Pyruvic acid is the final product of glycolysis, which is the first stage of cellular respiration. Why is pyruvic acid never the end product of fermentation.

Pyruvic acid enters fermentation to recycle NADH since NAD + is required to continue the process of glycolysis. During fermentation, pyruvate is reduced in single or multi step process and NADH is electron donor.

Explanation:

Fermentation occurs when oxygen is not available to oxidize NADH and FADH2. NADH oxidation is particularly required to continue the process of glycolysis since NAD + serves as electron acceptor during glycolytic reactions. Therefore, the end product of glycolysis (pyruvate) enters fermentation to recycle NADH.

During lactic acid fermentation, pyruvate is reduced in lactate and NADH serves as an electron donor. Thereby, the end product of lactate fermentation is lactate, not pyruvate. Similarly, pyruvate enters decarboxylation and forms acetaldehyde which in turn is reduced into ethanol and NADH serves as an electron donor. This makes ethanol the end product of ethanol fermentation.