On Mar 30, 9:51=A0pm, Aqua <a...@[EMAIL PROTECTED]
> wrote:
> My vague memories of undergraduate biochemistry was that the first
> photosynthetic pigments did indeed primarily absorb yellow and green
> wavelengths - perhaps the same as those still used by purple
> photosynthetic bacteria.
I *think* (somebody stomp on me if I'm wrong on this one) that the
only primary photosynthetic pigment is chlorophyll (and variations on
that ****phyrin ring), with the exception of a rhodopsin-based system
in some halobacteria (& this last, at least so far, lacks a carbon-
fixing system, so properly it's not photosynthesis, but photosynthesis
is really two (or more) separate processes anyway).
> Green chlorophyll was then a secondary pigment, to mop up the red and
> blue wavelengths the primary pigment wasn't getting - perhaps a niche
> organism.
So... evolution eliminated the pigments that could capture most of the
energy, while preserving the pigment that doesn't? That seems a
strange adaptation on the surface.
> All photosynthesis needs a reducing agent. =A0Purple photosynthetic
> bacteria use hydrogen sulphide or hydrogen or various other chemicals.
Actually, purple non-sulfur bacteria don't use an external reducing
agent in the capture of light energy. The only use a photosystem to
produce ATP, thus there's no need for a "source" for protons - you
just keep recycling them. It's only the carbon-fixing ****tions of
photosynthesis that require a source for a reducing agent... and if
you have enough ATP, that could be anything you like.
>=A0But the water -> oxygen reaction requires more energy than can
> be provided by one (light wavelength) photon.
Actually, as others have discussed far better than I, no. It's just
the PSI - PSII photosystem isn't very efficient at capturing a lot of
energy, so it ends up being a grafted together photosystem that seems
to have come from a number of different places.
>=A0It seems the water -> oxygen stunt was only pulled once.
Or from a different sort of view, it seems there are a few enzymes
that are difficult for evolution to hit upon more than once - cracking
water seems to be one of those, so every water-based photosystem ends
up with the same water-splitting enzyme. Some thing with fixing
nitrogen out of N2, and to some extent acquiring CO2 using something
as inefficient as RuBisCO.
--
Brian Davis
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