TCA cycle: energy yield: 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 · TCA cycle: energy yield

Sample item

Using P/O ratios of 2.5 for NADH and 1.5 for FADH2, how many ATP equivalents are generated by oxidation of one acetyl-CoA in the TCA cycle?

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

The answer and the reasoning

D. 10

One turn of the cycle yields 3 NADH, 1 FADH2 and 1 GTP (from succinyl-CoA synthetase). This gives 3 x 2.5 = 7.5, plus 1.5 from FADH2, plus 1 from GTP, which totals 10 ATP equivalents.

Option by option

  • A. 12

    Twelve uses the older P/O ratios of 3 and 2, which the question excludes.

  • B. 7.5

    7.5 counts only the three NADH and omits FADH2 and GTP.

  • C. 8

    Eight does not correspond to any correct accounting of the NADH, FADH2 and GTP produced per turn.

  • D. 10 (correct)

    7.5 from three NADH, 1.5 from FADH2 and 1 from GTP gives 10.

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.

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Questions about this Biochemistry question

What is the answer to this Biochemistry question?

The best answer is D, 10. One turn of the cycle yields 3 NADH, 1 FADH2 and 1 GTP (from succinyl-CoA synthetase).

Why is "12" not the answer?

Twelve uses the older P/O ratios of 3 and 2, which the question excludes.

Why is "7.5" not the answer?

7.5 counts only the three NADH and omits FADH2 and GTP.

Why is "8" not the answer?

Eight does not correspond to any correct accounting of the NADH, FADH2 and GTP produced per turn.

What concept does this question test?

It tests TCA cycle: energy yield in Biochemistry. This gives 3 x 2.5 = 7.5, plus 1.5 from FADH2, plus 1 from GTP, which totals 10 ATP equivalents.

How hard is this question?

We tag it as a multi-step question: it asks you to combine more than one idea. 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 TCA cycle: energy yield?

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.