Light reaction of Photosynthesis | NMDCAT 2021 | Cyclic and non-cyclic photophosphorylation

About This Video 1. When photosystem II absorbs light, an electron excited to a higher energy level in the reaction center chlorophyll P680 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 illed. 2. This hole is illed by the electrons which are extracted, by an enzyme, from water. This reaction splits a water molecules into two hydrogen ions and an oxygen atom, which immediately combines with another oxygen atom to form O2 . This water splitting step of photosynthesis that releases oxygen is called photolysis. The oxygen produced during photolysis is the main source of replenishment of atmospheric oxygen. 3. Each photoexcited electron passes from the primary electron acceptor of photosystem II to photosystem 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 thylakoid membrane to produce ATP. This ATP synthesis is called photophosphorylation because it is driven by light energy. Speciically, ATP synthesis during non-cyclic electron low is called non- cyclic photophosphorylation. This ATP generated by the light reactions will provide chemical energy for the synthesis of sugar during the Calvin cycle, the second major stage of photosynthesis. 5. The electron reaches the “bottom” of the electron transport chain and ills an electron “hole” in P700, the chlorophyll a molecules in the reaction center of photosystem I. This hole 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 photoexcited electrons to a second electron transport chain, which tmasmits them to ferredoxin (Fd), an iron containing protein. An enzyme called NADP reductase then transfers the electrons from Fd to NADP. This is the redox reaction that stores the high-energy electrons in NADPH. The NADPH molecule will provide reducing power for the synthesis of sugar in the Calvin cycle. The path of electrons through the two photosystems during non-cyclic photophosphorylation is known as Z-scheme from its shape.