Oxidative phosphorylation: practice question

A Biochemistry question in the style of the NEET PG paper. Answer it first, then read why the right option is right and why each of the others is wrong.

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Biochemistry · Oxidative phosphorylation

Sample item

Which complex of the electron-transport chain does not pump protons across the inner mitochondrial membrane?

Original practice item, not a recalled exam question. Nothing here is recorded.

The answer and the reasoning

B. Complex II

Complex II, succinate dehydrogenase, transfers electrons from FADH2 to coenzyme Q but does not pump protons. Complexes I, III, and IV contribute to the proton gradient.

Option by option

  • A. Complex IV

    Complex IV pumps protons and reduces oxygen to water.

  • B. Complex II (correct)

    Correct. Complex II is the non-proton-pumping complex.

  • C. Complex I

    Complex I pumps protons while accepting electrons from NADH.

  • D. Complex III

    Complex III pumps protons through the Q cycle.

Source note: Original NextPrep sample item — pending expert review. This item is original practice material awaiting independent expert review, so confirm important facts in a standard textbook.

More Biochemistry questions

Questions about this Biochemistry question

What is the answer to this Biochemistry question?

The best answer is B, Complex II. Complex II, succinate dehydrogenase, transfers electrons from FADH2 to coenzyme Q but does not pump protons.

Why is "Complex IV" not the answer?

Complex IV pumps protons and reduces oxygen to water.

Why is "Complex I" not the answer?

Complex I pumps protons while accepting electrons from NADH.

Why is "Complex III" not the answer?

Complex III pumps protons through the Q cycle.

What concept does this question test?

It tests Oxidative phosphorylation in Biochemistry. Complexes I, III, and IV contribute to the proton gradient.

How hard is this question?

We tag it as an application question: it asks you to use a fact on a patient or a finding. The tag is our own editorial judgement, not an official difficulty rating, so read the reason for every option, not only the answer.

How should I study Oxidative phosphorylation?

Build one table of enzyme deficiency, disease, inheritance and key finding, then revise it by spaced recall. Then use the flashcards and the other Biochemistry questions on this site to test what stuck.

Is this a real NEET PG question?

No. It is an original practice item (editorial status: sample, version 1). NextPrep does not publish recalled or leaked questions, and independent faculty review of this item has not been completed yet, so confirm important facts in a standard textbook.