Oxygen is the second most abundant gas in Earth’s atmosphere and an essential element of most organic molecules. Though oxygen is passed between the the lithosphere, biosphere and atmosphere in a variety of ways, photosynthesizing vegetation is largely responsible for oxygen found in the atmosphere. The cycling of oxygen through the Earth system is also accomplished by weathering of carbonate rock. Some atmospheric oxygen is bound to water molecules from plant transpiration and evaporation. Oxygen is also bound to carbon dioxide and released into the atmosphere during animal respiration.