NON-Cyclic Phosphorylation

• 1.       When Photosystem II absorbs light, an electron excited to a higher energy level in the reaction center and is captured by the primary electron acceptor of PS II. The oxidized chlorophyll is now a very strong oxidizing agent; its electron “hole” must be filled.
• 2.       This hole is filled by electrons, which are extracted by an enzyme from water. The spitting of water in photosynthesis that releases oxygen is called photolysis.
• 3.       Each photo excited electron passes from the electron acceptor of photosyntem II to photosyntem I via an electron transport chain. This chain consists of an electron carrier called plastoquinone (PQ), a complex of two cytochromes and a copper containing protein called plastocyanin (PC).
• 4.       As electrons move down the chain their energy goes on decreasing and is used by thylakoids membrane to produce ATP (photophosporylation).
• 5.       The electron reaches “bottom” of the electron transport chain and fills an electron “hole” in P700 which is created when light energy is absorbed by molecules of P700 and drives an electron from P700 to the primary acceptor of Photosystem I.
• 6.       The primary electron acceptor of Photosystem I passes the photo excited electrons to a second electron transport chain, which transmits them to ferredoxin (Fd) and then to form NADPH which provide redacting power for the synthesis of sugar in the Calvin Cycle.

This path of electrons through the two Photosystem during non-cyclic phosphorylation is known as Z- scheme from its shape.