# Explain how the chemiosmotic theory accounts for ATP synthesis during aerobic cell respiration.

> IB DP Biology Higher Level (2023 syllabus) — C1.2 Cell respiration · Explain · 4 marks

## Mark scheme (4 marks)

1. Electrons from NADH (and FADH₂) are passed along protein complexes of the electron transport chain, releasing energy
2. This released energy is used to actively pump protons (H⁺) from the mitochondrial matrix into the intermembrane space, establishing a proton / electrochemical gradient
3. Protons flow back down their concentration / electrochemical gradient into the matrix through ATP synthase (by facilitated diffusion / chemiosmosis)
4. The energy released by the movement of protons through ATP synthase drives the phosphorylation of ADP + Pᵢ to form ATP (oxidative phosphorylation)

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