Dear Gaber,

I think we should separate the two processes since 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.

http://www.majordifferences.com/2013/02/difference-between-cyclic-and-non.html#.Vuc_NOIrLIU

http://www.tutorvista.com/content/biology/biology-

iv/photosynthesis/photophosphorylation.php

Hoping this will be helpful,

Rafik

Thanks Rafik,

But the two processes do occur in plants. That why I am asking.

It is thought that cyclic photo-phosphorylation is a source of ATP required for chloroplast activities over and above that required in the carbon reduction cycle. since the non cyclic  photo-phosphorylation result in the production of ATP and NADPH,whereas cyclic photo-phosphorylation does no generate  NADPH, switching between cyclic and non cyclic photo-phosphorylation also represents a mechanism by which chloroplast can regulate the stromal ATP/NADPH ratios, which is important in the maintenance of chloroplast metabolic activity.

Dear Gaber,

Assimilation of atmospheric CO2 by photosynthetic organisms such as plants, cyanobacteria and green algae, requires the production of ATP and NADPH in a ratio of 3:2. The oxygenic photosynthetic chain can function following two different modes: the linear electron flow which produces reducing power and ATP, and the cyclic electron flow which only produces ATP. Some regulation between the linear and cyclic flows is required for adjusting the stoichiometric production of high-energy bonds and reducing power. In the green alga Chlamydomonas reinhardtii, the onset of the cyclic electron flow during a continuous illumination under aerobic conditions. In mutants devoid of Rubisco or ATPase, where the reducing power cannot be used for carbon fixation, a stimulation of the cyclic electron flow is observed. Recent studies show that the cyclic electron flow can operate under aerobic conditions and support a simple competition model where the excess reducing power is recycled to match the demand for ATP.

For more information, please use the following link:

http://www.sciencedirect.com/science/article/pii/S0005272809002321

Hoping this will be helpful,

Rafik

Besides the last statement Of Rafiq answer, it is pertinent to know  that what are the possible condition for plants to have the cyclic one. In non  cyclic, there is generation of oxy free radicals , along with ATP.NADPH, O2 depending upon intensity of light.This is very common assault  faced by field plants, But the generation of  free   radicals are not probable in cyclic, now here is the significance & essentiality of cyclic electron movement that is to meet the requirement of ATP as well as drive the chloroplastic activity in terms of C- assimilation &many others in absolute harsh condition, as the process does not require higher energy for e- movement & generation of ATP. It's matter of survival as well. This might be reason to have cyclic way of photosynthesis in plants.

Both cyclic and non cyclic linear electron transport occur in higher plants chloroplast to maintain the required metabolic rate resulted in  production of ATP and NADPH. The cyclic produces only ATP and no reducing powers. The metabolically active mesophyll cells normally require the ATP and NADP in 3:2 ratio, therefore there is an adjustment of  the ETC across the photosystems. If any inhibition or electron transport impairment occurs at PS II level predominantly the cyclic around PS I is evident.

Recently a concept involves cyclic electron flow around PS II also postulated by many workers if more than water is available  as electron donor at PS II. One such natural electron donor is ascorbate. Thus, the significance of various cyclic and non cyclic linear ETC are just to maintain the balance between the required level of reducing power and  source of excited electrons in the photosynthetic apparatus. A phenomenon of excess electrons handling is inevitable to prevent ROS generation and related oxidative stress in plants. 

Annamalai Nathan

the answer which i wrote is the answer to your question. please see any recent text book on plant physiology,you will find the answer.

It seems that pH and energy dissipation are the main uses of cyclic electron flow at least in PGR5 down mutants http://ac.els-cdn.com/S009286740200867X/1-s2.0-S009286740200867X-main.pdf?_tid=aa510f36-ec18-11e5-9905-00000aab0f01&acdnat=1458202818_a1ebdc260df17e9aafc6db22b57c62f3

In addition to the NADPH equilibrium, extra ATP requirement through cyclic PP is also welcome to the C4 plants in which CO2 generation is driven by ATP energy.

Thanks colleagues, your contributions really help.

Based on the above answers, can we expect that cyclic electron transfer is faster in C4 plants since they need extra energy for primary CO2 fixation, a process that does not occur in C3 plants?

It has been a matter of debate as to whether and to what extent cyclic photophosphorylation occurs in a leaf under normal physiological conditions.

Recent evaluations of the proton stoichiometry of photophosphorylation suggest that the yield of ATP in noncyclic electron transport is not sufficient for the requirements of CO2 assimilation, and therefore cyclic photophosphorylation seems to be required to fill the gap.

Moreover, cyclic photophosphorylation must operate at very high rates in the bundle sheath chloroplasts of certain C4 plants. These cells have a high demand for ATP and they contain high PS I activity but very little PS II