Electron Transport Chain & Oxidative Photophosphorylation

ELECTRON TRANSPORT CHAIN (ETC)
OXIDATIVE PHOTOPHOSPHORYLATION
CYCLIC PHOTOPHOSPHORYLATION
NON-CYCLIC PHOTOPHOSPHORYLATION
PHOTOLYSIS OF WATER
DIFFERENCE BETWEEN CYCLIC &NON-CYCLIC PHOTOPHOSPHORYLATION
ASSIMILATORY POWERS - ATP & NADPH

1. SALIMA K 5 th sem BSC Plant Science NSS College Manjeri PHOTOSYNTHETIC ELECTRON TRANSPORT & PHOTOPHOSPHORYLATION

2. CONTENTS INTRODUCTION ELECTRON TRANSPORT CHAIN (ETC) OXIDATIVE PHOTOPHOSPHORYLATION CYCLIC PHOTOPHOSPHORYLATION NON-CYCLIC PHOTOPHOSPHORYLATION PHOTOLYSIS OF WATER DIFFERENCE BETWEEN CYCLIC &NON-CYCLIC PHOTOPHOSPHORYLATION ASSIMILATORY POWERS - ATP & NADPH

3. INTRODUCTION The pigment systems PS-I and PS-II differ each other with regard the electron affinity. PS-I has lesser affinity for electrons than PS-II When these pigment systems are excited simultaneously with photons, ejection of electron occurs first from PS-1. The ejected high-energy electron moves down the electron transport chain. During this, oxidative photophosphorylation, ATP synthesis, and reduction of NADP occur.

4. ELECTRON TRANSPORT CHAIN (ETC) The electron transport chain (ETC) is a series of protein complexes that transfer electrons from electron donors to electron acceptors via redox reactions and couples this electron transfer with the transfer of protons (H+ ions) across a membrane. The electron transport chain is built up of peptides, enzymes, and other molecules.

5. ELECTRON TRANSPORT CHAIN (ETC) The flow of electrons through the electron transport chain is an exergonic process. The energy from the redox reactions create an electrochemical proton gradient that drives the synthesis of adenosine triphosphate (ATP) .

6. OXIDATIVE PHOTOPHOSPHORYLATION Photophosphorylation of ADP is the photosynthetic production of ATP from ADP and Pi, using light energy Oxidative photophosphorylation is the process in which ATP is formed as a result of the transfer of electrons by a series of electron carriers. Only two sources of energy are available : sun light energy from redox reaction. It is completed in two stages, namely cyclic photophosphorylation and non-cyclic photophosphorylation

7. CYCLIC PHOTOPHOSPHORYLATION In cyclic photophosphorylation, the high-energy electrons released from P 700 of PS- I flow down in a cyclic pathway. This results in a series of redox reactions. At first, the electrons move to a primary acceptor (X) and then to ferredoxin( Fd ). It is believed to be chemical nature primary acceptor is pheophytin which is a colourless , Mg- lacking chlorophyll-a. Fd is an iron-containing protein

8. CYCLIC PHOTOPHOSPHORYLATION From Fd the electrons cycle back to P 700 in a step-by-step manner through a chain of electron carrier .This is called cyclic electron transfer or cyclic electron flow . The major electron carriers of this cycle include cytochrome b 6 , plastaquinon (PQ-a terpenoid), cytochrome-f and plastocyanin (PC - a copper-containing protein). During this cyclic flow, electrons releases energy twice. ADP molecules capture this released energy and undergo photophosphorylation, resulting in the formation of ATP.

9. CYCLIC PHOTOPHOSPHORYLATION Since this phosphorylation is always coupled with the cyclic flow of electrons (or cyclic redox reactions), it is called cyclic oxidative photophosphorylation. After phosphorylation, the de-energized electrons return to the reaction center of PS-I to get re-energized and also to replace the electrons lost from it.

10. NON-CYCLIC PHOTOPHOSPHORYLATION Non-cyclic photophosphorylation is initiated by the light falling on PS-ll or P 680. In this case, high-energy electrons flow down non-cyclically from PS-ll to PS-I. This causes a series of redox reactions and the subsequent production of ATP. The generalized pathway of non-cyclic photophosphorylation is also called Z- pathway , considering its shape.

11. NON-CYCLIC PHOTOPHOSPHORYLATION Energized electrons from the P 680 of PS-II are at first accepted by a primary electron acceptor (P). From this reduced acceptor, electrons are shuttled to P 700 of PS-I through a chain of electron carriers. This is called non-cyclic electron transfer or non-cyclic electron flow . The major electron carriers between PS-II and PS-I include plastoquinone (PQ), cytochrome-b 6 , cytochrome-f and plastocyanin (PC).

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13. NON-CYCLIC PHOTOPHOSPHORYLATION During the non-cyclic flow, electrons release energy only one and this energy is trapped by ADP for the production of ATP through phosphorylation. Since this phosphorylation occurs during a non-cyclic electron flow, it is called non-cyclic photophosphorylation . After that, the de-energized electrons are transferred to P 700+. From P 700 electrons move in pairs non cyclically to NADP through a primary acceptor (X) and ferredoxin.

14. NON-CYCLIC PHOTOPHOSPHORYLATION This occurs only when electrons are initially made available from PS-II. Each NADP molecule accepts a pair of electrons. Simultaneously, two protons also enter it. This reduces NADP to NADPH + H and balances the negativity of electrons. This reduction of NADP to NADPH, using photon energy, is called photo-reduction of NADP . NADP + +2e-+ 2H  NADPH + H The NADPH would be used in the dark reaction for the fixation of CO 2 .

15. NON-CYCLIC PHOTOPHOSPHORYLATION The electrons and protons required for the photo- reduction of NADP are obtained by the photolysis of water. These electrons are supplied not directly, but indirectly through PS-II. But, the protons are directly available. The non-cyclic electron transport is important in photosynthesis, since it supplies assimilatory power in the form of NADPH, in addition to ATP, for CO 2 assimilation.

16. PHOTOLYSIS OF WATER Photolysis is the splitting of water molecules into proton, electron and oxygen in the presence of light . It takes place during non-cyclic photophosphorylation. The flow of electrons from PS-II causes electron deficiency in P 680 .This is often called " electron gap ” or " electron hole “. This deficiency is soon made up by the electrons released by the photolysis of water.

17. PHOTOLYSIS OF WATER H 2 O H + +OH - OH -  1 / 2 O 2 +H + +2e - H 2 O2H + +2e - + 1 / 2 O 2 The protons are used to reduce NADP and electrons are fed to photosystem II. Oxygen is released as a waste product. The electrons, released by the photolysis of water, are first carried to P 680 by a carrier system, namely Mn - protein system.

18. PHOTOLYSIS OF WATER From P 680 they flow to P 700 through the electron transporting chain. In P 700 they get boosted in high-energy levels by illumination. From there, they move to NADP through a second electron transport chain.

19. DIFFERENCE BETWEEN CYCLIC & N0N-CYCLIC PHOTOPHOSPHORYLATION CYCLIC PHOTOPHOSPHORYLATION NON-CYCLIC PHOTOPHOSPHORYLATION It refers to the synthesis of ATP during the light reaction of photosynthesis, coupling to a cyclic passage of electrons to and from P 700 It is the synthesis of ATP during the light reaction of photosynthesis in which a electron donor is required and oxygen is produced as a by product Only PS-I is involved Both PS I and PS II involved Reaction center is P700 Reaction center is P 680 Electron moves in a cyclic pattern. Electron moves in linear pattern Electrons are first-expelled from the reaction center of PS-I Electrons are first-expelled from the reaction center of PS II

20. CYCLIC PHOTOPHOSPHORYLATION NON-CYCLIC PHOTOPHOSPHORYLATION Electrons return to the P700 after passing through ETS Electrons return to the reaction center of P680 and are accepted by NADP+ Final electron acceptor is P 700 Final electron acceptor is NADP + It does not require an external electron donor Requires external electron donor like donor H 2 O or H 2 S Photolysis does not occur Photolysis occurs Oxygen is not produced Oxygen is produced Only ATP is produced Both ATP and NADPH + H + are synthesized Phosphorylation takes place at two places Phosphorylation takes place at only one place It is not sensitive to di chloro di methyl urea (DCMI) It is sensitive to DCMI and inhibits electron flow

21. CYCLIC PHOTOPHOSPHORYLATION NON-CYCLIC PHOTOPHOSPHORYLATION Occurs in plants, algae, and cyanobacteria Occurs in isolated chloroplasts and photosynthetic bacteria Occurs in anoxygenic photosynthesis Occurs in oxygenic photosynthesis

22. ASSIMILATORY POWERS The assimilatory power in photosynthesis is the power produced in the form of ATP and NADPH. Arnon (1956) used the term assimilatory power to refer ATP and NADPH. The molecules are formed during the evolution of oxygen and the photolysis Of water in the light reaction of photosynthesis. Nicotinamide adenine dinucleotide phosphate is the complete name of NADPH. And that of ATP is adenosine triphosphate - ATP & NADPH

23. ASSIMILATORY POWERS The process of formation of ATP from ADP and a inorganic phosphate utilizing light energy is called photophosphorylation. Assimilatory power helps in carbon dioxide fixation to form storage carbohydrate for the plant during the dark phase reactions ATP is commonly known as the energy currency of the cell. NADPH is an essential electron donor in all organisms, provides the reducing power for anabolic reactions and redox balance. - ATP & NADPH

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