The citric acid cycle is a metabolic pathway common to aerobic cells by which carbohydrates, fats and amino acids are oxidized to carbon dioxide and water. Enzymes catalyse the condensation of two- and four-carbon compounds to citric acid, and then enzymatic oxidations and decarboxylations regenerate the four-carbon starting compound plus CO₂ and H₂O. The energy available from the downhill flow of electrons to oxygen is used indirectly for the synthesis of ATP, the energy currency of the cell.