Oxidative phosphorylationEdexcel A-Level Biology A: Revision notes

Section 1

Where oxidative phosphorylation takes place

Oxidative phosphorylation is the final stage of aerobic respiration, making most of the ATP. It takes place on the inner mitochondrial membrane, which is folded into cristae to give a large surface area.

The membrane contains the electron transport chain (a series of electron carriers) and many ATP synthase enzymes. The inner membrane is almost impermeable to protons (H⁺), and the space between the inner and outer membranes is the intermembrane space.

Key termsoxidative phosphorylationelectron transport chainATP synthasecristae

Section 2

Electron transport and the proton gradient

Reduced NAD and reduced FAD from glycolysis, the link reaction and the Krebs cycle deliver hydrogen to the chain. They are oxidised, releasing protons (H⁺) and electrons.

The electrons pass along the carriers in a series of redox reactions, losing energy at each step. This energy is used to pump protons by active transport from the matrix into the intermembrane space. Because the membrane is impermeable to protons, an electrochemical (proton) gradient builds up, with a high proton concentration in the intermembrane space.

Key termsredox reactionproton gradient

Common mistake

Do not say ATP is made directly by the electron transport chain. The chain pumps protons, and ATP synthase makes the ATP.

Section 3

Chemiosmosis and ATP synthase

Protons flow back down their gradient, from the intermembrane space into the matrix, through channels in ATP synthase. This movement of protons is chemiosmosis. The energy released is used by ATP synthase to join ADP and inorganic phosphate (Pi) to make ATP.

The same enzyme can only make ATP while the proton gradient is maintained, so the process depends on the continued supply of electrons.

Key termschemiosmosis

Exam tip

Use the words 'down the electrochemical gradient' and 'through ATP synthase' when describing ATP production.

Section 4

The role of oxygen

At the end of the chain, oxygen is the final electron acceptor. It combines with electrons and protons to form water.

$\frac{1}{2}O_2 + 2H^+ + 2e^- \rightarrow H_2O$

This removes electrons from the chain, so reduced NAD and reduced FAD can be reoxidised and NAD and FAD regenerated for the link reaction and Krebs cycle. Without oxygen (or if a poison such as cyanide stops electron transfer to oxygen) the chain stops, the proton gradient is lost and ATP synthesis by oxidative phosphorylation stops. Cells then rely on glycolysis and make lactate.

Key termsfinal electron acceptorcyanide

Common mistake

Oxygen is not used in the Krebs cycle itself, but the cycle stops without it because the coenzymes cannot be regenerated.

That's the notes covered.

Carry on to the next subtopic.

Exam questions on Oxidative phosphorylation

A student is revising the final stage of aerobic respiration. This stage takes place on the inner membrane of the mitochondrion, where reduced NAD and reduced FAD from the earlier stages deliver hydrogen to a series of electron carriers called the electron transport chain.
Explain why the Krebs cycle stops when no oxygen is available, even though oxygen is not used in the cycle.2 marks
A researcher suspends intact mitochondria in a solution containing oxygen, ADP and phosphate, and monitors the pH in the space between the inner and outer mitochondrial membranes. When reduced NAD is added, the pH of this space falls and ATP is made. When cyanide is then added, the pH rises again and ATP production stops.
Describe how ATP is made when protons move through ATP synthase.2 marks
The inner membrane of a mitochondrion is folded into cristae. It is almost impermeable to protons (hydrogen ions) and contains electron carriers arranged in sequence, together with many molecules of the enzyme ATP synthase. The outer membrane is permeable to small molecules.
Explain how the electron transport chain creates a proton gradient across the inner mitochondrial membrane.3 marks

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Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).