Concept 5 Review

Fermentation Pathways

In the presence of oxygen, the electrons that are removed from glucose are eventually passed to oxygen by electron carriers; therefore, glucose is oxidized and oxygen is reduced to water. But, in the absence of oxygen, many cells generate ATP through glycolysis and fermentation. In fermentation the reduced NADH produced in glycolysis is converted back to oxidized NAD+.

There are two types of fermentation. Both types of fermentation regenerate oxidized NAD+, which is necessary for glycolysis and consequently the continued production of ATP by that pathway.

The net energy gain in fermentation is 2 ATP molecules/glucose molecule. In both lactic acid and alcoholic fermentation, all the NADH produced in glycolysis is consumed in fermentation, so there is no net NADH production, and no NADH to enter the ETC and form more ATP.