Brian Davis wrote:
> On Mar 30, 8:24 pm, "Dan Goodman" <dsg...@[EMAIL PROTECTED]
> wrote:
>> The April issue of Scientific American has an article on
>> astronomical including what light would be used for
>> photosynthesis on planets of various star-types...
>
> Curious. What are they basing this on? Things like "take advantage of
> frequency where the tensity is the greatest" (green, for our Sun...
> oops, looks like evolution missed that)? It would seem so much depends
> on what first photoactive pigment gets fixed into the system that
> there's an awful lot of good old evolutionary luck in there. Not to
> mention there's going to be some frequencies that biology would have
> intense trouble tapping (it's tough to fix energy when the incoming
> energy is enough to break covalent bonds, for instance).
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.
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.
All photosynthesis needs a reducing agent. Purple photosynthetic
bacteria use hydrogen sulphide or hydrogen or various other chemicals.
However, the most commonly available reducing agent is water. But the
water -> oxygen reaction requires more energy than can be provided by
one (light wavelength) photon. The "clever trick" in photosynthesis
where the electron is "charged up" twice, using two photons, was
developed in a cyanobacteria, using green chlorophyll. I don't know if
there was any particular advantage, or if that part was luck.
Cyanobacteria, by having access to so much water, quickly began
outstripping other photosynthetic organisms, and pumping so much oxygen
into the atmosphere they killed off many of the earlier organisms.
The photosynthesis in green plants is performed by chloroplasts, which
are organelles descended from cyanobacteria. It seems the water ->
oxygen stunt was only pulled once.
So, yes, I had exactly the same thoughts Brian did, only with a bit more
technical background.
Aqua
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