The human body is made up of several chemical elements; the element present in the highest proportion (65%) in the body is oxygen. Oxygen is a chemical element with symbol O and atomic number 8. It is a member of the chalcogen group on the periodic table and is a highly reactive nonmetallic element and oxidizing agent that readily forms compounds with most elements. In nature, free oxygen is produced by the light-driven splitting of water during oxygenic photosynthesis. According to some estimates, green algae and cyanobacteria in marine environments provide about 70% of the free oxygen produced on Earth and the rest is produced by terrestrial plants. Other estimates of the oceanic contribution to atmospheric oxygen are higher, while some estimates are lower, suggesting oceans produce ~45% of Earth’s atmospheric oxygen each year. Photosynthesis is the process by which green plants manufacture their food in the presence of sunlight and water.
A simplified overall formula for photosynthesis is:
6 CO2 + 6 H2O + photons ? C6H12O6 + 6 O2