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This tutorial presents fermentation at a level appropriate for most undergraduate biology classes and the MCAT exam.

Click to view the video tutorial: Fermentation.

Fermentation allows glycolysis to continue in the absence of oxygen, yielding a small amount of ATP. During fermentation 2 electrons a proton are transferred from each NADH product of glycolysis to pyruvate or a derivative acceptor molecule to regenerate NAD+. NAD+ is used to continue glycolysis.

Byproducts of fermentation include lactate, butate and ethanol.


1. Fermentation is an energy yielding process because it allows glycolysis to continue in the absence of oxygen. Since a net amount of 2 ATP is produced by glycolysis, a net amount of 2 ATP is also produced by fermentation.

2. NAD+ is required to complete glycolysis. NAD+ must be present to accept hydrogen before any ATP can be produced.

3. During glycolysis two molecules of NAD+ are reduced to NADH.

4. During lactic acid fermentation pyruvate is reduced to lactic acid and NADH is oxidized to regenerate NAD+.

5. In lactic acid fermentation hydrogen atoms (along with two electrons) are added directly on to pyruvate converting it to lactic acid. Pyruvate is the terminal electron acceptor and lactic acid is the product.

adenosine triphosphate (ATP)
ethanol fermentation
lactic acid fermentation
nicotinamide adenine dinucleotide (NAD+ / NADH)
terminal electron acceptor

RELATED TOPICS (Note: These links will go active as videos appear online.)
oxidation-reduction reactions

1. AAMC. (2008). Topics for Biological Science Section of the MCAT. [PDF Brochure].
2. Robergs, R. A., Ghiasvand, F., & Parker, D. (2004). Biochemistry of exercise-induced metabolic acidosis. Am J Physiol Regul Integr Comp Physiol, 287, 502-516. doi:10.1152/ajpregu.00114.2004
3. Allen, D.G., Lamb, G.D., & Westerblad, H. (2008). Skeletal Muscle Fatigue: Cellular Mechanisms. Physiol Rev, 88, 287-332. doi:10.1152/physrev.00015.2007.
4. Lodish, H., Berk, A., Kaiser, C., Kreiger, M., Scott, M. P., Bretscher. A., & Ploegh, H. (2008). Molecular Cell Biology (6th ed.). New York: W. H. Freeman and Company.
5. Campell, N., Reece, J. B., Taylor, M., & Simon, E. (2008). Biology: Concepts and Connections (5th ed.). San Francisco: Benjamin Cummings.
6. Fox, S. I. (2002). Human Physiology. (7th ed.). New York: McGraw-Hill.