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Photophosphorylation

Photophosphorylation. Light as an Energy Source. Joseph Priestly in 18 th century (plant and candle in a Bottle). Validates Light as an Energy Source. hv. CO 2 + H 2 O. C n H 2n O n + O 2 (starch, sucrose).

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Photophosphorylation

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  1. Photophosphorylation

  2. Light as an Energy Source Joseph Priestly in 18th century (plant and candle in a Bottle) Validates Light as an Energy Source hv CO2 + H2O CnH2nOn + O2 (starch, sucrose) • Light energy is used to reduce C on CO2, reverse of oxidative phosphorylation • This amazing ability changed the biosphere as we know it today (most ground shattering events in earth’s history

  3. Visible Light Harnessed by life to sustain life (produce 1011 tons of carbs=1018 Kj of Energy)

  4. Photosynthesis is a two staged process by which light energy is harnessed to oxidize H2O Light Rxns hv H2O NADPH + O2 e- flow H+ H+ H+ ATP Dark Rxns Rubisco Ribulose-1,5-BiPhosphate + CO2 + NADPH + ATP 3-Phosphoglycerate Gluconeogenesis CnH2nOn

  5. The Chloroplast • very similar to the mitochondria (permeable outer mem, impermeable inner mem) • Stroma contains enzymes involved in dark rxns, DNA/RNA/ribosomes • Stroma surronds 3rd membrane of thylakoid • Thylakoid mem contains proteins involved in, ATP production, light harvesting, transport • Harvest light using Photoreceptors comprised of pigments

  6. Chlorophyll is a Porphyrin Like Heme • chlorophyll (chl) is the principle photoreceptor in plants • Variety of different chl exist throughout light harvesting organisms • R group greatly affects the absorption spectra • highly conjugated allowing strong absorption • Chla is the most common chl in the plant kingdom • Forms the LIGHT HARVESTING COMPLEX along with other pigments

  7. Light Harvesting Complex (LHC) hv H2O O2 LHC Photosynthetic rxn center (PRC) NADPH ATP e- mobilized chla is the principle photoreceptor Primary rxns of photosynthesis Highest amount of chla LHC also contains accessory pigments to cover spectral regions Chla cannot absorb

  8. Accessory Pigments Brilliant fall colours of deciduous plants Β-carotene Water dwelling photosynthetic (long λ) Red algae which can be found well below sea level (ocean flours)

  9. Light Rxns • Photosynthesis is the process by which excited e- from Chla are passed through acceptor molecules converting electrical energy into chemical energy • Energy of a photon can be determined by: E = hv = hc/λ Where E=energy, h=Plank’s constant c=speed of light, λ=wavelength Photons are absorbed by chromophores at a certain wavelengths, hence, the law of conservation of energy applies ( energy difference between the ground state and excited states of the e- must match energy of the photon) During photosynthesis, light excited e- may dissipate energy in two fashions A) Exciton transfer (LHC) B) Photooxidation (PRC)

  10. Upon absorption of photons, e- are excited to another E level This energy can be dissipated in several manners Such as Photooxidation or Exciton transfer

  11. Photosystem I (PS I) and Photosystem II (PS II) Photosynthesis is a noncyclic process that uses reducing power, provided by light energy, to oxidize H2O. These events allow the production of ATP and NADPH PS II = P 680, oxidize H2O produce O2 and QBH2 PS I = P 700, produce NADPH hv hv ATP ATP, NADPH, and O2 production are at maximum when both complexes are absorbing light

  12. Mediators of Light Rxns hv NOTE THE SIMILARITIES WITH OXPHOS

  13. Direction of e- flow DCMU 8H+ 4H+ Thylakoid Stroma Tyrosyl radical (Z) • the various prosthetic groups are organized in a Z-scheme • depicts two photochemical events that drive e- from H2O to NADP+

  14. The OEC, H2O oxidation, and PS II P 680+ most powerful oxidant in nature. Captures e- from OEC through tyrosyl radical (Z) High positive redox potential Mn (III-IV) during e- removal 4 step process requiring 4 photons

  15. PS II* donates excited e- to Plastoquinone Pheophytin a (Chl no Mg2+) Plastoquinone (PQ)-Fe(II) complex • PQ which accepts 2e- for transport to Cyt b6f • QBH2 dissociates allowing anothor QB to bind

  16. Cytochrome b6f • Striking resemblance to Cyt C: ubiquinone oxidoreductase • several prosthetics (including 2 hemes) • 1 e- accepted, 2H+ are translocated (recall OEC) • generates much of the electrochemical gradient that drives • ATP production • Plastocyanin (PC, a Cu containing protein) transfers electrons • from cyt b6f to PS I Std redox potential = 0.370V Strained due to tetrahedral geometry

  17. PS I Complex • Several peptides, numerous Chla (A0, A1) , 25 carotenoid molecules, series of [4Fe-4S] (Fx, Fa, Fb), and 25 carotenoids • passage 1 e- at a time (from PC to P 700+) • electrons are passaged one at a time to Fd-NADP+ reductase in the stroma (once FAD is reduced it passes the electrons and protons to NADPH • Ferredoxin (Fd) serves as the e- mediator between PS I and Fd-NADP reductase

  18. Photophosphorylation • Chloroplasts produce ATP much in the same as mitochondria • H+ gradient is dissipated • intact thylakoid membrane • H+ translocating ATPase (cFocF1) resembles the mitochondrial equivalent (FoF1) • cFo and Fo are hydrophobic membrane spanners with H+ translocating abilities • cF1 and F1 are hydrophillic peripheral membrane proteins with several subunits; Α3β3γδε • ATP production prevented by DCCD, oligomycin, and protonophores (2,4-DNP, CCCP) H+ translocation Thylakoid lumen (acidic) Stroma (alkaline) H+ translocation Matrix (alkaline) Intermembrane space (acidic)

  19. Summary

  20. Light rxns Directly dependent on light ATP, NADPH, O2 Thylakoid lumen Dark rxns (Calvin Cycle) Dependent on products of light rxns CnH2nOn Stroma Dark Rxns

  21. Rubisco (Ribulose-1,5 Biphosphate Carboxylase/Oxygenase Rubisco (Lys)

  22. Oxygen inhibits Rubisco: Photorespiration • O2 can compete for the CO2 binding site on Rubisco • Produce 3-Phosphoglycerate and 3-Phosphoglycolate • The 3-Phosphoglycolate dissipates NADPH and ATP through Photorespiration • Serves to protect the chloroplast from photooxidative damage when CO2 is NOT available

  23. Regulation of Dark Rxns • CO2 and O2 compete for Rubisco • Rubisco functions optimally at pH of 8 • 3-phophoglycerate (alkaline-PG3-, acidic-PG2-). PG2- is an allosteric inhibitor of Rubisco and is present when stroma is alkaline • Rubisco activity is dependent on Mg2+ • 2-carboxyarabinitol-1-phosphate (CA1P) • Thioredoxin system (activates Fructose-1,6 Biphosphatase and Sedohepulose-Biphosphatase)

  24. Thioredoxin System

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