Answers To Review Questions

1. D. Reactions with negative AG can proceed; those with positive AG cannot proceed. Enzymes are catalysts and can therefore speed up the rate of a reaction, but they cannot cause a reaction to proceed if the AG is positive.

2. B. Although glucose is the major extracellular fuel supplied by the blood to our cells, it is not used as an energy currency inside cells. Rather, on entering cells energy is supplied to glucose to convert it to glucose-6-phosphate.

3. B. The sodium/potassium ATPase is located on the plasma membrane.

4. E. Answers B and C are false: H+ is at a lower concentration in the mitochondrial matrix than in the cytosol, and thermogenin (an alternative route for H+ entry expressed in a few cell types, especially brown fat) will not act to increase the energy released by H+ as it enters. Of answers A, D, and E the last is the best answer because it states the complete source of the energy. It is true that hydrogen ions that enter the mitochondrial matrix through ATP synthase release energy because the mitochondrial matrix is electrically negative with respect to the cytosol, but this is not the only source of energy—the concentration gradient also favors H+ entry. Similarly it is true that hydrogen ions that enter the mitochondrial matrix through ATP synthase release energy because H+ is at a higher concentration in the cytosol than in the mitochondrial matrix, but this is not the only source of energy—the electrical gradient also favors H+ entry.

5. C. Answers A, B, and D are reactions that occur somewhere in the cell, but they are not the reaction catalyzed by the electron transport chain. Answer E is a nonsense equation; in particular, there is no compound NAD as written, and nicotine adenine dinucleotide is always found as NADH or as NAD+. Note that answers A, B, D, and E involve ATP: ATP is not a substrate or product of the electron transport chain.

6. D. Under anaerobic conditions mitochondria cannot generate ATP and are dependent on the rest of the cell for an ATP supply. In an anaerobic animal or yeast cell, the ATP is generated by glycolysis.

7. E. All of answers A through D are true. In particular, while ubiquinone and cytochrome c both function as electron carriers, ubiquinone is hydrophobic and remains within the lipid bilayer, while cytochrome c is a hydrophilic molecule found in the aqueous intermembrane space.

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