Test 3 Chapter 14

In stage 1 of photosynthesis, a proton gradient is generated and ATP is synthesized. Where do protons become concentrated in the chloroplast? (a) thylakoid space (b) stroma (c) inner membrane (d) thylakoid membrane

(a)

Which of the following statements describes the phosphorylation event that occurs during the process known as oxidative phosphorylation? (a) A phosphate group is added to ADP. (b) ATP is hydrolyzed in order to add phosphate groups to protein substrates. (c) A phosphate group is added to molecular oxygen. (d) Inorganic phosphate is transported into the mitochondrial matrix, increasing the local phosphate concentration.

(a)

Cytochrome c oxidase is an enzyme complex that uses metal ions to help coordinate the transfer of four electrons to O2. Which metal atoms are found in the active site of this complex? (a) two iron atoms (b) one iron atom and one copper atom (c) one iron atom and one zinc atom (d) one zinc atom and one copper atom

(b)

In oxidative phosphorylation, ATP production is coupled to the events in the electron-transport chain. What is accomplished in the final electron-transfer event in the electron-transport chain? (a) OH- is oxidized to O2 (b) pyruvate is oxidized to CO2 (c) O2 is reduced to H2O (d) NAD+ is reduced to NADH

(c)

Oxidative phosphorylation, as it occurs in modern eukaryotes, is a complex process that probably arose in simple stages in primitive bacteria. Which mechanism is proposed to have arisen first as this complex system evolved? (a) electron transfers coupled to a proton pump (b) the reaction of oxygen with an ancestor of cytochrome c oxidase (c) ATP-driven proton pumps (d) the generation of ATP from the energy of a proton gradient

(c)

Which of the following statements describes the mitochondrial intermembrane space? (a) It is permeable to molecules with molecular mass as high as 5000 daltons. (b) It contains transporters for ATP molecules. (c) It contains proteins that are released during apoptosis. (d) It contains enzymes required for the oxidation of fatty acids.

(c)

Stage 1 of oxidative phosphorylation requires the movement of electrons along the electron-transport chain coupled to the pumping of protons into the intermembrane space. What is the final result of these electron transfers? (a) OH- is oxidized to O2 (b) pyruvate is oxidized to CO2 (c) O2 is reduced to H2O (d) H- is converted to H2

(c) Contrary to what the term "oxidative phosphorylation" may imply, the phosphorylation event does not depend on an oxidative reaction, but rather on the reduction of molecular oxygen, converting it to water.

If you add a compound to illuminated chloroplasts that inhibits the NADP+ reductase, NADPH generation ceases, as expected. However, ferredoxin does not accumulate in the reduced form because it is able to donate its electrons not only to NADP+ (via NADP+ reductase) but also back to the cytochrome b6-f complex. Thus, in the presence of the compound, a "cyclic" form of photosynthesis occurs in which electrons flow in a circle from ferredoxin, to the cytochrome b6-f complex, to plastocyanin, to photosystem I, to ferredoxin. What will happen if you now also inhibit photosystem II? (a) Less ATP will be generated per photon absorbed. (b) ATP synthesis will cease. (c) Plastoquinone will accumulate in the oxidized form. (d) Plastocyanin will accumulate in the oxidized form.

(c) If you now inhibit photosystem II, you will deprive plastoquinone, which can still donate its electrons to the cytochrome b6-f complex, of an electron source. Hence, plastoquinone will accumulate in its oxidized form. In contrast, all of the other components downstream of plastoquinone will be able to cycle between their oxidized and reduced states. ATP synthesis will continue, because electrons are still being fed through the cytochrome b6-f complex, and the same amount of ATP will be generated.

Which of the phylogenetic trees in Figure Q14-71 is the most accurate? (The mitochondria and chloroplasts are from maize, but they are treated as independent "organisms" for the purposes of this question.) Figure Q14-71

(c) Mitochondria are most closely related to Bacillus, and chloroplasts to cyanobacteria. Maize (a eukaryote) is more closely related to Giardia (a simple eukaryote) than it is to bacteria (prokaryotes).

The ATP synthase found in chloroplasts is structurally similar to the ATP synthase in mitochondria. Given that ATP is being synthesized in the stroma, where will the F0 portion of the ATP synthase be located? (a) thylakoid space (b) stroma (c) inner membrane (d) thylakoid membrane

(d)

Which component of the electron-transport chain is required to combine the pair of electrons with molecular oxygen? (a) cytochrome c (b) cytochrome b-c1 complex (c) ubiquinone (d) cytochrome c oxidase

(d)

Which of the following statements describes the mitochondrial matrix? (a) It is permeable to molecules with molecular mass as high as 5000 daltons. (b) It contains transporters for ATP molecules. (c) It contains proteins that are released during apoptosis. (d) It contains enzymes required for the oxidation of fatty acids.

(d)

Experimental evidence supporting the chemiosmotic hypothesis was gathered by using artificial vesicles containing a protein that can pump protons in one direction across the vesicle membrane to create a proton gradient. Which protein was used to generate the gradient in a highly controlled manner? (a) cytochrome c oxidase (b) NADH dehydrogenase (c) cytochrome c (d) bacteriorhodopsin

(d) Bacteriorhodopsin is a transmembrane protein that pumps protons across the membrane when exposed to light. The other proteins pump protons but are part of the electron-transport chain, so they would not be good options to show that it is only the proton gradient that is required in this system rather than any specific intermediate in the electron-transport chain.

Which of the following is not part of the process known as oxidative phosphorylation? (a) Molecular oxygen serves as a final electron acceptor. (b) FADH2 and NADH become oxidized as they transfer a pair of electrons to the electron-transport chain. (c) The electron carriers in the electron-transport chain toggle between reduced and oxidized states as electrons are passed along. (d) ATP molecules are produced in the cytosol as glucose is converted into pyruvate.

(d) During glycolysis, ATP generation occurs simultaneously with two of the key reactions that convert glucose to pyruvate in the cytosol. This production of ATP in the absence of oxygen is not part of oxidative phosphorylation; it is referred to as substrate- level phosphorylation.

Electron-transfer reactions occur rapidly. Which of the following statements best describes how the diffusion of ubiquinone is controlled in order to ensure its proximity to the other enzyme complexes? (a) Ubiquinone is anchored directly in the inner mitochondrial membrane via its hydrocarbon tail, and can only diffuse laterally. (b) Ubiquinone is present at high concentrations, minimizing the impact of diffusion on the electron-transport chain. (c) Ubiquinone becomes covalently attached to the other enzyme complexes. (d) The intermembrane space in the mitochondrion is relatively small, and therefore the random diffusion of these molecules is not a problem.

(a)

Which of the following statements describes the mitochondrial outer membrane? (a) It is permeable to molecules with molecular mass as high as 5000 daltons. (b) It contains transporters for ATP molecules. (c) It contains proteins that are released during apoptosis. (d) It contains enzymes required for the oxidation of fatty acids.

(a)

In which of the four compartments of a mitochondrion are each of the following located? A. porin B. the mitochondrial genome C. citric acid cycle enzymes D. proteins of the electron-transport chain E. ATP synthase F. membrane transport protein for pyruvate

A. Porin is in the outer membrane. B. The mitochondrial genome is in the matrix. C. The citric acid cycle enzymes are in the matrix. D. The proteins of the electron-transport chain are in the inner membrane. E. ATP synthase is in the inner membrane. F. The transport protein for pyruvate is in the inner membrane.

Based upon what you know about metabolism, explain how electrons are stripped from food molecules and used to drive the electron-transport chain.

Food molecules are ultimately converted into acetyl CoA,. Electrons removed during the generation of acetyl CoA are added to the cofactors NAD+ and FAD to generate the reduced cofactors NADH and FADH2, respectively. The two carbon atoms in the acetyl group of acetyl CoA are then fed into the citric acid cycle, where they are oxidized to two molecules of CO2. The electrons removed during this oxidation are also captured by the activated carriers NADH and FADH2. The high-energy electrons in all of these activated carriers, which derived from carbons that were formerly part of food molecules are now transferred to the proteins in the electron-transport chain.

Explain how scientists used artificial vesicles to prove that the generation of ATP by the ATP synthase was not powered by a single high-energy intermediate but rather by a proton gradient. Be sure to describe the two experiments that were negative controls (no ATP generated), the positive control (ATP generated as expected), and a fourth experiment proving that the gradient is the required energy source.

In all the experiments, artificial liposomes were generated and exposed to light, and the surrounding solution was checked for an increase in ATP. In the first experiment, the liposomal membranes contained only bacteriorhodopsin, a bacterial protein that pumps protons and is activated by light. In this negative control, ATP was not expected to be produced, and it was not. In the second experiment, also a negative control, the liposomes contained only ATP synthase. Again, if the chemiosmotic hypothesis was correct, ATP should not have been generated, which was what was observed. In the third experiment, both bacteriorhodopsin and ATP synthase were present in the liposomal membrane. When exposed to light, protons were pumped into the vesicle and ATP was generated. In the fourth experiment, to show that the ATP production was solely a result of the proton gradient, an uncoupling agent was added to the solution containing liposomes with bacteriorhodopsin and ATP synthase. In this case, even though the protons were being pumped into the liposomes, a gradient did not build up; this was because of the presence of the uncoupling agent, which made the membrane permeable to protons. No ATP was generated, proving that it was the proton gradient that was the energy source for ATP synthesis.

Some bacteria can live both aerobically and anaerobically. How does the ATP synthase in the plasma membrane of the bacterium help such bacteria to keep functioning in the absence of oxygen?

In the absence of oxygen, the respiratory chain no longer pumps protons, and thus no proton electrochemical gradient is generated across the bacterial membrane. In these conditions, the ATP synthase uses some of the ATP generated by glycolysis in the cytosol to pump protons out of the bacterium, thus forming the proton gradient across the membrane that the bacterium requires for importing vital nutrients by coupled transport.

In the carbon-fixation process in chloroplasts, carbon dioxide is initially added to the sugar __________________. The final product of carbon fixation in chloroplasts is the three-carbon compound __________________. This is converted into __________________ (which can be used directly by the mitochondria), into __________________ (which is exported to other cells), and into __________________ (which is stored in the stroma). The carbon-fixation cycle requires energy in the form of __________________ and reducing power in the form of __________________. 3-phosphoglycerate pyruvate ATP ribose 1,5-bisphosphate glyceraldehyde 3-phosphate ribulose 1,5-bisphosphate NADH starch NADPH sucrose

In the carbon-fixation process in chloroplasts, carbon dioxide is initially added to the sugar ribulose 1,5-bisphosphate. The final product of carbon fixation in chloroplasts is the three-carbon compound glyceraldehyde 3-phosphate. This is converted into pyruvate (which can be used directly by the mitochondria), into sucrose (which is exported to other cells), and into starch (which is stored in the stroma). The carbon- fixation cycle requires energy in the form of ATP and reducing power in the form of NADPH.

Mitochondria can use both __________________ and __________________ directly as fuel. __________________ produced in the citric acid cycle donates electrons to the electron-transport chain. The citric acid cycle oxidizes __________________ and produces __________________ as a waste product. __________________ acts as the final electron acceptor in the electron-transport chain. The synthesis of ATP in mitochondria is also known as __________________. acetyl groups NADH carbon dioxide NADP+ chemiosmosis NADPH fatty acids oxidative phosphorylation glucose oxygen NAD+ pyruvate

Mitochondria can use both pyruvate and fatty acids directly as fuel. NADH produced in the citric acid cycle donates electrons to the electron-transport chain. The citric acid cycle oxidizes acetyl groups and produces carbon dioxide as a waste product. Oxygen acts as the final electron acceptor in the electron-transport chain. The synthesis of ATP in mitochondria is also known as oxidative phosphorylation.

The relationship of free-energy change (ΔG) to the concentrations of reactants and products is important because it predicts the direction of spontaneous chemical reactions. In the hydrolysis of ATP to ADP and inorganic phosphate (Pi), the standard free-energy change (ΔG°) is -7.3 kcal/mole. The free-energy change depends on concentrations according to the following equation: ΔG = ΔG° + 1.42 log10 ([ADP] [Pi]/[ATP]) In a resting muscle, the concentrations of ATP, ADP, and Pi are approximately 0.005 M, 0.001 M, and 0.010 M, respectively. What is the ΔG for ATP hydrolysis in resting muscle? (a) -11.1 kcal/mole (b) -8.72 kcal/mole (c) 6.01 kcal/mole (d) -5.88 kcal/mole

(a) The ΔG for ATP hydrolysis is -11.1 kcal/mole. This result is calculated by substituting values into the equation given: ΔG = -7.3 kcal/mole + 1.42 log10 ([0.001 M] [0.010 M]/[0.005 M]) = -7.3 kcal/mole + 1.42 log10 (0.002) = -11.1 kcal/mole.

In the electron-transport chain in chloroplasts, ________-energy electrons are taken from __________. (a) high; H2O. (b) low; H2O. (c) high; NADPH. (d) low; NADPH.

(b)

NADH contains a high-energy bond that, when cleaved, donates a pair of electrons to the electron-transport chain. What are the immediate products of this bond cleavage? (a) NAD+ + OH- (b) NAD+ + H- (c) NAD- + H+ (d) NAD + H

(b)

Osmosis describes the movement of water across a biological membrane and down its concentration gradient. In chemiosmosis, useful energy is harnessed by the cell from the movement of _______________ across the inner mitochondrial membrane into the matrix _________________ a concentration gradient. (a) ATP, against (b) protons, down (c) electrons, down (d) ADP, against

(b)

The mitochondrial ATP synthase consists of several different protein subunits. Which subunit binds to ADP + Pi and catalyzes the synthesis of ATP as a result of a conformational change? (a) transmembrane H+ carrier (b) F1 ATPase head (c) peripheral stalk (d) central stalk

(b)

The photosystems in chloroplasts contain hundreds of chlorophyll molecules, most of which are part of _______________. (a) plastoquinone. (b) the antenna complex. (c) the reaction center. (d) the ferredoxin complex.

(b)

Ubiquinone is one of two mobile electron carriers in the electron-transport chain. Where does the additional pair of electrons reside in the reduced ubiquinone molecule? (a) The electrons are added directly to the aromatic ring. (b) The electrons are added to each of two ketone oxygens on the aromatic ring. (c) The electrons are added to the hydrocarbon tail, which hides them inside the membrane bilayer. (d) Both electrons, and one proton, are added to a single ketone oxygen bound to the aromatic ring.

(b)

Which of the following components of the electron-transport chain does not act as a proton pump? (a) NADH dehydrogenase (b) cytochrome c (c) cytochrome c reductase (d) cytochrome c oxidase

(b)

Which of the following statements about "redox potential" is true? (a) Redox potential is a measure of a molecule's capacity to strip electrons from oxygen. (b) For molecules that have a strong tendency to pass along their electrons, the standard redox potential is negative. (c) The transfer of electrons from cytochrome c oxidase to oxygen has a negative redox potential. (d) A molecule's redox potential is a measure of the molecule's capacity to pass along electrons to oxygen.

(b)

Which of the following statements describes the mitochondrial inner membrane? (a) It is permeable to molecules with molecular mass as high as 5000 daltons. (b) It contains transporters for ATP molecules. (c) It contains proteins that are released during apoptosis. (d) It contains enzymes required for the oxidation of fatty acids.

(b)

If you shine light on chloroplasts and measure the rate of photosynthesis as a function of light intensity, you get a curve that reaches a plateau at a fixed rate of photosynthesis, x, as shown in Figure Q14-62. Figure Q14-62 Which of the following conditions will increase the value of x? (a) increasing the number of chlorophyll molecules in the antenna complexes (b) increasing the number of reaction centers (c) adding a powerful oxidizing agent (d) decreasing the wavelength of light used

(b) The rate of photosynthesis will increase with increasing light intensity until photons hit all of the reaction centers directly. At saturating levels of light, the number of reaction centers that are still capable of being excited limits the rate of photosynthesis, which can be increased only by increasing the number of reaction centers or by increasing the rate at which the reaction centers are restored to their low-energy state. Increasing the number of chlorophyll molecules in the antenna complexes, the energy per photon of light, or the rate at which chlorophyll molecules are able to transfer energy electrons to one another will have no effect on either of these parameters. Adding a powerful oxidizing agent might, if anything, interfere with the reduction of the reaction center back to its resting state.

Which of the following is not an electron carrier that participates in the electron- transport chain? (a) cytochrome (b) quinone (c) rhodopsin (d) copper ion

(c)

Which of the following statements about cytochrome c is true? (a) Cytochrome c shuttles electrons between the NADH dehydrogenase complex and cytochrome c reductase complex. (b) When cytochrome c becomes reduced, two cysteines (sulfur-containing amino acids) become covalently bound to a heme group. (c) The pair of electrons accepted by cytochrome c are added to the porphyrin ring of the bound heme group. (d) Cytochrome c is the last protein in the electron-transport chain, passing its electrons directly to molecular oxygen, a process that reduces O2 to H2O.

(c)

Which of the following statements about mitochondrial division is true? (a) Mitochondria divide in synchrony with the cell. (b) The rate of mitochondrial division is the same in all cell types. (c) Mitochondrial division is mechanistically similar to prokaryotic cell division. (d) Mitochondria cannot divide and produce energy for the cell at the same time.

(c)

The relationship of free-energy change (ΔG) to the concentrations of reactants and products is important because it predicts the direction of spontaneous chemical reactions. Consider, for example, the hydrolysis of ATP to ADP and inorganic phosphate (Pi). The standard free-energy change (ΔG°) for this reaction is -7.3 kcal/mole. The free-energy change depends on concentrations according to the following equation: ΔG = ΔG° + 1.42 log10 ([ADP] [Pi]/[ATP]) In a resting muscle, the concentrations of ATP, ADP, and Pi are approximately 0.005 M, 0.001 M, and 0.010 M, respectively. At [Pi] = 0.010 M, what will be the ratio of [ATP] to [ADP] at equilibrium? (a) 1.38 × 106 (b) 1 (c) 7.2 × 10-8 (d) 5.14

(c) At equilibrium, the ΔG is equal to zero by definition. The ratio of [ATP] to [ADP] at equilibrium is less than 1:107. This result is calculated by setting ΔG = 0, so that 1.42 log10 ([ADP] [Pi]/[ATP]) = -ΔG° = 7.3 kcal/mole. Solving for [ADP]/[ATP], the equation becomes log10 ([ADP] [0.010]/[ATP]) = 7.3/1.42 = 5.14; then [ADP]/[ATP] = (105.14)/(0.010) = 13.8 × 106. Thus, the reciprocal [ATP]/[ADP] is 7.2 × 10-8

Bongkrekic acid is an antibiotic that inhibits the ATP/ADP transport protein in the inner mitochondrial membrane. Which of the following will allow electron transport to occur in mitochondria treated with bongkrekic acid? (a) placing the mitochondria in anaerobic conditions (b) adding FADH2 (c) making the inner membrane permeable to protons (d) inhibiting the ATP synthase

(c) Inhibition of the ATP/ADP translocase prevents the export of ATP generated by oxidative phosphorylation in exchange for an import of ADP into the matrix. The ensuing buildup of ATP at the expense of ADP inhibits the ATP synthase. Because protons are no longer being used to power the ATP synthase, the proton gradient is not dissipated; the increasingly steep proton gradient makes it increasingly difficult for the electron- transport proteins to pump protons out of the matrix, and electron transport quickly stops. Hence, the inner membrane becomes permeable to protons, allowing electron transport to resume (although no ATP will be synthesized).

The enzyme ribulose bisphosphate carboxylase (Rubisco) normally adds carbon dioxide to ribulose 1,5-bisphosphate. However, it will also catalyze a competing reaction in which O2 is added to ribulose 1,5-bisphosphate to form 3- phosphoglycerate and phosphoglycolate. Assume that phosphoglycolate is a compound that cannot be used in any further reactions. If O2 and CO2 have the same affinity for Rubisco, which of the following is the lowest ratio of CO2 to O2 at which a net synthesis of sugar can occur? (a) 1:3 (b) 1:2 (c) 3:1 (d) 2:1

(c) Three molecules of O2 are required to form three molecules of 3-phosphoglycerate and three molecules of phosphoglycolate. To break even (that is, simply to keep the Calvin cycle going with no net sugar produced), you need to have enough 3-phosphoglycerate to synthesize ribulose 1,5-bisphosphate again. Therefore, for every three molecules of O2 that react with ribulose 1,5-bisphosphate, you need to generate two additional molecules of 3-phosphoglycerate. For every three molecules of CO2 that go into the Calvin cycle, one molecule of 3-phosphoglycerate is formed. So if you have at least six molecules of CO2 per three molecules of O2 going through the Calvin cycle, you will break even. Only if you have a ratio of CO2 to O2 higher than 6:3 (2:1) can you have a net synthesis of carbohydrate.

Below is a list of breakthroughs in energy metabolism in living systems. Which is the correct order in which they are thought to have evolved? A. H2O-splitting enzyme activity B. light-dependent transfer of electrons from H2S to NADPH C. the consumption of fermentable organic acids D. oxygen-dependent ATP synthesis (a) A, C, D, B (b) C, A, B, D (c) B, C, A, D (d) C, B, A, D

(d)

Photosynthesis is a process that takes place in chloroplasts and uses light energy to generate high-energy electrons, which are passed along an electron-transport chain. Where are the proteins of the electron-transport chain located in chloroplasts? (a) thylakoid space (b) stroma (c) inner membrane (d) thylakoid membrane

(d)

Stage 2 of photosynthesis, sometimes referred to as the dark reactions, involves the reduction of CO2 to produce organic compounds such as sucrose. What cofactor is the electron donor for carbon fixation? (a) H2O (b) NADH (c) FADH2 (d) NADPH

(d)

Which of the following statements is not true about the possible fates of glyceraldehyde 3-phosphate? (a) It can be exported from the chloroplast to the cytosol for conversion into sucrose. (b) It can be used to make starch, which is stored inside the stroma of the chloroplast. (c) It can be used as a precursor for fatty acid synthesis and stored as fat droplets in the stroma. (d) It can be transported into the thylakoid space for use as a secondary electron acceptor downstream of the electron-transport chain.

(d)

Electron transport is coupled to ATP synthesis in mitochondria, in chloroplasts, and in the thermophilic bacterium Methanococcus. Which of the following is likely to affect the coupling of electron transport to ATP synthesis in all of these systems? (a) a potent inhibitor of cytochrome c oxidase (b) the removal of oxygen (c) the absence of light (d) an ADP analog that inhibits ATP synthase

(d) All chemiosmotic coupling systems involve a proton gradient that ATP synthase uses to bind to ADP and phosphorylate it. Hence, agents that prevent ADP from binding the synthase or that dissipate the proton gradient affect all chemiosmotic systems. Cytochrome c oxidase and oxygen are required only for mitochondria and aerobic bacteria (not Methanococcus); light is required only for chloroplasts and photosynthetic bacteria (not Methanococcus).

The relationship of free-energy change (ΔG) to the concentrations of reactants and products is important because it predicts the direction of spontaneous chemical reactions. In the hydrolysis of ATP to ADP and inorganic phosphate (Pi), the standard free-energy change (ΔG°) is -7.3 kcal/mole. The free-energy change depends on concentrations according to the following equation: ΔG = ΔG° + 1.42 log10 ([ADP] [Pi]/[ATP]) In a resting muscle, the concentrations of ATP, ADP, and Pi are approximately 0.005 M, 0.001 M, and 0.010 M, respectively. What is the ΔG for ATP synthesis in resting muscle? (a) -6.01 kcal/mole (b) 5.88 kcal/mole (c) 8.72 kcal/mole (d) 11.1 kcal/mole

(d) The ΔG for ATP hydrolysis is -11.1 kcal/mole. This result is calculated by substituting values into the equation given: ΔG = -7.3 kcal/mole + 1.42 log10 ([0.001 M] [0.010 M]/[0.005 M]) = -7.3 kcal/mole + 1.42 log10 (0.002) = -11.1 kcal/mole. The ΔG for synthesis is +11.1 kcal/mole because the forward and reverse reactions always have the same numerical value for ΔG but with the sign reversed.

Describe how a standard flashlight battery can convert energy into useful work and explain how this is similar to the energy conversions in the mitochondria.

A battery contains chemicals that generate negatively charged ions at one pole, and it is able to cause the continuous transfer of electrons along a metal wire if that pole is connected to the other end of the battery. The energy released by the electron-transfer process driven by the battery can be harnessed to do useful work, as when it is used to run an electric motor. Likewise, the energy released by the electron transfers that occur between the protein complexes in the electron-transport chain does useful work when it drives the movement of protons to one side of the membrane, since the resulting proton gradient is then used to generate chemical energy in the form of ATP.

A. Match each equation in column A with the corresponding standard redox potential in column B. Column A Column B 1. H2O ↔ 1⁄2O2 + 2H+ + 2 e - A. +30 mV 2. reduced ubiquinone ↔ oxidized ubiquinone + 2H+ + 2 e - B. +820 mV 3. NADH ↔ NAD+ + H+ + 2 e - C. +230 mV 4. reduced cytochrome c ↔ oxidized cytochrome c + e - D. - 320 mV B. How do these standard redox potentials support our understanding of the stepwise electron transfers that occur in the electron-transport chain? C. Why would it not be advantageous for living systems to evolve a mechanism for the direct transfer of electrons from NADH to O2?

A. 1—B; 2—A; 3—D; 4—C B. Each successive member of the electron-transport chain is a better electron acceptor, which permits a unidirectional series of electron transfers until reaching O2, which is the best electron acceptor and the final destination of the electrons, forming water as oxygen is consumed. C. If NADH directly donated electrons to O2, a large amount of energy would be released as heat and lost, rather than used as a way for the cell to generate chemical energy in the form of ATP.

Human infants have a much larger portion of brown adipose tissue than adult humans. It was found that the mitochondria in brown adipocytes (brown fat cells) have a novel protein in the inner mitochondrial membrane. This protein, called the uncoupling protein (UCP), was found to transport protons from the intermembrane space into the matrix. A. What is the impact of UCP on oxidative phosphorylation in the mitochondria of brown fat? B. Propose an explanation for the higher proportion of brown fat cells in infants compared to adults.

A. A protein that transports protons into the mitochondrial matrix would diminish the proton gradient. Without the proton gradient, ATP will not be generated. However, the electron-transport chain can still work, as long as oxygen is present. The UCP, therefore, is a biological uncoupler of the oxidative phosphorylation process. The electron-transport chain will run in a futile cycle that does not convert the energy from redox reactions into chemical energy (ATP), but instead releases this energy as heat. B. The thermogenesis resulting from the action of UCP is important for helping infants maintain a constant body temperature. As our body mass increases with age, our body temperatures are probably less susceptible to fluctuations, and therefore adults do not require the same amount of brown fat as infants.

Indicate whether the following statements are true or false. If a statement is false, explain why it is false. A. The driving force that pulls protons into the matrix is called the proton- motive force, which is a combination of the large force due to the pH gradient and the smaller force that results from the voltage gradient across the inner mitochondrial membrane. B. Under anaerobic conditions, the ATP synthase can hydrolyze ATP instead of synthesizing it. C. ATP is moved out of the matrix, across the inner mitochondrial membrane, in a co-transporter that also brings ADP into the matrix. D. Brown fat cells make less ATP because they have an inefficient ATP synthase.

A. False. Although it is true that both the pH gradient and the voltage gradient are components of the proton-motive force, it is the voltage gradient (also referred to as the membrane potential) that is the greater of the two. B. True. C. True. D. False. The inner mitochondrial membranes in brown fat cells contain a transport protein that allows protons to move down their gradient without passing through the ATP synthase. As a result, less ATP is made and most of the energy from the proton gradient is released as heat.

Indicate whether the following statements are true or false. If a statement is false, explain why it is false. A. The dark reactions of photosynthesis occur only in the absence of light. B. Much of the glyceraldehyde 3-phosphate made in the chloroplast ends up producing the molecules needed by the mitochondria to produce ATP. C. Ribulose 1,5-bisphosphate is similar to oxaloacetate in the Krebs cycle in that they are both regenerated at the end of their respective cycles. D. Each round of the Calvin cycle uses five molecules of CO2 to produce one molecule of glyceraldehyde 3-phosphate and one of pyruvate.

A. False. The dark reactions are those involved in carbon fixation and are named as such because they do not require light. B. True. C. True. D. False. Three molecules of CO2 are required for each round of the Calvin cycle, and the product is one molecule of glyceraldehyde 3-phosphate and the recycling of the ribulose 1,5-bisphosphate molecule.

Indicate whether the following statements are true or false. If a statement is false, explain why it is false. A. Ubiquinone is associated with the inner mitochondrial membrane as a protein-bound electron carrier molecule. B. Ubiquinone can transfer only one electron in each cycle. C. The iron-sulfur centers in NADH dehydrogenase are relatively poor electron acceptors. D. Cytochrome c oxidase binds O2 using an iron-heme group, where four electrons are shuttled one at a time.

A. False. Ubiquinone is an aromatic compound that uses its long hydrocarbon tail to associate with the inner mitochondrial membrane. B. False. Ubiquinone can transfer one or two electrons. In the case in which only one electron is transferred, the molecule contains an unpaired electron, which is highly reactive. C. True. D. True.

Mitochondrial structure and the reaction products generated inside the matrix are critical for generating stores of energy. Answer the following questions based on what you know about mitochondrial structure and processes. A. The gradients used to generate ATP are maintained across the inner mitochondrial membrane. Why don't we observe a similar gradient generation across the outer mitochondrial membrane? B. The proton-motive force created by the electrochemical proton gradient is the source of free energy utilized in ATP formation. Describe the two components contributing to the total proton-motive force.

A. The outer mitochondrial membrane contains large, channel-forming proteins called porins, which makes this membrane permeable to small molecules. Protons, other ions, nucleotides, and many other small molecules flow freely across this membrane, making it impossible to establish a gradient of any of these molecules on either side of this membrane. B. One component of the proton-motive force is the concentration gradient of protons (or pH gradient) across the membrane. This pH gradient makes it energetically favorable for protons to flow back into the matrix. A second component of the proton-motive force is a charge differential across the membrane, referred to as the membrane potential. Because the matrix side of the membrane has a net negative charge, and protons have a positive charge, this component of the proton-motive force also drives the movement of protons back into the matrix.

The respiratory chain is relatively inaccessible in the experimental manipulation of intact mitochondria. After disrupting mitochondria with ultrasound, however, it is possible to isolate functional submitochondrial particles, which consist of broken cristae that have resealed inside-out into small, closed vesicles. In these vesicles, the components that originally faced the matrix are now exposed to the surrounding medium. A. How might such an arrangement aid in the study of electron transport and ATP synthesis? B. Consider an anaerobic preparation of such submitochondrial particles. If a small amount of oxygen is added, do you predict that the preparation will consume oxygen in respiration reactions? Will the medium outside the particles become more acidic or more basic? What, if anything, will change if the flow of protons through ATP synthase is blocked by an inhibitor? Explain your answer.

A. This arrangement of components within the vesicles allows the experimental manipulation of the medium surrounding the vesicles, which permits the consequences of different conditions in the mitochondrial matrix to be examined. The medium can be altered by changing pH, adding electron carriers and oxygen, and providing ADP and Pi, for example. The oxidation of electron carriers, the consumption of oxygen, and the production of ATP can be measured in the medium. By changing the composition of the medium, it should be possible, for example, to identify the electron carriers that can donate electrons from the matrix to the transport chain (the side of the membrane that normally faces the matrix is now on the outside), to assess the redox potentials of various components of the transport chain, and to determine the dependence of ATP synthesis on the pH gradient across the membrane and on the ATP/ADP ratio. B. Respiration reactions will rapidly consume at least some of the added oxygen. During the anaerobic conditions, the electron carriers in the electron-transport chain were reduced; on the addition of oxygen, electrons will be transferred to oxygen, thereby reducing the oxygen and oxidizing the carriers. Concomitantly with the electron flow, protons will be pumped from the medium into the vesicles, thereby making the medium slightly more basic and the inside of the vesicles acidic. Inhibition of the ATP synthase will not have an immediate effect on oxygen consumption or proton pumping. However, the proton concentration inside the vesicles will quickly become too high to continue the activity of the electron-transport-coupled proton pumping, and thus electron transport and oxygen consumption will cease.

Indicate whether the following statements are true or false. If a statement is false, explain why it is false. A. Carbon fixation can be described as a process by which gaseous carbon- containing molecules are captured and incorporated into biological hydrocarbon molecules. B. The electron-transport proteins, utilized in stage 1 of photosynthesis, reside in the inner membrane of the chloroplast. C. Similar to oxidative phosphorylation, the electrons passed along the chloroplast electron-transport chain are ultimately passed on to a molecule of O2, to produce H2O. D. Stage 2 of photosynthesis involves a cycle of reactions that does not directly depend on energy derived from sunlight.

A. True. B. False. The electron-transport system in chloroplasts resides in the thylakoid membrane. C. False. The recipient of electrons in the chloroplast electron-transport chain is the NADP+ cofactor, which becomes reduced to NADPH. D. True.

Indicate whether the following statements are true or false. If a statement is false, explain why it is false. A. The number and location of mitochondria within a cell can change, depending on both the cell type and the amount of energy required. B. The inner mitochondrial membrane contains porins, which allow pyruvate to enter for use in the citric acid cycle. C. The inner mitochondrial membrane is actually a series of discrete, flattened, membrane-enclosed compartments called cristae, similar to what is seen in the Golgi apparatus. D. The intermembrane space of the mitochondria is chemically equivalent to the cytosol with respect to pH and the small molecules present.

A. True. B. False. The outer mitochondrial membrane contains porins, allowing the passage of all molecules with a mass of less than 5000 daltons. Although pyruvate must pass through the inner membrane, it does so in a highly regulated manner via specific transporter channels. C. False. Although the cristae do look like individual compartments on the basis of the images of the inner structure of the mitochondria, the inner membrane is a single, albeit highly convoluted, membrane. D. True.

For each of the following sentences, choose one of the options enclosed in square brackets to make a correct statement. An electron bound to a molecule with low affinity for electrons is a [high/low]- energy electron. Transfer of an electron from a molecule with low affinity to one with higher affinity has a [positive/negative] ΔG° and is thus [favorable/unfavorable] under standard conditions. If the reduced form of a redox pair is a strong electron donor with a [high/low] affinity for electrons, it is easily oxidized; the oxidized member of such a redox pair is a [weak/strong] electron acceptor.

An electron bound to a molecule with low affinity for electrons is a high-energy electron. Transfer of an electron from a molecule with low affinity to one with higher affinity has a negative ΔG° and is thus favorable under standard conditions. If the reduced form of a redox pair is a strong electron donor with a low affinity for electrons, it is easily oxidized; the oxidized member of such a redox pair is a weak electron acceptor.

The link between bond-forming reactions and membrane transport processes in the mitochondria is called __________________. (a) chemiosmotic coupling. (b) proton pumping. (c) electron transfer. (d) ATP synthesis.

Choice (a) is correct. Choices (b), (c), and (d) are individual parts of the overall process of chemiosmotic coupling.

Modern eukaryotes depend on mitochondria to generate most of the cell's ATP. How many molecules of ATP can a single molecule of glucose generate? (a) 30 (b) 2 (c) 20 (d) 36

Choice (a) is correct. Glycolysis of a single glucose molecule generates 2 ATP molecules. Oxidative phosphorylation in the mitochondria generates an additional 28 ATP molecules, making a total of 30 ATP molecules for each glucose molecule.

Which ratio of NADH to NAD+ in solution will generate the largest positive redox potential? (a) 1:10 (b) 10:1 (c) 1:1 (d) 5:1

Choice (a) is correct. NAD+ is the electron acceptor; NADH is the electron donor. If there is an excess of NAD+ in solution, there is less capacity to donate electrons and more capacity to accept electrons. This is reflected by a redox potential that is more positive than the alternative conditions.

Which of the following reactions has a sufficiently large free-energy change to enable it to be used, in principle, to provide the energy needed to synthesize one molecule of ATP from ADP and Pi under standard conditions? See Table Q14-47. Recall that ΔG° = -n (0.023) ΔE0′, and ΔE0′ = E0′ (acceptor) - E0′ (donor). (a) the reduction of a molecule of pyruvate by NADH (b) the reduction of a molecule of cytochrome b by NADH (c) the reduction of a molecule of cytochrome b by reduced ubiquinone (d) the oxidation of a molecule of reduced ubiquinone by cytochrome c

Choice (b) is correct. For a reaction to drive ATP synthesis under standard conditions, the ΔG° of the reaction must be less than -7.3 kcal/mole. Because ΔG° = -n (0.023) ΔE0′, the value of ΔE0′ must be greater than 317 mV/n, where n is the number of electrons transferred. ΔE0′ is 130 mV for the reduction of a molecule of pyruvate by NADH, 390 mV for the reduction of a molecule of cytochrome b by NADH, 40 mV for the reduction of a molecule of cytochrome b by ubiquinone, 200 mV for the oxidation of a molecule of ubiquinone by cytochrome c, and 590 mV for the oxidation of cytochrome c by oxygen. The numbers of electrons transferred in each of the above reactions are two, one, one, one, and one, respectively. Thus, only the second and fifth of these reactions are sufficient to drive ATP synthesis.

Which of the following types of ion movement might be expected to require co- transport of protons from the mitochondrial intermembrane space to the matrix, inasmuch as it could not be driven by the membrane potential across the inner membrane? (Assume that each ion being moved is moving against its concentration gradient.) (a) import of Ca2+ into the matrix from the intermembrane space (b) import of acetate ions into the matrix from the intermembrane space (c) exchange of Fe2+ in the matrix for Fe3+ in the intermembrane space (d) exchange of ATP from the matrix for ADP in the intermembrane space

Choice (b) is the correct answer. Because the inside of the membrane (the mitochondrial matrix) is more negative than the outside, in principle any traffic resulting in an increase in the positive charge in the matrix can be driven by the membrane potential. Hence, exchange of Fe2+ (or ATP) in the matrix for Fe3+ (or ADP) in the intermembrane space can be driven by the membrane potential and need not require the co-transport of protons down the pH gradient. The same applies to the import of Ca2+. But import of acetate ions into the matrix and exchange of Ca2+ in the matrix for Na+ in the intermembrane space result in an increase in the amount of negative charge in the matrix and they therefore cannot be driven by the charge difference between the two mitochondrial compartments.

Which of the following statements is true? (a) The NADH dehydrogenase complex can pump more protons than can the cytochrome b-c1 complex. (b) The pH in the mitochondrial matrix is higher than the pH in the intermembrane space. (c) The proton concentration gradient and the membrane potential across the inner mitochondrial membrane tend to work against each other in driving protons from the intermembrane space into the matrix. (d) The difference in proton concentration across the inner mitochondrial membrane has a much larger effect than the membrane potential on the total proton-motive force.

Choice (b) is the correct answer. The pumping of protons out of the matrix into the intermembrane space creates a difference in proton concentration between the two sides of the membrane, with the matrix at a higher pH (i.e., more alkaline) than the intermembrane space, which tends to equilibrate with the cytosol (which has a neutral pH). The electrons in NADH are at a higher energy than the electrons in reduced ubiquinone, but proton pumping is not determined simply by the energy of the electron donors [choice (a)]. Instead, the number of protons that can be pumped by each complex is determined by the difference in energy between the electrons in each substrate-product pair (i.e., the difference between the electrons in NADH and reduced ubiquinone, compared with that between reduced ubiquinone and reduced cytochrome c). The proton concentration gradient and the membrane potential generated by the electron-transport chain work in the same direction [choice (c)], creating a steep electrochemical gradient for protons across the membrane. For choice (d), the difference in proton concentration has a smaller effect than the membrane potential on the total proton-motive force.

NADH and FADH2 carry high-energy electrons that are used to power the production of ATP in the mitochondria. These cofactors are generated during glycolysis, the citric acid cycle, and the fatty acid oxidation cycle. Which molecule below can produce the most ATP? Explain your answer. (a) NADH from glycolysis (b) FADH2 from the fatty acid cycle (c) NADH from the citric acid cycle (d) FADH2 from the citric acid cycle

Choice (c) is correct. NADH produced in glycolysis does not contribute directly to ATP production in the mitochondria because it cannot be imported into the matrix. If the energy is transferred to a different carrier, some of the stored energy is lost. FADH2, from either the fatty acid cycle or the citric acid cycle, contributes less energy than NADH from the citric acid cycle because the electrons are donated further down the chain. Fewer electron transfers means that fewer protons are pumped across the membrane.

Which of the following statements is true? (a) Only compounds with negative redox potentials can donate electrons to other compounds under standard conditions. (b) Compounds that donate one electron have higher redox potentials than those compounds that donate two electrons. (c) The ΔE0′ of a redox pair does not depend on the concentration of each member of the pair. (d) The free-energy change, ΔG, for an electron-transfer reaction does not depend on the concentration of each member of a redox pair.

Choice (c) is the correct answer. By definition, E0′ refers to the standard state of equal concentrations of each member of the redox pair. Therefore, ΔE0′ does not vary with the actual concentrations. Compounds with positive redox potentials can donate electrons to other compounds under standard conditions, so long as the electron acceptor has a higher (more positive) redox potential; thus, option (a) is incorrect. Compounds that are able to donate only one electron do not necessarily have higher redox potentials than compounds that are able to donate two electrons; thus. option (b) is incorrect. (Water, for example, has a very high redox potential.) Although the ΔE0′ of a reaction is directly proportional to the ΔG° of a reaction and both are independent of the concentrations of substrates and products, the ΔG depends on these concentrations; thus, option (d) is incorrect.

Which of the following statements is true? (a) Ubiquinone is a small, hydrophobic protein containing a metal group that acts as an electron carrier. (b) A 2Fe2S iron-sulfur center carries one electron, whereas a 4Fe4S center carries two. (c) Iron-sulfur centers generally have a higher redox potential than do cytochromes. (d) Mitochondrial electron carriers with the highest redox potential generally contain copper ions and/or heme groups.

Choice (d) is the correct answer. Cytochrome c oxidase, which is the last carrier in the mitochondrial electron-transport chain and therefore has the highest redox potential, contains copper ions and a heme group. Ubiquinone is not a protein and does not contain a metal group [choice (a)]. Both 2Fe2S and 4Fe4S centers carry one electron [choice (b)]. Iron-sulfur centers generally have a lower redox potential than do cytochromes [choice (c)]. The heme group in cytochrome c contains a charged iron ion. The interiors of proteins are often hydrophobic, favoring a relatively high redox potential, because reduction of the iron ion decreases its charge, and charges are energetically unfavorable in a hydrophobic environment.

NADH donates electrons to the __________________ of the three respiratory enzyme complexes in the mitochondrial electron-transport chain. __________________ is a small protein that acts as a mobile electron carrier in the respiratory chain. __________________ transfers electrons to oxygen. Electron transfer in the chain occurs in a series of __________________ reactions. The first mobile electron carrier in the respiratory chain is__________________. cytochrome c plastoquinone cytochrome c oxidase reduction first second NADH dehydrogenase the cytochrome b-c1 complex oxidation third oxidation-reduction ubiquinone phosphorylation

NADH donates electrons to the first of the three respiratory enzyme complexes in the mitochondrial electron-transport chain. Cytochrome c is a small protein that acts as a mobile electron carrier in the respiratory chain. Cytochrome c oxidase transfers electrons to oxygen. Electron transfer in the chain occurs in a series of oxidation- reduction reactions. The first mobile electron carrier in the respiratory chain is ubiquinone.

Photons from sunlight that are in the ______________ wavelength range are preferentially absorbed by chlorophyll molecules to raise the energy levels of electrons in the __________ ring. The __________ reflected are lower in energy, which is indicated in the ________, green wavelengths detected by the human eye. benzene longer porphyrin blue orange red electrons photons shorter heme

Photons from sunlight that are in the red wavelength range are preferentially absorbed by chlorophyll molecules to raise the energy levels of electrons in the porphyrin ring. The photons reflected are lower in energy, which is indicated in the longer, green wavelengths detected by the human eye.

Explain how the F0 complex of ATP synthase harnesses the proton-motive force to help synthesize ATP. What would happen if the proton gradient were reversed?

Protons flow through a channel that exists between the subunits of the transmembrane H+ carrier, which forms a ring (the rotor). The flow of protons through this carrier makes the rotor and its attached stalk rotate. As the stalk rotates, it rubs against proteins in the stationary F1 portion of the ATP synthase. The resulting mechanical deformation produces a conformational change in the subunits of the F1 ATPase that causes them to produce ATP. When the proton gradient is reversed, the F1 portion of the ATP synthase catalyzes the hydrolysis of ATP to ADP and Pi, rather than the revers reaction of ATP synthesis; this causes protons to be pumped out of the matrix against their electrochemical gradient, as the rotor and its stalk rotate in the direction opposite to that involved in ATP synthesis.

The citric acid cycle generates NADH and FADH2, which are then used in the process of oxidative phosphorylation to make ATP. If the citric acid cycle (which does not use oxygen) and oxidative phosphorylation are separate processes, as they are, then why is it that the citric acid cycle stops almost immediately when O2 is removed?

The citric acid cycle stops almost immediately when oxygen is removed because several steps in the cycle require the oxidized forms of NAD+ and FAD. In the absence of oxygen, these electron carriers can be reduced by the reactions of the citric acid cycle but cannot be reoxidized by the electron-transport chain that participates in oxidative phosphorylation.

Consider a redox reaction between molecules A and B. Molecule A has a redox potential of -100 mV and molecule B has a redox potential of +100 mV. For the transfer of electrons from A to B, is the ΔG° positive or negative or zero? Under what conditions will the reverse reaction, transfer of electrons from B to A, occur?

The ΔG° is negative. The sign of ΔG° is the opposite of that of ΔE0′ = E0′ (acceptor) - E0′ (donor). The acceptance of electrons by B from A has a ΔE0′ = 100 - (-100) = 200. The reverse reaction, the donation of electrons from B to A, has a positive ΔG° and is therefore unfavorable under standard conditions. Remember that, by definition, the concentrations of A and its redox pair A′ are equal under standard conditions; similarly, the concentration of B is equal to the concentration of its redox pair B′. B will be able to donate electrons to A only when [B] > [B′] and/or [A] < [A′] to such an extent that the ΔG for electron transfer becomes negative.

In 1925, David Keilin used a simple spectroscope to observe the characteristic absorption bands of the cytochromes that participate in the electron-transport chain in mitochondria. A spectroscope passes a very bright light through the sample of interest and then through a prism to display the spectrum from red to blue. If molecules in the sample absorb light of particular wavelengths, dark bands will interrupt the colors of the rainbow. His key discovery was that the absorption bands disappeared when oxygen was introduced and then reappeared when the samples became anoxic. Subsequent findings demonstrated that different cytochromes absorb light of different frequencies. When light of a characteristic wavelength shines on a mitochondrial sample, the amount of light absorbed is proportional to the amount of a particular cytochrome present in its reduced form. Thus, spectrophotometric methods can be used to measure how the amounts of reduced cytochromes change over time in response to various treatments. If isolated mitochondria are incubated with a source of electrons such as succinate, but without oxygen, electrons enter the respiratory chain, reducing each of the electron carriers almost completely. When oxygen is then introduced, the carriers oxidize at different rates, as can be seen from the decline in the amount of reduced cytochrome (see Figure Q14-67). Note that cytochromes a1 and a3 cannot be distinguished and thus are listed as cytochrome (a1 + a3). How does this result allow you to order the electron carriers in the respiratory chain? What is their order? Figure Q14-67

This result allows you to order the electron carriers in the respiratory chain because when oxygen is added, the last carrier in the chain will be oxidized first. This is because oxygen is the final sink for the electrons that flow through the chain, and it participates directly in a redox reaction with the last electron carrier. The wave of oxidation will then proceed backward through the chain toward the first electron carrier in the chain; this is because the oxidation of each carrier will convert it to a form that can accept electrons from the "upstream" carrier in the chain, thereby oxidizing each upstream carrier sequentially. The order of cytochromes in the respiratory chain is the reverse of the order in which they are oxidized (that is, the order in which the reduced form is lost). Listed from first to last, the cytochromes in the chain are b, c1, c, and (a1 + a3).