The citric acid cycle is the second stage of respiration. In the citric acid cycle, the two carbons from acetyl-CoA are combined with oxaloacetate to form a six carbon compound (citrate).

In the first part of the citric acid cycle, two carbons from citrate are oxidized to CO2, yielding an activated four carbon compound (succinyl-CoA).

In the second part of the citric acid cycle, succinyl-CoA is converted back to oxaloacetate (see also Figure 14.3), thus making it possible to begin the process again.

In each turn of the citric acid cycle, two carbons are added from acetyl-CoA and two carbons are oxidized as CO2. Thus, there is no net incorporation of carbons from acetyl-CoA in each turn of the cycle.