Question

Why NADP + H+

is not synthesized during cyclic photo-phosphorylation ?

Answer 1

the cyclic phosphorylation occur in bacteria and the non-cyclic in plant. In nature, I do not have any information that both processes occur in the same species.

Let me just draw the differences between both processes:

Non - Cyclic Photophosphorylation  

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.

Cyclic Photophosphorylation

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. 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.

Answer 2

Cyclic photo-phosphorylation

Explanation:

• It is thought that cyclic photo-phosphorylation could be a supply of adenosine triphosphate needed for plastid activities over and higher than that needed within the carbon reduction cycle.
• Since the non cyclic photo-phosphorylation end in the assembly of adenosine triphosphate and NADPH,whereas cyclic photo-phosphorylation will no generate NADPH, change between cyclic and non cyclic photo-phosphorylation additionally represents
• A mechanism by that plastid will regulate the stromal ATP/NADPH ratios, that is very important within the maintenance of plastid metabolic activity.