LIGHT REACTION ( Non-Cyclic Photophosphorylation )

Light dependent phase of photosynthesis involves the absorption of light by the photosystems, excitation and flow of electrons through an electron transport chain, chemiosmotic synthesis of ATP, and reduction of NADP to NADPH. The flow of excited electrons through an electron transport chain during light reaction is of two different types Le., non-cyclic and cyclic. In non- cyclic electron flow, the excited electrons after leaving a particular photosystem do not comeback; these electrons after losing their energy are incorporated into another molecule. On the other hand, in cyclic electron flow, the excited electrons after leaving a particular photosystem finally comeback to their photosystem again. The most important event in light reaction is the production of ATP. This production of ATP during light reaction is called photophosphorylation and the mechanism is called chemiosmosis. There are two types of photophosphorylation. 

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(a) Non-cyclic photophosphorylation 

It is predominant pathway of light reaction in higher plants that occurs in routine. In this process both photosystems i.e., PS-I and PS-II are utilized and two electron transport chains are involved. When PS-ll absorbs light, its excited electrons after flowing through an electron transport chain are transferred to PS-I. Similarly, the excited electrons which are liberated from PS-I are finally accepted by NADP. Therefore it is called non-cyclic electron flow. The events of non-cyclic photophosphorylation are continuous but here they are discussed in steps for convenience.

 Absorption of light by PS-il and excitation of its electrons

 When just two photons strike the antenna complex of PS-II, the two electrons become excited and begin to move along the atoms of different pigments within photosystem. Ultimately, the absorbed energy reaches the reaction centre of PS-II (P680) and causes its two electrons to be excited. These excited electrons are captured by the primary electron acceptor of PS-ll and leave two "electron holes in the photosystem behind making chlorophyll a strong oxidizing agent.

 Photolysis of water

 The electron holes of photosystem must be filled so that in the presence of water splitting enzyme reactions can proceed. When water reacts with oxidized state of chlorophyll in photosystem, it breaks up into 2H ions, 2e and %0. Since this breakdown occurs in the presence of sunlight therefore, it is termed as photolysis of water. The electrons released from water are used to fill the "electron holes" of PS-II.

 Electron flow from PS-II to PS-I

 The excited/energized electrons which have been released from PS-II and captured by primary electron acceptor now begin to flow to PS-I through an electron transport chain. The electrons move from primary electron acceptor to the plastoquinone (PQ). From PQ the electrons flow through a complex of the cytochromes (Cyt) which consist of Cyt-b, and Cyt-f.

 The cytochrome complex is not only an electron carrier but it also works as proton pump. The electron flow through the cytochrome complex stimulates it to pump the protons from stroma to the thylakoid inner space. In this way the energy of flowing electrons is transformed into a gradient of protons (H) in the thylakoid inner space. The proton gradient activates an enzyme in thylakoid membrane called ATP synthase which not only moves the protons back into the stroma but also catalyzes a reaction in which ADP and Pi are combined to form ATP (photophosphorylation). This whole mechanism which involves flow of electron, pumping of protons and generation of ATP by thylakoid membranes is called chemiosmosis. This ATP, generated by light reactions will provide chemical energy for the synthesis of sugar during Calvin cycle. The energized electrons after losing their energy, move from cytochrome complex o the plastocyanin (PC) and finally incorporated into the PS-I

 Absorption of light by PS-I and excitation of its electrons On the other hand, when P700 in the reaction centre of PS-I molecule absorbs two photon of light, electrons are boosted to a higher energy level. P700 molecule passes these excited electrons to a primary electron acceptor of PS-1, creating "electron holes". The electron holes of P700 are filled by the pair of electrons received from the P680 (photosystem II) via electron transport chain.

 Electron flow from PS-I to NADP

 The primary electron acceptor of photosystem I passes the photoexcited electrons to a second electron transport chain: The electrons are accepted by ferredoxin (Fd). It is an iron containing protein. An enzyme called NADP reductase (flavoprotein enzyme) transfers the electrons from Fd to NADP NADP combines with electrons and hydrogen ions to form NADPH (reduced). The NADPH will provide reducing power for the synthesis DOAR of sugar in the Calvin cycle.

The path of electron transport through the two photosystems during non-cyclic photophosphorylation is known as Z-Scheme due to its conceptual zigzag shape.