Glycolysis occurs in the cytoplasm, begins cellular respiration, and breaks down glucose into two molecules of a three-carbon compound called pyruvate (aka pyruvic acid).

In steps 1–4, the energy investment phase, energy is consumed as two ATP molecules are used to energize a glucose molecule, which is then split into two small sugars that are now primed to release energy.

In steps 5–9, the energy payoff, two NADH molecules are produced for each initial glucose molecule and Four ATP molecules are generated (2 net).

ATP is formed in glycolysis by substrate-level phosphorylation during which an enzyme transfers a phosphate group from a substrate molecule to ADP and ATP is formed. The compounds that form between the initial reactant, glucose, and the final product, pyruvate, are called intermediates.