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Chapter Four: Cell Nutrition and Respiration
Anaerobic processes can actually occur in both anaerobic and aerobic organisms. Anaerobic cells use these processes whether or not oxygen is present, whereas aerobic organisms will use them only if there is very little or no oxygen in the environment. The usefulness of anaerobic processes is that they permit the NADH molecules formed during glycolysis to become NAD+ molecules. This allows the process of glycolysis to occur again so that more energy can be produced.
The most well-known anaerobic pathway is known as fermentation, which means the production of alcohol from sugar (in this case, glucose). Yeast cells utilize fermentation; that's why they are used to convert the sugary juice of grapes into wine.
The first step of fermentation is the removal of CO2 (carbon dioxide) from the molecules of pyruvic acid formed through glycolysis. The resulting molecule has two carbon atoms and is called acetaldehyde. Next, the coenzyme NADH (which was formed during glycolysis) releases its hydrogen atom to acetaldehyde. The acetaldehyde becomes ethyl alcohol, also called ethanol, and the NADH becomes NAD+. The NAD+ is then used in glycolysis.
A second anaerobic process involves the conversion of the pyruvic acid molecules to lactic acid. This is accomplished simply by an NADH molecule releasing its hydrogen atom to the pyruvic acid molecule, which then becomes lactic acid. Just as in fermentation, the NADH coenzyme becomes NAD+, which is used in glycolysis to produce more energy.