23. Energy Transformations: Respiration and Photosynthesis   Previous PageNext Page
       The Light Reactions: Trapping Solar Energy

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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