Fermentation is the breakdown of pyruvic acid without the use ofoxygen. Some scientists refer to the combined processes of glycolysisand fermentation as anaerobic respiration. The metabolism of pyruvicacid during fermentation does not produce any ATP. The function offermentation is to break down pyruvic acid and regenerate NAD forreuse in glycosis, where ATP is formed. Fermentation occurs in theforms of lactic acid fermentation and alcoholic fermentation. Lactic acid fermentation occurs in animal cells and in someunicellular organisms when oxygen is in short of supply. As oxygenis consumed in aerobic respiration, oxygen becomes scarce. Whenthis happens, NAD + H donates its hydrogen atoms to pyruvic acid,and lactic acid soon accumulates in tissues. Lactic acid makes

muscles feel tired and sore. This makes people feel muscle fatigueafter good work outs or strenuous exercise. Lactic acid is thentransported to the liver where it is converted into glucose. Alcoholic fermentation is a process that occurs in some plant cells andsome unicellular organisms, such as yeasts. In alcoholic fermentationpyruvic acid is converted into ethyl alcohol. Just as lactic acid accumulatesduring lactic acid fermentation, ethyl alcohol accumulates during alcoholicfermentation. The alcohol in wine and beer is produced by fermentationcarried out by some microorganisms and by some plants such as wheat andbarley. These fermentation ways don t supply cells with any additional ATPmolecules. Most of the energy remains in the fermentation products suchas lactic acid or ethyl alcohol.