# Explain the role of substrate-level phosphorylation in cell respiration, including where it occurs and why it remains essential even when oxidative phosphorylation is fully functional.

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

## Mark scheme (4 marks)

1. Substrate-level phosphorylation is the direct transfer of a phosphate group from a phosphorylated intermediate to ADP, forming ATP without the involvement of the electron transport chain.
2. Substrate-level phosphorylation occurs in glycolysis (in the cytoplasm/cytosol) and in the Krebs cycle (in the mitochondrial matrix).
3. Glycolysis produces a net yield of 2 ATP by substrate-level phosphorylation, and the Krebs cycle produces 1 ATP (or GTP) per turn / 2 ATP per glucose by substrate-level phosphorylation.
4. Substrate-level phosphorylation is essential because glycolysis (and the ATP it produces) occurs in the cytoplasm and is the only ATP source for cells that are transiently anaerobic or in tissues/organelles lacking access to oxidative phosphorylation, ensuring ATP supply is maintained even if the mitochondrial proton gradient is disrupted.

## Key terms

- [substrate-level phosphorylation](https://www.gradenine.co.uk/glossary/substrate-level-phosphorylation)
- [oxidative phosphorylation](https://www.gradenine.co.uk/glossary/oxidative-phosphorylation)

## Related

- [Revision notes for IB DP Biology Higher Level (2023 syllabus)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
- [Practice this with AI marking (free)](https://www.gradenine.co.uk/start)

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Source: [GradeNine](https://www.gradenine.co.uk/q/explain-the-role-of-substrate-level-phosphorylation-bb476bca) · Published by Druglandscape Ltd.