The Ability to Convert Light Into Chemical Energy Is Ancient

II. Transducing and Storing Energy
19. The Light Reactions of Photosynthesis
19.5. Accessory Pigments Funnel Energy Into Reaction Centers
Figure 19.33. Location of Photosynthesis Components. Photosynthetic assemblies are differentially distributed in the
stacked (appressed) and unstacked (nonapressed) regions of thylakoid membranes. [After a drawing kindly provided by
Dr. Jan M. Anderson and Dr. Bertil Andersson.]
II. Transducing and Storing Energy
19. The Light Reactions of Photosynthesis
19.6. The Ability to Convert Light Into Chemical Energy Is Ancient
The ability to convert light energy into chemical energy is a significant evolutionary advantage. Indeed,
photosynthesis arose early in the history of life on Earth, which began about 3.5 billion years ago. Geolog- ical
evidence suggests that oxygenic photosynthesis became important approximately 2 billion years ago. Anoxygenic
photosynthetic systems existed even earlier (Table 19.1). The photosynthetic system of the nonsulfur purple bacterium
Rhodospeudomonas viridis has most features common to oxygenic photosynthetic systems and clearly predates them.
Green sulfur bacteria such as Chlorobium thiosulfatophilum carry out a reaction that also seems to have appeared before
oxygenic photosynthesis and is even more similar to oxygenic photosynthesis than that of R. viridis . Reduced sulfur
species such as H2S are electron donors in the overall photosynthetic reaction:
Nonetheless, photosynthesis did not evolve immediately at the origin of life. The failure to discover photosynthesis in the
domain of Archaea implies that photosynthesis evolved exclusively in the domain of Bacteria. Eukaryotes appropriated
through endosymbiosis the basic photosynthetic units that were the products of bacterial evolution. All domains of life
do have electron-transport chains in common, however. As we have seen, components such as the ubiquinonecytochrome c oxidoreductase and cytochrome bf family are present in both respiratory and photosynthetic electrontransport chains. These components were the foundations on which light-energy-capturing systems evolved.
II. Transducing and Storing Energy
19. The Light Reactions of Photosynthesis
19.6. The Ability to Convert Light Into Chemical Energy Is Ancient
Table 19.1. Major Groups of Photosynthetic Prokaryotes
Bacteria
Photosynthetic electron donor
O2 use
Green sulfur
H2, H2S, S
Anoxygenic
Green nonsulfur Variety of amino acids and organic acids Anoxygenic
Anoxygenic
Purple sulfur
H2, H2S, S
Purple nonsulfur Usually organic molecules
Anoxygenic