The link reaction

The link reaction is the stage of aerobic respiration that joins glycolysis to the Krebs cycle. It happens in the matrix of the mitochondria. Each pyruvate from glycolysis loses a carbon dioxide molecule and two hydrogen atoms, becoming a two-carbon acetate, which then joins coenzyme A to make acetyl coenzyme A. The hydrogen reduces NAD, and no ATP is made.

Where does the link reaction take place?

Glycolysis happens in the cytoplasm and makes pyruvate. If oxygen is available, the pyruvate is moved into the mitochondrial matrix by active transport. The link reaction and the Krebs cycle after it both happen there, so the link reaction is the "link" between the stage in the cytoplasm and the stages in the mitochondria.

What happens in the link reaction

1. Pyruvate enters the matrix by active transport. Pyruvate has 3 carbon atoms.
2. Decarboxylation: a carbon dioxide molecule is removed from the pyruvate. This leaves 2 carbon atoms.
3. Dehydrogenation (oxidation): hydrogen is removed. It is picked up by the coenzyme NAD, which becomes reduced NAD. The pyruvate has been oxidised to acetate.
4. Acetate joins with coenzyme A, making acetyl coenzyme A (acetyl CoA).
5. Acetyl CoA carries the acetyl group into the Krebs cycle, where it combines with a four-carbon molecule to make a six-carbon molecule.

The link reaction equation

For one molecule of pyruvate:

Some books write reduced NAD as NADH or NADH + H⁺. They mean the same thing. AQA uses "reduced NAD".

Products of the link reaction

One glucose molecule makes two pyruvate molecules in glycolysis, so the link reaction happens twice for every glucose. Double everything:

The reduced NAD isn't wasted: it carries hydrogen to the electron transfer chain on the inner mitochondrial membrane, where it powers the production of ATP in oxidative phosphorylation. The carbon dioxide diffuses out of the cell as a waste product.

Check your understanding.

Why the link reaction needs oxygen

The link reaction does not use oxygen directly. But it uses up NAD, and NAD is only turned back from reduced NAD when the electron transfer chain passes its electrons to oxygen, the final electron acceptor. Without oxygen, the electron transfer chain stops, reduced NAD builds up, NAD runs out, and the link reaction and Krebs cycle stop too. That is why the link reaction is part of aerobic respiration only. In anaerobic respiration, pyruvate stays in the cytoplasm and is turned into lactate (in animals) or ethanol (in plants and yeast) instead.

Worked examples

Example 1: following the carbon atoms

A glucose molecule has 6 carbon atoms. Glycolysis splits it into 2 pyruvates of 3 carbons each. In the link reaction each pyruvate loses one carbon as CO₂, so 2 of the 6 carbon atoms leave as carbon dioxide here. The other 4 enter the Krebs cycle as 2 acetyl groups, and leave as CO₂ there.

Example 2: counting reduced NAD

How many molecules of reduced NAD are made from one glucose by the end of the link reaction? Glycolysis makes 2 and the link reaction makes 2 (one per pyruvate), so the total is 4.

Example 3: an inhibitor

A poison stops the electron transfer chain. Explain why the link reaction stops. Reduced NAD can no longer be oxidised back to NAD, so the supply of NAD runs out. The link reaction needs NAD to accept hydrogen from pyruvate, so it cannot continue.

Common mistakes

• Saying the link reaction happens in the cytoplasm. It is in the mitochondrial matrix.
• Saying ATP is made. It isn't; the link reaction makes no ATP.
• Forgetting to double for glucose. There are two pyruvates per glucose, so 2 CO₂, 2 reduced NAD and 2 acetyl CoA.
• Calling the product acetate in the final step. Acetate combines with coenzyme A: the product that enters the Krebs cycle is acetyl coenzyme A.
• Writing that oxygen is used in the link reaction. Oxygen is only used as the final electron acceptor.

Practice questions

1. Name the molecule that enters the link reaction.
2. Name the two-carbon compound formed when pyruvate is oxidised.
3. How many molecules of carbon dioxide are produced in the link reaction from 3 molecules of glucose?
4. Explain why the link reaction is described as an oxidation.

Answers: 1) Pyruvate. 2) Acetate (which then joins coenzyme A). 3) 3 glucose make 6 pyruvate, so 6 CO₂. 4) Hydrogen is removed from pyruvate and accepted by NAD, which is reduced.

Practise with a short timed paper

Respiration in detail is in the second year of A-level Biology. The daily A-level Biology: Year 2 paper is a short timed paper of 8 questions, with a new one every day. For the big picture of respiration, see the cellular respiration guide.

Read next

Biology · Respiration and photosynthesisGlycolysis for A-level Biology: stages, products, ATPGlycolysis for A-level Biology: where it happens, the three stages from glucose to pyruvate, the net gain of ATP and reduced NAD, and what happens next.