A Question

Joe Berry joeberry at biosphere.Stanford.EDU
Sun Sep 28 22:58:57 EST 1997

Dear Photosynthesis Researchers,

I received an interesting question that might be a useful topic for
discussion on the photosynthesis net.  The question comes from an
astronomer via Maxine Singer, President of the Carnegie Institution.

     Allan Sandage at the Observatories sent me the following
question.      Can you help with an answer?  He doesn't 'do' email, so
email me     the answer and I will send it on to him.

     "Why are plants green??  (I suppose this means and not yellow or
    blue or red)  What evolutionary advantage does green have re
     THanks, Maxine

I would appreciate hearing the thoughts of other photosynthesis
researchers.  I have included my answer and a response from Winslow
Briggs below.
Joe Berry
Carnegie Institution of Washington
Stanford, CA 94305
joeberry at biosphere.stanford.edu

An Answer:
Here is one way to look at it: Chlorophyll's absorption is at
wavelenths <700 and >400 nm. This "window" was probably prescribed by
the chemistry of the primordial oceans. These are thought to have
contained high concentrations of Fe+2 ion (which absorbs strongly at
wavelengths >700 nm) and dissolved organic compounds (which absorb in
the blue and near UV).  Thus, chlorophyll is a pigment that "fits"
into a window of available light energy. In this sense, it is ideally
suited for photosynthesis. On the other hand, chlorophyll is green
because it dosen't completely fill the window.  This is not an
advantage, and plants have evolved a number of accessory pigments to
fill the hole in the chorophyll absorption spectrum.  These pigments
donate absorbed photon energy to chorophyll.
Subject: Re: (Fwd) Question
Author:  "Winslow Briggs" <BRIGGS at andrew.stanford.edu> at Internet
Date:    9/25/97 11:26 AM

Let me add to Joe's comment:

There aren't any conjugated double bond pigments that I know that have
extremely broad absorption bands. Below 400nm, the increasing energy
of the photons raise the spectre of photochemical damage. Beyond 700
nm, the energy levels are sufficiently low that except in exceptional
cases they are insufficient for effectively driving photochemistry. A
compromise: an absorption band safely above the UV, and one
sufficiently down in the red that useful photochemistry is still
possible. My guess is that a single band in either wavelength region
would probably be selected against. The situation in higher plants is
not perfect, as Joe points out, and accessory pigments are made in
some algae to fill in the gaps. Even higher plants use carotenoids,
absorbing in the blue, to enhance energy capture, but these still do
not extend too far into the green window left by chlorophyll.

It seems to me that given the properties of conjugated double bond
systems in absorbing light energy, making a molecule with two major
bands within the biologically constrained wavelength range is not all
that simple, and chlorophyll is an ideal solution.

(Note the waving of hands!).

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