During respiration, which can be important in the earliest phases of the alcoholic fermentation and in all phases of commercial yeast production, most of the energy of the catabolism of carbon compounds is captured in the form of ATP. This is the result of the functioning of two metabolic pathways – the tricarboxylic or citric acid cycle and the electron transport chain. A further substrate level phosphorylation occurs during the succinyl-CoA-synthase reaction.