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news:c8bj99$1ijd$1{at}darwin.ediacara.org...
> > From: "irr" 
> > this idea of a transition or "variant" from H2S
utilization to H2O
> > utilization by proxy is completely incorrect; biochemically, these
> > two mechanisms are totally unrelated.
>
> So photosynthesis using H2O evolved from some totally different
> metabolic pathway, *not* at all from earlier H2S photosynthesis? No
> wonder it took so very long! What an amazing chance event we have to
> thank for our own kind of life being possible at all (or as the
> religious folk would term it, what a m******)! How well is it currently
> know which specific old metabolic pathway got mutated to yield the
> H-from-H2O part of the photosynthetic process? I seem to recall reading
> some detailed description of various kinds of photosynthesis whereby
> one process uses light to activate a chemical that shoves either an
> electron or proton across a membrane creating an electrostatic
> potential, then somewhere else along that membrane that potential is
> used to drive the ADP + PO4+ -> ATP process, or maybe the corresponding
> NADPH process, I forget which. But another form of photosynthesis
> doesn't make any across-membrane-potential at all, rather directly uses
> solar energy within a single electron-transport chain to make ATP or
> NADPH. Do I remember at least that part correctly?

There certainly was some initial excitement in the discovery of organisms -- 
including some cyanobacteria -- that could use H2S to re-reduce the
photosynthetic reaction center (Schopf in particular harps on this point in
his OoL book).  But it turns out this reduction is via the quinone pool and
through several membrane bound and then soluble cytochromes, then back to
the reaction center.

There are a difficulties in using H2O as a reductant, that make
photosynthetic water oxidation one of the most remarkable reactions in all
of biochemistry.  To oxidize H2O through one electron steps requires an
enormous initial energy -- around 2.75V -- that is roughly 2.5x more energy
than can be generated by chlorophyll oxidation (IIRC, photooxidized
chlorophyll is the strongest oxidant known in biology).  On the other hand,
you can oxidize H2O to O2 with a paltry .82V, but the problem is this is a
four electron process.  So the really amazing thing about oxygenic
photosynthesis (or the so-called oxygen evolving complex) is this ability to
function as a capacitor, to turn four single-electron chlorophyll
photooxidations into a concerted four electron "dump" onto H2O.

There are some ideas on how two, two-electron transfer systems (manganese
catalase, for example) might have come together to form an evolutionary
precursor, but I'm not sure what the current state of things here is.  Maybe
someone else can provide more insight on this question?

Your memory of different types of photosynthesis is indeed correct.
Chlorophyll-type photosynthesis is able in all cases (that I can think of
off hand) to generate NAD(P)H and so can be used by organisms to fix carbon.
Protons generated during the process -- during water oxidation, for
example -- can also be used to make ATP via ATP synthase.
Bacteriorhodopsin-type photosynthesis is basically a light-driven proton
pump and so only does ATP synthesis (and, strictly speaking, is phototrophy
rather than photosynthesis, as carbon isn't being fixed).
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