Answers To Review Questions

1. B. It is the sum of all of the catabolic and anabolic reactions in a cell.

2. B. The Krebs cycle oxidizes acetyl-CoA and two carbon dioxides are lost for each turn of the cycle. The cycle does not use pyruvate, rather, pyruvate is first converted to acetyl-CoA by pyruvate dehydrogenase, and it is acetyl-CoA that is fed into the cycle. Concerning answer D: the electron transport chain drives H+ out of the mitochondrial matrix. ATP synthase can also (by running in reverse) perform this function, but the enzymes of the Krebs cycle certainly do not. Concerning answer E: the Krebs cycle does not use either GTP or ATP to supply phosphate groups, though it does generate one GTP from GDP (step 5 in Fig. 13.2).

3. B. Glycolysis generates pyruvate. Pyruvate can then be converted to acetyl-CoA, but this takes place in mitochondria, not in thecytosol, and is not regarded as being part of the glycolysis pathway.

4. D. f oxidation produces acetyl-CoA from fatty acids and also produces NADH and FADH2.

5. D. Glucose cannot be produced from any fatty acid, including palmitate.

6. C. Photosynthesis takes place in chloroplasts.

7. D. One of the genes within the lux operon is luxI, which codes for the enzyme that makes N-acyl-HSL. This is therefore an example of positive feedback. Concerning the other examples (A and C): these are examples of negative feedback. Activation of phosphofructokinase will cause more fuel to be sent to the mitochondria and will therefore help the mitochondria convert AMP and ADP to ATP. Inhibiting transcription of the trp operon will reduce the rate at which the cell synthesizes tryptophan. (B) This is a sensible control mechanism, causing the cell to use glucose when it is available in preference to lactose. However, there is no feedback involved: the products of the lac operon are concerned with lactose metabolism, while cAMP levels in bacteria are affected by the glucose concentration.

8. B. A coenzyme is a molecule that acts as a second substrate for many enzymes; ATP is a substrate for a wide range of enzymes; some we have already met are polynucleotide kinase (page 139), hexokinase (page 284), phosphofructokinase (page 285), glycogen phosphorylase kinase (page 305), and cAMP-dependent protein kinase (page 305). Answer A is false: As H+ enters the mitochondria ADP is converted to ATP, not the other way around. Answer C is false: ATP is a highly charged ion and will certainly not pass through membranes by simple diffusion; it crosses the outer mitochon-drial membrane by passing through porin. Answer D is false: in the absence of oxygen ATP can be made by anaerobic glycolysis. Answer E is false: in a healthy cell none of the energy currencies are allowed to run down.

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