Explain how the products of the Krebs cycle are used in subsequent stages of aerobic respiration to maximise ATP yield.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
The Krebs cycle generates the reduced co‑enzymes NADH and FADH₂, which carry high‑energy electrons to the electron transport chain (ETC) in the inner mitochondrial membrane.
At the ETC, oxidation of NADH and FADH₂ releases energy that is used to pump protons (H⁺) from the matrix into the intermembrane space, creating an electrochemical gradient.
Protons then flow back into the matrix through ATP synthase (chemiosmosis), driving the phosphorylation of ADP to ATP – this is oxidative phosphorylation.
Because each NADH contributes to a larger proton gradient (entering at Complex I) than each FADH₂ (entering at Complex III), the overall ATP yield is maximised.
CO₂ produced in the cycle is a waste product and does not contribute to ATP synthesis.
At the ETC, oxidation of NADH and FADH₂ releases energy that is used to pump protons (H⁺) from the matrix into the intermembrane space, creating an electrochemical gradient.
Protons then flow back into the matrix through ATP synthase (chemiosmosis), driving the phosphorylation of ADP to ATP – this is oxidative phosphorylation.
Because each NADH contributes to a larger proton gradient (entering at Complex I) than each FADH₂ (entering at Complex III), the overall ATP yield is maximised.
CO₂ produced in the cycle is a waste product and does not contribute to ATP synthesis.
Examiner tips
- Use the exact terms NADH, FADH₂, ETC, proton gradient, ATP synthase, oxidative phosphorylation
- Show the flow from Krebs to ETC to ATP synthase
- Explain why NADH gives more ATP than FADH₂
Common mistakes
- Confusing FADH₂ with NADH or vice‑versa
- Saying CO₂ produces ATP
- Omitting the role of the proton gradient or ATP synthase
Mark scheme (4 marks)
- The Krebs cycle produces NADH (and FADH2) as reduced coenzymes that carry high-energy electrons to the electron transport chain (inner mitochondrial membrane).
- 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.
- Protons flow back into the matrix through ATP synthase (chemiosmosis), driving the phosphorylation of ADP to ATP (oxidative phosphorylation).
- 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 in this question
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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