Explanation

Cyclic-photophosphorylation involves only pigment system-I. When the photons activate PS-I, a pair of electrons are raised to higher energy level. They are captured by primary acceptor, which passes them on to ferredoxin, plastoquinone, cytochrome complex, plastocyanin and finally back to reaction centre of PS-I, i.e.,$P_{700}$. At each step of electron transfer, the electrons lose potential energy. Their trip down hill is caused by the transport chain to pump $H^{+}$ across the thylakoids membrane. The proton gradient thus established is responsible for forming ATP (2 molecules). But no reduction of NADP to NADPH+$H^{+}$ takes place.