Review the Concepts

1. The proton-motive force is essential for both mitochondrial and chloroplast function. What produces the proton-motive force, and what is its relationship to ATP? The compound 2,4-dinitrophenol (DNP), which was used in diet pills in the 1930s but later shown to have dangerous side effects, allows protons to diffuse across membranes. Why is it dangerous to consume DNP?

2. Mitochondria and chloroplasts are thought to have evolved from symbiotic bacteria present in nucleated cells. What is the experimental evidence from this chapter that supports this hypothesis?

3. The inner mitochondrial membrane exhibits all of the fundamental characteristics of a typical cell membrane, but it also has several unique characteristics that are closely associated with its role in oxidative phosphorylation. What are these unique characteristics? How does each contribute to the function of the inner membrane?

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4. Maximal production of ATP from glucose involves the reactions of glycolysis, the citric acid cycle, and the electron-transport chain. Which of these reactions requires O₂, and why? Which, in certain organisms or physiological conditions, can proceed in the absence of O₂?

5. Fermentation permits the continued extraction of energy from glucose in the absence of oxygen. If glucose catabolism is anaerobic, why is fermentation necessary for glycolysis to continue?

6. Describe the step-by-step process by which electrons from glucose catabolism in the cytoplasm are transferred to the electron-transport chain in the inner mitochondrial membrane. In your answer, note whether the electron transfer at each step is direct or indirect.

7. Mitochondrial oxidation of fatty acids is a major source of ATP, yet fatty acids can be oxidized elsewhere. What organelle, besides the mitochondrion, can oxidize fatty acids? What is the fundamental difference between oxidation occurring in this organelle and mitochondrial oxidation?

8. Each of the cytochromes in the mitochondrion contains prosthetic groups. What is a prosthetic group? Which type of prosthetic group is associated with the cytochromes? What property of the various cytochromes ensures unidirectional electron flow along the electron-transport chain?

9. The electron-transport chain consists of a number of multiprotein complexes, which work in conjunction to pass electrons from an electron carrier, such as NADH, to O₂. What is the role of these complexes in ATP synthesis? It has been demonstrated that respiration supercomplexes contain all the protein components necessary for respiration. Why is this beneficial for ATP synthesis, and what is one way that the existence of supercomplexes has been demonstrated experimentally? Coenzyme Q (CoQ) is not a protein, but a small, hydrophobic molecule. Why is it important for the functioning of the electron-transport chain that CoQ is a hydrophobic molecule?

10. It is estimated that each electron pair donated by NADH leads to the synthesis of approximately three ATP molecules, whereas each electron pair donated by FADH₂ leads to the synthesis of approximately two ATP molecules. What is the underlying reason for the difference in yield for electrons donated by FADH₂ versus NADH?

11. Describe the main functions of the different components of the ATP synthase enzyme in the mitochondrion. A structurally similar enzyme is responsible for the acidification of lysosomes and endosomes. Given what you know about the mechanism of ATP synthesis, explain how this acidification might occur.

12. Much of our understanding of ATP synthase is derived from research on aerobic bacteria. What makes these organisms useful for this research? Where do the reactions of glycolysis, the citric acid cycle, and the electron-transport chain occur in these organisms? Where is the proton-motive force generated in aerobic bacteria? What other cellular processes depend on the proton-motive force in these organisms?

13. An important function of the inner mitochondrial membrane is to provide a selectively permeable barrier to the movement of water-soluble molecules and thus to generate different chemical environments on either side of the membrane. However, many of the substrates and products of oxidative phosphorylation are water soluble and must cross the inner membrane. How does this transport occur?

14. The Q cycle plays a major role in the electron-transport chain of mitochondria, chloroplasts, and bacteria. What is the function of the Q cycle, and how does it carry out this function? What electron-transport components participate in the Q cycle in mitochondria, in purple bacteria, and in chloroplasts?

15. True or false: Since ATP is generated in chloroplasts, cells capable of undergoing photosynthesis do not require mitochondria. Explain. Name and describe the idea that explains how mitochondria and chloroplasts are thought to have originated in eukaryotic cells.

16. Write the overall reaction of oxygen-generating photosynthesis. Explain the following statement: The O₂ generated by photosynthesis is simply a by-product of the pathway’s generation of carbohydrates and ATP.

17. Photosynthesis can be divided into multiple stages. What are the stages of photosynthesis, and where does each occur within the chloroplast? Where is the sucrose produced by photosynthesis generated?

18. The photosystems responsible for absorption of light energy are each composed of two linked components, the reaction center and an antenna complex. What is the pigment composition and role of each component in the process of light absorption? What evidence exists that the pigments found in these components are involved in photosynthesis?

19. Photosynthesis in green and purple bacteria does not produce O₂. Why? How can these organisms still use photosynthesis to produce ATP? What molecules serve as electron donors in these organisms?

20. Chloroplasts contain two photosystems. What is the function of each? For linear electron flow, diagram the flow of electrons from photon absorption to NADPH formation. What does the energy stored in the form of NADPH synthesize?

21. The Calvin cycle “dark” reactions, which fix CO₂, do not function in the dark. What are the likely reasons for this? How are these reactions regulated by light?

22. Rubisco, which may be the most abundant protein on Earth, plays a key role in the synthesis of carbohydrates in organisms that use photosynthesis. What is rubisco, where is it located, and what function does it serve?