Photophosphorylation is the process of creating ATP using a Proton gradient created by the Energy gathered from sunlight. The process of creating the Proton gradient resembles that of the electron transport chain of Respiration. But since formation of this proton gradient is light-dependent, the process is called Photophosphorylation.
Chemiosmosis - Chemiosmosis is the process of using Proton movement to join ADP and Pi. This is accomplished by enzymes called ATP synthases or ATPases. The CF1-ATPase of the Thylakoid membrane is shown on the left. As protons pass through this enzyme ADP and Pi are joined to make ATP. The movement of the Protons through this enzyme provides the Energy needed to make ATP.
Noncyclic Photophosphorylation really refers to the ATP generated by Protons moved across the Thylakoid membranes during the Z-scheme. The Cytb6-f complex acts as an electron transport chain. As the electrons lose Energy (during a series of re/dox reactions) Protons are moved into the Thylakoid space. This Proton gradient can be used to generate ATP chemiosmotically.
The electrons lost by P680 (PS-II) are taken up by P700 (PS-I) and do not get back to P680 i.e., unidirectional and hence it is called non- cyclic phosphorylation. The electrons pass through the primary acceptor, plastoquinone (PQ), cytochrome complex, plastocyanin (PC) and finally to P700. The electrons given out by P700 are taken up by primary acceptor and are ultimately passed on to NADP. The electrons combine with H+ and reduce NADP to NADPH2. The hydrogen ions also called protons are made available by splitting up of water. Non-cyclic photophosphorylation needs a constant supply of water molecules. The net result of non-cyclic phosphorylation is the formation of oxygen, NADPH and ATP molecules. Oxygen is produced as a waste product of photosynthesis
During Cyclic Photophosphorylation the electrons are recycled, hence the name cyclic photophosphorylation. The excited electrons resulting from the absorption of light in photosystem I are received by the primary electron acceptor and then transferred to the cytb6-f complex which acts as an electron transport chain. The electrons return back to the reaction center of Photosystem I, where the cycle is ready to start all over. The electrons are using to translocate Protons which the ATPase uses to synthesize ATP. No reduction of NADP+ occurs in Cyclic Photophosphorylation.
Non-cyclic Photophosphorylation - During the movement of electrons shown in red, H+ moves across the membrane. The movement of electrons "drives" the reactions shown in blue.
Cyclic Photophosphorylation - During the movement of electrons shown in red, H+ moves across the membrane. These electrons don't generate NADPH, but the H+ transport can produce .
The electrons released by P700 of PS-I in the presence of light are taken up by the primary acceptor and are then passed on to ferredoxin (Fd), plastoquinone (PQ), cytochrome complex, plastocyanin (PC) and finally back to P700 i.e., electrons come back to the same molecule after cyclic movement.Cyclic Photophosphorylation
The cyclic photophosphorylation also results in the formation of ATP molecules just like in non - cyclic photo phosphorylation.
As the electrons move downhill in the electron transport chain, they lose potential energy and ATP molecules are formed in the same way as in mitochondria during respiration.
During cyclic photophosphorylation, electrons from photosystem - I are not passed to NADP from the electron acceptor. Instead the electrons are transferred back to P700. This downhill movement of electrons from an electron acceptor to P700 results in the formation of ATP and this is termed as cyclic photophosphorylation. It is very important to note that oxygen and NADPH2 are not formed during cycle photophosphorylation.