Question

who proposed the hypothesis that all photosynthetic organisms require a source of hydregen and in plants this source wan water and the oxygen is evolved by splitting of water ? What observation led to propose this hypothesis?

Answer 1

Van Niel in 1931 proposed the hypothesis that all photosynthetic organisms require a source of hydrogen and in plants this source is water and the oxygen is evolved by the splitting of water. 

(i) Van Niel observed that green sulphur photosynthetic bacteria fixed carbon dioxide in presence of hydrogen sulphide. 

(ii) Oxygen was not evolved instead globules of sulphur was formed. 

(iii) From this observation he concluded that during bacterial photosynthesis CO₂ does not split, instead hydrogen sulphide splits and acts as a hydrogen donor to CO₂ to form carbohydrates and molecular sulphur is released as one of the products. 

6CO₂ + 12H₂S \(\overset{PEP\,case}{\rightarrow}\) C₆H₁₂O₆ + 6H₂O + 12S 

This observation on photosynthetic bacteria led Van Niel to propose above given hypothesis.