Tuesday, December 22, 2009
oxygenic photosynthesis- fob-btechbiotechnology-7th chapter-photosynthesis
Oxygenic and Anoxygenic Photosynthesis - In the Cyanobacteria and Pro chlorophyta photosynthesis is oxygenic, i.e. there is evolution of oxygen.
There are two linked photosystems involved in photosynthesis. The electron donor is H2O, and oxygen is the ultimate product of oxidation. They are, therefore, aerobic phototrophs. The photosynthetic apparatus of the Cyanobacteria is remarkably similar in structure and function to the eukaryote chloroplast. Their light harvesting pigments, Chl a and phycobiliproteins, are homologous to those of the chloroplast of Rhodophyta (red algae).
Oxygenic photosynthesis is the principal producer of both oxygen and organic matter on earth. The primary step in this process — the conversion of sunlight into chemical energy — is driven by four, multisubunit, membrane-protein complexes that are known as photosystem I, photosystem II, cytochrome b 6 f and F-ATPase. Structural insights into these complexes are now providing a framework for the exploration not only of energy and electron transfer, but also of the evolutionary forces that shaped the photosynthetic apparatus.
Type b Cytochrome - There is evidence that a b-type cytochrome IS present in the photosynthetic electron transport system.
Cytochrome b has been demonstrated in Rhodopseudomons sphaeroides (E°‘ = + 50, mV) and R. capsulata (E°' = + 60 mV), and there are indications that it is adjacent to cytochrome c in the cyclic system. It is likely that cytochrome b may be present even in organisms where it has been previously reported to be absent.
Small amounts of cytochrome b could be masked by other substances. Chromatium vinosum and Chlorobium thiosulphatophilum lire reported to contain one molecule of cytochrome b per reaction centre (Knaff and Buchanan, 1975).
It is likely that cytochrome b has a role in cyclic photosynthetic flow in the Chromatiaceae, Chlorobiaceae as well as in the Rhodospirillaceae.
Type c Cytochrome - A number of different c type cytochromes have been found in the electron transport system of photosynthetis bacteria. In the purple nonsulphur bacterium Rhodospirillum rubrum a soluble c type cytochrome is associated with P870 of the reaction centre. This cytochrome is referred to as cytochrome c2, and has a high mid point potential of about + 30.0 mV.
Cytochrome c2 appears to be the electron donor to P870. In PSI of higher plants, plastocyanin (PC), a copper protein, is the electron donor to P700. Algae apparently represent an intermediate stage in evolution, since their electron donor to P700 is a c type cytochrome, although plastocyanin can also be present. Rhodospirillum also contains cytochrome cc' with two different haeme groups.
Purple sulphur bacteria like Chromatium contain cytochrome c552 in addition to cytochromes C2 and cc’. Cytochrome C552 (MW 72,000) has two haeme groups and one FMN.
The green sulphur bacterium Chlorobium has three cytochromes of the c type, but none of these has the high redox potential of cytochrome, c2 of purple bacteria and cytochrome f of higher green plants.