Explain the Krebs cycle with reaction.

The Krebs cycle (citric acid cycle) is the series of reactions in the mitochondria by which the acetyl group from pyruvic acid is completely oxidised to \(\mathrm{CO_2}\), producing NADH₂, FADH₂ and ATP.

Link reaction: pyruvic acid from glycolysis enters the mitochondria and is converted into acetyl-CoA:

\[\mathrm{Pyruvic\ acid + CoA + NAD}\] \[\mathrm{\rightarrow Acetyl\text{-}CoA + CO_2 + NADH_2}\]

Steps of the cycle:

1. Acetyl-CoA (2C) + oxaloacetic acid (4C) \(\rightarrow\) citric acid (6C).
2. Citric acid \(\rightarrow\) isocitric acid (rearrangement).
3. Isocitric acid \(\rightarrow\) α-ketoglutaric acid (5C) + \(\mathrm{CO_2}\) + NADH₂.
4. α-ketoglutaric acid \(\rightarrow\) succinyl-CoA (4C) + \(\mathrm{CO_2}\) + NADH₂.
5. Succinyl-CoA \(\rightarrow\) succinic acid, forming 1 ATP (GTP).
6. Succinic acid \(\rightarrow\) fumaric acid + FADH₂.
7. Fumaric acid + \(\mathrm{H_2O}\) \(\rightarrow\) malic acid.
8. Malic acid \(\rightarrow\) oxaloacetic acid + NADH₂. Oxaloacetic acid combines with the next acetyl-CoA, so the cycle continues.

Products of one turn: 2 \(\mathrm{CO_2}\), 3 NADH₂, 1 FADH₂ and 1 ATP. One glucose gives 2 pyruvic acid, so the cycle turns twice per glucose.

NADH₂ and FADH₂ then pass their hydrogen to the electron transfer chain, where oxygen accepts it to form water and most of the ATP is made.