Explain how chemiosmosis results in the synthesis of ATP during aerobic cell respiration.

IB DP Biology Standard Level (2023 syllabus) — C1.2 Cell respiration · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (4 marks)

Electrons are transferred through the electron transport chain on the inner mitochondrial membrane, releasing energy. This energy is used to pump protons from the matrix into the intermembrane space, creating a proton gradient. Protons then flow back into the matrix through ATP synthase, and the energy of this flow drives the phosphorylation of ADP to ATP (oxidative phosphorylation).

Examiner tips

  • Use the word ‘electron transport chain’ and ‘ATP synthase’ explicitly.
  • Show the sequence: electron transfer → proton pumping → gradient → proton flow → ATP synthesis.
  • Mention ‘oxidative phosphorylation’ to link the process to ATP production.
  • Keep the answer concise, covering all four points.

Mark scheme (4 marks)

  1. Electrons are passed along the electron transport chain (on the inner mitochondrial membrane), releasing energy.
  2. This energy is used to actively pump protons (H⁺ ions) from the mitochondrial matrix into the intermembrane space, establishing a proton gradient.
  3. Protons flow back into the matrix down their concentration gradient through ATP synthase.
  4. The flow of protons through ATP synthase drives the phosphorylation of ADP to ATP (oxidative phosphorylation).

Key terms in this question

chemiosmosis

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