Cyclic photophosphorylation is one of two light-dependent reactions and involves photosystem I only. Light is absorbed by the chlorophyll a molecule at the reaction centre. This excites electrons which gain enough energy to leave the molecule.The electrons are passed along a series of electron carriers in a series of redox reactions of reducing energy. The redox reactions are coupled with the active transport of hydrogen ions into the thylakoid space to harness the extra energy as a proton gradient. The energy of the proton gradient is used to synthesise ATP from ADP and inorganic phosphate by chemiosmosis. The energy-depleted electrons are returned to the chlorophyll a molecule.