Question

Which one of the following equations suggests that $O _{2}$ released during photosynthesis comes from water?

• A. $6 CO _{2}^{18}+12 H _{2} \rightarrow 6 O _{2}^{18}+ C _{6} H _{12} O _{6}+6 H _{2} O ^{18}$
• B. $6 CO _{2}+12 H _{2} O ^{18} \rightarrow 6 O _{2}+ C _{6} H _{12} O _{6}+6 H _{2} O ^{18}$
• C. $6 CO _{2}^{18}+12 H _{2} \rightarrow 6 O _{2}^{18}+ C _{6} H _{12} O _{6}+6 H _{2} O$
• D. $6 CO _{2}+12 H _{2} O ^{18} \rightarrow 6 O _{2}^{18}+ C _{6} H _{12} O _{6}+6 H _{2} O$

Answer 1

Answer: (D)

Ruben and Kamen (1941), while working on chlorella (unicellular green alga ) found that oxygen liberated during photosynthesis comes from water.
(i) When normal $H _{2} O$ radioactive $CO _{2}$ (i.e. $CO _{2}^{18}$ ) were used, normal $O _{2}$ is evolved.
$nCO _{2}^{18}+ nH _{2} O \xrightarrow[\text { Chlorophyll }]{\text { Light }}\left( CH _{2} O ^{18}\right)_{ n }+ nO _{2} \uparrow($ normal $)+ H _{2} O$
(ii) When normal $CO _{2}$ radioactive $H _{2} O \left(\right.$ i.e. $\left.H _{2} O ^{18}\right)$ were used, radioactive $O _{2}\left(\right.$ i.e. $\left.O _{2}^{18}\right)$ is evolved.
$nCO _{2}+ nH _{2} O ^{18} \rightarrow\left( CH _{2} O \right)_{ n }+ O _{2}^{18} \uparrow$ (radioactive oxygen) $+ H _{2} O$