Q.Explain why photosynthesis is considered the most important process in the biosphere.

The question of where the oxygen released during photosynthesis actually comes from — water or carbon dioxide — was settled by a sequence of classic experiments. In 1931, Van Niel, studying photosynthetic bacteria that use H2S instead of water ($6CO_2 + 12H_2S \xrightarrow{\text{Light}} C_6H_{12}O_6 + 6H_2O + 12S\downarrow$), proposed that green-plant photosynthesis follows the same underlying pattern but with water substituting for H2S, releasing O2 in place of sulphur. In 1941, Ruben confirmed this directly in Chlorella using water labelled with the heavy oxygen isotope 18O; the oxygen gas evolved carried that same 18O label, proving that the O2 released in photosynthesis originates from water, giving the fully confirmed equation $6CO_2 + 12H_2^{18}O \xrightarrow{\text{Light}} C_6H_{12}O_6 + 6H_2O + 6\,^{18}O_2$. Separately, in 1937, R. Hill cultured isolated chloroplasts in a medium with water but with no dissolved CO2 at all, plus a ferric compound; on illumination, the ferric compound was reduced (changed colour) by hydrogen split from water — since no CO2 was present, this proved that O2 evolution does not require CO2 at all, and must come purely from the light-driven splitting (photolysis) of water. This light-driven water-splitting reaction, independent of CO2 reduction, is called the Hill reaction, and it establishes that (i) photosynthetic oxygen comes from water, and (ii) the electrons that eventually reduce CO2 also originate from water. Together, these experiments underpin the modern view of photosynthesis as a light-driven oxidation-reduction process — water is oxidised, releasing O …