Solution Manual for Principles of Animal Physiology, 3rd Edition

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ANSWERKEYPrinciples ofAnimal PhysiologyThird EditionChristopher D. Moyes, Ph.D.Queen’sUniversityPatricia M. Schulte, Ph.D.University of British Columbia

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1Chapter 1Introduction to Physiological PrinciplesAnswerstoConcept CheckQuestions1. How would you define physiology?AnswerIt is the study ofthefunctioning of biological structures and systems, or how organisms work.2. What is a model organism in the context of physiological research?AnswerThe August Krogh Principle suggests that“for any biological question, there is an organism on which it can be mostconveniently studied.”A modelorganismis a convenient animal on which to study a biological question. Squid, forexample, 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?AnswerTemperature increases the thermal energy of molecules and increases the number of collisions between molecules.Becausemost reactions require molecular collisions, increasing the rate of collisions will increase the rate ofreactions. This increase does not continue to infinity as temperature increases because at high temperaturesmanyofthe intermolecular bonds that stabilize protein structure start to break, causing proteins to unfold and denature. Whenproteins are unfolded they are unable to perform their functions.Becausemost biochemical reactions only occur athigh rates because of the actions of protein catalysts, reaction rates decline when the catalysts begin to denature.4. What is allometric scaling?AnswerSome processes or structureschangein direct proportion to body mass, which is called isometric scaling. If theprocess or structure changes disproportionately with body mass, it is consideredto scaleallometrically.5.What is an adaptation?AnswerAn adaptation is a trait that arose via a process such as natural selection andthatcauses an increase in reproductivesuccess.6. Distinguish between homology and analogy.AnswerHomologydescribes a pattern wherea traitthat ispresent in two taxa is inherited from a common ancestor. Thesetraits may or may not be similar in appearance and function in the two taxa. For example,bird wings and human armsappear 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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2Analogydescribes a patternwherea trait is used for a similar function in two taxa, but is not inherited from a commonancestor. For example,the camera-type eyes of vertebrates and cephalopods were derived from the non-camera eyesof the closest relatives of each of these groups. This independent derivation suggests that these eyes are nothomologous, but instead are analogous.7. What is homeostasis?AnswerHomeostasis is the regulation or maintenance of internal conditions within a narrow range,despitechanges in theexternal environment.8. Distinguish between acclimation, acclimatization, polyphenism, and phenotypic plasticity.AnswerAcclimatization 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 inresponse to a singleenvironmental variable, usually in an artificial (e.g. laboratory) environment.Polyphenism occurs when different environments lead todiscrete alternativephenotypes.For example, developmentalplasticity 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 asa result of the environment;thus acclimation, acclimatization,and polyphenism are all types of phenotypic plasticity.

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1Chapter 1Introduction to Physiological PrinciplesAnswers to Review Questions1. Where would organelles such as the mitochondrion fit in the levels of organization shown in Figure 1.3?AnswerOrganelles such asmitochondria would fit between the molecular and the cellular levels in Figure 1.3. Moleculeswork together in complex biochemical pathways to form the complex structures that are assembled to makeorganelles. These organelles are parts of a cell.2. What is the Krogh principle, and why is it useful for animal physiologists?AnswerThe Krogh Principle states that for every biological problem there is an organism on which it can be mostconveniently studied. This means that medical physiologists interested in a function in humans (or veterinaryphysiologists interested in cows) might choose to study these functions in an animal such as a mouse, because they aremuch smaller and easier to maintain.3. All organisms have a maximum temperature at which they can function. Suggest a possible physical basis for thisobservation.AnswerThe maximum temperature that an organism can function at must be the lowest temperature at which a critical processrequired to sustain life fails. Not all physiological functions are likely to fail at exactly the same temperature, but whena critical process fails, life will end. Processes could fail at high temperatures for a variety of reasons, but ultimatelythis 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?AnswerSurface area increaseslesswith size than does volume, so surface area for exchange becomes limiting as sizeincreases, unless there is a change in shape. This pattern explains why physiological systems such as the respiratorysystem have extremely high surface area comparedwiththeir volume. This increases the area available for gasexchange.5. What are three fundamental requirements for adaptive evolution of a trait to occur?AnswerFor adaptive evolution to occur:1.There must be variation among individuals in the trait.2.The trait must be heritable.3.The trait must increase the fitness.

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26. Are the eyes of vertebrates and cephalopod mollusks homologous or analogous? Justify your answer.AnswerThe eyes of vertebrates and cephalopod mollusks are analogous. Both are camera-type eyes that can form a focusedimage, but they arose independently from different ancestors that lacked camera-type eyes. Thus,they did not share acommon ancestor and cannot be homologous,despite their similarity of function.7. Whatis the main benefitofhavingantagonistic controls in physiological systems?AnswerAntagonistic controls allow more precise control over a physiological system by having two separate controlmechanisms: one that increases, and one that decreases, the activity of the system. This is analogous to having both abrake 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.AnswerPositive feedback loops are organized so that the output of the system tends to increase the activity of the system. Thisresults in a rapid response and an amplification of the output of the system, resulting in a large response to a smallstimulus. 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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1Chapter 1Introduction to Physiological PrinciplesAnswers to Synthesis Questions1.What physical, chemical, or physiological constraints might lead to allometric scaling?AnswerAllometric scaling refers to variables that do not change linearly with body mass;thus,a given trait in a 1-kg animalwill 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 asdiffusion 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 notalwayshave a scaling exponent of 0.67, as would be expected if surface area-to-volume ratios were the only importantfactor.A variety of physical, chemical, and physiological mechanisms have been proposed to explain this pattern, butthe actual mechanism remains a matter of debate among physiologists.2.Why do physiologists need to understand evolution?AnswerThere is a great deal of diversity in physiological traits expressed by animals. These traits (an animal’s phenotype) arethe result of interactions between the animal’s genotype and its environment. An individual’s genotype reflects itsevolutionary history, shaped by natural selection and genetic drift. As a consequence, closely related organisms sharemore traits than distantly related ones. Evolution therefore allows physiologists to understand the significance of thediversity of physiological traits (both the proximal and ultimate causes).3.Compare and contrast adaptive evolution and genetic drift.AnswerBoth adaptive evolution and genetic drift can result in changes in the genotype of populations over time. For adaptiveevolution, a genetic mutation that results in a trait that increases the reproductive successcan be acted on by naturalselection andbecome widespread after multiple generations have passed.Alternatively, a population may have aunique trait, not because it improves fitness, but because the mutation increased in frequency as a result of randomfactorssuch as genetic drift. Such random factors are more likely when a population is small, but can occur in anysituation. Genetic drift can oppose selection, so that even a favorable mutation may be eliminated from a populationby chance.4. When might an adaptation become detrimental?AnswerIn a heterogeneous population, many genotypes will be present. When the environment changes, a subset of thegenotypes may do better, and that genotypemaybecome more abundant in the population. If the environment changesagain, 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.AnswerAtemperature sensor such as a thermometermeasures air temperature andsends a signal to a thermostat thatcomparesthe measured temperatureto the desired temperature, or setpoint. If the measured air temperature is belowthe setpoint, the thermostat will send a signal to turn on the heating system. When air temperature rises to the set

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2point, the thermostat will stop sending the “on” signal to the heating system. This process qualifies as a negativefeedback because the response (turning on the heat to warm the air) is in the opposite direction to the stimulus (coldair temperature).6. Make an argument for or against adopting the use of the term allostasis.AnswerArgument for: The term allostasis is useful because it points out that set points for homeostatic systems can changewith the environment, for example between seasons or with the phases of the reproductive cycle, which is notexplicitly included in the definition of homeostasis.Argument against: The term allostasis is not necessary because the original definition of homeostasis already includesthe 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 youexperimentally distinguish between these two possibilities?AnswerIt is probably an example of acclimatization,asthe observation was not made under controlled conditions in thelaboratory, and multiple factors likely changed in the spring (temperature, day length, amount of time spent outsideand exercising, and probably many others). However, it might also be a developmentally programmed event that isindependent of environmental factors. The way to test this experimentally would be to take a dog into the laboratoryand expose it to changes in each of the possible environmental factors individually and in combination and then toobserve the dog’s hair to see whether the thickness of the coat increases or decreases. Ideally, this experiment shouldbe done with many different individual dogs to see whether it is a consistent response for the species.

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1Chapter 2 Physiological Evolution of AnimalsAnswers toConcept CheckQuestions1.Distinguishbetween the terms protist, protozoan, metazoan, and eumetazoan.AnswerUnicellular eukaryotes are collectively called protists, though they are a polyphyletic group. Protozoans are thoseprotists with“animal-like”abilities to move. These twotermsinclude many unrelated organisms. Metazoan can beused 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 discretetissue layers.2. What is a coelom?AnswerIn triploblastic animals the early stages of embryonic development lead to three distinct tissue layers: endoderm,mesoderm, and ectoderm. In coelomate animals, a gap can occur within the early embryo, either through splitting ofthe mesoderm or folding of the mesoderm. The gap isa coelom.A pseudocoelom is like a coelom in that it is ainternal body cavity. However, a true coelom is lined by mesoderm, whereas a pseudocoelom is only partially lined bymesoderm.3. Distinguish between metamersand tagmata.AnswerMetamers are repeating segments of an animal. Tagmata arise when two or more metamers fuse and become aspecialized region.4.Which group of fish gave rise to the tetrapod lineage?AnswerThe sarcopterygian fish, represented by modern coelocanths (Coelocanthomorpha) and lungfish (Dipnoi), gave rise totetrapods.5.What is an amniote?AnswerTetrapods with an egg possessing four extraembryonic membranes are amniotes. They include reptiles, birds, andmammals, but exclude amphibians.6.What is the phylogenetic relationship between mammals, birds, and reptiles?AnswerMammals 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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2Myosin appeared in unicellular eukaryotes long before animals arose. It retains a role in cytoskeletaltrafficking, but inanimals it has derived roles in muscle. The myosin family amplified many times in animals, leading to the origin ofdifferent myosin isoforms.8.When did collagen evolve and what is its significance?AnswerCollagen first appeared in the earliest animals, and is used to construct many forms of extracellular matrix.9.What are cadherins?AnswerCadherins are cell membrane adhesion proteins. Though they appear in animals, their origins predate the appearanceof metazoans. Cadherin genes occur in protists, in particular the choanoflagellates that are thought to share a commonancestor with metazoans.10.What is cephalization?AnswerCephalization is an evolutionary trend whereby animals concentrate important sensory, nervous, and digestive systemcomponents toward the anterior, giving rise to a discrete head region.11.Why is metabolism more diverse in bacteria thaninanimals?AnswerBacteria have little capacity to evolve structurally because of their structural simplicity.Theirbest evolutionary optionfor 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 adaptationsare important in many aspects of animal evolutionary physiology.

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1Chapter 2 Physiological Evolution of AnimalsAnswersto Review Questions1. What is the significance of the similarity between choanoflagellates and choanocytes?AnswerIt is likely that choanoflagellates are the modern protist that is most closely related to the earliest metazoan. There is astriking similarity in the general structure of choanoflagellates and choanocytes of sponges.2. Why aren’t protozoans considered animals?AnswerProtozoans 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-celledeukaryotic organism could not be ananimal. Some protists have the animal-like trait of locomotion, but that is not an indicator of how closely a protist isrelated to the ancestors of animals.3. Which animals are diploblasts?AnswerDiploblasts have only two embryonic cell layers, endoderm and ectoderm. Sponges lack cell layers, and most animalshave three embryonic cell layers. Only cnidarians and probably ctenophores are diploblasts.4. Explain why arthropods are considered Ecdysozoans.AnswerEcdysozoans share the ability to moult at some stage in their development. Arthropods moult.5. Did all jawed animals evolve from the same agnathan ancestors?AnswerNo.Agnathans are a large group of jawless fish that gave rise to multiple lineages of fish and theirdescendants.Itappears likely that sharks and bony fish arose from different agnathan ancestors, which were distinct from those thatgave rise to lamprey and hagfish. It now appears likely that hagfish andlamprey, despite their divergence, shared acommon ancestor.6. How many times did terrestriality arise in animal lineages?AnswerMany 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 groupsthat live on land, although in many cases they live in wet environments, such as moss, or live as endoparasites in thefluids of land animals. Of course, select groups of fish can spend periods of time on land.

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27. What is meant by the term“a new head”?AnswerThe“new head”hypothesis suggests that the evolutionary diversification and success of vertebrates was madepossible by the development of structures anterior to the notochord. In different lineages, these structures becomeimportant musculoskeletal features such as the jaw.8. What is the significance of the evolution of the Na+/K+ATPase?AnswerThe evolution of the Na+/K+ATPase gave animals a mechanismby whichto pump Na+and K+across their cellmembranes, creating the electrochemical potential that is used todrive many transport processesand permit theformation of excitable tissues. The transporters linked indirectly to Na+/K+ATPase help animals regulate their cellvolume, which was important in this group of organisms because they lack cell walls.9. When did endothermy arise in animal evolution?AnswerEndothermy arose several times in animals.Endothermy aroseindependentlyin the ancestors of two groups ofvertebrates, 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.Theacquisition of endothermy in these groups is much more recent.10. Which came first, hormones or hormone receptors?AnswerIt is likely that the answer may differ between hormonesandhormone receptors, but in the case of the aldosteronereceptor discussed, it is likely that the receptor was present and capable of responding to other steroids beforealdosterone came on the scene.

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1Chapter 2 Physiological Evolution of AnimalsAnswerstoSynthesisQuestions1. Speculate on how animals might have evolved if the ancestral protist possessed a cell wall.AnswerRather than giving you possible answers, consider thesignificance of the cell wall in other organisms. It providesresistance to cell swelling, and how is cell volume controlled in animals? It provides a physical barrier that makes itdifficult for large molecules to cross, so how might that affect cell-to-cell signaling? It is a rigid structural support forcells that can be connected together to make tissues.It also precludes connections between adjacent cells, obviatingdirect cell-cell communication.2. What critical events led to the origin and diversification of tetrapods?AnswerTo answer this you probably need to draw on material fromoutside thischapter andfromother courses. Think aboutthe diversity in tetrapods, and how those differences may have arisen. Within vertebrates, the two whole-genomeduplications created the genetic raw material for specialization of genes. A number of events occurred in tetrapodevolution 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 andseasonality contributed to constraints on range limits;evolutionary changes in anatomy and physiology permitted theinvasion of new environments.3. Would you expect the underlying metabolic pathways to be similar or different in animal models of metabolic arrest?AnswerMetabolic 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 arisefrom fundamentally similarmetabolic pathways. Thus, factors such as energy balance depend on energy stores andenergetic costs, and the ways animals deal with these challenges differ widely. Molecular protection pathways, such aschaperone proteins and cryoprotectants,are also very similar among animals. Where animals tend to differ is in themagnitude of responses.Animals that undergo metabolic depression are likely to be very good at regulating pathwaysshared by all animals.

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1Chapter 3 Chemistry, Biochemistry, and Cell PhysiologyAnswers toConcept CheckQuestions1.What are the five main forms of energy used by animals? Provide biological and nonbiological examples of processes thatrepresent conversion of energy fromone form to another.AnswerThe five main forms of energy used by animals are radiant, mechanical, electrical, thermal,and chemical energy.Ifyou are having difficulty coming up with examples, consider Figure 3.2 as a starting point.2.What is the difference betweenthermal energy, heatand temperature?AnswerThermal energyof a systemisthe totalenergy associated with molecular motionof that system: vibration, rotation,and translocation.Heat is the transfer ofthermalenergy between objects or systems at a different temperature.Whenananimal’sbody heats the air around it, thermal energy is transferred from the molecules of the body to the moleculesof the surrounding air. Both thermal energy and heat are forms of energy, measured inJoules.Temperature is aconvenientmeasure of theaveragethermal energy of a system, assessed in units of Celcius, Farenheit, or Kelvin.Thedifference between these parametersisillustrated by considering a large and small animal, each at the sametemperature,20degrees warmer than the air. The large animal has more thermal energy because, despite having asimilar average thermal energy, it has a greater total thermal energy.Likewise, the large animal is able to transfermore heat to the environment.3. How are weak bonds affected by temperature?AnswerHydrophobic bonds are strengthened at warmer temperatures,whereas the other weak bonds arestrengthened at lowertemperatures.4. What change in pH has a greater effect on proton concentration: pH 6 to 7 or pH 7 to 8?AnswerThough each represents a tenfold decrease in proton concentration, the change from 6 to 7decreases protonconcentration by 9x10-7M, whereas the increase in pH from 7 to 8 decreases protonconcentration10-fold less, by9x10-8M.5. Describe a cup of coffee (no milk, one sugar) in terms of solute, solvent, and solution.AnswerThe chemicals released from the coffee beans are solutes dissolved in a solvent of water to produce the solution calledcoffee.

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26. What is the difference between osmolarity and tonicity?AnswerOsmolarity describes the number ofosmotically active solutes (osmolytes) in a solution, whereas tonicity refers to asolution in terms of its effects on a cell.7.Distinguish between allosteric and covalent regulation of enzymes.AnswerAllosteric regulation occurs when a molecule reversibly binds an enzyme at a site somewhere distant from the activesite, and causes a change in structure that influences function.Covalent modification of an enzyme is when anothermolecule 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 byenzymes.8.Distinguish between the following types of reactions: anabolic, catabolic, andamphibolic.AnswerAnabolic reactions are synthetic, catabolic are degradative, amphibolic are a combination of both synthetic anddegradative processes.9. Why is ATP considered a“high-energy”molecule?AnswerThe breakdown of ATP to ADP and phosphate liberates energy because of the thermodynamics of the reaction and therelative concentrations of the reactants.10. Distinguish between primary, secondary, tertiary, and quaternary structure.AnswerFor a protein, the primary structure is the amino acid sequence. This strand can be arranged into spirals or sheets toform the secondary structure. The protein can then be foldedin ways that change the relationship between theelements of secondary structure, creating the tertiary structure. The individual folded proteins may interact with otherproteins to form the quaternary structure.11. Why does temperature affect the three-dimensional structure of proteins?AnswerTemperature causes changes in the weak bonds that confer the secondary, tertiary,and quaternary structures. Heatingmay cause a protein to unfold, or become more compact, depending on the relative importance of hydrophobicinteractions.12. What is a molecular chaperone?AnswerA 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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313. Distinguish betweenstarch,glycogen, andcellulose.AnswerAll three are polysaccharides formed from glucose.Starch is a mixture of amylose, which has few branches, andamylopectin, which has some branches. Glycogenissimilar in structure but much morehighly branched.Both starchand glycogen are composed of glucose chains linked by α1,4 glycoside bonds. Cellulose is also a chain of glucose,butthese are linked by β1,4 glycoside bonds.14. What is the purpose of a redox shuttle?AnswerNADH produced in the cytoplasm cannot enter mitochondria. A series of cyclic enzymatic reactions allowsthetransfer of reducing energy across the mitochondrial membrane. NADH is oxidized to reduce a metabolite that can betransported across the inner mitochondrial membrane. When reoxidized in the mitochondria, the resulting NADH canbe oxidized by the electron transport chain.15. What happens to glycolytic pyruvate under hypoxic conditions?AnswerPyruvate can be reduced to form lactate, oxidizing NADH in the process. Lactate may be subsequently converted toother alternate end products. In some species, pyruvate can be processed to form ethanol, oxidizing NADH.16. How is energy derived from triglyceride breakdown?AnswerThe triglyceride can be broken down into glycerol and three fatty acids. Glycerol can enter the glycolytic pathwayviaconversion to glycerol 3-phosphate. The fatty acids enter beta-oxidation, with ATP produced by the oxidation of theNADH and FADH2produced.17. What are the of triglyceridebreakdown?AnswerStarting with triglyceride, the removal of one fatty acid generates diacylglyceride. Further breakdown of thediglyceride yields another fatty acid and monoglyceride. When monoglycerides are further degraded, removal of thelast fatty acid leaves glycerol. Thus, complete breakdown of triglyceride yields glycerol and three fatty acids.18. What are the main classes of phospholipid?AnswerAnimal 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 aminoacids?AnswerAcetyl CoA

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420. What is the proton motive force?AnswerThe proton motive force is the combination of a pH gradient and membrane potential. This electrochemical gradient isproduced by the electron transport chain and utilized for ATP synthesis.21.How is oxidation coupled to phosphorylation in mitochondrial oxidative phosphorylation?AnswerOxidation is coupled to phosphorylation in mitochondrial oxidative phosphorylation via theelectrontransportsystem.The regeneration of NAD+and FAD via the actions ofcomplexes I to IV andcytochromeccreates a proton gradient,whose energy is utilized, viacomplex V, to phosphorylate ADP to ATP. This process requires the reduction of oxygenatcomplex IV, so that the electrons thecomplex IV received fromcytochromeccan be passed on and new electronsaccepted.22.How does glucose catabolism differ under(a) high versus low energy conditions and (b) normal versus low oxygenconditions?AnswerAnaerobic glycolysis may be advantageous under high energy and/or low oxygen conditions as it produces ATPrapidly from glucose stores and does not require access to oxygen. Oxidative metabolism of glucose may beadvantageous under low energy and/or normal oxygen conditions,as it produces considerably more ATP and heat perunit 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?AnswerUnder low oxygen conditions, anaerobic pathways can be used to metabolize carbohydrates but lipid metabolicpathways will be limited due to their dependency onoxygen.24. What is a respiratory quotient?AnswerIt 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.AnswerMembranes are composedmainlyof mixtures of proteins and phospholipids. The proteins include transporters,enzymes, and proteins that are anchors for cytoplasmic and extracellular matrix proteins. The phospholipids includephosphoglycerides and sphingolipids. Aswell, membranes possess lesser amounts offatty acids, triglycerides,steroids,and steroid derivatives.26.How can cells alter the fluidity of membranes and why is this capacity important to cellular function?AnswerCells can alter membrane fluidity by changing the phospholipid composition in such a way that the weak bonds(vander Waals forces)between the fatty acid tails of the phospholipids are either strengthened or weakened—for example,

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5by favoring phospholipids with longer or shorter tails or with greater or lesser degrees of saturation. Increasing theconcentration of cholesterol also increases the fluidity of the membrane.27.What is the relationship between the Nernst equation and the equilibrium potential?AnswerThe Nernst equation can be solved to determine the membrane potential at which net movement of a specific ionceases at a given chemical gradient, i.e. the equilibrium potential for that ion.28.Distinguish between depolarization, repolarization, and hyperpolarization.AnswerDepolarization is a change in membrane potential toward a value of 0mV. After depolarization, a cell should return toits 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 thanat rest.29.Summarize the roles of the different subcellular compartments within a cell, and discuss how they influence physiologicalfunction.AnswerCompartments that should be discussed here include the nucleus, the mitochondria, the rough and smoothendoplasmic reticulum,and theGolgi apparatus.30. What does the Na+/ K+ATPase do in a typical cell?AnswerThe sodium pump, or Na+/ K+ATPase, uses the energy associated with ATP hydrolysis to pump 3Na+out of a cell inexchange for 2K+. As a result, this pumphelps maintain ion gradients and the membrane potential.31. Distinguish between paracellular transport and transcellular transport.AnswerBoth modes of transport refer to movement of a molecule across a cell layer. Paracellular transport describes themovement of a molecule between cells, whereas transcellular transport occurs through a cell.32. Distinguish between transcription and translation.AnswerThe production of RNA from DNA is transcription. Translation is when mRNA is read in producing protein.33. Distinguish betweenaparalog andanortholog.AnswerBoth terms refer to duplicates of genes. Paralogs are copies of a gene found within anindividual, encoded at differentloci (i.e., they are not alleles of a gene but rather two separate genes of common ancestry). Orthologs are copies ofgenes separated by a speciation event. For example, humans have LDH-A and LDH-B genes (paralogs). The LDH-Agenes of chimps and humans are orthologs.

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634.What are whole-genome duplications and when did they occur in animal evolution?AnswerMany 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 withtime the duplicate genes diverged and the progeny were considered diploid, retaining extra paralogs of many genes.

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1Chapter 3 Chemistry, Biochemistry, and Cell PhysiologyAnswers toQuantitativeQuestions1. What is the proton concentration of a solution at pH 7.4? At what temperature would this solution be neutral?AnswerpH=-log([H+]) rearranged as [H+]=10-pHifpH=7.4, then [H+]=10-7.4=4x10-8There is insufficient information to answer the effect of temperature on neutrality because a) the solution could bealready neutral (neutrality does not mean pH of 7.0, only that [H+]=[OH-]), because b) we don't know the temperatureat which the pH of 7.4 was measured and c) we don't know what is in the solution.2.What isthe basis for an RQ = 1 for carbohydrate oxidation. Why does palmitate oxidation give an RQ = 0.7?AnswerThe number of CO2produced depends on number of carbon in the molecule. Number of O2consumed depends on thepathway for oxidation. Using glucose as an example for carbohydrates, complete oxidation yields 10 NADH and 2FADH2, requiring the consumption of 6 O2in oxidative phosphorylation. Because glucose has 6 carbons, 6 CO2willbe produced(2 in oxidation of pyruvate, 4 in the tricarboxcylic acid cycle), leading to an RQ of 1:C6H12O6+ 6O2= 6H2O+ 6CO2RQ = 6CO2/6O2=1Using plamitate as an example for fatty acids, complete oxidation yields 31 NADH and 15 FADH2, requiring theconsumption of 23 O2. Because palmitate has 16 carbons, the RQ will be 0.7:C16H32O2+ 23O2= 16H20 + 16CO2RQ=16CO2/23O2=0.7.3. What rate of oxygen consumption would you expect in a tissue with a metabolic rate of 30μmol ATP/g/min?AnswerThe relationship between the rates of ATP and O2consumption depend on the fuel source.Oxidation of reducingequivalents by the ETS releases energy that is used to pump protons, but the amount of proton pumping depends onwhether or not Complex I is used.Theoretically,NADH oxidation, initiated by Complex I, liberates enough energy topump 3 H+per oxygen atom (or 6H+per O2),whereas oxidation of FADH2, initiated by Complex II, liberates enoughenergy to pump 2 H+per oxygen atom (or 4H+per O2).The actual stoichiometries of ATP production and O2consumption are lower and variable due to factors that influence the leak of protons across the mitochondrialmembrane. However, in theory, oxidation ofcarbohydratesgenerates mostlyNADH,and thus the rate of 30μmolATP/g/minwould be supported by a respiration rate of about 5μmolO2/g/min.Oxidation oflipidsrelies on both theFADHand NADH pathways, and thus a higher rate of respiration would be required to support the observed rate ofATP synthesis, between 5 and 7.5μmolO2/g/min.

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1Chapter 3 Chemistry, Biochemistry, and Cell PhysiologyAnswers toReviewQuestions1.What are the four types of weak bonds and how do they differ from each other and from covalent bonds?AnswerThe four types of weak bonds are van der Waalsforces, hydrogen bonds, ionic bonds,and hydrophobicinteractions.Covalent bonds are stronger, meaning more energy is required to break them.Hydrophobic interactions arestrengthened at high temperature whereasvan der Waalsforces, hydrogen bonds, ionic bondsare weakened at hightemperature.2. Why are reaction rates influenced by temperature?AnswerThe influence of temperature on chemical reactions dependson many factors. For a simple chemical reaction thatrequires energy to proceed spontaneously, increasing temperature will increase the reaction ratebecause a greaterfraction of the substrate molecules will possess activation energy.For enzymatic reactions, other factors come intoplay 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 greaterfraction to reach the activation energy barrier. Some reactions are very temperature sensitive in vivo andacceleratewhen temperature increases. Typically, a10oC temperature increaseincreases reaction rate 2-to 3-fold.. However,many enzymes, such as catalase, have almost no thermal sensitivity and proceed at the same rate regardless oftemperature. 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 theaffinity 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 marineand freshwater environments?AnswerOne of water’s unique properties is that it reaches a maximal density at 4°C; water density is lower above or belowthat temperature. As a result, ice floats on liquid water. In cold climates, surface ice formation insulates the underlyingwater from the cold air. This prevents bodies of water that are deep enough from freezing completely and provides astable thermal environment for aquatic organisms during winter. Another consequence of this property is that deepwaters tend to have a very stable temperature of 4°C, while surface waters can undergo large latitudinal and seasonalvariations in temperature. Therefore, aquatic and marine organisms that live in deep water are exposed to a very stablethermal environment.4. Discuss the mechanism by which cells can use transporters to change their osmotic and ionic properties.AnswerActive 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 acrossthe plasma membrane; a typical example is the Na+/K+ATPase (or Na+/K+pump) that helps generate anelectrochemical gradient across the plasma membrane. In secondary active transport, the active transport of amolecule is used to drive the movement of another. For instance, the Na+electrochemical gradient generated by theNa+/K+ATPase can be used to transport glucoseintointestinal cells using a symport or cotransporter membraneprotein, or to transport H+out of nephron cells using an antiport or exchanger membrane protein to regulate pH.

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25. Distinguish between the types of polysaccharides relevant to animals.AnswerPolysaccharides are simply chains of monosaccharides and in animals the most important ones are polymers ofglucosecreated when α-D-glucose molecules are attached between carbons 1 and 4 (α1,4 glycoside bonds).Amyloseis a plant polysaccharide with relativelyfew branches,whereasamylopectinhasa side branch approximately everythirty glucose molecules. Glycogen, produced by animals,is like amylopectin but withmore frequentbranches.Cellulose, another plant-derived glucose polymer, is essentially indigestible in animals because the glucose units areconnected by β1,4 glycoside bonds.Other polysaccharidescan be made from different sugars.Arthropods build their exoskeletons with chitin, apolysaccharide of N-acetyl-glucosamine. Vertebrates secrete hyaluronate, a polymer of N-acetyl-glucosamine andglucuronic 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 andkeratan sulfate.6. Compare the structures of phospholipids.AnswerAnimal cells produce two classes of phospholipids:phosphoglycerides and sphingolipids. Phosphoglycerides arebuilt on a glycerol backbone, with fatty acid chains on two of the glycerol carbons, with the third position occupied bya polar head group composed of an organic molecule linked via a phosphate.Sphingolipidshave a similar three-dimensional shape but are constructed differently. The backbone is sphingosine, which has a structure that resemblesglycerol with a single fatty acid. Afatty acid chain is also attached to the sphingosine backbone. Like aphosphoglyceride,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 theequilibrium constant. What happens to the mass action ratio if you add more enzyme? What happens when you addmore of A? What do you need to know to predict what would happen if temperature changed?AnswerThe mass action ratio depends on the relative concentrations of substrates and products. Enzymes accelerate reactionsby lowering the activation energy, but do not affect the direction of a given reaction. Therefore, adding enzymeswould have no effect on the mass action ratio.Adding more substrate (A) would decrease the mass action ratio below the equilibrium constant and promote theformation 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), andwe need how the affinity of the enzyme to the substrates and products is affected by temperature.8. Distinguish betweentheparametersthat describeenzyme kinetics anddiscussthe ways that cells control enzyme kinetics.AnswerEnzyme kinetic properties define how an enzyme will function under various conditions. The maximal rate is definedas theVmax. The affinity of the enzyme for substrate is reflected in the Michaelis constant (Km). Cells alter enzymekinetics by allosteric and covalent regulators. Allosteric regulators bind to the enzyme at a site different than thesubstrates and products, altering enzyme shape and function indirectly. Covalent regulation adds moieties to theenzyme 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-modifyingenzymes. Changes in the concentrations of substrates and products can change the reaction rates, through mass actioneffects, but these do not changeKmorVmax, which are properties of the enzyme.

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39. What metabolic conditions can affect the values of the respiratory quotient? What metabolic conditions affect therelationship between ATP produced and oxygen consumed?AnswerThe respiratory quotient is the ratio of CO2produced to O2consumed. While the amount of CO2produced depends onthe number of carbons in the nutrient, the O2consumed depends on the oxidation pathway. The oxidation ofcarbohydrates leadsto RQ values of 1, while the oxidation of fatty acids leadsto RQ values around 0.7. Becausecarbohydrates can be mobilized and oxidized rapidly, periods of short-term high-energy demands are characterized byhigh RQ values, close to 1. Conversely, lipids are the major fuel source during sustained energy demands or duringstarvation, 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, reliesmore on FAD-linkedenzymes for which the ATP/O ratio is 2. Therefore, the oxidation of carbohydrates leads to more ATP produced pervolume of oxygen consumed. Animals exposed to low levels of oxygen therefore favor the use of carbohydrates as afuel.10. How do the pathways of gluconeogenesis and glycolysis overlap?AnswerMost of the enzymes are shared between the two pathways, with these steps operating in opposite directions forglycolysis and gluconeogenesis. However, there are three steps that are specific to each pathway.(i) glucosetoglucose 6-phosphate. In glycolysis, glucose is phosphorylated by hexokinase, consuming ATP andreleasing ADP. In gluconeogenesis, the reverse step is catalyzed by glucose6-phosphatase, which cleaves off thephosphate 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 theenzyme fructose bisphosphatase, which removes the phosphate group.(iii)phosphoenolpyruvate and pyruvate.In glycolysis,pyruvate is produced from phosphoenolpyruvate by the enzymepyruvate kinase. In gluconeogenesis, the reverse is a two-step pathway, with pyruvate carboxylase formingoxaloacetate and phosphoenolpyruvate carboxykinase converting oxaloacetate to phosphoenolpyruvate.11.Discuss the ways in which a cell is able to alter its interactions with other cells.AnswerCells connect to each other through direct contacts mediated by proteins, as well as indirect contacts by attachment tothe extracellular matrix that they share. Cells can alter these relationships by internalizing the membrane proteins thatmediate the connections to other cells or the extracellular matrix. Alternately, they can secrete enzymes that digest theextracellular matrix, typically matrix metalloproteinases. All of these processesare underthecontrol of cell signalingpathways.12. Discuss the mechanism by which cells can use transporters to change their osmotic and ionic properties.AnswerTransporters can be stimulated or inhibited to alter the movement of ions, either by active transport or facilitateddiffusion. Many of these processes are dependent upon theelectrochemical gradients that are generated through activetransport and dissipated through ion channels. When cells move solutes, water has a tendency to move in relation tothe osmotic gradients. This is regulated in part by the expression of the aquaporin proteins that serve as waterchannels.Changes in the levels of transporters can arise by cells altering the synthesis and localization of the proteinsthat mediate transport, and these processes are under the control of cell signaling pathways.

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413. Many physiological processes require a change in the levels of proteins, such as membrane transporters. Discuss theprocesses that cells can use to change the protein levels. Discuss how the subcellular compartment influences this pathway.AnswerThelevels of proteins can be controlled at multiple steps of synthesis and degradation. Some of these steps andprocesses are shared by all proteins, regardless of their final destination. Other steps are unique to the finalcompartment in which the protein finds itself. At the genetic level, cells can control whether specific genes aretranscribed or not. Transcription can be controlled through proteins that modify the three-dimensional organization ofDNA into nucleosomes. Once the promoter is accessible, DNA-binding proteins and coactivators control theformation of the transcriptional machinery. The levels of transcription factors and chromatin remodeling enzymes areoften 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 thenucleus.Once mRNA is synthesized and processed, it is exported to the cytoplasm,where it can be transcribed and eventuallydegraded by RNases. For some proteins, the rate of protein synthesis is controlled by regulation of RNA degradation.Few specific proteins are controlled through controls on translation. Cells alter global protein synthesis through thephosphorylation of initiation and elongation factors, which affects the overall rate of translation in the cell. Forcytoplasmic and mitochondrial proteins, the mRNA is translated in the cytoplasm. In contrast, those proteins found inthe cell membrane and internal membrane network are translated into the ER. Once inside the ER, the protein is sentto the correct location via transport vesicles.Once proteins are synthesized, their levels can be regulated by degradation (proteolysis). Again, the compartmentdetermines 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 byproteasomes. The level of cell membrane proteins can be altered through exocytosis and endocytosis. Once invesicles, the protein may be sent to lysosomes for degradation.14. Other physiological processes require changes in theactivitiesof proteins. While this can arise through changes in thelevelsof proteins, it can also change through regulation of protein function. Discuss the various ways that cells can alter theactivity of enzymes or transporters.AnswerCompetitive inhibitors bind to the active sites of enzymes, preventing the binding of substrates and thereforedecreasing enzymatic activity. The binding of some molecules to enzymesin areas other than the active site can resultin 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 ofphosphorylation (by a protein kinase)ordephosphorylation (by a protein phosphatase) involves modifying enzymaticactivity (again by altering its three-dimensional shape) by the addition (or removal) of a phosphate group to an aminoacid of the enzyme.15. Discuss the origins of genetic variation.AnswerDuring the process of meiosis, chromosomes can experience mutations that produce genes that are different thaneither parent. These variants can accumulate in populations, forming a pool ofalleles of genes with differentsequences, which may affect the coding region (and consequently protein sequence) or regulatory region (andconsequently transcription).The frequency of alleles within a population may change as a result of evolutionary forcesthat act upon the alleles differentially.Over generations populations or species may experience different patterns ofselection 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 duplicatedgenes, known as paralogs, can diverge in sequence overmany generations,and theduplicated genes can follow manytrajectories.A paralogmight incur mutations in the promoter or coding region that prevent it from being transcribed

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5into 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 appearsspecialized for a specific cellular environment, this divergence iscalledsubfunctionalization.In some cases,a parlogcanmutate and diverge, resulting in a proteinthat does a fundamentally different function. This is calledneofunctionalization.These processes, originating early in animal evolution and operating at the level of individualcells, provide animals with physiological flexibility.16.How does genetic variation provide physiological flexibility?AnswerGenetic variation of direct relevance to physiology can arise in both coding and regulatory regions. Many genes existas duplicates and over evolutionary time, the duplicates (paralogs) diverge into genes that differ in expression patternsand/or protein structure. Some animals will express one paralog under one set of conditions,then switch to anotherwhen conditions change. This might influence how a given tissue responds to a challenge or allow the animal toproduce different forms in separate tissues (white versus red muscle) or regions of a tissue (small versus largeintestine). Within a population of a species, there may be polymorphisms of a particular gene (alleles) that affect geneexpression or protein structure. This variation may be unimportant under most circumstances but it may becomeimportant when conditions change, such that one allele is better suited than another. This could lead to naturalselection of the favorable genotype, increasing its frequency in the population.

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1Chapter 3 Chemistry, Biochemistry, and Cell PhysiologyAnswers toSynthesisQuestions1. Describe, in chemical terms, how antacids work.AnswerMost antacids are weak bases, for instance in the form of carbonates (ex. sodium bicarbonate, NaHCO3) orhydroxides(ex. magnesium hydroxide, Mg(OH)2). The reaction between a weak base and acid found in the stomach leads to theformation of a salt, water and sometimes CO2gas depending on the base (ex. NaHCO3), therefore reducing theconcentration 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 swimin the ocean? Describe these environments using the terminology of osmolarity and tonicity.AnswerFreshwater is hyposmotic and hypotonic relative to body fluids (both intra-and extracellular compartments).Therefore, bathing in fresh water causes water to enterinto 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 leaveareas 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 foranimals that experience changes in body temperature?AnswerpH is the negative log of the proton concentration. pK is the negative log of the equilibrium constant. For an acid or abase, 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 arerequired 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 andheterodimers can be formed from these six proteins?AnswerLabel the monomes A, B, C, D, E, and F.Homodimers are formed when two identical proteins join. In this case, therecould be 6 homodimers(AA,BB, CC, DD, EE, FF). Heterodimers are formed by two different proteins; in this casethere 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 theKmvalue for metabolic enzymes. For example, theconcentration of pyruvate is often close to theKmvalue for LDH. Why might this be advantageous, in terms of kineticregulation?AnswerThe value ofKmis the substrate concentration at which reaction velocity is half the maximal rate (Vmax). Whensubstrate concentration increases above theKmvalue of the metabolic enzyme, reaction rates approachVmaxandeventually will stop increasing because enzymes will reach saturation. By keeping substrate concentrations near orbelowKm, animal cells have the capacity tochangereaction rates when substrate concentrationchanges, for instanceas a result of achangein overall metabolic activity.

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26.Trace the path of a protein hormone, such as insulin, from its gene in the nucleus to secretion out of the cell.AnswerIn the nucleus, a gene, which is a segment of DNA, is transcribed into a mRNA. The mRNA transcript is processed toremove introns and moves out of the nucleus into the rough endoplasmic reticulum. Here, the mRNAistranslated byribosomes into a chain of amino acids, which will be folded with the help of chaperones and exportedviaa vesicle tothe Golgi apparatus. From the Golgi apparatus, the protein will be repackaged in a vesicle that will undergo exocytosisto 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.AnswerAn important part of the function of many cells is the ability to affect how it interacts with other cell types. Many ofthese interactions arephysical connections, mediated through cell membrane receptors and extracellular matrixproteins. Cells can control their interactions with surrounding cells by changing the types of receptors they produce, orby altering the nature of the surrounding extracellular matrix. The levels of membrane proteins and secreted proteinscan be controlled bysynthesis (transcription, translation) degradation (proteolysis) and membrane trafficking(exocytosis, endocytosis).

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1Chapter 4Cell Signaling and Endocrine RegulationAnswers to Concept Check Questions1. Compare and contrast paracrine and endocrine communication in terms of the three main steps of indirect signaling.AnswerParacrine and endocrine communication are very similar in terms ofStep 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. Inparacrine 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.AnswerHydrophobic messengers diffuse readilyacross cell membranes, because they are soluble in membrane lipids.However, because they are insoluble in aqueous solutions, theyaretypicallytransported bound to carrier proteins inthe blood.At the target cell, they can bind to extracellular receptors, or diffuse across the cell membrane and bind tointracellular receptors.Hydrophilic messengers are released from the signaling cell via exocytosisbecause their lowsolubility in lipids means that they cannot diffuse across the membrane. They travel dissolved in the blood, becausethey are readily soluble in aqueous solutions. At the target cell, they cannot cross the cell membraneandinstead mustbind 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 ofthe three main steps of indirect signaling?AnswerPeptide messengers are typically hydrophilic. They cannot pass directly through the membrane, so they are releasedby exocytosis. They dissolve in extracellular fluids and blood, so they do not need carrier proteins. They must bind totransmembrane receptors, because they cannot cross the cell membrane to interact with intracellular receptors.4. What is a preprohormone?AnswerA preprohormone is a larger protein molecule that is subsequently post-translationally modified by cleavage into thesmaller active hormone.5. What are the three main classes of steroid hormones in vertebrates?AnswerMineralocorticoids, glucocorticoids, and reproductive hormones6. Why are steroids usually bound to carrier proteins when transported in the blood?AnswerBecause they are hydrophobic (lipophilic) they do not dissolve well in aqueous solutions such as blood. Carrierproteins surround the hydrophobic messenger and isolate it from the surrounding aqueous solution.

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27. Are amines hydrophilic or hydrophobic messengers? How does this affect their release, transport, and signaling?AnswerMost amines are hydrophilic messengers (thyroid hormone is an exception). They are, therefore, usually released viaexocytosis and signal via membrane receptors.8. Outline the ways in which thyroid hormone release, transport, and signaling differ from that of other biogenic amines.AnswerUnlike the other amines, which are mostly hydrophilic, thyroid hormones are lipophilic. Thus, thyroid hormones candiffuse across the cell membrane when they are released into the blood. In the blood they are transported by a carrierprotein (which is not necessary for the other amines). They bind to intracellular receptors at the target cell, rather thanthe transmembrane receptors that are typical for other amines.9. What are eicosanoids?AnswerEicosanoids are lipid chemical messengers derived from fatty acids (typically arachidonic acid). They can function asneurotransmitters and paracrines.10. Name three gaseous signaling molecules, and one function that they share.AnswerCarbon monoxide, hydrogen sulfide,andnitric 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?AnswerCells 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 maintainhomeostasis?AnswerUp-regulation involves an increase in the number of active receptors, while down-regulation involves a decrease in thenumber of active receptors. Up-regulation may occur when a cell is exposed to a receptor antagonist (something thatblocks 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 helpskeep 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 theregulation of thousands of genes. Explain how this can be the case.AnswerHydrophobic messengers interact with intracellular receptors, activating them sothattheyfunction as transcriptionfactors.The activated receptor binds to a hormone-responsive element within the promoter of the target genes. Thesimplest way for a particular messenger to cause changes in a large number of genes would be to have a hormoneresponsive element for that messenger present in a very large number of genes in the genome.Another possible wayfor a single messenger to affect the transcription of many genes would be for the messenger to activate a

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3transcriptional cascade (as in Figure 4.20). If a messenger activates a gene that encodes a transcription factor, thatgene 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 responsesto a hydrophobic ligand?AnswerNongenomic responses occur when a hydrophobic ligand binds to a membrane-bound receptor and causes a signal inthe 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?AnswerThe primary response is opening or closing of the ion channel, resulting in a change in the membrane potential or achange 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.AnswerGenerally, signaling via a ligand-gated ion channel will be faster than signaling via an intracellular receptor becausechanges in membrane potential (which are the result of signal transduction via a ligand-gated ion channel) are nearlyinstantaneous. In contrast, intracellular receptors function as transcription factors, so they must first alter genetranscription, which is then followed by a change in translation, and thus a change in the amount of a protein, whichultimately has an effect on the target cell. Although these processescan be fairly rapid, they are not instantaneous, andsometimes they can be quite slow.17. What are the three main parts of a receptor-enzyme?AnswerThey contain an extracellular ligand-binding domain, a transmembrane domain, and an intracellular catalytic domain.18. How do receptor-enzymes amplify incoming signals?AnswerThe catalytic domain catalyzes a phosphorylation reaction that post-translationally modifies the target proteins byadding a phosphate group. A single activated receptor-enzyme can phosphorylate many target proteins, thusamplifying the signal.19. Outline the five main steps in Gprotein–coupled receptor signaling.Answer(1) Ligand bindsto the receptor.(2) The receptor signals to the Gprotein to release GDP and bind GTP.(3) Theactivated G protein interacts with an amplifier enzyme, activating it.(4) The amplifier enzyme converts a secondmessenger from an inactive to an active form.(5) The activated second messenger activates or inhibits cellularpathways.

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420. What is a second messenger?AnswerSecond messengers are small diffusible signaling molecules that are activated by transmembrane receptors (such as Gprotein–coupled receptors). Some second messengers are hydrophilic and work in the cytoplasm (such as Ca2+andcAMP), 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?AnswerNegative feedback is where the product inhibits further production of the stimulus. Positive feedback is where theproduct 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?AnswerAntagonistic 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 exertrapid andprecise control over the relevantphysiological function.23. Provide an example of a hormone controlled by athird-order endocrine pathway, and outline each step in the regulatorycascade.AnswerManyof the hormonessecreted 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 bytheneurohormone corticotropin-releasing hormone, which is releasedfrom the hypothalamus. The ACTH thencauses therelease of glucocorticoid hormones from the adrenalglands into the circulation.Thus this pathway is calledthehypothalamicpituitaryadrenal(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?AnswerInvertebrate endocrine systems rely heavily on neurohormones, whereas vertebrate endocrine systems have manyspecialized endocrine glands that produce hormones.25. How have gene duplications played a role in the evolution of the vertebrate endocrine system? Support your answer with atleast two examples.AnswerGene duplications have been important in the evolution of many vertebrate hormones and their receptors. Vasopressinand oxytocin provide an example of gene duplication. Only a single gene is present in the jawless fish, while jawedvertebrates have two. The glucocorticoid and mineralocorticoid receptors provide another example of a pair of genethat arose during the whole-genome duplication leading to the vertebrates. In this case, there has also been a shift inthe ligands binding to these receptors between fish and tetrapods. Prolactin and growth hormone provide a thirdexample.

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