Pyruvate Oxidation and the Krebs Cycle

The CO2 you breathe out comes from your last meal, not your last breath, because carbon dioxide is literally the carbon skeleton of glucose, chopped apart and released as the cell oxidizes it. After glycolysis splits glucose into two pyruvate molecules in the cytoplasm, each pyruvate crosses into the mitochondrial matrix. There it meets pyruvate dehydrogenase, a huge enzyme complex that strips off one carbon as CO2, hands the remaining two-carbon fragment to coenzyme A to form acetyl-CoA, and loads one NADH. This is pyruvate oxidation: one turn per pyruvate, so two per glucose.

Acetyl-CoA then feeds into the Krebs cycle (also called the citric acid cycle), a loop of eight reactions in the matrix. The two-carbon acetyl group combines with a four-carbon molecule to make six-carbon citrate, and as the cycle turns, two carbons leave as CO2, electrons get stripped off and caught by NAD+ and FAD to make NADH and FADH2, and one ATP-equivalent (GTP) forms directly. By the end of one turn the starting four-carbon molecule is regenerated, ready to accept another acetyl group. Since each glucose produces two acetyl-CoA, the cycle turns twice.

Count the carbons leaving as CO2: one per pyruvate oxidation times two, plus two per Krebs turn times two, equals six CO2 per glucose, matching the six carbons glucose started with. The cycle itself makes only a little ATP directly through substrate-level phosphorylation. Its real payoff is the stockpile of NADH and FADH2, which carry electrons to the electron transport chain, where most of glucose's ATP is actually generated through chemiosmosis.