Shprt and actual defination of oxygen …

 The Oxygen-Cycle: 21% of Oxygen is found in the elemental form in the atmosphere. It also occurs extensively in the combined form in the Earth’s crust as well as also in the air in the form of carbon dioxide. In the crust, it is found as the oxides of most metals and silicon, and also as carbonate, sulphate, nitrate and other minerals. It is an essential component of most biological molecules like carbohydrates, proteins, nucleic acids and fats (or lipids). The oxygen-cycle is the cycle that maintains the levels of oxygen in the atmosphere. Oxygen from the atmosphere is used up in three processes, namely combustion, respiration and in the formation of oxides of nitrogen. Oxygen is returned to the atmosphere by photosynthesis.