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.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
Substrate‑level phosphorylation is the direct transfer of a phosphate group from a phosphorylated intermediate to ADP, forming ATP without the electron transport chain.
It occurs in glycolysis in the cytoplasm and in the Krebs cycle in the mitochondrial matrix.
In glycolysis a net of 2 ATP are produced per glucose, and the Krebs cycle produces 1 ATP (or GTP) per turn, giving 2 ATP per glucose.
It remains essential because the ATP it generates is available in the cytoplasm and in tissues or organelles that cannot use oxidative phosphorylation, ensuring an ATP supply when the mitochondrial proton gradient is disrupted or when cells are transiently anaerobic.
It occurs in glycolysis in the cytoplasm and in the Krebs cycle in the mitochondrial matrix.
In glycolysis a net of 2 ATP are produced per glucose, and the Krebs cycle produces 1 ATP (or GTP) per turn, giving 2 ATP per glucose.
It remains essential because the ATP it generates is available in the cytoplasm and in tissues or organelles that cannot use oxidative phosphorylation, ensuring an ATP supply when the mitochondrial proton gradient is disrupted or when cells are transiently anaerobic.
Examiner tips
- Use the exact phrase "direct transfer of a phosphate group" to gain the first point.
- Mention both glycolysis and the Krebs cycle and specify their locations.
- State the ATP yields clearly (2 ATP from glycolysis, 1 ATP/GTP per cycle).
- Explain why it is essential – cytoplasmic/anaerobic ATP supply – to cover the final point.
Common mistakes
- Confusing substrate‑level with oxidative phosphorylation; forgetting the cytoplasmic location of glycolysis.
- Giving only the ATP yield from the Krebs cycle and omitting the 2 ATP from glycolysis.
- Failing to mention that it is essential when the proton gradient is lost or in anaerobic conditions.
Mark scheme (4 marks)
- 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.
- Substrate-level phosphorylation occurs in glycolysis (in the cytoplasm/cytosol) and in the Krebs cycle (in the mitochondrial matrix).
- 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.
- 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 in this question
substrate-level phosphorylation · oxidative phosphorylation
Related
- All IB DP Biology Higher Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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