Anaerobic fermentation is a process used by cells to extract energy from carbohydrates when oxygen is not available. It follows glycolysis and restores NAD+ to continue generating ATP. There are two types: ethanol fermentation and lactic acid fermentation. Ethanol fermentation is used by some bacterial and yeast cells to produce ethanol and carbon dioxide, while lactic acid fermentation is used by animal muscle cells to produce L-lactate. In heterolactic fermentation, pyruvate molecules undergo several chemical reactions. The lactate byproduct is transported to the liver, where it is converted back into glucose in the Cori cycle.
Anaerobic fermentation is a method used by cells 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 does not use oxygen but uses molecules that accept electrons that come from outside the cell. The process can follow glycolysis as the next step in breaking down glucose and other sugars to produce adenosine triphosphate (ATP) molecules 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 required to continue glycolysis. Anaerobic fermentation relies on enzymes to add a phosphate group to a single molecule of adenosine diphosphate (ADP) to produce ATP, meaning 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 main types of anaerobic fermentation: ethanol fermentation and lactic acid fermentation. Both restore NAD+ to allow a cell to continue generating ATP through glycolysis. The fermentation of ethanol converts two molecules of pyruvate, the products of glycolysis, into two molecules of ethanol and two molecules of carbon dioxide. The reaction is a two-step process in which pyruvate is first converted into acetaldehyde and carbon dioxide by the enzyme pyruvate decarboxylase.
In the second step, alcohol dehydrogenase converts acetaldehyde into ethanol. This metabolic process occurs in some types of bacterial cells and yeast cells. This makes yeast popular for making bread, beer and wine, using carbon dioxide or 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 pyruvate molecules undergo the same chemical reactions, and this form of lactic acid fermentation occurs in animal muscle cells and red blood cells.
In heterolactic fermentation, pyruvate molecules undergo several 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 of 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 lactate back into glucose, where it can be recycled by the body. Glucose is usually carried back to the muscles, where it can be stored as glycogen for future energy needs.
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