Oxidative phosphorylationAQA A-Level Biology: Revision notes
Section 1
Where and what
Oxidative phosphorylation is the synthesis of ATP that is associated with the transfer of electrons down the electron transfer chain and the passage of protons across the inner mitochondrial membrane. It is catalysed by ATP synthase embedded in the inner membrane, which is folded into cristae to provide a large surface area.
Section 2
Reduced coenzymes donate electrons
Reduced NAD and reduced FAD, produced in glycolysis, the link reaction and the Krebs cycle, deliver hydrogen to the electron transfer chain. They are oxidised to NAD and FAD (which can be used again) and release electrons that pass down the chain of carriers in a series of oxidation-reduction reactions.
Section 3
Proton pumping
The electrons lose energy as they pass along the carriers. This energy is used to pump protons (H⁺) from the matrix across the inner membrane into the intermembrane space. This creates a higher proton concentration in the intermembrane space than in the matrix: an electrochemical (proton) gradient.
The electrons do not make ATP directly. Their energy builds the proton gradient and the gradient drives ATP synthase.
Section 4
Chemiosmosis and ATP synthase
Protons flow back into the matrix down their gradient, through channels in ATP synthase. The energy of this flow is used by the enzyme to join ADP and Pi to make ATP. This idea, that ATP synthesis is driven by flow of protons across a membrane, is the chemiosmotic theory.
Protons are pumped into the intermembrane space by the chain, and flow back into the matrix through ATP synthase. Do not reverse these.
Section 5
Role of oxygen
Oxygen is the final electron acceptor. It combines with electrons and protons to form water. Without oxygen the chain cannot pass electrons on, so it stops, no proton gradient is formed and ATP is not made by oxidative phosphorylation. Reduced NAD and FAD would accumulate, so the Krebs cycle would stop too.
Section 6
Predicting effects
Think in order: electron flow, proton pumping, gradient, flow through ATP synthase, ATP. An inhibitor of the chain stops pumping and ATP synthesis. An inhibitor of ATP synthase makes the gradient steepen until pumping stops. A leaky membrane lets protons return without making ATP, so oxygen use continues but energy is lost as heat.
Always name the compartment (matrix or intermembrane space) when describing proton movement.
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Oxidative phosphorylation
- Mitochondria were isolated from heart muscle and the processes taking place at their inner membranes were investigated.Describe the role of reduced NAD in oxidative phosphorylation.2 marks
- In respiring mitochondria the intermembrane space is found to be more acidic than the matrix.Explain why the intermembrane space is more acidic than the matrix in respiring mitochondria.2 marks
- Two inhibitors are added, separately, to suspensions of respiring mitochondria that are supplied with reduced NAD and oxygen. Inhibitor X blocks the channel in ATP synthase so that protons cannot pass through it. Inhibitor Y blocks the last carrier of the electron transfer chain so that electrons cannot be passed to oxygen.Predict the effects of inhibitor X on ATP synthesis, on the proton concentration in the intermembrane space and on oxygen consumption, giving reasons.3 marks
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).