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PHOTOPHOSPHORYLATION

PHOTOPHOSPHORYLATION. LIGHT-DRIVEN SYNTHESIS OF ATP. ATP is synthesized in PSII. Cytochrome bf pumps protons. Quinones and Plastocyanin are mobile transport agents. Pheophytin (ph). A o  A 1  F X  F A/B  F D  F DR. Plastoquinones. Cyto bf complex. Plastocyanin

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PHOTOPHOSPHORYLATION

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  1. PHOTOPHOSPHORYLATION LIGHT-DRIVEN SYNTHESIS OF ATP ATP is synthesized in PSII Cytochrome bf pumps protons Quinones and Plastocyanin are mobile transport agents

  2. Pheophytin (ph) Ao  A1  FX  FA/B  FD  FDR Plastoquinones Cyto bf complex Plastocyanin (Cu2+) H+ -1.3 -0.8 NADP+ NADPH (P700) PS-I (P680) PS-II

  3. PHOTOSYSTEMS 1. Each photosystem is an electron transport chain 2. Initial acceptor of excited electron has highest reducing potential of the system 3. As electron falls, free energy is made available to pump protons (PSII) or reduce NADP+ (PSI) 4. A water molecule provides the electron (PSII) 5. O2 (oxidized H2O) is the product

  4. Soluble Chlorophyll acceptor Phylloquinone (Vitamin K1) Ferredoxin Reductase Photosystem One (PSI) 1. Absorption maximum is at 700 nm NADP+ 2. Ferredoxin is recipient 3. Operates between +0.4 and -1.3 volts 4. Sequence is: Ao A1 FX FA/FB FD NADP+ FDR Iron-Sulfur Proteins NADPH

  5. E E E E E E E E E E O Mn Mn O Mn O O Mn WATER-SPLITTING COMPLEX O2 H2O H2O 2H+ 2H+ Tyrosine Z One electron at a time

  6. 4 P680 + 4H++ 2QB + 4 Photons 4 P680+ + 2QBH2 4 P680+ + 4Z 4 P680 + 4Z+ 4Z+ + [Mn complex]0 4Z + [Mn complex]4+ [Mn complex]4+ + 2H2O [Mn complex]0+ 4H+ + O2 Equations Light Reaction of PSII 2H2O + 2QB + 4 Photons O2 + 2QBH2

  7. How many “flashes” (photons) are required to evolve one oxygen molecule O2 per flash 0 4 8 12 16 20 Answer: 4

  8. Non-Cyclic Electron Flow 1. PSI is the more primitive system 2. PSI cannot make ATP 3. PSII replaces the electron displaced by PSI 4. PSII gets its electron from H2O 5. Z scheme is non-cyclic photophosphorylation

  9. Cyclic Electron Flow 1. Electrons do not go to NADP+ 2. Electrons go from FD Cyt bf PC 3. Cyt bf and NADP+ compete for electrons 4. NADP+ concentration controls the shunt 5. High NADPH/ NADP+ ratio favors Cyt bf 6. One ATP for 2 electrons shunted The purpose of cyclic photophosphorylation is to match ATP levels with NADPH levels to optimize the dark reaction processes.

  10. One electron carrier Out of visible range Photosynthetic Electron Transport System in purple photosynthetic bacteria 2 photons are required to reduce Q to QH2

  11. Arnon’s Observation Some of the energy captured by the photosynthetic systems of chloroplasts is transformed into phosphate bond energy of ATP Daniel Arnon, 1954 Jagendorf’s Observation A pH gradient across the thylakoid membrane is capable of furnishing the driving force to generate ATP. Andre Jagendorf, 1966

  12. light light 2H+ NADP+ Fd PS II Q NADPH PSI Cyt bf QH2 PC 2H2O 4H+ Lumen O2 CFo Stroma CF1 Proton is pumped out of thylakoid lumen into stroma ATP ADP + Pi H+

  13. Chloroplasts H+ H+ H+ CFo-CF1 ATPase ATPase is oriented out H+ H+ H+ H+ Jagendorf’s Experiment pH 4 buffer No light H+ H+ H+ H+ H+ H+ H+ ADP + 32P Quickly Raise to pH 8 ADP~32P (ATP) Chloroplasts synthesize ATP with a proton gradient

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