Bio Test 2 Questions

1. How do action potentials and graded potentials differ?

1. A graded potential has a magnitude that varies with stimulus strength, whereas an action potential has an all-or-none magnitude that is independent of stimulus strength.

1. What is a protein kinase, and what is its role in a sig- nal transduction pathway?

1. A protein kinase is an enzyme that transfers a phosphate group from ATP to a protein, usually activating that protein (often a second type of protein kinase). Many signal transduction pathways include a series of such interactions, in which each phosphorylated protein kinase in turn phosphorylates the next protein ki- nase in the series. Such phosphorylation cascades carry a signal from outside the cell to the cellular protein(s) that will carry out the response.

1. Many spontaneous reactions occur very slowly. Why don't all spontaneous reactions occur instantly?

1. A spontaneous reaction is a reaction that is exergonic. However, if it has a high activation energy that is rarely attained, the rate of the reaction may be low.

1. How does ATP typically transfer energy from exer- gonic to endergonic reactions in the cell?

1. ATP usually transfers energy to endergonic processes by phosphorylating (adding phosphate groups to) other molecules. (Exergonic processes phosphorylate ADP to regenerate ATP.)

2. Why do enzymes act only on very specific substrates?

2. Only the specific substrate(s) will fit properly into the active site of an enzyme, the part of the enzyme that carries out catalysis.

1. How can a target cell's response to a single hormone molecule result in a response that affects a million other molecules?

1. At each step in a cascade of sequential activations, one molecule or ion may activate numerous molecules functioning in the next step.

How do you think a cell performing cellular respira- tion rids itself of the resulting CO2?

1. CO2 is a nonpolar molecule that can diffuse through the plasma membrane. As long as it diffuses away so that the concentration remains low outside the cell, it will continue to exit the cell in this way. (This is the opposite of the case for O2, described in this section.)

Cellular respiration uses glucose and oxygen, which have high levels of free energy, and releases CO2 and water, which have low levels of free energy. Is cellular respiration spontaneous or not? Is it exergonic or en- dergonic? What happens to the energy released from glucose?

1. Cellular respiration is a spontaneous and exergonic process. The energy re- leased from glucose is used to do work in the cell or is lost as heat.

1. Give an example of apoptosis during embryonic de- velopment, and explain its function in the develop- ing embryo.

1. In formation of the hand or paw in mammals, cells in the regions between the digits are programmed to undergo apoptosis. This serves to shape the digits of the hand or paw so that they are not webbed.

1. Under what circumstances could ions flow through ion channels from regions of low ion concentration to regions of high ion concentration?

1. Ions can flow against a chemical concentration gradient if there is an opposing electrical gradient of greater magnitude.

1. How is it possible for a particular neurotransmitter to produce opposite effects in different tissues?

1. It can bind to different types of receptors, each triggering a specific response in postsynaptic cells.

1. Nerve growth factor (NGF) is a water-soluble signal- ing molecule. Would you expect the receptor for NGF to be intracellular or in the plasma membrane? Why?

1. NGF is water-soluble (hydrophilic), so it cannot pass through the lipid mem- brane to reach intracellular receptors, as steroid hormones can. Therefore, you'd expect the NGF receptor to be in the plasma membrane—which is, in fact, the case.

Two molecules that can cross a lipid bilayer without help from membrane proteins are O2 and CO2. What property allows this to occur?

1. O2 and CO2 are both nonpolar molecules that can easily pass through the hy- drophobic interior of a membrane.

1. Describe the basic pathway of information flow through neurons that causes you to turn your head when someone calls your name.

1. Sensors in your ear transmit information to your brain. There the activity of in- terneurons in processing centers enables you to recognize your name. In response, sig- nals transmitted via motor neurons cause contraction of muscles that turn your neck.

1. How do an activator and an inhibitor have different effects on an allosterically regulated enzyme?

1. The activator binds in such a way that it stabilizes the active form of an en- zyme, whereas the inhibitor stabilizes the inactive form.

Sodium-potassium pumps help nerve cells establish a voltage across their plasma membranes. Do these pumps use ATP or produce ATP? Explain.

1. The pump uses ATP. To establish a voltage, ions have to be pumped against their gradients, which requires energy.

How does the second law of ther- modynamics help explain the diffusion of a substance across a membrane? See Figure 7.13 on page 132.

1. The second law is the trend toward randomization, or increasing entropy. When the concentrations of a substance on both sides of a membrane are equal, the distribution is more random than when they are unequal. Diffusion of a sub- stance to a region where it is initially less concentrated increases entropy, mak- ing it an energetically favorable (spontaneous) process as described by the second law. This explains the process seen in Figure 7.13.

1. Explain how signaling is involved in ensuring that yeast cells fuse only with cells of the opposite mating type.

1. The two cells of opposite mating type (a and ) each secrete a certain signaling molecule, which can only be bound by receptors carried on cells of the opposite mat- ing type. Thus, the a mating factor cannot bind to another a cell and cause it to grow toward the first a cell. Only an cell can "receive" the signaling molecule and respond by directed growth (see Figure 11.17 for more information).

The carbohydrates attached to some proteins and lipids of the plasma membrane are added as the membrane is made and refined in the ER and Golgi apparatus. The new membrane then forms transport vesicles that travel to the cell surface. On which side of the vesicle membrane are the carbohydrates?

1. They are on the inner side of the transport vesicle membrane.

Chapter 7 answer key

1.b 2.c 3.a 4.d 5.b

Chapter 8 answer key

1.b 2.c 3.b 4.a 5.c 6.e 7.c

Chapter 11 answer key

1.c 2.d 3.a 4.b 5.a 6.d 7.c 8.c

Chapter 48 answer key

1.c 2.c 3.c 4.e 5.b 6.a

2. WHAT IF? How might increased branching of an axon help coordinate responses to signals communi- cated by the nervous system?

2. Increased branching would allow control of a greater number of postsynaptic cells, enhancing coordination of responses to nervous system signals.

2. When a signal transduction pathway involves a phos- phorylation cascade, how does the cell's response get turned off?

2. Protein phos- phatases reverse the effects of the kinases.

2. In multiple sclerosis (from the Greek skleros, hard), myelin sheaths harden and deteriorate. How would this affect nervous system function?

2. Loss of the insulation provided by myelin sheaths leads to a disrup- tion of action potential propagation along axons. Voltage-gated sodium channels are restricted to the nodes of Ranvier, and without the insulating effect of myelin, the inward current produced at one node during an action potential cannot depolarize the membrane to the threshold at the next node.

2. WHAT IF? Suppose a cell's membrane potential shifts from 70 mV to 50 mV. What changes in the cell's permeability to K or Na could cause such a shift?

2. A decrease in permeability to K, an in- crease in permeability to Na, or both

2. Which of the following combinations has more free energy: glutamic acid ammonia ATP, or gluta- mine ADP P i? Explain your answer.

2. A set of coupled reactions can transform the first combination into the second. Since this is an exergonic process overall, ΔG is negative and the first combination must have more free energy (see Figure 8.9).

2. WHAT IF? Imagine you are a pharmacological re- searcher who wants to design a drug that inhibits a particular enzyme. Upon reading the scientific litera- ture, you find that the enzyme's active site is similar to that of several other enzymes. What might be a good approach to developing your inhibitor drug?

2. An inhibitor that binds to the active site of the enzyme you want to inhibit could also bind to and block the enzymes with similar structures, causing significant side effects. For this reason, you would be better off choosing to screen chemical compounds that bind allosterically to the enzyme in question, because allosteric regulatory sites are less likely to share similarity with other enzymes.

Explain why the sodium-potassium pump in Figure 7.18 would not be considered a cotransporter.

2. Each ion is being transported against its electrochemical gradient. If either ion were transported down its electrochemi- cal gradient, this would be considered cotransport.

2. WHAT IF? What types of protein defects could re- sult in apoptosis occurring when it should not? What types could result in apoptosis not occurring when it should?

2. If a receptor protein for a death- signaling molecule were defective such that it was activated even in the absence of the death signal, this would lead to apoptosis when it wouldn't normally oc- cur. Similar defects in any of the proteins in the signaling pathway, which would activate these relay or response proteins in the absence of interaction with the previous protein or second messenger in the pathway, would have the same ef- fect. Conversely, if any protein in the pathway were defective in its ability to re- spond to an interaction with an early protein or other molecule or ion, apoptosis would not occur when it normally should. For example, a receptor protein for a death-signaling ligand might not be able to be activated, even when ligand was bound. This would stop the signal from being transduced into the cell.

2. WHAT IF? If two cells have different scaffolding proteins, explain how they might behave differently in response to the same signaling molecule.

2. Scaffolding pro- teins hold molecular components of signaling pathways in a complex with each other. Different scaffolding proteins would assemble different collections of pro- teins, leading to different cellular responses in the two cells.

Describe the forms of energy found in an apple as it grows on a tree, then falls, then is digested by some- one who eats it.

2. The apple has po- tential energy in its position hanging on the tree, and the sugars and other nu- trients it contains have chemical energy. The apple has kinetic energy as it falls from the tree to the ground. Finally, when the apple is digested and its mole- cules broken down, some of the chemical energy is used to do work, and the rest is lost as thermal energy.

2. WHAT IF? What would the effect be if a cell made defective receptor tyrosine kinase proteins that were unable to dimerize?

2. The cell with the faulty receptor would not be able to respond appropri- ately to the signaling molecule when it was present. This would most likely have dire consequences for the cell, since regulation of the cell's activities by this re- ceptor would not occur appropriately.

the soil immediately around hot springs is much warmer than that in neighboring regions. Two closely related species of native grasses are found, one in the warmer region and one in the cooler re- gion. If you analyzed their membrane lipid composi- tions, what would you expect to find? Explain.

2. The grasses liv- ing in the cooler region would be expected to have more unsaturated fatty acids in their membranes because those fatty acids remain fluid at lower temperatures. The grasses living immediately adjacent to the hot springs would be expected to have more saturated fatty acids, which would allow the fatty acids to "stack" more closely, making the membranes less fluid and therefore helping them to stay intact at higher temperatures. (Cholesterol could not be used to moderate the effects of temperature on membrane fluidity because it is not found within plant cell membranes.)

2. Explain how nerve cells provide examples of both local and long-distance signaling.

2. The secre- tion of neurotransmitter molecules at a synapse is an example of local signaling. The electrical signal that travels along a very long nerve cell and is passed to the next nerve cell can be considered an example of long-distance signaling. (Note, however, that local signaling at the synapse between two cells is necessary for the signal to pass from one cell to the next.)

In the supermarket, produce is often sprayed with water. Explain why this makes vegetables look crisp.

2. The water is hypotonic to the plant cells, so the plant cells take up water. Thus, the cells of the vegetable remain turgid rather than plasmolyzing, and the vegetable (for example, lettuce or spinach) re- mains crisp and not wilted.

2. Organophosphate pesticides work by inhibiting acetyl- cholinesterase, the enzyme that breaks down the neuro- transmitter acetylcholine. Explain how these toxins would affect EPSPs produced by acetylcholine.

2. These toxins would prolong the EPSPs that acetyl- choline produces because the neurotransmitter would remain longer in the synaptic cleft.

2. Why is a transport protein needed to move water mole- cules rapidly and in large quantities across a membrane?

2. Water is a polar molecule, so it cannot pass very rapidly through the hydrophobic region in the middle of a phospholipid bi- layer.

s you saw in Figure 7.20 on page 137, a key process in metabolism is the trans- port of hydrogen ions (H) across a membrane to cre- ate a concentration gradient. Other processes can result in an equal concentration of H on each side. Which situation allows the H to perform work in this system? How is the answer consistent with what is shown in regard to energy in Figure 7.20?

2. When the H concentrations are the same, the system is at equilibrium and can do no work. Hydrogen ions can perform work only if their concentrations on each side of a membrane differ—in other words, when a gradient is present. This is consis- tent with Figure 7.20, which shows that an energy input (provided by ATP hy- drolysis) is required to establish the concentration gradient (the H gradient) that can in turn perform work.

3. MAKE CONNECTIONS How is ligand binding similar to the process of allosteric regulation of enzymes? See Figure 8.19 on page 158.

3. Binding of a ligand to a receptor changes the shape of the receptor, altering the ability of the receptor to transmit a signal. Binding of an allosteric regulator to an enzyme changes the shape of the enzyme, either promoting or inhibiting enzyme activity.

3. MAKE CONNECTIONS Consider how communication occurs in a colony of bacteria (see Figure 11.3, p. 207). In what general ways is that communication similar to and different from transmission of a nerve impulse by a neuron?

3. Communication by bacteria involves all the cells in a colony, whereas communication by neurons in- volves just a few cells in the animal body. In addition, neurons direct signals from one location to another, whereas bacterial cells communicate in all directions.

3. WHAT IF? When epinephrine is mixed with glyco- gen phosphorylase and glycogen in a test tube, is glu- cose 1-phosphate generated? Why or why not?

3. Glucose 1-phosphate is not generated, because the activation of the enzyme requires an intact cell, with an intact receptor in the membrane and an intact signal transduction pathway. The enzyme cannot be acti- vated directly by interaction with the signaling molecule in the test tube.

3. MAKE CONNECTIONS Review the discussion of pro- tein phosphatases on page 216, and see Figure 11.10 on page 215. Some human diseases are associated with malfunctioning protein phosphatases. How would such proteins affect signaling pathways?

3. A malfunction- ing protein phosphatase would not be able to dephosphorylate a particular receptor or relay protein. As a result, the signaling pathway, once activated, would not be able to be terminated. (In fact, one study found altered protein phosphatases in cells from 25% of colorectal tumors.)

3. MAKE CONNECTIONS Considering what you learned in Concepts 7.3 and 7.4 (pp. 134-136), does Figure 8.10a show passive or active transport? Explain.

3. Active transport: The solute is being transported against its concentration gradient, which requires energy, provided by ATP hydrolysis.

3. WHAT IF? Malonate is an inhibitor of the enzyme succinate dehydrogenase. How would you determine whether malonate is a competitive or noncompetitive inhibitor?

3. In the presence of malonate, increase the concentration of the normal substrate (succinate) and see whether the rate of reaction increases. If it does, malonate is a competitive in- hibitor.

3. WHAT IF? If a drug mimicked the activity of GABA in the CNS, what general effect on behavior might you expect? Explain.

3. Since GABA is an inhibitory neurotransmitter in the CNS, this drug would be expected to decrease brain activity. A decrease in brain activity might be expected to slow down or reduce behavioral activity. Many sedative drugs act in this fashion.

If a Paramecium caudatum swims from a hypotonic to an isotonic environment, will its con- tractile vacuole become more active or less? Why

3. The activity of Paramecium caudatum's contrac- tile vacuole will decrease. The vacuole pumps out excess water that accumulates in the cell; this accumulation occurs only in a hypotonic environment.

3. WHAT IF? Ouabain, a plant substance used in some cultures to poison hunting arrows, disables the sodium-potassium pump. What change in the resting potential would you expect to see if you treated a neuron with ouabain? Explain.

3. The activity of the sodium-potassium pump is essential to maintain the resting potential. With the pump inactivated, the sodium and potassium concentration gradients would gradually disappear, resulting in a greatly reduced resting potential.

3. MAKE CONNECTIONS Aquaporins exclude passage of hydronium ions (H3O; see pp. 52-53). Recent re- search on fat metabolism has shown that some aqua- porins allow passage of glycerol, a three-carbon alcohol (see Figure 5.10, p. 75), as well as H2O. Since H3O is much closer in size to water than is glycerol, what do you suppose is the basis of this selectivity?

3. The hydronium ion is charged, while glycerol is not. Charge is probably more significant than size as a basis for exclusion by the aquaporin channel.

Review the characteristics of the lysosome in Concept 6.4 (pp. 106-107). Given the in- ternal environment of a lysosome, what transport protein might you expect to see in its membrane

3. The internal environment of a lysosome is acidic, so it has a higher concentration of H than does the cyto- plasm. Therefore, you might expect the membrane of the lysosome to have a pro- ton pump such as that shown in Figure 7.20 to pump H into the lysosome.

3. WHAT IF? Suppose a mutation caused gated sodium channels to remain inactivated longer after an action potential. How would this affect the frequency at which action potentials could be generated? Explain.

3. The maxi- mum frequency would decrease because the refractory period would be extended.

Some night-time partygoers wear glow- in-the-dark necklaces. The necklaces start glowing once they are "activated," which usually involves snapping the necklace in a way that allows two chemicals to react and emit light in the form of chemiluminescence. Is the chemical reaction exer- gonic or endergonic? Explain your answer.

3. The reaction is exergonic because it releases en- ergy—in this case, in the form of light. (This is a nonbiological version of the bio- luminescence seen in Figure 8.1.)

3. What is the actual "signal" that is being transduced in any signal transduction pathway, such as those shown in Figures 11.6 and 11.10? In what way is this information being passed from the exterior to the in- terior of the cell?

3. The signal that is being transduced is the information that a signaling molecule is bound to the cell-surface receptor. Information is transduced by way of sequential protein-protein interactions that change protein shapes, causing them to function in a way that passes the signal along.

If you place a teaspoon of sugar in the bottom of a glass of water, it will dissolve completely over time. Left longer, eventually the water will disap- pear and the sugar crystals will reappear. Explain these observations in terms of entropy.

3. The sugar crystals become less ordered (entropy in- creases) as they dissolve and become randomly spread out in the water. Over time, the water evaporates, and the crystals form again because the water vol-ume is insufficient to keep them in solution. While the reappearance of sugar crystals may represent a "spontaneous" increase in order (decrease in entropy), it is balanced by the decrease in order (increase in entropy) of the water molecules, which changed from a relatively compact arrangement as liquid water to a much more dispersed and disordered form as water vapor.

4. MAKE CONNECTIONS Figure 7.13, on page 132, illus- trates diffusion by dye molecules. Could diffusion eliminate the concentration gradient of a dye that has a net charge? Explain.

4. Charged dye molecules could equilibrate only if other charged molecules could also cross the membrane. If not, a membrane potential would develop that would counterbalance the chemical gradient.

4. In liver cells, glycogen phosphorylase acts in which of the three stages of the signaling pathway associ- ated with an epinephrine-initiated signal?

4. Glyco- gen phosphorylase acts in the third stage, the response to epinephrine signaling.

4. MAKE CONNECTIONS In nature, what conditions could lead to natural selection favoring bacteria with enzymes that could break down the fucose-containing disaccharide discussed above? See the discussion of natural selection in Concept 1.2, pages 14-16.

4. If lactose wasn't present in the environment as a source of food and the fucose-containing disaccharide was available, bacteria that could digest the latter would be better able to grow and multiply than those that could not.

4. MAKE CONNECTIONS A change in the concentration of calcium ions is important for fertilization in sea urchins and other animals (see Figure 47.3, on p. 1023). What membrane activity is common to fertilization and neurotransmitter release?

4. Membrane fusion

4. WHAT IF? Upon activation of phospholipase C by the binding of a ligand to a receptor, what effect does the IP3-gated calcium channel have on Ca2 concen- tration in the cytosol?

4. The IP3-gated channel opens, allowing calcium ions to flow out of the ER, which raises the cytosolic Ca2 concentration.

4. Which of the following processes includes all others?

a. osmosis b. diffusion of a solute across a membrane c. facilitated diffusion d. passive transport e. transport of an ion down its electrochemical gradient

2. Binding of a signaling molecule to which type of receptor leads directly to a change in the distribution of ions on opposite sides of the membrane?

a. receptor tyrosine kinase b. G protein-coupled receptor c. phosphorylated receptor tyrosine kinase dimer d. ligand-gated ion channel e. intracellular receptor

8. Protein phosphorylation is commonly involved with all of the following except

a. regulation of transcription by extracellular signaling molecules. b. enzyme activation. c. activation of G protein-coupled receptors. d. activation of receptor tyrosine kinases. e. activation of protein kinase molecules.

1. Phosphorylation cascades involving a series of protein kinases are useful for cellular signal transduction because

a. they are species specific. b. they always lead to the same cellular response. c. they amplify the original signal manyfold. d. they counter the harmful effects of phosphatases. e. the number of molecules used is small and fixed

2. According to the fluid mosaic model of membrane structure, proteins of the membrane are mostly

a. spread in a continuous layer over the inner and outer sur- faces of the membrane. b. confined to the hydrophobic interior of the membrane. c. embedded in a lipid bilayer. d. randomly oriented in the membrane, with no fixed inside- outside polarity. e. free to depart from the fluid membrane and dissolve in the surrounding solution.

3. Where are neurotransmitter receptors located?

a. the nuclear membrane b. the nodes of Ranvier c. the postsynaptic membrane d. synaptic vesicle membranes e. the myelin sheath

5. Some bacteria are metabolically active in hot springs because

a. they are able to maintain a lower internal temperature. b. high temperatures make catalysis unnecessary. c. their enzymes have high optimal temperatures. d. their enzymes are completely insensitive to temperature. e. they use molecules other than proteins or RNAs as their main catalysts.

7. Which observation suggested to Sutherland the involve- ment of a second messenger in epinephrine's effect on liver cells?

a. Enzymatic activity was proportional to the amount of calcium added to a cell-free extract. b. Receptor studies indicated that epinephrine was a ligand. c. Glycogen breakdown was observed only when epinephrine was administered to intact cells. d. Glycogen breakdown was observed when epinephrine and glycogen phosphorylase were combined. e. Epinephrine was known to have different effects on differ- ent types of cells.

As a cell grows, its plasma membrane expands. Does this involve endocytosis or exocytosis? Explain.

Exocytosis. When a transport vesicle fuses with the plasma membrane, the vesicle membrane becomes part of the plasma membrane.

In Concept 6.7 (pp. 119-120), you learned that animal cells make an extracellular matrix (ECM). Describe the cellular pathway of syn- thesis and deposition of an ECM glycoprotein.

The glycoprotein would be synthesized in the ER lumen, move through the Golgi apparatus, and then travel in a vesicle to the plasma membrane, where it would undergo exocytosis and become part of the ECM.

3. Which of the following metabolic processes can occur with- out a net influx of energy from some other process?

a. ADP+ Pi =ATP+H2O b. C6H12O6 +6O2 =6CO2 + 6H2O c. 6CO2 + 6H2O= C6H12O6 + 6O2 d. amino acids=protein e. glucose + fructose = sucrose

6. If an enzyme is added to a solution where its substrate and product are in equilibrium, what will occur?

a. Additional product will be formed. b. Additional substrate will be formed. c. The reaction will change from endergonic to exergonic. d. The free energy of the system will change. e. Nothing; the reaction will stay at equilibrium.

1. In what way do the membranes of a eukaryotic cell vary?

a. Phospholipids are found only in certain membranes. b. Certain proteins are unique to each membrane. c. Only certain membranes of the cell are selectively permeable. d. Only certain membranes are constructed from amphi- pathic molecules. e. Some membranes have hydrophobic surfaces exposed to the cytoplasm, while others have hydrophilic surfaces facing the cytoplasm.

5. Why are action potentials usually conducted in one direction?

a. The nodes of Ranvier conduct potentials in one direction. b. The brief refractory period prevents reopening of voltage- gated Na channels. c. The axon hillock has a higher membrane potential than the terminals of the axon. d. Ions can flow along the axon in only one direction. e. Voltage-gated channels for both Na and K open in only one direction

1. What happens when a resting neuron's membrane depolarizes?

a. There is a net diffusion of Na out of the cell. b. The equilibrium potential for K (EK) becomes more positive. c. The neuron's membrane voltage becomes more positive. d. The neuron is less likely to generate an action potential. e. The cell's inside is more negative than the outside.

3. Which of the following factors would tend to increase mem- brane fluidity?

a. a greater proportion of unsaturated phospholipids b. a greater proportion of saturated phospholipids c. a lower temperature d. a relatively high protein content in the membrane e. a greater proportion of relatively large glycolipids com- pared with lipids having smaller molecular masses

4. If an enzyme in solution is saturated with substrate, the most effective way to obtain a faster yield of products is to

a. add more of the enzyme. b. heat the solution to 90°C. c. add more substrate. d. add an allosteric inhibitor. e. add a noncompetitive inhibitor.

4. Temporal summation always involves

a. both inhibitory and excitatory inputs. b. synapses at more than one site. c. inputs that are not simultaneous. d. electrical synapses. e. multiple inputs at a single synapse.

5. Consider this pathway: epinephrine → G protein-coupled receptor → G protein → adenylyl cyclase → cAMP. Identify the second messenger.

a. cAMP b. G protein c. GTP d. adenylyl cyclase e. G protein-coupled receptor

2. A common feature of action potentials is that they

a. cause the membrane to hyperpolarize and then depolarize. b. can undergo temporal and spatial summation. c. are triggered by a depolarization that reaches the threshold. d. move at the same speed along all axons. e. require the diffusion of Na and K through ligand-gated channels to propagate.

5. Based on Figure 7.21, which of these experimental treatments would increase the rate of sucrose transport into the cell?

a. decreasing extracellular sucrose concentration b. decreasing extracellular pH c. decreasing cytoplasmic pH d.adding an inhibitor that blocks the regeneration of ATP e. adding a substance that makes the membrane more perme- able to hydrogen ions

3. The activation of receptor tyrosine kinases is character- ized by

a. dimerization and phosphorylation. b. dimerization and IP3 binding. c. a phosphorylation cascade. d. GTP hydrolysis. e. channel protein shape change.

1. Choose the pair of terms that correctly completes this sen- tence: Catabolism is to anabolism as _______ is to _______.

a. exergonic; spontaneous b. exergonic; endergonic c. free energy; entropy d. work; energy e. entropy; enthalpy

6. Apoptosis involves all but which of the following?

a. fragmentation of the DNA b. cell-signaling pathways c. activation of cellular enzymes d. lysis of the cell e. digestion of cellular contents by scavenger cells

2. Most cells cannot harness heat to perform work because

a. heat is not a form of energy. b. cells do not have much heat; they are relatively cool. c. temperature is usually uniform throughout a cell. d. heat can never be used to do work. e. heat must remain constant during work.

4. Lipid-soluble signaling molecules, such as testosterone, cross the membranes of all cells but affect only target cells because

a. only target cells retain the appropriate DNA segments. b. intracellular receptors are present only in target cells. c. most cells lack the Y chromosome required. d. only target cells possess the cytosolic enzymes that trans- duce the testosterone. e. only in target cells is testosterone able to initiate the phosphorylation cascade leading to activated transcription factor.