why 12 molecules of water as substrate are used in the equation of photosynthesis???

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.

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Eqn of photosynthesis:

6CO2 + 12H2OSunlightC6H12O6 + 6H2O + 6O2

glucose(C6H12O6) is a of photosynthesis which contains 12 atoms of Hand 6 moleculesof O further, some molecules of water also is being lost ofcouse in the form of water vapours which is also considered as a product of photosynthesis so for the equation to b balanced 12 molecules of water is required .

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