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Learn7402Paper 2 · Paper 2: Topics 5–8Energy transfers in and between organismsKrebs cycle and oxidative phosphorylation
Energy transfers in and between organisms · Subtopic 3 of 5

Krebs Cycle and Oxidative Phosphorylation

Link reaction (matrix): pyruvate (3C) is decarboxylated (CO₂) and dehydrogenated (reduced NAD), forming acetyl (2C) which joins coenzyme A. Krebs cycle (matrix): acetyl CoA + oxaloacetate (4C) → citrate (6C); citrate is decarboxylated twice and dehydrogenated four times back to oxaloacetate. Per turn: 2 CO₂, 3 reduced NAD, 1 reduced FAD, 1 ATP (substrate-level). Two turns per glucose. Oxidative phosphorylation (inner membrane, cristae): reduced NAD/FAD release electrons to the electron transport chain; energy pumps H⁺ into the intermembrane space; H⁺ flow back through ATP synthase (chemiosmosis) makes ATP. Oxygen is the final electron acceptor, forming water. Total ≈ 32 ATP per glucose. Redox indicators (DCPIP, methylene blue) accept electrons and lose colour.

Cambridge 9700 Paper 4Edexcel YBI11 WBI15AQA 7402 Paper 2≈ 55 minRevision 1 · updated 2026-11-02
Section 1 of 14

Learning objectives

  • Describe the link reaction and Krebs cycle.
  • Explain chemiosmosis and the role of oxygen.
  • Count products per glucose.
  • Use respirometers and redox indicators.
Pause and think

Cyanide blocks the last protein in the electron transport chain. Why is it so deadly?

Section 2 of 14

Core idea

Start hereWhere most ATP comes from

Glycolysis gives 2 ATP. Mitochondria give about 30 more from the same glucose.

The big idea

The link reaction and Krebs cycle strip hydrogen from carbon compounds; the electron transport chain uses it to build a proton gradient that drives ATP synthase.

Aerobic respiration in the mitochondrion

Matrix, then inner membrane.

Worked exampleCounting per glucose

Total reduced NAD per glucose?

Goal
Number.
Strategy
Add each stage, doubling per pyruvate.
  1. glycolysis=2\text{glycolysis}=2
    Per glucose.
  2. link=2, Krebs=6\text{link}=2,\ \text{Krebs}=6
    1 and 3 per pyruvate, ×2.
  3. 2+2+6=102+2+6=10
    Total.
Answer10 reduced NAD (plus 2 reduced FAD).
✓ CheckMatches standard totals ✓.

Summary

  • Link: decarboxylation, dehydrogenation.
  • Krebs in matrix.
  • Chemiosmosis; O₂ final acceptor.
Key words
Chemiosmosis
ATP synthesis driven by protons flowing through ATP synthase.
Decarboxylation
Removal of CO₂.
HistoryKrebs and Mitchell. Hans Krebs worked out the cycle in 1937; Peter Mitchell's chemiosmotic theory (1961) was doubted for years, then won the 1978 Nobel Prize.
Fun factBrown fat. Brown fat cells let protons leak back across the membrane without making ATP, releasing the energy as heat to keep babies warm.
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