Cellular respiration is the process by which organisms obtain energy from nutrients like glucose. There are two types of cellular respiration: aerobic respiration, which requires oxygen, and anaerobic respiration, which does not require oxygen. Anaerobic respiration occurs in two steps - glycolysis and fermentation. Glycolysis breaks down glucose into pyruvic acid, producing 2 ATP. Fermentation further breaks down pyruvic acid into either alcohol and CO2 or lactic acid, but produces no additional ATP. Aerobic respiration occurs in three steps - glycolysis, the Krebs cycle, and the electron transport chain - and yields much more ATP, around 36 ATP per glucose molecule. Aerobic respiration is thus much more efficient at producing