Humans and other living organisms rely on glucose and other sources of nutrition to provide for cellular energy needs. Depending on the organism, some cells depend on oxygen to run reactions that extract energy from glucose, while other cells do not. Metabolic reactions that yield energy from glucose in the absence of oxygen are called fermentation reactions, and if these reactions produce lactic acid as a product, they fall into the category of lactic acid fermentation.
Significance
In humans and many organisms, cells have two metabolic strategies available to them with regard to extracting energy from glucose. The first strategy yields large amounts of energy, but depends on oxygen. The process of glycolysis splits sugar into two pyruvate molecules, explain Drs. Reginald Garrett and Charles Grisham. With available oxygen, the cells then go on to convert pyruvate into water and carbon dioxide. Alternately, if oxygen isn't abundant, cells can convert pyruvate into lactic acid--this branch of metabolism is called lactic acid fermentation.
Function
The purpose of lactic acid fermentation isn't to yield energy, since converting pyruvate to lactic acid doesn't generate any. Instead, lactic acid fermentation uses molecular components removed from pyruvate in the process of converting it to lactic acid to convert another molecule, called NADH, into NAD+. NAD+ is an important molecule, note Drs. Mary Campbell and Shawn Farrell, because without it, cells can't convert glucose to pyruvate, or pyruvate into carbon dioxide. Fermentation "recharges" the pool of reagents necessary to run the energy-yielding process of glycolysis.
Considerations
Compared to respiration, the process by which pyruvate is converted into carbon dioxide and water, fermentation is very inefficient. Overall, fermentation yields only about 1/15 of the energy that respiration does, meaning that it's not possible for large, multicellular organisms to use fermentation as their sole metabolic strategy. Humans, in fact, depend on respiration to fulfill the majority of their energy needs. Only for short periods of time, and in small areas of the body, do cells use fermentation instead, notes Dr. Lauralee Sherwood in her book, "Human Physiology."
Effects
The effects of lactic acid fermentation, aside from production of a very small amount of energy and regeneration of NAD+, include production of lactic acid, which irritates tissues. Humans use lactic acid fermentation in cells that are working very hard, under conditions in which demand outstrips oxygen supply. Muscles, for instance, often switch to fermentation during sprint or power efforts, notes Dr. Sherwood. The irritant lactic acid builds up, leading to the burning sensation common in hard-working muscles.
Expert Insight
Some bacteria metabolize glucose by lactic acid fermentation, note Drs. Garrett and Grisham. Lactobacillus, for instance, is a genus of bacteria that feeds on milk and produces lactic acid. The acid curdles the milk, leaving a sour flavor and creamy or slightly curdy texture. Humans enjoy the effects of bacterial lactic acid fermentation whenever they consume the curdled milk products yogurt and cheese, both of which are the result of Lactobacillus activity.
References
- "Biochemistry"; Reginald Garrett, Ph.D. and Charles Grisham, Ph.D.; 2007
- "Biochemistry"; Mary Campbell, Ph.D. and Shawn Farrell, Ph.D.; 2005
- "Human Physiology"; Lauralee Sherwood, Ph.D.; 2004
