Cellular respiration -- the process by which cells break down molecules to gain energy -- occurs through three pathways: glycolysis, the citric acid cycle and the electron transport chain. The primary function of glycolysis is to break down glucose, or sugar, into two pyruvate molecules. Pyruvate is a ketone of great importance, as it is the base material for the next step in respiration, the citric acid cycle. The process does yield other products, however
Glycolysis takes place in the cell's cytoplasm. The process of oxidizing glucose into pyruvate requires fuel. The cell uses two units of adenosine triphosphate, or ATP, per glucose molecule.
The glycolysis of one glucose molecule has a net yield of two pyruvates, four ATPs and two electron transport molecules called NADH. But since two ATPs were expended in the process, glycolysis really only yields two ATPs total. Most of the cell's needed ATP is generated in the third step of cellular respiration -- the electron transport chain.