Energy Transduction in Anoxygenic Photosynthesis

Abstract

Thomas (1974) has supposed that a dispassionate extraterrestrial might perceive the Earth as populated by two symbiotic forms. The chloroplasts would be seen as cellular structures built from carbon compounds, ranging from the pro-karyotic cyanobacteria at the simplest level through to those occupying the algae and green plants at the most elaborate. It would be clear that these species were equipped to harness solar energy to “fix” CO₂ by reduction to carbohydrate and thence to more elaborate molecules of the cell. Water would be identified as the source of electrons to reduce the CO₂, explaining the byproduct, dioxygen, that dominates the Earth’s atmosphere. The partner to this symbiotic pair, the mitochondrion, would be recognized as respiratory bacteria in its simplest, prokaryotic form, through the likes of those found in yeast to those occupying animal and plant cells. The mitochondrion would be seen to utilize the chloroplast-derived, reduced carbon compound material, not only for its own biosynthetic needs but also, in collaboration with O₂, as a source of fuel to provide free energy to drive these biosynthetic reactions, reforming CO₂ and H₂O and completing the cycle. Thus while the mitochondrion is able to capitalize on the photosynthetic industry of the chloroplast, the chloroplast benefits from the CO₂ recycling activities of the mitochondrion. The plantmitochondrial systems are outlined as a flow sheet in Fig. 1A and B.