Anaerobic respiration occurs without oxygen and involves the consumption of another molecule (nitrate, sulfur, iron, carbon dioxide, etc.) instead of oxygen. Unlike in fermentation, anaerobic respiration involves the formation of an electrochemical gradient by an electron transport system that results in the production of a number of ATP molecules. Unlike in aerobic respiration, the final electron recipient is a molecule other than oxygen. Many anaerobic organisms are obligate anaerobes; they don’t perform oxidative phosphorylation and die in the presence of oxygen. Others are facultative anaerobes and can also perform aerobic respiration when oxygen is available.