Now, the problem with this is glycolysis uses up NAD plus molecules. But in order for glycolysis to continually take place, we have to somehow continually regenerate the NAD plus molecules from the NADH molecules. And the way that we regenerate these NAD plus molecules that are needed by glycolysis is by the process of fermentation. So once again, under anaerobic conditions, glycolysis still occurs, which means that we break down glucose into two pyruvate molecules and produce a net of two ATP molecules. In the process, we also deplete our pool, our concentration of NAD plus molecules. But we need these NAD plus molecules for glycolysis to continually take place.