In glycolysis, glucose (a six carbon sugar) is split into two molecules of a three-carbon sugar. Glycolysis yields two molecules of ATP (free energy containing molecule), two molecules of pyruvic acid and two high energy electron carrying molecules of NADH. Glycolysis can occur with or without oxygen. In the presence of oxygen, glycolysis is the first stage of cellular respiration. Without oxygen, glycolysis allows cells to make small amounts of ATP. This process is called fermentation.