Well, remember that glycolysis, which is basically used to form ATP molecules and Pyruvate molecules from glucose, is not a perfect process. And what that means is it actually uses up important molecules known as mad plus molecules, nicotine amide, adenine, dinucleotides. And these molecules are needed by glycolysis to actually continue the process of glycolysis. But glycolysis doesn't regenerate these molecules once it takes place. And that's precisely why under aerobic conditions, when we have plenty of oxygen inside the cell, that Pyruvate enters the mitochondria and we form more ATP molecules. And we also regenerate those NAD plus molecules from the breakdown of Pyruvate under aerobic conditions.