0.26 Bis2a 07.2 fermentation  (Page 2/5)

Alcohol fermentation

Another familiar fermentation process is alcohol fermentation ( [link] ), which produces ethanol, an alcohol. The alcohol fermentation reaction is the following:

In the first reaction, a carboxyl group is removed from pyruvic acid, releasing carbon dioxide as a gas. The loss of carbon dioxide reduces the molecule by one carbon atom, making acetaldehyde. The second reaction removes an electron from NADH, forming NAD ⁺ and producing ethanol from the acetaldehyde, which accepts the electron. The fermentation of pyruvic acid by yeast produces the ethanol found in alcoholic beverages ( [link] ). If the carbon dioxide produced by the reaction is not vented from the fermentation chamber, for example in beer and sparkling wines, it remains dissolved in the medium until the pressure is released. Ethanol above 12 percent is toxic to yeast, so natural levels of alcohol in wine occur at a maximum of 12 percent.

Other fermentation methods occur in bacteria. Many bacteria are facultatively aerobes. This means that they can switch between aerobic and anaerobic growth depending on the availability of oxygen. Certain bacteria, like Clostridia bacteria, are obligate anaerobes. Obligate anaerobes live and grow in the absence of molecular oxygen. Oxygen is a poison to these microorganisms and kills them upon exposure. It should be noted that many forms of fermentation, an exception is lactic acid fermentation, produce gas, usually CO ₂ and acids, such as lactate or acetate. The production of particular types of gas is used as an indicator of the fermentation of specific carbohydrates, which plays a role in the laboratory identification of the bacteria. The various methods of fermentation are used by different organisms to ensure an adequate supply of NAD ⁺ for the sixth step in glycolysis. Without these pathways, that step would not occur, and no ATP would be harvested from the breakdown of glucose.

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• Alcohol and lactic acid fermentations

Section summary

If NADH cannot be metabolized through aerobic respiration, another electron acceptor is used. Most organisms will use some form of fermentation to accomplish the regeneration of NAD ⁺ , ensuring the continuation of glycolysis. The regeneration of NAD ⁺ in fermentation is not accompanied by ATP production; therefore, the potential for NADH to produce ATP using an electron transport chain is not utilized.

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