The Krebs Cycle
Biology: AQA A Level, topic 5.2.5
The Krebs Cycle
The Krebs cycle takes place in the matrix of the mitochondria. The products of the cycle are two coenzymes (NADH and FADH2), ATP and CO2.
The Krebs Cycle
Acetyl coenzyme A (coA)
- Acetyl coenzyme A acts as a carrier for the two-carbon acetyl group. It reacts with oxaloacetate (a four-carbon molecule) to produce citrate (a six-carbon molecule).
- CoA is now available to be recycled and reused in the link reaction.
- The production of citrate allows the Krebs cycle to begin.
6C → 5C
- Citrate is converted to a five-carbon molecule (5C) by decarboxylation and dehydrogenation.
- CO2 is produced as a by-product.
- NAD is reduced to NADH.
5C → 4C
- The five-carbon molecule is decarboxylated and dehydrogenated again to a four-carbon compound.
- CO2 is produced.
- NAD is reduced to NADH.
- ATP is also produced by substrate-level phosphorylation.
Regeneration of oxaloacetate
- This 4C molecule is then dehydrogenated again to produce another molecule of NADH. FAD is also reduced to FADH2.
- No decarboxylation takes place at this stage.
- These intermediate reactions regenerate oxaloacetate. This allows the cycle to continue again.
Net gain
- The net gain of the Krebs cycle is:
- 2 CO2 molecules.
- 3 NADH molecules.
- 1 ATP molecule.
- 1 FADH2 molecules.
- For each molecule of glucose, there are two cycles (this is because two molecules of pyruvate are produced in glycolysis).
Other respiratory substrates
- Fatty acids and amino acids can also be used as respiratory substrates in aerobic respiration.
- The substrates are converted to molecules that can easily enter the Krebs cycle.
Test yourself on The Krebs Cycle
Where does the Krebs cycle take place?
Which of the following statements about the Krebs cycle are true?
What is the name of the cycle shown in this diagram?