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Chlorophyll and the Photoelectric Effect

  1. Jul 26, 2010 #1
    1. The problem statement, all variables and given/known data

    Chlorophyll and the Photoelectric Effect

    I have been trying to put together a lesson on the photoelectric effect applied to biology and chemistry concepts, and have been unable to find an explanation for some things. Here is some background:

    When chlorophyll is extracted from a green plant (say, with acetone) the resulting solution will fluoresce bright red when exposed to UV light. I gather that the magnesium atom in the center of chlorophyll's porphyrin ring captures the UV photon and excites an electron via the photoelectric effect, and in the presence of the cytochrome complex will donate the excited electron into the ETC. This explains why ordinary leaves do not fluoresce, but ground up leaves - in which some the chloroplasts have been ruptured - will fluoresce.

    My questions:

    #1) Obviously, red is a lower energy photon than UV, so where does that missing energy go? Is there first an emission of a slightly lower-energy UV, followed by a red? That's the only thing I can think of to explain the rather large amount of missing energy.

    Or maybe the energy of the red photons corresponds to the excess energy that the excited electron would have had if it had been transferred to photosystem II?

    #2) Why red? It stands to reason that specific red color corresponds to the active wavelength of P680, the pigment present in photosystem II, but I have no basis for why that is, or how it occurs. Which I suppose makes this a corollary to question #1, specifically HOW that particular color is emitted.

    I realize that (a) my knowledge is patchy at best and therefore likely to be wrong, and (b) a suitable answer would be very long. Any help and/or references would be appreciated. Thanks
  2. jcsd
  3. Jul 27, 2010 #2
  4. Jul 27, 2010 #3


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    Sorry, I do not know anything about the energy levels of Chlorophyll, so I write only general things.

    Chlorophyll, like all atoms and molecules have a lot of allowed energy levels, but transition between them can happen with higher and lower probability. A molecule can be exited to a high energy level with UV light, and then it tends back to reach its ground state. It is possible that the direct transition is less probable than going down in more smaller steps, and the wavelength of the photons emitted during these transitions are outside the visible range. Chlorophyll has got a very strong absorption band in the red, that is why we see the leaves green, complementer of red. Absorbing that red photon the molecule gets into at a special excited state. If it arrives in this state after the UV excitation, it will radiate a red photon when returning to the ground level , unless there is an other more favourable way to get rid of energy.


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  5. Jul 27, 2010 #4
    there is no photoelectric effect here.
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