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Darren Goetze goetze at UNIXG.UBC.CA
Mon May 30 13:03:26 EST 1994

Well, actually yes i suppose it is a bit of a waste in some sense but on 
the other hand, plants have harnessed carotenoids as secondary light 
harvesters to broaden their absorption cross section in this region. 
carotenoids absorb most strongly in the region just red shifted from the 
blue absorption band of chl (not exactly green, more like yellow) but 
this energy can be transferred to chl's Qx transition via a dexter electron 
exchange mechanism. so at least some of this region can be used to effect 
photosynthesis, as many types of action spectra have shown.

as for why chl doesn't use this green region, the reason lies in the 
electronic configuration of the huge conjugated system about its Mg atom. 
this system can only be affected by photon waveforms (via foerster 
resonance) in two transitions Qx and Qy, corresponding to the blue and 
red absorption bands respectively. green light just doesn't fit the bill! 
however, its wavelength is just right to be reflected off the conjugated 
system, hence chl's green colour. note that different forms of chl (a, b, 
c, etc) both absorb and reflect slightly differnet colours of light.


On 28 May 1994, Carolyn S. Ogren wrote:

> The absorption spectrum for plant chlorophyll shows that there is
> little absorption of green light. I understand that is why plants
> are green. But why don't plants absorb, use, these wavelengths?
> Isn't this a "waste of a valuable resource?
> -- 
> q
> ?

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