Glycolysis can be divided into two phases—the energy requiring phase and the energy releasing phase. In the first phase, two ATP molecules are used to split the glucose into two molecules of glyceraldehyde-3-phosphate. In the second phase, the intermediate sugar is further catabolized to produce four ATP, two NADH, and two pyruvate molecules. Because 2 ATPs are consumed in the first phase, the net yield of glycolysis is two ATPs, two pyruvates, and two NADHs. But glycolysis extracts less than a quarter of the energy stored in the glucose molecule. So, in the presence of oxygen, pyruvate can enter mitochondria for further oxidation. Once inside, pyruvate is converted to acetyl-coenzyme-A, which can then enter the citric acid cycle producing more NADH molecules. All the NADHs then donate their electrons to the electron transport chain, producing ATP, and completing the oxidation of glucose.