The aerobic process of respiration takes place only in the presence of oxygen, but the role of oxygen is limited to the terminal stage of the process. During terminal oxidation, oxygen acts as final hydrogen acceptor and combines with H+ and forms the water. Pyruvate is formed by glycolytic catabolism of carbohydrates in the cytosol. In fermentation, there is incomplete oxidation of glucose under anaerobic conditions. During the conversion of succinyl-CoA to succinic acid, a molecule of GTP is synthesised.