Fermentation is a metabolic process used by many organisms to generate energy from sugars when oxygen is not available, classifying it as an anaerobic pathway. This ancient process remains a foundational biological mechanism. Humans have harnessed this chemical exchange for millennia, using it in the production of foods like yogurt, cheese, and bread, and beverages such as beer and wine. The process relies on a coupled redox reaction, where one molecule loses electrons (oxidation) while another gains them (reduction). This exchange allows cells to sustain minimal energy production under low-oxygen conditions.
Glycolysis: Setting Up the Reaction
Fermentation is always preceded by glycolysis, the initial stage of sugar breakdown occurring in the cell’s cytoplasm. During glycolysis, a single six-carbon glucose molecule is broken down into two molecules of the three-carbon compound, pyruvate. This pathway generates a small net gain of two molecules of adenosine triphosphate (ATP), the cell’s primary energy currency.
A significant event during glycolysis is the reduction of the electron carrier NAD+ (Nicotinamide Adenine Dinucleotide) to NADH. This occurs when NAD+ accepts high-energy electrons and a hydrogen ion stripped from glucose intermediates. The cell’s supply of NAD+ is limited. Without a process to recycle it from NADH, glycolysis would quickly halt.
Under normal, aerobic conditions, NADH transfers its electrons to the electron transport chain, regenerating NAD+ and producing a large amount of ATP. When oxygen is absent, however, this electron chain cannot operate, leading to a buildup of NADH and a shortage of NAD+. Fermentation serves one primary purpose: to regenerate the NAD+ supply so that glycolysis can continue to provide its modest ATP yield.
NADH Oxidation and NAD+ Regeneration
The molecule oxidized during fermentation is NADH. Oxidation refers to the loss of electrons; NADH gives up the high-energy electrons collected during glycolysis. By losing these electrons and a hydrogen ion, NADH is converted back into its oxidized form, NAD+.
This regeneration step is the defining action of fermentation, as it frees up the NAD+ molecules needed to sustain glycolysis. The conversion of NADH to NAD+ allows the small energy production of glycolysis to continue. The NAD+ is then immediately available to accept more electrons from the next round of glucose breakdown, creating a continuous, low-yield energy loop.
NAD+ is the oxidized, electron-accepting form, while NADH is the reduced, electron-carrying form. The oxidation of NADH involves transferring its electrons to another molecule. This acceptor is typically pyruvate or its derivatives in the final stage of fermentation. This electron transfer balances the chemical equation, ensuring electrons removed earlier are deposited onto a final organic acceptor.
The Role of Pyruvate and Final Products
Since NADH is oxidized (loses electrons), a different molecule must be reduced (gains electrons) to complete the reaction. The primary molecule acting as this final electron acceptor is pyruvate, the three-carbon product of glycolysis, or a compound derived from it. This reduction of pyruvate produces the characteristic end products of fermentation.
Lactic Acid Fermentation
In lactic acid fermentation, which occurs in human muscle cells during intense exercise or in bacteria used to make yogurt, pyruvate directly accepts the electrons from NADH. The reduction of pyruvate converts it into lactic acid, simultaneously oxidizing NADH back to NAD+. The build-up of lactate can cause temporary muscle soreness and fatigue.
Alcoholic Fermentation
Alternatively, alcoholic fermentation, carried out by yeast and some bacteria, involves two steps. First, pyruvate is modified by removing carbon dioxide, forming the intermediate molecule acetaldehyde. Acetaldehyde then accepts the electrons from NADH, reducing it to the final product, ethanol (alcohol), and regenerating NAD+. This conversion is the basis for brewing and winemaking, and the released carbon dioxide causes bread dough to rise.