On Thu, 6 May 2004 17:44:03 +0000 (UTC), 
RobertMaas{at}YahooGroups.Com  wrote:
>> From: jimmenegay{at}sbcglobal.net (Jim Menegay)

>> > In that case, it's obvious that all the oxygen in the resultant glucose
>> > came from the CO2, since there isn't any oxygen in H2S in the first
>> > place. See an analagous question later...

>> It might be true, but it is far from obvious.  The complete process of
>> photosynthesis is MUCH more complicated than is indicated by the
>> stoichiometry.  The part of the process that turns CO2 into glucose is
>> called the "dark reactions" since the coupling to the
light-absorbing
>> aspect of photosynthesis is very indirect.  The various steps of the
>> dark reactions involve oxygen from CO2, from water, and even from
>> phosphate.

>> The water molecules that provide the O2 at the opposite end of the
>> chain of reactions are, conceptually at least, completely different from
>> the ones involved in the dark reactions.  The hydrogen atoms are also
>> different.
> 
> I think I was making the, perhaps unwarranted, assumption that all the
> pathways in H2S and H2O photosynthesis are identical except for the
> point(s) where hydrogens are removed from that hydrogen donor, where
> the source molecule and the resultant atomic S or O are different. If
> that assumption is false, if there are other differences in the two
> mechanisms, very different pathways that somehow merge in overall
> effect, i.e. all the other changes in inputs and outputs cancel, to
> make the stoichiometry come out identical except for those two
> differences, then indeed it's two unrelated problems to study. But if
> the pathways are the same as I assumed, then mathematically we would
> have a contradiction to that pathways-same premise if my conclusion
> were false.
> 
> So do "we" (the experts in this field, the body of knowledge so-far)
> know for sure these two sets of pathways well enough to say whether
> they are identical except for hydrogen donor and reaction product, or
> whether they are different but with all the other terms cancelling?

There seems to be several confusing ideas here.

First, photosynthesis, strictly speaking, is the capture of light energy
coupled to the synthesis of ATP (or NADPH). The synthesis of molecules
such as glucose (or amino acids, or DNA, or lipids) is really an entirely 
separate process that uses the ATP (NADPH) generated by photosynthesis. 
This confusion arose because in most plants the primary use of ATP is to
make glucose and starch which is then transported to other cells.  It wasn't 
clear back in the 1950's that the actual photosynthesis reactions and
glucose synthesis were separate. Even today, most textbooks still refer 
to the Calvin cycle (CO2 fixation) as part of photosynthesis even though 
they should know better.

Second, there are several different mechanisms for coupling ATP synthesis
to the uptake of photons of light. In the most simple systems, an excited
electron is transferred from chlorophyll to a cytochrome bc complex where
it passes down a simple electron transport chain and returns to the 
chlorophyll molecule. As the excited electron loses energy it drives the
transport of hydrogen ions across a membrane. The membrane gradient that
is set up is used by ATP synthase to make ATP. In such a cyclic system
there is no requirement for any external source of electrons. The best
examples of such a simple photosystem are in purple bacteria and green
filamentous bacteria.

In some cases, excited electrons move from chlorophyll to NADPH. 


   NADP+  +  2 electrons  +  H+   --->  NADPH

This is a more efficient process since NADPH carries more energy than ATP 
and can be used in several biosynthesis pathways. However, this type of
photosynthesis is non-cyclic and some outside source of electrons has
to be found in order to regenerate the electron deficient chlorophyll
molecules. In cyanobacteria and plants the source of electrons is water
and the product of the reaction is oxygen. In green sulfur bacteria and
heliobacteria the electrons are extracted from various reduced sulfur 
compounds such as H2S and S2O3-- and others. 

Species that have evolved to use water as an electron source have a
more complex photosystem that is intimately associated with an enzyme 
called the oxygen evolving complex (OEC). Species that use other sources 
of electrons (e.g. H2S) have a very different set of enzymes that reduce 
cytochrome cofactors and these reduced cofactors carry electrons to the 
chlorophyll molecules.



Larry Moran
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