Fatty acid oxidation: stoichiometry: 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 · Fatty acid oxidation: stoichiometry

Sample item

Stearic acid (18 carbons) is completely oxidised by mitochondrial beta-oxidation. How many cycles occur, and how many acetyl-CoA, NADH and FADH2 are produced?

A9 cycles: 9 acetyl-CoA, 9 NADH and 9 FADH2B8 cycles: 9 acetyl-CoA, 8 NADH and 8 FADH2C9 cycles: 18 acetyl-CoA, 9 NADH and 9 FADH2D8 cycles: 8 acetyl-CoA, 8 NADH and 8 FADH2

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

The answer and the reasoning

B. 8 cycles: 9 acetyl-CoA, 8 NADH and 8 FADH2

A saturated fatty acid with n carbons undergoes (n/2) minus 1 cycles, which for 18 carbons is 9 minus 1, giving 8 cycles. Each cycle yields one NADH and one FADH2, so 8 of each. The last cycle splits a four-carbon unit into two acetyl-CoA, so the total number of acetyl-CoA is 18/2, which is 9.

Option by option

A. 9 cycles: 9 acetyl-CoA, 9 NADH and 9 FADH2
Nine cycles would be needed for a 20-carbon chain; the final cycle yields two acetyl-CoA from the 4-carbon unit.
B. 8 cycles: 9 acetyl-CoA, 8 NADH and 8 FADH2 (correct)
The 18-carbon chain gives 9 acetyl-CoA after 8 cycles, with 8 NADH and 8 FADH2.
C. 9 cycles: 18 acetyl-CoA, 9 NADH and 9 FADH2
Each acetyl-CoA contains two carbons, so 18 acetyl-CoA would need 36 carbons.
D. 8 cycles: 8 acetyl-CoA, 8 NADH and 8 FADH2
Eight acetyl-CoA would account for only 16 carbons.

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.

Questions about this Biochemistry question

What is the answer to this Biochemistry question?

The best answer is B, 8 cycles: 9 acetyl-CoA, 8 NADH and 8 FADH2. A saturated fatty acid with n carbons undergoes (n/2) minus 1 cycles, which for 18 carbons is 9 minus 1, giving 8 cycles.

Why is "9 cycles: 9 acetyl-CoA, 9 NADH and 9 FADH2" not the answer?

Nine cycles would be needed for a 20-carbon chain; the final cycle yields two acetyl-CoA from the 4-carbon unit.

Why is "9 cycles: 18 acetyl-CoA, 9 NADH and 9 FADH2" not the answer?

Each acetyl-CoA contains two carbons, so 18 acetyl-CoA would need 36 carbons.

Why is "8 cycles: 8 acetyl-CoA, 8 NADH and 8 FADH2" not the answer?

Eight acetyl-CoA would account for only 16 carbons.

What concept does this question test?

It tests Fatty acid oxidation: stoichiometry in Biochemistry. Each cycle yields one NADH and one FADH2, so 8 of each. The last cycle splits a four-carbon unit into two acetyl-CoA, so the total number of acetyl-CoA is 18/2, which is 9.

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 Fatty acid oxidation: stoichiometry?

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.