Question

Choose the correct statement(s) for light reaction:
I. Oxygen released during photosynthesis comes from molecules of carbon dioxide.
II. NADPH is a product of light reaction.
III. Photo-oxidation of water forms H+, O2 and e- .
IV. One photosynthetic unit consists of a photochemical reaction centre and antennae complex

• A. I, III, IV
• B. II, III, IV
• C. Only II
• D. Only I and II

Answer 1

The correct option is B II, III, IV

Oxygen released during photosynthesis comes from splitting of water molecules. It was proved initially with the help of indirect approach and later, by a direct evidence.

Indirect approach:
Photosynthesis in anoxygenic bacteria was compared with the photosynthesis in oxygenic bacteria.
${\mathrm{nCO}}_2+{\mathrm{nH}}_{2}O\stackrel{\mathrm{Chlorophyll}\&\mathrm{light}}{\to }{(\mathrm{CH}}^{}$₂O)n + nO2(Oxygenic organisms)
${\mathrm{nCO}}_2+{\mathrm{nH}}_{2}S\stackrel{\mathrm{Chlorophyll}\&\mathrm{light}}{\to }{\left({\mathrm{CH}}_{2}O\right)}_n+\mathrm{nS}$(Anoxygenic photosynthesis)

Direct evidence:
Two isotopes of oxygen - $O^{16}\mathrm{and}{O}^{18}$ were used.
${{\mathrm{nCO}}_2}^{18}+{\mathrm{nH}}_2{O}^{16\stackrel{\mathrm{Chlorophyll}\&\mathrm{light}}{\to }}{\left({\mathrm{CH}}_2{O}^{18}\right)}_n+{{\mathrm{nO}}_2}^{16}$