Besides ATP and pyruvate, glycolysis produces the reducing agent NADH2. Unless this were reoxidized to NAD, the cell's supply of NAD would be depleted. Also, pyruvate is retained by the cell. A buildup of pyruvate would decrease the rate of the glycolysis by the law of mass action. Many cells use the pyruvate and NADH2 in respiration with oxygen (described in the next section) to produce more ATP. In the absence of oxygen, cells use a different pathway, in which the NADH2 is used to reduce the pyruvate to various products that can leak out of the cell as waste products. This process, which forms such partially oxidized by-products to regenerate NAD, is called fermentation. More generally, fermentation is an anaerobic biochemical process in which organic compounds serve as both electron donors and acceptors. Thus, some organics become reduced (e.g., NAD) and others are oxidized (e.g., ethanol to acetic acid), and no inorganic electron acceptor (such as oxygen) is needed.

In animals, the rapid generation of ATP needed for muscle power can only be generated by glycolysis. Animal cells eliminate pyruvate and regenerate NAD by a single-step reaction that forms lactic acid. However, lactic acid is a "blind alley'' in animals and cannot be used further. But unlike pyruvate, it can diffuse out of the cell, where the blood transports it to the liver. There it is converted back to pyruvate to enter other pathways.

Figure 5.5 Glycolysis, fermentation, and the Krebs cycle. Inputs and outputs from each cycle are shown, as well as connections with the metabolism of other biochemical compounds.

The accumulation of lactic acid in the muscles causes the pain that results from vigorous exercise. Rest allows time for the elimination and conversion of the lactic acid.

Bacteria and yeast can produce other products. The most important is ethanol, which is formed by yeast in two steps, with acetaldehyde as an intermediate. Specific organism and cultivation conditions yield specific end products. Yeast can produce glycerol in addition to ethanol, although it is derived from intermediates of glycolysis and not from pyruvate. Clostridium produces acetone, isopropanol, butyrate, and butanol. Proprionic acid bacteria produce proprionate. Coliforms produce formic acid, acetic acid, hydrogen, and CO2. Enterobacter produces ethanol, 2,3-butanediol, formic acid, and lactic acid.

Fermentation produces other compounds of commercial importance, some of which may originate with pyruvate, such as penicillin. However, although they are produced during fermentation, they are not end products used by the cell. Thus, these are properly referred to as secondary metabolites, not fermentation products.

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