And when the concentration of NAD plus basically runs out, the process of glycolysis will come to a stop and the cell will no longer be able to actually produce those ATP molecules. And the way that our cells fix the problem is by taking the Peruvate molecule and actually using that Peruvate molecule, metabolizing it to regenerate those NAD plus molecules so in order for Glycolysis to proceed over and over and over, the NAD plus must be regenerated. And so the cell regenerates the NAD plus by breaking down, metabolizing those pyruvate molecules produced at the end of Glycolysis. Now, there are two major pathways. We have an aerobic pathway and an anaerobic pathway. Now, in aerobic, that simply means we're going to use oxygen.