Explain how the structure of the inner mitochondrial membrane is adapted to maximise the efficiency of ATP synthesis during aerobic respiration.

IB DP Biology Higher 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 is highly folded into cristae, which increases the surface area for the electron transport chain and ATP‑synthase complexes.
The membrane is impermeable to protons, allowing a proton gradient to be maintained across it.
Electron‑transport‑chain proteins (e.g. NADH dehydrogenase, cytochrome complexes) are embedded in the membrane and pump protons from the matrix into the intermembrane space, creating the electrochemical gradient.
ATP synthase, also embedded in the membrane, uses the flow of protons back into the matrix to drive phosphorylation of ADP to ATP.

Examiner tips

  • Use the word ‘embedded’ to show membrane localisation. Mention the proton gradient and chemiosmosis. Show the sequence: cristae → surface area → ETC → proton pumping → ATP synthase.
  • common_mistakes
  • :
  • Failing to state that the membrane is impermeable to protons. Omitting the role of cristae in increasing surface area. Using vague terms like ‘efficient’ without explaining the mechanism.

Mark scheme (4 marks)

  1. The inner mitochondrial membrane is highly folded into cristae, greatly increasing the surface area available for the electron transport chain and ATP synthase complexes.
  2. The inner membrane is impermeable to protons (H⁺/hydrogen ions), so a proton gradient can be maintained across it, which is essential for chemiosmosis.
  3. Proteins of the electron transport chain (e.g. NADH dehydrogenase / cytochrome complexes) are embedded in the inner membrane and pump protons from the matrix into the intermembrane space, establishing the electrochemical / proton gradient.
  4. ATP synthase is embedded in the inner membrane and uses the flow of protons back into the matrix (down their electrochemical gradient) to drive the phosphorylation of ADP to ATP.

Key terms in this question

inner mitochondrial membrane

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