What is the function of the citric acid cycle in transforming fuel molecules into ATP? Recall that fuel molecules are carbon compounds that are capable of being oxidized—of losing electrons. These fuel molecules are first processed to acetyl CoA, the actual fuel for the citric acid cycle. The citric acid cycle includes a series of oxidation–reduction reactions that ultimately result in the oxidation of the acetyl group to two molecules of carbon dioxide. These oxidations generate high-transfer-potential or high-energy electrons that will be used to power the synthesis of ATP. The function of the citric acid cycle is the harvesting of high-energy electrons from carbon fuels.

We begin with an examination of the cycle itself, noting that it consists of two parts: one part oxidizes carbon atoms to CO₂ and the other regenerates oxaloacetate. We then see how this metabolic hub is regulated, and we end with an examination of the glyoxylate cycle, a cycle unique to plants and some microorganisms that uses reactions of the citric acid cycle.