Explain how the compartmentalisation provided by the inner mitochondrial membrane contributes to ATP synthesis during aerobic respiration.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
The inner mitochondrial membrane is impermeable to protons, preventing their free diffusion.
Electron carriers embedded in this membrane pump protons from the matrix into the intermembrane space, creating a proton gradient (electrochemical or proton‑motive force).
Protons can only return to the matrix through ATP synthase, where the energy of the gradient drives the phosphorylation of ADP to ATP.
This compartmentalisation is essential because it allows a high‑concentration proton gradient to be maintained between the intermembrane space and the matrix, which would not be possible in an unpartitioned system.
Electron carriers embedded in this membrane pump protons from the matrix into the intermembrane space, creating a proton gradient (electrochemical or proton‑motive force).
Protons can only return to the matrix through ATP synthase, where the energy of the gradient drives the phosphorylation of ADP to ATP.
This compartmentalisation is essential because it allows a high‑concentration proton gradient to be maintained between the intermembrane space and the matrix, which would not be possible in an unpartitioned system.
Examiner tips
- Use the term ‘impermeable to protons’ and ‘proton‑motive force’. Show the sequence: impermeability → proton pumping → gradient → chemiosmosis via ATP synthase → ATP synthesis. Mention the two compartments explicitly (matrix and intermembrane space).
Common mistakes
- Confusing the inner membrane with the outer membrane. Saying protons diffuse freely across the membrane. Omitting the role of ATP synthase or the two distinct mitochondrial compartments.
Mark scheme (4 marks)
- The inner mitochondrial membrane is impermeable to protons (H⁺ / hydrogen ions), so protons cannot diffuse freely across it.
- Electron carriers in the inner membrane pump protons from the matrix into the intermembrane space, establishing a proton gradient (or electrochemical / proton-motive force).
- Protons can only re-enter the matrix through ATP synthase (chemiosmosis), so the energy of the gradient drives ATP synthesis from ADP and inorganic phosphate (Pᵢ).
- Compartmentalisation is essential because it allows a localised, concentrated proton gradient to be maintained between two distinct spaces (intermembrane space and matrix), which would be impossible in an unpartitioned system.
Key terms in this question
compartmentalisation · inner mitochondrial membrane
Related
- All IB DP Biology Higher Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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