Anaerobic

Under anaerobic conditions, the pyruvate molecule can follow other anaerobic pathways to regenerate the NAD⁺ necessary for glycolysis to continue. These include alcoholic fermentation and lactate fermentation. In the absence of oxygen the further reduction or addition of hydrogen ions and electrons to the pyruvate molecules that were produced during glycolysis is termed fermentation. This process recycles the reduced NADH to the free NAD⁺ coenzyme which once again serves as the hydrogen acceptor enabling glycolysis to continue. Alcoholic fermentation, characteristic of some plants and many microorganisms, yields alcohol and carbon dioxide as its products. Yeast is used by the biotechnology industries to generate carbon dioxide gas necessary for bread-making and in the fermentation of hops and grapes to produce alcoholic beverages. Depending on the yeast variety used, the different alcohol levels realized act as a form of population control by serving as the toxic element which kills the producers. Birds have been noted to fly erratically after they have gorged themselves on the fermenting fruit of the Pyracantha shrub.

Reduction of pyruvate by NADH to release the NAD⁺ necessary for the glycolytic pathway can also result in lactate fermentation, which takes place in some animal tissues and in some microorganisms. Lactic acid-producing bacterial cells are responsible for the souring of milk and production of yogurt. In working animal muscle cells, lactate fermentation follows the exhaustion of the ATP stores. Fast twitch muscle fibers store little energy and rely on quick spurts of anaerobic activity, but the lactic acid that accumulates within the cells eventually leads to muscle fatigue and cramp.