Explain the role of the mitochondrial inner membrane in the production 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)

The inner mitochondrial membrane houses the electron transport chain, a series of electron carriers. As electrons move through these carriers, energy is released and used to pump protons from the matrix into the intermembrane space, creating a proton‑electrochemical gradient across the membrane. Protons then flow back into the matrix via ATP synthase, driving the synthesis of ATP from ADP and inorganic phosphate.

Examiner tips

  • Mention the electron transport chain first; link proton pumping to the gradient; state that ATP synthase uses the gradient; keep answer concise and use correct terminology.

Common mistakes

  • Omitting the electron transport chain; confusing the direction of proton flow; not specifying that ATP synthase uses the gradient to phosphorylate ADP.

Mark scheme (4 marks)

  1. The inner mitochondrial membrane contains electron carriers / the electron transport chain
  2. Electrons are passed along the carrier proteins, releasing energy that is used to actively pump protons (H⁺) from the matrix into the intermembrane space
  3. A proton / electrochemical gradient is established across the inner membrane
  4. Protons flow back into the matrix through ATP synthase, and this drives the synthesis of ATP from ADP and inorganic phosphate

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