Fermentation of glucose is a metabolic process that takes place in the absence of oxygen. There are two possible cellular procedures that result in glucose fermentation, one of which is called ethanolic fermentation, and the other of which is called lactic acid fermentation. Cells either use ethanolic or lactic acid fermentation--no organism or cell relies upon both procedures. Fermentation is much less energy efficient overall than the oxygen-dependent metabolic strategy for extracting energy from glucose, called respiration, and as such, it's used by small organisms far more frequently than by larger ones.
Glycolysis
Regardless of whether an organism uses ethanolic or lactic acid fermentation, the first cellular procedure required for fermentation is the same--cells must split glucose into two smaller molecules by a process called glycolysis. Explain Drs. Reginald Garrett and Charles Grisham in their book, "Biochemistry," glycolysis literally means "glucose splitting," and breaks the six-carbon glucose molecule into two three-carbon molecules called pyruvate. This process yields a very small amount of energy, and requires no oxygen. Glycolysis is the only part of fermentation that yields energy. The process of glycolysis is dependent upon a cellular reactant molecule called NAD+, which is required for the cell to be able to split glucose.
Ethanolic Fermentation
Some organisms, including certain bacterial species and yeast, then enter the procedure of ethanolic fermentation. While the fermentation doesn't yield energy, it's a necessary cellular procedure because it further processes pyruvate and, in the process, regenerates the cell's supply of NAD+, note Drs. Mary Campbell and Shawn Farrell in their book, "Biochemistry." In ethanolic fermentation, each pyruvate molecule is broken into a molecule of ethanol, or drinking alcohol, and a molecule of carbon dioxide. The cell then rids itself of both products, which are waste. Beer, which is a fermented glucose broth, contains both the ethanol and carbon dioxide waste products of ethanolic fermentation by yeast, giving it both its alcohol and carbonation.
Lactic Acid Fermentation
Other organisms, including many bacteria as well as animals and humans, use lactic acid fermentation to process pyruvate. Like ethanolic fermentation, lactic acid fermentation yields no additional energy from glucose. Instead, explain Drs. Campbell and Farrell, the only purpose of conversion of pyruvate into lactic acid is to regenerate NAD+. Like pyruvate, lactic acid has three carbon atoms. This means that the process of lactic acid fermentation doesn't give off carbon dioxide, in contrast to ethanolic fermentation. Lactic acid, like ethanol, is a cellular waste product, and organisms that produce it through the glucose fermentation procedures dispose of or recycle it.
References
- "Biochemistry"; Reginald Garrett, Ph.D. and Charles Grisham, Ph.D.; 2007
- "Biochemistry"; Mary Campbell, Ph.D. and Shawn Farrell, Ph.D.; 2005
