Fermentation

How do organisms generate energy when oxygen is not available?

Recall from earlier in this chapter that two benefits of glycolysis are that it can produce ATP quickly and that it does not require oxygen. However, when a cell generates large amounts of ATP from glycolysis, it runs into a problem. In just a few seconds, all of the cell's available NAD⁺ molecules are filled up with electrons. Without oxygen, the electron transport chain does not run, so there is nowhere for the NADH molecules to deposit their electrons. Thus, NADH does not get converted back to NAD⁺. Without NAD⁺, the cell cannot keep glycolysis going, and ATP production stops. That's where a process called fermentation comes in.

When oxygen is not present, glycolysis is followed by a pathway that makes it possible to continue to produce ATP without oxygen. The combined process of this pathway and glycolysis is called fermentation. In the absence of oxygen, fermentation releases energy from food molecules by producing ATP.

During fermentation, cells convert NADH to NAD⁺ by passing high-energy electrons back to pyruvic acid. This action converts NADH back into the electron carrier NAD⁺, allowing glycolysis to produce a steady supply of ATP. Fermentation is an anaerobic process that occurs in the cytoplasm of cells. Sometimes, glycolysis and fermentation are together referred to as anaerobic respiration. There are two slightly different forms of the process—alcoholic fermentation and lactic acid fermentation, as seen in Figure 9–8.

In Your Notebook Make a compare/contrast table in which you compare alcoholic fermentation to lactic acid fermentation.