Non-Cyclic Photophosphorylation

Non-cyclic photophosphorylation is the main pathway of the light-dependent reactions. It uses both photosystems, and electrons flow in one direction, from water to NADP. It is often drawn as a 'Z-scheme'.1. Light is absorbed by photosystem II. Electrons in its reaction centre are excited and pass to an electron acceptor.
2. The electrons pass along an electron transport chain to photosystem I, losing energy. The energy pumps protons into the thylakoid space, and ATP is made by chemiosmosis through ATP synthase.
3. Light absorbed by photosystem I excites the electrons again, to a higher energy level. They pass via another electron acceptor to NADP, which is reduced with protons from the stroma to form reduced NADP.
4. Photosystem II has now lost electrons. They are replaced by the photolysis of water:2H₂O → 4H⁺ + 4e⁻ + O₂The electrons lost by photosystem I are in turn replaced by those arriving from photosystem II.The flow is called non-cyclic because the electrons end up on NADP and do not return to the photosystem they came from.The products are:
• ATP
• reduced NADP
• oxygen (from water, a by-product)ATP and reduced NADP are released into the stroma and used in the Calvin cycle: ATP provides energy and reduced NADP provides the hydrogen to reduce GP.