Explain why the rate of ATP production in a cell is significantly reduced when a metabolic poison blocks the transfer of electrons along the electron transport chain.

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 cellular respiration discovered that a newly isolated fungal toxin causes a rapid and dramatic fall in ATP concentration in aerobic cells, even when glucose supply remains constant. Further investigation revealed the toxin acts by permanently inhibiting a protein complex in the inner mitochondrial membrane that accepts electrons from reduced NAD and reduced FAD.

Model answer (4 marks)

Electron transfer along the chain is coupled to the active pumping of H⁺ ions across the inner mitochondrial membrane; blocking electron transfer stops this pumping.

When the proton gradient cannot be established or maintained, the electrochemical potential across the membrane collapses.

Without a proton motive force, protons cannot flow back through ATP synthase (chemiosmosis), so ATP synthase cannot phosphorylate ADP to ATP.

In addition, reduced NADH and FADH₂ accumulate because they cannot be re‑oxidised, which slows the Krebs cycle and the link reaction, further reducing ATP production from substrate‑level phosphorylation (glycolysis alone remains).

Examiner tips

  • Use the chain‑to‑proton pumping link first, then the gradient, then ATP synthase, and finally the effect on upstream cycles.
  • Show the logical sequence: electron block → no proton pumping → no gradient → no ATP synthase activity → reduced NADH/FADH₂ re‑oxidation → lower Krebs cycle activity.
  • Include the phrase "chemiosmosis" and "proton motive force" to match the mark scheme.

Mark scheme (4 marks)

  1. Electron transfer along the chain is linked to active pumping of protons (H⁺ ions) across the inner mitochondrial membrane, so blocking electron transfer stops proton pumping.
  2. Blocking electron transfer prevents the establishment / maintenance of the proton gradient (electrochemical / chemiosmotic gradient) across the inner mitochondrial membrane.
  3. Without a proton gradient, protons cannot flow back through ATP synthase (chemiosmosis), so ATP synthase cannot phosphorylate ADP to ATP.
  4. Reduced NAD and reduced FAD accumulate / cannot be reoxidised, so the Krebs cycle and link reaction are also inhibited, further reducing ATP yield (substrate-level phosphorylation from glycolysis alone remains).

Key terms in this question

electron transport chain · metabolic poison

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