Cellular respiration happens in two ways. Aerobic respiration is oxygen-based cellular respiration. Anaerobic respiration is cellular respiration that occurs without oxygen. The process of anaerobic respiration begins with breaking down glucose (sugar molecules) that produces pyruvic acid. The pyruvic acid then undergoes a fermentation process to produce ATP, which cells employ for energy production. Human beings are capable of performing both aerobic and anaerobic cellular respiration.
Anaerobic respiration is believed to be the oldest form of cellular respiration. It is still employed by many single celled organisms, particularly in anoxic (lacking oxygen) environments such as some riverbeds.
The anaerobic respiration of some plants and yeasts produces ethanol, which is the basis of consumable alcohol production.
Anaerobic respiration is a less efficient way of cellular energy production, producing only two molecules of ATP to the 40 produced in aerobic respiration, but it is a faster method.
The anaerobic respiration of yeast and the carbon dioxide causes bread to rise.
During intense physical activity, muscles use anaerobic respiration, which produces lactic acid. Lactic acid buildup is the reason muscles become weak during exertion and sore afterwards.