Even if there is no oxygen present, glycolysis can continue to generate ATP. However, for glycolysis to continue to produce ATP, there must be NAD+ present, which is responsible for oxidizing glucose. This is achieved by recycling NADH back to NAD+. When NAD+ is reduced to NADH, the electrons from NADH are eventually transferred to a separate organic molecule, transforming NADH back to NAD+. This process of renewing the supply of NAD+ is called fermentation, which falls into two categories.