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Glycolysis is a fundamental metabolic pathway that converts glucose, a six-carbon sugar, into two molecules of pyruvate, a three-carbon compound. This process occurs in the cytoplasm of virtually all living cells, both prokaryotic and eukaryotic, and is the initial stage of cellular respiration. Glycolysis does not require oxygen, making it an anaerobic process.

The pathway involves a series of ten enzymatic reactions. These reactions can be broadly divided into two phases: the energy investment phase and the energy payoff phase. In the energy investment phase, two molecules of ATP are consumed to prime the glucose molecule, making it unstable and ready for cleavage. Following this, the six-carbon molecule is split into two three-carbon molecules. The energy payoff phase then involves the conversion of these three-carbon molecules into pyruvate, generating a net gain of two ATP molecules and two molecules of NADH. NADH is an electron carrier that will later be used in the electron transport chain to produce more ATP if oxygen is present.

The pyruvate produced at the end of glycolysis can enter different metabolic fates depending on the availability of oxygen. In aerobic conditions, pyruvate is further processed through the citric acid cycle and oxidative phosphorylation to yield a large amount of ATP. In anaerobic conditions, pyruvate is converted into lactate or ethanol, a process known as fermentation, which regenerates NAD+ allowing glycolysis to continue. Glycolysis serves as a primary source of ATP for many cells and provides crucial intermediates for other biosynthetic pathways.