What happens to pyruvate in the link reaction?

It is oxidised to acetate, losing CO₂, and NAD is reduced to reduced NAD.

What does acetylcoenzyme A react with in the Krebs cycle?

A four-carbon molecule, to form a six-carbon molecule and release coenzyme A.

What is lost in the Krebs cycle?

Carbon dioxide (decarboxylation) and hydrogen (dehydrogenation).

How is ATP made in the Krebs cycle?

By substrate-level phosphorylation.

How many turns of the Krebs cycle occur per glucose?

Two, because glycolysis produces two pyruvate.

How many CO₂ are released per glucose in the link reaction and Krebs cycle?

Six: two from the link reaction and four from the cycle.

Why does the Krebs cycle need oxygen indirectly?

Oxygen accepts electrons at the end of the electron transfer chain, so reduced NAD and FAD are re-oxidised.

What is the difference between substrate-level and oxidative phosphorylation?

Substrate-level transfers a phosphate directly from a substrate; oxidative uses the proton gradient and ATP synthase.

Link reaction and Krebs cycleAQA A-Level Biology: Flashcards

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How does pyruvate enter the mitochondrion?

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How does pyruvate enter the mitochondrion?: By active transport into the matrix.
Where do the link reaction and Krebs cycle occur?: In the mitochondrial matrix.
What happens to pyruvate in the link reaction?: It is oxidised to acetate, losing CO₂, and NAD is reduced to reduced NAD.
What is formed when acetate combines with coenzyme A?: Acetylcoenzyme A.
What does acetylcoenzyme A react with in the Krebs cycle?: A four-carbon molecule, to form a six-carbon molecule and release coenzyme A.
What is lost in the Krebs cycle?: Carbon dioxide (decarboxylation) and hydrogen (dehydrogenation).
Which reduced coenzymes does the Krebs cycle produce?: Reduced NAD and reduced FAD.
How is ATP made in the Krebs cycle?: By substrate-level phosphorylation.
How many turns of the Krebs cycle occur per glucose?: Two, because glycolysis produces two pyruvate.
How many CO₂ are released per glucose in the link reaction and Krebs cycle?: Six: two from the link reaction and four from the cycle.
Why does the Krebs cycle need oxygen indirectly?: Oxygen accepts electrons at the end of the electron transfer chain, so reduced NAD and FAD are re-oxidised.
What is the difference between substrate-level and oxidative phosphorylation?: Substrate-level transfers a phosphate directly from a substrate; oxidative uses the proton gradient and ATP synthase.

Exam questions on Link reaction and Krebs cycle

Pyruvate produced by glycolysis in the cytoplasm of a liver cell is taken up by the mitochondria when oxygen is available.
Explain, with reference to carbon atoms and NAD, what happens to pyruvate when it is oxidised to acetate in the link reaction.2 marks
The Krebs cycle was studied in isolated mitochondria supplied with acetylcoenzyme A and oxygen.
Explain why the Krebs cycle would stop if the isolated mitochondria were deprived of oxygen.2 marks
Each glucose molecule gives two pyruvate molecules in glycolysis. Each pyruvate goes through the link reaction, which releases one carbon dioxide molecule and forms one reduced NAD, and each acetylcoenzyme A then enters one turn of the Krebs cycle. One turn of the Krebs cycle releases two carbon dioxide molecules and produces three reduced NAD, one reduced FAD and one ATP.
Calculate the number of carbon dioxide, reduced NAD and ATP molecules produced by the link reaction and Krebs cycle for each glucose molecule.3 marks

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Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).