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.

Fermentation is a type of metabolic process that occurs in the absence of oxygen, where organic molecules such as glucose are broken down to produce energy. During this process, the glucose molecule is initially converted into two molecules of pyruvate through a series of chemical reactions. This process is called glycolysis and is the same as the first step in aerobic respiration. In the absence of oxygen, the pyruvate molecules are then converted into different organic molecules through a variety of pathways depending on the organism involved.

One key aspect of fermentation is the regeneration of NAD⁺ from NADH. During glycolysis, NAD⁺ is converted into NADH, and the continued functioning of glycolysis requires the regeneration of NAD⁺. In the absence of oxygen, fermentation is the process that regenerates NAD⁺ by transferring electrons from NADH to an organic molecule, such as pyruvate, resulting in the formation of a new organic molecule and NAD⁺.

There are two main types of fermentation based on the end products of the reaction: lactic acid fermentation and alcohol fermentation. In mammals, lactic acid fermentation takes place in red blood cells that cannot respire aerobically due to the 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 a 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.