Anaerobic Cellular Respiration: Lactate Fermentation and Ethanol Fermentation Sec. 7.4

Fermentation Fermentation is a metabolic pathway that includes glycolysis and one or two reactions in which NADH is oxidized to NAD+ by reducing pyruvate to other compounds. Fermentation is much less efficient at producing ATP than aerobic cellular respiration. Two common types of fermentation are lactate fermentation and ethanol fermentation.

Lactate Fermentation When some single-celled organisms and some animal cells are temporarily without oxygen they carry out lactate fermentation. They use NADH from glycolysis to convert pyruvate into a molecule called lactate (also called lactic acid). The resulting NAD+ is recycled back into glycolysis to continue the process.

Lactate Fermentation Lactate fermentation occurs in muscle cells when they are working strenuously. During such times, the demand for energy exceeds what can be produced aerobically, so glycolysis is increased to the point where it exceeds the oxygen supply.

Lactate Fermentation When there is not enough oxygen, pyruvate starts to accumulate, because it cannot be broken down fast enough in the Krebs cycle. To sustain glycolysis, the muscle removes some of the excess pyruvate by converting it into lactate.

Lactate Fermentation Lactate is temporarily stored in the muscle cells. If enough lactate builds up the muscle fatigues, and cramps. When there is enough oxygen present, the lactate is converted back to pyruvate, which may then be used in regular aerobic cellular respiration.

Ethanol Fermentation Some organisms are able to function aerobically as well as anaerobically. When these organisms function anaerobically, they carry out ethanol fermentation. In ethanol fermentation pyruvate is broken into a two carbon compound called acetaldehyde and CO 2 by an enzyme called pyruvate decarboxylase.

Ethanol Fermentation The acetaldehyde is then reduced by NADH to produce ethanol. Ethanol fermentation is performed by baker’s yeast as well as brewer’s yeast when making bread and alcohol.