All subjects · Biology · WBI15 (Unit 5)

The link reaction and the Krebs cycle

With oxygen available, pyruvate enters the mitochondrion. The link reaction and Krebs cycle strip out carbon dioxide and hydrogen.

Mitochondria

Outer membrane and a folded inner membrane forming cristae, which give a large surface area for the electron transport chain and ATP synthase.
Matrix: contains the enzymes of the link reaction and Krebs cycle, plus mitochondrial DNA and ribosomes.
Intermembrane space: where H+ builds up during oxidative phosphorylation.

The link reaction

Pyruvate is actively transported into the matrix.

It is decarboxylated (CO2 removed) and dehydrogenated (hydrogen removed, reducing NAD).
The remaining 2-carbon acetyl group combines with coenzyme A to form acetyl coenzyme A.

Per pyruvate: 1 CO2, 1 reduced NAD, 1 acetyl CoA. Per glucose, double this.

The Krebs cycle

In the matrix:

1. Acetyl CoA (2C) combines with oxaloacetate (4C) to form citrate (6C). Coenzyme A is released to be reused.

2. Citrate is decarboxylated and dehydrogenated in a series of steps, regenerating oxaloacetate.

Per turn (one acetyl CoA): 2 CO2, 3 reduced NAD, 1 reduced FAD, 1 ATP (made directly, by substrate-level phosphorylation).

One glucose gives two pyruvate, so two turns per glucose.

Per glucose	Link reaction	Krebs cycle
CO2	2	4
Reduced NAD	2	6
Reduced FAD	0	2
ATP	0	2

Worked example

How many molecules of CO2 are produced from one glucose molecule in the link reaction and Krebs cycle together?

Link reaction: 1 CO2 per pyruvate, 2 per glucose.
Krebs cycle: 2 CO2 per turn, 2 turns: 4.
2 + 4 = 6

Answer: 6 CO2, matching the 6CO2 in the overall equation.

Key idea: Link reaction (matrix): pyruvate decarboxylated and dehydrogenated, joins coenzyme A to make acetyl CoA. Krebs cycle: acetyl CoA + oxaloacetate → citrate; per turn 2 CO2, 3 reduced NAD, 1 reduced FAD, 1 ATP; two turns per glucose.

Check yourself

How many CO2 molecules are released by the Krebs cycle for 3 glucose molecules?
Show answer
12. 4 per glucose (2 turns x 2): 12.
How many reduced NAD are made in the link reaction and Krebs cycle combined, per glucose?
Show answer
8. 2 from the link reaction + 6 from the Krebs cycle = 8.
Where does the Krebs cycle take place?
- Cytoplasm
- Intermembrane space
- Mitochondrial matrix
- Outer mitochondrial membrane
Show answer
Mitochondrial matrix. Krebs enzymes are in the matrix.
What happens to pyruvate in the link reaction?
- It is converted to lactate
- It is decarboxylated and dehydrogenated, forming acetyl CoA
- It is phosphorylated using ATP
- It combines with oxaloacetate directly
Show answer
It is decarboxylated and dehydrogenated, forming acetyl CoA. CO2 and hydrogen are removed; acetyl joins coenzyme A.
Which molecule combines with acetyl CoA at the start of the Krebs cycle?
- Pyruvate
- Glucose
- Citrate
- Oxaloacetate
Show answer
Oxaloacetate. 4C oxaloacetate + 2C acetyl = 6C citrate.
What does decarboxylation mean?
- Removal of hydrogen
- Removal of carbon dioxide
- Addition of phosphate
- Addition of water
Show answer
Removal of carbon dioxide. De- = remove; carboxyl groups leave as CO2.
Why is the inner mitochondrial membrane folded into cristae?
- To store glucose
- To hold the Krebs cycle enzymes
- To make the mitochondrion divide
- To increase the surface area for the electron transport chain and ATP synthase
Show answer
To increase the surface area for the electron transport chain and ATP synthase. More membrane means more ATP synthesis.

Practise with instant feedback

Sources

Pearson Edexcel International Advanced Level in Biology specification (XBI11/YBI11, first teaching 2018)

Last updated 7 November 2026. Spotted a mistake? Tell us.