Biology XI:13 Photosyn in higher plants(2 Light Reaction of Photosy

Light Reaction of Photosynthesis & Cyclic and Non-Cyclic Photophosphorylation Light Reaction (Photochemical Phase) This phase directly depends on light. The pigments absorb light energy and produce ATP. Includes: Light absorption Water splitting Oxygen release Formation of ATP and NADPH, which is then used in the biosynthetic phase Pigment molecules bound to the proteins form LHC (light harvesting complexes). LHC are located within two photosystems − PSI and PSII Each photosystem has two parts: Reaction centre − consisting of chlorophyll a molecule Antennae − consisting of accessory pigments, which increase the efficiency of photosynthesis by absorbing different wavelengths of light Reaction centre is different in both photosystems: PSI − P700; since chlorophyll a has absorption peak at 700 nm here PSII − P680; since chlorophyll a has absorption peak at 680 nm here. Photo-Phosphorylation The process of formation of ATP in chloroplast in the presence of sunlight Photo-phosphorylation is of two types: Non-cyclic photo-phosphorylation Cyclic photo-phosphorylation Non-Cyclic Photo-Phosphorylation PSII absorbs 680 nm wavelength of red light, causing electrons to become excited and jump into an orbit farther from the nucleus. These electrons are then accepted by an electron acceptor, which sends them to an electron transport system. Electron transport system transfers the electrons to PSI. Electrons in PSI are simultaneously excited on receiving a wavelength of 700 nm. These electrons are again transferred to another electron acceptor having a greater redox potential. From the electron acceptor, electrons are transferred to the molecule of NADP+. Addition of these electrons reduces the NADP+ to NADPH+ H+. During this process, electrons move downhill, i.e., towards the systems at greater redox potential. The flow of electrons assumes the shape of the letter ‘Z’ when all carriers are placed according to their redox potential. Hence, the process is called Z scheme. Since the electrons lost by PSII do not come back to it, this process of formation of ATP is called non-cyclic photo-phosphorylation. Cyclic Photo-Phosphorylation In this scheme, only PSI is functional. Hence, the electrons are circulated within the photosystem. This results in a cyclic flow of electrons. This scheme could possibly be occurring in stroma lamellae because it lacks both PSII and NADP reductase enzyme. This cyclic flow results only in the synthesis of ATP, and not of NADPH + H+. Splitting Of Water The electrons being transferred in the photo-phosphorylation reactions are generated by the splitting of water. Water splitting complex is associated with PSII. Manganese, chlorine, etc., play an important role. The light-dependent splitting of water is called photolysis2H2O → 4H+ + O2 + 4e− Electrons formed are used for replacing the electrons lost from P680. Oxygen is liberated as a byproduct of photosynthesis. Protons are used for the formation of reducing power NADP to NADPH+.