated within cells. Conditions where signaling pathways are blocked or defective can be deleterious, preventative, or prophylactic; examples include diabetes, heart disease, autoimmune disease, toxins, anesthetics, and birth control pills. Information presented and the examples highlighted in the section support concepts and learning objectives outlined in Big Idea 3 and Big Idea 2 of the AP® Biology Curriculum Framework. The Learning Objectives listed in the Curriculum Framework provide a transparent foundation for the AP® Biology course, an inquiry-based laboratory experience, instructional activities, and AP® exam questions. A learning objective merges required content with one or more of the seven science practices. Big Idea 3 Enduring Understanding 3.D Essential Knowledge Science Practice Learning Objective Essential Knowledge Living systems store, retrieve, transmit and respond to information essential to life processes. Cells communicate by generating, transmitting and receiving chemical signals. 3.D.4 Changes in signal transduction pathways can alter cellular response. 1.5 The student can re-express key elements of natural phenomena across multiple representations in the domain. 3.36 The student is able to describe a model that expresses the key elements of signal transduction pathways by which a signal is converted to a cellular response. 3.D.4 Changes in signal transduction pathways can alter cellular response. This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 9 | Cell Communication 383 Science Practice Learning Objective Essential Knowledge Science Practice Learning Objective Big Idea 2 Enduring Understanding 2.E 6.1 The student can justify claims with evidence. 3.37 The student is able to justify claims based on scientific evidence that changes in |
signal transduction pathways can alter cellular response. 3.D.4 Changes in signal transduction pathways can alter cellular response. 6.2 The student can construct explanations of phenomena based on evidence produced through scientific practices. 3.39 The student is able to construct an explanation of how certain drugs affect signal reception and, consequently, signal transduction pathways. Biological systems utilize free energy and molecular building blocks to grow, to reproduce, and to maintain dynamic homeostasis. Many biological processes involved in growth, reproduction and dynamic homeostasis include temporal regulation and coordination. Essential Knowledge 2.E.1 Timing and coordination of specific events are necessary for the normal development of an organism, and these events are regulated by a variety of mechanisms. Science Practice Learning Objective 7.1 The student can connect phenomena and models across spatial and temporal scales. 2.34 The student is able to describe the role of programmed cell death in development and differentiation, the reuse of molecules, and the maintenance of dynamic homeostasis. The Science Practice Challenge Questions contain additional test questions for this section that will help you prepare for the AP exam. These questions address the following standards: [APLO 3.33][APLO 3.35] Inside the cell, ligands bind to their internal receptors, allowing them to directly affect the cell’s DNA and protein-producing machinery. Using signal transduction pathways, receptors in the plasma membrane produce a variety of effects on the cell. The results of signaling pathways are extremely varied and depend on the type of cell involved as well as the external and internal conditions. A small sampling of responses is described below. Gene Expression Some signal transduction pathways regulate the transcription of RNA. Others regulate the translation of proteins from mRNA. An example of a protein that regulates translation in the nucleus is the MAP kinase ERK. ERK is activated in a phosphorylation cascade when epidermal growth factor (EGF) binds the EGF receptor (see Figure 9.10). Upon phosphorylation, ERK enters the nucleus and activates a protein kinase that, in turn, regulates protein translation (Figure 9.14). 384 Chapter 9 | Cell Communication Figure 9.14 ERK is a MAP kinase that activates translation when it is phosphorylated. ERK phosphorylates MNK1, which in turn phosphorylates eIF-4E, an elongation initiation factor that, with other initiation factors, is associated with mRNA. When eIF-4E becomes phosphorylated, the mRNA unfolds, |
allowing protein synthesis in the nucleus to begin. (See Figure 9.10 for the phosphorylation pathway that activates ERK.) The second kind of protein with which PKC can interact is a protein that acts as an inhibitor. An inhibitor is a molecule that binds to a protein and prevents it from functioning or reduces its function. In this case, the inhibitor is a protein called Iκ-B, which binds to the regulatory protein NF-κB. (The symbol κ represents the Greek letter kappa.) When Iκ-B is bound to NF-κB, the complex cannot enter the nucleus of the cell, but when Iκ-B is phosphorylated by PKC, it can no longer bind NF-κB, and NF-κB (a transcription factor) can enter the nucleus and initiate RNA transcription. In this case, the effect of phosphorylation is to inactivate an inhibitor and thereby activate the process of transcription. Increase in Cellular Metabolism The result of another signaling pathway affects muscle cells. The activation of β-adrenergic receptors in muscle cells by adrenaline leads to an increase in cyclic AMP (cAMP) inside the cell. Also known as epinephrine, adrenaline is a hormone (produced by the adrenal gland attached to the kidney) that readies the body for short-term emergencies. Cyclic AMP activates PKA (protein kinase A), which in turn phosphorylates two enzymes. The first enzyme promotes the degradation of glycogen by activating intermediate glycogen phosphorylase kinase (GPK) that in turn activates glycogen phosphorylase (GP) that catabolizes glycogen into glucose. (Recall that your body converts excess glucose to glycogen for short-term storage. When energy is needed, glycogen is quickly reconverted to glucose.) Phosphorylation of the second enzyme, glycogen synthase (GS), inhibits its ability to form glycogen from glucose. In this manner, a muscle cell obtains a ready pool of glucose by activating its formation via glycogen degradation and by inhibiting the use of glucose to form glycogen, thus preventing a futile cycle of glycogen degradation and synthesis. The glucose is then available for use by the muscle cell in response to a sudden surge of adrenaline—the “fight or flight” reflex. Cell Growth Cell signaling pathways also play a major role in cell division. Cells do not normally divide unless they are stimulated by signals from other cells. The lig |
ands that promote cell growth are called growth factors. Most growth factors bind to cell-surface receptors that are linked to tyrosine kinases. These cell-surface receptors are called receptor tyrosine kinases (RTKs). Activation of RTKs initiates a signaling pathway that includes a G-protein called RAS, which activates the MAP kinase pathway described earlier. The enzyme MAP kinase then stimulates the expression of proteins that interact with other cellular components to initiate cell division. This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 9 | Cell Communication 385 Cancer biologists study the molecular origins of cancer with the goal of developing new prevention methods and treatment strategies that will inhibit the growth of tumors without harming the normal cells of the body. As mentioned earlier, signaling pathways control cell growth. These signaling pathways are controlled by signaling proteins, which are, in turn, expressed by genes. Mutations in these genes can result in malfunctioning signaling proteins. This prevents the cell from regulating its cell cycle, triggering unrestricted cell division and cancer. The genes that regulate the signaling proteins are one type of oncogene, which is a gene that has the potential to cause cancer. The gene encoding RAS is an oncogene that was originally discovered when mutations in the RAS protein were linked to cancer. Further studies have indicated that 30 percent of cancer cells have a mutation in the RAS gene that leads to uncontrolled growth. If left unchecked, uncontrolled cell division can lead to tumor formation and metastasis, the growth of cancer cells in new locations in the body. Cancer biologists have been able to identify many other oncogenes that contribute to the development of cancer. For example, HER2 is a cell-surface receptor that is present in excessive amounts in 20 percent of human breast cancers. Cancer biologists realized that gene duplication led to HER2 overexpression in 25 percent of breast cancer patients and developed a drug called Herceptin (trastuzumab). Herceptin is a monoclonal antibody that targets HER2 for removal by the immune system. Herceptin therapy helps to control signaling through HER2. The use of Herceptin in combination with chemotherapy has helped to increase the overall survival rate of patients with metastatic breast cancer. More information on cancer biology research can be found at the National Cancer Institute website (http://openstaxcollege.org/l/32NCI). Cell Death When a cell is damaged |
, superfluous, or potentially dangerous to an organism, a cell can initiate a mechanism to trigger programmed cell death, or apoptosis. Apoptosis allows a cell to die in a controlled manner that prevents the release of potentially damaging molecules from inside the cell. There are many internal checkpoints that monitor a cell’s health; if abnormalities are observed, a cell can spontaneously initiate the process of apoptosis. However, in some cases, such as a viral infection or uncontrolled cell division, the cell’s normal checks and balances fail. External signaling can also initiate apoptosis. For example, most normal animal cells have receptors that interact with the extracellular matrix, a network of glycoproteins that provides structural support for cells in an organism. The binding of cellular receptors to the extracellular matrix initiates a signaling cascade within the cell. However, if the cell moves away from the extracellular matrix, the signaling ceases, and the cell undergoes apoptosis. This system keeps cells from traveling through the body and proliferating out of control. Another example of external signaling that leads to apoptosis occurs in T-cell development. T-cells are immune cells that bind to foreign macromolecules and particles, and target them for destruction by the immune system. Normally, T-cells do not target “self” proteins (those of their own organism), a process that can lead to autoimmune diseases. In order to develop the ability to discriminate between self and non-self, immature T-cells undergo screening to determine whether they bind to so-called self proteins. If the T-cell receptor binds to self proteins, the cell initiates apoptosis to remove the potentially dangerous cell. Apoptosis is also essential for normal embryological development. In vertebrates, for example, early stages of development include the formation of web-like tissue between individual fingers and toes (Figure 9.15). During the course of normal development, these unneeded cells must be eliminated, enabling fully separated fingers and toes to form. A cell signaling mechanism triggers apoptosis, which destroys the cells between the developing digits. 386 Chapter 9 | Cell Communication Figure 9.15 The histological section of a foot of a 15-day-old mouse embryo, visualized using light microscopy, reveals areas of tissue between the toes, which apoptosis will eliminate before the mouse reaches its full gestational age at 27 days. (credit: modification of work by Michal Mañas) Termination of the Signal Cascade The aberrant signaling |
often seen in tumor cells is proof that the termination of a signal at the appropriate time can be just as important as the initiation of a signal. One method of stopping a specific signal is to degrade the ligand or remove it so that it can no longer access its receptor. One reason that hydrophobic hormones like estrogen and testosterone trigger longlasting events is because they bind carrier proteins. These proteins allow the insoluble molecules to be soluble in blood, but they also protect the hormones from degradation by circulating enzymes. Inside the cell, many different enzymes reverse the cellular modifications that result from signaling cascades. For example, phosphatases are enzymes that remove the phosphate group attached to proteins by kinases in a process called dephosphorylation. Cyclic AMP (cAMP) is degraded into AMP by phosphodiesterase, and the release of calcium stores is reversed by the Ca2+ pumps that are located in the external and internal membranes of the cell. Activity Explain the mechanism by which a specific disease is caused by a defective signaling pathway. Then, investigate online how a specific drug works by blocking a signaling pathway. 9.4 | Signaling in Single-Celled Organisms In this section, you will explore the following questions: • How do single-celled yeasts use cell signaling to communicate with each other? • How does quorum sensing allow some bacteria to form biofilms? Connection for AP® Courses Cell signaling allows bacteria to respond to environmental cues, such as nutrient levels and quorum sensing (cell density). This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 9 | Cell Communication 387 Yeasts are eukaryotes (fungi), and the components and processes found in yeast signals are similar to those of cell-surface receptor signals in multicellular organisms. For example, budding yeasts often release mating factors that enable them to participate in a process that is similar to sexual reproduction. Information presented and the examples highlighted in the section support concepts and Learning Objectives outlined in Big Idea 3 of the AP® Biology Curriculum Framework. The Learning Objectives listed in the Curriculum Framework provide a transparent foundation for the AP® Biology course, an inquiry-based laboratory experience, instructional activities, and AP® Exam questions. A Learning Objective merges required content with one or more of the seven Science Practices. Big Idea 3 Enduring Understanding 3.D Living systems store, retrieve, transmit and respond to |
information essential to life processes. Cells communicate by generating, transmitting and receiving chemical signals. Essential Knowledge 3.D.1 Cell communication processes share common features that reflect a shared evolutionary history. Science Practice Learning Objective 1.5 The student can re-express key elements of natural phenomena across multiple representations in the domain. 3.36 The student is able to describe a model that expresses the key elements of signal transduction pathways by which a signal is converted to a cellular response. Essential Knowledge 3.D.1 Cell communication processes share common features that reflect a shared evolutionary history. Science Practice Learning Objective 6.1 The student can justify claims with evidence. 3.37 The student is able to justify claims based on scientific evidence that changes in signal transduction pathways can alter cellular response. The Science Practice Challenge Questions contain additional test questions for this section that will help you prepare for the AP exam. These questions address the following standards: [APLO 3.31][APLO 3.37] Within-cell signaling allows bacteria to respond to environmental cues, such as nutrient levels. Some single-celled organisms also release molecules to signal to each other. Signaling in Yeast Yeasts are eukaryotes (fungi), and the components and processes found in yeast signals are similar to those of cell-surface receptor signals in multicellular organisms. Budding yeasts (Figure 9.16) are able to participate in a process that is similar to sexual reproduction that entails two haploid cells (cells with one-half the normal number of chromosomes) combining to form a diploid cell (a cell with two sets of each chromosome, which is what normal body cells contain). In order to find another haploid yeast cell that is prepared to mate, budding yeasts secrete a signaling molecule called mating factor. When mating factor binds to cell-surface receptors in other yeast cells that are nearby, they stop their normal growth cycles and initiate a cell signaling cascade that includes protein kinases and GTP-binding proteins that are similar to G-proteins. 388 Chapter 9 | Cell Communication Figure 9.16 Budding Saccharomyces cerevisiae yeast cells can communicate by releasing a signaling molecule called mating factor. In this micrograph, they are visualized using differential interference contrast microscopy, a light microscopy technique that enhances the contrast of the sample. Signaling in Bacteria Signaling in bacteria enables bacteria to monitor extracellular conditions, ensure that there are sufficient amounts of nutrients, and ensure that hazardous situations are avoided. |
There are circumstances, however, when bacteria communicate with each other. The first evidence of bacterial communication was observed in a bacterium that has a symbiotic relationship with Hawaiian bobtail squid. When the population density of the bacteria reaches a certain level, specific gene expression is initiated, and the bacteria produce bioluminescent proteins that emit light. Because the number of cells present in the environment (cell density) is the determining factor for signaling, bacterial signaling was named quorum sensing. In politics and business, a quorum is the minimum number of members required to be present to vote on an issue. Quorum sensing uses autoinducers as signaling molecules. Autoinducers are signaling molecules secreted by bacteria to communicate with other bacteria of the same kind. The secreted autoinducers can be small, hydrophobic molecules such as acyl-homoserine lactone (AHL) or larger peptide-based molecules; each type of molecule has a different mode of action. When AHL enters target bacteria, it binds to transcription factors, which then switch gene expression on or off (Figure 9.17). The peptide autoinducers stimulate more complicated signaling pathways that include bacterial kinases. The changes in bacteria following exposure to autoinducers can be quite extensive. The pathogenic bacterium Pseudomonas aeruginosa has 616 different genes that respond to autoinducers. This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 9 | Cell Communication 389 Figure 9.17 Autoinducers are small molecules or proteins produced by bacteria that regulate gene expression. Which of the following statements about quorum sensing is false? a. Autoinducers must bind to receptors to turn on transcription of genes responsible for the production of more autoinducers. b. Autoinducers can only act on a different cell. It cannot act on the cell in which it is made. c. Autoinducers turn on genes that enable the bacteria to form a biofilm. d. The receptor stays in the bacterial cell, but the autoinducers diffuse out. Some species of bacteria that use quorum sensing form biofilms, complex colonies of bacteria (often containing several species) that exchange chemical signals to coordinate the release of toxins that will attack the host. Bacterial biofilms (Figure 9.18) can sometimes be found on medical equipment; when biofilms invade implants such |
as hip or knee replacements or heart pacemakers, they can cause life-threatening infections. The ability of certain bacteria to form biofilms has evolved because of a selection of genes that enable cell-cell communication confers an evolutionary advantage. When bacterial colonies form biofilms, they create barriers that prevent toxins and antibacterial drugs from affecting the population living in the biofilm. As a result, these populations are more likely to survive, even in the presence of antibacterial agents. This often means that bacteria living in biofilms have higher fitness than bacteria living on their own. 390 Chapter 9 | Cell Communication Think About It Why is signaling in multicellular organisms more complicated than signaling in single-celled organisms such as microbes? Figure 9.18 Cell-cell communication enables these (a) Staphylococcus aureus bacteria to work together to form a biofilm inside a hospital patient’s catheter, seen here via scanning electron microscopy. S. aureus is the main cause of hospital-acquired infections. (b) Hawaiian bobtail squid have a symbiotic relationship with the bioluminescent bacteria Vibrio fischeri. The luminescence makes it difficult to see the squid from below because it effectively eliminates its shadow. In return for camouflage, the squid provides food for the bacteria. Free-living V. fischeri do not produce luciferase, the enzyme responsible for luminescence, but V. fischeri living in a symbiotic relationship with the squid do. Quorum sensing determines whether the bacteria should produce the luciferase enzyme. (credit a: modifications of work by CDC/Janice Carr; credit b: modifications of work by Cliff1066/Flickr) Free-living V. fischeri do not luminesce. Why? a. The squid provides certain nutrients that allow the bacteria to luminesce. b. The squid produces the luminescent luciferase enzyme, so bacteria living outside the squid do not luminesce. c. The ability to luminesce does not benefit free-living bacteria, so free-living bacteria do not produce luciferase. d. Luciferase is toxic to free-living bacteria, so free-living bacteria do not produce this enzyme. Research on the details of quorum sensing has led to advances in growing bacteria for industrial purposes. Recent discoveries suggest that it may be possible to exploit bacterial signaling pathways to control bacterial growth; this process could replace or supplement antibiotics that are no |
longer effective in certain situations. This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 9 | Cell Communication 391 Watch geneticist Bonnie Bassler discuss her discovery (http://openstaxcollege.org/l/bacteria_talk) of quorum sensing in biofilm bacteria in squid. What does bioluminescence show about communication in bacteria? a. Bacteria interact by physical signals among a colony. b. Bacterium interact by chemical signals when it is alone. c. Bacterium interact by physical signals when it is alone. d. Bacteria interact by chemical signals among a colony. The first life on our planet consisted of single-celled prokaryotic organisms that had limited interaction with each other. While some external signaling occurs between different species of single-celled organisms, the majority of signaling within bacteria and yeasts concerns only other members of the same species. The evolution of cellular communication is an absolute necessity for the development of multicellular organisms, and this innovation is thought to have required approximately 2.5 billion years to appear in early life forms. Yeasts are single-celled eukaryotes, and therefore have a nucleus and organelles characteristic of more complex life forms. Comparisons of the genomes of yeasts, nematode worms, fruit flies, and humans illustrate the evolution of increasingly complex signaling systems that allow for the efficient inner workings that keep humans and other complex life forms functioning correctly. Kinases are a major component of cellular communication, and studies of these enzymes illustrate the evolutionary connectivity of different species. Yeasts have 130 types of kinases. More complex organisms such as nematode worms and fruit flies have 454 and 239 kinases, respectively. Of the 130 kinase types in yeast, 97 belong to the 55 subfamilies of kinases that are found in other eukaryotic organisms. The only obvious deficiency seen in yeasts is the complete absence of tyrosine kinases. It is hypothesized that phosphorylation of tyrosine residues is needed to control the more sophisticated functions of development, differentiation, and cellular communication used in multicellular organisms. Because yeasts contain many of the same classes of signaling proteins as humans, these organisms are ideal for studying signaling cascades. Yeasts multiply quickly and are much simpler organisms than humans or other multicellular animals. Therefore, the signaling cascades are also simpler and easier to study, although they contain |
Subsets and Splits
No community queries yet
The top public SQL queries from the community will appear here once available.