The link reaction and the Krebs cycle

Respiration, Muscles and the Internal Environment · Stages of respiration · note 3 of 5

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The link reaction and the Krebs cycleSpec 7.3

In short

The link reaction and the Krebs cycle take place in the mitochondrial matrix and completely oxidise glucose. Pyruvate is decarboxylated and dehydrogenated, then acetyl groups enter the Krebs cycle, which releases carbon dioxide, makes ATP by substrate-level phosphorylation and produces reduced NAD and reduced FAD by dehydrogenation.

Unlike glycolysis, which happens in the cytoplasm, the link reaction and the Krebs cycle take place in the mitochondria (the matrix). Pyruvate from glycolysis is actively transported into the matrix. Together they achieve the complete oxidation of glucose. Names of the other compounds are not required.

The link reaction

Pyruvate (3C) is decarboxylated: one carbon is lost as CO₂.
It is also dehydrogenated: hydrogen is removed and passed to NAD, forming reduced NAD.
The remaining 2C acetyl group combines with coenzyme A to form acetyl CoA, which carries it into the Krebs cycle.
This happens twice per glucose, because glycolysis makes two pyruvate.

The Krebs cycle

The acetyl group (2C) from acetyl CoA joins a 4C compound to form a 6C compound. The 6C compound is then converted back to the 4C compound in a series of enzyme-controlled steps, so the cycle can start again. In each turn:

two carbon atoms are lost as CO₂ by decarboxylation;
hydrogen is removed by dehydrogenation and accepted by NAD (3 reduced NAD) and by FAD (1 reduced FAD);
one ATP is made by substrate-level phosphorylation.

The cycle turns twice for each glucose molecule, so the totals are doubled.

Products of the link reaction and Krebs cycle per glucose molecule
Stage	CO₂	Reduced NAD	Reduced FAD	ATP
Link reaction (2 pyruvate)	2	2	0	0
Krebs cycle (2 turns)	4	6	2	2
Total for both	6	8	2	2

Exam tip:

Reduced NAD and reduced FAD are the main products: they carry hydrogen (electrons) to the electron transport chain, where most ATP is made. Only the cycle itself makes ATP directly.

The link reaction converts pyruvate (3C) to acetyl CoA (2C) with CO₂ and reduced NAD released; acetyl CoA joins oxaloacetate (4C) to make citrate (6C) in the Krebs cycle, which releases two CO₂, three reduced NAD, one reduced FAD and one ATP per turn, all in the mitochondrial matrix. — The link reaction and Krebs cycle in the mitochondrial matrix, labelled by carbon number.

Written and checked against the Edexcel International A Level Biology (YBI11) specification · Updated October 2026

Frequently asked questions

What are the stages of aerobic respiration in order?

The stages are glycolysis in the cytoplasm, the link reaction and the Krebs cycle in the mitochondrial matrix, and oxidative phosphorylation on the inner mitochondrial membrane. Glucose is split, carbon is lost as carbon dioxide, hydrogen is carried by reduced NAD and reduced FAD, and ATP is made.

Where does the Krebs cycle take place?

The Krebs cycle takes place in the matrix of the mitochondrion. The link reaction also happens in the matrix, whereas glycolysis happens in the cytoplasm. Oxidative phosphorylation takes place on the inner mitochondrial membrane, which is folded into cristae to give a large surface area.

What is chemiosmosis in respiration?

Chemiosmosis is the movement of protons down their concentration gradient through ATP synthase. The electron transport chain pumps protons from the matrix into the intermembrane space. When they flow back through ATP synthase, the energy released is used to join ADP and inorganic phosphate to make ATP.

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