Q. Which equation is correct to prove that $O_{2}$ comes from water during photosynthesis?
A) $6CO_{2}^{\ \ 18} + 12H_{2}O \rightarrow 6O_{2}^{\ 18} + C_{6}H_{12}O_{6} + 6H_{2}O$
B) $6CO_{2} + 12H_{2}O^{\ 18} \rightarrow 6O_{2} + C_{6}H_{12}O_{6} + 6H_{2}O^{\ 18}$
C) $6CO_{2}^{\ \ 18} + 12H_{2}O \rightarrow 6{CO}_{2}^{\ 18} + C_{6}H_{12}O_{6}$
D) $6CO_{2} + 12H_{2}O^{18} \rightarrow 6O_{2}^{\ 18} + C_{6}H_{12}O_{6} + 6H_{2}O$
Explanation
Ruben and Kamen (1941) and Ruben et al (1941) suspended Chlorella in water having non-radioactive heavy isotope of oxygen $\ ^{18}O$, instead of natural oxygen ($\ ^{16}O$). The suspension was illuminated. Oxygen evolved was tested by means of mass spectrometer. It was found to be having isotope, $O^{18}$. This is possible only if, oxygen evolved during photosynthesis comes from splitting of water $6CO_{2} + 12H_{2}O^{18}\underset{Chlorophyll}{\overset{\ \ \ \ \ \ \ \ \ Light\ \ \ \ \ \ \ \ \ }{\rightarrow}}C_{6}H_{12}O_{6} + 6H_{2}O + 6\ O_{2}^{18}$
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.