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Q & A: light and photosynsthesis

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Most recent answer: 04/20/2011
Q:
How is it possible that light being a electromagnetic wave be able to be used in photosynthesis?
- Joshua Johnson (age 17)
Tahlequah Oklahoma
A:
You can start to think about this a little using classical physics. The chlorophyll molecules have electrons which can circulate around, making them a bit like loop antennas. The electromagnetic waves are picked up by these antennas.

To go any further, you need to consider the quantum nature of the molecules and the light. The possible states of the electrons in the chlorophyll have certain specific energies. (There's actually an effective spread of these energies for chlorophylls in the practical environment of a photosynthesizing unit, in which various thermal vibrations can contribute to the energy.)  The energy from the light is transferred in units of hf, where f is the light frequency and h is Planck's constant. The frequency of light has to approximately match the energy difference between the lowest energy states of the chlorophyll and the next batch of higher-energy states.  A photon (hf) of light gets absorbed, putting an electron in one of those higher-energy states. That's the quantum version of an antenna picking up some energy from the wave. It differs from a classical version in that the low-frequency light basically never gets picked up by this quantum antenna, unlike the behavior of a classical antenna.

Mike W.


(published on 04/20/2011)

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