A complete Definition of Photosynthesis: An anabolic process by which chloroplast of green plants and other phototroph’s synthesize carbohydrates (glucose) and evolve molecular O2 as by-product, using CO2, H2O and sunlight.
Till 1930, it was believed that photosynthesis is just the reverse of respiration.
An experiment conducted by Ruben, Randall and kamen (1940) proved that O2 evolved during photosynthesis comes from water, not from CO2.
For the experiment, they used heavy radioisotopes of Oxygen (O18) in water (H2O18) and CO218
Experiment 1: When photosynthesis occurs in the presence of H2O18 and normal CO2, the heavy isotope was found in the evolved O2
Experiment 2: When photosynthesis occurs in the presence of normal H2O and CO218, the isotopic oxygen is not evolved.
The experiment for the first time proved that the oxygen evolved during photosynthesis comes from water.
Then the conventional equation was modified as 6 molecules of H2O is not sufficient for the evolution of 6 molecules of O2.
Now the correct balanced equation of photosynthesis is
Reference: Ruben, S., Randall, M., Kamen, M., & Hyde, J. L. (1941). Heavy oxygen (O18) as a tracer in the study of photosynthesis. Journal of the American Chemical Society, 63(3), 877-879.