Alcoholic fermentation is mainly used by various yeast species to make energy.
If there is no oxygen available, the yeasts have in the alcoholic fermentation another possibility of energy supply. But they can - as compared with cellular respiration - recover substantially less energy from glucose, in the form of adenosine triphosphate (ATP): by complete oxidation, a molecule of glucose provides 36 molecules of ATP, but by alcoholic fermentation only 2 molecules of ATP. These two molecules are obtained in glycolysis, the first step in the chain of reactions for both cellular respiration and fermentation.
The two additional steps of the fermentation, and thus the production of ethanol serve not to make energy, but the regeneration of the NAD + cofactor used by the enzymes of glycolysis. As NAD + is available in limited quantities, it is converted by the NADH reduced state fermentation enzymes to the NAD + oxidized state by reduction of acetaldehyde to ethanol.
