Glycolysis is a universal process that provides energy in the form of ATP molecules. It requires two molecules of NAD+, which are reduced to NADH during glycolysis. Thus, regeneration of NAD+ is necessary as if NAD+ is absent, glycolysis cannot be able to continue.
During anaerobic respiration (respiration in the absence of oxygen), fermentation takes place to regenerate NAD+ used in the process of glycolysis.
