Krebs Cycle

Biology Unit 5

Krebs Cycle

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Biology Unit 5
Revision Notes
Topic 7: Run for your life
9. Describe the role of the Krebs cycle in the complete
oxidation of glucose and formation of carbon dioxide,
ATP, reduced NAD and reduced FAD and that
respiration is a many-stepped process with each step
controlled and catalysed by a specific intracellular
In aerobic respiration, the pyruvate (from glycolysis) is completely oxidised into carbon dioxide
and water using oxygen.
Aerobic respiration takes place in two stages:
First pyruvate is oxidised into carbon dioxide and hydrogen (accepted by the coenzymes
NAD and FAD). This takes place in the matrix of the mitochondria and involves the Krebs
The second stage involves the ETC, oxidative phosphorylation and chemiosmosis. It will be
seen later.
The Krebs Cycle
The Krebs cycle occurs in the matrix of the mitochondria. The main purpose is to supply a continuous
flow of hydrogen (and therefore electrons) to the electron transport chain for use in the synthesis of
ATP by oxidative phosphorylation.
1. The acetyl coA (2C) combines with an acid (4C) to form citric acid (citrate) (6C).
2. The citric acid (6C) breaks down to form another acid (5C) and a CO molecule, reducing NAD
to NADH in the process.
3. This acid (5C) is broken down into the original 4-carbon acid and another molecule of CO,
4. This acid (4C) then reacts to form another molecule of citric acid and so the cycle is complete.
For 1 molecule of Glucose, the Krebs cycle will turn twice (as 2 x 3C pyruvate go on to the
Each molecule of the 2C acetyl coA from the link reaction is used to generate:
3 x reduced NAD
1 x reduced FAD
2 x CO
1 x ATP by substrate-level phosphorylation (synthesised directly from the energy released
by reorganising chemical bonds)
1 x 4C compound, which is regenerated to accept an acetyl group and start the cycle again
Text Book: p. 136 - 137

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