Explain how the ultrastructure of mitochondria is related to their function in aerobic respiration.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
The inner membrane is folded into cristae, which increases the surface area for the electron transport chain and ATP synthase complexes, allowing more ATP to be produced.
The matrix contains the enzymes and co‑enzymes (e.g. NAD⁺) required for the Krebs cycle and the link reaction, so the products of glycolysis are oxidised to CO₂ and high‑energy electrons.
Protons pumped from the matrix into the intermembrane space create a proton gradient across the inner membrane; ATP synthase uses this electrochemical gradient (chemiosmosis) to synthesise ATP from ADP and Pi.
Mitochondria possess their own circular DNA and 70S ribosomes, enabling synthesis of some of their proteins, reflecting their endosymbiotic origin and semi‑autonomous function.
The matrix contains the enzymes and co‑enzymes (e.g. NAD⁺) required for the Krebs cycle and the link reaction, so the products of glycolysis are oxidised to CO₂ and high‑energy electrons.
Protons pumped from the matrix into the intermembrane space create a proton gradient across the inner membrane; ATP synthase uses this electrochemical gradient (chemiosmosis) to synthesise ATP from ADP and Pi.
Mitochondria possess their own circular DNA and 70S ribosomes, enabling synthesis of some of their proteins, reflecting their endosymbiotic origin and semi‑autonomous function.
Examiner tips
- Mention cristae and surface area first; link matrix to Krebs cycle; explain proton gradient and chemiosmosis; note mitochondrial DNA for completeness.
Common mistakes
- Confusing the inner membrane with the outer membrane; forgetting the role of the matrix; omitting the proton gradient mechanism; not mentioning mitochondrial DNA or ribosomes.
Mark scheme (4 marks)
- The inner mitochondrial membrane is folded into cristae, which greatly increases the surface area available for the electron transport chain and ATP synthase complexes.
- The matrix is the site of the Krebs cycle / link reaction, and contains the necessary enzymes and co-enzymes (e.g. NAD) for these reactions.
- The intermembrane space accumulates protons (H⁺ ions) pumped from the matrix, establishing an electrochemical / proton gradient across the inner membrane that drives ATP synthase by chemiosmosis.
- Mitochondria contain their own circular DNA and ribosomes (70S), allowing them to synthesise some of their own proteins, consistent with endosymbiotic origin and semi-autonomous function.
Related
- All IB DP Biology Higher Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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