Respiration is the process by which chemical energy in organic molecules is released. This energy is made available to living cells in form of ATP. We can divide it in three pathway: [a] Aerobic: Here oxygen is used to completely oxidise the organic food to CO3 and H2O with high energy release. C6H12O6+6O26CO2+6H2O+686kcal [b] Anaerobic: In the cytoplasm, in absence of O3 the organic substrate is incompletely breaks down. [c] Fermentation: It is extracellular incomplete breakdown of glucose into ethyl alcohol or lactic acid.