Cyclic Photophosphorylation:
Here only PS I participates. The light energy absorbed by the light harvesting complex is Transferred by inductive resonance to P700. P700 can accept light energy only of a wavelength longer than 680 nm. The excited electron of PQ700 is first accepted by an unknown acceptor (x) which in turn transfers its electron to FRS (Ferredoxin reducing substance) and then to ferredoxin. This causes ferredoxin to release one of its own electrons (energy rich), which shuttles through the chain consisting of Cyt b6, Cyt f, plastocyanin and finally back to P700. The P700 now returns to the ground state and is now ready to receive one more unit of radinat energy.
The electorn released from the chlorophyll (P700) is returned in a continuous chain i.e., it travels in a cyclic fashion, hence the name cyclic electron transport or cyclic photophosphorylation.
Synthesis of ATP during cyclic photophosphorylation:
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The electron ejected from P700 has an energy of 0.42 v which gets reduced to 0.417 when it reaches ferredoxin. Here the electrons passes through 3 downhill migration (Cyt b6, Cyt f and Pi) steps before it is returned to P700. The potential gap between Ferredoxin to P700 (ground state) is about one volt which is sufficient to reproduce at least two molecules of ATP. ATP molecules are formed at two states – a) between ferredoxin and Cyt b6 and b) between Cyt b6 and Cytf.
In cyclic photophosphorylation water does not participate; hence there is neither evolution of O2 nor is there any formation of reduced NADPH molecule (for there is no hydrogen donor). Further PS II is not involved in a stage.
Ramriez et al (1968) opine that cyclic photophosphorylation is of limited occurrence in plants and a high rate of cyclic reaction may even retard the CO2 fixation as NADPH will not be available. They argue that cyclic reaction is not involved in the main path of carbohydrate synthesis as it can only provide part of ATP requirement for CO2 fixation. Describe non cyclic photophosphorylation?
This is the major pathway of light reaction involving both PS I and PS II Non-cyclic photophosphorylation is also called the ‘Z’ scheme electron transport because of the zig zag fashion of electron travel. It takes place as follows.
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a) The radiant energy is trapped by PS I (P700) which ejects one if its ajectrons and is trapped by X, FRS and finally goes to ferredoxin. The re- i ferredoxin now transfers its electrons to NADPH. NADPH gets reduced PS I however remains in the excited state as it has not got back its electron.
b) Meanwhile PS II absorbs light energy (680nm) and ejects one of its own electrons which is trapped by an unknown quinone acceptor (Q). From Q- the electron moves downhill to cyt b6, plastoquimne, cyt f, Plastocyanin (PC) and finally to PS I. PS I which is in an excited state, now returns to the ground state as it has got an electron from PS II.
How does PS II come back to the ground state now that it has sent its electron to fill up the ‘hole’ in PS I? Because unless the PS II returns to the normal state, the system cannot continue functioning. The answer lies in the dissociation of water molecules. Water splits it into H+ and OH ions, what is the mechanism of this splitting of water? Some physiologists call it photolysis of water, others are not sure of the mechanism but state that Mn+ and CI ions are essential for the breakdown of water.
In any case the excited PS II returns to the normal state by getting an electron from the OH ions of water. According to Salisbury and Ross (1986), the excited PS II returns to the ground state by attracting an electron from an adjacent Mn- protein, which in turn gets an electron from the OH- of water. The H+ ions released by water are accepted by NADPH to become NADPH H. Thus a reduced NADPH is formed.
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The electron transport system or photophosphorylation is called non cyclic because the excited PS I returns to the normal state by getting an electron from PS II, while the excited PS II returns to the ground state by receiving an electron from water. Hence the movement of electron is not in a cyclic fashion.
ATP synthesis during non-cyclic photophosphorylation:
The electron (+0.8 v) released by PS II is accepted by a quinone from where the downhill migration begins to reach PS I via Cyt b6, Cyt f and PC. One molecule of ATP is synthesized when the electron shuttles between PQ and Cyt f. Release of oxygen and formation of NADPH+ + h+: Four molecules of water split up for every turn of non-cyclic photophosphorylation. This is necessary (see fig 4.9) to provide four electrons to the excited PS II to bring it back to the ground state. Similarly, four H+ ions are required to reduce 2 molecules of NADPH. These reactions take place as follows.
(four electrons are required to reduce 2 molecules of NADPH and these come from the excited PSI via ferredoxin)
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Thus the end products of non-cyclic photophosphorylation are:
a) One molecule of ATP
b) Two molecules of reduced NADPH
c) 2 molecules of water and
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d) one molecule of O2.