In aerobic respiration, the final electron acceptor is an oxygen molecule, O₂. If aerobic respiration occurs, then approximately 38 molecules of ATP will be produced using the energy of the high-energy electrons carried by NADH or FADH₂ to the electron transport chain. If aerobic respiration does not occur, NADH must be reoxidized to NAD⁺ for reuse as an electron carrier for glycolysis to continue.

How is this done?

• Some organisms use an organic molecule (such as methane) as the final electron acceptor instead of oxygen. Processes that use an organic molecule to regenerate NAD⁺ from NADH are collectively referred to as fermentation.
• In contrast, some living systems use an inorganic molecule (such as nitrate or sulfur) as a final electron acceptor to regenerate NAD^+.

Both of these methods are anaerobic (do not require oxygen) to achieve NAD⁺ regeneration and enable organisms to convert energy for their use in the absence of oxygen.

Lactic Acid Fermentation

The fermentation method used by animals and some bacteria like those in yogurt is lactic acid fermentation (Figure 1). This occurs routinely in mammalian red blood cells and in skeletal muscle that does not have enough oxygen to allow aerobic respiration to continue (such as in muscles after hard exercise). In muscles, lactic acid produced by fermentation must be removed by the blood circulation and brought to the liver for further metabolism. The chemical reaction of lactic acid fermentation is the following:

The build-up of lactic acid causes muscle stiffness and fatigue. Once the lactic acid has been removed from the muscle and is circulated to the liver, it can be converted back to pyruvic acid and further catabolized for energy.

Figure 1 Lactic acid fermentation is common in muscles that have become exhausted by use.

Alcohol Fermentation

Another familiar fermentation process is alcohol fermentation (Figure 2), which produces ethanol, an alcohol. The alcohol fermentation reaction is the following:

Figure 2 The reaction resulting in alcohol fermentation is shown.

The fermentation of pyruvic acid by yeast produces the ethanol found in alcoholic beverages (Figure 3). 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.

Figure 3 Fermentation of grape juice to make wine produces CO₂ as a byproduct. Fermentation tanks have valves so that pressure inside the tanks can be released.

References

Unless otherwise noted, images on this page are licensed under CC-BY 4.0 by OpenStax.

Text adapted from: OpenStax, Concepts of Biology. OpenStax CNX. May 18, 2016 http://cnx.org/contents/b3c1e1d2-839c-42b0-a314-e119a8aafbdd@9.10