Explain how the chemiosmotic gradient across the inner mitochondrial membrane is established and used to synthesise ATP during aerobic respiration.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
Electrons from NADH and FADH₂ are transferred through the electron transport chain, releasing energy.
This energy powers protein complexes (Complex I, III, IV) to pump H⁺ from the matrix into the intermembrane space, creating a proton/electrochemical gradient.
Protons flow back into the matrix via ATP synthase (by chemiosmosis), down the gradient.
The proton flow drives rotation of the γ‑subunit of ATP synthase, coupling it to phosphorylation of ADP + Pᵢ to form ATP (oxidative phosphorylation).
This energy powers protein complexes (Complex I, III, IV) to pump H⁺ from the matrix into the intermembrane space, creating a proton/electrochemical gradient.
Protons flow back into the matrix via ATP synthase (by chemiosmosis), down the gradient.
The proton flow drives rotation of the γ‑subunit of ATP synthase, coupling it to phosphorylation of ADP + Pᵢ to form ATP (oxidative phosphorylation).
Examiner tips
- Use the exact terms: electron transport chain, proton gradient, chemiosmosis, ATP synthase, oxidative phosphorylation.
- Show the sequence of events: electron transfer → proton pumping → gradient → proton flow → ATP synthesis.
- Mention both the electrochemical gradient and the role of ATP synthase rotation.
Common mistakes
- Confusing the direction of proton flow or saying it moves against the gradient.
- Using vague terms like ‘energy’ without linking to proton pumping.
- Omitting the role of ATP synthase or the phosphorylation of ADP + Pᵢ.
Mark scheme (4 marks)
- Electrons from reduced coenzymes (NADH/FADH₂) are passed along the electron transport chain, releasing energy
- This energy is used by protein complexes to actively pump protons (H⁺) from the matrix into the intermembrane space, creating a proton/electrochemical gradient
- Protons flow back into the matrix through ATP synthase (down the electrochemical gradient / by facilitated diffusion / chemiosmosis)
- The flow of protons causes rotation of ATP synthase, driving the phosphorylation of ADP + Pᵢ to produce ATP (oxidative phosphorylation)
Key terms in this question
chemiosmotic gradient · 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 →
More Cell respiration questions
- Explain why the net ATP yield from the complete aerobic oxidation of one glucose…
- Explain the role of the electron transport chain in linking the oxidation of NAD…
- Explain how the products of the Krebs cycle are used in subsequent stages of aer…
- Explain the role of substrate-level phosphorylation in cell respiration, includi…
- Explain how acetyl-CoA links the products of glycolysis to the Krebs cycle, incl…
- Explain why lactate fermentation allows glycolysis to continue during intense ex…