The citric acid cycle is a means of breaking pyruvate down to CO₂, and transferring hydrogen atoms and free energy to molecules of reduced carriers: NADH and FADH₂. The respiratory chain then accepts reduced carrier molecules from any source - citric acid cycle or glycolysis - reoxidizes them with 0₂, and uses the free energy to synthesize ATP molecules. In essence, the citric acid cycle takes the 546 kcal quantity of energy represented by pyruvate, and breaks it down into a series of 53 kcal (NADH) and 36 kcal (FADH₂) packages.

The cycle is diagrammed opposite, and the free energy steps are shown in the graph on page 18. The cycle also is known as the tricarboxylic acid cycle, or the Krebs cycle after its discoverer, Hans Krebs. In the operation of the cycle, pyruvate first is oxidized and converted to a primed form of acetate, acetyl coenzyme A (coenzyme A diagrammed opposite). This is combined with oxaloacetate to make citrate, and this molecule then is degraded in a series of steps to produce oxaloacetate again, which is ready to combine with more primed acetate. During the course of the cycle, two carbon atoms are removed as C0₂, and four pairs of hydrogen atoms are used to reduce NAD⁺ and FAD, with the storage of free energy. These energy-removing steps, which are the reason for the existence of the cycle, are labeled 4, 5, 7, and 9 in the diagram on page 16.