The reactants of photosynthesis are the products of cellular respiration. The reactant of cellular respiration are the products of photosynthesis. Photosynthesis happens in chloroplasts and cellular respiration happens in the mitochondria. The equation for photosynthesis is 6CO2 + 6H2O + light energy = C6H12O6 + 6O2. The equation for cellular respiration is C6H12O6 + 6O2= 6CO2 + 6H2O + ATP.