Tuesday, December 22, 2009
bacterial photosynthesis- fob-btechbiotechnology-7th chapter-photosynthesis
In prokaryotes (blue green bacteria, Prochlorophyta, purple and green bacteria) the photosynthetic structures are chromatophores.
In bacterial photosynthesis, a quantum of visible light is absorbed by a molecule of chlorophyll a or a molecule of a carotenoid and the energy is then transferred to another chlorophyll in a special reaction centre causing the ejection of an electron. This electron is then accepted by ferrodoxin and the oxidized chlorophyll oxidizes the terminal cytochrome of the electron transport system.
The resulting charged system, with a large potential difference between its termini is used to produce ATP (photophosphorylation) and the reducing power in the form of TPNH, which are both used in the reduction of CO2 .
The amount of ATP and TPNH formed in this system varies depending on the photophosphorylation system. In cyclic photophosphorylation, the electrons are transferred from the reduced ferredoxin to oxidized cytochrome through a chain of quinones and cytochromes in a close circle which leads to the conversion of a part of the absorbed light into energy. In the "noncyclic photophosphorylation", the electrons at the reducing end of the chain are used to reduce NADP and to complete the electron transport, electrons must be supplied from an another source. In photosynthetic bacteria, these electrons are derived by the oxidation of a substrate other than water and therefore no oxygen is released.
Photosynthesizing bacteria are generally obligate anaerobes. Bacterial photosynthesis is also more primitive in that the photosynthetic apparatus is not found in chloroplasts but in particles attached to the extensions of the plasma membrane. With the use of energy and the reducing power supplied by photosynthesis or chemosynthesis, the autotrophic bacteria reduce CO2 to derive their (carbon requirement. Although this process is fundamentally biosynthetic, it is ultimately linked to autotrophic energy metabolism.
In the Rhodospirillaceae (purple non sulphur bacteria) and the Chromatiaceae (purple sulphur bacteria) the thylakoids are extensions of the cell membrane. They may be in the form of vesicles, tubular bodies or lamellae. In the Chlorobiaceae (green sulphur bacteria) the sacs forming the photosynthetic apparatus are not continuous with the cell membrane