<P> Dioxygen is used in cellular respiration and many major classes of organic molecules in living organisms contain oxygen , such as proteins , nucleic acids , carbohydrates , and fats , as do the major constituent inorganic compounds of animal shells , teeth , and bone . Most of the mass of living organisms is oxygen as a component of water , the major constituent of lifeforms . Oxygen is continuously replenished in Earth 's atmosphere by photosynthesis , which uses the energy of sunlight to produce oxygen from water and carbon dioxide . Oxygen is too chemically reactive to remain a free element in air without being continuously replenished by the photosynthetic action of living organisms . Another form ( allotrope ) of oxygen , ozone ( O ) , strongly absorbs ultraviolet UVB radiation and the high - altitude ozone layer helps protect the biosphere from ultraviolet radiation . However , ozone present at the surface is a byproduct of smog and thus a pollutant . </P>
<P> Dioxygen is used in cellular respiration and many major classes of organic molecules in living organisms contain oxygen , such as proteins , nucleic acids , carbohydrates , and fats , as do the major constituent inorganic compounds of animal shells , teeth , and bone . Most of the mass of living organisms is oxygen as a component of water , the major constituent of lifeforms . Oxygen is continuously replenished in Earth 's atmosphere by photosynthesis , which uses the energy of sunlight to produce oxygen from water and carbon dioxide . Oxygen is too chemically reactive to remain a free element in air without being continuously replenished by the photosynthetic action of living organisms . Another form ( allotrope ) of oxygen , ozone ( O ) , strongly absorbs ultraviolet UVB radiation and the high - altitude ozone layer helps protect the biosphere from ultraviolet radiation . However , ozone present at the surface is a byproduct of smog and thus a pollutant . </P>
where does most of the oxygen come from on earth
where does most of the oxygen come from on earth
photosynthesis , which uses the energy of sunlight to produce oxygen from water and carbon dioxide
photosynthesis , which uses the energy of sunlight to produce oxygen from water and carbon dioxide