The Krebs cycle: Higher Level only: This statement is Higher Level only. Learn what happens and what is made; only citrate and oxaloacetate need naming.
Practise this as you read
- Outline the Krebs cycle.
- Compare the stages that release carbon dioxide or reduce NAD.
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The big idea: In the matrix, each acetyl group joins oxaloacetate (4C) to form citrate (6C). Citrate is decarboxylated twice and oxidized four times, back to oxaloacetate. Each turn releases 2 carbon dioxide and makes 3 reduced NAD, 1 reduced FAD and 1 ATP.
The points to remember
- Acetyl (2C) + oxaloacetate (4C) → citrate (6C).
- Two decarboxylations: 6C → 5C → 4C, releasing 2 CO₂.
- Four oxidations: 3 reduced NAD + 1 reduced FAD per turn.
- 1 ATP per turn, made directly.
- Oxaloacetate is regenerated; two turns per glucose.
Remember it as: Krebs per turn: 2, 3, 1, 1 (CO₂, reduced NAD, reduced FAD, ATP).
Real example: where the carbon dioxide you breathe out comes from. Of the six carbon dioxide molecules released from one glucose, two come from the link reaction and four from the two turns of the Krebs cycle.
Trap: The cycle itself makes only 1 ATP per turn: most of its value is the reduced NAD and FAD, which feed the electron transport chain.
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How it is asked: Expect a statement or table question on which stages decarboxylate or reduce NAD, and an Outline of the cycle.
IB-style questionOutline[4 marks]
Outline the Krebs cycle.
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