descriptions of the importance of each reactant and product in photosynthesis

Photosynthesis: 12 H2O + 6 CO2 -> 6 O2 + C6H12O6 + 6 H2O 

12 water molecules and 6 carbon dioxide molecules are the reactants. 6 oxygen molecules, 1 glucose molecule, and 6 water molecules are the products. 

Light dependent reaction happens at the thylakoid membrane, made up of lipid bilayers, which is conveniently located in the chloroplast -> Light-independent reaction (the Calvin-Benson Cycle) happens in the stroma -> creation of glucose molecule. That's a much abridged explanation of photosynthesis.

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