Fermentation is a metabolic process that occurs inside cells when oxygen is not available, allowing organisms to generate energy under anaerobic conditions. This pathway is widely utilized in nature, from muscle cells in the human body to the yeast used in brewing and the bacteria that produce yogurt. The process involves chemical transformations, including both oxidation and reduction reactions. To keep the energy-producing pathway running, the cell must perform a chemical reduction step, which is the core of fermentation’s function. This article explains the central chemical necessity that drives fermentation and identifies which molecules are reduced to sustain the process.
The Chemical Necessity of Fermentation
The initial step of energy extraction from sugar, known as glycolysis, is the foundation for both aerobic respiration and fermentation. During glycolysis, a molecule of glucose is broken down into two molecules of pyruvate, generating a small amount of usable energy in the form of adenosine triphosphate (ATP). A byproduct of this reaction is the reduction of the coenzyme Nicotinamide Adenine Dinucleotide (\(\text{NAD}^+\)) to \(\text{NADH}\).
The cell has a limited supply of the oxidized form, \(\text{NAD}^+\), which is required to continue glycolysis. If oxygen is present, \(\text{NADH}\) is recycled back to \(\text{NAD}^+\) through the electron transport chain, but in an anaerobic environment, this recycling stops. Without converting \(\text{NADH}\) back into \(\text{NAD}^+\), the cell runs out of \(\text{NAD}^+\), halting glycolysis and stopping all ATP production.
Fermentation serves the singular purpose of regenerating the \(\text{NAD}^+\) supply so that glycolysis can continue to produce ATP. This is accomplished by transferring the electrons carried by \(\text{NADH}\) to another molecule. By dropping its electrons, the \(\text{NADH}\) is oxidized back to \(\text{NAD}^+\), making the coenzyme available for reuse in glycolysis. The molecule that accepts these electrons is, by definition, the one that is reduced.
The Role of Pyruvate as the Electron Acceptor
The molecule that accepts the electrons from \(\text{NADH}\) is typically the pyruvate molecule, which is the end product of glycolysis. When a molecule gains electrons, it is chemically reduced, and in fermentation, pyruvate (or a molecule derived from it) fulfills this role as the final electron acceptor. This reduction of pyruvate is what allows \(\text{NADH}\) to be oxidized to \(\text{NAD}^+\), ensuring the cellular energy cycle remains active.
In the most direct form of fermentation, the two molecules of pyruvate generated from glucose accept the electrons carried by \(\text{NADH}\). This transfer of electrons reduces the pyruvate molecule and converts the \(\text{NADH}\) back into \(\text{NAD}^+\). This molecular exchange is the central chemical action of fermentation, keeping the supply of \(\text{NAD}^+\) high enough for the cell to sustain its minimal energy output.
Manifestations of Reduction in Common Pathways
The reduction of the final electron acceptor leads to the creation of various organic end products, with the specific molecule formed depending on the organism and the enzymes present. The two most common pathways are lactic acid fermentation and alcoholic fermentation. Both of these processes achieve the same goal of \(\text{NAD}^+\) regeneration but yield different reduced products.
Lactic Acid Fermentation
In lactic acid fermentation, pyruvate is reduced directly by \(\text{NADH}\) to form lactate, the ionized form of lactic acid. This process occurs in human muscle cells during intense exercise when oxygen is scarce. It is also utilized by certain bacteria, like the Lactobacillus species, to produce cheese and yogurt.
Alcoholic Fermentation
Alcoholic fermentation involves a two-step process where pyruvate is converted into its final reduced product. First, the pyruvate is decarboxylated, meaning a carbon atom is released as carbon dioxide (\(\text{CO}_2\)), which creates an intermediate molecule called acetaldehyde. Acetaldehyde then acts as the final electron acceptor, being reduced by \(\text{NADH}\) to form ethanol. This pathway is utilized by yeast in the production of alcoholic beverages and is responsible for the rising of bread dough due to the \(\text{CO}_2\) gas produced.