Organisms require a constant supply of energy, primarily obtained through the breakdown of organic molecules. When oxygen is plentiful, cells employ aerobic cellular respiration to generate adenosine triphosphate (ATP). When oxygen becomes scarce, cells switch to alternative, less efficient metabolic pathways. Fermentation is an anaerobic process that allows for the continued, limited generation of ATP. Lactic acid fermentation is a specific metabolic route used by various organisms and cell types for energy conversion under low-oxygen conditions.
Glycolysis: The Precursor Pathway
Before lactic acid fermentation, the initial sugar molecule, typically glucose, must be processed through glycolysis. This foundational sequence takes place in the cell’s cytoplasm and represents the first stage of both aerobic respiration and fermentation. During glycolysis, a single six-carbon glucose molecule is broken down into two molecules of the three-carbon compound called pyruvate. This breakdown requires the input of two ATP molecules to initiate the reaction.
The process yields a net gain of two ATP molecules, providing immediate energy for the cell. It also involves the reduction of two molecules of the coenzyme nicotinamide adenine dinucleotide (NAD+) to form two molecules of NADH. This NADH carries high-energy electrons. Glycolysis provides the cell with a small amount of energy and the primary carbon skeleton molecule—pyruvate—which directly enters the next stage of anaerobic metabolism. Pyruvate thus serves as the direct predecessor to the reactant molecules.
Identifying the Immediate Reactants
The final stage of lactic acid fermentation relies on the interaction of two specific molecules generated during glycolysis. The first reactant is the three-carbon molecule pyruvate. Pyruvate acts as the acceptor molecule for high-energy electrons, serving as the substrate transformed into the final product.
The second required molecule is NADH, the reduced form of the coenzyme NAD+. NADH carries the electrons and a proton captured during glycolysis. This NADH must be converted back to NAD+ to keep glycolysis operational. Therefore, the immediate reactants for the final fermentation reaction are pyruvate and the electron carrier, NADH.
The Lactic Acid Conversion Step
The conversion of the reactants into the final products is catalyzed by the enzyme lactate dehydrogenase (LDH). This enzyme facilitates the direct transfer of a hydrogen atom and electrons from NADH to pyruvate. The chemical transformation results in pyruvate being reduced, meaning it gains electrons and a proton, yielding the product lactate, which is the ionized form of lactic acid.
This reaction is chemically represented by the conversion of pyruvate and NADH into lactate and NAD+. The regeneration of NAD+ is the biochemical point of this anaerobic step. The newly formed NAD+ cycles back to the glycolysis pathway to accept more electrons. This ensures that glycolysis continues to break down glucose and generate ATP when oxygen is absent.
Biological Context and Outcomes
Lactic acid fermentation plays a distinct role in both microbial life and complex animal physiology. In the human body, this process is associated with muscle cells during intense exercise. When oxygen demand exceeds the supply, muscle cells switch to anaerobic metabolism to sustain rapid ATP production. The buildup of lactate was historically believed to be the cause of muscle fatigue and the burning sensation during high-intensity activity.
Beyond human physiology, lactic acid fermentation is a defining metabolic characteristic of certain bacteria, notably those within the genus Lactobacillus. These microorganisms are obligate or facultative anaerobes that rely on this pathway for their energy needs. The lactate they produce is harnessed extensively in the food industry to create a variety of common products.
For instance, the fermentation of lactose in milk by these bacteria creates the characteristic texture and tangy flavor of yogurt and certain cheeses. The same metabolic action converts sugars in cabbage and cucumbers into lactic acid, creating preserved foods like sauerkraut and pickles. This metabolic route serves a dual purpose: providing emergency energy in animal cells and acting as a foundational process in microbial food preservation.