Question

explain why 12 molecules of water used as a substrate in eqn of photosynthesis

Answer 1

6CO2+ 12H2O → C6H12O6+ 6H2O + 6O2What this equation on the top means is the fact that  the light reaction thats needed in dark reactions, which is created to make a molecule of glucose which is 12 molecules of NADPH and and 18 molecules of ATP. So in the end you add 2 water molecules and finally the finishing product is the demand of 4 electrons and protons which can finally be finished.

Answer 2

6CO2+ 12H2O → C6H12O6+ 6H2O + 6O2