The citric acid cycle is the stage in cellular respiration in which fuel molecules are completely oxidized. Specifically, the acetyl group of acetyl-CoA is completely oxidized to carbon dioxide and the chemical energy is transferred to ATP by substrate-level phosphorylation and to the reduced electron carriers NADH and FADH₂. In this way, the citric acid cycle supplies electrons to the electron transport chain, leading to the production of much more energy in the form of ATP than is obtained by glycolysis alone.