(a) Most accepted mechanism of photosynthesis is supposed to occur in two steps. In the first step, $H_2O$ decomposes to give $H_2$ and $O_2$ in presence of chlorophyll and the $H_2$ thus produced reduces $CO_2$ to $C_6H_{12}O_6$ and $H_2O$ molecules are also produced as shown below : $12H_2O_(l)$ → $12H_{2(g)}+6O_{2(g)}$ ... (i) $6CO_{2(g)}+12H_{2(g)}$ → $C_6H_{12}O_{6(s)}+6H_2O_(l)$ ... (ii) –––––––––––––––––––––––– $6CO_{2(g)}+12H_{2(g)}$ → $C_6H_{12}O_{6(s)}+6H_2O_(l)+6O_{2(g)}$ Therefore, it is more appropriate to write the equation for photosynthesis as (iii) because it emphasises that 12 molecules of $H_2O$ are used per molecule of carbohydrate formed and 6 molecules of $H_2O$ are produced during the process. (b) The purpose of writing $O_2$ two times suggests that $O_2$ is being obtained from each of the two reactants. $O_{3(g)}$ → $O_{2(g)}+O_(g)$ $H_2O_{2(g)}+O_(g)$ → $H_2O_(l)+O_{2(g)}$ ––––––––––––––––––––––– $O_{3(g)} + H_2O_{2(l)}$ → $H_2O_(l)+O_{2(g)}+O_{2(g)}$ The path of reactions (a) and (b) can be determined by tracer technique method using labelled H or O, $H_2O^{18}$ or $D_2O$ in reaction (a) or by using $H_2O^{18}_2$ or $O^{18}_3$ in reaction (b).