H2O comes out of PS?

Bill Purves purves at THUBAN.AC.HMC.EDU
Thu Mar 16 15:08:23 EST 2000


At 05:28 PM 3/16/00 +0000, Gerry Deitzer wrote:

... a great discussion of the "hydrodynamics" of photosynthesis.
I'd like to add just another couple of thoughts to it.

First, why do we try to write that particular balanced equation?
Because it traces back to Cornelis Bernardus Van Niel and the
generalization he made from comparing plant and (certain) bacterial
photosynthesis:

  CO2 + 2 H2A ----> (CH2O) + 2A + H2O

His proposal in the 1930's, followed by the Ruben/Kamen isotope
experiment early in the 1940's, set us up...

We include those items in our teaching because they are elegant.
We get in trouble when we assume things should "just balance out."
As Gerry said, water is not generated in the Calvin cycle.
In fact, it is generated (net) in photophosphorylation!  The
formation of 18 molecules of ATP releases 18 molecules of water
(that's a condensation reaction, or, if you prefer, a dehydration
synthesis).  Of course, when the ATP is hydrolyzed in the Calvin
cycle some molecules of water are gobbled up.

Watch my hands:

Returning to Gerry:
 
>1) 12 H2O + 12 NADP + 18 ADP + 18 Pi ---> 6 O2 + 12 NADPH2 + 18 ATP

Note that, in order for that to balance, 18 H2O's should appear on
the right side of equation (1).

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

AND, in order for that one to balance, 12 H2O's should appear on
the left side of equation (2).  (Think about it. The number IS 12.
We're just adding up atoms.)

Now let's add them up, with those added waters.  Note that we
see 6 CO2 go in on the left and 6 O2 and C6H12O6 come out on the
right.  Watch the waters:

1) 12 go in and 18 come out

2) 12 go in and none come out

Summing the two equations gives 24 H2O going in and 18 coming out,
which accounts for all the biochemistry.  Now, if you are compelled
to regenerate the Van Niel equation, subtract 12 H2O from each side,
giving you 12 going in and 6 coming out.  Voila!  BUT that's sort
of a cop out, because there is no BIOCHEMICAL justification for
taking away the waters on the two sides.
 
(bill)


William K. Purves      Vice President/Editorial Director
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