Light reaction of photosynthesis

BOOK REFERENCE
1.TAIZ AND ZEIGER
2.LEHNINGER
3.PLANT PHYSIOLOGY BY R.K SINHA
4.HOPKINS

1. Light reaction of photosynthesis NAME: ARIF MAJID EN. NO.:GI3854 CLASS: MSc. F 1 ARIF MAJID MSc Final Advanced Plant Physiology

2. 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. 2 2

3. Chloroplast is the site of photosynthesis 3 Thylakoids : Site of light Reactions Stroma : Site of dark reactions

4. LIGHT REACTION The requirements are: Photosynthetic pigments Solar light (light of visible spectrum) Water Electron transporters Electron acceptors Proton acceptors 4

5. PHOTOSYNTHETIC PIGMENTS These 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 5

6. PHOTOSYNTHETIC PIGMENTS CHLOROPHYLLS 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 6

7. 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 7

8. 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 8

9. 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 9

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11. PHOTOSYSTEMS Large,multimolecular pigment protein complexes. Components of photosynthetic ETC. Oxygenic organisms contain both PSI & PSII. Anoxygenic organisms contain PSI only. PSI consists of reaction centre P 700 and higher ratio of chl a to chl b. PSII consists of reaction centre P 680 and equal no of chlorophyll a and b molecules. PSII have high efficiency than PSI. 11

12. Function of PSI & PSII 12

13. PHOTOCHEMICAL REACTION Absorption 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. 13

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15. 15 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

16. 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 O 2 , 4 electrons and 4 protons. Mn takes all the four electrons . Protons are utilized in photophosphorylation. 16

17. Z-SCHEME 17

18. Oxidation of water PSII is the only known complex that can oxidise H 2 O resulting in the release of O 2 into atmosphere. Water oxidation requires 2 molecules of H 2 O & involves 4 sequential turn over of the reaction centre. O 2 released by PSII occur with 4 flashes of light Each photochemical reaction creates an oxidant that removes 1eˊ. 18

19. Photolysis of water: KOK cycle 19

20. 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. 20

21. PHOTOPHOSPHORYLATION ATP 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. 21

22. Photophosphorylation 22

23. Herbicides blocking photosynthetic electron flow Dichlorophenyl dimetyl urea (DCMU) Paraquat 23

24. Conclusion of Light Reaction ATP 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. 24

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