# Explain how the ultrastructure of mitochondria is related to their function in aerobic respiration.

> IB DP Biology Higher Level (2023 syllabus) — A2.2 Cell structure · Explain · 4 marks

## 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 complexes.
2. 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.
3. 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.
4. 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.

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