Photosynthesis Converts light to chemical energy 6 CO 2 + 6 H 2 O + light energy C 6 H 12 O 6 + 6 O 2.

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    13-Jan-2016

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  • Photosynthesis Converts light to chemical energy6 CO2 + 6 H2O + light energy C6H12O6 + 6 O2

  • Photosynthesis 2 sets of rxns in separate parts of chloroplast

  • Photosynthesis 1) Light rxnsuse light to pump H+use pH to make ATP by chemiosmosis

  • Photosynthesis 1) Light rxnsuse light to pump H+use pH to make ATP by chemiosmosis2) Light-independent (dark) rxns use ATP &NADPH from light rxnsto make organics

  • Photosynthesis 1) Light rxnsuse light to pump H+use pH to make ATP by chemiosmosis2) Light-independent (dark) rxns use ATP &NADPH from light rxnsto make organicsonly link: each providessubstrates needed by theother

  • Important structural features of chloroplastsvery large organelles: 5-10 m long, 2-4 m wide

  • Important structural features of chloroplasts3 membranes1) outer envelopepermeable to molecules up to 10 kDa due to porins

  • Important structural features of chloroplasts3 membranes1) outer envelope2) inner envelopeimpermeable: all import/export is via transporters

  • Important structural features of chloroplasts1) outer envelope2) inner envelope3) thylakoids: Stromal membranes

  • Important structural features of chloroplasts3) thylakoids: Stromal membranesa) grana: stacks of closely appressed membranesb) stromal lamellae: single thylakoids linking grana

  • Important structural features of chloroplastsAll cp membranes have MGDG, DGDG & SLthylakoids only have MGDG, DGDG, SL & PGthylakoid lipids have many trienoic fatty acids most fluid membranes known

  • Important structural features of chloroplastsStroma is pH 8.0 in lightthylakoid lumen is < 5Stroma is full of proteinalso contains DNA & genetic apparatus

  • Light Rxns3 stages1) Catching a photon (primary photoevent)

  • Light Rxns3 stages1) Catching a photon (primary photoevent)2) ETS

  • Light Rxns3 stages1) Catching a photon (primary photoevent)2) ETS3) ATP synthesis by chemiosmosis

  • Catching photonsphotons: particles of energy that travel as wavesEnergy inversely proportional to wavelength () visible light ranges from 400 -700 nm

  • Catching photonsPhotons: particles of energy that travel as wavescaught by pigments: molecules that absorb light

  • PigmentsCan only absorb certain photons

  • PigmentsCan only absorb certain photonsPhoton has exact energy to push an e- to an outer orbital

  • PigmentsCan only absorb certain photonsPhoton has exact energy to push an e- to an outer orbitalfrom ground to excited state

  • PigmentsPhoton has exact energy to push an e- to an outer orbitalfrom ground to excited stateeach pigment has an absorption spectrum: l it can absorb

  • PigmentsChlorophyll a is most abundant pigmentchlorophyll a looks green-> absorbs all l but greenReflects green

  • Accessory Pigments absorb l which chlorophyll a misses chlorophyll b is an importantaccessory pigment

  • Accessory Pigments absorb l which chlorophyll a misseschlorophyll b is an important accessory pigmentothers include xanthophylls & carotenoids

  • Accessory Pigments action spectrum shows use of accessory pigments l used for photosynthesis

  • Accessory Pigments action spectrum shows use of accessory pigments l used for photosynthesisplants use entire visible spectruml absorbed by chlorophyll work best

  • Light Reactions1) Primary photoevent: pigment absorbs a photon

  • Light Reactions1) Primary photoevent: pigment absorbs a photone- is excited -> moves to outer orbital

  • Light Reactions4 fates for excited e-:1) returns to ground state emitting heat & longer light = fluorescence

  • Light Reactions4 fates for excited e-:1) fluorescence2) transfer to another molecule

  • Light Reactions4 fates for excited e-:1) fluorescence2) transfer to another molecule3) Returns to ground state dumping energy as heat

  • 4 fates for excited e-:1) fluorescence2) transfer to another molecule3) Returns to ground state dumping energy as heat4) energy is transferred by inductive resonanceexcited e- vibrates and induces adjacent e- to vibrate at same frequency

  • 4 fates for excited e-:4) energy is transferred by inductive resonanceexcited e- vibrates and induces adjacent e- to vibrate at same frequencyOnly energy is transferred

  • 4 fates for excited e-:4) energy is transferred by inductive resonanceexcited e- vibrates and induces adjacent e- to vibrate at same frequencyOnly energy is transferrede- returns to ground state

  • PhotosystemsPigments are bound to proteins arranged in thylakoids in photosystems arrays that channel energy absorbed by any pigment to rxn center chlorophylls

  • PhotosystemsPigments are bound to proteins arranged in thylakoids in photosystems arrays that channel energy absorbed by any pigment to rxn center chlsNeed 2500 chlorophyll to make 1 O2

  • PhotosystemsArrays that channel energy absorbed by any pigment to rxn center chls2 photosystems : PSI & PSIIPSI rxn center chl a dimer absorbs 700 nm = P700

  • PhotosystemsArrays that channel energy absorbed by any pigment to rxn center chls2 photosystems : PSI & PSIIPSI rxn center chl a dimer absorbs 700 nm = P700 PSII rxn center chl a dimerabsorbs 680 nm = P680

  • PhotosystemsEach may have associated LHC (light harvesting complex) (LHC can diffuse within membrane)PSI has LHCI: ~100 chl a, a few chl b & carotenoids

  • PhotosystemsEach may have associated LHC (light harvesting complex) (LHC can diffuse within membrane)PSI has LHCI: ~100 chl a, a few chl b & carotenoidsPSII has LHCII: ~250 chl a, many chl b & carotenoidsProteins of LHCI & LHCII also differ

  • PhotosystemsPSI performs cyclic photophosphorylationAbsorbs photon & transfers energy to P700

  • cyclic photophosphorylationAbsorbs photon & transfers energy to P700transfers excited e- from P700 to fd

  • cyclic photophosphorylationAbsorbs photon & transfers energy to P700transfers excited e- from P700 to fdfd returns e- to P700 via PQ, cyt b6/f & PC

  • cyclic photophosphorylationAbsorbs photon & transfers energy to P700transfers excited e- from P700 to fdfd returns e- to P700 via PQ, cyt b6/f & PC Cyt b6/f pumps H+

  • Cyclic PhotophosphorylationTransfers excited e- from P700 to fdFd returns e- to P700 via cyt b6-f & PCCyt b6-f pumps H+Use PMF to make ATP

  • Cyclic photophosphorylationfirst step is from P700 to A0 (another chlorophyll a)charge separation prevents e- from returning to ground state = true photoreaction

  • Cyclic photophosphorylationfirst step is from P700 to A0 (another chlorophyll a)next transfer e- to A1 (a phylloquinone)next = 3 Fe/S proteins

  • Cyclic photophosphorylationfirst step is from P700 to A0 (another chlorophyll a)next transfer e- to A1 (a phylloquinone)next = 3 Fe/S proteinsfinally ferredoxin

  • Cyclic photophosphorylationFerredoxin = branchpoint: in cyclic PS FD reduces PQ

  • Cyclic photophosphorylationFerredoxin reduces PQPQH2 diffuses to cyt b6/f2) PQH2 reduces cyt b6 and Fe/S, releases H+ in lumensince H+ came from stroma, transports 2 H+ across membrane (Q cycle)

  • Cyclic photophosphorylation3) Fe/S reduces plastocyanin via cyt fcyt b6 reduces PQ to form PQ-

  • Cyclic photophosphorylation 4) repeat process, Fe/S reduces plastocyanin via cyt fcyt b6 reduces PQ- to form PQH2

  • Cyclic photophosphorylation 4) repeat process, Fe/S reduces plastocyanin via cyt fcyt b6 reduces PQ- to form PQH2Pump 4H+ from stroma to lumen at each cycle (per net PQH2)

  • Cyclic photophosphorylation 5) PC (Cu+) diffuses to PSI, where it reduces an oxidized P700

  • Cyclic photophosphorylationenergetics: light adds its energy to e--> excited stateEo' P700 = +0.48 VEo' P700* = -1.3 V

  • Cyclic photophosphorylationenergetics: light adds its energy to e--> excited stateEo' P700 = +0.48 VEo' P700* = -1.3 VEo' fd = - 0.42 V

  • Cyclic photophosphorylationenergetics: light adds its energy to e--> excited stateEo' P700 = +0.48 VEo' P700* = -1.3 VEo' fd = - 0.42 VEo' cyt b6/f = +0.3V

  • Cyclic photophosphorylationenergetics: light adds its energy to e--> excited stateEo' P700 = +0.48 VEo' P700* = -1.3 VEo' fd = - 0.42 VEo' cyt b6/f = +0.3VEo' PC = +0.36V

  • Cyclic photophosphorylationenergetics: light adds its energy to e--> excited stateEo' P700 = +0.48 VEo' P700* = -1.3 VEo' fd = - 0.42 VEo' cyt b6/f = +0.3VEo' PC = +0.36Ve- left in excited statereturns in ground state

  • Cyclic photophosphorylatione- left in excited statereturns in ground stateEnergy pumped H+

  • Cyclic photophosphorylationLimitations Only makes ATP

  • Cyclic photophosphorylationLimitations Only makes ATPDoes not supply electrons for biosynthesis = no reducing power

  • PhotosystemsPSI performs cyclic photophosphorylationMakes ATP but not NADPH: exact mech for PQ reduction unclear, but PQ pumps H+

  • Photosystem IIEvolved to provide reducing power-> added to PSI

  • Photosystem IIEvolved to provide reducing powerAdded to PSIrxn center absorbs 680 nm (cf 700 nm)

  • Photosystem IIrxn center absorbs 680 nm (cf 700 nm)can oxidize H2Oredox potential of P680+ is + 1.1 V (cf + 0.82 V for H2O)

  • Photosystem IIrxn center absorbs 680 nm (cf 700 nm)can oxidize H2Oredox potential of P680+ is + 1.1 V (cf + 0.82 V for H2O)Use e- from H2O to reduce NADP+ (the e- carrier used for catabolic reactions)

  • Photosystem IIrxn center absorbs 680 nm (cf 700 nm)can oxidize H2Oredox potential of P680+ is + 1.1 V (cf + 0.82 V for H2O)Use e- from H2O to reduce NADP+ (the e- carrier used for catabolic reactions)use NADPH c.f. NADH to prevent cross-contaminating catabolic& anabolic pathways

  • PSI and PSII work together in the Z-scheme - a.k.a. non-cyclic photophosphorylationGeneral idea: redox potential from H2O to NADP+ is so great that must boost energy of H2O e- in 2 steps

  • PSI and PSII work together in the Z-scheme General idea: redox potential from H2O to NADP+ is so great that must boost energy of H2O e- in 2 stepseach step uses a photon

  • PSI and PSII work together in the Z-scheme General idea: redox potential from H2O to NADP+ is so great that must boost energy of H2O e- in 2 stepseach step uses a photon2 steps = 2 photosystems

  • PSI and PSII work together in the Z-scheme 1) PSI reduces NADP+

  • PSI and PSII work together in the Z-scheme 1) PSI reduces NADP+ e- are replaced by PSII

  • PSI and PSII work together in the Z-scheme 2) PSII gives excited e- to ETS ending at PSI

  • PSI and PSII work together in the Z-scheme 2) PSII gives excited e- to ETS ending at PSIEach e- drives cyt b6/f

  • PSI and PSII work together in the Z-scheme 2) PSII gives excited e- to ETS ending at PSIEach e- drives cyt b6/fUse PMF to make ATP

  • PSI and PSII work together in the Z-scheme 2) PSII gives excited e- to ETS ending at PSIEach e- drives cyt b6/fUse PMF to make ATPPSII replaces e- from H2O forming O2

  • PSI and PSII work together in the Z-scheme Light absorbed by PS II makes ATPLight absorbed by PS I makes reducing power

  • cyclic non-cyclicUltimate e- sourceNonewaterO2 released?NoyesTerminal e- acceptorNoneNADP+Form in which energy isATPATP &temporarily capturedNADPHPhotosystems requiredPSIPSI & PSII

  • Z-scheme energetics

  • Physical organization of Z-schemePS II consists of: P680 (a dimer of chl a) ~ 30 other chl a & a few carotenoids> 20 proteins D1 & D2 bind P680 & all e- carriers

  • Physical organization of Z-schemePSII has 2 groups of closely associated proteins1) OEC (oxygen evolving complex) on lumen side, near rxn center Ca2+, Cl- & 4 Mn2+

  • Physical organization of Z-schemePSII also has two groups of closely associated proteins1) OEC (oxygen evolving complex) on lumen side, near rxn center Ca2+, Cl- & 4 Mn2+2) variable numbers of LHCII complexes

  • Physical organization of Z-scheme2 mobile carriersplastoquinone : lipid similar to ubiquinone

  • Physical organization of Z-scheme2 mobile carriers1) plastoquinone : lipid similar to ubiquinone headgroup alternates between quinone & quinol

  • Physical organization of Z-scheme2 mobile carriers1) plastoquinone : lipid similar to ubiquinone headgroup alternates between quinone & quinolCarries 2 e- & 2 H+

  • Physical organization of Z-scheme2 mobile carriers1) plastoquinone : hydrophobic molecule like ubiquinone headgroup alternates between quinone and quinolCarries 2 e- & 2 H+ diffuses within bilayer

  • Physical organization of Z-scheme2 mobile carriers1) plastoquinone 2) plastocyanin (PC) : peripheral membrane protein of thylakoid lumen

  • Physical organization of Z-scheme2) plastocyanin (PC) : peripheral membrane protein of thylakoid lumenCu is alternately oxidized & reducedcarries 1 e- & 1 H+

  • Physical organization of Z-scheme3 protein complexes (visible in EM of thylakoid)1) PSI2) PSII3) cytochrome b6/f 2 cytochromes & an Fe/S protein

  • Physical organization of Z-scheme2 mobile carriers1) plastoquinone 2) plastocyanin (PC)3 protein complexes 1) PSI2) PSII3) cytochrome b6/f ATP synthase (CF0-CF1 ATPase) is also visible in E/M

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