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Chapter 19 Oxidative Phosphorylation and Photophosphorylation
231
Short Answer Questions
44. As you read and answer this question, you are (presumably) consuming oxygen. What single reaction
accounts for most of your oxygen consumption?
45. Show the path of electrons from ubiquinone (Q or coenzyme Q) to oxygen in the mitochondrial
respiratory chain. One of the two compounds (Q and O2) has a standard reduction potential (E'°) of
0.82 V, and the other, 0.045 V. Which value belongs to each compound? How did you deduce this?
46. Diagram the path of electron flow from NADH to the final electron acceptor during electron transport
in mitochondria. For each electron carrier, indicate whether only electrons, or both electrons and
protons, are accepted/donated by that carrier. Also, indicate where electrons from succinate oxidation
enter the chain of carriers.
47. During electron transfer through the mitochondrial respiratory chain, the overall reaction is:
NADH + 1/2 O2 + H+ → NAD+ + H2O. The difference in reduction potentials for the two half-
reactions (E'°) is +1.14 V. Show how you would calculate the standard free-energy change, G'°,
for the reaction as written above. (The Faraday constant, , is 96.48 kJ/V·mol.)
48. The standard reduction potential for ubiquinone (Q or coenzyme Q) is 0.045 V, and the standard
reduction potential (E'°) for FAD is –0.219 V. Using these values, show that the oxidation of FADH2
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by ubiquinone theoretically liberates enough energy to drive the synthesis of ATP. The Faraday
constant, , is 96.48 kJ/V·mol. G'° for ATP synthesis is +30.5 kJ/mol.
49. A recently discovered bacterium carries out ATP synthesis coupled to the flow of electrons through a
chain of carriers to some electron acceptor. The components of its electron transfer chain differ from
those found in mitochondria; they are listed below with their standard reduction potentials.
Electron carriers in the newly discovered bacterium:
—————————————————————————————————————
Electrons E'°
Oxidant Reductant transferred (V)
—————————————————————————————————————
NAD+ NADH 2 –0.32
flavoprotein b (FPb) flavoprotein b 2 –0.62
(oxidized) (reduced)
cytochrome c (Fe3+) cytochrome c (Fe2+) 1 +0.22
Fe-S protein Fe-S protein 2 +0.89
(oxidized) (reduced)
flavoprotein a (FPa) flavoprotein a 2 +0.77
(oxidized) (reduced)
—————————————————————————————————————
(a) Place the electron carriers in the order in which they are most likely to act in carrying electrons.
(b) Is it likely that O2 (for which E'° = 0.82 V) is the final electron acceptor in this organism? Why or
why not? (c) How would you calculate the maximum number of ATP molecules that could
theoretically be synthesized, under standard conditions, per pair of electrons transfered through this
chain of carriers? (The Faraday constant, , is 96.48 kJ/V·mol.) G'° for ATP synthesis is +30.5
kJ/mol.
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50. Describe, in simple diagrams and a few words, the chemiosmotic theory for coupling oxidation to
phosphorylation in mitochondria.
51. Compound X is an inhibitor of mitochondrial ATP synthesis. It was observed that when compound X
was added to cells, the NAD+/NADH ratio decreased. Would you expect X to be an uncoupling agent
or an inhibitor of respiratory electron transfer? Explain.
52. Cyanide ion (CN–) blocks electron transfer in mitochondria at the level of cytochrome a + a3. 2,4-
Dinitrophenol (DNP) is an uncoupler of mitochondrial oxidative phosphorylation. Draw graphs
(ATP synthesized vs. time and O2 consumed vs. time) with labeled curves to show the effects of
adding each of these compounds separately to a suspension of mitochondria supplied with O2,
succinate, ADP, and Pi. Use an arrow to indicate the time of drug addition.
53. Give an example of (a) an uncoupler of oxidative phosphorylation, and (b) an inhibitor of respiration.
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(c) Describe the difference in the effects of such uncouplers and inhibitors on mitochondrial function.
54. Mitochondria carrying out oxidative phosphorylation consume oxygen. Explain what happens to this
oxygen, and describe the effect of an uncoupling agent such as 2,4-dinitrophenol on the rate of
oxygen consumption. Assume there is a sufficient supply of oxidizable substrate, ADP, and Pi.
55. The compound 2,4-dinitrophenol (DNP), an uncoupler, was briefly used as a weight-loss drug. Some
of its effects in people who took the drug included weight loss and higher than normal body
temperature. Some people even died. Explain the first two effects of the compound in biochemical
terms.
56. Although molecular oxygen (O2) does not participate directly in any of the reactions of the citric acid
cycle, the cycle operates only when O2 is present. Explain this observation.
57. The skunk cabbage (Symphocarpus foetidus) can maintain a temperature of 10–25 °C higher than the
temperature of the surrounding air. Suggest a mechanism for this.
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58. Consider a liver cell carrying out the oxidation of glucose under aerobic conditions. Suppose that we
added a very potent and specific inhibitor of the mitochondrial ATP synthase, completely inhibiting
this enzyme. Indicate whether each of the following statements about the effect of this inhibitor is
true or false; if false, explain in a sentence or two why it is false.
____ (a) ATP production in the cell will quickly drop to zero.
____ (b) The rate of glucose consumption by this cell will decrease sharply.
____ (c) The rate of oxygen consumption will increase.
____ (d) The citric acid cycle will speed up to compensate.
____ (e) The cell will switch to fatty acid oxidation as an alternative to glucose oxidation, and the
inhibitor will therefore have no effect on ATP production.
59. When the F1 portion of the ATP synthetase complex is removed from the mitochondrial membrane
and studied in solution, it functions as an ATPase. Why does it not function as an ATP synthetase?
60. When the G'° of the ATP synthesis reaction is measured on the surface of the ATP synthase
enzyme, it was found to be close to zero. Describe briefly why this is so.
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61. Explain briefly the current model for how the proton motive force that is generated by electron
transport is used to drive the ATP synthesis reaction.
62. For each of the following explain whether they are transported into or out of the mitochondrion and
comment briefly on the mechanism for each:
(1) NADH
(2) Inorganic phosphate
(3) ADP
63. Describe and explain the two different ratios that affect the rate of respiration in mitochondria. What
is accomplished by these control mechanisms?
64. In his studies of alcoholic fermentation by yeast, Louis Pasteur noted that the sudden addition of
oxygen (O2) to a previously anaerobic culture of fermenting grape juice resulted in a dramatic
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decrease in the rate of glucose consumption. This “Pasteur effect” can be counteracted by the
addition of 2,4-dinitrophenol (DNP), an uncoupler of oxidative phosphorylation. (a) Why would the
yeast cells consume less glucose in the presence of oxygen? Can you estimate how much less glucose
they would use? (b) Why would DNP counteract or prevent the Pasteur effect?
65. Describe the role of hypoxia-inducible factor (HIF-1) in reducing reactive oxygen species (ROS).
66. Describe the role of cytochrome c in apoptosis.
67. Mutations in mitochondrial genes frequently produce diseases that affect the brain and skeletal
muscle (mitochondrial encephalomyopathies). Why are these two tissues particularly sensitive to
mitochondrial mutations?
68. Discuss three lines of evidence that support the theory that mitochondria evolved from
endosymbiontic bacteria.
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69. Photophosphorylation differs from oxidative phosphorylation in that the former requires the input of
energy in the form of ___________ to create a good electron donor. In photophosphorylation,
electrons flow through a series of membrane-bound carriers including ___________ , ___________ ,
and ___________ proteins, whereas ___________ are pumped across a membrane to create an
___________ potential.
70. Describe the effect(s) that a mitochondrial uncoupler such as 2,4-dinitrophenol (DNP) would have on
photophosphorylation.
71. Discuss how “accessory pigments” are able to extend the range of light absorption of the
chlorophylls. Name some accessory pigments.
72. What is an action spectrum, and what do peaks in an action spectrum signify? Show a typical action
spectrum plot for photosynthesis.
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73. Describe what happens at photosystem I from the point where an antenna chlorophyll molecule
absorbs a photon of light to the passage of an electron to NADP+.
74. Give five general classes of electron carriers that function in both mitochondrial electron transfer to
O2 and photosynthetic electron transfer.
75. The processes of oxidative phosphorylation coupled with electron transfer (in mitochondria) and
photophosphorylation (in chloroplasts) resemble each other in certain respects. Describe five ways in
which the two processes are similar, and describe three significant differences between the two
processes.
76. Show the path of electrons from photosystem II to NADPH in the chloroplast. What is the source of
the energy that moves electrons through this path? Show where oxygen is involved in this pathway.
77. Plants carrying out photosynthesis produce O2. Describe the source of this O2, and explain, with
chemical equations or schematic diagrams, why O2 production occurs only during daylight hours.
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78. During photophosphorylation in plants, electrons flow through a series of carriers in the chloroplast.
What is the ultimate donor of electrons, and what is the ultimate acceptor? What provides the energy
to move those electrons?
79. Describe what happens when a photon is absorbed by photosystem II; end the description of electron
flow at plastoquinone.
80. DCMU is an herbicide that acts by blocking photosynthetic electron flow from photosystem II (PSII)
to the cytochrome b6f complex. Predict the effect of DCMU on O2 production and on ATP synthesis
in the chloroplasts of plants sensitive to DCMU.
81. Given that ~8 photons are required to produce one molecule of O2, roughly how many photons are
required to produce one molecule of glucose.
82. Some photosynthetic bacteria contain the protein bacteriorhodopsin, which absorbs light and pumps
protons out of the cell directly. Briefly describe how such a cell could use bacteriorhodopsin and an
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H+ATPase to make ATP using light.
