11th Class Biology Photosynthesis in Higher Plants Mechanism Of Photosynthesis

On the basis of discovery of Nicolas de Saussure that "The amount of \[{{O}_{2}}\] released from plants is equal to the amount of \[C{{O}_{2}}\] absorbed by plants", it was considered that \[{{O}_{2}}\] released in photosynthesis comes from \[C{{O}_{2}},\] but Ruben proved that this concept is wrong.

In 1930, C.B. Van Niel proved that, sulphur bacteria use \[{{H}_{2}}S\](in place of water) and \[C{{O}_{2}}\] to synthesize carbohydrates as follows:

\[6C{{O}_{2}}+12{{H}_{2}}S\xrightarrow{\,\,\,\,}{{C}_{6}}{{H}_{12}}{{O}_{6}}+6{{H}_{2}}O+12S\]

This led Van Niel to the postulation that in green plants, water \[({{H}_{2}}O)\] is utilized in place of \[{{H}_{2}}S\] and \[{{O}_{2}}\] is evolved in place of sulphur (S). He indicated that water is electron donar in photosynthesis.

\[6C{{O}_{2}}+12{{H}_{2}}O\xrightarrow{\,\,\,\,}{{C}_{6}}{{H}_{12}}{{O}_{6}}+6{{H}_{2}}O+6{{O}_{2}}\]

This was confirmed by Ruben and Kamen in 1941 using Chlorella a green alga.

They used isotopes of oxygen in water, i.e., \[{{H}_{2}}^{18}O\] instead of \[{{H}_{2}}O\] (normal) and noticed that liberated oxygen contains \[^{18}O\] of water and not of \[C{{O}_{2}}.\] The overall reaction can be given as under :

\[6C{{O}_{2}}+12{{H}_{2}}^{18}O\underset{\text{Chlorophyll}}{\mathop{\xrightarrow{\text{Light}}}}\,{{C}_{6}}{{H}_{12}}{{O}_{6}}+{{6}^{18}}{{O}_{2}}+6{{H}_{2}}O\]