Photophosphorylation is the synthesis of ATP from ADP and inorganic phosphate in the presence of light. When two photosystems work in a series, first PS II and then PS I, a process called noncyclic photophosphorylation occurs. When only PS I is functional, the electron is circulated within the photosystem and phosphorylation occurs due to cyclic flow of electrons, it is cyclic photophosphorylation.