Explain how the ultrastructure of a mitochondrion is related to its role in aerobic cellular respiration.
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, ATP synthase and oxidative phosphorylation.
The matrix contains the enzymes of the Krebs cycle and pyruvate oxidation, so the reactions of aerobic respiration take place inside the organelle.
The double‑membrane structure creates an intermembrane space; protons pumped across the inner membrane build a proton gradient that drives ATP synthesis by chemiosmosis.
Mitochondria have their own circular DNA and ribosomes, enabling synthesis of some respiratory proteins and self‑replication, reflecting their endosymbiotic origin.
The matrix contains the enzymes of the Krebs cycle and pyruvate oxidation, so the reactions of aerobic respiration take place inside the organelle.
The double‑membrane structure creates an intermembrane space; protons pumped across the inner membrane build a proton gradient that drives ATP synthesis by chemiosmosis.
Mitochondria have their own circular DNA and ribosomes, enabling synthesis of some respiratory proteins and self‑replication, reflecting their endosymbiotic origin.
Examiner tips
- Use the word ‘inner membrane’ and ‘cristae’ to show knowledge of structure. Link each structural feature directly to its functional role (e.g. surface area → electron transport).
- Mention the matrix enzymes and the proton gradient explicitly – these are key points in the scheme.
- Include the DNA/ribosome point to demonstrate understanding of evolutionary context.
Mark scheme (4 marks)
- The inner mitochondrial membrane is folded into cristae, which increases the surface area available for the electron transport chain / ATP synthase / oxidative phosphorylation.
- The matrix is the site of the Krebs cycle / contains enzymes for the Krebs cycle and pyruvate oxidation, so reactions of aerobic respiration occur within the organelle.
- The double membrane / two membranes create a compartment (intermembrane space) that allows a proton / H⁺ gradient to be established, which drives ATP synthesis via chemiosmosis.
- Mitochondria contain their own circular DNA and ribosomes, allowing synthesis of some of the proteins required for respiration / self-replication, consistent with endosymbiotic origin.
Key terms in this question
Related
- All IB DP Biology Standard Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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