What happens to NADH produced during glycolysis when oxygen is scarce?

When oxygen is scarce, NADH produced during glycolysis is utilized in fermentation pathways to regenerate NAD+. In aerobic conditions, NADH can readily donate its electrons to the electron transport chain, which in turn allows for the regeneration of NAD+ and the continuation of glycolysis. However, when oxygen levels are low, the electron transport chain cannot function efficiently.

To prevent the depletion of NAD+—which is essential for glycolysis to continue—cells convert the NADH back to NAD+ through fermentation processes. In muscle cells, this often results in the production of lactic acid, while in yeast, ethanol and carbon dioxide are produced. This regeneration of NAD+ allows glycolysis to persist, providing the necessary ATP energy for the cell, even in anaerobic conditions.

This mechanism is vital as it allows cells to continue generating energy from glucose despite the lack of oxygen, ensuring cellular metabolism can sustain temporary anaerobic conditions.