Alcoholic fermentation and lactic acid fermentation.

Glycolysis is the first process in cellular respiration. If free oxygen is not available in its free state, glycolysis is followed by lactic acid fermentation or alcoholic fermentation. These processes are anaerobic – meaning they do not require free oxygen. These processes both produce a little ATP (energy) for the cell. Aerobic processes usually produce many more ATPs.

Alcoholic fermentation is the process by which certain cells convert pyruvate (a product of glycolysis) into ethyl alcohol and carbon dioxide. Yeast in bread carries out this process, and the CO2 causes the bread to rise.

During lactic acid fermentation, pyruvate is used to form lactic acid. This is the process that the dairy industry uses to produce yogurt and cheese. Lactic acid builds up in the muscles when the body doesn’t get enough oxygen, causing the burning sensation you get when you exercise.