A form of * anaerobic respiration in which glucose is broken down to form ethanol and carbon dioxide. It is carried out by yeasts and various other fungi and by certain bacteria. Fermentation takes place outside the organism and is catalysed by enzymes of the zymase complex. These are either secreted by living cells or released on cell death. Fermentation usually stops due to cell poisoning when the alcohol level reaches about 15%. The process is central to the brewing, wine-making, and baking industries. Since free oxygen is not available as a hydrogen acceptor acetaldehyde is used instead. Pyruvic acid, formed by glycolysis, is broken down to acetaldehyde and carbon dioxide. The acetaldehyde is then reduced by NADH2 to form ethanol and NAD. The process yields about 72 kcals from each glucose molecule. This is only about 10% of the energy that would be released by complete oxidation of glucose, as in * aerobic respiration. |
