Most organisms undergo cellular respiration to produce energy. However when there is an absence of oxygen, an organism will go through a process called fermentation.

Fermentation does not create glucose, however it does allow glycolysis to continue so that ATP is produced. There are two types of fermentation: Lactic Acid Fermentation and Alcohol Fermentation.

Lactic Acid Fermentation This process occurs in your muscle cells. The build up of lactic acid is what causes muscles to “burn” during exercise.

What happens?  The cells undergo glycolysis to break down glucose into 2 pyruvates. In this process, 2 ATP and 2 NADHs are formed.  Two NADH molecules provide energy to convert pyruvate into lactic acid. As the NADH is used, it is converted back into NAD+.  Two molecules of NAD+ are recycled back to glycolysis, so that NADH can be reformed.

Alcoholic Fermentation  Occurs in yeast and some plants.

What happens?  The cells undergo glycolysis to break down glucose into 2 pyruvates. In this process, 2 ATP and 2 NADHs are formed.  Two NADH molecules provide energy to break down pyruvate into an alcohol and carbon dioxide. As the NADH is used, it is converted back into NAD+.  Two molecules of NAD+ are recycled back to glycolysis, so that NADH can be reformed.