During fermentation,
yeast breaks down sugars to form pyruvate molecules via glycolysis.
The two pyruvate are then reduced to two molecules of ethanol and 2CO2. Without oxygen, the carbon molecules can not be further metabolized. Fermentation yields  much less ATP than aerobic respiration. 

Fermentation
This week, we'll explore anaerobic respiration by yeast = fermentation

Bio 111 fermentation lab