A water-splitting enzyme extracts electrons from water and passes them to oxidized P680, which has a high affinity for electrons. As water is oxidized, the removal of electrons splits water into two hydrogen ions and an oxygen atom. The oxygen atom immediately combines with a second oxygen atom to form O 2 . It is this water-splitting step of photosynthesis that releases O 2 . This coupling mechanism is chemiosmosis .
Some electron carriers can only transport electrons in the company of protons.
The protons are picked up on one side of the thylakoid membrane and deposited on the opposite side as the electrons move to the next member of the transport chain. This process is called photophosphorylation because the energy required is light. This form of ATP production is called noncyclic photophosphorylation .