In the equation of PHOTOSYNTHESIS, why 12 molecules of water as substrate are used?
The net reaction of photosynthesis is
6CO2 + 12H2O → C6H12O6 + 6H2O + 6O2
From this reaction, it is clear that to form one molecule of glucose, six molecules of carbon dioxide are required.
To answer why 12 molecules of water are required, here is the explanation.
Products of light reaction required in dark reactions to form one molecule of glucose are 12 molecules of NADPH and 18 molecules of ATP.
For forming 2 molecules of NADPH from NADP+ , 2 molecules of water are required so that demand of 4 electrons and protons can be satisfied.
Two molecules of water split to form1 molecule of oxygen and the respective number of electrons and hydrogen ions are also formed as shown in the reaction given below.
Now for producing 12 molecules of NADPH , total 12 molecules of water are required.