To capture the available light energy, photosynthetic organisms have evolved a set of pigments that efficiently absorb visible and near-infrared light. These pigments are sometimes referred to as chromophores - compounds that absorb light of specific wavelength. Structures of a few of the most important photosynthetic chromophores are shown in Figure 17.7.
Chlorophyll and some of the accessory pigments are contained in the thylakoid membranes of the chloroplast. The assemblies of light-harvesting pigments in the thylakoid membrane, together with their associated proteins, are organized into well-defined photosystems, structural units dedicated to the task of absorbing light photons and recovering some of their energy in a chemical form. The first part of this process takes place in what are referred to as light-harvesting complexes. Each is a multisubunit protein complex containing multiple antenna pigment molecules (chlorophylls and some accessory pigments) and a pair of chlorophyll molecules that act as the reaction center, trapping energy quanta excited by the absorption of light.
Most of the chlorophyll molecules are not directly engaged in the photochemical process itself but act, instead, as antenna molecules of the light-harvesting complexes. Antenna molecules absorb photons, and the energy is passed by resonance transfer to specific chlorophyll molecules in a relatively few reaction centers. In other words, the energy of a photon absorbed by any antenna molecule in a photosystem wanders about the system randomly (Figure 17.11). Eventually (meaning in about 10-10 s), the energy finds its way to a chlorophyll molecule in the reaction center. This molecule is like the other chlorophylls, but it is in a somewhat different environment, so that its excited state energy level is a bit lower. Thus, it acts as a trap for quanta of energy absorbed by any of the other pigment molecules. It is the excitation of this reaction center that begins the actual photochemistry of the light reactions, for it starts a series of electron transfers.
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