Cyclic and Noncyclic photophosphorylation and mechanism of ATP synthesis

Cyclic and Noncyclic photophosphorylation:

Chlorophylls are present inAnoxygenic photosynthetic bacteria called bacteriochlorophylls, that alter from the chlorophylls of the plants in structure and In light absorbing properties. In the infrared region (725-1,035 nm) Bacteriochlorophylls absorb light. They should not contained in chloroplasts but they are found in extensive membrane systems throughout the bacterial cell.

When a quantum of light is absorbed by a molecule of bacteriochlorophyll , the light energy raises the molecule to an excited state. An electron is given off by bacteriochlorophyll in exicited state . Then the bacteriochlorophyll becomes positively charged. It then helps as an electron trap or strong oxidizing agent.

Ferredoxin is known as the electron , carrying some of the energy absorbed from light , is transferred to an iron-containing heme protein .from where it can be passed successively to ubiquinone , to cytochrome b, and to cytochrome f, and finally back to the positively charged bacteriochlorophyll. Necessarly , the electron has gone around in a cycle , beginning with , and returning to , bacteriochlorophyll . this relatively simple process is illustrated

In between cytochrome b and cytochrome f energy is released in the step is used for photophosphorylation –the production of ATP from ADP and inorganic phosphate.

In these reactionsno NADP⁺ has been reduced . The NADP⁺ reduction in photosynthetic bacteria is accomplished not by the priocess of photosynthesis but by using power from constituents of the environmental sources, such as H ₂S and other important inorganic and organic compounds .These types of reduced compounds usually abound in the anaerobic environment of photosynthetic bacteria.

It may be added that light of higher energy than that absorbed by bacteriochlorophylls can contribute to bacterial photosynthesis since are carotenoids and other accessory pigments in the bacterial cells which absorb light at shorter wavelengths and transfer the energy to the bacteriochlorophylls.

Inoxygenic photosynthetic bacteria like plants , algae, and cyanobacteria , The process of photosynthesis occurs in non-cyclic photophosphorylation. In the process , when (a molecule in pigment system II )one of two systems of light reactions absorbs light , this type of energy raises the molecule to an excited state and the molecule produces an electron . The electron is shifted to plastoquinone , to cytochrome b, to cytochrome f, and lastly to pigment system I. The generation of ATP from ADP and inorganic phosphate occurs through phosphorylation in the step between cytochrome b and cytochrome f. when pigment system I absorbs light than it releases an electron . This electron is transferred from ferredoxin to flavoprotein , to NADP⁺. Photophosphorylation occurs again between the release of the electron from pigment system I to ferredoxin. also, note that NADP⁺ is reduced in this part of the process (see fig. 10-13) . the process differs from cyclic photophosphorylation because the electron lost by pigment system II is not cycled back to it. Instead, electrons are replaced in pigment system II by the light generated the breakdown of water photolysis. There is some evidence that this scheme of noncyclic photophosphorylation, shown fig. 10-13 , may have to be modified . it appears that system II pigments alone can carry out the entire process of noncyclic photophosphorylation. Thus the noncyclic reduction of ferredoxin need not involve system I pigments. Further, the most important role of plastoquinone is in the transport of protons originating from the water. This modified process has been termed oxygenic photophosphorylation. Further evidence is needed for its confirmation.

The mechanism of ATP synthesis

The important chemical reactions which lead to the synthesis of ATP are now easily known. But the transfer of electrons through the respiratory transport chain is coupled to the synthesis of ATP is not very clearly known. There are Several alternate hypotheses have been put forward to explain about the how energy released during electron transport hypotheses advanced in 1961 by peter Mitchell, a British biochemist. In 1978 Mitchell was awarded the Nobel prize for his work in this field . The flow of electrons through the system of carrier molecules produce energy which drives positively charged ions (H⁺), or protons , through the membranes of chloroplasts, mitochondria, and bacterial cells is given by this theory. The movement of hydrogen ions results in the acidification of the surroundings medium and the generation of a pH gradient (a difference in pH) across the organelle or cell membrane. In addition , such hydrogen-ion lead to the formation of an electric potential gradient (a difference in charge) across the membrane(since an electric charge is carried by the proton). In this way, the energy released during the transfer of electrons through the respiratory chain is conserved as a “Protonmotive force”; the electric potential gradients are produced by pumping hydrogen ions across to membrane.

References

Michael J.Pleczar JR, Chan E.C.S. and Noel R. Krieg. Microbiology. Tata Mc GrawHill, 1993.

Powar. and Daginawala. General Microbiology.

Rangaswami and Bagyaraj D.J. Agricultural Microbiology.

Debey, RC and D K Maheshwari. A textbook of Microbiology. India: s.chand and company Ltd., 1999.