H2O comes out of PS?

Gerald F. Deitzer gd3 at umail.umd.edu
Thu Mar 16 12:28:59 EST 2000

Hi John, Scott et al.,

Sorry to be so late in adding my two cents worth to this discussion, but I
just want to clarify one thing to perhaps remove some of the

Neither equation is correct, but both are correct, depending on what you want
to explain.  The fact is that the 12 water molecules that are split in PSII to
produce 6 molecules of Oxygen and twelve molecules of NADPH2, all come from
the thylakoid (light) reactions and there is no generation of water in the
stromal (Calvin Cycle) reactions.  The only place water is generated is in the
mitochondria during respiration.  Water participates in many biochemical
reactions, but has nothing to do with the current equation.  In fact, it gets
worse if you try to account for all of the water in photosynthesis, because
the fixation of CO2 to RubP takes H2O as well as CO2.  However, it is only the
water molecules split in PSII that you should be concerned with.

Stop trying to balance an oversimplified equation as if the CO2 molecules all
polymerized with water to make a C6H12O6 molecule.  This  never happens!  The
closest you come to a hexose sugar is Fructose-6-Phosphate which is C6H11O5P
(close enough) or about 1/2 of a sucrose molecule (C12H22O11) that is made in
the cytoplasm.

I suggest that you leave the equation as:

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

or stop trying to fit everything into this equation and separate the equation

1) 12 H2O + 12 NADP + 18 ADP + 18 Pi ---> 6 O2 + 12 NADPH2 + 18 ATP

2.) 6 CO2 + 12 NADPH2 + 18 ATP -----> C6H12O6 + 12 NADP + 18 ADP + 18 Pi

Add these two together and you wind up with the first equation.  Of course you
will still have to explain how you arrived at these two equations and why you
don't have to worry about the other 6 H2O molecules.  Good luck.

Gerry Deitzer

John Hewitson wrote:

> Scott J. Brenkert wrote on 11/3/00 8:49 am
> >I think the formula is:  6CO2 + 6H2O --> C6H12O6 + 6O2
> I teach that the formula is actually 6CO2 + 12H2O --> C6H12O6 + 6H2O + 6O2
> Light is used to split H2O releasing O2 and producing ATP (from ADP + Pi)
> and reduced NADPH2 - this takes place on the thlakoids of the chloroplast.
> The NADPH2 is used in the Calvin cycle to reduce CO2 and one of the
> byproducts of one (?) of the stages of the Calvin cycle of reactions is
> H2O.  The H in this water will have come via the NADPH2 and the O in this
> water has come from CO2.  This takes place in the stroma of the
> chloroplast.
> I bet this is an oversimplification, but it satisfies me and most 17 year
> olds.
> Dr. John Hewitson (Oundle School)
> Bramston Garden Cottage, Oundle, Peterborough PE8 4BG UK
> Phone and Fax +44 (0) 1832 272209
> email john_hewitson at breathe.co.uk
> ---
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