In aerobic cells from animal and certain other species, the major pathway for the complete oxidation of acetyl coenzyme A (the thioester of acetic acid with coenzyme A); also known as the Krebs cycle or tricarboxylic acid cycle. Reduced electron carriers generated in the cycle are reoxidized by oxygen via the electron transport system; water is formed, and the energy liberated is conserved by the phosphorylation of adenosine diphosphate (ADP) to adenosine triphosphate (ATP). Reactions of the cycle also function in metabolic processes other than energy generation. The role of the cycle in mammalian tissues will be emphasized in this article.