Oxygen has an uniquely important place in the composition of both the animate and inanimate world. It is the only element that is present in high concentrations in the crust, atmosphere, hydrosphere and biosphere. The oxygen in the atmosphere is chemically reactive. The oxygen is mainly present as the free element, in the form of the dioxygen molecule, O2. The presence of dioxygen in the atmosphere means that the reactions occurring on the Earth's surface are mainly carried out under oxidizing conditions. Photosynthesis is arguably the most important step in the oxygen cycle. Water is the most common hydrogen donor, with the hydrogen being transferred to the carbon atoms obtained from the carbon dioxide, and the oxygen from the water being liberated as dioxygen. Ultraviolet radiation is also involved in the conversion of dioxygen into the triatomic molecule ozone, O3.