Anaerobic fermentation is a method cells use to extract energy from carbohydrates when oxygen or other electron acceptors are not available in the surrounding environment. This differentiates it from anaerobic respiration, which doesn’t use oxygen but does use electron-accepting molecules that come from outside of the cell. The process can follow glycolysis as the next step in the breakdown of glucose and other sugars to produce molecules of adenosine triphosphate (ATP) that create an energy source for the cell.
Through this method, a cell is able to regenerate nicotinamide adenine dinucleotide (NAD+) from the reduced form of nicotinamide adenine dinucleotide (NADH), a molecule necessary to continue glycolysis. Anaerobic fermentation relies on enzymes to add a phosphate group to an individual adenosine diphosphate (ADP) molecule to produce ATP, which means it is a form of substrate-level phosphorylation. This contrasts with oxidative phosphorylation, which uses energy from an established proton gradient to produce ATP.
There are two major types of anaerobic fermentation: ethanol fermentation and lactic acid fermentation. Both restore NAD+ to allow a cell to continue generating ATP through glycolysis. Ethanol fermentation converts two pyruvate molecules, the products of glycolysis, to two molecules of ethanol and two molecules of carbon dioxide. The reaction is a two-step process in which pyruvate is converted to acetaldehyde and carbon dioxide first, by the enzyme pyruvate decarboxylase.
In the second step, alcohol dehydrogenase converts acetaldehyde to ethanol. This metabolic process occurs in certain types of bacteria cells and in yeast cells. This makes yeast popular for making bread, beer, and wine, by using either the carbon dioxide or the ethanol from fermentation.
Lactic acid fermentation is another form of anaerobic fermentation, and is commonly used by muscle cells during times of stress when not enough oxygen is available. These cells convert the two molecules of pyruvate from glycolysis into two molecules of L-lactate using the enzyme lactate dehydrogenase. This process is known as homolactic fermentation, because two molecules of pyruvate undergo the same chemical reactions, and this form of lactic acid fermentation occurs in animal muscle cells and red blood cells.
In heterolactic fermentation, the pyruvate molecules undergo different chemical reactions. One is converted to lactate, while the other is converted to ethanol and carbon dioxide. This process occurs in some species of anaerobic organisms.
In animals, the lactate byproduct from anaerobic fermentation is pumped into the bloodstream, where it is transported to the liver. In a process called the Cori cycle, the liver uses its own set of enzymes to convert the lactate back to glucose, where it can be recycled by the body. The glucose is usually transported back to the muscles, where it can be stored as glycogen for future energy needs.