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MCAT Citric Acid Cycle, Electron Transport and ATP Yield Practice Questions

By StudyStore · Published 2026-10-20 · 8 original questions · about 12 min

Eight free MCAT-style practice questions on pyruvate dehydrogenase, the citric acid cycle, the electron transport chain, uncouplers, proton gradients and ATP accounting, with explanations.

Quick concept primer

Aerobic respiration is a chain of handoffs. Pyruvate dehydrogenase links glycolysis to the citric acid cycle by converting pyruvate to acetyl-CoA, CO2 and NADH. Each acetyl-CoA turning the cycle yields 3 NADH, 1 FADH2, 1 GTP (or ATP) and 2 CO2. NADH and FADH2 then give electrons to the electron transport chain in the inner mitochondrial membrane, where complexes I, III and IV pump protons from the matrix into the intermembrane space. Complex II (succinate dehydrogenase) passes electrons to the chain but pumps no protons.

ATP synthase uses the proton gradient: protons flow back into the matrix through F0 and drive ATP formation at F1. Using the common modern estimates of about 2.5 ATP per NADH and 1.5 per FADH2, complete oxidation of one glucose yields roughly 30 to 32 ATP, depending on which shuttle moves cytosolic NADH electrons into the mitochondrion.

Traps: respiration is coupled, so blocking ATP synthase also slows oxygen consumption; and uncouplers do the opposite, speeding up oxygen consumption while ATP production falls.

8 practice questions

Try each question before opening the explanation. Click "Show answer and explanation" to check yourself. All questions are original and written for study practice.

Question 1

Which of the following is NOT a cofactor or coenzyme of the pyruvate dehydrogenase complex?

  1. FAD
  2. Biotin
  3. Lipoic acid
  4. Thiamine pyrophosphate
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Answer: B. Biotin

The pyruvate dehydrogenase complex uses five cofactors: thiamine pyrophosphate (B1), lipoic acid, coenzyme A (pantothenate, B5), FAD (riboflavin, B2) and NAD+ (niacin, B3). Biotin is the cofactor for carboxylases such as pyruvate carboxylase and acetyl-CoA carboxylase, not for PDH.

Question 2

One molecule of pyruvate is completely oxidized to CO2 by pyruvate dehydrogenase and one turn of the citric acid cycle. How many NADH, FADH2 and GTP are produced in total?

  1. 4 NADH, 1 FADH2, 1 GTP
  2. 3 NADH, 1 FADH2, 1 GTP
  3. 5 NADH, 1 FADH2, 0 GTP
  4. 4 NADH, 2 FADH2, 2 GTP
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Answer: A. 4 NADH, 1 FADH2, 1 GTP

PDH makes 1 NADH from pyruvate. The cycle then makes 3 NADH (isocitrate dehydrogenase, alpha-ketoglutarate dehydrogenase, malate dehydrogenase), 1 FADH2 (succinate dehydrogenase) and 1 GTP (succinyl-CoA synthetase). The total for one pyruvate is 4 NADH, 1 FADH2 and 1 GTP. Choice B forgets the PDH step.

Question 3

Which complex of the electron transport chain accepts electrons from FADH2 (via succinate) but does NOT pump protons?

  1. Complex I
  2. Complex II
  3. Complex III
  4. Complex IV
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Answer: B. Complex II

Complex II is succinate dehydrogenase, the one citric acid cycle enzyme embedded in the inner membrane. It reduces ubiquinone using electrons from succinate (via FAD) but has too little free energy change to pump protons. This is why FADH2 yields fewer ATP than NADH, which feeds complex I and takes part in three proton-pumping steps.

Question 4

Isolated, well-coupled mitochondria are given substrate and oxygen. Three agents are tested separately.

AgentO2 consumptionATP synthesisProton gradient
Wdecreaseddecreasedincreased
Xdecreaseddecreaseddecreased
Yincreaseddecreaseddecreased

Which agent is most consistent with a classic uncoupler such as 2,4-dinitrophenol?

  1. W
  2. None of them
  3. X
  4. Y
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Answer: D. Y

An uncoupler makes the inner membrane leaky to protons, so the gradient collapses and the electron transport chain runs faster in an attempt to rebuild it, raising oxygen consumption while little ATP is made and the energy is released as heat. Agent W matches an ATP synthase inhibitor such as oligomycin (the gradient builds, back-pressure slows the chain). Agent X matches an electron transport inhibitor such as cyanide (no pumping, no gradient, no oxygen use).

Question 5

During oxidative phosphorylation, which statement about the proton gradient across the inner mitochondrial membrane is correct?

  1. The gradient is used only to heat the cell
  2. The intermembrane space is more basic than the matrix
  3. The intermembrane space is more acidic than the matrix, and protons flow down their gradient through ATP synthase into the matrix
  4. Protons flow through F0 of ATP synthase from the matrix to the intermembrane space
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Answer: C. The intermembrane space is more acidic than the matrix, and protons flow down their gradient through ATP synthase into the matrix

Complexes I, III and IV pump protons out of the matrix, acidifying the intermembrane space and leaving the matrix relatively alkaline and negative. Protons return through the F0 channel of ATP synthase, and the energy of that flow drives ATP synthesis in F1. Heat generation by gradient dissipation occurs only with uncoupling proteins or uncouplers, not as the primary use.

Question 6

Using 2.5 ATP per NADH, 1.5 ATP per FADH2 and 1 ATP per GTP, how many ATP-equivalents are obtained from the complete oxidation of one acetyl-CoA by the citric acid cycle and oxidative phosphorylation?

  1. 12
  2. 10
  3. 7.5
  4. 15
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Answer: B. 10

One turn of the cycle yields 3 NADH, 1 FADH2 and 1 GTP. Then 3 x 2.5 = 7.5, 1 x 1.5 = 1.5 and 1 x 1 = 1, for a total of 10. The older textbook value of 12 comes from assuming 3 ATP per NADH and 2 per FADH2. Choice C counts only the NADH, and choice D (15) is the older value for a whole pyruvate (including the PDH NADH, at 3 ATP per NADH and 2 per FADH2).

Question 7

Which condition would most likely slow the citric acid cycle in a resting cell?

  1. A rise in matrix calcium
  2. High ADP concentration
  3. Low ATP to ADP ratio
  4. High NADH to NAD+ ratio
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Answer: D. High NADH to NAD+ ratio

NADH and ATP inhibit key dehydrogenases (isocitrate dehydrogenase, alpha-ketoglutarate dehydrogenase, PDH). They signal that the chain is backed up and the energy charge is high. ADP and calcium activate those same enzymes, and a low ATP to ADP ratio signals demand for more ATP.

Question 8

Brown adipose tissue contains uncoupling protein 1 (UCP1). When it is active in an infant exposed to cold, which outcome is expected?

  1. Protons return to the matrix through UCP1, bypassing ATP synthase, so more of the energy of electron transport is released as heat
  2. ATP synthesis rises and heat production falls
  3. Oxygen consumption falls
  4. The electron transport chain stops
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Answer: A. Protons return to the matrix through UCP1, bypassing ATP synthase, so more of the energy of electron transport is released as heat

UCP1 is a proton channel in the inner mitochondrial membrane. Dissipating the gradient lowers the back-pressure on the chain, so substrate oxidation and oxygen consumption speed up, and the energy that would have gone into ATP appears as heat, a process called nonshivering thermogenesis. ATP production does not rise, and the chain does not stop.

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Written by StudyStore from standard scientific facts as a study aid. These are not official exam questions and are not affiliated with or endorsed by any testing organization. Always check values against the reference provided with your exam or course.