Explain how the structural features of the mitochondrion are related to its role in aerobic cell respiration.

IB DP Biology Higher Level (2023 syllabus) — B2.2 Organelles and compartmentalisation · 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 membrane is highly folded into cristae, giving a large surface area for the electron transport chain and ATP synthase.
The matrix contains the enzymes, co‑enzymes and mitochondrial DNA/ribosomes for the Krebs cycle.
Protons pumped into the intermembrane space by the electron transport chain create a proton gradient that drives ATP synthesis through ATP synthase.
The double membrane keeps the matrix and intermembrane space chemically distinct, allowing each stage of aerobic respiration to occur in its own environment.

Examiner tips

  • Use the word ‘folded’ for cristae, ‘matrix’ for Krebs cycle, ‘intermembrane space’ for proton gradient, and ‘double membrane’ for compartmentalisation.
  • Show the causal link: structure → function → energy production.
  • Keep each point short and directly tied to the marks listed.

Common mistakes

  • Confusing the outer membrane with the inner membrane; the outer membrane is not involved in the electron transport chain.
  • Lacking the detail that the matrix contains DNA and ribosomes, or that the proton gradient drives ATP synthase.
  • Using vague terms like ‘large area’ instead of ‘folded into cristae’ or ‘double membrane’.”

Mark scheme (4 marks)

  1. The inner mitochondrial membrane is folded into cristae, which greatly increases the surface area available for the electron transport chain and ATP synthase.
  2. The matrix is the site of the Krebs cycle and contains the enzymes, coenzymes, and DNA/ribosomes needed for these reactions.
  3. The intermembrane space accumulates protons (H⁺) pumped by the electron transport chain, establishing a proton gradient that drives ATP synthesis via ATP synthase.
  4. The double membrane compartmentalises the mitochondrion, maintaining the distinct chemical environments needed for each stage of aerobic respiration.

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