In the absence of oxygen, organisms continue to carry out glycolysis, since glycolysis does not use oxygen in its chemical process. But glycolysis does require NAD+. In aerobic respiration, the electron transport chain turns NADH back to NAD+ with the aid of oxygen, thereby averting any NAD+ shortage and allowing glycolysis to take place. In anaerobic respiration, cells must find another way to turn NADH back to NAD+.
This “other way” is called fermentation. Fermentation’s goal is not to produce additional energy, but merely to replenish NAD+ supplies so that glycolysis can continue churning out its slow but steady stream of ATP. Because pyruvates are not needed in anaerobic respiration, fermentation uses them to help regenerate NAD+. While employing the pyruvates in this way does allow glycolysis to continue, it also results in the loss of the considerable energy contained in the pyruvate sugars.
There are two principle forms of fermentation, lactic acid fermentation and alcoholic fermentation. For the SAT II Biology, remember that no matter what kind of fermentation occurs, anaerobic respiration only produces 2 net ATP in glycolysis.
Lactic Acid Fermentation
In lactic acid fermentation, pyruvate is converted to a three-carbon compound called lactic acid:
pyruvate + NADH
lactic acid + NAD+
In this reaction, the hydrogen from the NADH molecule is transferred to the pyruvate molecule.
Lactic acid fermentation is common in fungi and bacteria. Lactic acid fermentation also takes place in human muscle cells when strenuous exercise causes temporary oxygen shortages. Since lactic acid is a toxic substance, its buildup in the muscles produces fatigue and soreness.
Alcoholic Fermentation
Another route to NAD+ produces alcohol (ethanol) as a by-product:
pyruvate + NADH
ethyl alcohol + NAD+ + CO2
Alcoholic fermentation is the source of ethyl alcohol present in wines and liquors. It also accounts for the bubbles in bread. When yeast in bread dough runs out of oxygen, it goes through alcoholic fermentation, producing carbon dioxide. These carbon dioxide bubbles create spaces in the dough and cause it to rise.
Like lactic acid, the ethanol produced by alcoholic fermentation is toxic. When ethanol levels rise to about 12 percent, the yeast dies.
