H2O comes out of PS?

david walker david at alegba.demon.co.uk
Mon Apr 10 12:09:34 EST 2000

Re: H2O comes out of PS?-- A final word?
Personally I prefer a nominal [CH2O] to C6H12O6 if only because it
simplifies arithmetic. The principal end-product of chloroplast
photosynthesis is not, as we all know,  glucose (nor any free sugar) but
triose phosphate. If you add up all the reactions of the Benson-Calvin Cycle
(including the 5 molecules of H20 consumed per 3CO2 fixed) plus the
associated photochemical reactions and simplify by cancelling things that
appear on both sides of the equation you are left with

3C02 + 2H20 + H3PO4 = CH20H.CO.CH2.OPO(OH)2 + 302

This equation not only balances but doesn't create some of the problems
have been discussed. However, if you don't wish to get into phosphate and
all that stuff, you can still write

2H20 + 2NADP = 2NADPH2 + O2

for the sum of the photochemical reactions and

CO2 + 2NADPH2  = [CH2O] +  2NADP + H2O

for the sum of the Benson-Calvin Cycle reactions.

Add these together , cancel out the NADPs and NADPH2s,  and you are left

CO2 + 2H20 = [CH2O] + O2 + H20

as the overall equation for photosynthesis. Eureka!

Of course you can simplify this still further by cancelling out one molecule
of water on either side but then you are back, in all essentials, to one
sixth of the traditional

6CO2 + 6H20 = C6H1206 + 6O2

and your students might again be driven into thinking, wrongly but not
unreasonably, that some of the oxygen which is evolved must be directly
derived from CO2. 

David Walker



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