Chapter 14: Energy Generation in Mitochondria and Chloroplasts
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) -11.1 kcal/mole The δG for 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 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) thylakoid space
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) low; H2O
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) the antenna complex
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) 3:1 Three molecules of O2 are required to form three molecules of 3-phosphoglycerate and three molecules of phosphoglycolate. To break even (i.e., 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.
Oxidative phosphorylation, as it occurs in modern eucaryotes, 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 oxidase (c) ATP-driven proton pumps (d) the generation of ATP from the energy of a proton gradient
(c) ATP-driven proton pumps
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) Plastoquinone will accumulate in the oxidized form. 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 following is not an electron carrier that participates in the electron-transport chain? (a) cytochrome (b) quinone (c) rhodopsin (d) copper ion
(c) rhodopsin
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) C, B, A, 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) NADPH
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) thylakoid membrane
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) thylakoid membrane
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. Outer membrane B. The Matrix C. The matrix D. Inner membrane E. Inner membrane F. Inner membrane
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.
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.
Indicate whether the following statements are true or false. If a statement is false, explain why it is false. 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.
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.
Indicate whether the following statements are true or false. If a statement is false, explain why it is false. D. Brown fat cells make less ATP because they have an inefficient ATP synthase.
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. B. The inner mitochondrial membrane contains porins, which allow pyruvate to enter for use in the citric acid cycle.
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.
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 the both the cell type and the amount of energy required.
True
Indicate whether the following statements are true or false. If a statement is false, explain why it is false. B. Under anaerobic conditions, the ATP synthase can hydrolyze ATP instead of synthesizing it.
True
Indicate whether the following statements are true or false. If a statement is false, explain why it is false. C. ATP is moved out of the matrix, across the inner mitochondrial membrane, in a co-transporter that also brings ADP into the matrix.
True
Indicate whether the following statements are true or false. If a statement is false, explain why it is false. D. The intermembrane space of the mitochondria is chemically equivalent to the cytosol with respect to pH and the small molecules present.
True
Modern eucaryotes 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
a) 30 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.
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
a) chemiosmotic coupling
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) NAD+ + H-
Which of the following types of ion movement might be expected to require co-transport of protons from the 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
b) import of acetate ions into the matrix from the intermembrane space
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 Q14-45. Figure Q14-45 Which of the following conditions will increase the value of x? (a) increasing the number of chlorophyll molecules in the antennae complexes (b) increasing the number of reaction centers (c) adding a powerful oxidizing agent (d) decreasing the wavelength of light used
b) increasing the number of reaction centers 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 antennae 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.
Cytochrome 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) one iron atom and one copper atom
Which of the following statements is true? (a) Because the electrons in NADH are at a higher energy than the electrons in reduced ubiquinone, 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.
b) the pH in the matrix is higher than the intermembrane space
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) making the inner membrane permeable to protons
Which of the following reactions have a large enough 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 14-23. Recall that δG° = -n (0.023) δE′0 and δE′0 = E′0 (acceptor) - E′0 (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 there a table
b) the reduction of a molecule of cytochrome b by NADH For a reaction to drive ATP synthesis under standard conditions, the δG°′ of the reaction must be less than -7.3 kcal/mol. Because δG°′ = -n (0.023) δE′0, the value of δE′0 must be greater than 317 mV/n, where n is the number of electrons transferred. δE′0 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 reactions (b) and (e) are sufficient to drive ATP synthesis.
The F1 portion of the mitochondrial ATP synthase comprises several different protein subunits. Which subunit binds to ADP + Pi and catalyzes the synthesis of ATP as a result of a conformational change? (a) α (b) β (c) δ (d) ε
b) β
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) 7.2 x 10-8 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.
Which of the phylogenetic trees in Figure Q14-53 is the most accurate? (The mitochondria and chloroplasts are from maize, but they are treated as independent "organisms" for the purposes of this question.)
c) Mitochondria are most closely related to Bacillus, and chloroplasts to cyanobacteria. Maize (a eucaryote) is more closely related to Giardia (a simple eucaryote) than it is to bacteria (procaryotes).
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 molecuale 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
c) NADH from the citric acid cycle 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 mean that fewer protons are pumped across the membrane.
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) O2 is reduced to H2O
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) O2 is reduced to H2O 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.
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 of compounds that donate two electrons. (c) The δE′0 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.
c) The δE′0 of a redox pair does not depend on the concentration of each member of the pair. By definition, E′0 refers to the standard state of equal concentrations of each member of the redox pair. Therefore δE′0 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 δE′0 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.
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 kcal/mole
d) 11 kcal/mole The δG for 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.
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
d) 5:1 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 (which would reflect a negative redox potential) and more capacity to accept electrons (which would reflect a positive redox potential).
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.
d) Mitochondrial electron carriers with the highest redox potential generally contain copper ions and/or heme groups. Cytochrome 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.
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 oxidase (b) the removal of oxygen (c) the absence of light (d) an ADP analogue that inhibits ATP synthase
d) an ADP analogue that inhibits ATP synthase 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 oxidase and oxygen are required only for mitochondria and aerobic bacteria (not Methanococcus); light is required only for chloroplasts and photosynthetic bacteria (not Methanococcus).
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 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 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) cytochrome c oxidase