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? Well, basically because without NAD plus molecules, glycolysis will not take place. So things are pretty simple during the presence of oxygen, when we have enough oxygen present in the cell. But what happens if we do not have any oxygen present in the cell?