When photosystem II absorbs light, an electron excited to a higher energy level in the reaction centre chlorophyll (P680) is captured by the primary electron acceptor. The oxidized chlorophyll is now a very strong oxidizing agent. This reaction splits a water molecule into two hydrogen ions and an oxygen atom, which immediately combines with another oxygen atom to form O₂. Each photoexcited electron (energized by light) passes from the primary electron acceptor in photosystem II to photosystem I via an electron transport chain. As electron move down the chain, their lower energy level is harnessed by the thylakoid membrane to produce ATP (by chemiosmosis) called non-cyclic photophosphorylation. Under certain conditions, the photoexcited electrons take an alternative path called cyclic electron flow, which uses photosystem I (P₇₀₀) but not photosystem II (P₆₈₀).  This is called cyclic photophosphorylation.  The chloroplast shifts to this process when the ATP supply drops and the level of NADPH rises.