>>>In article <199611011420.IAA21137 at mari.acc.stolaf.edu>,
>>ceumb at STOLAF.EDU wrote:
..
Ceumb>A friend asked me a question the other day about chlorophyll. He was
Ceumb>wondering why blue and red but not green were absorbed. ...
>>Marder>This is not really accurate. Chlorophyll DOES absorb green light, but
Marder>less efficiently than red and blue light. Shine white light through a
Marder>single leaf and you will see some of the green light coming through.
Marder>Shine light through a thickness of several leaves and virtually no
Marder>green light will get through.
In article <v01530501aea26b5aec21@[129.93.135.96]>,
markwell at UNLINFO.UNL.EDU (John Markwell) wrote:
>I would argue that this is not quite accurate either. The effect from
>stacking several leaves is probably due more to the combination of
>reflection and scattering, than absorption by the chlorophyll. Chlorophyll
>DOES absorb a small amount of green light (~550 nm), but the optical
>properties of a leaf must also be considered.
I agree that optical properties are important - due to scattering, light
pathlengths in the leaf can be quite long which *increases* the chance that
green light is absorbed. Photosynthesis action spectra for leaves usually show
substantial activity in the green.
>>Regarding why plants have not evolved an absorbing species capable of
>absorbing the green light, I will provide a speculative explanation.
..
>Because Chloorphylls can
>absorb photons with two different energies (i.e. red and blue) in their
>first and second excited singlet states, they are already efficient
>light-harvesters with a high molar absorptivity. A second component of the
>answer is that as plants evolved their photosynthetic machinery, they began
>to utilize carotenoids to harvest light. The light energy captured by the
>carotenoids is transfered to the second excited singlet state of
>chlorophyll. Thus, it a mutation were to change the electronic structure
>of the chlorophyll and cause it to absorb different wavelengths of light,
>it would probably cause a loss of the light-harvesting function of the
>carotenoids. So perhaps the evolution of light-harvesting by carotenoids
>'locked' the chlorophylls into their present bioenergetic identities?
>>Anyone consider this reasonable?
>I agree that the interplay between chlorophyll and carotenoids may be crucial.
However, can I suggest looking at it the other way round? When chlorophyll
singlets aren't harnessed by photosynthesis, triplets can form, which is
extremely destructive. Part of carotenoid function is to safely quench the
triplets. However, we now have strong hints that carotenoids (mainly
xanthophylls) can also quench the chlorophyll singlets (non-photochemical
quenching - NPQ). Furthermore, the degree of this quenching is adjusted
according to the physiological state of the plant and light conditions.
I speculate that some of the chlorophyll antenna proteins were originally
"shade" proteins with carotenoid-based NPQ permanently switched on. These
evolved into antennae once plants learned to control the degree of NPQ.
This is of course HIGHLY speculative:)
Jonathan B. Marder , Department of Agricultural Botany
E-mail: MARDER at agri.huji.ac.il | The Hebrew University of Jerusalem
Phone: (08 or +9728) 9481918 | /\/ Faculty of Agriculture
Fax: (08 or +9728) 9467763 |/ \ P.O.Box 12, Rehovot 76100, ISRAEL
http://www.agri.huji.ac.il/~marder
