Now, glycolysis breaks down our glucose into two pyruvate molecules as well as a net of two ATP molecules. And in the presence of oxygen, those two pyruvate molecules will then go into the mitochondrial matrix where they will undergo a series of processes that eventually produce NADH molecules. And those NADH molecules then go onto the electron transport chain, give off their electrons onto the enzymes of the electron transport chain, and the NADH molecules are oxidized back into the NAD plus molecules. And these NAD plus molecules can then be reused in the process of glycolysis, pyruvate decarboxylation and the citric acid cycle. So we see that under aerobic conditions, basically the electron transport chain is where we synthesize the majority of the ATP molecules and it's also the location where we oxidize the NADH back into the NAD plus. Now, the question is, why do we have to actually regenerate these NAD plus molecules?