Fermentation is an anaerobic (not using oxygen) alternative to cellular respiration.  In fermentation, glycolysis is the metabolic pathway that generates ATP. This produces only 2 ATP. However, there is more to the process. Fermentation provides an anaerobic step that recycles NADH back into NAD+.  Alcohol fermentation, a process used in baking and winemaking, is a type of fermentation often used by yeasts. Yeasts are single-celled fungi that normally use aerobic respiration to process their food. But they can survive in anaerobic conditions when glucose can continue the glycolysis process. In the alcohol fermentation process, pyruvate is converted to carbon dioxide and two ethanol. During this process, the oxidation of NADH recharges the cell with NAD+ that keeps glycolysis running. Ethanol, the two-carbon product, is toxic to the organisms that produce it. Yeasts release their alcohol wastes to the surroundings. Muscle cells and certain bacteria regenerate NAD+ through lactic acid fermentation. NADH is oxidized to NAD+ as pyruvate is reduced to lactate. There is a conversion of pyruvate to lactate with no carbon dioxide release. The dairy industry lactic acid fermentation by bacteria to produce yogurt and cheese, pictured here.


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