Explain why the rate of ATP production in a cell would decrease significantly if the inner mitochondrial membrane became freely permeable to hydrogen ions (protons).

OCR A-Level Biology A (H420) — 5.7 Respiration · Explain · 4 marks · View as Markdown

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

Researchers studying mitochondrial function discovered that certain toxins act by making the inner mitochondrial membrane permeable to hydrogen ions, effectively uncoupling electron transport from ATP synthesis.

Model answer (4 marks)

The proton gradient across the inner mitochondrial membrane would be lost, so protons would no longer flow through ATP synthase by chemiosmosis. Without this proton motive force, ATP synthase cannot catalyse the phosphorylation of ADP to ATP, meaning oxidative phosphorylation stops. Because oxidative phosphorylation normally produces the majority of ATP in aerobic respiration, the overall ATP yield would decrease significantly.

Examiner tips

  • Use the term ‘proton gradient’ or ‘electrochemical gradient’ first. Explain the link between the gradient, chemiosmosis and ATP synthase. Mention that oxidative phosphorylation is the main ATP source. Keep the answer concise – 4 points only.

Common mistakes

  • Confusing the inner membrane with the outer membrane. Saying the cell simply ‘loses ATP’ without explaining the mechanism. Forgetting to mention that oxidative phosphorylation is the major ATP source.

Mark scheme (4 marks)

  1. The proton gradient (electrochemical / chemiosmotic gradient) across the inner mitochondrial membrane would be lost
  2. Protons would no longer be forced through ATP synthase (chemiosmosis would not occur)
  3. ATP synthase cannot catalyse the phosphorylation of ADP to ATP (oxidative phosphorylation does not occur)
  4. Overall ATP yield is greatly reduced because oxidative phosphorylation accounts for the majority of ATP produced in aerobic respiration

Key terms in this question

inner mitochondrial membrane

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