Cellular respiration refers to a process by which cells convert food into energy. Fermentation is a specific chemical reaction within the respiration cycle. It takes place when the cells do not have access to oxygen, a condition also known as anaerobic respiration. The process of fermentation generates far less energy than aerobic, or oxygen-based, respiration.
Glycolysis is the first step of the respiration cycle. It occurs in both aerobic and anaerobic respiration. During glycolysis, glucose molecules are broken down into a substance called pyruvate. If oxygen is available, the pyruvate is further broken down and goes through many more chemical reactions to generate a large amount of energy. If oxygen is not available, the pyruvate goes through the process of fermentation, which generates a small amount of energy along with alcohol or lactic acid.
According to Chemistry for Biologists, fermentation generates only about 10 percent of the energy generated by aerobic respiration, enough to continue performing glycolysis. Oxygen-based organisms (such as human beings) can survive for a very limited amount of time on the minuscule amount of energy generated by fermentation.
Fermentation is far more common in plants than in animals. In plants, ethanol is a desirable byproduct of fermentation. Manufacturers of alcoholic beverages require ingredients to ferment to obtain the alcoholic content of the beverages. For example, beer is made from fermented grain.
When people exercise, they often use up oxygen faster than it can be restored. For this reason, muscle cells are capable of engaging in temporary anaerobic respiration. When adequate oxygen is not available due to exercising, muscle cells ferment, producing lactic acid. Lactic acid then builds in the muscles, causing cramps, soreness and fatigue.
Aerobic respiration powers the majority of organs and cells in human beings. The large amount of energy generated allows vital muscles such as the heart and involuntary breathing muscles to continue moving. Aerobic respiration is also required for brain functions as well as for the movement of voluntary muscles.
