Citric acid cycle
A cycle in the mitochondrial matrix that fully oxidises the carbon from pyruvate to carbon dioxide. It mainly produces electron carriers, NADH and FADH2.
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Analogy
A mill wheel that keeps turning and loads electron carriers onto trucks.
Example
2 turns of the cycle.6 NADH.2 FADH2.2 ATP.4 CO2 (plus 2 before).
In practice
Each glucose gives two turns of the cycle. Together they yield 6 NADH, 2 FADH2 and 2 ATP.
Practice
Flashcards
- Where does the citric acid cycle happen?In the mitochondrial matrix.
Interview questions
- Outline the stages of aerobic respiration and say where each occurs and what it produces.
- Glycolysis in cytoplasm: 2 ATP, 2 NADH
- Citric acid cycle in matrix: NADH, FADH2, CO2
- Electron transport and chemiosmosis in inner membrane: most ATP
- Oxygen is final electron acceptor
Questions people ask
Why is it called a cycle?
Because the last molecule, oxaloacetate, is the same one that picks up carbon at the start. Acetyl-CoA joins it, the molecule is slowly dismantled, and oxaloacetate is rebuilt, ready for the next turn. Nothing runs out because the starting molecule is regenerated every time.
Does the cycle use oxygen directly?
No. Oxygen is not used in the cycle itself. But the cycle depends on the electron transport chain to turn NADH and FADH2 back into NAD+ and FAD, and the chain needs oxygen. So with no oxygen, the cycle soon stops as well.