Turning Krebs' cycle.

Turn the citric acid cycle one reaction at a time in the matrix: carbon count 6C to 5C to 4C as each CO₂ leaves, with NADH, FADH₂ and the one substrate-level GTP lighting up where they form.

Count, for one glucose with link reaction included, the NADH, FADH₂, CO₂ and GTP, and match the ledger.

The GTP of Krebs' cycle is oxidative phosphorylation; the cycle runs on the inner membrane.

• 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.

Teaching this idea? Put the simulation on your own page or LMS with this code.