Carbohydrate metabolism: ATP yield from glycogen: 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 · Carbohydrate metabolism: ATP yield from glycogen

Sample item

During intense exercise, a muscle fibre degrades glycogen to lactate under anaerobic conditions. What is the net ATP gain per glucose unit taken from glycogen?

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

The answer and the reasoning

B. 3 ATP

Phosphorylase releases glucose-1-phosphate, which becomes glucose-6-phosphate without using ATP. Only the phosphofructokinase-1 step consumes ATP, so four ATP formed by substrate-level phosphorylation minus one gives a net of three, compared with two when starting from free glucose.

Option by option

  • A. 4 ATP

    Four ATP is the gross yield of substrate-level phosphorylation before subtracting the phosphofructokinase-1 investment.

  • B. 3 ATP (correct)

    Correct. Glycogen skips the hexokinase ATP investment, so net yield is 3 ATP.

  • C. 2 ATP

    Two ATP is the net yield from free glucose, which needs hexokinase and so spends one more ATP.

  • D. 1 ATP

    One ATP is not the net result of anaerobic glycolysis from glycogen or from free glucose.

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 B, 3 ATP. Phosphorylase releases glucose-1-phosphate, which becomes glucose-6-phosphate without using ATP.

Why is "4 ATP" not the answer?

Four ATP is the gross yield of substrate-level phosphorylation before subtracting the phosphofructokinase-1 investment.

Why is "2 ATP" not the answer?

Two ATP is the net yield from free glucose, which needs hexokinase and so spends one more ATP.

Why is "1 ATP" not the answer?

One ATP is not the net result of anaerobic glycolysis from glycogen or from free glucose.

What concept does this question test?

It tests Carbohydrate metabolism: ATP yield from glycogen in Biochemistry. Only the phosphofructokinase-1 step consumes ATP, so four ATP formed by substrate-level phosphorylation minus one gives a net of three, compared with two when starting from free glucose.

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 Carbohydrate metabolism: ATP yield from glycogen?

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.