The equation for photosynthesis is

$\ce{6CO2 + 6H2O + Energy -> C6H12O6 + 6O2}$

My confusion rests in the following: during the photolysis (water-splitting) step of the light reactions that occur in Photosystem II, water gets split into 2 H+ ions, 2 electrons (which enter the photosystem), and one Oxygen ion. But if Oxygen ($O_2$) is comprised of two oxygen ions covalently bonded to each other, doesn't this mean that

$\ce{6H2O -> 3O2}$ ?

Could someone please clarify this for me?

  • $\begingroup$ There are 12 oxygen atoms in CO2 and 6 in H2O giving you a total of 18 Oxygens on the reactants side of the equation. On the products side, you have 6 Oxygen atoms in glucose and 12 in gaseous oxygen for a total of 18. The reactants and products balance. You are forgetting about CO2. $\endgroup$
    – AMR
    Oct 29, 2015 at 16:59
  • $\begingroup$ These representative reactions are a bit misleading. CO₂ fixation is not directly coupled with production of oxygen. This may not be an answer to your question but my suggestion would be to just dump these oversimplified explanations in the trash can and have a look at the actual stoichiometries in the Hill reactions. $\endgroup$
    Oct 30, 2015 at 5:22

1 Answer 1


Equation you have mentioned is balanced chemical equation. In reality these are series of Redox reactions, major two as follows,

Oxidation of oxygen from water in presence of light (energy from photon),

$ 2H_2O \xrightarrow{Photons} O_2 + 4H^*$

Ions produced from above reaction reduces carbon dioxide ,

$4H^* + CO_2 \rightarrow (CH_2O) + H_2O$

So both equations combined, we can write general form as

$nCO_2 + nH_2O \xrightarrow{Photons} (CH_2O)n + nO_2$

In simple words to answer your doubt, in final equation 2 oxygen molecule coming from per 2 $H_2O$ molecules and 1 coming from per $CO_2$ molecule. Hence final balanced equation is

$6CO_2 + 6H_2O \xrightarrow{Photons} 6(CH_2O) + 6O_2$

  • $\begingroup$ Reactions 3 and 4 do not use photons. It is a crude representation of the Calvin-Benson cycle. $\endgroup$
    Oct 30, 2015 at 5:19
  • $\begingroup$ Reaction 3 is general form to represent complete reaction (1+2) $\endgroup$
    – Dexter
    Oct 30, 2015 at 5:29
  • $\begingroup$ But it is as misleading as the OP's reaction. It is an extreme oversimplification as you know very well. These reactions are not even directly coupled. I think you should address this point. $\endgroup$
    Oct 30, 2015 at 5:34
  • $\begingroup$ Are you sure we should add that detail here ? It might confuse OP more, considering OP is concern about where is extra oxygen coming from. If so, you can convert this to community wiki and edit answer. $\endgroup$
    – Dexter
    Oct 30, 2015 at 5:44
  • $\begingroup$ I think the "representative reaction" is the source of all confusion. People have to realize that it is incorrect. In reality these are not coupled reactions and therefore the stoichiometry cannot be exactly balanced. The approximate stoichiometries can be obtained from analysis of the metabolic flux. $\endgroup$
    Oct 30, 2015 at 5:47

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