In the presence of air and various cofactors and enzymes, fatty acids are degraded to acetyl-CoA . The pathway is called beta-oxidation . Each cycle of beta-oxidation shortens the fatty acid chain by two carbon atoms and produces one equivalent each of NADH and one FADH 2 . The NADH and FADH 2 are used to generate ATP by oxidative phosphorylation. Dozens of ATP equivalents are generated by the beta-oxidation of a single long acyl chain.  The acetyl-CoA produced by beta-oxidation can be subsequently metabolized by the citric acid cycle, yielding further equivalents of ATP.