The ion nicotinamide adenine dinucleotide, or NAD, is a crucial electron acceptor during the process of glycolysis. Glycolysis is the first step in breaking down glucose for use in cellular respiration, and it is the main process used in fermentation. In fermentation, NAD is the only electron acceptor, doing the same job that oxygen performs in respiration, albeit far less efficiently.
Beyond its function in glycolosis, nicotinamide adenine dinucleotide also serves a few other cellular functions, most notably as a substrate for enzymes modifying existing proteins. The molecule is composed of two nucleotide groups joined by their phosphate groups. The molecule left over after glycolysis, NADH, has an additional hydrogen and two additional electrons in its molecular structure relative to NAD.
This addition must be removed before NAD is ready to be used again, but the normal mechanisms for this type of reaction are found in the mitochondria. This is a problem, because glycolysis occurs in the cytoplasm, and NAD and NADH cannot enter the mitochondria. The body overcomes this difficulty by using glycerol phosphate, a molecule that can pass across the mitochondrial membranes. In fermentation, the hydrogen is instead removed when NADH helps convert pyruvic acid to lactic acid.