Explain how the chemiosmotic theory accounts for ATP synthesis during aerobic cell 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 protein complexes of the electron transport chain, releasing energy. This energy is used to actively pump protons from the mitochondrial matrix into the intermembrane space, creating a proton/electrochemical gradient. Protons then flow back into the matrix through ATP synthase by chemiosmosis, and the energy released by this proton movement drives the phosphorylation of ADP + Pᵢ to form ATP (oxidative phosphorylation).
Examiner tips
- Use the exact terminology: electron transport chain, proton gradient, ATP synthase, chemiosmosis, oxidative phosphorylation.
- Show the sequence of events in the correct order.
- Mention both NADH and FADH₂ as electron donors.
- Link the proton flow to ATP synthesis directly.
Common mistakes
- Confusing chemiosmosis with diffusion; forgetting to mention the proton gradient.
- Using vague terms like ‘energy’ without specifying the proton gradient and ATP synthase.
- Omitting the role of NADH and FADH₂ as electron donors.
Mark scheme (4 marks)
- Electrons from NADH (and FADH₂) are passed along protein complexes of the electron transport chain, releasing energy
- This released energy is used to actively pump protons (H⁺) from the mitochondrial matrix into the intermembrane space, establishing a proton / electrochemical gradient
- Protons flow back down their concentration / electrochemical gradient into the matrix through ATP synthase (by facilitated diffusion / chemiosmosis)
- The energy released by the movement of protons through ATP synthase drives the phosphorylation of ADP + Pᵢ to form ATP (oxidative phosphorylation)
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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