In the process of photophosphorylation, high energy electrons (e-) released from chlorophyll molecules are carried through various carriers, lose energy and return to the same chlorophyll molecule, it is called cyclic photophosphorylation. Cyclic photophosphorylation process is as follows.
(i) Only photosystem-I participates in the cyclic photophosphorylation process. Photosystem-II absorbs light energy from the sun. This light energy is transferred to the reaction center P-700.
(ii) Two high energy electrons (2e-) are emitted from P-700. Electrons are accepted by the two carriers ferridoxine (Pd).
(iii) Two electrons from ferridoxine come to cytochrome b6 (Cyt. b6). Electrons from cytochrome-b6 pass two very quickly to plastoquinin (PQ).
(iv) Then two electrons come from plastoquinin to cytochrome-f (Cyt. f). At this time the two electrons lose some energy. Under the influence of this energy, ATP is produced by combining ADP and Pi.
(v) Finally electrons pass from cytochrome-f to photosystem-I via plastocyanin. Thus photosystem-I regains its lost electrons.