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whereas isobilateral symmetry is observed in dicot leaves. d. Monocots have leaves with reticulate, net-like venation and dicot leaves have parallel venation. 80. How does a compound leaf give a selective advantage to avoid herbivory? This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 23 | Plant Form and Physiology 1005 a. Compound leaves produce certain types of chemical compounds that are harmful to herbivores. b. It is more efficient for large herbivores to eat large, simple leaves. c. Compound leaves are thicker than simple leaves. d. It is more efficient for large herbivores to eat the small leaflets of compound leaves. examined. Assume that the contribution of gravity and matric potential are negligible and can be ignored. Recall that the overall water potential for a system is represented by the equation: Ψsystem = Ψtotal = Ψs + Ψp + Ψg + Ψm overall Ψ of the soil: -2.1MPa solute potential of the plant’s cell contents: -0.12MPa pressure potential (Ψp) of the plant’s cells: -2.3 MPa Is Plant X a good candidate for introduction to the salt marsh? 81. Stomata are usually found in higher numbers on the abaxial or bottom surface of a leaf. What is the advantage of such an arrangement? a. Presence of stomata on the abaxial or bottom surface ensures that no, or very little, water is lost due to guttation. b. The abaxial or bottom surface receives more sunlight and water evaporates faster by transpiration. c. Herbivores do not prefer to eat leaves with stomata on the abaxial or bottom surface. d. The adaxial or upper surface receives more sunlight and water evaporates faster by transpiration. 82. Which plants have leaves that are adapted to cold temperatures? a. Conifers such as spruce, fir, and pine have ovalshaped leaves with sunken stomata, helping to reduce water loss. b. Succulents such as aloes and agaves have waxy cuticles with sunken stomata, helping to reduce water loss. c. Conifers such as spruce, orchids, and pine have needle- |
shaped leaves with sunken stomata, helping to reduce water loss. d. Conifers such as spruce, fir, and pine have needle-shaped leaves with sunken stomata, helping to reduce water loss. 83. How is a leaf different from a leaflet? a. A leaf petiole attaches directly to the stem at a bud node, whereas a leaflet petiole is attached to the main petiole or the midrib, not the stem. a. Yes, because the overall water potential of the plant is less negative than the water potential of the soil. b. No, because the overall water potential of the plant is less negative than the water potential of the soil. c. Yes, because the overall water potential of the plant is more negative than the water potential of the soil. d. No, because the overall water potential of the plant is more negative than the water potential of the soil. 85. What organs in humans are similar in function to the vascular tissues of vascular plants? 86. Apoptosis, or programmed cell death, is an important step in the development of xylem. How does apoptosis contribute to xylem development? 87. A florist decided to paint the leaves of poinsettia with a gold paint to embellish them. The plant soon wilted and the leaves drooped. What explains this damage? a. The paint clogged the stomata. Without photosynthesis, the plant could not pull water from the soil. b. The paint clogged the stomata. Without transpiration, the plant could not pull water from the soil. c. The paint clogged the hydathodes. Without transpiration, the plant could not pull water from the soil. d. The paint clogged the stomata. Without guttation, the plant could not pull water from the soil. b. A leaf has reticulate venation whereas leaflets show parallel venation. 88. The process of bulk flow transports fluids in a plant. What are the two main bulk flow processes? c. A leaf petiole attaches to the main petiole or the midrib, not the stem, whereas a leaflet petiole attaches directly to the stem at a bud node. d. A leaf has parallel venation whereas leaflets show reticulate venation. 84. Scientists on a new project to restore a damaged salt marsh are investigating several plants that could be introduced. Plant X is considered a possible |
candidate. Before the decision is made, the following data are a. Movement of water up the xylem and movement of solutes up and down the phloem b. Movement of water up the phloem and movement of solutes up and down the xylem. c. Movement of water up and down the xylem and movement of solutes up the phloem d. Movement of solutes up the xylem and movement of water up and down the phloem 1006 Chapter 23 | Plant Form and Physiology 89. During a severe drought, the soil becomes dry and its water potential decreases. Many plants will wilt in such an environment. Consider that the overall water potential for a system is represented by the equation: Ψsystem = Ψtotal = Ψs + Ψp + Ψg + Ψm What is one reason that plants are unable to draw water from the soil? a. The water potential of the soil becomes lower than the water potential of the plants. b. The water potential of the soil becomes lower than the solute potential of the plants. c. The water potential of the soil becomes higher than the water potential of the plants. d. The solute potential of the soil becomes lower than the water potential of the plants. 90. A botanist compares the number of stomata between two plants. One plant, a eucalyptus, has stomata equally distributed on both sides of the leaf. The other plant has most of its stomata on the underside of the leaf. What does the positioning of the stomata indicate about which leaf surfaces on the two plants receive light in their natural environment? a. The first plant receives light only on the upper surface of the leaves whereas the leaves of the second plant are equally exposed to sunlight. b. The first plant receives light only on the lower surface whereas the second plant receives light only on the upper surface. c. The first plant receives light only on the upper surface whereas the second plant receives light only on the lower surface. d. The first plant has leaves that are equally exposed to sunlight whereas the second plant receives light only on the upper surface. 91. In the Northern Hemisphere, owners and managers of plant nurseries have to plan lighting schedules for a longday plant that will flower in February. What lighting periods and color will be most effective? a. Long periods of illumination with light enriched in the red range of the spectrum b. Short periods of illumination with |
light enriched in the red range of the spectrum c. Long periods of illumination with light enriched in the far-red range of the spectrum d. Short periods of illumination with light enriched in the far-red range of the spectrum 92. Why do plants that cannot detect gravity show stunted growth with tangled roots and trailing stems? This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 a. Without gravitropism, both roots and seedlings would grow upward. b. Without gravitropism, roots would grow in all directions and seedlings would grow upward. c. Without gravitropism, roots would grow upward but seedlings would not grow upward toward the surface. d. Without gravitropism, roots would grow in all directions but seedlings would not grow upward toward the surface. 93. Storage facilities for fruits and vegetables are usually refrigerated and well ventilated. Why are these conditions advantageous? a. Refrigeration slows chemical reactions, including fruit ripening. Ventilation adds the ethylene gas that speeds up fruit maturation. b. Refrigeration slows chemical reactions, including fruit maturation. Ventilation removes the ethylene gas that reduces fruit ripening. c. Refrigeration slows chemical reactions, including fruit maturation. Ventilation removes the ethylene gas that speeds up fruit ripening. d. Refrigeration removes the ethylene gas that speeds up fruit ripening. Ventilation slows chemical reactions, including fruit maturation. 94. A Venus fly trap has a very low sensitivity threshold, yet it can tell the difference between the light touch of an insect and a drop of rainwater or wind. How can the Venus fly trap differentiate between a random stimulus and an actual prey? a. Hair-like appendages on the surface of the leaves respond to repeated contact. b. Hair-like appendages on the surface of the leaves respond to a single contact. c. Hair-like appendages on the surface of the leaves respond to chemical stimulus from the insect. d. Hair-like appendages on the surface of the leaves respond to the electrical stimulus from the insect. 95. Stomata close in response to bacterial infection. This response is a defense mechanism because it ________, and the hormone involved is ________. b. a. restricts the entry of O2; gibberellin restricts the entry of CO2; abscisic acid c. prevents further entry of pathogens; aux |
in d. prevents further entry of pathogens; abscisic acid 96. Why is shade avoidance an important survival mechanism for plants? Would you expect seeds with large energy storage to display as strong a response of shade avoidance as small seeds with limited reserves? Chapter 23 | Plant Form and Physiology 1007 a. A seedling growing in the shade of a mature plant will not have enough light to promote meristematic growth. A seed with large storage will be able to sustain growth until its seedling can reach enough light for photosynthesis. b. A seedling growing in the shade of a mature plant will not have enough light to promote photosynthesis. Small seeds with limited reserve will be able to sustain growth until seedlings can reach enough light for photosynthesis. c. A seedling growing in the shade of a mature plant will not have enough light to promote photosynthesis. A seed with large storage will be able to sustain growth until its seedling can reach enough light for photosynthesis. d. A seedling growing in the shade of a mature plant will not have enough light to promote respiration. Small seeds with limited reserve will be able to sustain growth until their seedlings can reach enough light for photosynthesis. TEST PREP FOR AP® COURSES 97. A plant has a measured pressure potential Ψp = 0.21MPa and a solute potential Ψs =-3.50MPa. The soil is saturated with water because it rained. How will the water move? After three months of dry weather, the soil has dried out. How will the water potential of the soil compare to the water potential measured immediately before the rain? How will the stomata respond to the change in weather? a. The water will move from the plant to the soil. Dry soil has a lower water potential than wet soil. Under drought conditions, the stomata close to conserve water and leaves may also be shed if the drought continues. b. The water will move from the soil to the plant. Dry soil has a higher water potential than wet soil. Under drought conditions, the stomata close to conserve water and leaves may also be shed if the drought continues. c. The water will move from the soil to the plant. Dry soil has a lower water potential than wet soil. Under drought conditions, the stomata open its pores wider in order to perform a better rate of transpiration. d. The water will move from the soil to the plant. Dry soil has a |
lower water potential than wet soil. Under drought conditions, the stomata close to conserve water and leaves may also be shed if the drought continues. 98. Plants lose water from their aboveground surfaces in the process of transpiration. Most of this water is lost from stomata. Excess loss of water has severe consequences and may be fatal for the plant. The table shows data collected on a sunny day. What is the best explanation for the transpiration rates leveling off and declining at temperature higher than 27°C? a. The plant ran out of water. b. The plant needs less water as temperature increases, so transpiration slows down to limit water uptake by the roots. c. Stomata close to conserve water, slowing down transpiration. d. The amount of water in the leaves decreases at high temperature and less is available for evaporation. 99. Humidity is an environmental factor that affects transpiration rate. Which statement accurately explains the 1008 Chapter 23 | Plant Form and Physiology shape of the curve obtained when increasing humidity is plotted against constant temperature to find the rate of transcription? a. b. c. d. Increasing humidity leads to reduced evaporation rates due to increased difference in water vapor pressure between leaf and atmosphere. Increasing humidity leads to reduced evaporation rates due to decreased difference in water vapor pressure between leaf and soil. Increasing humidity leads to reduced evaporation rates due to decreased difference in water vapor pressure between leaf and atmosphere. Increasing humidity leads to increased evaporation rates due to decreased difference in water vapor pressure between leaf and atmosphere. 100. Plants sense drought through the decrease in water potential in the ground. This graph shows concentrations of several hormones that were measured during a drought period and plotted versus time. According to the data in the graph, which hormone shows the strongest response to drought? a. auxin b. abscisic acid c. cytokinin d. gibberellins 101. This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 When drought conditions are forecast, fields are sprayed with a hormone that will promote a stress response. According to the graph, which hormone should be sprayed and why? a. Gibberellins, to promote plant growth before the plants are damaged b. Abscisic acid, to promote plant growth before the plants are damaged c. Abscisic acid, to promote protective response to drought before the plants are damaged |
. d. Gibberellins, to promote protective response to drought before the plants are damaged. 102. Seeds were germinated in the dark on three plates. Plate A was irradiated with a short pulse of red light; plate B was irradiated with a short pulse of red light followed by a pulse of far-red light; and plate C was the control and was maintained in the dark. After three days, the plates were scored for percentage of germination, as shown in this table. What conclusion can be drawn from the experiment? Chapter 23 | Plant Form and Physiology 1009 a. Darkness inhibits germination. b. Red light promotes germination. c. Far-red light promotes germination. d. Germination is independent from light irradiation. 103. a. The sealant stopped evaporation. b. The plants with sealed cuts grew new branches. c. The plants with unsealed cuts were infected by pathogens that entered through the cuts. d. The plants with unsealed cuts lost photosynthates through bleeding of sap. 105. Seeds were germinated in the dark on three plates. Plate A was irradiated with a short pulse of red light; plate B was irradiated with a short pulse of red light followed immediately by a pulse of far-red light; plate D was irradiated by a short pulse of red light followed one hour later by a pulse of far-red light; and plate C was the control and was maintained in the dark. After three days, the plates were scored for percentage of germination, as shown in this table. What hypothesis do the results suggest about the mechanism of action of red light? a. Red light converts the phytochrome to its active form Pr which can be converted to the inactive form Pfr by far red light. After one hour, cascade of events initiated by Pfr has already begun promoting germination and hence, it cannot be reversed even by the pulse of far light. b. Red light converts the phytochrome to its active form Pfr, which can be converted to the inactive form Pr by far-red light. After one hour, cascade of events initiated by Pr has already begun promoting germination and, hence, it cannot be reversed even by the pulse of far light. c. Far red light converts the phytochrome to its active form Pfr, which can be converted to the inactive form Pr by red light. After one hour, the cascade of events initiated by Pr has already begun |
promoting germination and, hence, it cannot be reversed even by the pulse of far light. d. Red light converts the phytochrome to its active form Pfr which can be converted to the inactive form Pr by far red light. After one hour, the cascade of events initiated by Pfr has already begun promoting germination and, hence, it cannot be reversed even by the pulse of far light. 104. After branches of woody saplings were trimmed, half of the cuts were covered with a sealant and the other half were left untouched. The plants with sealed cuts fared much better after several weeks. What is the likely reason? Jasmonate is produced in plants as a response to injury. Researchers compared the response to infection of mutant plants that were unable to produce jasmonate (Ja-) with the response of normal plants (Ja+) from the same species. Leaves were inoculated with spores from pathogenic molds. The size of the wounds was examined 48 hours after application. The plants were also infected with moths and the weight of the larvae was determined aafter 48 hours. This table shows the results. According to the results of the experiment, what conclusion can the researchers draw about the specificity of jasmonate protection? a. b. c. d. Jasmonate protects against infection from a variety of pathogens. Jasmonate protects against infection from one pathogen. Jasmonate cannot provide protection against infection. Jasmonate provides specific defense in winters and the defense is non-specific in summers. 106. In the Northern Hemisphere, a florist grows shrubs of the same species of woody plant under two different light schedules for three weeks. The first set is maintained under 15 hours of light and 9 hours of dark daily. The second set is maintained under 9 hours of light followed by 14 hours of dark daily. The first set of plants does not form flowers, but the second set of plants blooms. What can you conclude about these plants? 1010 107. a. This species of shrub does not flower if the day is short. b. They bloom early in the year (around February). c. They bloom mid-summer (around June). d. The critical dark period is 9 hours. Chapter 23 | Plant Form and Physiology Heliotropism is the description of a response to the light of the sun. Seedlings of sunflowers were exposed to sunlight for 15 days. Following the 15 days of exposure to sunlight, the seedlings |
were transferred to complete darkness and their movement was monitored. This graph plots the movement of the seedlings in the dark versus time. What conclusion can be drawn about the light dependence of the movement of sunflowers from the graph? a. The movement does require light once it is set but it will eventually slow down, suggesting that a “clock” molecule is degraded over time. b. The movement does not require light once it is set and it will keep showing this upward and downward trend in the same manner. A student randomly chose 40 tobacco seeds of the same species from a packet. He placed 20 seeds on moist paper towels in each of two petri dishes. He wrapped dish A completely in an opaque cover to exclude all light. He did not wrap dish B. He placed the dishes equidistant from a light source set to a cycle of 14 hours of light and 10 hours of dark. All other conditions were the same for the two dishes. He examined the dishes after 7 days, and permanently removed the opaque cover from dish A. This table shows the student’s data. The most probable cause for the difference in mean stem length between plants in dish A and plants in dish B is ____. a. shortening of cells in the stem in response to the lack of light b. elongation of the stem in response to the lack of light c. enhancement of stem elongation by light d. genetic differences between the seeds c. The movement does not require light once it is 109. set and it will eventually slow down, suggesting that a “clock” molecule never degrades. d. The movement does not require light once it is set and it will eventually slow down, suggesting that a “clock” molecule is degraded over time. 108. This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 23 | Plant Form and Physiology 1011 Groups of 20 seedlings from the same plant species were treated with gibberellins. Each group received a different concentration of hormone. The seedlings were grown under the same environmental conditions. After 15 days of growth, the internode distances between the first and second sets of leaves were measured in each group of seedlings. On this graph, the mean internode distance for each group is plotted against the concentration of gibberellins that the group received. According to the results, why is this effect of gibberellins on internode |
length used in agriculture to spray grapes with oversized fruit? a. b. c. d. to lengthen the internode distance and accommodate larger fruit to shorten the internode distance and accommodate larger fruit to lengthen the internode distance and accommodate more flowers to shorten the internode distance and accommodate smaller fruit SCIENCE PRACTICE CHALLENGE QUESTIONS 110. The net photosynthetic production rate (NPP) is the difference between the rate of carbon fixation by photosynthesis (P) and the respiration rate (R). Each of these rates can be expressed in units of grams of carbon per day (gC/d). Vascular plants convert fixed carbon that is not released as carbon dioxide into biomass with a growth rate (G). A. Draw areas within the box to represent the rates of growth (G) and respiration (R) to show the limit of each on the overall growth rate. The area of the box represents the rate of photosynthesis (P). Figure 23.42 When the dependences on temperature of photosynthetic and respiration rates of a vascular plant are measured, the results depend on the species but have the general form shown in the figure. In these measurements, the temperature is maintained for several hours. The plant is then returned to 25 °C for several hours before the next set of measurements is made at a slightly higher temperature. Figure 23.43 B. Evaluate these data to approximately predict the quantitative effect on the NPP and free energy availability in a deciduous forest ecosystem with a 3–5-°C increase in temperature. This is the expected temperature increase by the year 2100. Assume the current average summer temperature of the forest ecosystem is 25 °C. In other experiments, rather than returning the plants to 25 °C, the plant is grown for several days at a constant higher 1012 Chapter 23 | Plant Form and Physiology temperature. Under these conditions, the maximum photosynthetic rate shifts towards the temperature of the new growing conditions. However, there is little change in the temperature dependence of respiration rate. This is referred to as temperature acclimation, an effect of great importance to predictions of future climate change. C. Pose two scientific questions whose pursuit could lead to either an improved understanding of the mechanisms of temperature acclimation or improvements in models of atmospheric carbon dioxide concentrations that control temperature. According to the graph, growth is predicted to increase when acclimation is taken into account and the average temperature increases of Earth’s surface increases |
by the expected 3-5°C. Growth enhancement may be reduced, however, if respiration increases more rapidly than photosynthesis, particularly under periods of drought and stress. Thus, climate warming may result in positive, negative, or potentially no effect on the free energy availability in forest ecosystems. D. In the figure below, the response to temperate change in terms of the rates of photosynthesis and respiration are sketched as a function of time from the very short-term (seconds) to the longer-term (decades) changes. Acclimation in the laboratory occurs in days. Analyze the graphs; in the box bounded by a dashed line, sketch curves for responses of both processes beyond the acclimation observed in the laboratory that are consistent with a neutral effect on free energy availability and provide your reasoning. Figure 23.44 E. Analyze the long-term effect of a rate of respiration that exceeds the rate of photosynthesis in terms of dynamic homeostasis. 111. A disruption of dynamic homeostasis in the relationship between vascular plants and insects is occurring as global climate changes. The reduction in the yield of soybeans is plotted against leaf area removed by two insects, beetles and aphids. Soybean blooms begin to This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 develop in the week of 13 July. Prior to that time, there is no effect of leaf removal on yield, even with complete loss of leaves. In the week of 18 August, plants are beginning to form seeds, and loss of leaves can be devastating. Figure 23.45 A. One observed effect of climate change is the shift toward earlier development in many insects. Quantitatively describe the worst possible consequences for yield, assuming plant developmental timing is not altered by warming temperatures, if the peak abundance of Japanese beetles is shifted from 18 July to 13 July, and 80% of leaf area is lost. The expression of genes involved in seed development is temperature dependent, unlike the scenario suggested in part A. More than 90% of soybean seeds planted in 2015 in the soybean-corn ecosystem of the central United States are the herbicide-resistant, genetically modified “Roundup Ready” variety. The seed has a patented genome. It produces seeds that are sterile and must be purchased each spring from the patent holder. B. Predict how the use of Roundup Ready seeds affects the selection of expression regulated in response to increasing temperature. Roundup is an |
herbicide whose active chemical component is glyphosphate. This molecule disrupts the synthesis of phenylalanine, tyrosine, and tryptophan. By inserting a gene from Agrobacteria, a Roundup Ready seed can synthesize these amino acids in the presence of the herbicide. C. Pose two scientific questions that must be considered to estimate the long-term effectiveness of this strategy for weed management. 112. By increasing the photosynthetic surface area, a plant increases the rate of capture of free energy. For every carbon atom fixed into carbohydrates, between 200 and 400 water molecules are released through stomata to the Chapter 23 | Plant Form and Physiology 1013 Chlorophyta, green algae, during the Devonian period (which began about 400 million years ago). The three most significant structural innovations in that process are responses to selection through the availability of water resources: 1) the cuticle, a waxy covering of the epidermis that retains water; 2) stomata, openings that penetrate the cuticle through which water and carbon dioxide are transported; and 3) a vascular system, plant tissues through which water moves. Measurements of gases trapped in ice cores provide atmospheric concentrations of the distant, as well as the recent, past. Life must adapt to changes in the environment. Woodward examined samples from the Cambridge herbarium of several trees (Nature, 327, 1987) to determine the stomatal index (percentage of epidermal cells that contain a stoma). In 1720, when the herbarium samples were collected, the carbon dioxide concentration in Earth’s atmosphere was 225 ppm. In the year of the study, 1987, it was 340 ppm (it is 370 ppm in 2016). The following table presents some of Woodward’s reported results. Tree Genus Acer Quercus Rumex Table 23.1 CO2 (ppm) Stomatal Index (%) 225 340 225 340 225 340 14.9 ± 0.8 6.7 ± 1.1 17.4 ± 1.1 9.6 ± 1 15.5 ± 0.7 11.8 ± 0.9 Teng and co-workers (PLoS ONE, 2009) followed the dependence of Arabidopsis, a member of the Brassica family of vascular plants, grown under a range of elevated CO2 concentrations for 15 generations. They found elevated stomatal densities for each generation that were not heritable. Engineer and co-workers (Nature, 513, 2014 |
) discovered a mutant Arabidopsis in which stomatal density increases as CO2 concentration increases. Measurements of a component of the set of mRNA molecules for epidermal patterning factor 2 (EPF2), responsible for stomatal density, are shown for plants grown in low and high CO2 concentrations. atmosphere. A simple geometric model can be used to estimate the minimum number of leaves on a tree, as shown. Figure 23.46 A. Identify and justify the data needed to describe the relationship between the free energy captured and the water transpired by a tree with dimensions D, L, and W. Use these data to construct a mathematical model of the relationship between transpiration rate and the rate of free energy captured when a single carbon atom is fixed. The diversity of vascular plants decreases with increasing latitude. Equatorial ecosystems have greater plant diversity than do ecosystems further south or north. One of several explanations offered to account for this observation is the energy-equivalence model—as free energy increases, population size increases. As population size increases, mutations increase. One bit of evidence for the energyequivalence model is the correlation of family-level diversity with actual evapotranspiration, the sum of water transferred by both transpiration and evaporation of surface water. This property is reported in mm of water per square meter of surface area. B. Explain the relationship between free energy exchange and latitude that is the basis of the energy-equivalence model. Shared ancestry is indicated by taxonomic classification in which a family of organisms contains many genera, and within each genus there are many species. A survey of tree flora (Latham and Ricklefs, Oikos, 67, 1993) at comparable latitudes in a temperate eastern Asia forest ecosystem (729 species in 177 genera and 67 families) and an eastern North America forest ecosystem (253 species in 90 genera and 46 families) had no species in common, but there were 20 common genera and 40 common families. Actual evapotranspiration for the two ecosystems are 850 ± 200 mm (eastern North America) and 730 ± 160 mm (eastern Asia). C. Analyze these data to test the validity of the energyequivalence model. 113. The evolution of vascular plants followed the colonization of terrestrial habitats by ancestors of 1014 Chapter 23 | Plant Form and Physiology Figure 23.47 A. Analyze these data in terms of the likelihood that the effect of carbon dioxide concentration on stomatal density involves |
negative feedback at the level of i) translation, ii) post-transcription, or iii) changes in genotype. B. Changes in precipitation patterns are expected to accompany an increase in atmospheric carbon dioxide. Predict the effect on a forest where trees that have matured over decades are suddenly under drought stress. Justify your prediction in terms of positive or negative feedback where stomatal density is high and a drought occurs. In a favorable environment, trees continue to accumulate biomass and increase in height until the flow rate of water through the xylem (plant vascular tissue that transports water and minerals from roots to shoots) is no longer sufficient to support the negative water potential at the interface between root and soil. Fluid dynamic models predict that increasing the diameter, d, of the xylem greatly increases the rate of flow of water, leading to greater productivity when water is abundant. Under conditions of drought stress, the water potential is reduced, and an air bubble can disrupt the flow of water in that vessel entirely. A larger stem diameter permits a larger number of small vessels. C. Describe a model of the evolution of xylem in trees in terms of selection under conditions of unlimited and limited water resources. Olson and Rosell (New Phytologist, 197, 2013) investigated the question of whether xylem diameter was determined by water availability or by plant height and, consequently, stem diameter. A summary of their data is shown with lines of best fit through data with the corresponding color. This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Figure 23.48 D. Analyze these data and summarize the pattern that addresses their question. Note: As x increases, log(x) increases. 114. Like the animal intestine, the organ system principally responsible for nutrient and water uptake, the plant root system, is home to a microbiome upon which the host depends. One important role for the root microbiome is innate immunity. Wheat take-all is a disease caused by the fungus Gaeumannomyces graminis that attacks plant roots and blocks root water channels. When a major outbreak occurs in a wheat field, susceptibility remains high in the following year. But after four to six continued crops of wheat in the same field, susceptibility to the disease declines. This resistance can be transferred with the soil. Burning the soil surface or rotation with another crop returns susceptibility to the next wheat crop. The Fusarium (a fungus) wilt disease of strawberries and |
potato scab caused by Streptomyces scabies (a bacteria) show a similar disease progression and transferability of resistance (Weller, Ann. Rev. Plant Phytopath, 26, 1988). A. Plants, like animals, have immune defenses that may involve cooperative interactions between organisms. Describe a model of immune response that accounts for these behaviors. In plants, the first line of defense is the cell wall. Animal cells lack this protective barrier. Adaptive immunity of vertebrates to pathogens uses specific defenses that are transportable within the organism, such as T-cells, and retains information about earlier infections, such as T-cell receptors. Unlike adaptive immunity, the innate responses of plants are much less effective in defending against necrotrophic (colonizing dead tissue) than against biotrophic (infecting living tissue) pathogens. In animal Chapter 23 | Plant Form and Physiology 1015 tissue, the response to infection is inflammation, the recruitment of resources to protect the tissue. In plant tissue, the response is apoptosis. plants and animals that express each of these differences in terms of these strategies: cell boundary, immunological memory, and tissue repair. B. Describe contrasting models of defense strategies for 1016 Chapter 23 | Plant Form and Physiology This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 24 | The Animal Body: Basic Form and Function 1017 24 | THE ANIMAL BODY: BASIC FORM AND FUNCTION Figure 24.1 An arctic fox is a complex animal, well adapted to its environment. It changes coat color with the seasons, and has longer fur in winter to trap heat. (credit: modification of work by Keith Morehouse, USFWS) Chapter Outline 24.1: Animal Form and Function 24.2: Animal Primary Tissues 24.3: Homeostasis Introduction The structures of animals consist of primary tissues that make up more complex organs and organ systems. Homeostasis allows an animal to maintain a balance between its internal and external environments. The arctic fox is an example of a complex animal that is well adapted to its environment and illustrates the relationships between an animal’s form and function. According to researchers, animals living millions of years ago in the Himalayan Mountains of Tibet are ancestors to many of today’s cold-adapted animals. For example, a type of Tibetan fox from 3–5 million years ago is the ancestor to |
the arctic fox. More about this research can be found at the Science Daily website (http://openstaxcollege.org/l/32arcticfox). [1] 1. Xiaoming Wang, Zhijie Jack Tseng, Qiang Li, Gary T. Takeuchi, Guangpu Xie, From ‘third pole’ to north pole: a Himalayan origin for the arctic fox. Proceedings of the Royal Society B: Biological Sciences. June 11, 2014. 1018 Chapter 24 | The Animal Body: Basic Form and Function 24.1 | Animal Form and Function In this section, you will explore the following questions: • What are the various types of body plans that occur in animals? • What are the limits on animal size and shape? • How do bioenergetics relate to body size, levels of activity, and the environment? Connection for AP® Courses As you have learned, specialized cells in the animal body are organized into tissues, organs, and organ systems, which efficiently localize functions, such as the digestion of food and the elimination of wastes. As we explore the information in this section, our primary focus is homeostasis—the ability to maintain dynamic equilibrium around a set point. Animals need to maintain their “normal” internal environments while also responding to external environmental changes. In our study of biology thus far, we have seen numerous examples of structure-function relationships, and the design of the animal body is no exception. Specialization in multicellular animals contributes to efficiency in cell processes. For example, animals must be able to procure nutrients and eliminate wastes, and cells that line the small intestine allow for diffusion. Furthermore, the relationship between metabolic rate and body mass is typically an inverse one: The smaller the animal, the higher its metabolism, with mice having a higher metabolic rate than, for example, elephants. Because mice have a greater surface area-to-volume ratio for their mass than larger animals, they lose heat at a faster rate and, consequently, require more energy to maintain constant body temperature. Speaking of temperature, we learned that the body temperature of ectothermic animals varies according to environmental temperatures. When snakes need to warm up, they bask in the sun; when they need to cool down, they go into the shade. Other animals, including mice, kangaroos and humans, are endothermic because they are able to maintain a fairly constant internal body temperature despite environmental temperatures; for example, shivering generates heat, whereas sweating returns |
our body temperature to its normal set point of 37◦C. We will explore the control of these responses in more detail in the Homeostasis section. The information presented and the examples highlighted in the section support concepts outlined in Big Idea 2 of the AP® Biology Curriculum Framework. The AP® 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 Essential Knowledge Science Practice Learning Objective Essential Knowledge Science Practice 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. 2.A.1 All living systems require constant input of free energy. 6.2 The student can construct explanations of phenomena based on evidence produced through scientific practices. 2.1 The student is able to explain how biological systems use free energy based on empirical data that all organisms require constant energy input to maintain organization, to grow and to reproduce. 2.A.1 All living systems require constant input of free energy. 6.1: The student can justify claims with evidence. This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 24 | The Animal Body: Basic Form and Function 1019 Learning Objective Essential Knowledge Science Practice Learning Objective Essential Knowledge Science Practice Learning Objective Essential Knowledge Science Practice Learning Objective 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. 2.A.1 All living systems require constant input of free energy. 4.2 The student can design a plan for collecting data to answer a particular scientific question. 2.35 The student is able to design a plan for collecting data to support the scientific claim that timing and coordination of physiological events involve regulation 2.A.1 All living systems require constant input of free energy. 6.1 The student can justify claims with evidence. 2.36 The student is able to justify scientific claims with evidence to show how timing and coordination of physiological events involve regulation. 2.A.1 All living systems require constant input of free energy. 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. 2.37 The student is able to connect concepts that describe mechanisms that regulate the timing and coordination of physiological events. Animals vary in form and function. From a sponge to a worm to a goat, an organism has a distinct body plan that limits its size and shape. Animals’ bodies are also designed to interact with their environments, whether in the deep sea, a rainforest canopy, or the desert. Therefore, a large amount of information about the structure of an organism's body (anatomy) and the function of its cells, tissues and organs (physiology) can be learned by studying that organism's environment. Body Plans Figure 24.2 Animals exhibit different types of body symmetry. The sponge is asymmetrical, the sea anemone has radial symmetry, and the goat has bilateral symmetry. Animal body plans follow set patterns related to symmetry. They are asymmetrical, radial, or bilateral in form as illustrated in Figure 24.2. Asymmetrical animals are animals with no pattern or symmetry; an example of an asymmetrical animal is a sponge. Radial symmetry, as illustrated in Figure 24.2, describes when an animal has an up-and-down orientation: any plane cut along its longitudinal axis through the organism produces equal halves, but not a definite right or left side. This plan is found mostly in aquatic animals, especially organisms that attach themselves to a base, like a rock or a boat, and extract their food from the surrounding water as it flows around the organism. Bilateral symmetry is illustrated in the same 1020 Chapter 24 | The Animal Body: Basic Form and Function figure by a goat. The goat also has an upper and lower component to it, but a plane cut from front to back separates the animal into definite right and left sides. Additional terms used when describing positions in the body are anterior (front), posterior (rear), dorsal (toward the back), and ventral (toward the stomach). Bilateral symmetry is found in both land-based and aquatic animals; it enables a high level of mobility. Limits on Animal Size and Shape Animals with bilateral symmetry that live in water tend to have a fusiform shape: this is a tubular shaped body that is tapered at both ends. This shape decreases the drag on the body as it moves through water and allows the animal to swim at high speeds. Table 24.1 lists the maximum speed of various animals. Certain types |
of sharks can swim at fifty kilometers an hour and some dolphins at 32 to 40 kilometers per hour. Land animals frequently travel faster, although the tortoise and snail are significantly slower than cheetahs. Another difference in the adaptations of aquatic and land-dwelling organisms is that aquatic organisms are constrained in shape by the forces of drag in the water since water has higher viscosity than air. On the other hand, land-dwelling organisms are constrained mainly by gravity, and drag is relatively unimportant. For example, most adaptations in birds are for gravity not for drag. Maximum Speed of Assorted Land Marine Animals Animal Speed (kmh) Speed (mph) Cheetah Quarter horse Fox Shortfin mako shark Domestic house cat Human Dolphin Mouse Snail Table 24.1 113 77 68 50 48 45 32–40 13 0.05 70 48 42 31 30 28 20–25 8 0.03 Most animals have an exoskeleton, including insects, spiders, scorpions, horseshoe crabs, centipedes, and crustaceans. Scientists estimate that, of insects alone, there are over 30 million species on our planet. The exoskeleton is a hard covering or shell that provides benefits to the animal, such as protection against damage from predators and from water loss (for land animals); it also provides for the attachments of muscles. As the tough and resistant outer cover of an arthropod, the exoskeleton may be constructed of a tough polymer such as chitin and is often biomineralized with materials such as calcium carbonate. This is fused to the animal’s epidermis. Ingrowths of the exoskeleton, called apodemes, function as attachment sites for muscles, similar to tendons in more advanced animals (Figure 24.3). In order to grow, the animal must first synthesize a new exoskeleton underneath the old one and then shed or molt the original covering. This limits the animal’s ability to grow continually, and may limit the individual’s ability to mature if molting does not occur at the proper time. The thickness of the exoskeleton must be increased significantly to accommodate any increase in weight. It is estimated that a doubling of body size increases body weight by a factor of eight. The increasing thickness of the chitin necessary to support this weight limits most animals with an exoskeleton to a relatively small size. The same principles apply to endoskeletons, but they are more efficient |
because muscles are attached on the outside, making it easier to compensate for increased mass. This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 24 | The Animal Body: Basic Form and Function 1021 Figure 24.3 Apodemes are ingrowths on arthropod exoskeletons to which muscles attach. The apodemes on this crab leg are located above and below the fulcrum of the claw. Contraction of muscles attached to the apodemes pulls the claw closed. An animal with an endoskeleton has its size determined by the amount of skeletal system it needs in order to support the other tissues and the amount of muscle it needs for movement. As the body size increases, both bone and muscle mass increase. The speed achievable by the animal is a balance between its overall size and the bone and muscle that provide support and movement. Limiting Effects of Diffusion on Size and Development The exchange of nutrients and wastes between a cell and its watery environment occurs through the process of diffusion. All living cells are bathed in liquid, whether they are in a single-celled organism or a multicellular one. Diffusion is effective over a specific distance and limits the size that an individual cell can attain. If a cell is a single-celled microorganism, such as an amoeba, it can satisfy all of its nutrient and waste needs through diffusion. If the cell is too large, then diffusion is ineffective and the center of the cell does not receive adequate nutrients nor is it able to effectively dispel its waste. An important concept in understanding how efficient diffusion is as a means of transport is the surface area to volume ratio. Recall that any three-dimensional object has a surface area and volume; the ratio of these two quantities is the surfaceto-volume ratio. Consider a cell shaped like a perfect sphere: it has a surface area of 4πr2, and a volume of (4/3)πr3. The surface-to-volume ratio of a sphere is 3/r; as the cell gets bigger, its surface area to volume ratio decreases, making diffusion less efficient. The larger the size of the sphere, or animal, the less surface area for diffusion it possesses. The solution to producing larger organisms is for them to become multicellular. Specialization occurs in complex organisms, allowing cells to become more efficient at doing fewer tasks. For example, circ |
ulatory systems bring nutrients and remove waste, while respiratory systems provide oxygen for the cells and remove carbon dioxide from them. Other organ systems have developed further specialization of cells and tissues and efficiently control body functions. Moreover, surface area-tovolume ratio applies to other areas of animal development, such as the relationship between muscle mass and cross-sectional surface area in supporting skeletons, and in the relationship between muscle mass and the generation of dissipation of heat. 1022 Chapter 24 | The Animal Body: Basic Form and Function Visit this interactive site (http://openstaxcollege.org/l/nanoscopy) to see an entire animal (a zebrafish embryo) at the cellular and sub-cellular level. Use the zoom and navigation functions for a virtual nanoscopy exploration. Zebrafish have bilateral symmetry. What does that mean? a. Bilaterally symmetric means that a plane cut from the front to back of the organism produces distinct left and right sides that are mirror images of each other. b. Bilaterally symmetric means that a plane cut from the top to the bottom of the organism produces distinct left and right sides that are mirror images of each other. c. Bilaterally symmetric means that a plane cut from the front to back of the organism produces distinct left and right sides that are not mirror images of each other. d. Bilaterally symmetric means that a plane cut along its longitudinal axis produces equal halves, but not definite right or left sides. Animal Bioenergetics All animals must obtain their energy from food they ingest or absorb. These nutrients are converted to adenosine triphosphate (ATP) for short-term storage and use by all cells. Some animals store energy for slightly longer times as glycogen, and others store energy for much longer times in the form of triglycerides housed in specialized adipose tissues. No energy system is one hundred percent efficient, and an animal’s metabolism produces waste energy in the form of heat. If an animal can conserve that heat and maintain a relatively constant body temperature, it is classified as a warm-blooded animal and called an endotherm. The insulation used to conserve the body heat comes in the forms of fur, fat, or feathers. The absence of insulation in ectothermic animals increases their dependence on the environment for body heat. The amount of energy expended by an animal over a specific time is called its metabolic rate. The rate is measured variously in joules, calories, or kilocal |
ories (1000 calories). Carbohydrates and proteins contain about 4.5 to 5 kcal/g, and fat contains about 9 kcal/g. Metabolic rate is estimated as the basal metabolic rate (BMR) in endothermic animals at rest and as the standard metabolic rate (SMR) in ectotherms. Human males have a BMR of 1600 to 1800 kcal/day, and human females have a BMR of 1300 to 1500 kcal/day. Even with insulation, endothermal animals require extensive amounts of energy to maintain a constant body temperature. An ectotherm such as an alligator has an SMR of 60 kcal/day. Energy Requirements Related to Body Size Smaller endothermic animals have a greater surface area for their mass than larger ones (Figure 24.4). Therefore, smaller animals lose heat at a faster rate than larger animals and require more energy to maintain a constant internal temperature. This results in a smaller endothermic animal having a higher BMR, per body weight, than a larger endothermic animal. This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 24 | The Animal Body: Basic Form and Function 1023 Figure 24.4 The mouse has a much higher metabolic rate than the elephant. (credit “mouse”: modification of work by Magnus Kjaergaard; credit “elephant”: modification of work by “TheLizardQueen”/Flickr) Energy Requirements Related to Levels of Activity The more active an animal is, the more energy is needed to maintain that activity, and the higher its BMR or SMR. The average daily rate of energy consumption is about two to four times an animal’s BMR or SMR. Humans are more sedentary than most animals and have an average daily rate of only 1.5 times the BMR. The diet of an endothermic animal is determined by its BMR. For example: the type of grasses, leaves, or shrubs that an herbivore eats affects the number of calories that it takes in. The relative caloric content of herbivore foods, in descending order, is tall grasses > legumes > short grasses > forbs (any broad-leaved plant, not a grass) > subshrubs > annuals/biennials. Energy Requirements Related to Environment Animals adapt to extremes of temperature or food availability through torpor. Torpor is a |
process that leads to a decrease in activity and metabolism and allows animals to survive adverse conditions. Torpor can be used by animals for long periods, such as entering a state of hibernation during the winter months, in which case it enables them to maintain a reduced body temperature. During hibernation, ground squirrels can achieve an abdominal temperature of 0° C (32° F), while a bear’s internal temperature is maintained higher at about 37° C (99° F). If torpor occurs during the summer months with high temperatures and little water, it is called estivation. Some desert animals use this to survive the harshest months of the year. Torpor can occur on a daily basis; this is seen in bats and hummingbirds. While endothermy is limited in smaller animals by surface to volume ratio, some organisms can be smaller and still be endotherms because they employ daily torpor during the part of the day that is coldest. This allows them to conserve energy during the colder parts of the day, when they consume more energy to maintain their body temperature. Activity Read about how scientists developed a method using today’s technology to collect data on heart rates in hibernating bears at this website (http://openstaxcollege.org/l/32bears). Design an experiment that would allow you to collect body temperature and heart rate at the same time. Discuss how combining data on body temperature with heart rate can give you information on the animal’s overall metabolism. Think About It • Small mammals, such as squirrels need to eat at least once a week during hibernation. Why is it impossible for them to go through the entire winter without eating, as bears do? Also, why must smaller mammals, like squirrels, store food for the winter while larger mammals, like bears, do not? • Hummingbirds lower their metabolic rate and body temperature at night, an example of torpor. What advantage does torpor provide hummingbirds on a nightly basis? Think about the high metabolic rates of hummingbirds. Animal Body Planes and Cavities A standing vertebrate animal can be divided by several planes. A sagittal plane divides the body into right and left portions. 1024 Chapter 24 | The Animal Body: Basic Form and Function A midsagittal plane divides the body exactly in the middle, making two equal right and left halves. A frontal plane (also called a coronal plane) separates the belly (ventral) or stomach from the back (dorsal). |
A transverse plane (or, horizontal plane) is perpendicular to the sagittal planes and the long axis of the body. This is sometimes called a cross section, and, if the transverse cut is at an angle, it is called an oblique plane. Figure 24.5 illustrates these planes on a goat (a four-legged animal) and a human being. Figure 24.5 Shown are the planes of a quadruped goat and a bipedal human. The midsagittal plane divides the body exactly in half, into right and left portions. The frontal plane divides the front and back, and the transverse plane divides the body into upper and lower portions. Vertebrate animals have a number of defined body cavities, as illustrated in Figure 24.6. Two of these are major cavities that contain smaller cavities within them. The dorsal cavity contains the cranial and the vertebral (or spinal) cavities. The ventral cavity contains the thoracic cavity, which in turn contains the pleural cavity around the lungs and the pericardial cavity, which surrounds the heart. The ventral cavity also contains the abdominopelvic cavity, which can be separated into the abdominal and the pelvic cavities. This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 24 | The Animal Body: Basic Form and Function 1025 Figure 24.6 Vertebrate animals have two major body cavities. The dorsal cavity, indicated in green, contains the cranial and the spinal cavity. The ventral cavity, indicated in yellow, contains the thoracic cavity and the abdominopelvic cavity. The thoracic cavity is separated from the abdominopelvic cavity by the diaphragm. The thoracic cavity is separated into the abdominal cavity and the pelvic cavity by an imaginary line parallel to the pelvis bones. (credit: modification of work by NCI) Physical Anthropologist Physical anthropologists study the adaption, variability, and evolution of human beings, plus their living and fossil relatives. They can work in a variety of settings, although most will have an academic appointment at a university, usually in an anthropology department or a biology, genetics, or zoology department. Non-academic positions are available in the automotive and aerospace industries where the focus is on human size, shape, and anatomy. Research by these professionals might range from studies of how the human body reacts to |
car crashes to exploring how to make seats more comfortable. Other non-academic positions can be obtained in museums of natural history, anthropology, archaeology, or science and technology. These positions involve educating students from grade school through graduate school. Physical anthropologists serve as education coordinators, collection managers, writers for museum publications, and as administrators. Zoos employ these professionals, especially if they have an expertise in primate biology; they work in collection management and captive breeding programs for endangered species. Forensic science utilizes physical anthropology expertise in identifying human and animal remains, assisting in determining the cause of death, and for expert testimony in trials. 1026 Chapter 24 | The Animal Body: Basic Form and Function 24.2 | Animal Primary Tissues In this section, you will explore the following questions: • What are characteristics of epithelial tissues? • What are the different types of connective tissues in animals? • What are differences among the three types of muscle tissue? • What are characteristics of nervous tissue? Connection for AP® Courses The content described in this section is not within the scope of AP®. However, we have already learned that the relationship between structure and function includes the cellular level, and we will continue to reinforce that when we explore the nervous system later. The tissues of multicellular, complex animals are four primary types: epithelial, connective, muscle, and nervous. Recall that tissues are groups of similar cells group of similar cells carrying out related functions. These tissues combine to form organs—like the skin or kidney—that have specific, specialized functions within the body. Organs are organized into organ systems to perform functions; examples include the circulatory system, which consists of the heart and blood vessels, and the digestive system, consisting of several organs, including the stomach, intestines, liver, and pancreas. Organ systems come together to create an entire organism. Epithelial Tissues Epithelial tissues cover the outside of organs and structures in the body and line the lumens of organs in a single layer or multiple layers of cells. The types of epithelia are classified by the shapes of cells present and the number of layers of cells. Epithelia composed of a single layer of cells is called simple epithelia; epithelial tissue composed of multiple layers is called stratified epithelia. Table 24.2 summarizes the different types of epithelial tissues. Cell shape Different Types of Epithelial Tissues Description Location squamous flat, irregular round shape simple: lung alveoli, |
capillaries stratified: skin, mouth, vagina cuboidal cube shaped, central nucleus glands, renal tubules columnar tall, narrow, nucleus toward base tall, narrow, nucleus along cell simple: digestive tract pseudostratified: respiratory tract transitional round, simple but appear stratified urinary bladder Table 24.2 Squamous Epithelia Squamous epithelial cells are generally round, flat, and have a small, centrally located nucleus. The cell outline is slightly irregular, and cells fit together to form a covering or lining. When the cells are arranged in a single layer (simple epithelia), they facilitate diffusion in tissues, such as the areas of gas exchange in the lungs and the exchange of nutrients and waste at blood capillaries. This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 24 | The Animal Body: Basic Form and Function 1027 Figure 24.7 Squamous epithelia cells (a) have a slightly irregular shape, and a small, centrally located nucleus. These cells can be stratified into layers, as in (b) this human cervix specimen. (credit b: modification of work by Ed Uthman; scale-bar data from Matt Russell) Figure 24.7a illustrates a layer of squamous cells with their membranes joined together to form an epithelium. Image Figure 24.7b illustrates squamous epithelial cells arranged in stratified layers, where protection is needed on the body from outside abrasion and damage. This is called a stratified squamous epithelium and occurs in the skin and in tissues lining the mouth and vagina. Cuboidal Epithelia Cuboidal epithelial cells, shown in Figure 24.8, are cube-shaped with a single, central nucleus. They are most commonly found in a single layer representing a simple epithelia in glandular tissues throughout the body where they prepare and secrete glandular material. They are also found in the walls of tubules and in the ducts of the kidney and liver. Figure 24.8 Simple cuboidal epithelial cells line tubules in the mammalian kidney, where they are involved in filtering the blood. Columnar Epithelia Columnar epithelial cells are taller than they are wide: they resemble a stack of columns in an epithelial layer, and are most commonly found in a single-layer arrangement. The nuclei of columnar epithelial cells in the digestive tract appear to be lined |
up at the base of the cells, as illustrated in Figure 24.9. These cells absorb material from the lumen of the digestive tract and prepare it for entry into the body through the circulatory and lymphatic systems. 1028 Chapter 24 | The Animal Body: Basic Form and Function Figure 24.9 Simple columnar epithelial cells absorb material from the digestive tract. Goblet cells secret mucous into the digestive tract lumen. Columnar epithelial cells lining the respiratory tract appear to be stratified. However, each cell is attached to the base membrane of the tissue and, therefore, they are simple tissues. The nuclei are arranged at different levels in the layer of cells, making it appear as though there is more than one layer, as seen in Figure 24.10. This is called pseudostratified, columnar epithelia. This cellular covering has cilia at the apical, or free, surface of the cells. The cilia enhance the movement of mucous and trapped particles out of the respiratory tract, helping to protect the system from invasive microorganisms and harmful material that has been breathed into the body. Goblet cells are interspersed in some tissues (such as the lining of the trachea). The goblet cells contain mucous that traps irritants, which in the case of the trachea keep these irritants from getting into the lungs. Figure 24.10 Pseudostratified columnar epithelia line the respiratory tract. They exist in one layer, but the arrangement of nuclei at different levels makes it appear that there is more than one layer. Goblet cells interspersed between the columnar epithelial cells secrete mucous into the respiratory tract. Transitional Epithelia Transitional or uroepithelial cells appear only in the urinary system, primarily in the bladder and ureter. These cells are arranged in a stratified layer, but they have the capability of appearing to pile up on top of each other in a relaxed, empty bladder, as illustrated in Figure 24.11. As the urinary bladder fills, the epithelial layer unfolds and expands to hold the volume of urine introduced into it. As the bladder fills, it expands and the lining becomes thinner. In other words, the tissue transitions from thick to thin. This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 24 | The Animal Body: Basic Form and Function 1029 |
Figure 24.11 Transitional epithelia of the urinary bladder undergo changes in thickness depending on how full the bladder is. An empty bladder is composed of piled up transitional cells with a folded epithelial lining. What would you predict happens to those cells as the bladder fills with urine? a. The epithelial lining unfolds and becomes thicker. b. The epithelial lining remains folded with the cells piled up. c. The epithelial lining unfolds and becomes thinner. d. The epithelial lining unfolds, but remains the same thickness. Connective Tissues Connective tissues are made up of a matrix consisting of living cells and a non-living substance, called the ground substance. The ground substance is made of an organic substance (usually a protein) and an inorganic substance (usually a mineral or water). The principal cell of connective tissues is the fibroblast. This cell makes the fibers found in nearly all of the connective tissues. Fibroblasts are motile, able to carry out mitosis, and can synthesize whichever connective tissue is needed. Macrophages, lymphocytes, and, occasionally, leukocytes can be found in some of the tissues. Some tissues have specialized cells that are not found in the others. The matrix in connective tissues gives the tissue its density. When a connective tissue has a high concentration of cells or fibers, it has proportionally a less dense matrix. The organic portion or protein fibers found in connective tissues are either collagen, elastic, or reticular fibers. Collagen fibers provide strength to the tissue, preventing it from being torn or separated from the surrounding tissues. Elastic fibers are made of the protein elastin; this fiber can stretch to one and one half of its length and return to its original size and shape. Elastic fibers provide flexibility to the tissues. Reticular fibers are the third type of protein fiber found in connective tissues. This fiber consists of thin strands of collagen that form a network of fibers to support the tissue and other organs to which it is connected. The various types of connective tissues, the types of cells and fibers they are made of, and sample locations of the tissues is summarized in Table 24.3. 1030 Chapter 24 | The Animal Body: Basic Form and Function Connective Tissues Tissue Cells Fibers Location loose/areolar dense, fibrous connective tissue fibroblasts, macrophages, some lymphocytes, some neutrophils few: collagen, elastic, reticular around blood vessels; anchors epit |
helia fibroblasts, macrophages, mostly collagen irregular: skin regular: tendons, ligaments hyaline: few collagen fibrocartilage: large amount of collagen shark skeleton, fetal bones, human ears, intervertebral discs some: collagen, elastic vertebrate skeletons few none adipose (fat) blood cartilage chondrocytes, chondroblasts bone osteoblasts, osteocytes, osteoclasts adipose adipocytes red blood cells, white blood cells blood Table 24.3 Loose/Areolar Connective Tissue Loose connective tissue, also called areolar connective tissue, has a sampling of all of the components of a connective tissue. As illustrated in Figure 24.12, loose connective tissue has some fibroblasts; macrophages are present as well. Collagen fibers are relatively wide and stain a light pink, while elastic fibers are thin and stain dark blue to black. The space between the formed elements of the tissue is filled with the matrix. The material in the connective tissue gives it a loose consistency similar to a cotton ball that has been pulled apart. Loose connective tissue is found around every blood vessel and helps to keep the vessel in place. The tissue is also found around and between most body organs. In summary, areolar tissue is tough, yet flexible, and comprises membranes. Figure 24.12 Loose connective tissue is composed of loosely woven collagen and elastic fibers. The fibers and other components of the connective tissue matrix are secreted by fibroblasts. Fibrous Connective Tissue Fibrous connective tissues contain large amounts of collagen fibers and few cells or matrix material. The fibers can be arranged irregularly or regularly with the strands lined up in parallel. Irregularly arranged fibrous connective tissues are found in areas of the body where stress occurs from all directions, such as the dermis of the skin. Regular fibrous connective tissue, shown in Figure 24.13, is found in tendons (which connect muscles to bones) and ligaments (which connect bones This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 24 | The Animal Body: Basic Form and Function 1031 to bones). Figure 24.13 Fibrous connective tissue from the tendon has strands of collagen fibers lined up in parallel. Cartilage Cartilage is a connective tissue with a large amount of the matrix and variable amounts of |
fibers. The cells, called chondrocytes, make the matrix and fibers of the tissue. Chondrocytes are found in spaces within the tissue called lacunae. A cartilage with few collagen and elastic fibers is hyaline cartilage, illustrated in Figure 24.14. The lacunae are randomly scattered throughout the tissue and the matrix takes on a milky or scrubbed appearance with routine histological stains. Sharks have cartilaginous skeletons, as does nearly the entire human skeleton during a specific pre-birth developmental stage. A remnant of this cartilage persists in the outer portion of the human nose. Hyaline cartilage is also found at the ends of long bones, reducing friction and cushioning the articulations of these bones. Figure 24.14 Hyaline cartilage consists of a matrix with cells called chondrocytes embedded in it. The chondrocytes exist in cavities in the matrix called lacunae. Elastic cartilage has a large amount of elastic fibers, giving it tremendous flexibility. The ears of most vertebrate animals contain this cartilage as do portions of the larynx, or voice box. Fibrocartilage contains a large amount of collagen fibers, giving the tissue tremendous strength. Fibrocartilage comprises the intervertebral discs in vertebrate animals. Hyaline cartilage found in movable joints such as the knee and shoulder becomes damaged as a result of age or trauma. Damaged hyaline cartilage is replaced by fibrocartilage and results in the joints becoming “stiff.” Bone Bone, or osseous tissue, is a connective tissue that has a large amount of two different types of matrix material. The organic matrix is similar to the matrix material found in other connective tissues, including some amount of collagen and elastic fibers. This gives strength and flexibility to the tissue. The inorganic matrix consists of mineral salts—mostly calcium 1032 Chapter 24 | The Animal Body: Basic Form and Function salts—that give the tissue hardness. Without adequate organic material in the matrix, the tissue breaks; without adequate inorganic material in the matrix, the tissue bends. There are three types of cells in bone: osteoblasts, osteocytes, and osteoclasts. Osteoblasts are active in making bone for growth and remodeling. Osteoblasts deposit bone material into the matrix and, after the matrix surrounds them, they continue to live, but in a reduced metabolic state as osteocytes. Osteocytes |
are found in lacunae of the bone. Osteoclasts are active in breaking down bone for bone remodeling, and they provide access to calcium stored in tissues. Osteoclasts are usually found on the surface of the tissue. Bone can be divided into two types: compact and spongy. Compact bone is found in the shaft (or diaphysis) of a long bone and the surface of the flat bones, while spongy bone is found in the end (or epiphysis) of a long bone. Compact bone is organized into subunits called osteons, as illustrated in Figure 24.15. A blood vessel and a nerve are found in the center of the structure within the Haversian canal, with radiating circles of lacunae around it known as lamellae. The wavy lines seen between the lacunae are microchannels called canaliculi; they connect the lacunae to aid diffusion between the cells. Spongy bone is made of tiny plates called trabeculae these plates serve as struts to give the spongy bone strength. Over time, these plates can break causing the bone to become less resilient. Bone tissue forms the internal skeleton of vertebrate animals, providing structure to the animal and points of attachment for tendons. This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 24 | The Animal Body: Basic Form and Function 1033 Figure 24.15 (a) Compact bone is a dense matrix on the outer surface of bone. Spongy bone, inside the compact bone, is porous with web-like trabeculae. (b) Compact bone is organized into rings called osteons. Blood vessels, nerves, and lymphatic vessels are found in the central Haversian canal. Rings of lamellae surround the Haversian canal. Between the lamellae are cavities called lacunae. Canaliculi are microchannels connecting the lacunae together. (c) Osteoblasts surround the exterior of the bone. Osteoclasts bore tunnels into the bone and osteocytes are found in the lacunae. Adipose Tissue Adipose tissue, or fat tissue, is considered a connective tissue even though it does not have fibroblasts or a real matrix and only has a few fibers. Adipose tissue is made up of cells called adipocytes that collect and store fat |
in the form of triglycerides, for energy metabolism. Adipose tissues additionally serve as insulation to help maintain body temperatures, allowing animals to be endothermic, and they function as cushioning against damage to body organs. Under a microscope, adipose tissue cells appear empty due to the extraction of fat during the processing of the material for viewing, as seen in Figure 24.16. The thin lines in the image are the cell membranes, and the nuclei are the small, black dots at the edges of the cells. 1034 Chapter 24 | The Animal Body: Basic Form and Function Figure 24.16 Adipose is a connective tissue is made up of cells called adipocytes. Adipocytes have small nuclei localized at the cell edge. Blood Blood is considered a connective tissue because it has a matrix, as shown in Figure 24.17. The living cell types are red blood cells (RBC), also called erythrocytes, and white blood cells (WBC), also called leukocytes. The fluid portion of whole blood, its matrix, is commonly called plasma. Figure 24.17 Blood is a connective tissue that has a fluid matrix, called plasma, and no fibers. Erythrocytes (red blood cells), the predominant cell type, are involved in the transport of oxygen and carbon dioxide. Also present are various leukocytes (white blood cells) involved in immune response. The cell found in greatest abundance in blood is the erythrocyte. Erythrocytes are counted in millions in a blood sample: the average number of red blood cells in primates is 4.7 to 5.5 million cells per microliter. Erythrocytes are consistently the same size in a species, but vary in size between species. For example, the average diameter of a primate red blood cell is 7.5 um, a dog is close at 7.0 um, but a cat’s RBC diameter is 5.9 um. Sheep erythrocytes are even smaller at 4.6 um. Mammalian erythrocytes lose their nuclei and mitochondria when they are released from the bone marrow where they are made. Fish, amphibian, and avian red blood cells maintain their nuclei and mitochondria throughout the cell’s life. The principal job of an erythrocyte is to carry and deliver oxygen to the tissues. Leukocytes are the predominant white blood cells found in the peripheral blood. |
Leukocytes are counted in the thousands in the blood with measurements expressed as ranges: primate counts range from 4,800 to 10,800 cells per µl, dogs from 5,600 to 19,200 cells per µl, cats from 8,000 to 25,000 cells per µl, cattle from 4,000 to 12,000 cells per µl, and pigs from 11,000 to 22,000 cells per µl. Lymphocytes function primarily in the immune response to foreign antigens or material. Different types of lymphocytes make antibodies tailored to the foreign antigens and control the production of those antibodies. Neutrophils are phagocytic cells and they participate in one of the early lines of defense against microbial invaders, aiding in the removal of bacteria that has entered the body. Another leukocyte that is found in the peripheral blood is the monocyte. Monocytes give rise to This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 24 | The Animal Body: Basic Form and Function 1035 phagocytic macrophages that clean up dead and damaged cells in the body, whether they are foreign or from the host animal. Two additional leukocytes in the blood are eosinophils and basophils—both help to facilitate the inflammatory response. The slightly granular material among the cells is a cytoplasmic fragment of a cell in the bone marrow. This is called a platelet or thrombocyte. Platelets participate in the stages leading up to coagulation of the blood to stop bleeding through damaged blood vessels. Blood has a number of functions, but primarily it transports material through the body to bring nutrients to cells and remove waste material from them. Muscle Tissues There are three types of muscle in animal bodies: smooth, skeletal, and cardiac. They differ by the presence or absence of striations or bands, the number and location of nuclei, whether they are voluntarily or involuntarily controlled, and their location within the body. Table 24.4 summarizes these differences. Type of Muscle Striations Nuclei Control Location Types of Muscles no yes yes single, in center involuntary visceral organs many, at periphery voluntary skeletal muscles single, in center involuntary heart smooth skeletal cardiac Table 24.4 Smooth Muscle Smooth muscle does not have striations in its cells. It has a single, centrally located nucleus, as shown in Figure 24.18. Constriction of |
smooth muscle occurs under involuntary, autonomic nervous control and in response to local conditions in the tissues. Smooth muscle tissue is also called non-striated as it lacks the banded appearance of skeletal and cardiac muscle. The walls of blood vessels, the tubes of the digestive system, and the tubes of the reproductive systems are composed of mostly smooth muscle. Figure 24.18 Smooth muscle cells do not have striations, while skeletal muscle cells do. Cardiac muscle cells have striations, but, unlike the multinucleate skeletal cells, they have only one nucleus. Cardiac muscle tissue also has intercalated discs, specialized regions running along the plasma membrane that join adjacent cardiac muscle cells and assist in passing an electrical impulse from cell to cell. Skeletal Muscle Skeletal muscle has striations across its cells caused by the arrangement of the contractile proteins actin and myosin. These muscle cells are relatively long and have multiple nuclei along the edge of the cell. Skeletal muscle is under voluntary, somatic nervous system control and is found in the muscles that move bones. Figure 24.18 illustrates the histology of skeletal muscle. Cardiac Muscle Cardiac muscle, shown in Figure 24.18, is found only in the heart. Like skeletal muscle, it has cross striations in its cells, but cardiac muscle has a single, centrally located nucleus. Cardiac muscle is not under voluntary control but can be influenced by the autonomic nervous system to speed up or slow down. An added feature to cardiac muscle cells is a line than extends along the end of the cell as it abuts the next cardiac cell in the row. This line is called an intercalated disc: it assists in passing electrical impulse efficiently from one cell to the next and maintains the strong connection between neighboring cardiac cells. Nervous Tissues Nervous tissues are made of cells specialized to receive and transmit electrical impulses from specific areas of the body and to send them to specific locations in the body. The main cell of the nervous system is the neuron, illustrated in Figure 1036 Chapter 24 | The Animal Body: Basic Form and Function 24.19. The large structure with a central nucleus is the cell body of the neuron. Projections from the cell body are either dendrites specialized in receiving input or a single axon specialized in transmitting impulses. Some glial cells are also shown. Astrocytes regulate the chemical environment of the nerve cell, and oligodendrocytes insulate the axon so the electrical nerve impulse is transferred more |
efficiently. Other glial cells that are not shown support the nutritional and waste requirements of the neuron. Some of the glial cells are phagocytic and remove debris or damaged cells from the tissue. A nervous tissue consists of neurons and glial cells. Figure 24.19 The neuron has projections called dendrites that receive signals and projections called axons that send signals. Also shown are two types of glial cells: astrocytes regulate the chemical environment of the nerve cell, and oligodendrocytes insulate the axon so the electrical nerve impulse is transferred more efficiently. Click through the interactive review (http://openstaxcollege.org/l/tissues) to learn more about epithelial tissues. Why would a single layer of flat epithelial cells rather than cuboidal cells (cube-shaped) cells function more efficiently in diffusion? a. As a single layer of flat epithelia are more tightly knit than cuboidal cells. b. As diffusion of nutrients and gas is easier across a single layer of flat epithelial cells than cuboidal cells. c. As diffusion of only gases is easier across a single layer of flat epithelial cells than cuboidal cells. d. As active transport of nutrients and gas is easier across a single layer of flat epithelial cells than cuboidal cells. This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 24 | The Animal Body: Basic Form and Function 1037 Pathologist A pathologist is a medical doctor or veterinarian who has specialized in the laboratory detection of disease in animals, including humans. These professionals complete medical school education and follow it with an extensive post-graduate residency at a medical center. A pathologist may oversee clinical laboratories for the evaluation of body tissue and blood samples for the detection of disease or infection. They examine tissue specimens through a microscope to identify diseases. Some path<|endoftext|>ologists perform autopsies to determine the cause of death and the progression of disease. 24.3 | Homeostasis In this section, you will explore the following questions: • What is homeostasis? • What factors affect homeostasis? • What are differences between negative and positive feedback mechanisms used in homeostasis? • What are differences between thermoregulation mechanisms in endothermic and ectothermic animals? Connection for AP® Courses Animals must be able to maintain homeostasis—the ability to maintain dynamic equilibrium around a set point |
—while also being able to respond to changing conditions. For example, as an endotherm, your body temperature remains fairly constant around 37◦C or 98.6◦F. If your temperature climbs above the set point, you sweat to cool off; if your temperature drops below the set point, you shiver to warm up. Your blood glucose levels also remain fairly constant because the liver removes glucose from the blood and converts it to glycogen; when the body cells require glucose, glycogen is broken down. (You can probably hypothesize how your liver will respond if you eat a dozen jelly donuts!) The failure to maintain homeostasis can be detrimental and can even cause death. Consequently, negative and/or positive feedback loops regulate homeostasis. Negative feedback mechanisms result in slight fluctuations above and below the set point. For example, if you were to consume a dozen jelly donuts, your blood sugar level would rise, and your pancreas would release insulin, a hormone involved in the conversion of glucose to glycogen, thus returning your blood glucose level to its appropriate set point. By comparison, positive feedback amplifies responses in the same direction, with the variable initiating the response moving the system even further away from the set point. There are fewer examples of positive feedback, but one is the onset of labor in childbirth when uterine contractions increase in strength with the secretion of oxytocin, another hormone. However, the loss of internal equilibrium due to positive feedback can be detrimental; for example, a small area of damaged heart tissue can precipitate a heart attack which, in turn, damages even more cardiac muscle. Information presented and the examples highlighted in the section support concepts outlined in Big Idea 2 of the AP Biology® Curriculum Framework. The AP® 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. 1038 Chapter 24 | The Animal Body: Basic Form and Function Essential Knowledge 2.C.1 Organisms use feedback mechanisms to maintain their internal environments and respond to external environmental changes. Science Practice Learning Objective 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. 2.16 The student is able to connect how organisms use negative feedback to maintain their internal environments. Essential Knowledge 2.C.1 Organisms use feedback mechanisms to maintain their internal environments and respond to external environmental changes. Science Practice Learning Objective 5.3 The student can evaluate the evidence provided by data sets in relation to a particular scientific question. 2.17 The student is able to evaluate data that show the effect(s) of changes in concentrations of key molecules on negative feedback mechanisms. Essential Knowledge 2.C.1 Organisms use feedback mechanisms to maintain their internal environments and respond to external environmental changes. Science Practice Learning Objective 6.4 The student can make claims and predictions about natural phenomena based on scientific theories and models. 2.18 The student is able to make predictions about how organisms use negative feedback mechanisms to maintain their international environments. Essential Knowledge 2.C.1 Organisms use feedback mechanisms to maintain their internal environments and respond to external environmental changes. Science Practice Learning Objective 6.4 The student can make claims and predictions about natural phenomena based on scientific theories and models. 2.19 The student is able to make predictions about how positive feedback mechanisms amplify activities and processes in organisms based on scientific theories and models. Essential Knowledge 2.C.1 Organisms use feedback mechanisms to maintain their internal environments and respond to external environmental changes. Science Practice Learning Objective 6.1 The student can justify claims with evidence. 2.20 The student is able to justify that positive feedback mechanisms amplify responses in organisms. Animal organs and organ systems constantly adjust to internal and external changes through a process called homeostasis (“steady state”). These changes might be in the level of glucose or calcium in blood or in external temperatures. Homeostasis means to maintain dynamic equilibrium in the body. It is dynamic because it is constantly adjusting to the changes that the body’s systems encounter. It is equilibrium because body functions are kept within specific ranges. Even an animal that is apparently inactive is maintaining this homeostatic equilibrium. Homeostatic Process The goal of homeostasis is the maintenance of equilibrium around a point or value called a set point. While there are normal fluctuations from the set point, the body’s systems will usually attempt to go back to this point. A change in the internal or external environment is called a stimulus and is detected by a receptor; the response of |
the system is to adjust the deviation parameter toward the set point. For instance, if the body becomes too warm, adjustments are made to cool the animal. If the blood’s glucose rises after a meal, adjustments are made to lower the blood glucose level by getting the nutrient into tissues that need it or to store it for later use. Control of Homeostasis When a change occurs in an animal’s environment, an adjustment must be made. The receptor senses the change in the environment, then sends a signal to the control center (in most cases, the brain) which in turn generates a response that is signaled to an effector. The effector is a muscle (that contracts or relaxes) or a gland that secretes. Homeostatsis is This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 24 | The Animal Body: Basic Form and Function 1039 maintained by negative feedback loops. Positive feedback loops actually push the organism further out of homeostasis, but may be necessary for life to occur. Homeostasis is controlled by the nervous and endocrine system of mammals. Negative Feedback Mechanisms Any homeostatic process that changes the direction of the stimulus is a negative feedback loop. It may either increase or decrease the stimulus, but the stimulus is not allowed to continue as it did before the receptor sensed it. In other words, if a level is too high, the body does something to bring it down, and conversely, if a level is too low, the body does something to make it go up. Hence the term negative feedback. An example is animal maintenance of blood glucose levels. When an animal has eaten, blood glucose levels rise. This is sensed by the nervous system. Specialized cells in the pancreas sense this, and the hormone insulin is released by the endocrine system. Insulin causes blood glucose levels to decrease, as would be expected in a negative feedback system, as illustrated in Figure 24.20. However, if an animal has not eaten and blood glucose levels decrease, this is sensed in another group of cells in the pancreas, and the hormone glucagon is released causing glucose levels to increase. This is still a negative feedback loop, but not in the direction expected by the use of the term “negative.” Another example of an increase as a result of the feedback loop is the control of blood calcium. If calcium levels decrease, specialized cells in the parathy |
roid gland sense this and release parathyroid hormone (PTH), causing an increased absorption of calcium through the intestines and kidneys and, possibly, the breakdown of bone in order to liberate calcium. The effects of PTH are to raise blood levels of the element. Negative feedback loops are the predominant mechanism used in homeostasis. Figure 24.20 Blood sugar levels are controlled by a negative feedback loop. (credit: modification of work by Jon Sullivan) Positive Feedback Loop A positive feedback loop maintains the direction of the stimulus, possibly accelerating it. Few examples of positive feedback loops exist in animal bodies, but one is found in the cascade of chemical reactions that result in blood clotting, or coagulation. As one clotting factor is activated, it activates the next factor in sequence until a fibrin clot is achieved. The direction is maintained, not changed, so this is positive feedback. Another example of positive feedback is uterine contractions during childbirth, as illustrated in Figure 24.21. The hormone oxytocin, made by the endocrine system, stimulates the contraction of the uterus. This produces pain sensed by the nervous system. Instead of lowering the oxytocin and causing the pain to subside, more oxytocin is produced until the contractions are powerful enough to produce childbirth. 1040 Chapter 24 | The Animal Body: Basic Form and Function Figure 24.21 The birth of a human infant is the result of positive feedback. State whether each of the following processes is regulated by a positive or negative feedback loop. a. A person feels satiated after eating a large meal. b. The blood has plenty of red blood cells. As a result, erythropoietin, a hormone that stimulates the production of new red blood cells, is no longer released from the kidney. a. a. This is regulated by a positive feedback loop as the stimulus (hunger) has changed direction in response to a signal (fullness). b. This is regulated by a positive feedback loop as the stimulus (red blood cell release) has changed direction in response to a signal (presence of enough red blood cells). b. a. This is regulated by a negative feedback loop as the stimulus (hunger) has changed direction in response to a signal (fullness). b. This is regulated by a positive feedback loop as the direction of the stimulus has been maintained. c. a. This is regulated by a positive feedback loop as the stimulus (hunger) has changed direction in response to a signal |
(fullness). b. This is regulated by a negative feedback loop as the stimulus (red blood cell release) has changed direction in response to a signal (presence of enough red blood cells). d. a. This is regulated by a negative feedback loop as the stimulus (hunger) changed direction in response to a signal (fullness). b. This is regulated by a negative feedback loop as the stimulus (red blood cell release) changed direction in response to a signal (presence of enough red blood cells). Set Point It is possible to adjust a system’s set point. When this happens, the feedback loop works to maintain the new setting. An example of this is blood pressure: over time, the normal or set point for blood pressure can increase as a result of continued increases in blood pressure. The body no longer recognizes the elevation as abnormal and no attempt is made to return to the lower set point. The result is the maintenance of an elevated blood pressure that can have harmful effects on the body. Medication can lower blood pressure and lower the set point in the system to a more healthy level. This is called a process of alteration of the set point in a feedback loop. Changes can be made in a group of body organ systems in order to maintain a set point in another system. This is called acclimatization. This occurs, for instance, when an animal migrates to a higher altitude than it is accustomed to. In order to adjust to the lower oxygen levels at the new altitude, the body increases the number of red blood cells circulating in the This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 24 | The Animal Body: Basic Form and Function 1041 blood to ensure adequate oxygen delivery to the tissues. Another example of acclimatization is animals that have seasonal changes in their coats: a heavier coat in the winter ensures adequate heat retention, and a light coat in summer assists in keeping body temperature from rising to harmful levels. Feedback mechanisms can be understood in terms of driving a race car along a track: watch a short video lesson (http://openstaxcollege.org/l/feedback_loops) on positive and negative feedback loops. Voltage-gated sodium channels occur in the cell membranes of nerve cells. They open in response to sodium entering the cell, which in turn, allows more sodium to enter the cell. Is this a positive or negative feedback loop and why? a. This |
is a positive feedback loop as voltage-gated sodium channels open in response to sodium influx and then close when enough sodium has entered through the channels. b. This is a negative feedback loop as voltage-gated sodium channels open in response to sodium influx and then close when enough sodium has entered through the channels. c. This is a positive feedback loop as voltage-gated sodium channels open in response to sodium influx, which allows more sodium to go in through the channels. d. This is a negative feedback loop as voltage-gated sodium channels open in response to sodium influx, which allows more sodium to go in through the channels. Think About It How are negative feedback loops used to regulate body homeostasis? How is a condition such as diabetes a good example of the failure of a set point in humans? Hypothesize and draw a diagram that shows what you think is the feedback failure for a person with diabetes. Homeostasis: Thermoregulation Body temperature affects body activities. Generally, as body temperature rises, enzyme activity rises as well. For every ten degree centigrade rise in temperature, enzyme activity doubles, up to a point. Body proteins, including enzymes, begin to denature and lose their function with high heat (around 50oC for mammals). Enzyme activity will decrease by half for every ten degree centigrade drop in temperature, to the point of freezing, with a few exceptions. Some fish can withstand freezing solid and return to normal with thawing. 1042 Chapter 24 | The Animal Body: Basic Form and Function Watch this Discovery Channel video (http://openstaxcollege.org/l/thermoregulate) on thermoregulation to see illustrations of this process in a variety of animals. How does the loose skin of an elephant help it regulate body temperature? a. Loose skin is thicker, which allows the excess heat to dissipate quickly through the skin. b. Loose skin brings more heat and blood to the body surface, facilitating heat loss. c. Loose skin contains greater skin area, which allows excess heat to dissipate as heat loss occurs through the skin. d. Loose skin has smaller skin area, which allows excess heat to dissipate as heat loss occurs through the skin. Endotherms and Ectotherms Animals can be divided into two groups: some maintain a constant body temperature in the face of differing environmental temperatures, while others have a body temperature that is the same as their environment and thus varies with the environment. Animals that |
do not control their body temperature are ectotherms. This group has been called cold-blooded, but the term may not apply to an animal in the desert with a very warm body temperature. In contrast to ectotherms, which rely on external temperatures to set their body temperatures, poikilotherms are animals with constantly varying internal temperatures. An animal that maintains a constant body temperature in the face of environmental changes is called a homeotherm. Endotherms are animals that rely on internal sources for body temperature but which can exhibit extremes in temperature. These animals are able to maintain a level of activity at cooler temperature, which an ectotherm cannot due to differing enzyme levels of activity. Heat can be exchanged between an animal and its environment through four mechanisms: radiation, evaporation, convection, and conduction (Figure 24.22). Radiation is the emission of electromagnetic “heat” waves. Heat comes from the sun in this manner and radiates from dry skin the same way. Heat can be removed with liquid from a surface during evaporation. This occurs when a mammal sweats. Convection currents of air remove heat from the surface of dry skin as the air passes over it. Heat will be conducted from one surface to another during direct contact with the surfaces, such as an animal resting on a warm rock. This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 24 | The Animal Body: Basic Form and Function 1043 Figure 24.22 Heat can be exchanged by four mechanisms: (a) radiation, (b) evaporation, (c) convection, or (d) conduction. (credit b: modification of work by “Kullez”/Flickr; credit c: modification of work by Chad Rosenthal; credit d: modification of work by “stacey.d”/Flickr) Figure 24.23 The body temperature of ectotherms varies with the environment. For that reason, reptiles, such as this American alligator, bask in the sun to warm themselves. If an American alligator has been basking but gets too hot, how might the alligator cool itself? a. b. increase vasodilation sweat c. move into shade d. increase metabolic rate 1044 Chapter 24 | The Animal Body: Basic Form and Function Heat Conservation and Dissipation Animals conserve or dissipate heat in a variety of ways. In certain climates, endothermic animals have |
some form of insulation, such as fur, fat, feathers, or some combination thereof. Animals with thick fur or feathers create an insulating layer of air between their skin and internal organs. Polar bears and seals live and swim in a subfreezing environment and yet maintain a constant, warm, body temperature. The arctic fox, for example, uses its fluffy tail as extra insulation when it curls up to sleep in cold weather. Mammals have a residual effect from shivering and increased muscle activity: arrector pili muscles cause “goose bumps,” causing small hairs to stand up when the individual is cold; this has the intended effect of increasing body temperature. Mammals use layers of fat to achieve the same end. Loss of significant amounts of body fat will compromise an individual’s ability to conserve heat. Endotherms use their circulatory systems to help maintain body temperature. Vasodilation brings more blood and heat to the body surface, facilitating radiation and evaporative heat loss, which helps to cool the body. Vasoconstriction reduces blood flow in peripheral blood vessels, forcing blood toward the core and the vital organs found there, and conserving heat. Some animals have adaptions to their circulatory system that enable them to transfer heat from arteries to veins, warming blood returning to the heart. This is called a countercurrent heat exchange; it prevents the cold venous blood from cooling the heart and other internal organs. This adaption can be shut down in some animals to prevent overheating the internal organs. The countercurrent adaption is found in many animals, including dolphins, sharks, bony fish, bees, and hummingbirds. In contrast, similar adaptations can help cool endotherms when needed, such as dolphin flukes and elephant ears. Some ectothermic animals use changes in their behavior to help regulate body temperature. For example, a desert ectothermic animal may simply seek cooler areas during the hottest part of the day in the desert to keep from getting too warm. The same animals may climb onto rocks to capture heat during a cold desert night. Some animals seek water to aid evaporation in cooling them, as seen with reptiles. Other ectotherms use group activity such as the activity of bees to warm a hive to survive winter. Many animals, especially mammals, use metabolic waste heat as a heat source. When muscles are contracted, most of the energy from the ATP used in muscle actions is wasted energy that translates into heat. Severe cold elicits a |
shivering reflex that generates heat for the body. Many species also have a type of adipose tissue called brown fat that specializes in generating heat. Neural Control of Thermoregulation The nervous system is important to thermoregulation, as illustrated in Figure 24.22. The processes of homeostasis and temperature control are centered in the hypothalamus of the advanced animal brain. This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 24 | The Animal Body: Basic Form and Function 1045 Figure 24.24 The body is able to regulate temperature in response to signals from the nervous system. When bacteria are destroyed by leukocytes, pyrogens are released into the blood. Pyrogens reset the body’s thermostat to a higher temperature, resulting in fever. How do pyrogens cause body temperature to rise? a. Pyrogens circulate to the hypothalamus to reset the body’s “thermostat,” causing a rise in temperature. b. Pyrogens circulate to the thalamus to reset the body’s “thermostat,” causing a rise in temperature. c. Pyrogens cause an increase in the activity of the animal’s enzymes, which results in the temperature rise. d. Pyrogens entering the blood release some lipid substances, which ultimately cause the rise in temperature. The hypothalamus maintains the set point for body temperature through reflexes that cause vasodilation and sweating when the body is too warm, or vasoconstriction and shivering when the body is too cold. It responds to chemicals from the body. When a bacterium is destroyed by phagocytic leukocytes, chemicals called endogenous pyrogens are released into the blood. These pyrogens circulate to the hypothalamus and reset the thermostat. This allows the body’s temperature to increase in what is commonly called a fever. An increase in body temperature causes iron to be conserved, which reduces a nutrient needed by bacteria. An increase in body heat also increases the activity of the animal’s enzymes and protective cells while inhibiting the enzymes and activity of the invading microorganisms. Finally, heat itself may also kill the pathogen. A fever that was once thought to be a complication of an infection is now understood to be a normal defense mechanism. 1046 Chapter 24 | The Animal Body: Basic Form and Function KEY TERMS acclimatization alteration in a body system in response |
to environmental change alteration change of the set point in a homeostatic system apodeme ingrowth of an animal’s exoskeleton that functions as an attachment site for muscles asymmetrical describes animals with no axis of symmetry in their body pattern basal metabolic rate (BMR) metabolic rate at rest in endothermic animals canaliculus microchannel that connects the lacunae and aids diffusion between cells cartilage type of connective tissue with a large amount of ground substance matrix, cells called chondrocytes, and some amount of fibers chondrocyte cell found in cartilage columnar epithelia epithelia made of cells taller than they are wide, specialized in absorption connective tissue type of tissue made of cells, ground substance matrix, and fibers cuboidal epithelia epithelia made of cube-shaped cells, specialized in glandular functions dorsal cavity body cavity on the posterior or back portion of an animal; includes the cranial and vertebral cavities ectotherm animal incapable of maintaining a relatively constant internal body temperature endotherm animal capable of maintaining a relatively constant internal body temperature epithelial tissue tissue that either lines or covers organs or other tissues estivation torpor in response to extremely high temperatures and low water availability fibrous connective tissue type of connective tissue with a high concentration of fibers frontal (coronal) plane plane cutting through an animal separating the individual into front and back portions fusiform animal body shape that is tubular and tapered at both ends hibernation torpor over a long period of time, such as a winter homeostasis dynamic equilibrium maintaining appropriate body functions lacuna space in cartilage and bone that contains living cells loose (areolar) connective tissue type of connective tissue with small amounts of cells, matrix, and fibers; found around blood vessels matrix component of connective tissue made of both living and non-living (ground substances) cells midsagittal plane plane cutting through an animal separating the individual into even right and left sides negative feedback loop feedback to a control mechanism that increases or decreases a stimulus instead of maintaining it osteon subunit of compact bone positive feedback loop feedback to a control mechanism that continues the direction of a stimulus pseudostratified layer of epithelia that appears multilayered, but is a simple covering sagittal plane plane cutting through an animal separating the individual into right and left sides This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 24 |
| The Animal Body: Basic Form and Function 1047 set point midpoint or target point in homeostasis simple epithelia single layer of epithelial cells squamous epithelia type of epithelia made of flat cells, specialized in aiding diffusion or preventing abrasion standard metabolic rate (SMR) metabolic rate at rest in ectothermic animals stratified epithelia multiple layers of epithelial cells thermoregulation regulation of body temperature torpor decrease in activity and metabolism that allows an animal to survive adverse conditions trabecula tiny plate that makes up spongy bone and gives it strength transitional epithelia epithelia that can transition for appearing multilayered to simple; also called uroepithelial transverse (horizontal) plane plane cutting through an animal separating the individual into upper and lower portions ventral cavity body cavity on the anterior or front portion of an animal that includes the thoracic cavities and the abdominopelvic cavities CHAPTER SUMMARY 24.1 Animal Form and Function Animal bodies come in a variety of sizes and shapes. Limits on animal size and shape include impacts to their movement. Diffusion affects their size and development. Bioenergetics describes how animals use and obtain energy in relation to their body size, activity level, and environment. 24.2 Animal Primary Tissues The basic building blocks of complex animals are four primary tissues. These are combined to form organs, which have a specific, specialized function within the body, such as the skin or kidney. Organs are organized together to perform common functions in the form of systems. The four primary tissues are epithelia, connective tissues, muscle tissues, and nervous tissues. 24.3 Homeostasis Homeostasis is a dynamic equilibrium that is maintained in body tissues and organs. It is dynamic because it is constantly adjusting to the changes that the systems encounter. It is in equilibrium because body functions are kept within a normal range, with some fluctuations around a set point for the processes. REVIEW QUESTIONS 1. The pleural cavity is part of which cavity? a. dorsal b. thoracic c. abdominal d. pericardial 2. A plane that divides an animal into dorsal and ventral portions is the ____ plane. 3. What is the term for a hard covering or shell that provides protection and muscle attachment? a. apodeme b. fusiform c. exoskeleton d. endotherm 4. Which organism has a fusiform shape |
? a. sagittal b. midsagittal c. d. frontal transverse a. elephant b. dolphin c. spider d. human 1048 Chapter 24 | The Animal Body: Basic Form and Function 5. Which type of animal maintains a constant internal body temperature? plasma membrane called intercalated discs. What is the role of intercalated discs? a. efficiently pass electrical impulses between cardiac cells b. facilitate immune response to foreign antigens c. cushion body organs from damage d. keeps blood vessels in place 13. The part of a neuron that contains the nucleus is the ____. a. axon b. dendrite c. cell body d. oligodendrocyte 14. Schwann cells or oligodendrocytes manufacture a lipid called myelin. Which statement best describes the function of this lipid? a. b. c. d. regulates the chemical environment sends input receives input improves signal transfer efficiency 15. Animals maintain an overall steady state of internal conditions by ___. a. ectothermy b. homeostasis c. basal metabolic rate d. standard metabolic rate 16. To what does the term “equilibrium” refer in the context of organismal homeostasis? a. control mechanisms that amplify a response b. control mechanisms that increase or decrease a stimulus c. the target point in homeostasis d. body functions are maintained within a given range 17. What type of feedback loop pushes an organism’s physiology further away from it normal setpoints? a. positive feedback loop b. negative feedback loop c. d. set point receptor 18. When faced with a sudden drop in environmental temperature, an endothermic animal will ____. a. endotherm b. ectotherm c. poikilotherm d. fusiform 6. Smaller endothermic animals have _______ surface area for their mass compared with larger endothermic animals. a. equal b. greater c. less d. no 7. What is the term for epithelial cells that are composed of multiple layers? a. b. c. d. simple stratified squamous transitional 8. Which type of epithelial cell is best adapted to aid diffusion? a. squamous b. cuboidal c. columnar d. transitional 9. Why do osteoclasts need to break down bone? a. b. c. d. to deposit bone material into the bone matrix to facilitate osteoclast persistence without using excess energy to provide access to calcium |
in the tissue to facilitate compact bone structure 10. Plasma is the ____. a. fibers in the blood b. matrix of the blood c. cell that phagocytizes bacteria d. cell that functions in response to antigens 11. Why is it necessary for most muscle cells to be under voluntary control? a. b. c. to facilitate response to local conditions of tissues to facilitate movement of bone to speed up or slow down the autonomic nervous system d. to facilitate movement of internal organs 12. Cardiac muscle contains specialized regions along the This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 24 | The Animal Body: Basic Form and Function 1049 a. experience a substantial drop in its body temperature 21. Which method of heat exchange occurs during direct contact between the source and the animal? b. c. d. find a warm rock on which to bask increase muscle activity to generate heat increase fur or fat to increase insulation 19. Homeostasis is primarily controlled by _______ feedback loops. a. positive b. negative c. acclimatization d. receptor 20. Which is an example of negative feedback? a. lowering of blood glucose after a meal b. blood clotting after an injury c. lactation during nursing d. uterine contractions during labor CRITICAL THINKING QUESTIONS 23. How does an asymmetrical body plan differ from radial or bilateral body plans? a. Asymmetrical organisms can produce equal halves if cut along a certain plane, whereas radially and bilaterally symmetric organisms have no distinct pattern. b. Asymmetrical organisms have no distinct pattern, whereas radially and bilaterally symmetric organisms can produce equal halves if cut along a certain plane. c. Asymmetrical organisms produce equal halves if cut along a certain plane with no definite right or left side, whereas radially and bilaterally symmetric organisms can produce equal halves. d. Asymmetrical organisms produce equal halves if cut along a certain plane with definite right and left sides, whereas radially and bilaterally symmetric organisms can produce equal halves. 24. Why are most organisms with exoskeletons relatively small? a. radiation b. evaporation c. convection d. conduction 22. Which of the following is a strategy that may be employed by an ectotherm to immediately increase body temperature? a. Consume more food to increase fat as insulation. b. c. Increase amount |
of vasodilation. Increase amount of muscle contraction. d. Sit on a warm rock. a. Increases in body weight increase body size by a factor of eight, and the chitin thickness of the exoskeleton has to significantly decrease to accommodate increase in body size. b. Doubling of body size increases body weight by a factor of eight, and the chitin thickness of the exoskeleton has to significantly decrease to accommodate weight increase. c. Increases in body weight increase body size by a factor of eight, and the chitin thickness of the exoskeleton has to significantly increase to accommodate increase in body size. d. Doubling of body size increases body weight by a factor of eight, and the chitin thickness of the exoskeleton has to significantly increase to accommodate weight increase. 25. What is the relationship between basal metabolic rate (BMR) and body size? Why? 1050 Chapter 24 | The Animal Body: Basic Form and Function a. BMR decreases with body size, because larger animals require more energy to maintain their size. However, smaller animals have relatively higher BMRs per body weight because they have greater surface area. b. BMR increases with body size, because smaller animals require more energy to maintain their size. However, larger animals have relatively higher BMRs per body weight because they have greater surface area. c. BMR increases with body size, because larger animals require more energy to maintain their size. However, smaller animals have relatively higher BMRs per body weight because they have greater surface area for their mass. d. BMR decreases with body size, because smaller animals require more energy to maintain their size. However, larger animals have relatively higher BMRs per body weight because they have greater surface area for their mass. 26. Radial symmetry is typically found in aquatic organisms. What is radial symmetry and why is it advantageous to certain aquatic organisms? a. Radially symmetric means that a plane cut from the front to back of the organism produces distinct left and right sides that are mirror images of each other. It helps certain aquatic organisms to extract food from surrounding environments. b. Radially symmetric means that a plane cut from the front to back of the organism produces distinct left and right sides that are mirror images of each other. It helps certain aquatic organisms to perform photosynthesis. c. Radially symmetric means that a plane cut along its longitudinal axis will produce equal halves, and there is no distinct left or right. It helps certain aquatic organisms |
to perform photosynthesis. d. Radially symmetric means that a plane cut along its longitudinal axis to produce equal halves, and there is no distinct left or right. It helps certain aquatic organisms to extract food from surrounding environments. 27. Columnar epithelial cells, which are typically found in a single-layer arrangement, are found along the digestive tract. What is the role of columnar epithelial cells in digestion? a. Columnar epithelial cells absorb material from the lumen of the digestive tract to prepare the material for entry into the body. b. Columnar epithelial cells release mucus for lubrication as well as antimicrobial agents in the digestive tract. c. Columnar epithelial cells secrete enzymes like salivary amylase which aid in digestion by the breakdown of carbohydrates in the body. d. Columnar epithelial cells help in the propulsion of food by peristalsis in the digestive tract of the body. 28. In vertebrates, cartilage is found in fetal bones, ears, and intervertebral discs, whereas bone is found in the skeleton. What are the similarities between cartilage and bone? a. Both are types of connective tissue in the body and cells of both are known as chondrocytes. b. Both are types of connective tissue in the body and have non-vascular organic matrix material that provides strength and flexibility. c. Both are types of connective tissue in the body and have organic matrix material that provides strength and flexibility. d. Both consist of bone marrow and have organic matrix material that provides strength and flexibility. 29. A friend sneaks up behind you and scares you, speeding up your heart rate. How and why did this event influence cardiac muscle contraction? a. Muscle contraction speed increases as the enteric nervous system responds to local conditions and makes muscle contraction speed up or slow down. b. Muscle contraction speed increases as the autonomic nervous system responds to local conditions and makes muscle contraction speed up or slow down. c. Muscle contraction speed increases as the somatic nervous system responds to local conditions and makes muscle contraction speed up or slow down. d. Muscle contraction speed increases as the central nervous system responds to local conditions and makes muscle contraction speed up or slow down. 30. Neurons have several specialized structures, including dendrites. What might happen if an individual has malformed dendrites? This OpenStax book is available for free at http://cnx.org/content/col12078/ |
1.6 Chapter 24 | The Animal Body: Basic Form and Function 1051 a. The individual’s neurons would not be able to receive input properly. 33. How can an environmental change result in an alteration of gland secretion? b. The individual’s neurons would not be able to synthesize proteins. c. The individual’s neurons would not be able to communicate with target neurons. d. The individual’s neurons would not be able to carry nerve signals. 31. How can squamous epithelia, which have a high surface area-to-volume ratio, both facilitate diffusion and prevent damage from abrasion? a. Single layers of squamous epithelia facilitate gas, nutrient or waste exchange, whereas stratified layers provide protection but are not replaceable following damage. b. Stratified layers of squamous epithelia facilitate gas, nutrient or waste exchange, whereas single layers provide protection and are replaceable following damage. c. Single layers of squamous epithelia facilitate gas, nutrient or waste exchange whereas stratified layers provide protection and are replaceable following damage. d. Single layers of squamous epithelia facilitate only exchange of gases by diffusion, whereas stratified layers provide protection and are replaceable following damage. 32. What is homeostasis and how does it help maintain equilibrium of various body functions throughout the body? a. A receptor detects change, sends a signal to the control center, which sends a signal to the gland to inhibit the gland secretions. b. A receptor detects change, sends a signal to the control center, which sends a signal to the gland to increase the secretions of the gland. c. A receptor detects change and sends a signal to the effector directly,which in this case is the gland. d. A receptor detects change, sends a signal to the control center, which in turn sends a signal to the effector, which in this case is the gland. 34. How is a condition such as diabetes a good example of the failure of a set point in humans? a. A negative feedback loop cannot proceed in diabetic individuals, as they do not produce enough functional insulin to lower blood sugar. b. Negative feedback loop cannot proceed in diabetic individuals, as they do not produce enough functional insulin to increase the blood sugar. c. Positive feedback loop cannot proceed in diabetic individuals, as they do not produce enough functional insulin to lower blood sugar. d. Positive feedback loop cannot proceed in diabetic individuals, as they do not produce enough functional |
insulin to increase the blood sugar. a. Homeostasis is the process of achieving stability, which occurs through behavioral changes. Equilibrium is maintained by that ensuring body functions remain within a certain range. b. Homeostasis is the process by which constant adjustments to changes in the body occur, and equilibrium is maintained by ensuring that body functions remain within a certain range. c. Homeostasis is the process that prevents blood loss from circulation when a blood vessel is ruptured, and equilibrium is maintained by ensuring that circulation of blood is kept within a normal range. d. Homeostasis is the process by which constant adjustment to changes in the body occurs, and equilibrium is maintained as body functions remain within a certain range without any fluctuations. 35. What are the roles of vasodilation and vasoconstriction in maintaining body temperature? a. Vasodilation allows for radiation and evaporative heat loss, and vasoconstriction brings blood to the core to conserve heat by vital organs. b. Vasodilation brings blood to the core to conserve heat by vital organs, and vasoconstriction results in radiation and evaporative heat loss. c. Vasodilation results in the formation of an insulating layer between skin and internal organs causing heat conservation and brings blood to the core to conserve heat. d. Vasodilation results in radiation and evaporative heat loss, and vasoconstriction transfers heat from arteries to veins to warm blood returning to the heart. TEST PREP FOR AP® COURSES 36. Maintaining body heat is important for maintaining body functions in animals. Which of the following statements provides an example of how an animal can actively generate body heat? 1052 Chapter 24 | The Animal Body: Basic Form and Function a. Triglycerides are used to store energy for later a. Temperature varies by latitude, and body size use. b. An animal produces metabolic waste energy in the form of heat. c. An animal has insulation, which helps it maintain a constant body temperature. d. An animal eats a large amount of high-fat foods to produce adipose tissue. 37. Ectotherms and endotherms have different strategies for generating and maintaining body heat. Explain why ectotherms are more dependent on the environment for body heat than endotherms and how endotherms are able to generate and maintain body temperature. a. Ectotherms use external thermal heat whereas endotherms use metabolically generated heat to help regulate and maintain body temperatures. b. Ectotherms |
use external heat to help regulate and maintain body temperatures whereas endotherms have constantly varying internal temperatures. c. Ectotherms use metabolically-generated heat to maintain a constant body temperature whereas endotherms use metabolically generated heat to regulate body temperature within a wider range. d. Ectotherms use external thermal energy to help regulate and maintain body temperatures whereas endotherms maintain a constant body temperature. 38. 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 tissues. b. Bacteria lack introns, while many eukaryotic genes contain many of these intervening sequences. c. Carnivores have more teeth that are specialized for grinding food. d. Plants generally use starch molecules for storage while animals use glycogen and fats for storage. 39. The body sizes of organisms vary and tends to be correlated with the region in which the organisms are found. Why do organisms at different latitudes tend to have different body sizes, and what is the relationship between heat loss and body size in an organism? This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 affects heat retention and loss. Smaller organisms lose heat at a slower rate than larger organisms because they have a smaller surface area for their mass. b. Temperature varies by latitude, and body size affects heat retention and loss. Smaller organisms lose heat at a faster rate than larger organisms because they have a greater surface area for their mass. c. Temperature varies by latitude, and body size affects heat retention and loss. Larger organisms lose heat at a faster rate than smaller organisms because they have a greater surface area for their mass. d. Temperature varies by latitude, and body size affects heat retention and loss. Smaller organisms lose heat at a faster rate than larger organisms because they have a smaller surface area for their mass. 40. If an American alligator has been basking but gets too hot, how might the alligator cool itself? a. b. increase vasodilation sweat c. move into shade d. increase metabolic rate 41. During torpor, arctic ground squirrels reduce their energy requirements by reducing their core body temperature and metabolic rate. |
Why would an active ground squirrel’s ATP synthesis also increase in proportion to metabolic rate when temperatures fall below 0°? a. Colder temperatures causes ATP to degrade. b. ATP is synthesized through cellular respiration, which provides body heat. c. ATP synthesis is needed to provide more oxygen to the cells. d. ATP is consumed by the cells to generate body heat. Chapter 24 | The Animal Body: Basic Form and Function 1053 In the data, BM = body mass, CD = cool-down time; WU = warm-up time, NBT = normal body temperature and BTH = body temperature during hibernation. What can you conclude from the data collected on five different animals as shown in the table above? a. The time it takes for animals to change body temperature is directly related to body size. b. The time it takes for animals to change their body temperature is indirectly related to their size. c. Larger animals hibernate for longer periods of time. d. Smaller animals hibernate for shorter periods of time 45. 42. Why is hibernation not a good option for small animals like the hummingbirds to help reduce its metabolic rate and conserve its need for food? a. Hummingbirds have a fast metabolic rate and a large surface area to volume ratio. b. Hummingbirds are unable to lower their metabolic rate and body temperature to enter hibernation. c. Hummingbirds migrate south for the winter. d. Hummingbirds live a short life. 43. How does hibernation differ in small animals such as ground squirrels and larger animals such as bears? a. Smaller animals can engage in torpor while larger animals cannot. b. Larger animals can engage in torpor while smaller animals cannot. c. Smaller animals cannot remain inactive throughout the entire winter while larger animals can. d. Larger animals cannot remain inactive throughout the entire winter while smaller animals can. 44. In the data, BM = body mass, CD = cool-down time; WU = warm-up time, NBT = normal body temperature and BTH = body temperature during hibernation. What can you conclude from about the time it takes to cool down versus the time it takes to warm up? 1054 Chapter 24 | The Animal Body: Basic Form and Function a. Larger animals consume more energy to maintain their body temperatures. b. Smaller animals can survive hibernation with less food reserves than larger animals. c. Smaller animals require more time to |
alter their body temperature. d. Larger animals require more time to alter their body temperature. 46. The endocrine system incorporates feedback mechanisms that maintain homeostasis. Which of the following demonstrates negative feedback by the endocrine system? a. During labor, the fetus exerts pressure on the uterine wall, inducing the production of oxytocin, which stimulates uterine wall contraction. The contractions cause the fetus to further push on the wall, increasing the production of oxytocin. b. After a meal, blood glucose levels become elevated, stimulating beta cells of the pancreas to release insulin into the blood. Excess glucose is then converted to glycogen in the liver, reducing blood glucose levels. c. At high elevation, atmospheric oxygen is scarcer. In response to signals that oxygen is low, the brain decreases an individual’s rate of respiration to compensate for the difference. d. A transcription factor binds to the regulating region of a gene, blocking the binding of another transcription factor required for expression. 47. This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 This figure depicts the process of calcium homeostasis. Describe how blood calcium control is an example of a negative feedback loop. a. Cells in parathyroid gland sense calcium decrease causing parathyroid hormone release and stimulating calcium absorption. Bone may also break down to release calcium. b. Cells in parathyroid gland sense calcium decrease causing calcitonin release and stimulating calcium absorption. Bone may also break down to release calcium. c. Cells in thyroid gland sense calcium decrease causing calcitonin release and stimulating calcium absorption. Bone may also break down to release calcium. d. Cells in parathyroid gland sense calcium increase causing parathyroid hormone release and stimulating calcium absorption. Bone may also break down to release calcium. 48. In organisms, homeostasis of various bodily processes, such as body temperature, blood glucose levels, and blood calcium levels, is essential for the maintenance of proper body functions. What role does insulin play in homeostasis? Chapter 24 | The Animal Body: Basic Form and Function 1055 a. When a fetus pushes against the uterine wall, insulin is released by the brain to stimulate uterine contractions. b. c. d. In the presence of decreased blood glucose levels, insulin is produced by the parathyroid to increase calcium absorption. Insulin activation activates other clotting factors until a fibrin |
clot is produced. Insulin is secreted by the pancreas in response to elevated blood glucose levels to remove glucose from the blood. 49. Proper blood glucose levels are necessary to maintain cellular function, because glucose is fuel for cells. Glucagon is an important component of blood glucose homeostasis, which is maintained by a negative feedback loop. Describe the role of glucagon in blood glucose homeostasis. a. When blood sugar is low, glucose and ATP produce glycogen. Excess blood sugar stimulates the release of glucagon, which in turn stimulates glycogen release to increase blood glucose levels. b. When there is excess blood sugar, excess glucose and ATP produce glucagon. A drop in blood glucose level stimulates the release of glycogen, which in turn stimulates glycogen release to increase blood glucose levels. c. When there is excess blood sugar, the excess glucose and ATP produce glycogen. A drop in blood glucose level stimulates the release of glucagon, which in turn stimulates the release of glycogen to increase blood glucose levels. d. When there is excess blood sugar, the excess glucose and ATP produce glycogen. A drop in blood glucose level stimulates the release of glucagon, which in turn releases more glucagon to increase blood glucose levels. 50. One process that is under the control of a negative feedback loop is red blood cell production. These cells carry oxygen to all of the body cells, and remove some carbon dioxide. What would most likely happen if an individual had a sufficient number of red blood cells? a. The individual would have increased red blood cell production. b. The individual’s body would start destroying the red blood cells. c. The individual’s body would cease production of new red blood cells. d. The individual would produce the same amount of red blood cells. 51. Diabetes results when either insulin cannot be produced or does not function properly. Consequently, diabetes can produce complications such as blindness, heart disease, and kidney disease. To help manage diabetes, a patient can get insulin injections. How do insulin injections promote a negative feedback loop to help maintain blood glucose production? a. b. c. d. Insulin injections allow transport and storage of glucose to increase blood glucose levels after consuming a large or high-glucose meal. Insulin injections allow only storage of glucose to decrease blood glucose levels after consuming a large or high-glucose meal. Insulin injections allow transport and storage of glucose to increase blood glucose levels before consuming a meal. Insulin |
injections allow transport and storage of glucose to decrease blood glucose levels after consuming a large or high-glucose meal. 52. Positive feedback loops amplify processes in organisms. Which of the following statements describes the role of the hormone oxytocin in a positive feedback loop for childbirth? a. Oxytocin halts uterine contractions when the fetus pushes on the uterine wall. b. Oxytocin maintains pain levels as the child is pushed through the birth canal. c. Oxytocin stimulates uterine contractions when the fetus pushes on the uterine wall. d. Oxytocin decreases pain levels as the child is pushed through the birth canal. 53. Birth is one of the few positive feedback loops observed in humans and is essential for the proper delivery of babies. Describe how a baby pushing against a pregnant woman’s cervix stimulates a positive feedback loop. a. Stretching stimulates nerve impulses to be sent to the brain, which releases oxytocin from the pituitary, which in turn causes uterine contractions. b. Stretching stimulates nerve impulses to be sent to the brain, which releases estrogen from the pituitary, which in turn causes uterine contractions. c. Stretching stimulates nerve impulses to be sent to the brain, which releases oxytocin from the parathyroid gland, which in turn causes uterine contractions. d. Stretching stimulates nerve impulses to be sent to the brain which releases progesterone from the pituitary, which in turn causes uterine contractions 54. Negative feedback mechanisms are far more prevalent in the human body than positive feedback loops because they help regulate homeostasis. However, there are some instances of positive feedback loops that can be observed in animals. Regulation of which of the following is an 1056 Chapter 24 | The Animal Body: Basic Form and Function example of a positive feedback loop? a. When body temperature gets too high, signals are sent to reduce body temperature. b. Increased blood glucose levels stimulate insulin production, which in turn sequesters glucose from the blood. c. Decreased calcium levels stimulate increased calcium absorption. d. Activation of one clotting factor stimulates production of other clotting factors until a fibrin clot is produced. 55. Both negative and positive feedback loops are essential for maintaining proper body functions. Blood calcium and blood clotting are under the control of different feedback loops. Which of these processes is maintained by a positive feedback loop and why? a. Blood clotting is maintained by |
a positive feedback loop, as clotting is amplified in response by increasing the amount of clotting factors when clotting factors are present. b. Blood clotting is maintained by a positive feedback loop, as clotting factors are maintained in a specific range and a positive loop helps return the conditions to the set point. c. Blood calcium is maintained by a positive feedback loop, as calcium levels are amplified in response by increasing the amount of calcium levels when calcium is present. d. Blood calcium is maintained by a positive feedback loop, as calcium levels are maintained in a specific range and a positive feedback loop helps return the conditions to the set point. This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 25 | Animal Nutrition and the Digestive System 1057 25 | ANIMAL NUTRITION AND THE DIGESTIVE SYSTEM Figure 25.1 For humans, fruits and vegetables are important in maintaining a balanced diet. (credit: modification of work by Julie Rybarczyk) Chapter Outline 25.1: Digestive Systems 25.2: Nutrition and Energy Production 25.3: Digestive System Processes 25.4: Digestive System Regulation Introduction All living organisms need nutrients to survive. Animals obtain their nutrients by the consumption of other organisms. At the cellular level, the biological molecules necessary for animal function are amino acids, lipid molecules, nucleotides, and simple sugars. However, the food consumed consists of protein, fat, and complex carbohydrates. Animals must convert these macromolecules into the simple molecules required for maintaining cellular functions, such as assembling new molecules, cells, and tissues. The conversion of the food consumed to the nutrients required is a multi-step process involving digestion and absorption. During digestion, food particles are broken down into smaller components; later, they are absorbed by the body. One of the challenges in human nutrition is maintaining a balance between food intake, storage, and energy expenditure. Imbalances can have serious health consequences. For example, eating too much food while not expending much energy 1058 Chapter 25 | Animal Nutrition and the Digestive System leads to obesity; this in turn will increase the risk of developing illnesses such as type-2 diabetes and cardiovascular disease. The recent rise in obesity and related diseases makes understanding the role of diet and nutrition in maintaining good health all the more important. Many health experts believe that nutrition education will improve the overall health of the entire population. In fact, one nutrition education program |
, the Supplemental Nutrition Assistance Program (SNAP), focused on preschool-aged children enrolled in a low-cost childcare setting and found that participating children were significantly more likely to eat more vegetables at home. You can read more about SNAP here (http://openstaxcollege.org/l/32SNAP). [1] 25.1 | Digestive Systems In this section, you will explore the following questions: • What are the differences between digestion and absorption? • What are different types of digestive systems in invertebrates and vertebrates? • What are the specialized functions of the organs involved in processing food in the human body? • How do organs work together to digest food and absorb nutrients? Connection for AP® Courses Much information in this chapter is not within the scope of AP®. However, the chapter provides us with the opportunity to review concepts we’ve explored previously, including structure and function, macromolecules, energy production, transport of substances across membranes, and enzyme activity. All living organisms require a source of energy and molecules needed to build cells, tissues, and organs. During digestion, food is broken down into smaller molecules for absorption and distribution to all cells of the body. Nutrients are required to carry out cellular processes and maintain homeostasis, and digestion and absorption require the participation of several organs. Different animals have evolved different types of digestive systems specialized to meet their dietary needs. You do not need to memorize details about the different types of animal digestive systems, but you might find it interesting to explore the evolution of the system through a few groups of animals, from intracellular digestion in simple invertebrates to a digestive tract and accessory organs in complex vertebrates. Using a human eating a turkey sandwich as an example, food is ingested through the mouth. The mouth is the location where both mechanical (chewing) and chemical breakdown of food begins via the enzyme amylase, which breaks down carbohydrates into simpler sugars. The food bolus then travels by peristalsis (alternating waves of contraction) down the pharynx and esophagus to the stomach. In the stomach, pepsinogen mixes with hydrochloric acid to form pepsin, which begins digesting proteins, such as turkey, into smaller chains of amino acids. Mucus in the stomach protects its lining from damage by acidity, and the tightening of a sphincter prevents stomach contents from regurgitating into the esophagus. Further digestion of the ingredients of the sandwich occurs |
in the small intestine aided by a variety of enzymes; for example, bile salts and pancreatic amylase dumped into the small intestine from the gallbladder and pancreas, respectively, help emulsify fats. Once the ingredients of the sandwich have been broken down into smaller nutrient molecules, including amino acids, glucose, and fatty acids, they are absorbed from the small intestine into the circulatory and lymphatic systems. The walls of the small intestine contain small, finger-like projections called villi and microvilli that increase surface area for absorption of nutrients by diffusion. The large intestine or colon does not produce digestive enzymes but functions to absorb water, salts, and some vitamins. Any nutrients from the sandwich are stored in the liver, and wastes are eliminated. Information presented and the examples highlighted in the section support concepts outlined in Big Idea 2 and Big Idea 4 of the AP® Biology Curriculum Framework. The AP® 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. 1. P. A. Williams et al. Nutrition-education program improves preschoolers’ at-home diet: a group randomized trial. Journal of the Academy of Nutrition and Dietetics, 2014; 114 (7): 1001. This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 25 | Animal Nutrition and the Digestive System 1059 Enduring Understanding 2.A Growth, reproduction and maintenance of living systems require free energy and matter. Essential Knowledge 2.A.3 Organisms must exchange matter with the environment to grow, reproduce and maintain organization. Science Practice Learning Objective 2.2 The student can apply mathematical routines to quantities that describe natural phenomena. 2.6 The student is able to use calculated surface area-to-volume ratios to predict which cell(s) might eliminate wastes or procure nutrients faster by diffusion. Essential Knowledge 2.A.3 Organisms must exchange matter with the environment to grow, reproduce and maintain organization. Science Practice Learning Objective Big Idea 4 Enduring Understanding 4.A 6.2 The student can construct explanations of phenomena based on evidence produced through scientific practices. 2.7 The |
student will be able to explain how cell size and shape affects the overall rate of nutrient intake and the rate of waste elimination. Biological systems interact, and these systems and their interactions possess complex properties. Interactions within biological systems lead to complex properties. Essential Knowledge 4.A.4 Organisms exhibit complex properties due to interactions between their constituent parts. Science Practice Learning Objective 3.3 The student can evaluate scientific questions. 4.8 The student is able to evaluate scientific questions concerning organisms that exhibit complex properties due to the interaction of their constituent parts. Essential Knowledge 4.A.4 Organisms exhibit complex properties due to interactions between their constituent parts. Science Practice Learning Objective 6.4 The student can make claims and predictions about natural phenomena based on scientific theories and models. 4.9 The student is able to predict the effects of a change in a component(s) of a biological system on the functionality of an organism(s). Essential Knowledge 4.A.4 Organisms exhibit complex properties due to interactions between their constituent parts. Science Practice Learning Objective Enduring Understanding 4.B 1.3 The student can refine representations and models of natural or man-made phenomena and systems in the domain. 4.10 The student is able to refine representations and models to illustrate biocomplexity due to interactions of the constituent parts. Competition and cooperation are important aspects of biological systems. Essential Knowledge 4.B.2 Cooperative interactions within organisms promote efficiency in the use of energy and matter. Science Practice Learning Objective 1.4 The student can use representations and models to analyze situations or solve problems qualitatively and quantitatively. 4.18 The student is able to use representations and models to analyze how cooperative interactions within organisms promote efficiency in the use of matter and energy. 1060 Chapter 25 | Animal Nutrition and the Digestive System Animals obtain their nutrition by consuming other organisms. Depending on their diet, animals can be classified into the following categories: plant eaters (herbivores), meat eaters (carnivores), and those that eat both plants and animals (omnivores). The nutrients and macromolecules present in food are not immediately accessible to the cells. There are a number of processes that modify food within the animal body to make the nutrients and organic molecules accessible for cellular function. As animals evolved in complexity of form and function, their digestive systems have also evolved to accommodate their various dietary needs. Herbivores, Omnivores, and Carnivores Herbivores are animals whose primary food source is plant-based. |
Examples of herbivores, as shown in Figure 25.2 include vertebrates like deer, koalas, and some bird species, as well as invertebrates such as crickets and caterpillars. These animals have evolved digestive systems capable of handling large amounts of plant material. Herbivores can be further classified into frugivores (fruit-eaters), granivores (seed eaters), nectivores (nectar feeders), and folivores (leaf eaters). Figure 25.2 Herbivores, like this (a) mule deer and (b) monarch caterpillar, eat primarily plant material. (credit a: modification of work by Bill Ebbesen; credit b: modification of work by Doug Bowman) Carnivores are animals that eat other animals. The word carnivore is derived from Latin and literally means “meat eater.” Wild cats such as lions, shown in Figure 25.3a and tigers are examples of vertebrate carnivores, as are snakes and sharks, while invertebrate carnivores include sea stars, spiders, and ladybugs, shown in Figure 25.3b. Obligate carnivores are those that rely entirely on animal flesh to obtain their nutrients; examples of obligate carnivores are members of the cat family, such as lions and cheetahs. Facultative carnivores are those that also eat non-animal food in addition to animal food. Note that there is no clear line that differentiates facultative carnivores from omnivores; dogs would be considered facultative carnivores. This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 25 | Animal Nutrition and the Digestive System 1061 Figure 25.3 Carnivores like the (a) lion eat primarily meat. The (b) ladybug is also a carnivore that consumes small insects called aphids. (credit a: modification of work by Kevin Pluck; credit b: modification of work by Jon Sullivan) Omnivores are animals that eat both plant- and animal-derived food. In Latin, omnivore means to eat everything. Humans, bears (shown in Figure 25.4a), and chickens are example of vertebrate omnivores; invertebrate omnivores include cockroaches and crayfish (shown in Figure 25.4b). Figure 25.4 Omnivores like the (a) bear and (b) |
crayfish eat both plant and animal based food. (credit a: modification of work by Dave Menke; credit b: modification of work by Jon Sullivan) Invertebrate Digestive Systems Animals have evolved different types of digestive systems to aid in the digestion of the various foods they consume. The simplest example is that of a gastrovascular cavity and is found in organisms with only one opening for digestion. Platyhelminthes (flatworms), Ctenophora (comb jellies), and Cnidaria (coral, jelly fish, and sea anemones) use this type of digestion. Gastrovascular cavities, as shown in Figure 25.5a, are typically a blind tube or cavity with only one opening, the “mouth”, which also serves as an “anus”. Ingested material enters the mouth and passes through a hollow, tubular cavity. Cells within the cavity secrete digestive enzymes that break down the food. The food particles are engulfed by the cells lining the gastrovascular cavity. The alimentary canal, shown in Figure 25.5b, is a more advanced system: it consists of one tube with a mouth at one end and an anus at the other. Earthworms are an example of an animal with an alimentary canal. Once the food is ingested through the mouth, it passes through the esophagus and is stored in an organ called the crop; then it passes into the gizzard where it is churned and digested. From the gizzard, the food passes through the intestine, the nutrients are absorbed, and the waste is eliminated as feces, called castings, through the anus. 1062 Chapter 25 | Animal Nutrition and the Digestive System Figure 25.5 (a) A gastrovascular cavity has a single opening through which food is ingested and waste is excreted, as shown in this hydra and in this jellyfish medusa. (b) An alimentary canal has two openings: a mouth for ingesting food, and an anus for eliminating waste, as shown in this nematode. Vertebrate Digestive Systems Through evolution, vertebrate digestive systems have adapted to different diets. Some animals have a single stomach, while others have multi-chambered stomachs. Birds have developed a digestive system adapted to eating unmasticated food. Monogastric: Single-chambered Stomach As the word monogastric suggests, this type of digestive system consists of one (“ |
mono”) stomach chamber (“gastric”). Humans and many animals have a monogastric digestive system as illustrated in Figure 25.6ab. The process of digestion begins with the mouth and the intake of food. The teeth play an important role in masticating (chewing) or physically breaking down food into smaller particles. The enzymes present in saliva also begin to chemically break down food. The esophagus is a long tube that connects the mouth to the stomach. Using peristalsis, or wave-like smooth muscle contractions, the muscles of the esophagus push the food towards the stomach. In order to speed up the actions of enzymes in the stomach, the stomach is an extremely acidic environment, with a pH between 1.5 and 2.5. The gastric juices, which include enzymes in the stomach, act on the food particles and continue the process of digestion. Further breakdown of food takes place in the small intestine where enzymes produced by the liver, the small intestine, and the pancreas continue the process of digestion. The nutrients are absorbed into the blood stream across the epithelial cells lining the walls of the small intestines. The waste material travels on to the large intestine where water is absorbed and the drier waste material is compacted into feces; it is stored until it is excreted through the rectum. This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 25 | Animal Nutrition and the Digestive System 1063 Figure 25.6 (a) Humans and herbivores, such as the (b) rabbit, have a monogastric digestive system. However, in the rabbit the small intestine and cecum are enlarged to allow more time to digest plant material. The enlarged organ provides more surface area for absorption of nutrients. Rabbits digest their food twice: the first time food passes through the digestive system, it collects in the cecum, and then it passes through the anus as soft feces called cecotrophes. The rabbit re-ingests these cecotrophes to further digest them. Avian Birds face special challenges when it comes to obtaining nutrition from food. They do not have teeth and so their digestive system, shown in Figure 25.7, must be able to process un-masticated food. Birds have evolved a variety of beak types that reflect the vast variety in their diet, ranging from seeds and insects to fruits and |
nuts. Because most birds fly, their metabolic rates are high in order to efficiently process food and keep their body weight low. The stomach of birds has two chambers: the proventriculus, where gastric juices are produced to digest the food before it enters the stomach, and the gizzard, where the food is stored, soaked, and mechanically ground. The undigested material forms food pellets that are sometimes regurgitated. Most of the chemical digestion and absorption happens in the intestine and the waste is excreted through the cloaca. 1064 Chapter 25 | Animal Nutrition and the Digestive System Figure 25.7 The avian esophagus has a pouch, called a crop, which stores food. Food passes from the crop to the first of two stomachs, called the proventriculus, which contains digestive juices that break down food. From the proventriculus, the food enters the second stomach, called the gizzard, which grinds food. Some birds swallow stones or grit, which are stored in the gizzard, to aid the grinding process. Birds do not have separate openings to excrete urine and feces. Instead, uric acid from the kidneys is secreted into the large intestine and combined with waste from the digestive process. This waste is excreted through an opening called the cloaca. This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 25 | Animal Nutrition and the Digestive System 1065 Avian Adaptations Birds have a highly efficient, simplified digestive system. Recent fossil evidence has shown that the evolutionary divergence of birds from other land animals was characterized by streamlining and simplifying the digestive system. Unlike many other animals, birds do not have teeth to chew their food. In place of lips, they have sharp pointy beaks. The horny beak, lack of jaws, and the smaller tongue of the birds can be traced back to their dinosaur ancestors. The emergence of these changes seems to coincide with the inclusion of seeds in the bird diet. Seed-eating birds have beaks that are shaped for grabbing seeds and the two-compartment stomach allows for delegation of tasks. Since birds need to remain light in order to fly, their metabolic rates are very high, which means they digest their food very quickly and need to eat often. Contrast this with the ruminants, where the digestion of plant matter takes a very long time. Although both birds and humans are vertebrates, birds have a relatively higher metabolic rate |
than humans. a. Why is the metabolic rate of birds relatively higher than that of humans? b. How do birds compensate for such a high metabolism? a. a. Birds have smaller surfaces to lose heat than humans, so their metabolic rate must be higher. b. Birds need to eat greater amounts of food since they digest food quickly. b. a. Birds need to be light to fly, so they need to digest their food faster than humans. b. Birds need to eat greater amounts of food since they digest food quickly. c. a. Birds have smaller surfaces to lose heat than humans, so their metabolic rate must be higher. b. Birds need to eat often to maintain energy since they digest food quickly. d. a. Birds need to be light to fly, so they need to digest their food faster than humans. b. Birds need to eat often to maintain energy since they digest food quickly. Ruminants Ruminants are mainly herbivores like cows, sheep, and goats, whose entire diet consists of eating large amounts of roughage or fiber. They have evolved digestive systems that help them digest vast amounts of cellulose. An interesting feature of the ruminants’ mouth is that they do not have upper incisor teeth. They use their lower teeth, tongue and lips to tear and chew their food. From the mouth, the food travels to the esophagus and on to the stomach. To help digest the large amount of plant material, the stomach of the ruminants is a multi-chambered organ, as illustrated in Figure 25.8. The four compartments of the stomach are called the rumen, reticulum, omasum, and abomasum. These chambers contain many microbes that break down cellulose and ferment ingested food. The abomasum is the “true” stomach and is the equivalent of the monogastric stomach chamber where gastric juices are secreted. The four-compartment gastric chamber provides larger space and the microbial support necessary to digest plant material in ruminants. The fermentation process produces large amounts of gas in the stomach chamber, which must be eliminated. As in other animals, the small intestine plays an important role in nutrient absorption, and the large intestine helps in the elimination of waste. 1066 Chapter 25 | Animal Nutrition and the Digestive System Figure 25.8 Ruminant animals, such as goats and cows, have four stomachs. The first two stomachs, the rumen and the retic |
ulum, contain prokaryotes and protists that are able to digest cellulose fiber. The ruminant regurgitates cud from the reticulum, chews it, and swallows it into a third stomach, the omasum, which removes water. The cud then passes onto the fourth stomach, the abomasum, where it is digested by enzymes produced by the ruminant. Pseudo-ruminants Some animals, such as camels and alpacas, are pseudo-ruminants. They eat a lot of plant material and roughage. Digesting plant material is not easy because plant cell walls contain the polymeric sugar molecule cellulose. The digestive enzymes of these animals cannot break down cellulose, but microorganisms present in the digestive system can. Therefore, the digestive system must be able to handle large amounts of roughage and break down the cellulose. Pseudo-ruminants have a threechamber stomach in the digestive system. However, their cecum—a pouched organ at the beginning of the large intestine containing many microorganisms that are necessary for the digestion of plant materials—is large and is the site where the roughage is fermented and digested. These animals do not have a rumen but have an omasum, abomasum, and reticulum. Parts of the Digestive System The vertebrate digestive system is designed to facilitate the transformation of food matter into the nutrient components that sustain organisms. Oral Cavity The oral cavity, or mouth, is the point of entry of food into the digestive system, illustrated in Figure 25.9. The food consumed is broken into smaller particles by mastication, the chewing action of the teeth. All mammals have teeth and can chew their food. The extensive chemical process of digestion begins in the mouth. As food is being chewed, saliva, produced by the salivary glands, mixes with the food. Saliva is a watery substance produced in the mouths of many animals. There are three major glands that secrete saliva—the parotid, the submandibular, and the sublingual. Saliva contains mucus that moistens food and buffers the pH of the food. Saliva also contains immunoglobulins and lysozymes, which have antibacterial action to reduce tooth decay by inhibiting growth of some bacteria. Saliva also contains an enzyme called salivary amylase that begins the process of converting starches in the food into a dis |
accharide called maltose. Another enzyme called lipase is produced by the cells in the tongue. Lipases are a class of enzymes that can break down triglycerides. The lingual lipase begins the breakdown of fat components in the food. The chewing and wetting action provided by the teeth and saliva prepare the food into a mass called the bolus for swallowing. The tongue helps in swallowing—moving the bolus from the mouth into the pharynx. The pharynx opens to two passageways: the trachea, which leads to the lungs, and the esophagus, which leads to This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 25 | Animal Nutrition and the Digestive System 1067 the stomach. The trachea has an opening called the glottis, which is covered by a cartilaginous flap called the epiglottis. When swallowing, the epiglottis closes the glottis and food passes into the esophagus and not the trachea. This arrangement allows food to be kept out of the trachea. Figure 25.9 Digestion of food begins in the (a) oral cavity. Food is masticated by teeth and moistened by saliva secreted from the (b) salivary glands. Enzymes in the saliva begin to digest starches and fats. With the help of the tongue, the resulting bolus is moved into the esophagus by swallowing. (credit: modification of work by the National Cancer Institute) Esophagus The esophagus is a tubular organ that connects the mouth to the stomach. The chewed and softened food passes through the esophagus after being swallowed. The smooth muscles of the esophagus undergo a series of wave like movements called peristalsis that push the food toward the stomach, as illustrated in Figure 25.10. The peristalsis wave is unidirectional—it moves food from the mouth to the stomach, and reverse movement is not possible. The peristaltic movement of the esophagus is an involuntary reflex; it takes place in response to the act of swallowing. Figure 25.10 The esophagus transfers food from the mouth to the stomach through peristaltic movements. A ring-like muscle called a sphincter forms valves in the digestive system. The gastro-esophageal sphincter is located at the stomach |
end of the esophagus. In response to swallowing and the pressure exerted by the bolus of food, this sphincter opens, and the bolus enters the stomach. When there is no swallowing action, this sphincter is shut and prevents the contents of the stomach from traveling up the esophagus. Many animals have a true sphincter; however, in humans, there is no true sphincter, but the esophagus remains closed when there is no swallowing action. Acid reflux or “heartburn” occurs when the acidic digestive juices escape into the esophagus. 1068 Stomach Chapter 25 | Animal Nutrition and the Digestive System A large part of digestion occurs in the stomach, shown in Figure 25.11. The stomach is a saclike organ that secretes gastric digestive juices. The pH in the stomach is between 1.5 and 2.5. This highly acidic environment is required for the chemical breakdown of food and the extraction of nutrients. When empty, the stomach is a rather small organ; however, it can expand to up to 20 times its resting size when filled with food. This characteristic is particularly useful for animals that need to eat when food is available. Figure 25.11 The human stomach has an extremely acidic environment where most of the protein gets digested. (credit: modification of work by Mariana Ruiz Villareal) Which of the following statements about the digestive system is true? a. Bile is a mixture of food and digestive juices that is produced in the stomach. b. Food enters the large intestine before the small intestine. c. In the small intestine, chyme mixes with bile, which emulsifies fats. d. The large intestines are separated from the small intestines by the pyloric sphincter. The stomach is the major site for protein digestion in animals other than ruminants. Protein digestion is mediated by an enzyme called pepsin in the stomach chamber. Pepsin is secreted by the chief cells in the stomach in an inactive form called pepsinogen. Pepsin breaks peptide bonds and cleaves proteins into smaller polypeptides; it also helps activate more pepsinogen, starting a positive feedback mechanism that generates more pepsin. Another cell type—parietal cells—secretes hydrogen and chloride ions, which combine in the lumen to form hydrochloric acid, the primary acidic component of the stomach juices. Hydrochloric acid helps to |
convert the inactive pepsinogen to pepsin. The highly acidic environment also kills many microorganisms in the food and, combined with the action of the enzyme pepsin, results in the hydrolysis of protein in the food. Chemical digestion is facilitated by the churning action of the stomach. Contraction and relaxation of smooth muscles mixes the stomach contents about every 20 minutes. The partially digested food and gastric juice mixture is called chyme. Chyme passes from the stomach to the small intestine. Further protein digestion takes place in the small intestine. Gastric emptying occurs within two to six hours after a meal. Only a small amount of chyme is released into the small intestine at a time. The movement of chyme from the stomach into the small intestine is regulated by the pyloric sphincter. When digesting protein and some fats, the stomach lining must be protected from getting digested by pepsin. There are two points to consider when describing how the stomach lining is protected. First, as previously mentioned, the enzyme pepsin is synthesized in the inactive form. This protects the chief cells, because pepsinogen does not have the same enzyme functionality of pepsin. Second, the stomach has a thick mucus lining that protects the underlying tissue from the action of the digestive juices. When this mucus lining is ruptured, ulcers can form in the stomach. Ulcers are open wounds in or on This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 25 | Animal Nutrition and the Digestive System 1069 an organ caused by bacteria (Helicobacter pylori) when the mucus lining is ruptured and fails to reform. Small Intestine Chyme moves from the stomach to the small intestine. The small intestine is the organ where the digestion of protein, fats, and carbohydrates is completed. The small intestine is a long tube-like organ with a highly folded surface containing fingerlike projections called the villi. The apical surface of each villus has many microscopic projections called microvilli. These structures, illustrated in Figure 25.12, are lined with epithelial cells on the luminal side and allow for the nutrients to be absorbed from the digested food and absorbed into the blood stream on the other side. The villi and microvilli, with their many folds, increase the surface area of the intestine and increase absorption efficiency |
of the nutrients. Absorbed nutrients in the blood are carried into the hepatic portal vein, which leads to the liver. There, the liver regulates the distribution of nutrients to the rest of the body and removes toxic substances, including drugs, alcohol, and some pathogens. Figure 25.12 Villi are folds on the small intestine lining that increase the surface area to facilitate the absorption of nutrients. Which of the following statements about the small intestine is true? a. Absorptive cells that line the small intestine have small projections that increase surface area and aid in the absorption of food. b. The outside of the small intestine has many folds, called villi. c. Microvilli are lined with blood vessels as well as lymphatic vessels. d. The inside of the small intestine is called the lymphatic vessel. The human small intestine is over 6m long and is divided into three parts: the duodenum, the jejunum, and the ileum. The “C-shaped,” fixed part of the small intestine is called the duodenum and is shown in Figure 25.11. The duodenum is separated from the stomach by the pyloric sphincter which opens to allow chyme to move from the stomach to the duodenum. In the duodenum, chyme is mixed with pancreatic juices in an alkaline solution rich in bicarbonate that neutralizes the acidity of chyme and acts as a buffer. Pancreatic juices also contain several digestive enzymes. Digestive juices from the pancreas, liver, and gallbladder, as well as from gland cells of the intestinal wall itself, enter the duodenum. Bile is produced in the liver and stored and concentrated in the gallbladder. Bile contains bile salts which emulsify lipids while the pancreas produces enzymes that catabolize starches, disaccharides, proteins, and fats. These digestive juices break down the food particles in the chyme into glucose, triglycerides, and amino acids. Some chemical digestion of food takes place in the duodenum. Absorption of fatty acids also takes place in the duodenum. The second part of the small intestine is called the jejunum, shown in Figure 25.11. Here, hydrolysis of nutrients is continued while most of the carbohydrates and amino acids are absorbed through the intestinal lining. The bulk of chemical digestion and nutrient absorption occurs in the je |
junum. The ileum, also illustrated in Figure 25.11 is the last part of the small intestine and here the bile salts and vitamins are 1070 Chapter 25 | Animal Nutrition and the Digestive System absorbed into blood stream. The undigested food is sent to the colon from the ileum via peristaltic movements of the muscle. The ileum ends and the large intestine begins at the ileocecal valve. The vermiform, “worm-like,” appendix is located at the ileocecal valve. The appendix of humans secretes no enzymes and has an insignificant role in immunity. Figure 25.13 Transmission electron microscope image of a thin section cut through an epithelial cell from a human jejunum (segment of the small intestine). The image shows the apical end of an absorptive cell with some of the densely packed microvilli that make up the striated border. Each microvillus is approximately 1 μm long by 0.1 μm in diameter and contains a core of actin microfilaments. (credit: “Microvilli”, Wikimedia Commons) What is the role of microvilli in nutrient absorption? a. Microvilli form the inner layer of epithelial tissue in the small intestine and increase the absorption of nutrients from chyme. b. Microvilli are projections of absorptive cells that are involved in the absorption of bile salts and vitamin B12. c. Microvilli increase the surface area of absorptive cells, and therefore increase the amount of nutrients that can be absorbed. d. Microvilli use smooth muscle contractions to move the chyme, which contains nutrients, thereby increasing the rate of absorption. Large Intestine The large intestine, illustrated in Figure 25.14, reabsorbs the water from the undigested food material and processes the waste material. The human large intestine is much smaller in length compared to the small intestine but larger in diameter. It has three parts: the cecum, the colon, and the rectum. The cecum joins the ileum to the colon and is the receiving pouch for the waste matter. The colon is home to many bacteria or “intestinal flora” that aid in the digestive processes. The colon can be divided into four regions, the ascending colon, the transverse colon, the descending colon and the sigmoid colon. The main functions of the colon are to extract the water and mineral salts |
from undigested food, and to store waste material. Carnivorous mammals have a shorter large intestine compared to herbivorous mammals due to their diet. This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 25 | Animal Nutrition and the Digestive System 1071 Figure 25.14 The large intestine reabsorbs water from undigested food and stores waste material until it is eliminated. Rectum and Anus The rectum is the terminal end of the large intestine, as shown in Figure 25.14. The primary role of the rectum is to store the feces until defecation. The feces are propelled using peristaltic movements during elimination. The anus is an opening at the far-end of the digestive tract and is the exit point for the waste material. Two sphincters between the rectum and anus control elimination: the inner sphincter is involuntary and the outer sphincter is voluntary. Accessory Organs The organs discussed above are the organs of the digestive tract through which food passes. Accessory organs are organs that add secretions (enzymes) that catabolize food into nutrients. Accessory organs include salivary glands, the liver, the pancreas, and the gallbladder. The liver, pancreas, and gallbladder are regulated by hormones in response to the food consumed. The liver is the largest internal organ in humans and it plays a very important role in digestion of fats and detoxifying blood. The liver produces bile, a digestive juice that is required for the breakdown of fatty components of the food in the duodenum. The liver also processes the vitamins and fats and synthesizes many plasma proteins. The pancreas is another important gland that secretes digestive juices. The chyme produced from the stomach is highly acidic in nature; the pancreatic juices contain high levels of bicarbonate, an alkali that neutralizes the acidic chyme. Additionally, the pancreatic juices contain a large variety of enzymes that are required for the digestion of protein and carbohydrates. The gallbladder is a small organ that aids the liver by storing bile and concentrating bile salts. When chyme containing fatty acids enters the duodenum, the bile is secreted from the gallbladder into the duodenum. Activity Create a mini-poster that shows the procurement, digestion, absorption, and distribution of nutrients through the digestive |
systems of one invertebrate animal and one vertebrate animal. Explain how the organs of the system promote efficiency in the use of matter and energy. Think About It Explain how the villi and microvilli aid in absorption of nutrients from the small intestine into the circulatory system. 1072 Chapter 25 | Animal Nutrition and the Digestive System 25.2 | Nutrition and Energy Production In this section, you will explore the following questions: • Why must an animal have a balanced diet? • What are the primary components of food? • What are examples of essential nutrients required for cellular function that cannot be synthesized by the animal body? • How is energy produced through diet and digestion? • How are excess carbohydrates and energy stored in the body? Connection for AP® Courses Much of the content described in this module is not within the scope of AP®. However, as we learn in the chapter on biological macromolecules, in animals the organic molecules required for building cellular materials and tissues come from food. During digestion, complex carbohydrates are broken down into glucose and used to provide energy through metabolic pathways, such as cellular respiration (see the chapter on cellular respiration). Excess sugars in the body are stored as glycogen in the liver and muscles for later use. Another important requirement is nitrogen, and protein catabolism provides a source of nitrogen; amino acids from protein breakdown are building blocks for new proteins. The carbon and nitrogen derived from amino acids also become building blocks for nucleic acids. Excess nitrogen is excreted because it is toxic. Although the animal body can synthesize many of the molecules necessary for function from organic precursors, some essential nutrients must be consumed from food. Vitamins are another class of essential organic molecules that are required in small quantities for many enzymes to function. (No, you do not need to memorize the table of vitamins and their functions!) Deficiencies in nutrients can have detrimental effects on an animal’s health. For example, among other things, vitamin D is necessary for calcium absorption for bone development, and vitamin C is critical to multiple biochemical pathways, including immune function. As we learn in the chapter on cellular respiration, animals need free energy, primarily supplied by carbohydrates, to maintain homeostasis. ATP is the energy currency of the cell and is produced by the oxidative reactions in the cytoplasm and mitochondria, where carbohydrates, proteins, and fats undergo a series of metabolic reactions collectively called cellular respiration. When the amount of ATP available exceeds the |
body’s requirements, the liver uses the excess ATP and glucose to produce molecules of glycogen. The ability to store excess energy is an evolutionary adaptation that helps animals deal with mobility and food shortages. Information presented and the examples highlighted in the section support concepts outlined in Big Idea 2 of the AP® Biology Curriculum Framework. The AP® 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 Essential Knowledge Science Practice Learning Objective 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. 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. This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 25 | Animal Nutrition and the Digestive System 1073 Essential Knowledge 2.A.3 Organisms must exchange matter with the environment to grow, reproduce and maintain organization. Science Practice Learning Objective 4.1 The student can justify the selection of the kind of data needed to answer a particular scientific question. 2.8 The student is able to justify the selection of data regarding the types of molecules that an animal will take up as necessary building blocks. Given the diversity of animal life on our planet, it is not surprising that the animal diet would also vary substantially. The animal diet is the source of materials needed for building DNA and other complex molecules needed for growth, maintenance, and reproduction; collectively these processes are called biosynthesis. The diet is also the source of materials for ATP production in the cells. The diet must be balanced to provide the minerals and vitamins that are required for cellular function. Food Requirements What are the fundamental requirements of the animal diet? The animal diet should be well balanced and provide nutrients required for bodily function and the minerals and vitamins required for maintaining structure and regulation necessary for good health and reproductive capability. These requirements for a human are illustrated graphically in Figure 25.15 Figure 25.15 |
For humans, a balanced diet includes fruits, vegetables, grains, and protein. (credit: USDA) 1074 Chapter 25 | Animal Nutrition and the Digestive System The first step in ensuring that you are meeting the food requirements of your body is an awareness of the food groups and the nutrients they provide. To learn more about each food group and the recommended daily amounts, explore this interactive site (http://openstaxcollege.org/l/food_groups) by the United States Department of Agriculture. How many cups of vegetables per day are recommended for a 61-year-old woman who does not exercise on a regular basis? a. 1.5 cups b. 2 cups c. 2.5 cups d. 3 cups Let’s Move! Campaign Obesity is a growing epidemic and the rate of obesity among children is rapidly rising in the United States. To combat childhood obesity and ensure that children get a healthy start in life, former first lady Michelle Obama has launched the Let’s Move! campaign. The goal of this campaign is to educate parents and caregivers on providing healthy nutrition and encouraging active lifestyles to future generations. This program aims to involve the entire community, including parents, teachers, and healthcare providers to ensure that children have access to healthy foods—more fruits, vegetables, and whole grains—and consume fewer calories from processed foods. Another goal is to ensure that children get physical activity. With the increase in television viewing and stationary pursuits such as video games, sedentary lifestyles have become the norm. Learn more at www.letsmove.gov. How does increased activity help reduce obesity in individuals? a. A more active individual will burn more calories. b. A more active individual will consume less fat. c. A more active individual will consume more calories. d. The appetite of a more active individual will be less. Organic Precursors The organic molecules required for building cellular material and tissues must come from food. Carbohydrates or sugars are the primary source of organic carbons in the animal body. During digestion, digestible carbohydrates are ultimately broken down into glucose and used to provide energy through metabolic pathways. Complex carbohydrates, including polysaccharides, can be broken down into glucose through biochemical modification; however, humans do not produce the enzyme cellulase and lack the ability to derive glucose from the polysaccharide cellulose. In humans, these molecules provide the fiber required for moving waste through the large intestine and a healthy colon. The intestinal flora in the human gut are able to extract some nutrition from |
these plant fibers. The excess sugars in the body are converted into glycogen and stored in the liver and muscles for later use. Glycogen stores are used to fuel prolonged exertions, such as long-distance running, and to provide energy during food shortage. Excess glycogen can be converted to fats, which are stored in the lower layer of the skin of mammals for insulation and energy storage. Excess digestible carbohydrates are stored by mammals in order to survive famine and aid in mobility. This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 25 | Animal Nutrition and the Digestive System 1075 Another important requirement is that of nitrogen. Protein catabolism provides a source of organic nitrogen. Amino acids are the building blocks of proteins and protein breakdown provides amino acids that are used for cellular function. The carbon and nitrogen derived from these become the building block for nucleotides, nucleic acids, proteins, cells, and tissues. Excess nitrogen must be excreted as it is toxic. Fats add flavor to food and promote a sense of satiety or fullness. Fatty foods are also significant sources of energy because one gram of fat contains nine calories. Fats are required in the diet to aid the absorption of fat-soluble vitamins and the production of fat-soluble hormones. Essential Nutrients While the animal body can synthesize many of the molecules required for function from the organic precursors, there are some nutrients that need to be consumed from food. These nutrients are termed essential nutrients, meaning they must be eaten, and the body cannot produce them. The omega-3 alpha-linolenic acid and the omega-6 linoleic acid are essential fatty acids needed to make some membrane phospholipids. Vitamins are another class of essential organic molecules that are required in small quantities for many enzymes to function and, for this reason, are considered to be co-enzymes. Absence or low levels of vitamins can have a dramatic effect on health, as outlined in Table 25.1 and Table 25.2. Both fat-soluble and water-soluble vitamins must be obtained from food. Minerals, listed in Table 25.3, are inorganic essential nutrients that must be obtained from food. Among their many functions, minerals help in structure and regulation and are considered co-factors. Certain amino acids also must be procured from food and cannot be synthesized by the body. |
These amino acids are the “essential” amino acids. The human body can synthesize only 11 of the 20 required amino acids; the rest must be obtained from food. The essential amino acids are listed in Table 25.4. Water-soluble Essential Vitamins Function Deficiencies Can Lead To Sources Vitamin Vitamin B1 (Thiamine) Needed by the body to process lipids, proteins, and carbohydrates Coenzyme removes CO2 from organic compounds Vitamin B2 (Riboflavin) Takes an active role in metabolism, aiding in the conversion of food to energy (FAD and FMN) Vitamin B3 (Niacin) Used by the body to release energy from carbohydrates and to process alcohol; required for the synthesis of sex hormones; component of coenzyme NAD+ and NADP+ Muscle weakness, Beriberi: reduced heart function, CNS problems Cracks or sores on the outer surface of the lips (cheliosis); inflammation and redness of the tongue; moist, scaly skin inflammation (seborrheic dermatitis) Milk, meat, dried beans, whole grains Meat, eggs, enriched grains, vegetables Pellagra, which can result in dermatitis, diarrhea, dementia, and death Meat, eggs, grains, nuts, potatoes Vitamin B5 (Pantothenic acid) Assists in producing energy from foods (lipids, in particular); component of coenzyme A Fatigue, poor coordination, retarded growth, numbness, tingling of hands and feet Vitamin B6 (Pyridoxine) The principal vitamin for processing amino acids and lipids; also helps convert nutrients into energy Irritability, confusion, mouth sores or ulcers, anemia, muscular twitching Used in energy and amino acid metabolism, fat synthesis, and fat breakdown; helps the body use blood sugar Hair loss, dermatitis, numbness and tingling in the extremities; neuromuscular disorders Vitamin B7 (Biotin) Table 25.1 Meat, whole grains, milk, fruits, vegetables Meat, dairy products, whole grains, orange juice Meat, eggs, legumes and other vegetables 1076 Chapter 25 | Animal Nutrition and the Digestive System Vitamin Function Deficiencies Can Lead To Sources Water-soluble Essential Vitamins Vitamin B9 (Folic acid) Assists the normal development of cells, especially during fetal development; helps metabolize nucleic and amino acids Deficiency during pregnancy is associated with birth defects, such as |
neural tube defects and anemia Vitamin B12 (Cobalamin) Maintains healthy nervous system and assists with blood cell formation; coenzyme in nucleic acid metabolism Anemia, neurological disorders, numbness, loss of balance Vitamin C (Ascorbic acid) Helps maintain connective tissue: bone, cartilage, and dentin; boosts the immune system Scurvy, which results in bleeding, hair and tooth loss; joint pain and swelling; delayed wound healing Leafy green vegetables, whole wheat, fruits, nuts, legumes Meat, eggs, animal products Citrus fruits, broccoli, tomatoes, red sweet bell peppers Table 25.1 Fat-soluble Essential Vitamins Vitamin Function Deficiencies Can Lead To Sources Critical to the development of bones, teeth, and skin; helps maintain eyesight, enhances the immune system, fetal development, gene expression Night-blindness, skin disorders, impaired immunity Dark green leafy vegetables, yelloworange vegetables fruits, milk, butter Rickets, osteomalacia, immunity Cod liver oil, milk, egg yolk Deficiency is rare; anemia, nervous system degeneration Wheat germ oil, unrefined vegetable oils, nuts, seeds, grains Bleeding and easy bruising Leafy green vegetables, tea Vitamin A (Retinol) Vitamin D Critical for calcium absorption for bone development and strength; maintains a stable nervous system; maintains a normal and strong heartbeat; helps in blood clotting Vitamin E (Tocopherol) Lessens oxidative damage of cells,and prevents lung damage from pollutants; vital to the immune system Vitamin K (Phylloquinone) Table 25.2 Essential to blood clotting This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 25 | Animal Nutrition and the Digestive System 1077 Figure 25.16 A healthy diet should include a variety of foods to ensure that needs for essential nutrients are met. (credit: Keith Weller, USDA ARS) Minerals and Their Function in the Human Body Mineral Function Deficiencies Can Lead To Sources Needed for muscle and neuron function; heart health; builds bone and supports synthesis and function of blood cells; nerve function Osteoporosis, rickets, muscle spasms, impaired growth Milk, yogurt, fish, green leafy vegetables, legumes Needed for production of hydrochloric acid (HCl) in the stomach and nerve function; osmotic balance Muscle cramps, mood disturbances |
, reduced appetite Table salt Required component of many redox enzymes, including cytochrome c oxidase; cofactor for hemoglobin synthesis Copper deficiency is rare Liver, oysters, cocoa, chocolate, sesame, nuts Required for the synthesis of thyroid hormones Goiter Seafood, iodized salt, dairy products Required for many proteins and enzymes, notably hemoglobin, to prevent anemia Required co-factor for ATP formation; bone formation; normal membrane functions; muscle function Anemia, which causes poor concentration, fatigue, and poor immune function Red meat, leafy green vegetables, fish (tuna, salmon), eggs, dried fruits, beans, whole grains Mood disturbances, muscle spasms Whole grains, leafy green vegetables A cofactor in enzyme functions; trace amounts are required Manganese deficiency is rare Common in most foods *Calcium *Chlorine Copper (trace amounts) Iodine Iron *Magnesium Manganese (trace amounts) Molybdenum (trace amounts) Acts as a cofactor for three essential enzymes in humans: sulfite oxidase, xanthine oxidase, and aldehyde oxidase Molybdenum deficiency is rare Table 25.3 1078 Chapter 25 | Animal Nutrition and the Digestive System Minerals and Their Function in the Human Body Mineral Function Deficiencies Can Lead To Sources *Phosphorus A component of bones and teeth; helps regulate acid-base balance; nucleotide synthesis Weakness, bone abnormalities, calcium loss Milk, hard cheese, whole grains, meats *Potassium Vital for muscles, heart, and nerve function Cardiac rhythm disturbance, muscle weakness Legumes, potato skin, tomatoes, bananas Selenium (trace amounts) A cofactor essential to activity of antioxidant enzymes like glutathione peroxidase; trace amounts are required Selenium deficiency is rare Common in most foods *Sodium Zinc (trace amounts) Systemic electrolyte required for many functions; acid-base balance; water balance; nerve function Muscle cramps, fatigue, reduced appetite Table salt Required for several enzymes such as carboxypeptidase, liver alcohol dehydrogenase, and carbonic anhydrase Anemia, poor wound healing, can lead to short stature Common in most foods *Greater than 200mg/day required Table 25.3 Amino acids that must be consumed Amino acids anabolized by the body Essential Amino Acids isoleucine leucine lysine methionine phenylalanine tryptophan val |
ine histidine* threonine arginine* alanine selenocysteine aspartate cysteine glutamate glycine proline serine tyrosine asparagine *The human body can synthesize histidine and arginine, but not in the quantities required, especially for growing children. Table 25.4 This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 25 | Animal Nutrition and the Digestive System 1079 Think About It There are several nations where malnourishment is a common occurrence. What are some of the health challenges posed by malnutrition? What are examples of diseases caused by nutrient deficiencies? Food Energy and ATP Animals need food to obtain energy and maintain homeostasis. Homeostasis is the ability of a system to maintain a stable internal environment even in the face of external changes to the environment. For example, the normal body temperature of humans is 37°C (98.6°F). Humans maintain this temperature even when the external temperature is hot or cold. It takes energy to maintain this body temperature, and animals obtain this energy from food. The primary source of energy for animals is carbohydrates, mainly glucose. Glucose is called the body’s fuel. The digestible carbohydrates in an animal’s diet are converted to glucose molecules through a series of catabolic chemical reactions. Adenosine triphosphate, or ATP, is the primary energy currency in cells; ATP stores energy in phosphate ester bonds. ATP releases energy when the phosphodiester bonds are broken and ATP is converted to ADP and a phosphate group. ATP is produced by the oxidative reactions in the cytoplasm and mitochondrion of the cell, where carbohydrates, proteins, and fats undergo a series of metabolic reactions collectively called cellular respiration. For example, glycolysis is a series of reactions in which glucose is converted to pyruvic acid and some of its chemical potential energy is transferred to NADH and ATP. ATP is required for all cellular functions. It is used to build the organic molecules that are required for cells and tissues; it provides energy for muscle contraction and for the transmission of electrical signals in the nervous system. When the amount of ATP is available in excess of the body’s requirements, the liver uses the excess ATP and excess glucose to produce molecules called glycogen. Glycogen is a polymeric form of glucose and is stored in the liver and skeletal muscle cells |
. When blood sugar drops, the liver releases glucose from stores of glycogen. Skeletal muscle converts glycogen to glucose during intense exercise. The process of converting glucose and excess ATP to glycogen and the storage of excess energy is an evolutionarily important step in helping animals deal with mobility, food shortages, and famine. 1080 Chapter 25 | Animal Nutrition and the Digestive System Obesity Obesity is a major health concern in the United States, and there is a growing focus on reducing obesity and the diseases it may lead to, such as type-2 diabetes, cancers of the colon and breast, and cardiovascular disease. How does the food consumed contribute to obesity? Fatty foods are calorie-dense, meaning that they have more calories per unit mass than carbohydrates or proteins. One gram of carbohydrates has four calories, one gram of protein has four calories, and one gram of fat has nine calories. Animals tend to seek lipid-rich food for their higher energy content. The signals of hunger (“time to eat”) and satiety (“time to stop eating”) are controlled in the hypothalamus region of the brain. Foods that are rich in fatty acids tend to promote satiety more than foods that are rich only in carbohydrates. Excess carbohydrate and ATP are used by the liver to synthesize glycogen. The pyruvate produced during glycolysis is used to synthesize fatty acids. When there is more glucose in the body than required, the resulting excess pyruvate is converted into molecules that eventually result in the synthesis of fatty acids within the body. These fatty acids are stored in adipose cells—the fat cells in the mammalian body whose primary role is to store fat for later use. It is important to note that some animals benefit from obesity. Polar bears and seals need body fat for insulation and to keep them from losing body heat during Arctic winters. When food is scarce, stored body fat provides energy for maintaining homeostasis. Fats prevent famine in mammals, allowing them to access energy when food is not available on a daily basis; fats are stored when a large kill is made or lots of food is available. Which of the following statements about obesity is true? a. Carbohydrate-rich foods satisfy hunger better than fatty acid–rich food. b. Obesity is disadvantageous for organisms that live in cold climates. c. Fat has more calories than protein or carbohydrates. d. In the presence of excess blood glucose, fatty acids are synthesized and stored in skeletal muscle. 25 |
.3 | Digestive System Processes In this section, you will explore the following questions: • What is the process of digestion? • What steps are involved in digestion and absorption? • What is elimination? • What are the roles of the small and large intestines in absorption? Connection for AP® Courses Much of the information in this module is not within the scope of AP®. However, when we explored concepts about biological molecules in the chapter on biological macromolecules, we learn how macromolecules—carbohydrates, lipids, proteins, and nucleic acids—are synthesized from monomers. During digestion, these polymers are broken down into monomers, which are then absorbed and transported to all cells of the body. In the Digestive Systems module, we described the fate of the ingredients of a sandwich as they pass through the digestive tract. Food is ingested through the mouth, and digestion and absorption occur in a series of steps, with special enzymes playing important roles in digesting carbohydrates, proteins, and lipids. (You do not need to know the names of the specific enzymes involved in chemical digestion.) While most absorption of nutrients takes place in the small intestine, the remaining water, some vitamins, and any leftover salts are absorbed in the large intestine. Elimination describes the removal of undigested food. This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 25 | Animal Nutrition and the Digestive System 1081 Obtaining nutrition and energy from food is a multi-step process. For true animals, the first step is ingestion, the act of taking in food. This is followed by digestion, absorption, and elimination. In the following sections, each of these steps will be discussed in detail. Ingestion The large molecules found in intact food cannot pass through the cell membranes. Food needs to be broken into smaller particles so that animals can harness the nutrients and organic molecules. The first step in this process is ingestion. Ingestion is the process of taking in food through the mouth. In vertebrates, the teeth, saliva, and tongue play important roles in mastication (preparing the food into bolus). While the food is being mechanically broken down, the enzymes in saliva begin to chemically process the food as well. The combined action of these processes modifies the food from large particles to a soft mass that can be swallowed and can travel the length of the esophagus. Digestion and Abs |
orption Digestion is the mechanical and chemical break down of food into small organic fragments. It is important to break down macromolecules into smaller fragments that are of suitable size for absorption across the digestive epithelium. Large, complex molecules of proteins, polysaccharides, and lipids must be reduced to simpler particles such as simple sugar before they can be absorbed by the digestive epithelial cells. Different organs play specific roles in the digestive process. The animal diet needs carbohydrates, protein, and fat, as well as vitamins and inorganic components for nutritional balance. How each of these components is digested is discussed in the following sections. Carbohydrates The digestion of carbohydrates begins in the mouth. The salivary enzyme amylase begins the breakdown of food starches into maltose, a disaccharide. As the bolus of food travels through the esophagus to the stomach, no significant digestion of carbohydrates takes place. The esophagus produces no digestive enzymes but does produce mucous for lubrication. The acidic environment in the stomach stops the action of the amylase enzyme. The next step of carbohydrate digestion takes place in the duodenum. Recall that the chyme from the stomach enters the duodenum and mixes with the digestive secretion from the pancreas, liver, and gallbladder. Pancreatic juices also contain amylase, which continues the breakdown of starch and glycogen into maltose, a disaccharide. The disaccharides are broken down into monosaccharides by enzymes called maltases, sucrases, and lactases, which are also present in the brush border of the small intestinal wall. Maltase breaks down maltose into glucose. Other disaccharides, such as sucrose and lactose are broken down by sucrase and lactase, respectively. Sucrase breaks down sucrose (or “table sugar”) into glucose and fructose, and lactase breaks down lactose (or “milk sugar”) into glucose and galactose. The monosaccharides (glucose) thus produced are absorbed and then can be used in metabolic pathways to harness energy. The monosaccharides are transported across the intestinal epithelium into the bloodstream to be transported to the different cells in the body. The steps in carbohydrate digestion are summarized in Figure 25.17 and Table 25.5. Figure 25.17 Digestion of carbohydrates is performed by several enzymes. Starch |
and glycogen are broken down into glucose by amylase and maltase. Sucrose (table sugar) and lactose (milk sugar) are broken down by sucrase and lactase, respectively. 1082 Chapter 25 | Animal Nutrition and the Digestive System Digestion of Carbohydrates Enzyme Produced By Site of Action Substrate Acting On End Products Salivary amylase Salivary glands Mouth Pancreatic amylase Pancreas Oligosaccharidases Lining of the intestine; brush border membrane Small intestine Small intestine Polysaccharides (Starch) Disaccharides (maltose), oligosaccharides Polysaccharides (starch) Disaccharides (maltose), monosaccharides Disaccharides Monosaccharides (e.g., glucose, fructose, galactose) Table 25.5 Protein A large part of protein digestion takes place in the stomach. The enzyme pepsin plays an important role in the digestion of proteins by breaking down the intact protein to peptides, which are short chains of four to nine amino acids. In the duodenum, other enzymes—trypsin, elastase, and chymotrypsin—act on the peptides reducing them to smaller peptides. Trypsin elastase, carboxypeptidase, and chymotrypsin are produced by the pancreas and released into the duodenum where they act on the chyme. Further breakdown of peptides to single amino acids is aided by enzymes called peptidases (those that break down peptides). Specifically, carboxypeptidase, dipeptidase, and aminopeptidase play important roles in reducing the peptides to free amino acids. The amino acids are absorbed into the bloodstream through the small intestines. The steps in protein digestion are summarized in Figure 25.18 and Table 25.6. This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 25 | Animal Nutrition and the Digestive System 1083 Figure 25.18 Protein digestion is a multistep process that begins in the stomach and continues through the intestines. Digestion of Protein Produced By Stomach chief cells Site of Action Substrate Acting On End Products Stomach Proteins Peptides Pancreas Small intestine Proteins Peptides Enzyme Pep |
sin Trypsin Elastase Chymotrypsin Carboxypeptidase Pancreas Small intestine Peptides Amino acids and peptides Aminopeptidase Dipeptidase Table 25.6 Lining of intestine Small intestine Peptides Amino acids 1084 Lipids Chapter 25 | Animal Nutrition and the Digestive System Lipid digestion begins in the stomach with the aid of lingual lipase and gastric lipase. However, the bulk of lipid digestion occurs in the small intestine due to pancreatic lipase. When chyme enters the duodenum, the hormonal responses trigger the release of bile, which is produced in the liver and stored in the gallbladder. Bile aids in the digestion of lipids, primarily triglycerides by emulsification. Emulsification is a process in which large lipid globules are broken down into several small lipid globules. These small globules are more widely distributed in the chyme rather than forming large aggregates. Lipids are hydrophobic substances: in the presence of water, they will aggregate to form globules to minimize exposure to water. Bile contains bile salts, which are amphipathic, meaning they contain hydrophobic and hydrophilic parts. Thus, the bile salts hydrophilic side can interface with water on one side and the hydrophobic side interfaces with lipids on the other. By doing so, bile salts emulsify large lipid globules into small lipid globules. Why is emulsification important for digestion of lipids? Pancreatic juices contain enzymes called lipases (enzymes that break down lipids). If the lipid in the chyme aggregates into large globules, very little surface area of the lipids is available for the lipases to act on, leaving lipid digestion incomplete. By forming an emulsion, bile salts increase the available surface area of the lipids many fold. The pancreatic lipases can then act on the lipids more efficiently and digest them, as detailed in Figure 25.19. Lipases break down the lipids into fatty acids and glycerides. These molecules can pass through the plasma membrane of the cell and enter the epithelial cells of the intestinal lining. The bile salts surround long-chain fatty acids and monoglycerides forming tiny spheres called micelles. The micelles move into the brush border of the small intestine absorptive cells where the long-chain fatty acids and mon |
oglycerides diffuse out of the micelles into the absorptive cells leaving the micelles behind in the chyme. The long-chain fatty acids and monoglycerides recombine in the absorptive cells to form triglycerides, which aggregate into globules and become coated with proteins. These large spheres are called chylomicrons. Chylomicrons contain triglycerides, cholesterol, and other lipids and have proteins on their surface. The surface is also composed of the hydrophilic phosphate "heads" of phospholipids. Together, they enable the chylomicron to move in an aqueous environment without exposing the lipids to water. Chylomicrons leave the absorptive cells via exocytosis. Chylomicrons enter the lymphatic vessels, and then enter the blood in the subclavian vein. Figure 25.19 Lipids are digested and absorbed in the small intestine. Vitamins Vitamins can be either water-soluble or lipid-soluble. Fat soluble vitamins are absorbed in the same manner as lipids. It is important to consume some amount of dietary lipid to aid the absorption of lipid-soluble vitamins. Water-soluble vitamins This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 25 | Animal Nutrition and the Digestive System 1085 can be directly absorbed into the bloodstream from the intestine. This website (http://openstaxcollege.org/l/digest_enzymes) has an overview of the digestion of protein, fat, and carbohydrates. Which of the following events in digestion and absorption is incorrect? a. Carbohydrate digestion begins in the mouth. b. Protein is primarily digested in the stomach. c. Fats are primarily digested in the large intestine. d. Digested carbohydrate, fat, and protein molecules are absorbed by villi in the small intestine. 1086 Chapter 25 | Animal Nutrition and the Digestive System Figure 25.20 Mechanical and chemical digestion of food takes place in many steps, beginning in the mouth and ending in the rectum. Which of the following statements about digestive processes is true? a. Bile emulsifies lipids in the small intestine. b. Trypsin and lipase in the stomach digest protein. c. Amylase, maltase, and lactase in the mouth digest carbohydrates. d. Peptides are primarily absorbed in the large intestines. Elimination The |
final step in digestion is the elimination of undigested food content and waste products. The undigested food material enters the colon, where most of the water is reabsorbed. Recall that the colon is also home to the microflora called “intestinal flora” that aid in the digestion process. The semi-solid waste is moved through the colon by peristaltic movements of the muscle and is stored in the rectum. As the rectum expands in response to storage of fecal matter, it triggers the neural signals required to set up the urge to eliminate. The solid waste is eliminated through the anus using peristaltic movements of the rectum. This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 25 | Animal Nutrition and the Digestive System 1087 Common Problems with Elimination Diarrhea and constipation are some of the most common health concerns that affect digestion. Constipation is a condition where the feces are hardened because of excess water removal in the colon. In contrast, if enough water is not removed from the feces, it results in diarrhea. Many bacteria, including the ones that cause cholera, affect the proteins involved in water reabsorption in the colon and result in excessive diarrhea. Emesis Emesis, or vomiting, is elimination of food by forceful expulsion through the mouth. It is often in response to an irritant that affects the digestive tract, including but not limited to viruses, bacteria, emotions, sights, and food poisoning. This forceful expulsion of the food is due to the strong contractions produced by the stomach muscles. The process of emesis is regulated by the medulla. 25.4 | Digestive System Regulation In this section, you will explore the following questions: • How is neural regulation involved in the digestive processes? • How do hormones regulate digestion? Connection for AP® Courses The concepts presented in this module are not within the scope for AP® other than to note that the brain houses the control center for the sensation of hunger and satiety (fullness). The functions of the digestive system are regulated through neural and hormonal responses. In the chapter that discusses the endocrine system, we explore in detail the role of the endocrine system in maintaining homeostasis, including the normal functioning of the digestive system. The brain is the control center for the sensation of hunger and satiety. The functions of the digestive system are regulated through neural and hormonal responses. Neural Responses to |
Food In reaction to<|endoftext|> the smell, sight, or thought of food, like that shown in Figure 25.21, the first response is that of salivation. The salivary glands secrete more saliva in response to stimulation by the autonomic nervous system triggered by the food in preparation for digestion. Simultaneously, the stomach begins to produce hydrochloric acid to digest the food. Recall that the peristaltic movements of the esophagus and other organs of the digestive tract are under the control of the brain. The brain prepares these muscles for movement as well. When the stomach is full, the part of the brain that detects satiety signals fullness. There are three overlapping phases of gastric control—the cephalic phase, the gastric phase, and the intestinal phase—each requires many enzymes and is under neural control as well. 1088 Chapter 25 | Animal Nutrition and the Digestive System Figure 25.21 Seeing a plate of food triggers the secretion of saliva in the mouth and the production of HCL in the stomach. (credit: Kelly Bailey) Digestive Phases The response to food begins even before food enters the mouth. The first phase of ingestion, called the cephalic phase, is controlled by the neural response to the stimulus provided by food. All aspects—such as sight, sense, and smell—trigger the neural responses resulting in salivation and secretion of gastric juices. The gastric and salivary secretion in the cephalic phase can also take place due to the thought of food. Right now, if you think about a piece of chocolate or a crispy potato chip, the increase in salivation is a cephalic phase response to the thought. The central nervous system prepares the stomach to receive food. The gastric phase begins once the food arrives in the stomach. It builds on the stimulation provided during the cephalic phase. Gastric acids and enzymes process the ingested materials. The gastric phase is stimulated by (1) distension of the stomach, (2) a decrease in the pH of the gastric contents, and (3) the presence of undigested material. This phase consists of local, hormonal, and neural responses. These responses stimulate secretions and powerful contractions. The intestinal phase begins when chyme enters the small intestine triggering digestive secretions. This phase controls the rate of gastric emptying. In addition to gastric emptying, when chyme enters the small |
intestine, it triggers other hormonal and neural events that coordinate the activities of the intestinal tract, pancreas, liver, and gallbladder. Hormonal Responses to Food The endocrine system controls the response of the various glands in the body and the release of hormones at the appropriate times. One of the important factors under hormonal control is the stomach acid environment. During the gastric phase, the hormone gastrin is secreted by G cells in the stomach in response to the presence of proteins. Gastrin stimulates the release of stomach acid, or hydrochloric acid (HCl) which aids in the digestion of the proteins. However, when the stomach is emptied, the acidic environment need not be maintained and a hormone called somatostatin stops the release of hydrochloric acid. This is controlled by a negative feedback mechanism. In the duodenum, digestive secretions from the liver, pancreas, and gallbladder play an important role in digesting chyme during the intestinal phase. In order to neutralize the acidic chyme, a hormone called secretin stimulates the pancreas to produce alkaline bicarbonate solution and deliver it to the duodenum. Secretin acts in tandem with another hormone called cholecystokinin (CCK). Not only does CCK stimulate the pancreas to produce the requisite pancreatic juices, it also stimulates the gallbladder to release bile into the duodenum. This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 25 | Animal Nutrition and the Digestive System 1089 Visit this website (http://openstaxcollege.org/l/enteric_endo) to learn more about the endocrine system. Review the text and watch the animation of how control is implemented in the endocrine system. Which of the following statements about the enteric endocrine system is true? a. Gastrin stimulates pancreatic enzyme and bile secretion. b. The enteric endocrine system includes all endocrine cells of the gastrointestinal tract. c. All hormone-secreting cells in the enteric endocrine system are clustered together. d. Enteric endocrine system cells secrete enzymes in response to specific stimuli. Another level of hormonal control occurs in response to the composition of food. Foods high in lipids take a long time to digest. A hormone called gastric inhibitory peptide is secreted by the small intestine to slow down |
the peristaltic movements of the intestine to allow fatty foods more time to be digested and absorbed. Understanding the hormonal control of the digestive system is an important area of ongoing research. Scientists are exploring the role of each hormone in the digestive process and developing ways to target these hormones. 1090 Chapter 25 | Animal Nutrition and the Digestive System KEY TERMS alimentary canal tubular digestive system with a mouth and anus aminopeptidase protease that breaks down peptides to single amino acids; secreted by the brush border of small intestine anus exit point for waste material bile digestive juice produced by the liver; important for digestion of lipids bolus mass of food resulting from chewing action and wetting by saliva carboxypeptidase intestine protease that breaks down peptides to single amino acids; secreted by the brush border of the small carnivore animal that consumes animal flesh cephalic phase first phase of digestion, controlled by the neural response to the stimulus provided by food cholecystokinin hormone that stimulates the contraction of the gallbladder to release bile chylomicron small lipid globule chyme mixture of partially digested food and stomach juices chymotrypsin pancreatic protease digestion mechanical and chemical break down of food into small organic fragments dipeptidase protease that breaks down peptides to single amino acids; secreted by the brush border of small intestine duodenum first part of the small intestine where a large part of digestion of carbohydrates and fats occurs elastase pancreatic protease endocrine system system that controls the response of the various glands in the body and the release of hormones at the appropriate times esophagus tubular organ that connects the mouth to the stomach essential nutrient nutrient that cannot be synthesized by the body; it must be obtained from food gallbladder organ that stores and concentrates bile gastric inhibitory peptide hormone secreted by the small intestine in the presence of fatty acids and sugars; it also inhibits acid production and peristalsis in order to slow down the rate at which food enters the small intestine gastric phase materials digestive phase beginning once food enters the stomach; gastric acids and enzymes process the ingested gastrin hormone which stimulates hydrochloric acid secretion in the stomach gastrovascular cavity digestive system consisting of a single opening gizzard muscular organ that grinds food herbivore animal that consumes strictly plant diet ileum last part of the small intestine; connects the small intestine to the large intestine; |
important for absorption of B-12 ingestion act of taking in food intestinal phase third digestive phase; begins when chyme enters the small intestine triggering digestive secretions and This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 25 | Animal Nutrition and the Digestive System 1091 controlling the rate of gastric emptying jejunum second part of the small intestine lactase enzyme that breaks down lactose into glucose and galactose large intestine digestive system organ that reabsorbs water from undigested material and processes waste matter lipase enzyme that chemically breaks down lipids liver organ that produces bile for digestion and processes vitamins and lipids maltase enzyme that breaks down maltose into glucose mineral inorganic, elemental molecule that carries out important roles in the body monogastric digestive system that consists of a single-chambered stomach omnivore animal that consumes both plants and animals pancreas gland that secretes digestive juices pepsin enzyme found in the stomach whose main role is protein digestion pepsinogen inactive form of pepsin peristalsis wave-like movements of muscle tissue proventriculus glandular part of a bird’s stomach rectum area of the body where feces is stored until elimination roughage component of food that is low in energy and high in fiber ruminant animal with a stomach divided into four compartments salivary amylase enzyme found in saliva, which converts carbohydrates to maltose secretin hormone which stimulates sodium bicarbonate secretion in the small intestine small intestine organ where digestion of protein, fats, and carbohydrates is completed somatostatin hormone released to stop acid secretion when the stomach is empty sphincter band of muscle that controls movement of materials throughout the digestive tract stomach saclike organ containing acidic digestive juices sucrase enzyme that breaks down sucrose into glucose and fructose trypsin pancreatic protease that breaks down protein villi folds on the inner surface of the small intestine whose role is to increase absorption area vitamin organic substance necessary in small amounts to sustain life CHAPTER SUMMARY 25.1 Digestive Systems Different animals have evolved different types of digestive systems specialized to meet their dietary needs. Humans and many other animals have monogastric digestive systems with a single-chambered stomach. Birds have evolved a digestive system that includes a gizzard where the food is crushed into smaller pieces. This compensates for their inability to masticate. Ruminants that consume large amounts |
of plant material have a multi-chambered stomach that digests 1092 Chapter 25 | Animal Nutrition and the Digestive System roughage. Pseudo-ruminants have similar digestive processes as ruminants but do not have the four-compartment stomach. Processing food involves ingestion (eating), digestion (mechanical and enzymatic breakdown of large molecules), absorption (cellular uptake of nutrients), and elimination (removal of undigested waste as feces). Many organs work together to digest food and absorb nutrients. The mouth is the point of ingestion and the location where both mechanical and chemical breakdown of food begins. Saliva contains an enzyme called amylase that breaks down carbohydrates. The food bolus travels through the esophagus by peristaltic movements to the stomach. The stomach has an extremely acidic environment. An enzyme called pepsin digests protein in the stomach. Further digestion and absorption take place in the small intestine. The large intestine reabsorbs water from the undigested food and stores waste until elimination. 25.2 Nutrition and Energy Production Animal diet should be balanced and meet the needs of the body. Carbohydrates, proteins, and fats are the primary components of food. Some essential nutrients are required for cellular function but cannot be produced by the animal body. These include vitamins, minerals, some fatty acids, and some amino acids. Food intake in more than necessary amounts is stored as glycogen in the liver and muscle cells, and in fat cells. Excess adipose storage can lead to obesity and serious health problems. ATP is the energy currency of the cell and is obtained from the metabolic pathways. Excess carbohydrates and energy are stored as glycogen in the body. 25.3 Digestive System Processes Digestion begins with ingestion, where the food is taken in the mouth. Digestion and absorption take place in a series of steps with special enzymes playing important roles in digesting carbohydrates, proteins, and lipids. Elimination describes removal of undigested food contents and waste products from the body. While most absorption occurs in the small intestines, the large intestine is responsible for the final removal of water that remains after the absorptive process of the small intestines. The cells that line the large intestine absorb some vitamins as well as any leftover salts and water. The large intestine (colon) is also where feces is formed. 25.4 Digestive System Regulation The brain and the endocrine system control digestive processes. The brain controls the responses of hunger and sat |
iety. The endocrine system controls the release of hormones and enzymes required for digestion of food in the digestive tract. REVIEW QUESTIONS 1. When you eat an apple, it is first physically broken down into smaller fragments. What is the term for this process? a. elimination b. absorption c. mastication d. peristalsis 2. Which of the following statements is true? a. The majority of water is reabsorbed by the small intestines. b. Elimination is a process that occurs via diffusion. c. Absorption is the process that chemically breaks down food. d. The small intestines absorb nutrients. 3. Ruminants and pseudo-ruminants are both able to digest plant materials but have different mechanisms for doing so. Which of the following is a pseudo-ruminant? a. cow b. goat c. crow d. horse 4. Which of the following statements about animal digestion is true? a. Roughage is digested very quickly. b. Birds eat large quantities at one time. c. Birds have a four-chambered stomach. d. In pseudo-ruminants, roughage is digested in the cecum. 5. Chemical and mechanical digestion begins in the mouth, and food is prepared into a _____, which is then swallowed. a. bolus b. trachea c. peristalsis d. sphincter This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 25 | Animal Nutrition and the Digestive System 1093 6. Which of the following statements about digestion is true? a. Humans produce cellulase, which breaks down cellulose. a. Pepsin is converted to pepsinogen with the help of hydrochloric acid. b. Intestinal flora have enzymes that break down some of the fiber. b. Starch digestion begins in the mouth. c. Bile is released from the gallbladder to break c. Wave-like muscle movements called peristalsis move food from the stomach to the mouth. d. Amino acids are absorbed through the intestinal lining of the ileum. 7. Chyme is highly acidic. What is secreted by the pancreas to neutralize chyme? a. hydrochloric acid b. bicarbonate c. bile d. amylase 8. How does the liver assist in fat digestion? a. produces b |
icarbonate b. concentrates bile salts c. produces bile d. produces pepsin 9. When you eat food, it is vital that the nutrients be absorbed. How does absorption occur? a. Food is mechanically and chemically broken down into smaller molecules. b. Alternating waves of muscular contraction facilitate movement of food. c. Partially digested food flows into the small down fiber. d. In the stomach, pepsin is produced to break down plant material. 13. Which statement is not an example of how fat is beneficial? a. Fat helps absorb lipid-soluble vitamins. b. Fat helps produce lipid-soluble hormones. c. Fat has low energy density. d. Fat makes you feel full faster. 14. Certain molecules are required by but not produced by the body. Fat- and water-soluble _______ are organic molecules that cannot be produced by the body but are required for many enzymatic functions. a. minerals b. vitamins c. amino acids d. sugars 15. What is the result of insufficient amounts of the mineral iodine in the body? a. muscle weakness b. poor immune function c. mood disturbances intestine and food regurgitation is prevented. d. goiters d. Nutrients diffuse across the intestines. 10. Certain organs control the release of hormones that have vital roles in digestion. Which of the following controls hunger and satiety signals? a. thymus b. adrenal cortex c. thyroid d. hypothalamus 16. Adenosine triphosphate, or ATP, is the source of energy for cells. ATP stores energy in _______ bonds. a. carbohydrate b. glycolysis c. glycogen d. phosphodiester 17. Which of the following statements about glycogen is true? 11. One cup of which of the following has the most calories? a. When an individual is sedentary, glycogen is converted to glucose. a. spaghetti with tomato sauce b. The liver releases glycogen when blood sugar b. deep-fried zucchini c. mixed fruit d. scrambled eggs 12. Plant materials, such as fruits and vegetables, are difficult to digest because they are difficult to break down. How are humans able to obtain nutrients from fruits and vegetables? drops. c. ATP is produced by excess glycogen and glucose. d. During glycolysis, glycogen is converted to pyruvic acid. 18. What is produced from excess ATP and glucose? 1094 Chapter 25 | |
Animal Nutrition and the Digestive System a. glycogen b. pyruvate c. peptides d. essential nutrients 19. Which of the following is not a reason why ATP is required by animals? a. ATP is needed to build organic molecules. b. ATP provides energy for muscle contraction. c. ATP assists in electrical signal transmission. d. ATP is the body’s fuel source. 20. Different macromolecules have varying amounts of energy density. Which of the following is the least energy dense? a. protein b. c. fat fiber d. carbohydrates 21. Which of the following does not play a role in masticating food? a. teeth b. pharynx c. d. saliva tongue 22. Which of the following statements about the process of digestion is true? a. Organisms absorb large molecules through digestive cells. b. The last step of digestion is absorption. a. Aminopeptidase and dipeptidase break peptides into amino acids. b. Pepsin breaks proteins into peptides. c. Trypsin, elastase, and chymotrypsin break proteins into peptides. d. Carboxypeptidase breaks peptides into amino acids and peptides. 25. Water reabsorption is an essential component of processing food. Where is the majority of water reabsorbed? a. b. small intestines rectum c. colon d. anus 26. If you come down with the flu, you might experience emesis. What causes emesis? a. stomach muscle contractions b. neural signals that urge elimination c. inadequate water reabsorption d. excess water reabsorption 27. Not all organs involved in processing food are involved in digestion. Which of the following organs is not involved in digestion? a. mouth b. anus c. d. stomach small intestine 28. Which of the following statements about digestion of food in the large intestines is true? c. Food is only mechanically broken down in the a. Mechanical digestion occurs by bacteria. mouth. b. Semi-solid waste is moved by wave-like muscle d. Food is prepared into a bolus before it is contractions. swallowed. 23. Which of the following enzymes is involved in carbohydrate digestion? a. pancreatic amylase c. Most nutrients are absorbed. d. Peristaltic mixing occurs. 29. Taking in food, or ____, is the first step of gaining nutrients from food. b. elastase |
c. trypsin d. pepsin 24. In protein digestion, what happens in the stomach? a. digestion b. ingestion c. elimination d. absorption 30. What is the correct order of processes by which nutrients and energy are obtained from food? This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 25 | Animal Nutrition and the Digestive System 1095 33. Hormones are essential for digesting ingested food items. Which hormone controls the release of bile from the gallbladder? a. pepsin b. gastrin c. amylase d. cholecystokinin 34. What is the role of gastrin in food digestion and absorption? a. Gastrin stimulates release of stomach acid. b. Gastrin stimulates production of bicarbonate. c. Gastrin stimulates pancreatic juice production. d. Gastrin stops the release of stomach acid. 35. The gastric phase assists in processing ingested materials. When does the gastric phase begin? a. when food is smelled b. when food reaches the stomach c. when chyme enters the small intestines d. when food is ingested a. digestion → ingestion → absorption → elimination b. c. d. ingestion → absorption → digestion → elimination ingestion → digestion → absorption → elimination ingestion → digestion → elimination → absorption 31. Gastric control has three phases that assist in digesting food. Which phase is initiated by chyme? a. intestinal b. gastric c. cephalic d. digestive 32. Which of the following occurs during the cephalic phase of gastric control? a. Salivation is triggered. b. Food is processed by gastric acids and enzymes. c. Gastrin is produced. d. Digestive secretions are released. CRITICAL THINKING QUESTIONS 36. Explain how villi and microvilli aid in absorption. a. Villi and microvilli increase the surface area of the small intestines, which aids in the absorption of bile salts and vitamin B12. b. Villi and microvilli increase the surface area of the small intestine, which increases the absorption of nutrients by diffusion. c. Villi and microvilli form the inner layer of epithelial tissue in the small intestine and increase the absorption of nutrients from chyme. d. Villi and microvilli absorb food through the small intestine via smooth muscle contractions |
called peristalsis. 37. Ruminants, such as this goat, are able to digest large amounts of plant material. How is plant material passed through, digested, and absorbed in the ruminant digestive system? 1096 Chapter 25 | Animal Nutrition and the Digestive System a. Food is chewed in the mouth, then passes a. a. When the serosa layer of stomach ruptures through the esophagus into the rumen and then the reticulum, which contain microbes that break down cellulose and ferment the ingested plant material. The ruminant regurgitates cud from the rumen, and the food is passed into the omasum for water removal and then into the small and large intestines for nutrient and further water absorption. Waste is excreted through the anus. b. Food is chewed in the mouth, then passes through the esophagus into the rumen and then the reticulum, which contain microbes that break down cellulose and ferment the ingested plant material. The ruminant regurgitates cud from the rumen, and the food is passed into the abomasum for water removal and then into the small and large intestines for nutrient and further water absorption. Waste is excreted through the anus. c. Food is chewed in the mouth, then passes through the esophagus into the rumen and then the reticulum, which contain microbes that break down proteins and ferment the ingested plant material. Ruminants regurgitate cud from the rumen, and the food is passed into the omasum for water removal and then into the small and large intestines for nutrient and further water absorption. Waste is excreted through the anus. d. Food is chewed in the mouth then passes through the esophagus into the reticulum and then the rumen, which contain microbes that break down cellulose and ferment the ingested plant material. The ruminant regurgitates cud from the rumen, and the food is passed into the omasum for water removal and then into the small and large intestines for nutrient and further water absorption. Waste is excreted through the anus. 38. a. How does a stomach ulcer form? b. How could you prevent a stomach ulcer from forming in your stomach? and does not reform, an open wound is formed. It may be caused by bacteria b. Ulcers can be prevented by eliminating ingesting items that cause degradation of the |
mucus lining like foods that irritate the stomach. b. a. When the mucus lining of the stomach ruptures and does not reform, an open wound is formed. It may be caused by a virus. b. Ulcers can be prevented by eliminating ingesting items that cause degradation of the mucus lining, like foods that irritate the stomach. c. a. When the mucus lining of the stomach ruptures and does not reform, an open wound is formed. It may be caused by bacteria. b. Ulcers can be prevented by ingesting items that will increase the acid content of the stomach. d. a. When the mucus lining of the stomach ruptures and does not reform, an open wound forms. It may be caused by bacteria. b. Ulcers can be prevented by eliminating ingesting items that cause degradation of the mucus lining, such as foods that irritate the stomach. 39. How is the gallbladder involved in digestion, even though it is considered an accessory organ? a. The gallbladder secretes bile to the duodenum, which uses it to break down proteins. It is considered an accessory organ because food does not directly pass through it. b. The gallbladder secretes bile to the duodenum, which uses it to break down fats. It is considered an accessory organ because food does not directly pass through it. c. The gallbladder secretes bile to the ileum, which uses it to break down fats. It is considered an accessory organ because food does not directly pass through it. d. The gallbladder secretes bile to the ileum, which uses it to break down proteins. It is considered an accessory organ because only a very small amount of digestion takes place in the gallbladder. 40. What is the role of saliva in the digestive system? This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 25 | Animal Nutrition and the Digestive System 1097 a. Saliva contains an enzyme called amylase, which starts the chemical digestion in the mouth by breaking down proteins. b. Saliva contains an enzyme called lipase, which starts chemical digestion in the mouth by breaking down proteins. c. Saliva contains an enzyme called maltase, which starts chemical digestion in the mouth by breaking down carbohydrates. d. Saliva contains an enzyme called amylase, which starts chemical digestion in |
the mouth by breaking down carbohydrates. 41. What are the biological benefits of a balanced diet? a. A balanced diet provides excess energy to be stored in the body and nutrients to maintain good health and increase reproductive capability. b. A balanced diet allows excess energy to be stored in the body, thereby increasing the rate of metabolic reactions. c. A balanced diet provides nutrients needed to maintain proper bodily functions, and vitamins and minerals to maintain good health and reproductive capability. d. A balanced diet provides nutrients needed to maintain proper bodily functions, and vitamins and minerals to maintain good health and increase reproductive capability. 42. Why is it important to eat carbohydrates, which provide organic carbons? a. They are needed to provide insulation to mammals. b. They help to fight infections. c. They are needed to produce antibodies. d. They are needed to build cells and tissues. 43. a. Why is it necessary to consume essential nutrients? b. What are two examples of fat-soluble essential vitamins, and what are their functions in the human body? a. a. Essential nutrients are not synthesized by the body and are not necessary for proper body function. b. Vitamins B and C are two fat-soluble essential vitamins. Vitamin B helps maintain eyesight, and vitamin C is essential for blood clotting. b. a. Essential nutrients are not synthesized by the body but are necessary for proper body function. b. Vitamins A and K are two fat-soluble essential vitamins. Vitamin A helps maintain connective tissue, and vitamin K is essential for blood clotting. c. a. Essential nutrients are synthesized by the body and are necessary for proper body function. b. Vitamins D and K are two fat-soluble essential vitamins. Vitamin D helps maintain a stable nervous system, and vitamin K is essential for blood clotting. d. a. Essential nutrients are not synthesized by the body but are necessary for proper body function. b. Vitamins A and K are two fat-soluble essential vitamins. Vitamin A helps maintain eyesight, and vitamin K is essential for blood clotting. 44. What happens to glycogen when blood sugar drops? a. b. c. d. It stimulates the release of insulin, which can regulate the blood sugar level. It is released from the liver and converted to glucose to increase blood sugar levels. It is converted to starch, which breaks down to form glucose and increase blood sugar levels. It is released from the liver and converted to pyru |
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