Bioenergetics: NADH shuttles: 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 · Bioenergetics: NADH shuttles

Sample item

Cytosolic NADH from glycolysis of one glucose molecule enters mitochondria through the malate-aspartate shuttle in heart and through the glycerol-3-phosphate shuttle in skeletal muscle. Using P/O ratios of 2.5 for mitochondrial NADH and 1.5 for FADH2, how many more ATP are obtained per glucose with the malate-aspartate shuttle?

A4 more ATPBNo differenceC1 more ATPD2 more ATP

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

The answer and the reasoning

D. 2 more ATP

Glycolysis generates 2 cytosolic NADH per glucose. The malate-aspartate shuttle delivers them as mitochondrial NADH, giving 2 x 2.5 = 5 ATP. The glycerol-3-phosphate shuttle passes the electrons to FAD, giving 2 x 1.5 = 3 ATP. The difference is 5 minus 3, which is 2 ATP.

Option by option

A. 4 more ATP
Four would apply if the P/O ratio difference were 2 per NADH, but the difference is only 1 per NADH.
B. No difference
The shuttles differ because one feeds complex I and the other bypasses it to enter at coenzyme Q.
C. 1 more ATP
A difference of one would arise from a single NADH rather than the two produced per glucose.
D. 2 more ATP (correct)
Five ATP versus three ATP from two NADH gives a difference of two.

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 D, 2 more ATP. Glycolysis generates 2 cytosolic NADH per glucose.

Why is "4 more ATP" not the answer?

Four would apply if the P/O ratio difference were 2 per NADH, but the difference is only 1 per NADH.

Why is "No difference" not the answer?

The shuttles differ because one feeds complex I and the other bypasses it to enter at coenzyme Q.

Why is "1 more ATP" not the answer?

A difference of one would arise from a single NADH rather than the two produced per glucose.

What concept does this question test?

It tests Bioenergetics: NADH shuttles in Biochemistry. The malate-aspartate shuttle delivers them as mitochondrial NADH, giving 2 x 2.5 = 5 ATP. The glycerol-3-phosphate shuttle passes the electrons to FAD, giving 2 x 1.5 = 3 ATP. The difference is 5 minus 3, which is 2 ATP.

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 Bioenergetics: NADH shuttles?

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.