Fermentation
Fermentation is an extension of glycolysis that makes ATP without the use of oxygen, through substrate-level phosphorylation. Fermentation is used by organisms living in anaerobic environments, where access to oxygen is not possible, and therefore a complete cycle of cellular respiration is not possible. In this process, glycolysis functions the same way, producing two ATP. However, when the two pyruvate molecules are reduced, the NAD that causes glycolysis to begin is recycled, beginning the process once more. Cells do not run out of their NAD storage, repeatedly reproducing NAD through the transfer of electrons to the electron transfer chain. Through the process, two ATP can be produced. Two examples of fermentation are alcohol fermentation and lactic acid fermentation. Alcohol fermentation is the conversion of pyruvate to ethanol, while lactic fermentation is the conversion of pyruvate to lactate.