AP Biology: Cellular Respiration Regulation and Feedback Practice Questions
Use original cell models and enzyme experiments to connect energy demand with pathway activity. Distinguish a measured response from a proposed mechanism, and apply each experiment's stated conditions. This is independent study practice, not an official AP resource.
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1. In a cell model, ATP accumulation inhibits phosphofructokinase and slows glucose breakdown. ATP use then lowers ATP abundance and releases that inhibition. What makes this a negative feedback loop?
Explanation. The stated response opposes the original rise in ATP: accumulated ATP slows a pathway contributing to ATP supply. When ATP is used, inhibition relaxes. The loop adjusts supply in response to the output rather than permanently destroying the enzyme.
2. An enzyme requires ATP at its catalytic site, but excess ATP also binds a separate inhibitory site. A preparation speeds up as ATP rises from very low levels, then slows at much higher levels. Which explanation fits both observations?
Explanation. At low ATP availability, supplying more substrate can raise the rate. At high ATP, occupancy of the separate inhibitory site can reduce activity. Distinct binding roles explain why the effect need not follow one direction across all concentrations.
3. A purified glycolytic enzyme has a regulatory site that binds AMP and increases activity. With ATP, sugar substrate, pH, temperature, and enzyme amount fixed, adding AMP raises product formation. What does the experiment directly support?
Explanation. The controlled assay measures this enzyme's response to AMP. It supports activation under those conditions. A purified enzyme tube cannot establish changes in organelle number or predict the net ATP concentration of a whole cell with many other reactions.
4. Two phosphofructokinase preparations have equal activity at low citrate. High citrate reduces the normal enzyme's rate but leaves a variant's rate unchanged. Both retain the same catalytic response to substrate without citrate. Which hypothesis best fits?
Explanation. Preserved baseline catalysis with a changed response to citrate points to altered regulation. The data do not identify a particular amino acid or binding site, but they contradict claims that the variant cannot catalyze the reaction at all.
5. A simplified pathway converts S to X to Y to Z. Z binds and inhibits the enzyme for S to X. Starting from steady operation, Z is suddenly added while S supply stays constant. Before enzyme amounts change, which immediate response is predicted?
Explanation. The model explicitly assigns Z an inhibitory effect on the first enzyme. Adding Z therefore reduces that reaction's rate. This prediction concerns X formation, not necessarily X concentration, which also depends on how rapidly the next step consumes X.
6. In matched assays, an enzyme forms 30 micromoles of product per minute without an inhibitor and 18 with it. What is the percentage decrease relative to the uninhibited rate?
Explanation. The decrease is 30 minus 18, or 12 micromoles per minute. Dividing the decrease by the original rate gives 12/30 = 0.40, so the rate decreases by 40%. The remaining 18/30, or 60%, is the fraction of activity retained.
7. A model of intact, coupled mitochondria holds oxygen, electron donors, phosphate, and temperature adequate and constant. ADP is initially limiting. Adding ADP increases oxygen consumption. Which explanation best fits this model?
Explanation. ADP availability limits ATP synthesis in the stated model. Supplying ADP permits greater use of the proton gradient through ATP synthase and supports faster coupled electron transport. Oxygen remains the terminal electron acceptor; ADP does not replace it.
8. An isolated enzyme's rate falls when NADH is added while all other assay conditions are held constant. In a living cell, high NADH also accompanies slower pathway activity. Which additional claim would go beyond these observations?
Explanation. The controlled assay supports an effect of NADH on the isolated enzyme. It does not establish that NADH is the only influence in a living cell. Substrate supply, other regulators, enzyme abundance, and transport can also affect pathway activity.
9. A cell's modeled ATP pool receives ATP at 9 units per second and uses it at 6 units per second. ATP accumulation inhibits supply until production is 6 units per second, while use stays at 6. What is the predicted pool behavior before and after adjustment?
Explanation. Net pool change equals input minus use. Initially that is 9 minus 6 = 3 units per second. After feedback adjustment it is 6 minus 6 = 0. A zero net change means a stable pool, not an empty pool or a halt in ATP turnover.
10. A signaling treatment lowers an enzyme's activity within seconds while measured enzyme amount stays constant. A phosphatase treatment restores activity in the same preparation. Which proposed mechanism is most consistent with these results?
Explanation. The rapid response at constant enzyme amount points to regulation of existing molecules. Restoration by a phosphatase is consistent with removal of a regulatory phosphate. This is a supported mechanism hypothesis, not identification of the exact modified residue.
11. Two experimental pathways contain identical enzymes. In one, a downstream product inhibits an earlier step that contributes to its supply. In the other, an upstream intermediate activates a later step. How should these two arrangements be classified?
Explanation. A downstream product acting back on an earlier step forms feedback; the stated effect is inhibitory. An upstream intermediate promoting a later step acts forward along the pathway. Neither arrangement requires the regulator to serve as a consumed substrate.
12. A proposed feedback model predicts that high ATP reduces an enzyme's activity. Two buffers differ in both ATP concentration and pH, and the high-ATP buffer gives a lower rate. Which next comparison best isolates ATP's effect?
Explanation. The initial comparison confounds ATP with pH. Holding the other listed conditions constant while varying ATP provides a cleaner test of its effect on activity. Repeating only one buffer does not separate the two candidate explanations.
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FAQ
Does a stable ATP concentration mean respiration has stopped?
No. ATP production and use can continue at equal rates, leaving the pool approximately constant. Distinguish the amount present from the rate at which it is produced and consumed.
Can ATP be both a substrate and a regulator?
Yes. An enzyme can use ATP in catalysis while also responding to ATP at a separate regulatory site. The net activity depends on concentration and the conditions specified in the problem.
How should I evaluate an enzyme regulation experiment?
Identify the changed variable, the controlled conditions, and the measured rate. Use those observations to assess the proposed mechanism, and avoid treating an isolated enzyme result as proof of every whole-cell response.