Chlorophyll and Green Light

John Markwell markwell at UNLINFO.UNL.EDU
Sun Nov 3 10:48:53 EST 1996


>In article <199611011420.IAA21137 at mari.acc.stolaf.edu>,
>   ceumb at STOLAF.EDU wrote:
>>Dear Plant Edders,
>>
>>A friend asked me a question the other day about chlorophyll.  He was
>>wondering why blue and red but not green were absorbed.  After some
>>reading and chatting with colleagues I came to the short answer that
>>chlorphyll was so efficient with red and blue that there was no
>>evolutionary advantage in absorbing green and/or that changes needed
>>to absorb green would mean loss of efficiency absorbing blue or red.
>>
>
>This is not really accurate. Chlorophyll DOES absorb green light, but less
>efficiently than red and blue light. Shine white light through a single leaf
>and you will see some of the green light coming through. Shine light through a
>thickness of several leaves and virtually no green light will get through.
>
>In practice, a crop canopy, or several metres depth of algal culture will
>absorb a great deal of green light and use it for photosynthesis.
>

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.

Regarding why plants have not evolved an absorbing species capable of
absorbing the green light, I will provide a speculative explanation.  Algae
have been forced to evolve additional chlorophylls (e.g. Chl c),
carotenoids and bilin pigments to increase absorptive cross-section within
the visible spectrum, but they live in environments with less light than
terrestrial plants.  I feel that part of the answer lies in the relatively
high light intensities on the land surface.  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?


John Markwell                           markwell at unl.edu
Department of Biochemistry              Phone:  402-472-2924
Beadle Center                           FAX:    402-472-7842
Univeristy of Nebraska
Lincoln, NE  68588-0664





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