Oxidative phosphorylation

Respiration, Muscles and the Internal Environment · Stages of respiration · note 4 of 5

Spec 7.4

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Oxidative phosphorylationSpec 7.4

In short

Oxidative phosphorylation makes ATP using energy released as electrons pass along the electron transport chain on the inner mitochondrial membrane. This energy pumps protons into the intermembrane space; protons flow back through ATP synthase by chemiosmosis, driving ADP and phosphate to ATP. Oxygen is the final electron acceptor.

Most ATP is made by oxidative phosphorylation, which is linked to the electron transport chain on the inner mitochondrial membrane. The folded cristae give a large surface area for the carriers and ATP synthase.

Reduced NAD and reduced FAD deliver hydrogen to the chain. They are oxidised back to NAD and FAD, which can be used again in glycolysis, the link reaction and the Krebs cycle. The hydrogen splits into protons (H⁺) and electrons.
The electrons pass along a chain of electron carriers in a series of redox reactions. Each step releases a little energy.
This energy is used to pump protons from the matrix into the intermembrane space, building up a high concentration of H⁺ there: a proton gradient across the inner membrane.
Protons diffuse back into the matrix down their gradient through channels in the enzyme ATP synthase. This flow of protons is called chemiosmosis, and it provides the energy to join ADP and inorganic phosphate (Pi) to form ATP.
At the end of the chain, oxygen is the final electron acceptor. It combines with electrons and protons to form water.

Summary of aerobic respiration for one glucose molecule
Stage	Site	ATP made directly	Reduced NAD	Reduced FAD
Glycolysis	Cytoplasm	2 (net)	2	0
Link reaction	Matrix	0	2	0
Krebs cycle	Matrix	2	6	2
Oxidative phosphorylation	Inner membrane	Most of the ATP (from reduced NAD and FAD)	Oxidised	Oxidised

Exam tip:

Without oxygen the chain stops. Reduced NAD and reduced FAD are not oxidised, so NAD and FAD run out, and the link reaction and Krebs cycle also stop.

Electron transport chain in the inner mitochondrial membrane: reduced NAD gives up electrons, protons are pumped into the intermembrane space, ATP synthase lets protons flow back into the matrix making ATP, and oxygen accepts electrons and protons to form water. — Electrons passing along the chain pump protons across the membrane; protons flow back through ATP synthase (chemiosmosis).

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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