Oxygen is one of the major components of the life or protoplasm. It occurs in combination in H2O, carbohydrates lipids etc. The major supply of O2 to life is from atmosphere. The atmosphere gets oxygen from two sources viz., (a) photo dissociation of water vapour in which hydrogen escapes into the outer space (b) photosynthesis it is active only after the origin of life on the earth. The cycling of O2 is very complex. It circulates as a constituent of CO2 and H2O. It also combines with many elements to form their oxides. In photosyn­thesis, water is split to force oxygen. This oxygen is reconstituted during respiration. Some part of the oxygen in higher atmosphere is converted into ozone.