Question

If a photosynthesising plant releases oxygen containing more than the normal amount of ${}^{18}$O, it is concluded that the plant has been supplied with

• A. $C_6{H}_{12}{O}_6$ containing ${}^{18}O$.
• B. $H_{2}O$ containing ${}^{18}O$.
• C. $C{O}_2$ containing ${}^{18}O$.
• D. Oxygen in the form of ozone.

Answer 1

The correct option is B $H_{2}O$ containing ${}^{18}O$.

Since, the process of photosynthesis makes use of CO${}_2$ and water to give out carbohydrates ( sugar) and O${}_2$, studies have been conducted to find that the oxygen evolved in the process is from CO${}_2$ or H${}_2$O. To work this out, the scientist used radioactive ${}^{18}O$ isotope of oxygen. Carbon dioxide containing ${}^{18}O$ oxygen is left to react with normal H${}_2$O. The result was that, the oxygen given out did not contain the radioactive isotope confirming that the oxygen evolved comes from water. Nevertheless, if the plant gives out more than the normal amount of oxygen, the plant then has been supplied with H${}_2$O containing radioactive isotopes of oxygen.

So, the correct answer is option B.