The heart of the light-trapping apparatus is a collection of molecules
that have delocalized electrons: chlorophylls and b-carotene
(right) in green plants, and phycoerythrin and phycocyanin in red
and blue-green algae. The absorption spectra of these pigments are
shown above. The colors of each are understandable in terms of the
wavelengths of light that are not absorbed. Chlorophyll a
differs from b in having a –CH3 instead of a –CHO
at the upper right corner of the ring, as it is drawn on the right.
This diminishes the extent of delocalization by two atoms, increases
the energy-level separations, and shifts the main absorption from
the blue toward the violet (see spectra). The carotenes, phycoerythrin,
and phycocyanin are "antenna molecules" that trap light
at wavelengths at which the chlorophylls are inefficient, and pass
their electronic excitation on to chlorophyll.
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