What is the end product of glycolysis when oxygen is absent in cells?

During glycolysis, glucose is broken down into pyruvate, generating a small amount of ATP and reducing equivalents in the form of NADH. When oxygen is absent (anaerobic conditions), pyruvate cannot enter the mitochondrial matrix for aerobic respiration. Instead, it undergoes fermentation.

In mammalian cells (such as muscle cells), the process of fermentation converts pyruvate into lactate (lactic acid). This conversion is vital because it regenerates NAD+, allowing glycolysis to continue despite the lack of oxygen. It serves as a means of producing energy without relying on aerobic respiration, which is crucial for cells that may experience transient low oxygen levels.

In specific organisms, different pathways can occur in the absence of oxygen. For example, yeast cells can convert pyruvate into ethanol (ethyl alcohol) through alcoholic fermentation, but that is not the pathway in most mammalian systems. Therefore, when glucose is metabolized under anaerobic conditions in human cells, the primary end product is lactate.