Green plants and algae use two type of photo-system called PS-I and PS -II. In PS-I chlorophyll absorb maximum at 700 nm wavelength of light thus reaction centre is $\mathrm{P}_{700}$ and PS-II absorb maximum 680 nm wavelength of light thus reaction centre is $\mathrm{P}_{680} . \mathrm{P}_{680}$ (PS-II) extracts an electron from water, returning to its unexcited state because hydrolysis of $\mathrm{H}_2 \mathrm{O}$ occurs in PS-II. So, $\mathrm{O}_2$ evolved in PS-II come from water.