Simulation · Biology · Class 11
Turning Krebs' cycle
From the lesson Tricarboxylic acid cycle in Respiration in Plants. Change the values and watch what happens.
Turning Krebs' cycleBiology · Class 11
The idea behind it
NCERT §12.4.1
- The cycle is also called Krebs' cycle, after Hans Krebs, who first worked it out, or the citric acid cycle. It runs in the mitochondrial matrix.
- It starts when the acetyl group (2C) condenses with oxaloacetic acid (OAA, 4C) and water to give citric acid (6C). Citrate synthase catalyses this, and CoA is set free.
- Citrate is isomerised to isocitrate. Two decarboxylations follow in succession, giving α-ketoglutaric acid (5C) and then succinyl-CoA; each releases one CO₂.
- Succinyl-CoA is converted to succinic acid, making one GTP. This is substrate-level phosphorylation; in a coupled reaction the GTP becomes GDP as ADP becomes ATP.
- The rest of the cycle oxidises succinic acid, via malic acid, back to OAA so the cycle can go on.
- Each turn has three steps where NAD⁺ is reduced to NADH + H⁺ and one where FAD⁺ is reduced to FADH₂.
- For the cycle to keep running, OAA must be continually replenished, and NAD⁺ and FAD⁺ must be regenerated from NADH and FADH₂.
- Summary per pyruvic acid (matrix): pyruvic acid + 4NAD⁺ + FAD⁺ + 2H₂O + ADP + Pi → 3CO₂ + 4NADH + 4H⁺ + FADH₂ + ATP. This includes the link reaction.
- Per glucose, counting the link reaction, the story so far is 8 NADH + H⁺, 2 FADH₂ and 2 ATP (from the cycle), with 6 CO₂ released. Neither O₂ nor the large ATP yield has appeared yet.