Non-cyclic photophosphorylation is associated with the movement of electrons according to their redox potentials. The process involves two photosystems (PS I and PS II) and NADP+ reduction. When light is absorbed by PS II, the excited electrons enter into an electron transport chain to produce ATP. Photoactivation of PS I causes the release of electrons which reduce NADP+ to NADPH. The photolysis of water releases electrons which replace electrons lost by PS II. As a result, PS I electrons are replaced by PS II electrons. As electrons move from the higher energy level to lower, the scheme of the non-cyclic photophosphorylation forms a Z-like pattern and, as a result, is called the Z scheme.