chemical
Pyruvate still contains a great deal of chemical energy. In fact, two pyruvate molecules contain most of the chemical energy from the original glucose molecule. If oxygen is present, pyruvate enters the mitochondria for complete breakdown by the Krebs Cycle and electron transport chain. If oxygen is not present, cells must transform pyruvate to regenerate NAD + in order to continue making ATP. Keep in mind that glycolysis produces a net total of 2 ATP.