Essential Cell Biology

Bruce Alberts, Dennis Bray, Karen Hopkin

Chapter 14

Energy Generation in Mitochondria and Chloroplasts - all with Video Answers

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Chapter Questions

Problem 1

Dinitrophenol (DNP) is a small molecule that renders membranes permeable to protons. In the $1940 s,$ small amounts of this highly toxic compound were given to patients to induce weight loss. DNP was effective in melting away the pounds, especially promoting the loss of fat reserves. Can you explain how it might cause such loss? As an unpleasant side reaction, however, patients had an elevated temperature and sweated profusely during the treatment. Provide an explanation for these symptoms.

Nicole Hewett

Nicole Hewett

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Problem 2

Electron micrographs show that mitochondria in heart muscle have a much higher density of cristae than mitochondria in skin cells. Suggest an explanation for this observation.

Nicole Hewett

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Problem 3

When the drug dinitrophenol (DNP) is added to mitochondria, the inner membrane becomes permeable to protons $\left(\mathrm{H}^{+}\right) .$ In contrast, when the drug nigericin is added to mitochondria, the inner membrane becomes permeable to $\mathrm{K}^{+}$. (A) How does the electrochemical proton gradient change in response to DNP? (B) How does it change in response to nigericin?

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Problem 4

The remarkable properties that allow ATP synthase to run in either direction allow the interconversion of energy stored in the H' gradient and energy stored in ATP to proceed in either direction. (A) If ATP synthase making ATP can be likened to a water-driven turbine producing electricity, what would be an appropriate analogy when it works in the opposite direction? (B) Under what conditions would one expect the ATP synthase to stall, running neither forward nor backward?
(C) What determines the direction in which the ATP synthase operates?

Nicole Hewett

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Problem 5

Calculate the number of usable ATP molecules produced per pair of electrons transferred from NADH to oxygen, if (i) five protons are pumped across the inner mitochondrial membrane for each electron passed through the three respiratory enzyme complexes,
(ii) three protons must pass through the ATP synthase for each ATP molecule that it produces from ADP and inorganic phosphate inside the mitochondrion, and (iii) one proton is used to produce the voltage gradient needed to transport each ATP molecule out of the mitochondrion to the cytosol where it is used.

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Problem 6

At many steps in the electrontransport chain, Fe ions are used as part of heme or FeS clusters to bind the electrons in transit. Why do these functional groups that carry out the chemistry of electron transfer need to be bound to proteins? Provide several different reasons why this is necessary.

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Problem 7

Two different diffusible electron and carriers, ubiquinone cytochrome between the $c,$ shuttle electrons three protein complexes of the Could electron-transport chain. the same diffusible carrier, in principle, be used for both steps? Explain your answer.

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Problem 8

Chloroplasts have a third internal compartment, the thylakoid space, bounded by the thylakoid membrane. This membrane contains the photosystems, reaction centers, electron-transport chain, and ATP synthase. In contrast, mitochondria use their inner membrane for electron transport and ATP synthesis. In both organelles, protons are pumped out of the largest internal compartment (the matrix in mitochondria and the stroma in chloroplasts). The thylakoid space is completely sealed off from the rest of the cell. Why does this arrangement allow a larger $\mathrm{H}^{+}$ gradient in chloroplasts than can be achieved for mitochondria?

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Problem 9

Both NADPH and the related carrier molecule NADH are strong electron donors. Why might plant cells have evolved to rely on NADPH, rather than NADH, to provide the reducing power for photosynthesis?

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Problem 10

A. How do cells in plant roots survive, since they contain no chloroplasts and are not exposed to light?
B. Unlike mitochondria, chloroplasts do not have a transporter that allows them to export ATP to the cytosol. How, then, do plant cells obtain the ATP that they need to carry out energyrequiring metabolic reactions in the cytosol?

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Problem 11

Which of the following statements are correct? Explain your answers.
A. After an electron has been removed by light, the affinity for electrons of the positively charged chlorophyll in the reaction center of the first photosystem (photosystem II) is even greater than the electron affinity of $\mathrm{O}_{2}$
B. Photosynthesis is the light-driven transfer of an electron from chlorophyll to a second molecule that normally has a much lower affinity for electrons.
C. Because it requires the removal of four electrons to release one $\mathrm{O}_{2}$ molecule from two $\mathrm{H}_{2} \mathrm{O}$ molecules, the water-splitting enzyme in photosystem II has to keep the reaction intermediates tightly bound so as to prevent partly reduced, and therefore hazardous, superoxide radicals from escaping.

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Problem 12

Which of the following statements are correct? Explain your
answers.
A. Many, but not all, electron-transfer reactions involve metal ions.
B. The electron-transport chain generates an electrical potential across the membrane because it moves electrons from the intermembrane space into the matrix.
C. The electrochemical proton gradient consists of two components: a pH difference and an electrical potential.
D. Ubiquinone and cytochrome $c$ are both diffusible electron carriers.
E. Plants have chloroplasts and therefore can live without mitochondria.
F. Both chlorophyll and heme contain an extensive system of double bonds that allows them to absorb visible light.
G. The role of chlorophyll in photosynthesis is equivalent to that of heme in mitochondrial electron transport.
H. Most of the dry weight of a tree comes from the minerals that are taken up by the roots.

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Problem 13

A single proton moving down its electrochemical gradient into the mitochondrial matrix space liberates $4.6 \mathrm{kcal} /$ mole of free energy $(\Delta G)$. How many protons have to flow across the inner mitochondrial membrane to synthesize one molecule of ATP if the $\Delta G$ for ATP synthesis under intracellular conditions is between 11 and 13 kcal/mole? $(\Delta G$ is discussed in Chapter $3,$ pp. $90-100 .$ ) Why is a range given for this latter value, and not a precise number? Under which conditions would the lower value apply?

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Problem 14

In the following statement, choose the correct one of the alternatives in italics and justify your answer." If no $\mathrm{O}_{2}$ is available, all components of the mitochondrial electrontransport chain will accumulate in their reduced/oxidized form. If $\mathrm{O}_{2}$ is suddenly added again, the electron carriers in cytochrome c oxidase will become reduced/oxidized before/after those in NADH dehydrogenase."

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Problem 15

Assume that the conversion of oxidized ubiquinone to reduced ubiquinone by NADH dehydrogenase occurs on the matrix side of the inner mitochondrial membrane and that its oxidation by cytochrome c reductase occurs on the intermembrane space side of the membrane (see Figures $14-14$ and $14-23$ ). What are the consequences of this arrangement for the generation of the $\mathrm{H}^{+}$ gradient across the membrane?

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Problem 16

If a voltage is applied to two platinum wires (electrodes) immersed in water, then water molecules become split into $\mathrm{H}_{2}$ and $\mathrm{O}_{2}$ gas. At the negative electrode, electrons are donated and $\mathrm{H}_{2}$ gas is released; at the positive electrode, electrons are accepted and $\mathrm{O}_{2}$ gas is produced. When photosynthetic bacteria and plant cells split water, they produce $\mathrm{O}_{2},$ but no $\mathrm{H}_{2}$. Why?

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Problem 17

In an insightful experiment performed in the 1960 s, chloroplasts were first soaked in an acidic solution at $\mathrm{pH} 4$ so that the stroma and thylakoid space became acidified (Figure $Q 14-17$ ). They were then transferred to a basic solution $(\mathrm{pH} 8)$. This quickly increased the $\mathrm{pH}$ of the stroma to $8,$ while the thylakoid space temporarily remained at $\mathrm{pH} 4 .$ A burst of ATP synthesis was observed, and the pH difference between the thylakoid and the stroma then disappeared.
A. Explain why these conditions lead to ATP synthesis.
B. Is light needed for the experiment to work?
C. What would happen if the solutions were switched so that the first incubation is in the pH 8 solution and the second one in the pH 4 solution?
D. Does the experiment support or question the chemiosmotic model?
Explain your answers.

Nicole Hewett

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Problem 18

As your first experiment in the laboratory, your adviser asks you to reconstitute purified bacteriorhodopsin, a light-driven $\mathrm{H}^{+}$ pump from the plasma membrane of photosynthetic bacteria, and purified ATP synthase from ox-heart mitochondria together into the same membrane vesicles-as shown in Figure $014-18 .$ You are then asked to add $\mathrm{ADP}$ and $\mathrm{P}_{\mathrm{i}}$ to the external medium and shine light into the suspension of vesicles.
A. What do you observe?
B. What do you observe if not all the detergent is removed and the vesicle membrane therefore remains leaky to ions?
C. You tell a friend over dinner about your new experiments, and he questions the validity of an approach that utilizes components from so widely divergent, unrelated organisms: "Why would anybody want to mix vanilla pudding with brake fluid?" Defend your approach against his critique.

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Problem 19

$\mathrm{FADH}_{2}$ is produced in the citric acid cycle by a membrane-embedded enzyme complex, called succinate dehydrogenase, that contains bound FAD and carries out the reactions
$$\text { succinate }+\mathrm{FAD} \rightarrow \text { fumarate }+\mathrm{FADH}_{2}$$ and $$\mathrm{FADH}_{2} \rightarrow \mathrm{FAD}+2 \mathrm{H}^{+}+2 e^{-}$$
The redox potential of FADH $_{2}$, however, is only $-220 \mathrm{mV}$ Referring to Panel $14-1$ (p. 466 ) and Figure $14-24$, suggest a plausible mechanism by which its electrons could be fed into the electron-transport chain. Draw a diagram to illustrate your proposed mechanism.

Nicole Hewett

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Problem 20

Some bacteria have become specialized to live in an environment of high $\mathrm{pH}(\mathrm{pH} \sim 10) .$ Do you suppose that these bacteria use a proton gradient across their plasma membrane to produce their ATP? (Hint: all cells must maintain their cytoplasm at a pH close to neutrality.)

Nicole Hewett

Numerade Educator

Problem 21

Figure $014-21$ summarizes the circuitry used by mitochondria and chloroplasts to interconvert different forms of energy. Is it accurate to say
A. that the products of chloroplasts are the substrates for mitochondria?
B. that the activation of electrons by the photosystems enables chloroplasts to drive electron transfer from $\mathrm{H}_{2} \mathrm{O}$ to carbohydrate, which is the opposite direction of electron transfer in the mitochondrion?
C. that the citric acid cycle is the reverse of the normal carbon-fixation cycle?

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Problem 22

A manuscript has been submitted for publication to a prestigious scientific journal. In the paper, the authors describe an experiment in which they have succeeded in trapping an individual ATP synthase molecule and then mechanically rotating its head by applying a force to it. The authors show that upon rotating the head of the ATP synthase, ATP is produced, in the absence of an $\mathrm{H}^{+}$ gradient. What might this mean about the mechanism whereby ATP synthase functions? Should this manuscript be considered for publication in one of the best journals?

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Problem 23

You mix the following components in a solution. Assuming that the electrons must follow the path specified in Figure $14-14,$ in which experiments would you expect a net transfer of electrons to cytochrome c? Discuss why electron transfer does not occur in the other experiments.
A. reduced ubiquinone and oxidized cytochrome $c$
B. oxidized ubiquinone and oxidized cytochrome $c$
C. reduced ubiquinone and reduced cytochrome $c$
D. oxidized ubiquinone and reduced cytochrome $c$
E. reduced ubiquinone, oxidized cytochrome $c,$ and cytochrome c reductase complex
F. oxidized ubiquinone, oxidized cytochrome $c,$ and cytochrome c reductase complex
G. reduced ubiquinone, reduced cytochrome $c,$ and cytochrome c reductase complex
H. oxidized ubiquinone, reduced cytochrome $c,$ and cytochrome c reductase complex

Nicole Hewett

Numerade Educator