Lactic acid fermentation takes place under anaerobic conditions. During lactic acid fermentation in cells, the pyruvate produced during glycolysis is converted to lactic acid by oxidizing an electron carrier.
During cellular respiration, glycolysis reactions break down glucose, a molecule with six carbons, into two molecules of pyruvate, a molecule with three carbons. Glycolysis also reduces the electron carrier NAD+ (nicotinamide adenine dinucleotide) to NADH. Under normal circumstances, the pyruvate molecules are sent through the citric acid cycle and the NADH molecules are sent through the electron chain transport to be oxidized back to NAD+, which is then recycled and used in a new glycolysis reaction. The objective of the entire process of cellular respiration is to produce molecules of ATP (adenosine triphosphate), which is the energy currency of the cell.
However, when there is insufficient oxygen present in the cell, the pyruvate cannot enter the citric acid cycle, and the NADH cannot be sent to the electron transport chain. Usually such conditions occur during periods of intense physical activity, when the oxygen supply to muscle tissue does not meet the actual requirements of the tissue. In such cases, the pyruvate is fermented to lactic acid by an enzyme called lactate dehydrogenase. The reduction reaction that converts pyruvate to lactic acid in turn oxidizes the NADH to NAD+, which re-enters the glycolysis cycle. This process, which takes place under anaerobic conditions, is called lactic acid fermentation.