Explain cyclic photophosphorylation.

(N/A) When only $PS-I$ is functional,the electron is circulated within the photosystem and the phosphorylation occurs due to the cyclic flow of electrons. This is called cyclic photophosphorylation.
$A$ possible location where this occurs is in the stroma lamellae.
While the membrane or lamellae of the grana have both $PS-I$ and $PS-II$,the stroma lamellae lack $PS-II$ as well as the $NADP$ reductase enzyme.
The excited electron does not pass on to $NADP^+$.
It is cycled back to the $PS-I$ complex through the electron transport chain.
The cyclic flow,therefore,results only in the synthesis of $ATP$ but not of $NADPH + H^+$.
Cyclic photophosphorylation also occurs when only light of wavelengths beyond $680 \ nm$ is available for excitation.