The oxygen is the product of photosynthesis. Plants and algae use the energy of sunlight to turn water and carbon dioxide into sugar and oxygen. The plants use the sugar as food and turn some into cellulose to make cell walls. Oxygen is released into the atmosphere as a byproduct.
The earths atmosphere is made up of 78% nitrogen and 21% oxygen
97%
no
Oxygen gas
Free oxygen
Plants
oxygen
Nitrogen and oxygen,
no
mostly nitrogen with 21% oxygen
photosynthsis
The earths atmosphere is made up of 78% nitrogen and 21% oxygen