# Explain how the products of the Krebs cycle are used in subsequent stages of aerobic respiration to maximise ATP yield.

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

## Mark scheme (4 marks)

1. The Krebs cycle produces NADH (and FADH2) as reduced coenzymes that carry high-energy electrons to the electron transport chain (inner mitochondrial membrane).
2. Oxidation of NADH and FADH2 at the electron transport chain releases energy used to pump protons (H⁺) from the matrix into the intermembrane space, establishing an electrochemical/proton gradient.
3. Protons flow back into the matrix through ATP synthase (chemiosmosis), driving the phosphorylation of ADP to ATP (oxidative phosphorylation).
4. CO2 released by the Krebs cycle is a metabolic waste product and does not contribute to ATP synthesis; the ATP yield is maximised because each NADH yields more ATP than FADH2 (enters at a later complex), reflecting different entry points into the chain.

## Key terms

- [Krebs cycle](https://www.gradenine.co.uk/glossary/krebs-cycle)

## Related

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Source: [GradeNine](https://www.gradenine.co.uk/q/explain-how-the-products-of-the-8703aa7b) · Published by Druglandscape Ltd.