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Solution Manual for Principles of Animal Physiology, 3rd Edition

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    A NSWER K EY Principles of Animal Physiology Third Edition Christopher D. Moyes , Ph.D. Queen’s University Patricia M. Schulte , Ph.D. University of British Columbia

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    1 Chapter 1 Introduction to Physiological Principles Answers to Concept Check Questions 1. How would you define physiology? Answer It is the study of the functioning of biological structures and systems, or how organisms work . 2. What is a model organism in the context of physiological research? Answer The August Krogh Principle suggests that “ for any biological question, there is an organism on which it can be most conveniently studied .” A model organism is a convenient animal on which to study a biological question. Squid, for example, was an early model to study neuronal function because of the size of its giant axon. 3. Why do the rates of biochemical reactions increase as temperature increases? Do they do so infinitely? Answer Temperature increases the thermal energy of molecules and increases the number of collisions between molecules. Because most reactions require molecular collisions, increasing the rate of collisions will increase the rate of reactions. This increase does not continue to infinity as temperature increases because at high temperatures many of the intermolecular bonds that stabilize protein structure start to break, causing proteins to unfold and denature. When proteins are unfolded they are unable to perform their functions. Because most biochemical reactions only occur at high rates because of the actions of protein catalysts, reaction rates decline when the catalysts begin to denature. 4. What is allometric scaling? Answer Some processes or structures change in direct proportion to body mass, which is called isometric scaling. If the process or structure changes disproportionately with body mass, it is considered to scale allometric ally . 5. What is an adaptation? Answer An adaptation is a trait that arose via a process such as natural selection and that causes an increase in reproductive success. 6. Distinguish between homology and analogy . Answer Homology describes a pattern where a trait that is present in two taxa is inherited from a common ancestor. These traits may or may not be similar in appearance and function in the two taxa. For example , bird wings and human arms appear to be quite different, but they are actually homologous because they both evolved from the forelimbs of a four - legged ancestor.

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    2 Analogy describes a pattern where a trait is used for a similar function in two taxa, but is not inherited from a common ancestor. For example , the camera - type eyes of vertebrates and cephalopods were derived from the non - camera eyes of the closest relatives of each of these groups . This independent derivation suggests that these eyes are not homologous, but instead are analogous . 7. What is homeostasis? Answer Homeostasis is the regulation or maintenance of internal conditions within a narrow range , despite changes in the external environment . 8. Distinguish between acclimation, acclimatization, polyphenism, and phenotypic plasticity. Answer Acclimatization is a reversible phenotypic change produced from variations in natural environmental conditions, usually working in combination (for example , low temperature and short day length in the winter). Acclimation is similar but describes the process of reversible phenotypic adjustment in response to a single environmental variable, usually in an artificial (e.g. laboratory) environment . Polyphenism occurs when different environments lead to discrete alternative phenotypes. For example, developmental plasticity can lead to polyphenism that is often irreversible . Phenotypic plasticity is a general term that reflects the ability of a single genotype to result in multiple phenotypes as a result of the environment ; thus acclimation, acclimatization , and polyphenism are all types of phenotypic plasticity.

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    1 Chapter 1 Introduction to Physiological Principles Answers to Review Questions 1. Where would organelles such as the mitochondrion fit in the levels of organization shown in Figure 1.3? Answer Organelles such a s mitochondria would fit between the molecular and the cellular levels in Figure 1.3. Molecules work together in complex biochemical pathways to form the complex structures that are assembled to make organelles. These organelles are parts of a cell . 2. What is the Krogh principle, and why is it useful for animal physiologists? Answer The Krogh Principle states that f or every biological problem there is an organism on which it can be most conveniently studied. This means that medical physiologists interested in a function in humans (or veterinary physiologists interested in cows) might choose to study these functions in an animal such as a mouse, because they are much smaller and easier to maintain. 3. All organisms have a maximum temperature at which they can function. Suggest a possible physical basis for this observation. Answer The maximum temperature that an organism can function at must be the lowest temperature at which a critical process required to sustain life fails. Not all physiological functions are likely to fail at exactly the same temperature, but when a critical process fails, life will end. Processes could fail at high temperatures for a variety of reasons, but ultimately this failure must be due to changes in intermolecular interactions at high temperatures. 4. How might size - related changes in surface area - to - volume ratios affect physiological functions? Answer Surface area increases less with size than does volume, so surface area for exchange becomes limiting as size increases, unless there is a change in shape. This pattern explains why physiological systems such as the respiratory system have extremely high surface area compared with their volume. This increases the area available for gas exchange. 5. What are three fundamental requirements for adaptive evolution of a trait to occur? Answer For adaptive evolution to occur: 1. There must be v ariation among individuals in the trait. 2. The trait must be heritable . 3. The trait must increase the fitness .

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    2 6. Are the eyes of vertebrates and cephalopod mollus k s homologous or analogous? Justify your answer. Answer The eyes of vertebrates and cephalopod mollus k s are analogous. Both are camera - type eyes that can form a focused image, but they arose independently from different ancestors that lacked camera - type eyes. Thus , they did not share a common ancestor and cannot be homologous , despite their similarity of function. 7. What is the main benefit of having antagonistic controls in physiological systems? Answer Antagonistic controls allow more precise control over a physiological system by having two separate control mechanisms: one that increases, and one that decreases, the activity of the system. This is analogous to having both a brake and an accelerator on a car. 8. Explain why a positive feedback loop is unlikely to be involved in a control system that maintains homeostasis. Answer Positive feedback loops are organized so that the output of the system tends to increase the activity of the system. This results in a rapid response and an amplification of the output of the system, resulting in a large response to a small stimulus. This control mechanism is unlikely to be appropriate for a control system that maintains homeostasis, because homeostasis involves maintaining the system within a narrow range around the set point.

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    1 Chapter 1 Introduction to Physiological Principles Answers to Synthesis Questions 1. What physical, chemical, or physiological constraints might lead to allometric scaling? Answer Allometric scaling refers to variables that do not change linearly with body mass ; thus, a given trait in a 1 - kg animal will not be twice as large in a 2 - kg animal. A major constraint that leads to allometric scaling is the surface area - to - volume ratio, which decreases as mass increases. This affects many physical and chemical processes (such as diffusion rates and heat exchange) that in turn affect the scaling of physiological functions. But surface area - to - volume constraints do not fully explain all instances of allometric scaling. For example, metabolic rate does not always have a scaling exponent of 0.67, as would be expected if surface area - to - volume ratios were the only important factor. A variety of physical, chemical, and physiological mechanisms have been proposed to explain this pattern, but the actual mechanism remains a matter of debate among physiologists. 2. Why do physiologists need to understand evolution? Answer There is a great deal of diversity in physiological traits expressed by animals. These traits (an animal ’ s phenotype) are the result of interactions between the animal ’ s genotype and its environment. An individual ’ s genotype reflects its evolutionary history, shaped by natural selection and genetic drift. As a consequence, closely related organisms share more traits than distantly related ones. Evolution therefore allows physiologists to understand the significance of the diversity of physiological traits (both the proximal and ultimate causes). 3. Compare and contrast adaptive evolution and genetic drift. Answer Both adaptive evolution and genetic drift can result in changes in the genotype of populations over time. For adaptive evolution, a genetic mutation that results in a trait that increases the reproductive success can be acted on by natural selection and become widespread after multiple generations have passed. Alternatively , a population may have a unique trait, not because it improves fitness, but because the mutation increased in frequency as a result of random factors such as genetic drift . Such random factors are more likely when a population is small, but can occur in any situation. Genetic drift can oppose selection, so that even a favorable mutation may be eliminated from a population by chance. 4. When might an adaptation become detrimental? Answer In a heterogen e ous population, many genotypes will be present. When the environment changes, a subset of the genotypes may do better, and that genotype may become more abundant in the population. If the environment changes again, perhaps reverting back to the original condition, that dominant genotype may now be at a disadvantage. 5. Home heating systems such as a furnace are regulated via negative feedback. Describe how such a system might work. Answer A temperature sensor such as a thermometer measures air temperature and sends a signal to a thermostat that compares the measured temperature to the desired temperature, or set point. If the measured air temperature is below the set point, the thermostat will send a signal to turn on the heating system. When air temperature rises to the set

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    2 point, the thermostat will stop sending the “on” signal to the heating system. This process qualifies as a negative feedback because the response (turning on the heat to warm the air) is in the opposite direction to the stimulus (cold air temperature). 6. Make an argument for or against adopting the use of the term allostasis. Answer Argument for: The term allostasis is useful because it points out that set points for homeostatic systems can change with the environment, for example between seasons or with the phases of the reproductive cycle, which is not explicitly included in the definition of homeostasis. Argument against: The term allostasis is not necessary because the original definition of homeostasis already includes the concept that it is necessary to alter the functions of some physiological systems to maintain homeostasis in others. 7. Dogs typically shed some hair in the spring. Is this an example of acclimation or acclimatization? How might you experimentally distinguish between these two possibilities? Answer It is probably an example of acclimatization, as the observation was not made under controlled conditions in the laboratory, and multiple factors likely changed in the spring (temperature, day length, amount of time spent outside and exercising, and probably many others). However, it might also be a developmentally programmed event that is independent of environmental factors. The way to test this experimentally would be to take a dog into the laboratory and expose it to changes in each of the possible environmental factors individually and in combination and then to observe the dog’s hair to see whether the thickness of the coat increases or decreases. Ideally, this experiment should be done with many different individual dogs to see whether it is a consistent response for the species.

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    1 Chapter 2 Physiological Evolution of Animals Answers to Concept Check Questions 1. D istin guish between the terms protist, protozoan, metazoan, and eumetazoan . Answer Unicellular eukaryotes are collectively called protists, though they are a polyphyletic group. Protozoans are those protists with “ animal - like ” abilities to move. These two terms include many unrelated organisms. Metazoan can be used synonymously with “ animal ” though some researchers separate sponges from true metazoans, or eumetazoans. Inother words, eumetazoans is used to distinguish animals with tissue layers from sponges, which have no discrete tissue layers. 2. What is a coelom? Answer In t ripl o blastic animals the early stages of embryonic development lead to three distinct tissue layers: endoderm, mesoderm, and ectoderm. In coel o mate animals, a gap can occur within the early embryo, either through splitting of the mesoderm or folding of the mesoderm. The gap is a coelom. A pseudocoelom is like a coelom in that it is a internal body cavity. However, a true coelom is lined by mesoderm, whereas a pseudocoelom is only partially lined by mesoderm. 3. Distinguish between metamer s and tagmat a. Answer Metamers are repeating segments of an animal. Tagmata arise when two or more metamers fuse and become a specialized region. 4 . Which group of fish gave rise to the tetrapod lineage? Answer T he sarcopterygian fish, represented by modern coelocanths (Coelocanthomorpha) and lungfish (Dipnoi), gave rise to tetrapods. 5 . What is an amniote? Answer T etrapods with an egg possessing four extraembryonic membr a nes are amniotes. They include reptiles, birds, and mammals, but exclude amphibians. 6 . What is the phylogenetic relationship between mammals, birds, and reptiles? Answer Mammals and birds each had a reptilian ancestor, though their ancestors were only distantly related. 7 . Did myosin evolve as a muscle protein? Answer

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    2 Myosin appeared in unicellular eukaryotes long before animals arose. It retains a role in cytoskeletal trafficking, but in animals it has derived roles in muscle. The myosin family amplified many times in animals, leading to the origin of different myosin isoforms. 8 . When did collagen evolve and what is its significance? Answer Collagen first appeared in the earliest animals, and is used to construct many forms of extracellular matrix. 9 . What are cad h erins? Answer Cadherins are cell membrane adhesion proteins. Though they appear in animals, their origins predate the appearance of metazoans. Cadherin genes occur in protists, in particular the choanoflagellates that are thought to share a common ancestor with metazoans. 1 0 . What is cephalization? Answer Cephalization is an evolutionary trend whereby animals concentrate important sensory, nervous, and digestive system components toward the anterior, giving rise to a discrete head region. 1 1 . Why is metabolism more diverse in bacteria than in animals? Answer Bacteria have little capacity to evolve structurally because of their structural simplicity. Their best evolutionary option for surviving environmental stress is through their biochemistry. Animals are able to evolve changes in structure, which can offer different routes for resisting and surviving environmental stress. Nonetheless, biochemical adaptations are important in many aspects of animal evolutionary physiology.

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    1 Chapter 2 Physiological Evolution of Animals Answers to Review Questions 1. What is the significance of the similarity between choanoflagellates and choanocytes? Answer It is likely that choanoflagellates are the modern protist that is most closely related to the earliest metazoan. There is a striking similarity in the general structure of choanoflagellates and choanocytes of sponges. 2. Why aren ’ t protozoans considered animals? Answer Protozoans are a polyphyletic group of single - celled eukarytotes, which include groups that are plantlike, fungilike, and animal - like. Animals are, by definition, multicellular, so a single - cell ed eukaryotic organism could not be an animal. Some protists have the animal - like trait of locomotion, but that is not an indicator of how closely a protist is related to the ancestors of animals. 3. Which animals are diploblasts? Answer Diploblasts have only two embryonic cell layers, endoderm and ectoderm. Sponges lack cell layers, and most animals have three embryonic cell layers. Only cnidarians and probably ctenophores are diploblasts. 4. Explain why arthropods are considered Ecdysozoans. Answer Ecdysozoans share the ability to moult at some stage in their development. Arthropods moult. 5. Did all jawed animals evolve from the same agnathan ancestors? Answer No. Agnathans are a large group of jawless fish that gave rise to multiple lineages of fish and their descendants . It appears likely that sharks and bony fish arose from different agnathan a n cestors, which were distinct from those that gave rise to lamprey and hagfish. It now appears likely that hagfish and l amprey, despite their divergence, shared a common ancestor. 6. How many times did terrestriality arise in animal lineages? Answer Many times. Land was invaded by multiple arthropod lineages, including myriapods, hexapods, and chelicerates. Amphibians were the first tetrapod group that had a major terrestrial presence. Many invertebrate phyla have groups that live on land, although in many cases they live in wet environments, such as moss, or live as endoparasites in the fluids of land animals. Of course, select groups of fish can spend periods of time on land.

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    2 7. What is meant by the term “ a new head ” ? Answer The “ new head ” hypothesis suggests that the evolutionary diversification and success of vertebrates was made possible by the development of structures anterior to the notochord. In different lineages, these structures become important musculoskeletal features such as the jaw. 8. What is the significance of the evolution of the Na + /K + ATPase? Answer The evolution of the Na + /K + ATPase gave animals a mechanism by which to pump Na + and K + across their cell membrane s , creating the electrochemical potential that is used to drive many transport process es and permit the formation of excitable tissues. The transporters linked indirectly to Na + /K + ATPase help animals regulate their cell volume , which was important in this group of organisms because they lack cell walls . 9. When did endothermy arise in animal evolution? Answer Endothermy arose several times in animals. Endothermy arose independently in the ancestors of two groups of vertebrates, birds and mammals. Birds arose about 150 million years ago and mammals about 300 million years ago. Other groups have select species that are partially endothermic, such as bees, tuna, billfish, and large sharks. The acquisition of endothermy in these groups is much more recent. 10. Which came first, hormones or hormone receptors? Answer It is likely that the answer may differ between hormones and hormone receptors, but in the case of the aldosterone receptor discussed, it is likely that the receptor was present and capable of responding to other steroids before aldosterone came on the scene.

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    1 Chapter 2 Physiological Evolution of Animals Answers to Synthesis Questions 1. Speculate on how animals might have evolved if the ancestral protist possessed a cell wall. Answer Rather than giving you possible answers, consider the significance of the cell wall in other organisms. It provides resistance to cell swelling, and how is cell volume controlled in animals? It provides a physical barrier that makes it difficult for large molecules to cross, so how might that affect cell - to - cell signaling? It is a rigid structural support for cells that can be conne cted together to make tissues. It also precludes connections between adjacent cells, obviating direct cell - cell communication. 2. What critical events led to the origin and diversification of tetrapods? Answer T o answer this you probably need to draw on material from outside this chapter and from other courses. Think about the diversity in tetrapods, and how those differences may have arisen. Within vertebrates, the two whole - genome duplications created the genetic raw material for specialization of genes. A number of events occurred in tetrapod evolution that contributed to their diversity. From the developmental perspective, the flexibility associated with the “ new head ” allowed different configurations of jaws. Environmental changes, such as air density and oxygen levels, permitted some groups to elevate metabolic rate, and others to evolve flight. Changes in global temperature and seasonality contributed to constraints on range limits ; evolutionary changes in anatomy and physiology permitted the invasion of new environments. 3. Would you expect the underlying metabolic pathways to be similar or different in animal models of metabolic arrest? Answer Metabolic arrest comes in various forms, driven by diverse environmental factors such as water limitations, temperature, oxygen, or food. Many of the solutions are biochemical in nature, and the basic features of animals arise from fundamentally similar metabolic pathways. Thus, factors such as energy balance depend on energy stores and energetic costs, and the ways animals deal with these challenges differ widely. Molecular protection pathways, such as chaperone proteins and cryoprotectants , are also very similar among animals. Where animals tend to differ is in the magnitude of responses. A nimals that undergo metabolic depression are likely to be very good at regulating pathways shared by all animals.

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    1 Chapter 3 Chemistry, Biochemistry, and Cell Physiology Answers to Concept Check Questions 1. What are the five main forms of energy used by animals? Provide biological and nonbiological examples of processes that represent conversion of energy from one form to another. Answer The five main forms of energy used by animals are radiant, mechanical, electrical, thermal , and chemical energy. If you are having difficulty coming up with examples, consider Figure 3.2 as a starting point. 2 . What is the difference between thermal energy , heat and temperature? Answer Thermal energy of a system is the total energy associated with molecular motion of that system : vibration, rotation, and translocation. Heat is the transfer of thermal energy between objects or systems at a different temperature. When a n animal’s body heats the air around it, thermal energy is transferred from the molecules of the body to the molecules of the surrounding air. Both thermal energy and heat are forms of energy, measured in Joules. Temperature is a convenient measure of the average thermal energy of a system , assessed in units of Celcius, Farenheit, or Kelvin . The difference between these parameters is illustrated by considering a large and small animal, each at the same temperature, 20 degrees warmer than the air. The large animal has more thermal energy because, despite having a similar average thermal energy, it has a greater total thermal energy. Likewise, the large animal is able to transfer more heat to the environment. 3. How are weak bonds affected by temperature? Answer Hydrophobic bonds are strengthened at warmer temperatures , whereas the other weak bonds are strengthened at lower tem p e ratures. 4. What change in pH has a greater effect on proton concentration: pH 6 to 7 or pH 7 to 8? Answer Though each represents a tenfold decrease in proton concentration, t he change from 6 to 7 decreases proton concentration by 9 x 10 - 7 M, whereas the increase in pH from 7 to 8 decreases proton concentration 10 - fold less, by 9 x 10 - 8 M. 5. Describe a cup of coffee (no milk, one sugar) in terms of solute, solvent, and solution. Answer The chemicals released from the coffee beans are solutes dissolved in a solvent of water to produce the solution called coffee.

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    2 6. What is the difference between osmolarity and tonicity? Answer Osmolarity describes the number o f osmotically active solutes (osmolytes) in a solution, whereas tonicity refers to a solution in terms of its effects on a cell. 7 . Distinguish between allosteric and covalent regulation of enzymes. Answer Allosteric regulation occurs when a molecule reversibly binds an enzyme at a site somewhere distant from the active site, and causes a change in structure that influences function. Covalent modification of an enzyme is when another molecule is added to or removed from the enzyme. Common molecules include phosphate groups (phosphorylation), methyl groups (methylation), or acetate groups (acetylation). Covalent modifications are usually mediated by enzymes. 8 . Distinguish between the following types of reactions: anabolic, catabolic , and amphibolic. Answer Anabolic reactions are synthetic, catabolic are degradative, amphibolic are a combination of both synthetic and degradative processes. 9. Why is ATP considered a “ high - energy ” molecule? Answer The breakdown of ATP to ADP and phosphate liberates energy because of the thermodynamics of the reaction and the relative concentrations of the reactants. 10. Distinguish between primary, secondary, tertiary, and quaternary structure. Answer For a protein, the primary structure is the amino acid sequence. This strand can be arranged into spirals or sheets to form the secondary structure. The protein can then be folded i n ways that change the relationship between the elements of secondary structure, creating the tertiary str u cture. The individual folded proteins may interact with other proteins to form the quaternary structure. 11. Why does temperature affect the three - dimensional structure of proteins? Answer Temperature causes changes in the weak bonds that confer the secondary, tertiary , and quaternary structures. Heating may cause a protein to unfold , or become more compact, depending on the relative importance of hydrophobic interactions. 12. What is a molecular chaperone? Answer A molecular chaperone is a protein catalyst that uses ATP to change the three - dimensional structure of a protein. Typically, chaperones help a protein achieve its proper configuration.

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    3 13. Distinguish between starch, glycogen, and cellulose . Answer All three are polysaccharides formed from glucose. Starch is a mixture of amylose, which has few branches, and amylopectin, which has some branches. Glycogen is similar in structure but much more highly branched . Both starch and glycogen are composed of glucose chains linked by α1,4 glycoside bonds. Cellulose is also a chain of glucose , but these are linked by β1,4 glycoside bonds. 14. What is the purpose of a redox shuttle? Answer NADH produced in the cytoplasm cannot enter mitochondria. A series of cyclic enzymatic reactions allow s the transfer of reducing energy across the mitochondrial membrane. NADH is oxidized to reduce a metabolite that can be transported across the inner mitochondrial membrane. When reoxidized in the mitochondria, the resulting NADH can be oxidized by the electron transport chain. 15. What happens to glycolytic pyruvate under hypoxic conditions? Answer Py r uvate can be reduced to form lactate, oxidizing NADH in the process. Lactate may be subsequently converted to other alternate end products. In some species, pyruvate can be processed to form ethanol, oxidizing NADH. 16. How is energy derived from triglyceride breakdown? Answer The triglyceride can be broken down into glycerol and three fatty acids. Glycerol can enter the glycolytic pathway via conversion to glycerol 3 - phosphate. The fatty acids enter beta - oxidation, with ATP produced by the oxidation of the NADH and FADH 2 produced. 17. What are the of triglyceride breakdown ? Answer Starting with triglyceride, the removal of one fatty acid generates diacylglyceride. Further breakdown of the diglyceride yields another fatty acid and monoglyceride. When monoglycerides are further degraded, removal of the last fatty acid leaves glycerol. Thus, complete breakdown of triglyceride yields glycerol and three fatty acids . 18. What are the main classes of phospholipid? Answer Animal cells produce two classes of phospholipids: phosphoglycerides and sphingolipids . 19. Which molecule is at the convergence of pathways for oxidation of fatty acids, carbohydrate, lactate, and some amino acids? Answer Acetyl CoA

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    4 20. What is the proton motive force? Answer The proton motive force is the combination of a pH gradient and membrane potential. This electrochemical gradient is produced by the electron transport chain and utilized for ATP synthesis. 21 . How is oxidation coupled to phosphorylation in mitochondrial oxidative phosphorylation? Answer Oxidation is coupled to phosphorylation in mitochondrial oxidative phosphorylation via the e lectron t ransport s ystem. The regeneration of NAD + and FAD via the actions of c omplexes I to IV and c ytochrome c creates a proton gradient, whose energy is utilized, via c omplex V, to phosphorylate ADP to ATP. This process requires the reduction of oxygen at c omplex IV, so that the electrons the c omplex IV received from c ytochrome c can be passed on and new electrons accepted. 22. How does glucose catabolism differ under (a) high versus low energy conditions and (b) normal versus low oxygen conditions ? Answer Anaerobic glycolysis may be advantageous under high energy and/or low oxygen conditions as it produces ATP rapidly from glucose stores and does not require access to oxygen. Oxidative metabolism of glucose may be advantageous under low energy and/or normal oxygen conditions , as it produces considerably more ATP and heat per unit glucose and it does not produce lactic acid as a by - product. 23 . Under what conditions is it more advantageous to use carbohydrate rather than lipid as a metabolic fuel? Answer Under low oxygen conditions, anaerobic pathways can be used to metabolize carbohydrates but lipid metabolic pathways will be limited due to their dependency on oxygen. 24. What is a respiratory quotient? Answer It is the ratio of carbon dioxide produced to oxygen consumed in metabolism. The ratio is an index of fuel selection. An RQ of 0.7 results when lipid is the main fuel, and 1.0 when carbohydrate is being oxidized. 25. Discuss the composition of biological membranes. Answer Membranes are composed mainly of mixtures of proteins and phospholipids . The proteins include transporters, enzymes, and proteins that are anchors for cytoplasmic and extracellular matrix proteins. The phospholipids include phosphoglycerides and sphingolipids. As well , membranes possess lesser amounts of fatty acids, triglycerides, steroids , and steroid derivatives. 2 6 . How can cells alter the fluidity of membranes and why is this capacity important to cellular function? Answer Cells can alter membrane fluidity by changing the phospholipid composition in such a way that the weak bonds (van der Waals forces) between the fatty acid tails of the phospholipids are either strengthened or weakened — for example,

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    5 by favoring phospholipids with longer or shorter tails or with greater or lesser degrees of saturation. Increasing the concentration of cholesterol also increases the fluidity of the membrane. 27. What is the relationship between the Nernst equation and the equilibrium potential? Answer The Nernst equation can be solved to determine the membrane potential at which net movement of a specific ion ceases at a given chemical gradient , i.e. the equilibrium potential for that ion . 28. Distinguish between depolarization, repolarization, and hyperpolarization. Answer Depolarization is a change in membrane potential toward a value of 0 mV. After depolarization, a cell should return to its normal resting membrane potential via repolarization. Often, the nature of ion movements leads to an overshoot, causing hyperpolarization, where the membrane is even more polarized tha n at rest. 29 . Summarize the roles of the different subcellular compartments within a cell, and discuss how they influence physiological function. Answer Compartments that should be discussed here include the nucleus, the mitochondria, the rough and smooth endoplasmic reticulum , and the G olgi apparatus. 3 0 . What does the Na + / K + ATPase do in a typical cell? Answer The sodium pump, or Na + / K + ATPase, uses the energy associated with ATP hydrolysis to pump 3Na + out of a cell in exchange for 2K + . As a result, this pump helps maintain ion gradients and the membrane potential. 3 1 . Distinguish between paracellular transport and transcellular transport. Answer Both modes of transport refer to movement of a molecule across a cell layer. Paracellular transport describes the movement of a molecule between cells, whereas transcellular transport occurs through a cell. 3 2 . Distinguish between transcription and translation. Answer The production of RNA from DNA is transcription. Translation is when mRNA is read in producing protein. 3 3 . Distinguish between a paralog and an ortholog . Answer Both terms refer to duplicates of genes. Paralogs are copies of a gene found within an individual, encoded at different loci (i.e., they are not alleles of a gene but rather two separate genes of common ancestry ) . Orthologs are copies of genes separated by a speciation event. For example, humans have LDH - A and LDH - B genes (paralogs). The LDH - A genes of chimps and humans are orthologs.

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    6 3 4 . What are whole - genome duplications and when did they occur in animal evolution? Answer Many genes or chromosomal regions can be duplicated and persist as extra copies. At several points in evolution, animals experienced a duplication of the entire genome. For a period, the organism was essentially tetraploid but with time the duplicate genes diverged and the progeny were considered diploid, retaining extra paralogs of many genes.

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    1 Chapter 3 Chemistry, Biochemistry, and Cell Physiology Answers to Quantitative Questions 1. What is the proton concentration of a solution at pH 7.4? At what temperature would this solution be neutral? Answer pH= - log([H + ]) rearranged as [H + ]=10 - pH if pH=7.4, then [H + ]=10 - 7.4 =4x10 - 8 There is insufficient information to answer the effect of temperature on neutrality because a) the solution could be already neutral (neutrality does not mean pH of 7.0, only that [H + ]=[OH - ]), because b) we don't know the temperature at which the pH of 7.4 was measured and c) we don't know what is in the solution. 2. What is the basis for an RQ = 1 for carbohydrate oxidation. Why does palmitate oxidation give an RQ = 0.7? Answer The number of CO 2 produced depends on number of carbon in the molecule. Number of O 2 consumed depends on the pathway for oxidation. Using glucose as an example for carbohydrates, complete oxidation yields 10 NADH and 2 FADH 2 , requiring the consumption of 6 O 2 in oxidative phosphorylation . Because glucose has 6 carbons, 6 CO 2 will be produced (2 in oxidation of pyruvate, 4 in the tricarboxcylic acid cycle) , leading to an RQ of 1: C 6 H 12 O 6 + 6O 2 = 6H 2 O + 6CO 2 RQ = 6CO 2 /6O 2 =1 Using p l amitate as an example for fatty acids, complete oxidation yields 31 NADH and 15 FADH 2 , requiring the consumption of 23 O 2 . Because palmitate has 16 carbons, the RQ will be 0.7: C 16 H 32 O 2 + 23O 2 = 16H 2 0 + 16CO 2 RQ=16CO 2 /23O 2 =0.7. 3. What rate of oxygen consumption would you expect in a tissue with a metabolic rate of 30 μ mol ATP/ g/ min? Answer The relationship between the rates of ATP and O 2 consumption depend on the fuel source. Oxidation of reducing equivalents by the ETS releases energy that is used to pump protons, but the amount of proton pumping depends on whether or not Complex I is used. Theoretically, NADH oxidation, initiated by Complex I, liberates enough energy to pump 3 H + per oxygen atom (or 6 H + per O 2 ) , whereas oxidation of FADH 2 , initiated by Complex II, liberates enough energy to pump 2 H + per oxygen atom (or 4 H + per O 2 ). The actual stoichiometries of ATP production and O 2 consumption are lower and variable due to factors that influence the leak of protons across the mitochondrial membrane. However, in theory, o xidation of carbohydrates generates mostly NADH , and thus the rate of 30 μ mol ATP/ g/ min would be supported by a respiration rate of about 5 μ mol O 2 / g/ min. Oxidation of lipids relies on both the FADH and NADH pathways, and thus a higher rate of respiration would be required to support the observed rate of ATP synthesis , between 5 and 7.5 μ mol O 2 / g/ min .

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    1 Chapter 3 Chemistry, Biochemistry, and Cell Physiology Answers to Review Questions 1. What are the four types of weak bonds and how do they differ from each other and from covalent bonds? Answer The four types of weak bonds are van der Waals forces , hydrogen bonds, ionic bonds , and hydrophobic interactions . Covalent bonds are stronger, meaning more energy is required to break them. Hydrophobic interactions are strengthened at high temperature whereas van der Waals forces , hydrogen bonds, ionic bonds are weakened at high temperature. 2. Why are reaction rates influenced by temperature ? Answer The influence of temperature on chemical reactions depend s on many factors. For a simple chemical reaction that requires energy to proceed spontaneously, increasing temperature will increase the reaction rate because a greater fraction of the substrate molecules will possess activation energy. For enzymatic reactions, other factors come into play because of how temperature affects the thermodynamics and kinetics. As with simple chemical reactions, increasing temperature could increase reaction rates by imparting more energy to the substrates, enabling a greater fraction to reach the activation energy barrier. Some reactions are very temperature sensitive in vivo and accelerate when temperature increases. Typically, a 10 o C temperature increase increases reaction rate 2 - to 3 - fold. . However, many enzymes, such as catalase, have almost no thermal sensitivity and proceed at the same rate regardless of temperature. There are also effects of temperature on enzyme structure, which can alter its efficiency as an enzyme. Increasing temperature may cause the enzyme to undergo cyclic changes in shape faster (accelerating rates), alter the affinity for substrates or products, or cause the enzyme to partially unfold, making it a poorer catalyst. 3 . How does the density of water change in relation to temperature? How do these properties affect animals that live in marine and freshwater environments? Answer One of water ’ s unique properties is that it reaches a maximal density at 4°C; water density is lower above or below that temperature. As a result, ice floats on liquid water. In cold climates, surface ice formation insulates the underlying water from the cold air. This prevents bodies of water that are deep enough from freezing completely and provides a stable thermal environment for aquatic organisms during winter. Another consequence of this property is that deep waters tend to have a very stable temperature of 4°C, while surface waters can undergo large latitudinal and seasonal variations in temperature. Therefore, aquatic and marine organisms that live in deep water are exposed to a very stable thermal environment. 4. Discuss the mechanism by which cells can use transporters to change their osmotic and ionic properties. Answer Active transporters use energy released from an exergonic reaction (usually ATP hydrolysis) to, directly or indirectly, move molecules against concentration gradients. In primary active transport, energy is used to move a molecule across the plasma membrane; a typical example is the Na + /K + ATPase (or Na + /K + pump) that helps generate an electrochemical gradient across the plasma membrane. In secondary active transport, the active transport of a molecule is used to drive the movement of another. For instance, the Na + electrochemical gradient generated by the Na + /K + ATPase can be used to transport glucose in to intestinal cells using a symport or cotransporter membrane protein, or to transport H + out of nephron cells using an antiport or exchanger membrane protein to regulate pH.

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    2 5. Distinguish between the types of polysaccharides relevant to animals. Answer Polysaccharides are simply chains of monosaccharides and in animals the most important ones are polymers of glucose created when α - D - glucose molecules are attached between carbons 1 and 4 (α1,4 glycoside bonds). A mylose is a plant polysaccharide with relatively few branches, whereas amylopectin has a side branch approximately every thirty glucose molecules. Glycogen , produced by animals, is like amylopectin but with more frequent branches. Cellulose, another plant - derived glucose polymer, is essentially indigestible in animals because the glucose units are connected by β1,4 glycoside bonds. Other poly saccharides can be made from different sugars. Arthropods build their exoskeletons with chitin, a polysaccharide of N - acetyl - glucosamine. Vertebrates secrete hyaluronate, a polymer of N - acetyl - glucosamine and glucuronic acid, into the extracellular space, where its gel - like properties act as a spacer between cells and tissues. Hyaluronate is a member of a class of compounds called glycosaminoglycans that include chondroitin sulfate and keratan sulfate. 6. Compare the structures of phospholipids. Answer Animal cells produce two classes of phospholipids: phosphoglycerides and sphingolipids . Phosphoglycerides are built on a glycerol backbone, with fatty acid chains on two of the glycerol carbons, with the third position occupied by a polar head group composed of an organic molecule linked via a phosphate. S phingolipids have a similar three - dimensional shape but are constructed differently. The backbone is sphingosine, which has a structure that resembles glycerol with a single fatty acid. A fatty acid chain is a lso attached to the sphingosine backbone . Like a phosphoglyceride , a polar head group is attached to the backbone via a phosphate group. 7. If the enzymatic reaction A + B  → C + D is near equilibrium, then the mass action ratio is close to the equilibrium constant. What happens to the mass action ratio if you add more enzyme? What happens when you add more of A? What do you need to know to predict what would happen if temperature changed? Answer The mass action ratio depends on the relative concentrations of substrates and products. Enzymes accelerate reactions by lowering the activation energy, but do not affect the direction of a given reaction. Therefore, adding enzymes would have no effect on the mass action ratio. Adding more substrate (A) would decrease the mass action ratio below the equilibrium constant and promote the formation of more products, but only if the other substrate (B) is not in limited supply. For the effect of temperature, we need to know if the reaction is exothermic or endothermic (high temperatures favor endothermic reactions), and we need how the affinity of the enzyme to the substrates and products is affected by temperature. 8. Distinguish between the parameters that describe enzyme kinetics and discuss the ways that cells control enzyme kinetics. Answer Enzyme kinetic properties define how an enzyme will function under various conditions. The maximal rate is defined as the V max . The affinity of the enzyme for substrate is reflected in the Michaelis constant ( K m ). Cells alter enzyme kinetics by allosteric and covalent regulators. Allosteric regulators bind to the enzyme at a site different than the substrates and products, altering enzyme shape and function indirectly. Covalent regulation adds moieties to the enzyme structure via covalent bonds. The most common changes are phosphorylation, regulated by protein kinases. Many signal transduction pathways alter the activity of enzymes by stimulating or inhibiting such protein - modifying enzymes. Changes in the concentrations of substrates and products can change the reaction rates, through mass action effects, but these do not change K m or V max , which are properties of the enzyme.

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    3 9. What metabolic conditions can affect the values of the respiratory quotient? What metabolic conditions affect the relationship between ATP produced and oxygen consumed? Answer The respiratory quotient is the ratio of CO 2 produced to O 2 consumed. While the amount of CO 2 produced depends on the number of carbons in the nutrient, the O 2 consumed depends on the oxidation pathway. The oxidation of carbohydrates lead s to RQ values of 1, while the oxidation of fatty acids lead s to RQ values around 0.7. Because carbohydrates can be mobilized and oxidized rapidly, periods of short - term high - energy demands are characterized by high RQ values, close to 1. Conversely, lipids are the major fuel source during sustained energy demands or during starvation, which results in lower RQ values, near 0.7. The oxidation of different fuels results in different ATP/O ratios. Carbohydrate oxidation relies mostly on NADH - linked enzymes for which the ATP/O ratio is 3. Fatty acid oxidation, on the other hand, rel ies more on FAD - linked enzymes for which the ATP/O ratio is 2. Therefore, the oxidation of carbohydrates leads to more ATP produced per volume of oxygen consumed. Animals exposed to low levels of oxygen therefore favor the use of carbohydrates as a fuel. 10. How do the pathways of gluconeogenesis and glycolysis overlap ? Answer Most of the enzymes are shared between the two pathways, with these steps operating in opposite directions for glycolysis and gluconeogenesis. However, there are three steps that are specific to each pathway. (i) glucose to glucose 6 - phosphate. In glycolysis, glucose is phosphorylated by hexokinase, consuming ATP and releasing ADP. In gluconeogenesis, the reverse step is catalyzed by glucose 6 - phosphatase, which cleaves off the phosphate group. (ii) fructose 6 - phosphate and fructose bisphosphate. In glycolysis, a second phosphate is added to fructose 6 - phosphate by the enzyme phosphofructokinase, at the expense of ATP. In gluconeogenesis, this step is reversed by the enzyme fructose bisphosphatase, which removes the phosphate group. ( i ii) phosphoenolpyruvate and pyruvate. In glycolysis , pyruvate is produced from phosphoenolpyruvate by the enzyme pyruvate kinase. In gluconeogenesis, the reverse is a two - step pathway, with pyruvate carboxylase forming oxaloacetate and phosphoenolpyruvate carboxykinase converting oxaloacetate to phosphoenolpyruvate. 11. Discuss the ways in which a cell is able to alter its interactions with other cells. Answer Cells connect to each other through direct contacts mediated by proteins, as well as indirect contacts by attachment to the extracellular matrix that they share. Cells can alter these relationships by internalizing the membrane proteins that mediate the connections to other cells or the extracellular matrix. Alternately, they can secrete enzymes that digest the extracellular matrix, typically matrix metalloproteinases. All of these process es are under the control of cell signaling pathways. 12. Discuss the mechanism by which cells can use transporters to change their osmotic and ionic properties. Answer Transporters can be stimulated or inhibited to alter the movement of ions, either by active transport or facilitated diffusion. Many of these processes are dependent upon the electrochemical gradients that are generated through active transport and dissipated through ion channels. When cells move solutes, water has a tendency to move in relation to the osmotic gradients. This is regulated in part by the expression of the aquaporin proteins that serve as water channels. Changes in the levels of transporters can arise by cells altering the synthesis and localization of the proteins that mediate transport, and these processes are under the control of cell signaling pathways.

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    4 13. Many physiological processes require a change in the levels of proteins, such as membrane transporters. Discuss the processes that cells can use to change the protein levels. Discuss how the subcellular compartment influences this pathway. Answer The levels of proteins can be controlled at multiple steps of synthesis and degradation. Some of these steps and processes are shared by all proteins, regardless of their final destination. Other steps are unique to the final compartment in which the protein finds itself. At the genetic level, cells can control whether specific genes are transcribed or not. Transcription can be controlled through proteins that modify the three - dimensional organization of DNA into nucleosomes. Once the promoter is accessible, DNA - binding proteins and coactivators control the formation of the transcriptional machinery. The levels of transcription factors and chromatin remodeling enzymes are often controlled by signaling pathways that change either the activity of these proteins or their subcellular localization. For example, phosphorylation of a transcription factor might allow it dimerize or lead it to be imported into the nucleus. Once mRNA is synthesized and processed, it is exported to the cytoplasm , where it can be transcribed and eventually degraded by RNases. For some proteins, the rate of protein synthesis is controlled by regulation of RNA degradation. Few specific proteins are controlled th r ough controls on translation. Cells alter global protein synthesis through the phosphorylation of initiation and elongation factors, which affects the overall rate of translation in the cell. For cytoplasmic and mitochondrial proteins, the mRNA is translated in the cytoplasm. In contrast, those proteins found in the cell membrane and internal membrane network are translated into the ER. Once inside the ER, the protein is sent to the correct location via transport vesicles. Once proteins are synthesized, their levels can be regulated by degradation (proteolysis). Again, the compartment determines the pathway of proteolysis. Cytoplasmic proteins are degraded via the ubiquitin - proteasome pathway. Proteins that are damaged or targeted for degradation are tagged with the protein ubiquitin , then degraded by proteasomes. The level of cell membrane proteins can be altered through exocytosis and endocytosis. Once in vesicles, the protein may be sent to lysosomes for degradation. 14. Other physiological processes require changes in the activities of proteins. While this can arise through changes in the levels of proteins, it can also change through regulation of protein function. Discuss the various ways that cells can alter the activity of enzymes or transporters. Answer Competitive inhibitors bind to the active sites of enzymes, preventing the binding of substrates and therefore decreasing enzymatic activity. The binding of some molecules to enzyme s in areas other than the active site can result in a change in the three - dimensional shape of the enzyme, which will affect the affinity of the enzyme to its substrate. This is called allosteric regulation, and it can either increase or decrease the activity of an enzyme. The process of phosphorylation (by a protein kinase) or dephosphorylation (by a protein phosphatase) involves modifying enzymatic activity (again by altering its three - dimensional shape) by the addition (or removal) of a phosphate group to an amino acid of the enzyme. 15 . Discuss the origins of genetic variation. Answer During the process of meiosis, chromosomes can experience mutations that produce genes that are different than either parent. These variants can accumulate in populations, forming a pool of alleles of genes with different sequences, which may affect the coding region (and consequently protein sequence) or regulatory region (and consequently transcription). The frequency of alleles within a population may change as a result of evolutionary forces that act upon the alleles differentially. Over generations populations or species may experience different patterns of selection on the alleles, such that their progeny possess their own versions of the gene with different sequences. Variants of a given gene between species are known as orthologs. Many genes are present as gene families that arise from duplications of individual or groups of genes. The duplicated genes , known as paralogs, can diverge in sequence over many generations, and the duplicated genes can follow many trajectories . A paralog might incur mutations in the promoter or coding region that prevent it from being transcribed

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    5 into an mRNA that encodes a functional protein (pseudogenization) . The paralogs may be expressed at different times, in different tissues, or change in expression in response to a physiological challenge. Since each gene appears specialized for a specific cellular environment, this divergence is call ed subfunctionalization. In some cases, a parlog can mutate and diverge, resulting in a protein that does a fundamentally different function. This is called neofunctionalization . These processes, originating early in animal evolution and operating at the level of individual cells, provide animals with physiological flexibility. 16. How does genetic variation provide physiological flexibility? Answer Genetic variation of direct relevance to physiology can arise in both coding and regulatory regions. Many genes exist as duplicates and over evolutionary time, the duplicates (paralogs) diverge into genes that differ in expression patterns and/or protein structure. Some animals will express one paralog under one set of conditions , then switch to another when conditions change. This might influence how a given tissue responds to a challenge or allow the animal to produce different forms in separate tissues (white versus red muscle) or regions of a tissue (small versus large intestine). Within a population of a species, there may be polymorphisms of a particular gene (alleles) that affect gene expression or protein structure. This variation may be unimportant under most circumstances but it may become important when conditions change, such that one allele is better suited than another. This could lead to natural selection of the favorable genotype, increasing its frequency in the population.

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    1 Chapter 3 Chemistry, Biochemistry, and Cell Physiology Answers to Synthesis Questions 1. Describe, in chemical terms, how antacids work. Answer Most antacids are weak bases, for instance in the form of carbonates (ex. sodium bicarbonate, NaHCO 3 ) or hydroxides (ex. magnesium hydroxide, Mg(OH) 2 ). The reaction between a weak base and acid found in the stomach leads to the formation of a salt, water and sometimes CO 2 gas depending on the base (ex. NaHCO 3 ), therefore reducing the concentration of free hydrogen ions and increasing pH. 2 . Why do your hands get wrinkled if you spend too much time in the bathtub? Would the same thing happen when you swim in the ocean? Describe these environments using the terminology of osmolarity and tonicity. Answer Freshwater is hyposmotic and hypotonic relative to body fluids (both intra - and extracellular compartments). Therefore, bathing in fresh water causes water to enter in to the body in areas that are not waterproof (calluses), causing cellular swelling (the cause of wrinkles). Salt water is hyperosmotic and hypertonic, causing water to leave areas that are not waterproof, but this causes little, if any, visible signs. 3. What is the relationship between pK and pH? How does temperature influence the pK of water? What might this mean for animals that experience changes in body temperature? Answer pH is the negative log of the proton concentration. pK is the negative log of the equilibrium constant. For an acid or a base, pH = pK plus the log of the concentration of the anion divided by the concentration of the undissociated acid. pH = pK when these concentrations are equal. The pK of water increases as temperature increases. Higher pHs are required at higher temperatures if the ionization states of affected biological molecules are to stay the same. 4 . A type of protein comes in six different forms. Each form can dimerize with the other. How many unique homodimers and heterodimers can be formed from these six proteins? Answer Label the monomes A, B, C, D, E, and F. Homodimers are formed when two identical proteins join. In this case, there could be 6 homodimers (AA,BB, CC, DD, EE, FF) . Heterodimers are formed by two different proteins; in this case there could be 15 unique heterodimers formed by 6 different proteins (AB, AC, AD, AE,AF, BC, BD, BE, BE, BF, CD, CE, CF, DE, DF, EF). 5 . Many animals maintain metabolites at concentrations near the K m value for metabolic enzymes. For example, the concentration of pyruvate is often close to the K m value for LDH. Why might this be advantageous, in terms of kinetic regulation? Answer The value of K m is the substrate concentration at which reaction velocity is half the maximal rate ( V ma x ). When substrate concentration increases above the K m value of the metabolic enzyme, reaction rates approach V max and eventually will stop increasing because enzymes will reach saturation. By keeping substrate concentrations near or below K m , animal cells have the capacity to change reaction rates when substrate concentration changes , for instance as a result of a change in overall metabolic activity.

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    2 6. Trace the path of a protein hormone, such as insulin, from its gene in the nucleus to secretion out of the cell. Answer In the nucleus, a gene, which is a segment of DNA, is transcribed into a mRNA. The mRNA transcript is processed to remove introns and moves out of the nucleus into the rough endoplasmic reticulum. Here, the mRNA is translated by ribosomes into a chain of amino acids, which will be folded with the help of chaperones and exported via a vesicle to the Golgi apparatus. From the Golgi apparatus, the protein will be repackaged in a vesicle that will undergo exocytosis to release its content in the extra - cellular space. 7. Discuss the ways in which a cell is able to alter its interactions with other cells. Answer An important part of the function of many cells is the ability to affect how it interacts with other cell types. Many of these interactions are physical connections, mediated through cell membrane receptors and extracellular matrix proteins. Cells can control their interactions with surrounding cells by changing the types of receptors they produce, or by altering the nature of the surrounding extracellular matrix. The levels of membrane proteins and secreted proteins can be controlled by synthesis (transcription, translation) degradation (proteolysis) and membrane trafficking (exocytosis, endocytosis).

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    1 Chapter 4 Cell Signaling and Endocrine Regulation Answers to Concept Check Questions 1. Compare and contrast paracrine and endocrine communication in terms of the three main steps of indirect signaling. Answer Paracrine and endocrine communication are very similar in terms of Step 1 (release from the signaling cell) and step 3 (actions at the target cell) of indirect signaling , but step 2 (transport through the extracellular fluid) differs. In paracrine communication , transport is by diffusion through the extracellular fluid. In endocrine communication, transport is through the circulatory system. 2. Compare and contrast hydrophilic and hydrophobic messengers in terms of the three main steps of indirect signaling. Answer Hydrophobic messengers diffuse readily across cell membranes, because they are soluble in membrane lipids. However, because they are insoluble in aqueous solutions, they are typically transported bound to carrier proteins in the blood. At the target cell, they can bind to extracellular receptors, or diffuse across the cell membrane and bind to intracellular receptors. Hydrophilic messengers are released from the signaling cell via exocytosis because their low solubility in lipids means that they cannot diffuse across the membrane . They travel dissolved in the blood , because they are readily soluble in aqueous solutions. At the target cell, they cannot cross the cell membrane and instead must bind with membrane receptors on the target cell to stimulate a response. 3. Are peptide messengers hydrophilic or hydrophobic? How does this property influence the mechanisms involved in each of the three main steps of indirect signaling? Answer Peptide messengers are typically hydrophilic. They cannot pass directly through the membrane, so they are released by exocytosis. They dissolve in extracellular fluids and blood, so they do not need carrier proteins. They must bind to transmembrane receptors, because they cannot cross the cell membrane to interact with intracellular receptors. 4. What is a preprohormone? Answer A preprohormone is a larger protein molecule that is subsequently post - translationally modified by cleavage into the smaller active hormone. 5. What are the three main classes of steroid hormones in vertebrates? Answer Mineralocorticoids, glucocorticoids, and reproductive hormones 6. Why are steroids usually bound to carrier proteins when transported in the blood? Answer Because they are hydrophobic (lipophilic) they do not dissolve well in aqueous solutions such as blood. Carrier proteins surround the hydrophobic messenger and isolate it from the surrounding aqueous solution.

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    2 7. Are amines hydrophilic or hydrophobic messengers? How does this affect their release, transport, and signaling? Answer Most amines are hydrophilic messengers (thyroid hormone is an exception). They are, therefore, usually released via exocytosis and signal via membrane receptors. 8. Outline the ways in which thyroid hormone release, transport, and signaling differ from that of other biogenic amines. Answer Unlike the other amines, which are mostly hydrophilic, thyroid hormones are lipophilic. Thus, thyroid hormones can diffuse across the cell membrane when they are released into the blood. In the blood they are transported by a carrier protein (which is not necessary for the other amines). They bind to intracellular receptors at the target cell, rather than the transmembrane receptors that are typical for other amines. 9. What are eicosanoids? Answer Eicosanoids are lipid chemical messengers derived from fatty acids (typically arachidonic acid). They can function as neurotransmitters and paracrines. 10. Name three gaseous signaling molecules, and one function that they share. Answer Carbon monoxide, hydrogen sulfide, and nitric oxide are all involved in regulating the diameter of blood vessels. 11. Why do some cells respond to a chemical messenger while other cells ignore it? Answer Cells will only respond to chemical messengers for which they possess appropriate receptors. 12. Compare and contrast receptor up - regulation and down - regulation. How do these phenomena help to maintain homeostasis? Answer Up - regulation involves an increase in the number of active receptors, while down - regulation involves a decrease in the number of active receptors. Up - regulation may occur when a cell is exposed to a receptor antagonist (something that blocks the receptor) for long periods. Down - regulation may occur when a cell is exposed to a receptor agonist (something that activates the receptor) for long periods. This is a form of negative feedback on the receptor that helps keep the cellular responses to a signaling pathway within the homeostatically regulated range. 13. Some hydrophobic messengers alter the expression of only a few genes, while other messengers cause changes in the regulation of thousands of gene s . Explain how this can be the case. Answer Hydrophobic messengers interact with intracellular receptors , activating them so that they function as transcription factors. The activated receptor binds to a hormone - responsive element within the promoter of the target genes. The simplest way for a particular messenger to cause changes in a large number of genes would be to have a hormone responsive element for that messenger present in a very large number of genes in the genome. Another possible way for a single messenger to affect the transcription of many genes would be for the messenger to activate a

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    3 transcriptional cascade (as in Figure 4.20). If a messenger activates a gene that encodes a transcription factor, that gene could go on to activate many other genes. 14. Some responses to hydrophobic ligands are termed nongenomic responses. How do they differ from the typical responses to a hydrophobic ligand? Answer Nongenomic responses occur when a hydrophobic ligand binds to a membrane - bound receptor and causes a signal in the target cell via a signal transduction pathway that occurs in the cytoplasm. 15. What is the primary response of a cell when a ligand binds to a ligand - gated ion channel? Answer The primary response is opening or closing of the ion channel, resulting in a change in the membrane potential or a change in the concentration of a specific intracellular signaling molecule such as calcium . 16. Which would you predict to be faster, signaling via a ligand - gated ion channel or signaling via an intracellular receptor? Justify your answer. Answer Generally, signaling via a ligand - gated ion channel will be faster than signaling via an intracellular receptor because changes in membrane potential (which are the result of signal transduction via a ligand - gated ion channel) are nearly instantaneous. In contrast, intracellular receptors function as transcription factors, so they must first alter gene transcription, which is then followed by a change in translation, and thus a change in the amount of a protein, which ultimately has an effect on the target cell. Although these process es can be fairly rapid, they are not instantaneous, and sometimes they can be quite slow. 17. What are the three main parts of a receptor - enzyme? Answer They contain an extracellular ligand - binding domain, a transmembrane domain, and an intracellular catalytic domain. 18. How do receptor - enzymes amplify incoming signals? Answer The catalytic domain catalyzes a phosphorylation reaction that post - t ranslationally modifies the target proteins by adding a phosphate group. A single activated receptor - enzyme can phosphorylate many target proteins, thus amplifying the signal. 19. Outline the five main steps in G protein – coupled receptor signaling . Answer ( 1) Ligand bind s to the receptor . ( 2) The receptor signals to the G protein to release GDP and bind GTP . ( 3) The activated G protein interacts with an amplifier enzyme, activating it. ( 4) The amplifier enzyme converts a second messenger from an inactive to an active form . ( 5) The activated second messenger activates or inhibits cellular pathways.

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    4 20. What is a second messenger? Answer Second messengers are small diffusible signaling molecules that are activated by transmembrane receptors (such as G protein – coupled receptors). Some second messengers are hydrophilic and work in the cytoplasm (such as Ca 2+ and cAMP), whereas some are hydrophobic and work within the membrane (such as DAG). 21. Compare and contrast negative feedback and positive feedback. Which type of control allows maintenance of homeostasis? Answer Negative feedback is where the product inhibits further production of the stimulus. Positive feedback is where the product stimulates further production of the stimulus. Negative feedback allows maintenance of homeostasis. 22. What are antagonistic pairings? What are the advantages of this organization of control systems? Answer Antagonistic pairings are systems in which one hormone increases a particular phenomenon and the other decreases it. By acting together in this way, they allow the endocrine system to exert rapid and precise control over the relevant physiological function. 23. Provide an example of a hormone controlled by a third - order endocrine pathway, and outline each step in the regulatory cascade. Answer M any of the hormones secreted by the anterior pituitary are involved in third order endocrine pathways. For example, adrenocorticotropic hormone (ACTH) is regulated in this way . The release of ACTH is regulated by the neurohormone corticotropin - releasing hormone , which is released from the hypothalamus . The ACTH then causes the release of glucocorticoid hormones from the adrenal glands into the circulation . Thus this pathway is called the h ypothalamic p ituitary a drenal (HPA) axis because of the three steps involved in this third order feedback loop. 24. What is the major difference between invertebrate and vertebrate endocrine systems? Answer Invertebrate endocrine systems rely heavily on neurohormones, whereas vertebrate endocrine systems have many specialized endocrine glands that produce hor m ones. 25. How have gene duplications played a role in the evolution of the vertebrate endocrine system? Support your answer with at least two examples. Answer Gene duplications have been important in the evolution of many vertebrate hormones and their receptors. Vasopressin and oxytocin provide an example of gene duplication. Only a single gene is present in the jawless fish, while jawed vertebrates have two. The glucocorticoid and mineralocorticoid receptors provide another example of a pair of gene that arose during the whole - genome duplication leading to the vertebrates. In this case, there has also been a shift in the ligands binding to these receptors between fish and tetrapods. Prolactin and growth hormone provide a third example .

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