THE OXYGEN CYCLE

Earth is the only place in the solar system where free oxygen exists in significant quantities. This free oxygen, which constitutes nearly a fifth of our atmosphere by volume, is a product of photosynthesis, carried out over more than 3 billion years of earth history. In the process of respiration, free oxygen reacts rapidly with reduced organic material; most processes of this sort are now catalyzed by organisms. In the absence of photosynthesis, respiration would consume all organic material containing oxygen in about 50 years, but a large pool of oxygen would still remain in the atmosphere.