Photosynthesis Chlorophyll wavelength absorption

[GRB 080319B]

Why do the chlorophyll in land plants absorb primarily in the blue and red wavelengths, http://en.wikipedia.org/wiki/File:Solar_Spectrum.png" ? Isn't more energy available in 500nm green light vs 700nm red light? E =\frac{hc}{\lambda}

 

[alxm]

I suppose the only answer possible is "Because the mechanism that was evolved requires it."

Total energy is less important here than whether a single photon has sufficient energy to cause the ionization/charge-separation which is the first step in photosynthesis.
And that single photon doesn't have enough energy in itself either; it takes four photons to cause a reaction, while the energy is 'stored' in the manganese ion cluster of Photosystem II.

It's all a very intricate system, and it's somewhat telling that nature has only evolved this once; it's essentially the same PS2 in all green things.

Perhaps, for instance, a lower-energy photon would not cause a large enough charge separation, and the electron would too easily return back to the chlorophyll.