Fermentation

Fermentation is another anaerobic (non-oxygen-requiring) pathway for breaking down glucose, one that’s performed by many types of organisms and cells. In fermentation, the only energy extraction pathway is glycolysis, with one or two extra reactions tacked on at the end.

Fermentation and cellular respiration begin the same way, with glycolysis. In fermentation, however, the pyruvate made in glycolysis does not continue through oxidation and the citric acid cycle, and the electron transport chain does not run. Because the electron transport chain isn’t functional, the NADH made in glycolysis cannot drop its electrons off there to turn back into NAD+.

The purpose of the extra reactions in fermentation, then, is to regenerate the electron carrier NAD+ from the NAD+ in glycolysis. The extra reactions accomplish this by letting NADH drop its electrons off with an organic molecule (such as pyruvate, the end product of glycolysis). This drop-off allows glycolysis to keep running by ensuring a steady supply of NAD+.