Most eukaryotic organisms require oxygen to survive and function adequately. Such organisms produce large amounts of energy during aerobic respiration by metabolizing glucose and oxygen into carbon dioxide and water. However, most eukaryotes can generate some energy in the absence of oxygen by anaerobic metabolism.

Aerobic respiration proceeds through a series of oxidation-reduction reactions that end when oxygen–the final electron acceptor–is reduced to water. In the absence of oxygen, this reaction cannot proceed. Instead, cells regenerate NADH produced during glycolysis by using an organic molecule, such as pyruvate, as the final electron acceptor. The process of using an organic molecule to regenerate NAD⁺ from NADH is called fermentation.

There are two types of fermentation based on the end products of the reaction: 1) lactic acid fermentation and 2) alcohol fermentation. In mammals, lactic acid fermentation takes place in red blood cells that cannot respire aerobically due to lack of mitochondria, as well as in skeletal muscles during strenuous exercise. It also occurs in certain bacteria, like those found in yogurt. In this reaction, pyruvate and NADH are converted to lactic acid and NAD⁺.

Alcohol fermentation is a two-step process. In the first step, pyruvate is converted to carbon dioxide and acetaldehyde. In the second step, acetaldehyde acts as an electron acceptor and is reduced to ethanol with concomitant conversion of NADH into NAD⁺. Overall, alcohol fermentation converts pyruvate and NADH into ethanol, carbon dioxide, and NAD⁺. Yeasts use alcohol fermentation to convert sugars into carbon dioxide and ethanol. This process is harnessed to produce alcoholic beverages such as beer and wine.