Question

During photosynthesis, oxygen in glucose comes from

• A. oxygen in air
• B. water
• C. carbon dioxide
• D. both (b) and (c).

Answer 1

Answer: (C)

Ruben and Kamen $(1941)$ and Ruben et al $(1941)$ suspended Chlorella in water having nonradioactive 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 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.
$6C{O}_2+12H_2{}^{18}O\underset{\text{chlorophyll,enzymes}}{\overset{\text{ light }}{\to }}{C}_6{H}_{12}{O}_6+6H_{2}O+6{}^{18}{O}_2\uparrow$