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Light reaction of photosynthesis

NAME: ARIF MAJIDEN. NO.:GI3854CLASS: MSc. F

ARIF MAJID MSc Final

Advanced Plant Physiology

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PHOTOSYNTHESIS It is a process in which energy from sunlight is

transformed into chemical energy that is stored in carbohydrates and other organic molecules.

Two phases of photosynthesis 1. Light reaction or Light dependent phase. - The reactions occur in thylakoids. -The products are ATP and NADPH. 2. Dark Reactions or Light independent phase . - The reactions occur in stroma. -Products are Trioses, Tetroses, Pentoses, Hexoses

and Heptoses.

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Chloroplast is the site of photosynthesisThylakoids: Site of light ReactionsStroma: Site of dark reactions

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LIGHT REACTION The requirements are: Photosynthetic pigments Solar light (light of visible spectrum) Water Electron transporters Electron acceptors Proton acceptors

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PHOTOSYNTHETIC PIGMENTSThese are coloured complex organic molecules.Absorb solar energy in the visible range of

electromagnetic spectrum (B/w 400nm-700nm).Pigments absorb light due to possession of

conjugated double bonds (Polyenes).Types: - Chlorophylls: a,b ,c,d,e and bacteriochlorophylls - Carotenoids: Carotenes and Carotenols - Phycobilins: Phycoerythrin and Phcocyanin

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PHOTOSYNTHETIC PIGMENTSCHLOROPHYLLS

Distribution Absorption peaks(nm)

A All green plants 435,670-680

B All green plants 480,650

C Diatoms and brown algae

645

D Red algae 740

Protochlorophyll Etiolated plants

Bacterioviridin Green sulfur bacteria 750

Bacteriochlorophyll Purple sulphur bacteria 800,850,890

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PHOTOSYNTHETIC PIGMENTS

CAROTENES Distribution Absorption peaks

  α All green plants and red algae

420,440,470

  β All green plants 425,450,480

  γ Green sulphur bacteria 440,460,495

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PHOTOSYNTHETIC PIGMENTS CAROTENOLS Distribution Absorption peaks

Luteol All green plants and red algae

425,445,475

Violaxanthol All green plants 425,450,475

Fucoxanthol Diatoms and brown algae

425,450,475

Spirilloxanthol Purple bacteria 464,490,535

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PHOTOSYNTHETIC PIGMENTS PHYCOBILINS Distribution in plants Absorption peaks

Phycocyanins

Blue green algae 618

Phycoerythrins Red algae 490,546,576

Allophycocyanins Blue green and red algae

654

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PHOTOSYSTEMSLarge,multimolecular pigment protein complexes.Components of photosynthetic ETC.Oxygenic organisms contain both PSI & PSII.Anoxygenic organisms contain PSI only.PSI consists of reaction centre P700 and higher ratio of chl a

to chl b.PSII consists of reaction centre P680 and equal no of

chlorophyll a and b molecules.PSII have high efficiency than PSI.

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Function of PSI & PSII

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PHOTOCHEMICAL REACTIONAbsorption of light is the first step in any photochemical

reaction.Absorption of a photon excites chlorophyll molecules and

other (accessory) pigments, which funnel the energy into reaction centers.

In the reaction centers, photoexcitation results in a charge separation that produces a strong electron donor and a strong electron acceptor.

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1. Light excites an antenna molecules to a higher energy level.

2. The excited antenna molecule passes energy to a neighbouring chlorophyll molecule ,exciting it.

3. Ultimately this energy is transferred to reaction center exciting it.

4. The excited reaction center passes electron to an electron acceptor

5. The electron hole in reaction centre is filled by an electron from an electron donor.

The absorption of photon has caused charge separation in the reaction center

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Mechanism of Electron transport

Electron is shifted from PSII to Phaeophytin.PSII gets oxidized and Ph gets reduced.Tyrosine (Z) donates electron to PSII.Mn Donates electron to Z.After transfer of 4 electrons Water splits into O2, 4

electrons and 4 protons.Mn takes all the four electrons .Protons are utilized in photophosphorylation.

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Z-SCHEME

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Oxidation of water PSII is the only known complex that

can oxidise H2O resulting in the release of O2 into atmosphere.

Water oxidation requires 2 molecules of H2O & involves 4 sequential turn over of the reaction centre.

O2 released by PSII occur with 4 flashes of light

Each photochemical reaction creates an oxidant that removes 1eˊ.

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Photolysis of water: KOK cycle

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NADP⁺ REDUCTION

Electrons are continuously transferred from water through Mn, Z, PSII, Ph ,QA, QB, Cytbf, PC,PSI, AO, A1, Fd to NADP .⁺

Electrons are shifted as a result of redox reactions.NADP takes two electrons through ETC.⁺NADP takes one proton from stroma and changes ⁺

into NADPH.

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PHOTOPHOSPHORYLATIONATP synthesis with mediation of light.Protons are transferred from thylakoid space to stroma

through ATPase generating ATP.One ATP is formed after transfer of one pair of protons.Chemiosmosis is responsible in the formation of ATP.Chemiosmosis is induced by proton gradient and

protomotive force.

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Photophosphorylation

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Herbicides blocking photosynthetic electron flow

Dichlorophenyl dimetyl urea (DCMU) Paraquat

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Conclusion of Light ReactionATP is generated which is called Energy currency.NADP is reduced to NADPH called “Reducing ⁺

power’’.The two together are called “Assimilatory power’’.Both of these are utilized in dark phase.ATP is used in phosphorylation of 3PGA into 1,3 Bis-

PGA.NADPH reduces 1,3,bis-PGA into PG Ald.Oxygen is evolved.

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