# Explain why the net ATP yield from the complete aerobic oxidation of one glucose molecule is considerably lower than the theoretical maximum of 38 ATP.

> IB DP Biology Higher Level (2023 syllabus) — C1.2 Cell respiration · Explain · 4 marks

## Mark scheme (4 marks)

1. The theoretical maximum assumes 100% efficiency of the proton gradient / chemiosmotic coupling, but in reality the inner mitochondrial membrane is not perfectly impermeable to protons, allowing some proton leakage that dissipates the gradient without driving ATP synthesis.
2. The ATP:NADH and ATP:FADH₂ ratios (P/O ratios) used in the theoretical calculation assume fixed stoichiometry, but the actual number of ATP molecules synthesised per NADH oxidised is non-integer and variable (approximately 2.5 for NADH and 1.5 for FADH₂), so the true yield is lower.
3. Some of the proton-motive force is used to drive the transport of ADP and inorganic phosphate into the mitochondrial matrix (via the adenine nucleotide translocase and phosphate carrier), reducing the number of protons available exclusively for ATP synthase.
4. NADH produced in glycolysis (in the cytoplasm) must be shuttled into the mitochondria; the malate–aspartate shuttle or glycerol-3-phosphate shuttle transfers these electrons at the cost of one or more ATP equivalents, or feeds electrons into the chain at FADH₂ rather than NADH level, reducing overall yield.

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