Fermentation is the process that releases energy without using oxygen. In cellular respiration, anaerobic fermentation contrasts with aerobic cell respiration. Both processes convert glucose from food into adenosine triphosphate for energy for cells.
Every cell needs energy to survive and function properly. In animals, this energy comes from glucose molecules formed from the food they eat. The glucose molecules are broken down by glycolysis. If there is oxygen present from the bloodstream, the broken-down glucose moves into the cell's mitochondria and is converted to energy in the form of ATP. If there is no oxygen present, the broken-down glucose stays in the cell's cytoplasm, and fermentation takes place, forming ATP. Fermentation in the cytoplasm produces less ATP than regular cellular respiration in the mitochondria.