Photosynthesis Depends on the Photochemical Reactivity of Chlorophyll

With the exception of certain archaebacteria, all known photosynthetic organisms take advantage of the photochemical reactivity of one or another type of chlorophyll 7fig. 15.4). Chlorophylls resemble hemes but differ from them in four major respects  

The central metal atom is magnesium rather than iron. 

Cross-sectional view of a cell of Rhodospirillum rubrum, a purple photosynthetic bacterium. A double-membrane system surrounds the cell. The inner membrane is extensively invaginated into tubules (arrows). These look circular when they are cut in cross section. (Courtesy of Dr. Gerald Peters.) 

In the case of chlorophyll a and chlorophyll b, the two major chlorophylls in plants and cyanobacteria, one of the pyrrole rings (ring IV) is reduced by the addition of two hydrogens. In bacteriochlorophylls a and b, which occur in the purple and green bacteria, two of the rings are reduced (rings II and IV). 

The propionyl side chain of ring IV is esterified with a long-chain isoprenoid alcohol. 

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