MCAT Biological and Biochemical Foundations of Living Systems Quiz: 1d Tca Cycle Oxidative Phosphorylation
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1d Tca Cycle Oxidative PhosphorylationQuestion 1 of 20

A pharmacologic inhibitor blocks the conversion of succinyl-CoA to succinate. In treated cells, succinyl-CoA increases while succinate decreases; OCR decreases modestly.

Which change would most likely increase ATP production under these conditions?

Increase cytosolic NADH to drive mitochondrial ATP synthase directly
Inhibit Complex II to prevent succinate oxidation
Inhibit pyruvate dehydrogenase to reduce acetyl-CoA input
Restore substrate-level phosphorylation at succinyl-CoA synthetase to increase GTP/ATP formation
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MCAT Biological and Biochemical Foundations of Living Systems Quiz

MCAT Biological and Biochemical Foundations of Living Systems Quiz: 1d Tca Cycle Oxidative Phosphorylation

Practice 1d Tca Cycle Oxidative Phosphorylation in MCAT Biological and Biochemical Foundations of Living Systems with focused quiz questions that help you check what you know, review explanations, and build confidence with test-style prompts.

What this quiz covers

This quiz focuses on 1d Tca Cycle Oxidative Phosphorylation, giving you a quick way to practice the rules, question types, and explanations that matter most for MCAT Biological and Biochemical Foundations of Living Systems.

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Try each quiz question before looking at the correct answer. Use the explanations to review missed ideas, then come back to similar questions until the pattern feels familiar.

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Question 1

A pharmacologic inhibitor blocks the conversion of succinyl-CoA to succinate. In treated cells, succinyl-CoA increases while succinate decreases; OCR decreases modestly.

Which change would most likely increase ATP production under these conditions?

  1. Increase cytosolic NADH to drive mitochondrial ATP synthase directly
  2. Inhibit Complex II to prevent succinate oxidation
  3. Inhibit pyruvate dehydrogenase to reduce acetyl-CoA input
  4. Restore substrate-level phosphorylation at succinyl-CoA synthetase to increase GTP/ATP formation (correct answer)
Explanation: This question tests understanding of the Citric Acid Cycle and Oxidative Phosphorylation. Energy production includes substrate-level phosphorylation at succinyl-CoA synthetase, generating GTP/ATP in TCA. The stimulus shows inhibitor blocking succinyl-CoA to succinate, increasing succinyl-CoA, decreasing succinate, and modestly decreasing OCR, illustrating partial TCA block. The correct answer (D) follows as restoring substrate-level ATP compensates for reduced ETC ATP. A distractor like (B) fails by further blocking succinate oxidation, worsening flux. To verify, measure succinyl-CoA: accumulation confirms block at synthetase. Consider ATP production efficiency: blocking reduces TCA-linked ATP, but substrate-level can partially compensate.

Question 2

A compound inhibits citrate transport out of mitochondria. In proliferating cells, this leads to decreased cytosolic acetyl-CoA and increased mitochondrial citrate. OCR increases modestly.

Which mechanism most plausibly explains the increase in OCR?

  1. Cytosolic acetyl-CoA directly stimulates Complex IV activity
  2. Mitochondrial citrate accumulation favors continued TCA cycling and NADH production for the ETC (correct answer)
  3. Blocking citrate export increases glycolytic ATP, which increases OCR
  4. Citrate export inhibition uncouples oxidative phosphorylation by increasing proton leak
Explanation: This question tests understanding of the Citric Acid Cycle and Oxidative Phosphorylation. Energy production involves TCA generating NADH/FADH2, with citrate export for cytosolic uses, but retention may enhance mitochondrial flux. The stimulus shows inhibited citrate export decreasing cytosolic acetyl-CoA, increasing mitochondrial citrate, and modestly increasing OCR, illustrating retained citrate fueling TCA. The correct answer (B) follows as mitochondrial citrate accumulation promotes TCA cycling and NADH for ETC, boosting OCR. A distractor like (A) fails since cytosolic acetyl-CoA doesn't directly stimulate Complex IV. To verify, track citrate levels: mitochondrial buildup should correlate with OCR increase. Consider ATP production efficiency: inhibiting export may enhance oxidative ATP by retaining carbons in TCA.

Question 3

A study compared two conditions in isolated mitochondria with ADP present: Condition 1: pyruvate + malate Condition 2: succinate A Complex I inhibitor is added.

Which outcome is most consistent with the inhibitor's effect on ATP production across conditions?

  1. ATP production decreases in both conditions to the same extent
  2. ATP production decreases more in Condition 1 than in Condition 2 (correct answer)
  3. ATP production increases in Condition 1 because NADH accumulates
  4. ATP production increases in Condition 2 because succinate requires Complex I
Explanation: This question tests understanding of the Citric Acid Cycle and Oxidative Phosphorylation. Energy production varies by substrate: pyruvate + malate feed Complex I via NADH, succinate via Complex II via FADH2. The stimulus compares conditions with Complex I inhibitor, illustrating differential reliance on Complex I. The correct answer (B) follows as Condition 1 (NADH-linked) decreases more than Condition 2 (FADH2-linked bypassing I). A distractor like (D) fails since succinate bypasses, not requires, Complex I. To verify, test inhibitors: Complex I blocks NADH- but not succinate-oxidation. Consider ATP production efficiency: succinate yields ~1.5 ATP fewer per molecule than NADH substrates due to bypassed pumping.

Question 4

Cells were exposed to hypoxia (1% O2_2) for 2 hours. Compared with normoxia, OCR decreased and intracellular NADH/NAD+^+ increased. TCA intermediates showed increased succinate and decreased fumarate.

Based on the data, which enzyme is most likely rate-limited under hypoxia?

  1. Succinate dehydrogenase (Complex II) (correct answer)
  2. Citrate synthase
  3. Pyruvate kinase
  4. Hexokinase
Explanation: This question tests understanding of the Citric Acid Cycle and Oxidative Phosphorylation. Energy production under hypoxia limits O2 for ETC, causing NADH buildup and TCA slowdown. The stimulus shows decreased OCR, increased NADH/NAD+, elevated succinate, decreased fumarate under hypoxia, illustrating reverse SDH activity. The correct answer, succinate dehydrogenase (A), follows as it's rate-limited by high NADH driving reversal. A distractor like (B) fails since citrate synthase isn't directly O2-dependent or showing succinate buildup. To verify, measure redox state: high NADH/NAD+ inhibits forward SDH. Consider ATP production efficiency: hypoxia shifts to glycolysis, reducing ATP yield per glucose.

Question 5

Mitochondria were incubated with pyruvate + malate. Addition of a competitive inhibitor of citrate synthase caused decreased citrate formation and increased acetyl-CoA levels.

Which downstream effect on oxidative phosphorylation is most likely?

  1. Increased NADH production due to faster TCA cycling
  2. Decreased NADH production, reducing electron delivery to the ETC and ATP synthesis (correct answer)
  3. Increased proton pumping at Complex II due to acetyl-CoA accumulation
  4. No change in ATP production because glycolysis can occur in mitochondria
Explanation: This question tests understanding of the Citric Acid Cycle and Oxidative Phosphorylation. Energy production starts with citrate synthase condensing acetyl-CoA and OAA, fueling TCA NADH for ETC. The stimulus shows citrate synthase inhibitor decreasing citrate and increasing acetyl-CoA, illustrating blocked TCA entry. The correct answer (B) follows as decreased NADH production reduces ETC activity and ATP synthesis. A distractor like (C) fails since Complex II doesn't pump protons, not increasing pumping. To verify, monitor TCA flux: reduced citrate confirms downstream NADH drop. Consider ATP production efficiency: inhibiting entry reduces ATP per pyruvate by limiting reducing equivalents.

Question 6

A patient-derived fibroblast line carries a loss-of-function mutation in the E3 component of α\alpha-ketoglutarate dehydrogenase complex. Under aerobic conditions, metabolomics shows elevated α\alpha-ketoglutarate and decreased succinyl-CoA.

Which prediction is consistent with the effect of the mutation on cellular respiration?

  1. Increased NADH production in the TCA cycle, increasing proton pumping
  2. Decreased NADH generation, reducing electron flow through Complex I and ATP synthesis (correct answer)
  3. Increased glycolytic NADH directly increases mitochondrial Complex I flux without shuttles
  4. Increased FADH2_2 production at Complex II increases proton pumping at Complex II
Explanation: This question tests understanding of the Citric Acid Cycle and Oxidative Phosphorylation. Energy production relies on TCA enzymes like α-KGDH oxidizing α-ketoglutarate to succinyl-CoA, producing NADH for ETC. The stimulus shows E3 mutation elevating α-ketoglutarate and decreasing succinyl-CoA, illustrating blocked TCA at α-KGDH. The correct answer (B) follows as decreased NADH reduces ETC electron flow and ATP synthesis. A distractor like (D) fails since Complex II pumps no protons, not increasing pumping there. To verify, check downstream intermediates: depletion confirms flux block. Consider ATP production efficiency: α-KGDH mutation reduces NADH per TCA turn, lowering ATP yield.

Question 7

Isolated mitochondria were supplied with NADH-generating substrates and ADP. After addition of an uncoupler, OCR increased to 180% of baseline while ΔΨm\Delta\Psi_m decreased to 55% of baseline.

Which statement best explains the observed increase in OCR?

  1. Reduced proton backpressure accelerates electron transport, increasing O2_2 consumption (correct answer)
  2. Uncouplers directly donate electrons to Complex IV, increasing O2_2 consumption
  3. Uncouplers inhibit ATP synthase, causing NADH to accumulate and drive OCR up
  4. Uncouplers increase ATP yield per NADH, increasing OCR to meet ATP demand
Explanation: This question tests understanding of the Citric Acid Cycle and Oxidative Phosphorylation. Energy production uses ETC to create proton gradient, with OCR reflecting electron flow coupled to ATP synthesis. The stimulus shows uncoupler increasing OCR to 180% and decreasing ΔΨm to 55%, illustrating dissipation of gradient accelerating ETC. The correct answer (A) follows as reduced backpressure speeds electron transport, boosting O2 use. A distractor like (D) fails since uncouplers decrease ATP yield per NADH by wasting gradient heat. To verify, monitor ΔΨm: uncouplers collapse it while increasing OCR. Consider ATP production efficiency: uncoupling reduces efficiency, producing heat over ATP.

Question 8

Cells were treated with a selective inhibitor of mitochondrial pyruvate carrier (MPC). In the presence of glucose, investigators observed decreased acetyl-CoA labeling from 13^{13}C-glucose and decreased OCR, while lactate secretion increased.

Which change would most likely increase ATP production under these conditions?

  1. Increase cytosolic pyruvate by inhibiting pyruvate kinase
  2. Provide fatty acids to increase mitochondrial acetyl-CoA generation independent of MPC (correct answer)
  3. Inhibit lactate dehydrogenase to prevent NAD+^+ regeneration
  4. Inhibit citrate synthase to reduce acetyl-CoA utilization
Explanation: This question tests understanding of the Citric Acid Cycle and Oxidative Phosphorylation. Energy production requires pyruvate entry into mitochondria via MPC for conversion to acetyl-CoA, fueling TCA and ETC. The stimulus shows MPC inhibition decreasing acetyl-CoA from glucose, OCR, and increasing lactate, illustrating diverted pyruvate to fermentation. The correct answer (B) follows as fatty acids provide acetyl-CoA via beta-oxidation, bypassing MPC to restore TCA/ATP. A distractor like (D) fails by reducing acetyl-CoA use, worsening accumulation without increasing flux. To verify, measure acetyl-CoA sources: alternative substrates should rescue OCR. Consider ATP production efficiency: bypassing MPC with lipids maintains high-yield oxidative ATP over glycolysis.

Question 9

A small molecule selectively inhibits the mitochondrial dicarboxylate carrier, limiting malate import into the matrix. In intact cells grown on glucose, metabolomics shows decreased mitochondrial NADH and increased cytosolic NADH.

Which outcome is most likely for oxidative phosphorylation?

  1. Increased Complex I activity due to higher cytosolic NADH
  2. Decreased electron supply to the ETC, reducing ATP production (correct answer)
  3. Increased ATP production because malate import normally inhibits the TCA cycle
  4. No change in ATP production because FADH2_2 fully substitutes for NADH at Complex I
Explanation: This question tests understanding of the Citric Acid Cycle and Oxidative Phosphorylation. Energy production depends on shuttles like malate-aspartate transferring cytosolic NADH into mitochondria for ETC oxidation and ATP synthesis. The stimulus shows inhibition of dicarboxylate carrier limiting malate import, decreasing mitochondrial NADH and increasing cytosolic NADH, illustrating disrupted shuttle reducing mitochondrial reducing power. The correct answer (B) follows as decreased electron supply to ETC reduces proton gradient and ATP production. A distractor like (D) fails since FADH2 via Complex II doesn't substitute for NADH at Complex I and yields less ATP. To verify, assess shuttle activity: blocking malate import should decrease mitochondrial NADH oxidation. Consider ATP production efficiency: shuttle inhibition reduces ATP per glucose by limiting NADH access to ETC.

Question 10

A mutation reduces the activity of mitochondrial ATP synthase without affecting ETC complexes. In intact cells supplied with glucose, investigators observe decreased OCR and increased ΔΨm\Delta\Psi_m.

Which prediction is consistent with this mutation?

  1. ATP production increases because fewer protons are required per ATP
  2. Electron transport accelerates because ATP synthase no longer consumes ADP
  3. NADH oxidation increases because ΔΨm\Delta\Psi_m is higher
  4. Electron transport slows due to increased proton-motive force opposing further pumping (correct answer)
Explanation: This question tests understanding of the Citric Acid Cycle and Oxidative Phosphorylation. Energy production couples ETC proton pumping to ATP synthase proton flow, with synthase defects affecting respiratory control. The stimulus shows decreased OCR and increased ΔΨm in cells with ATP synthase mutation, illustrating impaired proton re-entry. The correct answer (D) follows as high ΔΨm creates backpressure, slowing electron transport. A distractor like (B) fails by incorrectly predicting accelerated transport without synthase activity. To verify, measure gradient: synthase defects increase ΔΨm and decrease OCR. Consider ATP production efficiency: reduced synthase lowers ATP yield despite intact ETC.

Question 11

Researchers measured citrate synthase flux in isolated mitochondria by tracking incorporation of 13^{13}C-acetyl-CoA into citrate. Under condition Y, acetyl-CoA incorporation decreased while oxaloacetate (OAA) concentration increased and NADH/NAD+^+ ratio increased.

Which change would most likely increase ATP production under condition Y?

  1. Increase cytosolic NADH by inhibiting lactate dehydrogenase
  2. Enhance NADH oxidation by adding an uncoupler to increase ΔΨm\Delta\Psi_m
  3. Promote NADH reoxidation by increasing electron transport chain capacity (correct answer)
  4. Inhibit pyruvate dehydrogenase to reduce acetyl-CoA entry into the TCA cycle
Explanation: This question tests understanding of the Citric Acid Cycle and Oxidative Phosphorylation. Energy production requires NAD+ regeneration via ETC for continued TCA flux and ATP synthesis from the proton gradient. The stimulus shows decreased acetyl-CoA incorporation into citrate under condition Y, with increased OAA and NADH/NAD+, illustrating high NADH inhibiting citrate synthase or upstream steps. The correct answer (C) follows as increasing ETC capacity reoxidizes NADH, restoring NAD+ for TCA and boosting ATP. A distractor like (D) fails by further reducing TCA flux, worsening NADH buildup, not improving it. To verify, measure NADH levels: interventions lowering NADH/NAD+ should increase flux. Consider ATP production efficiency: enhancing ETC increases ATP yield by alleviating redox backlog.

Question 12

A researcher compares mitochondrial ATP production in permeabilized cells supplied with either (i) pyruvate + malate or (ii) succinate, each with ADP and inorganic phosphate. In both conditions, a low dose of rotenone is present to prevent reverse electron transport. ATP production rates are shown. Based on the data, which conclusion best accounts for the substrate-dependent difference in ATP production?

  1. Pyruvate + malate yields more ATP because glycolysis directly generates mitochondrial ATP in permeabilized cells
  2. Succinate yields more ATP because Complex II pumps more protons than Complex I
  3. Pyruvate + malate yields less ATP because NADH oxidation bypasses Complex III
  4. Succinate yields less ATP because it donates electrons to the ETC downstream of Complex I, reducing proton pumping (correct answer)
Explanation: This question tests understanding of the Citric Acid Cycle and Oxidative Phosphorylation, specifically the differential ATP yield from NADH versus FADH2-linked substrates. The principle is that electrons from NADH enter the ETC at Complex I and result in proton pumping at Complexes I, III, and IV, while electrons from succinate (via FADH2) enter at Complex II and only result in proton pumping at Complexes III and IV. The data shows higher ATP production with pyruvate + malate (NADH-generating) compared to succinate (FADH2-generating), reflecting the additional proton pumping at Complex I. The correct answer D follows because succinate donates electrons downstream of Complex I, bypassing the first proton-pumping site and therefore generating less ATP per electron pair (~1.5 ATP) compared to NADH (~2.5 ATP). Answer B is incorrect because Complex II does not pump protons at all. A fundamental principle is that the P/O ratio (ATP per oxygen consumed) is higher for NADH-linked substrates due to the additional proton pumping at Complex I.

Question 13

A mitochondrial inner-membrane protonophore (Compound Z) was added to intact hepatocytes supplied with fatty acids and oxygen. Measurements were made 10 minutes after treatment.

Data table (Z relative to vehicle):

  • OCR: 1.8
  • Cellular ATP: 0.55
  • Mitochondrial membrane potential (Δψ\Delta\psi): 0.40
  • Matrix NADH/NAD+^+: 0.60

Based on the data, which interpretation best explains the observed changes?

  1. Compound Z inhibits Complex IV, decreasing OCR and increasing NADH/NAD+^+ due to blocked electron transfer
  2. Compound Z uncouples electron transport from ATP synthesis, increasing OCR while dissipating Δψ\Delta\psi and lowering ATP (correct answer)
  3. Compound Z inhibits ATP synthase, increasing Δψ\Delta\psi and decreasing OCR while ATP remains near baseline
  4. Compound Z activates glycolysis, increasing ATP and decreasing OCR by reducing mitochondrial substrate supply
Explanation: This question tests understanding of the Citric Acid Cycle and Oxidative Phosphorylation. Uncouplers dissipate the proton gradient, accelerating ETC without ATP synthesis, altering respiration and redox states. The stimulus data show increased OCR with decreased ATP, delta psi, and NADH/NAD+ after Compound Z, typical of protonophore action. This indicates uncoupling, boosting OCR by gradient dissipation while reducing ATP, as in choice B. A distractor like choice A suggests Complex IV inhibition, which decreases OCR, but data shows increased OCR, highlighting incorrect electron flow interpretation. To verify, monitor gradient-dependent changes: uncouplers uniquely elevate OCR while collapsing delta psi. Consider efficiency: uncoupling wastes energy as heat, reducing ATP yield per oxygen consumed.

Question 14

A patient-derived fibroblast line carries a missense mutation in SDHB (a Complex II subunit) that reduces succinate dehydrogenase activity. Cells were cultured under normoxia with glutamine present. Metabolites and respiration were measured.

Data table (mutant relative to control):

  • Succinate: 3.1
  • Fumarate: 0.55
  • CoQH$_2$/CoQ: 0.70
  • Basal OCR: 0.75

Which prediction is consistent with the effect of the mutation on cellular respiration?

  1. Electron entry at Complex II is reduced, decreasing CoQ reduction and lowering downstream oxygen consumption (correct answer)
  2. Complex II inhibition increases fumarate production, increasing OCR by accelerating the TCA cycle
  3. Succinate accumulation directly increases ATP synthase activity by increasing proton pumping at Complex II
  4. The primary defect is impaired glycolytic ATP production, explaining elevated succinate and reduced CoQH$_2$/CoQ
Explanation: This question tests understanding of the Citric Acid Cycle and Oxidative Phosphorylation. Complex II oxidizes succinate to fumarate, reducing CoQ for ETC electron flow. The stimulus shows succinate accumulation, low fumarate, CoQH2/CoQ, and OCR in SDHB mutants, signaling enzymatic defect. This reduces electron entry at Complex II, decreasing CoQ reduction and oxygen consumption, as in choice A. A distractor like choice B claims increased fumarate and OCR, but data shows low fumarate, indicating flawed TCA flux interpretation. To verify, assess metabolite ratios: high succinate with low CoQH2 confirms Complex II block. Consider ATP efficiency: impaired Complex II lowers electrons to ETC, reducing phosphorylation capacity.

Question 15

Malate dehydrogenase is inhibited. Immediately, which pair of TCA changes occurs?

  1. Malate accumulates; OAA falls (correct answer)
  2. Citrate accumulates; OAA rises
  3. Fumarate rises; malate falls
  4. Succinyl-CoA and citrate rise
Explanation: Malate dehydrogenase converts malate into OAA, so inhibiting it immediately lowers OAA production and causes malate to build up. The tempting wrong answer is that upstream intermediates like citrate rise, but the direct substrate of the blocked enzyme accumulates first, not distant earlier products.

Question 16

To assess metabolic flux, investigators pulsed cells with 13^{13}C-labeled pyruvate under normoxia and quantified labeling in TCA intermediates after 2 minutes. A selective inhibitor of the mitochondrial pyruvate carrier (MPC) was applied 15 minutes prior to the pulse.

Data table (fractional 13^{13}C enrichment; inhibitor vs vehicle):

  • Citrate: 0.20 vs 0.62
  • α\alpha-ketoglutarate: 0.18 vs 0.58
  • Malate: 0.22 vs 0.55
  • Lactate (cytosolic): 0.78 vs 0.40

Which change would most likely increase ATP production under the inhibitor condition?

  1. Decrease ADP availability to increase proton-motive force and stimulate ATP synthase flux
  2. Further increase lactate dehydrogenase activity to maximize cytosolic NADH delivery to Complex I
  3. Inhibit Complex II to force electron entry through Complex I and increase ATP yield per electron
  4. Increase mitochondrial pyruvate import by restoring MPC activity to raise TCA-derived NADH for oxidative phosphorylation (correct answer)
Explanation: This question tests understanding of the Citric Acid Cycle and Oxidative Phosphorylation. Pyruvate entry via MPC fuels TCA, producing NADH for ETC and ATP synthesis. The data show reduced 13C enrichment in TCA intermediates but increased lactate with MPC inhibition, indicating diverted pyruvate. Restoring MPC activity increases pyruvate import, enhancing TCA NADH and oxidative phosphorylation, matching choice D. A distractor like choice B suggests boosting lactate dehydrogenase, but this would further divert pyruvate from mitochondria, worsening ATP via incorrect pathway prioritization. To verify, trace labeled carbon: restored import elevates TCA labeling and respiration. Consider energy yield: mitochondrial pyruvate oxidation provides more ATP than cytosolic lactate production.

Question 17

Why is O2 required for continued citric acid cycle flux?

  1. To oxidize acetyl-CoA directly
  2. To power ATP synthase directly
  3. To remove CO2 from citrate
  4. To regenerate NAD+ and FAD (correct answer)
Explanation: The cycle's dehydrogenases transfer electrons to NAD+ and FAD, making NADH and FADH2. Without O2 as the final electron acceptor in the chain, these cofactors stay reduced, so NAD+ and FAD run out and the cycle stalls. The tempting wrong answer is that O2 powers ATP synthase directly; actually ATP synthase runs on the proton gradient, while O2 accepts electrons at the end of the chain.

Question 18

A mutation inactivates complex I. Which process is unaffected?

  1. NADH oxidation by the ETC
  2. ATP synthesis from NADH
  3. FADH2 oxidation by the ETC (correct answer)
  4. Proton pumping at complex I
Explanation: FADH2 delivers electrons at complex II, downstream of complex I, so inactivating complex I does not block its oxidation. NADH oxidation, proton pumping at complex I, and ATP synthesis from NADH all require complex I. The tempting wrong answer is ATP synthesis from NADH, which fails because NADH cannot enter the chain without complex I.

Question 19

ATP synthase is blocked. Which pair follows?

  1. H+ gradient same; O2 use up
  2. H+ gradient up; O2 use up
  3. H+ gradient down; O2 use same
  4. H+ gradient up; O2 use down (correct answer)
Explanation: Blocking ATP synthase stops protons from flowing back into the matrix, so the H+ gradient builds up. The electron transport chain cannot pump against that steep gradient, so electron flow halts and O2 consumption drops. A tempting wrong choice is that O2 use stays the same because the chain is separate, but the chain is coupled to gradient dissipation through ATP synthase.

Question 20

An uncoupler collapses the proton gradient. Which pair follows?

  1. O2 use falls; ATP unchanged
  2. O2 use rises; ATP falls (correct answer)
  3. O2 use rises; ATP unchanged
  4. O2 use falls; ATP falls
Explanation: Collapsing the proton gradient uncouples electron transport from ATP synthesis. Protons leak back through the membrane instead of driving ATP synthase, so ATP falls. At the same time the loss of the gradient removes back-pressure on the electron transport chain, so oxygen consumption rises to pump more protons. The tempting wrong answer is O2 use falls with ATP unchanged; that ignores that uncoupled respiration speeds up.