42. What shifts non cyclic Photophosphorylation to cyclic Photophosphorylation?

The saturation of NADP with hydrogens. Which means there are only so many carriers (NADP) for hydrogen and when the NADPs all have one atom of hydrogen, there are no more carriers. Not to waste the accumulated hydrogens, the event shifts from hydrogen Calvin transfer, ATP production, and oxygen release (non-cyclic) to strictly ATP production (cyclic).

Non cyclic photophosphorylation	Cyclic Photophosphorylation