Light (Hill) Reaction
There are two pathways involved in the light reaction.
In noncyclic photophosphorylation, a photon of light excites P680 which ejects two electrons. These electrons are picked up and passed through various proteins, eventually pumping two hydrogen ions across the thylakoid membrane, phosphorylating one ATP, and entering P700. A photon of light hitting P700 also causes it to eject two electrons. These, too, are passed through a protein chain, pumping through two hydrogen ions and forming one NADPH. These products then enter the dark, or light-independent, reaction.
In cyclic photophosphorylation, a photon of light excites P700 which ejects two electrons. These electrons are passed through a chain of proteins and two hydrogen are pumped through the thylakoid membrane. No ATP or NADPH is produced, however. The electrons then return to P700 (by the way of plastocyanin) and the cycle begins anew.
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