1. Before the pyruvates from glycolysis can feed into the Krebs cycle, they must undergo a transition reaction. The pyruvate is converted into a 2-carbon acetyl group as the third carbon is lost as CO₂. The acetyl group is attached to coenzyme A to form acetyl-CoA.
2. The 2-carbon acetyl-CoA combines with the 4-carbon oxaloacetate of the Krebs cycle to form 6-carbon citrate.
3. Citrate is converted to isocitrate.
4. The 6-carbon isocitrate is oxidized by NAD⁺ to produce NADH and 5-carbon alpha-ketoglutarate. (One carbon is lost as CO₂.)
5. The 5-carbon alpha-ketoglutarate is oxidized by NAD⁺ to produce NADH and 4-carbon succinyl-CoA. (One carbon is lost as CO₂.)
6. Oxidation of succinyl-CoA produces succinate and one ATP.
7. Oxidation of succinate by FAD produces FADH₂ and fumarate.
8. Fumarate is converted into malate.
9. Oxidation of malate by NAD⁺ produces NADH and oxaloacetate.

The two molecules of acetyl-CoA from the transition reaction (see Fig. 34C) enter the Krebs cycle. This results in the formation of 6 molecules of NADH, two molecules of FADH₂, two molecules of ATP, and four molecules of CO₂. The NADH and FADH₂ molecules then carry electrons to the electron transport system for further production of ATPs by chemiosmotic phosphorylation (see Fig. 36).