. 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 $P_{700}$ and PS-II absorb maximum $680 \,nm$ wavelength of light thus reaction centre is $P _{680} \cdot P _{680}$ (PS-II) extracts an electron from water, returning to its unexcited state because hydrolysis of $H _{2} O$ occurs in PS-II. So, $O _{2}$ evolved in PS-II come from water.