Antenna pigments, chlorophylls

The interiors of rhodopseudomonad bacteria are filled with photosynthetic vesicles, which are hollow, membrane-enveloped spheres. The photosynthetic reaction centers are embedded in the membrane of these vesicles. One end of the protein complex faces the Inside of the vesicle, which is known as the periplasmic side the other end faces the cytoplasm of the cell. Around each reaction center there are about 100 small membrane proteins, the antenna pigment protein molecules, which will be described later in this chapter. Each of these contains several bound chlorophyll molecules that catch photons over a wide area and funnel them to the reaction center. By this arrangement the reaction center can utilize about 300 times more photons than those that directly strike the special pair of chlorophyll molecules at the heart of the reaction center. 

Such a process can naturally be expected to play a certain part in the mechanism of directed energy transport in biological systems, in particular, in the transfer of absorbed energy from the antenna chlorophyll molecules to the reactive center in the photosynthetic system of plants. In Ref. [30], energy exchange between molecules of the photosynthetic pigments chlorophyll a and pheophytin a was studied experimentally with pigments introduced into the polar matrix. 

Figure 10.16 Solar energy transfer from accessory pigments to the reaction centre, (a) The photon absorption by a component of the antenna complex transfers to a reaction centre chlorophyll, or, less frequently, is reemitted as fluorescence, (b) The electron ends up on the reaction centre chlorophyll because its lowest excited state has a lower energy than that of the other antenna pigment molecules. (From Voet and Voet, 2004. Reproduced with permission from John Wiley Sons., Inc.)...

Light excites an antenna molecule (chlorophyll or iccessory pigment), raising an electron to a higher energy level. 

Other pigments, called antenna pigments, or accessory pigments, absorb light at other wavelengths. The accessory pigments are responsible for the brilliant colors of plants in the autumn (in the Northern Hemisphere). The breakdown of chlorophyll allows us to see the colors of the accessory pigments. 

The three primary photosynthetic pigments are (1) the chlorophylls, which absorb blue-violet and red wavelengths of light (2) the carotenoids, which serve as antenna pigments and protect from ROS and (3) the xanthophylls, which also serve as antenna pigments. 

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