# 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

> 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.

## 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

- [electron transport chain](https://www.gradenine.co.uk/glossary/electron-transport-chain)
- [metabolic poison](https://www.gradenine.co.uk/glossary/metabolic-poison)

## Related

- [Revision notes for OCR A-Level Biology A (H420)](https://www.gradenine.co.uk/learn)
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Source: [GradeNine](https://www.gradenine.co.uk/q/explain-why-the-rate-of-atp-97f4041e) · Published by Druglandscape Ltd.