Photophosphorylation is the synthesis of ATP from ADP and inorganic phosphate in the presence of light.

When only PS I is functional, the electron is circulated within the photosystem and the phosphorylation occurs due to cyclic flow of electrons.

While the membrane or lamellae of the grana have both PS I and PS II the stroma lamellae membranes lack PS II as well as NADP reductase enzyme. The excited electron does not pass on to NADP+ but is cycled back to the PS I complex through the electron transport chain. The cyclic flow hence, results only in the synthesis of ATP, but not of NADPH+ H⁺. Cyclic photophosphorylation also occurs when only light of wavelengths beyond 680 nm are available for excitation.