Re: Colors of alien plants -- Scientific American

Aqua
> 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.

One photon would suffice.  Possibly the reason that the
electron is done twice is that the new mechanism was
cobbled on top of the old mechanism.  Evolution always
works by adapting old systems, even when such an
approach is extremely stupid. 

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We have the right to defend ourselves and our property, because 
of the kind of animals that we are. True law derives from this 
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     James A. Donald