Chapter 23 - Evolution of Photosynthesis

Abstract

This chapter discusses photochemical reactions and their evolution. Photosynthesis depends on the photochemical reactivity of chlorophyll. Plants and other photosynthetic organisms convert about 1011 tons of carbon from carbon dioxide into organic compounds annually. All photosynthetic organisms that belong to the eubacterial and eukaryotic kingdoms take advantage of the photochemical reactivity of chlorophyll or bacteriochlorophyll. The electrons in a molecule are held in a set of molecular orbitals, each of which is associated with a definite energy. The chlorophyll that undergoes photo-oxidation is bound to a protein in a complex called a reaction center. Reaction centers of purple bacteria generally contain three polypeptides with molecular weights of approximately 28,000, 31,000, and 34,000. Electrons move from p870 to bacteriopheophytin and then to quinine. Cyclic electron-transport chain moves protons across the membrane. The reactive chlorophyll or bacteriochlorophyll molecules of P700, P680, or P870 make up only a small fraction of the total pigment in photosynthetic membranes. Photosynthesis arose early in evolution and is still widespread among eubacteria. The 16S-rRNA sequences indicate that various photosynthetic species are related more closely to nonphotosynthetic species of the same phylum than they are to the photosynthetic organisms of other phyla that have similar photosynthetic systems. Photosynthetic systems ultimately turned their attention to water as a source of electrons. Primitive photosynthesizers probably used reductants such as Fe (0H2), hydrogen, hydrogen sulfide, and other reduced sulfur compounds where they were abundant. PS-1 reaction centers may have evolved from the antenna complexes of PS-2 reaction centers. Chlorophyll and bacteriochlorophyll become strong reductants when excited by light.