At some points during this pathway, electrons travel in pairs. The cyclic pathway may be visualized as starting with Photosystem I absorbing two photons of light followed by a series of redox reactions which return the electrons along an electron transport chain in the thylakoid membrane to chlorophyll. With each redox reaction along the electron transport chain, electrons lose potential energy until they return to their ground-state orbital in the P700 reaction center . Absorption of another two photons of light by the pigments send a second pair of electrons through the cyclic pathway. This coupling mechanism is chemiosmosis.
Some electron carriers can only transport electrons in the company of hydrogen ions.
The hydrogen ions are picked up on one side of the thylakoid membrane and deposited on the opposite side as the electrons move to the next member of the transport chain. An ATP synthase enzyme in the thylakoid membrane uses this proton-motive force to make ATP in a process called photophosphorylation due to the energy required being light energy. This form of ATP production is referred to specifically as cyclic photophosphorylation as it occurs during the cyclic pathway. I to the excited state. Photosystem II supplies electrons of P700 to fill these holes.