The link reaction and the Krebs cycle

Molecules (Interaction and interdependence) · Cell respiration · note 6 of 8

Spec C1.2.11, C1.2.12

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The link reaction and the Krebs cycleSpec C1.2.11, C1.2.12

In short

In the link reaction, pyruvate in the mitochondrial matrix is oxidised and decarboxylated, and the acetyl group is transferred by coenzyme A to the Krebs cycle. In the cycle, the acetyl group joins oxaloacetate (4C) to form citrate (6C). Two decarboxylations and four oxidations regenerate oxaloacetate, releasing carbon dioxide and producing reduced NAD and ATP.

The link reaction

In aerobic respiration, pyruvate from glycolysis is transported into the mitochondrial matrix. There it undergoes oxidation (hydrogen removed, NAD reduced) and decarboxylation (carbon removed as CO₂). The remaining 2C acetyl group is attached to coenzyme A, forming acetyl CoA, which transfers the acetyl group to the Krebs cycle.

pyruvate (3C) + NAD + CoA → acetyl CoA (2C) + CO₂ + reduced NAD

Lipids and carbohydrates are both metabolised to form acetyl groups (2C), so this is where the pathways for different respiratory substrates join. Per glucose the link reaction happens twice.

The Krebs cycle

The acetyl group (2C) is transferred from coenzyme A to oxaloacetate (4C), forming citrate (6C). Coenzyme A is released to collect another acetyl group.
Citrate is converted back to oxaloacetate by a series of reactions that include two decarboxylations, releasing 2 CO₂.
The cycle includes four oxidations, which are dehydrogenation reactions. Three of them reduce NAD; the fourth reduces another hydrogen carrier, FAD. Reduced FAD also passes its electrons to the electron transport chain, but IB questions focus on NAD.
One ATP is produced directly in each turn.
Oxaloacetate is regenerated, so the cycle can accept another acetyl group.

Products per turn of the Krebs cycle (two turns per glucose)
Product	Per turn	Per glucose
CO₂	2	4
Reduced NAD	3	6
Reduced FAD	1	2
ATP	1	2

Link reaction and Krebs cycle in the mitochondrial matrix: pyruvate (3C) loses CO₂ and reduces NAD to form acetyl CoA (2C); the acetyl group joins oxaloacetate (4C) to form citrate (6C); around the cycle are two decarboxylations releasing CO₂, four oxidations (three reducing NAD and one reducing FAD) and one ATP formed, and oxaloacetate is regenerated. — Each turn of the Krebs cycle: 2 CO₂, 3 reduced NAD, 1 reduced FAD and 1 ATP; oxaloacetate is regenerated.

Exam tip:

Name only citrate and oxaloacetate. Remember the counts: two decarboxylations and four oxidations per turn, and two turns per glucose.

Written and checked against the IB Biology HL specification · Updated October 2026

Frequently asked questions

Why is ATP called the energy currency of the cell?

ATP is called the energy currency because it distributes energy within cells. It is soluble, stable without enzymes and stays inside the cell, and hydrolysis to ADP and phosphate releases enough energy for tasks such as active transport, building macromolecules and movement. It is quickly regenerated from ADP using energy from respiration.

What is the difference between cell respiration and gas exchange?

Cell respiration is a set of enzyme-catalysed reactions inside cells that produce ATP using energy from carbon compounds. Gas exchange is the diffusion of oxygen and carbon dioxide between an organism and its environment, for example in the lungs. Gas exchange supplies oxygen for aerobic respiration and removes the carbon dioxide it produces.

What are the differences between aerobic and anaerobic respiration?

Aerobic respiration needs oxygen and mitochondria, can use carbohydrates, lipids and amino acids, gives a large yield of ATP and produces carbon dioxide and water. Anaerobic respiration in humans needs no oxygen, happens only in the cytoplasm, uses only glucose, gives just 2 ATP per glucose and produces lactate.

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