Question

If green plant cells are incubated with $O^{18}$ - labeled water, which of the following will become radioactive when the cells are exposed to light?

• A. $O_2$
• B. $C{O}_2$
• C. $H_{2}O$
• D. Sugar

Answer 1

The correct option is A $O_2$

Ruben and Kamen (1941), while working on chlorella (unicellular green alga ) found that oxygen liberated during photosynthesis comes from water.

(i) When normal $H_{2}O$ and radioactive $C{O}_2$(i.e.$C{O}_2^{18}$) were used, normal $O_2$ is evolved.

$nC{O}_2^{18}+n{H}_{2}O\underset{Chlorophyll}{\overset{Light}{\to }}(C{H}_2{O}^{18}{)}_n+n{O}_2\uparrow (normal)+H_{2}O$

(ii) When normal $C{O}_2$ radioactive $H_{2}O$(i.e.$H_2{O}^{18}$) were used, radioactive $O_2$ (i.e.$O_2^{18}$) is evolved.

$nC{O}_2+n{H}_2{O}^{18}\to (C{H}_{2}O{)}_n+O_2^{18}\uparrow (radioactiveoxygen)+H_{2}O$

So the correct answer is '$O_2$'.