Solution:
Ruben and Kamen (1941) and Ruben et al (1941) suspended Chlorella in water having nonradioactive heavy isotope of oxygen, \({ }^{18} \mathrm{O}\), instead of natural oxygen \({ }^{16} \mathrm{O}\). The suspension was
illuminated. Oxygen evolved was tested by means of mass spectrometer. It was found to be heavy isotope, \({ }^{18}\) O.This is possible only if oxygen evolved during photos ynth es is comes from
splitting of water. The oxygen in glucose comes from carbon dioxide.
\(6 \mathrm{CO}_{2}+12 \mathrm{H}_{2}{ }^{18} \mathrm{O} \underset{\text { chlorophyl, enzymes }}{\text { light }} \mathrm{C}_{6} \mathrm{H}_{12} \mathrm{O}_{6}+6 \mathrm{H}_{2} \mathrm{O}+6{ }^{18} \mathrm{O}_{2} \uparrow\)
