When does lactic acid fermentation take place in human cells?

Lactic acid fermentation occurs in humans mostly during vigorous  activity or exercise.

Fermentation is the process of producing ATP in the absence of oxygen through glycolysis alone. Lactic acid fermentation converts  the 3-carbon pyruvate to the 3-carbon lactic acid and regenerates NAD+ in the process, allowing glycolysis  to continue to make ATP in low oxygen conditions.

Since there is a limited supply of of NAD+, available in  any given cell, this electron acceptor must be regenerated to allow ATP production to continue.

To achieve this, NADH donates its extra electrons to the pyruvate molecules, regenerating NAD+ .  When pyruvate accepts these electrons, it is reduced and becomes lactic acid.

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