Glycolysis
Glycolysis is a central metabolic pathway that breaks glucose into pyruvate while capturing energy as ATP and NADH.
What glycolysis is
Glycolysis is the enzyme-driven pathway that splits one molecule of glucose into two molecules of pyruvate. It sits near the center of metabolism because it can supply quick ATP, feed carbon into other pathways, and operate before oxygen-dependent respiration begins.
The pathway in outline
The pathway has ten main enzyme-catalyzed steps. Early reactions rearrange and phosphorylate glucose, while later reactions harvest energy from the two three-carbon molecules that were produced by splitting the original sugar.
Energy investment phase
The first half of glycolysis spends ATP to activate glucose and trap it inside the cell as phosphorylated intermediates. This investment makes the molecule easier to split and prepares the carbon skeleton for energy extraction.
Energy payoff phase
The second half of glycolysis generates ATP by substrate-level phosphorylation and reduces NAD+ to NADH. Because each glucose becomes two three-carbon intermediates, the payoff reactions happen twice for every glucose molecule.
ATP and NADH
The net yield from glycolysis is usually two ATP and two NADH per glucose. The ATP can be used immediately, while NADH carries high-energy electrons that may enter respiration or be recycled during fermentation.
Pyruvate fates
Pyruvate is a metabolic crossroads. In oxygen-rich cells it can be converted to acetyl-CoA for the citric acid cycle, while cells relying on fermentation reduce pyruvate or related molecules to regenerate NAD+ and keep glycolysis running.
Regulation
Cells regulate glycolysis at irreversible steps, especially reactions catalyzed by hexokinase, phosphofructokinase, and pyruvate kinase. These checkpoints help match glucose breakdown to energy demand and available fuel.
Why it matters
Glycolysis appears across bacteria, archaea, and eukaryotes, making it one of biology's most widely shared energy pathways. Its products link sugar metabolism to biosynthesis, respiration, fermentation, and disease-relevant changes in cell metabolism.