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 · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (4 marks)

The theoretical 38 ATP assumes 100 % efficiency of the proton gradient and fixed P/O ratios. In reality the inner mitochondrial membrane leaks protons, so part of the gradient is lost before ATP synthase uses it. The actual P/O ratios are ≈2.5 per NADH and ≈1.5 per FADH₂, not the integer values used in the calculation. Some of the proton‑motive force is consumed by the adenine nucleotide translocase and phosphate carrier to import ADP and Pi into the matrix, leaving fewer protons for ATP synthase. Finally, cytosolic NADH from glycolysis must be shuttled into mitochondria; the malate–aspartate shuttle is efficient but the glycerol‑3‑phosphate shuttle feeds electrons as FADH₂, costing an extra ATP equivalent. These factors together reduce the net yield to about 30–32 ATP.

Examiner tips

  • Mention proton leakage first, then P/O ratios, then transport costs, and finally the shuttle effect – this order mirrors the mark scheme.
  • Use the exact terms ‘P/O ratio’, ‘malate–aspartate shuttle’, ‘glycerol‑3‑phosphate shuttle’, and ‘adenine nucleotide translocase’ to gain full marks.

Common mistakes

  • Assuming the 38 ATP figure is correct; students forget to explain the loss mechanisms.
  • Using the wrong P/O ratios (e.g. 3 and 2) instead of 2.5 and 1.5.
  • Omitting the cost of importing ADP/Pi into the matrix.

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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