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.
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
