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Why plants are green?

  1. Aug 21, 2004 #1
    from a physical argument if plantes were black they would absorbe more energy from the sun then..why clorophile is green anbd why the plants are green?..
  2. jcsd
  3. Aug 21, 2004 #2


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    a very beautiful question
    I think someone got Nobel for asking how photosynthesis works
    which is a kindred question

    apparently the molecule is able to absorb two wavelengths which are on either side of green, and this alternative two kinds of excitement allow it to participate in two separate chemical reactions, each requiring energy.

    it was years ago I read about it, but if I can remember there is one absorbed wavelength shorter (bluer) than green
    and one wavelength longer (redder) than green

    each of these he can absorb and be energized to do some chemistry
    but green he cannot absorb
    so it comes back to us

    there may also be another version of the chlorophyll molecule that can absorb different wavelengths and do other chemical reactions that also have the same overall effect. but this version we have in common plants is not set up to use green----so it comes back to us and we see leaves green

    maybe more expert people will answer. i would like to hear more about this myself.

    did you not look in a college biology textbook to see the two kinds of photosynthesis chemistry diagrammed? I think it involves the energy carrier molecule ADP/ATP. this stuff is great and we depend on it!
  4. Aug 21, 2004 #3
    Let me start with the easy answers....

    Chlorophyll is green because there's a magnesium in the porphyrin group. (Iron, for example, in a porphyrin group typically results in a reddish color as one finds in hemoglobin.) Plants are green because chloroplasts are green, which is where photosynthesis in plants occurs. Chloroplasts are green since they contain chlorophyll. Chloroplasts are actually the results of a symbiotic alliance from the depths of evolutionary time between a eukaryotic cell and cyanobacteria (aka blue-green algae).

    Actually, there's a simple explanation for why photosynthetic organisms don't absorb across the visible explanation. It's the idea of a "spectral niche" - the first photosynthetic organism(s) had a delightful old time when they finally could do photosynthesis, as there was no competition for being able to harvest light from the sun. When other players showed up on the scene, it made no sense to try to outcompete the originals at their own game, so those which could tweak their absorption to different wavelengths were the ones who stayed in the game. As such, evolutionary pressures prevented the situation presented in the original post from occurring, although it makes sense in terms of the physics.
  5. Aug 23, 2004 #4

    That is Amazing!!
  6. Aug 23, 2004 #5
    There are other considerations, of course.

    If one looks at the solar spectrum, one finds a higher flux at lower energies (longer wavelengths). The spectrum, as I dimly recall, reaches a peak somewhere in the middle of the visible spectrum and instead of decaying right down, it levels off as it reaches into the infrared. So there's plenty of longer wavelength light to muck around with instead of trying to catch the more energetic - but comparatively less abundant - shorter wavelength light.

    If one digs about in the photosynthesis literature even for a short time, you'll notice that the chlorophyll dimer - "special pair" in photosynthesis parlance - where photochemistry is initiated is often indicated by the following scheme: P(insert number here). That number is the wavelength of EM radiation that the pair absorbs to reach its excited state and start the electron transfer process. They vary from organism to organism, sometimes a lot, sometimes a little. In plants and cyanobacteria, the water-splitting complex Photosystem II (which also is the one responsible for oxygen evolution) absorbs right around at 680 nm, while its companion, Photosystem I, has its special pair absorb at 700 nm. In the purple bacterial photosynthetic reaction center that is typically talked about, the special pair absorbs at 870 nm. (See again "spectral niche.")

    Also, it's important to remember that the cleavage of water and transport of electrons across the membrane sets up a proton gradient which of course powers ATP synthase, without which we'd all be in a world of trouble (if we were here at all). Nature is awfully efficient, I always say. Absorb light, shuttle electrons around so they can be useful reductants in the cell, and set up a proton gradient to power ATP synthesis.

    I could say more, but am not sure if people want to hear my random blatherings.

    o O (Why, yes, Virginia, I did work in a photosynthesis lab for two years.)
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