photosynthesis converts light to chemical energy 6 co 2 + 6 h 2 o + light energy c 6 h 12 o 6 + 6 o...
TRANSCRIPT
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Photosynthesis Converts light to chemical energy
6 CO2 + 6 H2O + light energy <=> C6H12O6 + 6 O2
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Photosynthesis 2 sets of rxns in separate parts of chloroplast
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Photosynthesis 1) Light rxnsuse light to pump H+
use ∆ pH to make ATP by chemiosmosis
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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
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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
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Important structural features of chloroplasts
very large organelles: 5-10 µm long, 2-4 µm wide
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Important structural features of chloroplasts3 membranes
1) outer envelopepermeable to molecules up to 10 kDa due to porins
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Important structural features of chloroplasts3 membranes
1) outer envelope2) inner envelope
impermeable: all import/export is via transporters
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Important structural features of chloroplasts
1) outer envelope
2) inner envelope
3) thylakoids:
Stromal membranes
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Important structural features of chloroplasts
3) thylakoids: Stromal membranes
a) grana: stacks of closely appressed membranes
b) stromal lamellae: single thylakoids linking grana
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Important structural features of chloroplastsAll cp membranes have MGDG, DGDG & SL
thylakoids only have MGDG, DGDG, SL & PGthylakoid lipids have many trienoic fatty acids most fluid membranes known
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Important structural features of chloroplasts
Stroma is pH 8.0 in light
thylakoid lumen is < 5
Stroma is full of protein
also contains DNA
& genetic apparatus
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Light Rxns3 stages
1) Catching a photon (primary photoevent)
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Light Rxns3 stages
1) Catching a photon (primary photoevent)2) ETS
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Light Rxns3 stages
1) Catching a photon (primary photoevent)2) ETS3) ATP synthesis by chemiosmosis
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Catching photonsphotons: particles of energy that travel as wavesEnergy inversely proportional to wavelength () visible light ranges from 400 -700 nm
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Catching photonsPhotons: particles of energy that travel as wavescaught by pigments: molecules that absorb light
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PigmentsCan only absorb certain photons
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PigmentsCan only absorb certain photonsPhoton has exact energy to push an e- to an outer orbital
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PigmentsCan only absorb certain photonsPhoton has exact energy to push an e- to an outer orbitalfrom ground to excited state
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PigmentsPhoton has exact energy to push an e- to an outer orbitalfrom ground to excited stateeach pigment has an absorption spectrum: it can absorb
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PigmentsChlorophyll a is most abundant pigmentchlorophyll a looks green-> absorbs all but greenReflects green
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Accessory Pigments absorb which chlorophyll a misses chlorophyll b is an importantaccessory pigment
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Accessory Pigments absorb which chlorophyll a misseschlorophyll b is an important accessory pigmentothers include xanthophylls & carotenoids
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Accessory Pigments action spectrum shows use of accessory pigments used for photosynthesis
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Accessory Pigments action spectrum shows use of accessory pigments used for photosynthesisplants use entire visible spectrum absorbed by chlorophyll work best
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Light Reactions1) Primary photoevent: pigment absorbs a photon
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Light Reactions1) Primary photoevent: pigment absorbs a photon
e- is excited -> moves to outer orbital
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Light Reactions4 fates for excited e-:1) returns to ground state emitting heat & longer light = fluorescence
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Light Reactions4 fates for excited e-:
1) fluorescence2) transfer to another molecule
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Light Reactions4 fates for excited e-:
1) fluorescence2) transfer to another molecule3) Returns to ground state dumping energy as heat
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4 fates for excited e-:1) fluorescence2) transfer to another molecule3) Returns to ground state dumping energy as heat4) energy is transferred by inductive resonance
excited e- vibrates and induces adjacent e- to vibrate at same frequency
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4 fates for excited e-:4) energy is transferred by inductive resonance
excited e- vibrates and induces adjacent e- to vibrate at same frequencyOnly energy is transferred
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4 fates for excited e-:4) energy is transferred by inductive resonance
excited e- vibrates and induces adjacent e- to vibrate at same frequencyOnly energy is transferrede- returns to ground state
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PhotosystemsPigments are bound to proteins arranged in thylakoids in photosystems arrays that channel energy absorbed by any pigment to rxn center chlorophylls
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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
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PhotosystemsArrays that channel energy absorbed by any pigment to rxn center chls2 photosystems : PSI & PSII
PSI rxn center chl a dimer absorbs 700 nm = P700
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PhotosystemsArrays that channel energy absorbed by any pigment to rxn center chls2 photosystems : PSI & PSII
PSI rxn center chl a dimer absorbs 700 nm = P700 PSII rxn center chl a dimerabsorbs 680 nm = P680
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PhotosystemsEach may have associated LHC (light harvesting complex) (LHC can diffuse within membrane)
PSI has LHCI: ~100 chl a, a few chl b & carotenoids
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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
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PhotosystemsPSI performs cyclic photophosphorylationAbsorbs photon & transfers energy to P700
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cyclic photophosphorylationAbsorbs photon & transfers energy to P700transfers excited e- from P700 to fd
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cyclic photophosphorylationAbsorbs photon & transfers energy to P700transfers excited e- from P700 to fdfd returns e- to P700 via PQ, cyt b6/f & PC
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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+
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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
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Cyclic photophosphorylationfirst step is from P700 to A0 (another chlorophyll a)charge separation prevents e- from returning to ground state = true photoreaction
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Cyclic photophosphorylationfirst step is from P700 to A0 (another chlorophyll a)next transfer e- to A1 (a phylloquinone)next = 3 Fe/S proteins
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Cyclic photophosphorylationfirst step is from P700 to A0 (another chlorophyll a)next transfer e- to A1 (a phylloquinone)next = 3 Fe/S proteinsfinally ferredoxin
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Cyclic photophosphorylation1) Ferredoxin = branchpoint: in cyclic PS FD reduces PQ
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Cyclic photophosphorylation1) Ferredoxin reduces PQ2) PQH2 diffuses to cyt b6/f2) PQH2 reduces cyt b6 and Fe/S, releases H+ in lumen
since H+ came from stroma, transports 2 H+ across membrane (Q cycle)
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Cyclic photophosphorylation3) Fe/S reduces plastocyanin via cyt fcyt b6 reduces PQ to form PQ-
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Cyclic photophosphorylation 4) repeat process, Fe/S reduces plastocyanin via cyt fcyt b6 reduces PQ- to form PQH2
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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)
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Cyclic photophosphorylation 5) PC (Cu+) diffuses to PSI, where it reduces an oxidized P700
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Cyclic photophosphorylation
energetics:
light adds its energy to e-
-> excited state
Eo' P700 = +0.48 V
Eo' P700* = -1.3 V
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Cyclic photophosphorylation
energetics:
light adds its energy to e-
-> excited state
Eo' P700 = +0.48 V
Eo' P700* = -1.3 V
Eo' fd = - 0.42 V
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Cyclic photophosphorylation
energetics:
light adds its energy to e-
-> excited state
Eo' P700 = +0.48 V
Eo' P700* = -1.3 V
Eo' fd = - 0.42 V
Eo' cyt b6/f = +0.3V
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Cyclic photophosphorylation
energetics:
light adds its energy to e-
-> excited state
Eo' P700 = +0.48 V
Eo' P700* = -1.3 V
Eo' fd = - 0.42 V
Eo' cyt b6/f = +0.3V
Eo' PC = +0.36V
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Cyclic photophosphorylation
energetics:
light adds its energy to e-
-> excited state
Eo' P700 = +0.48 V
Eo' P700* = -1.3 V
Eo' fd = - 0.42 V
Eo' cyt b6/f = +0.3V
Eo' PC = +0.36V
e- left in excited state
returns in ground state
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Cyclic photophosphorylation
e- left in excited state
returns in ground state
Energy pumped H+
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Cyclic photophosphorylationLimitations Only makes ATP
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Cyclic photophosphorylationLimitations Only makes ATPDoes not supply electrons for biosynthesis = no reducing power
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PhotosystemsPSI performs cyclic photophosphorylationMakes ATP but not NADPH: exact mech for PQ reduction unclear, but PQ pumps H+
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Photosystem II
Evolved to provide reducing power
-> added to PSI
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Photosystem IIEvolved to provide reducing powerAdded to PSIrxn center absorbs 680 nm (cf 700 nm)
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Photosystem II
rxn center absorbs 680 nm (cf 700 nm)
can oxidize H2O
redox potential of P680+ is
+ 1.1 V (cf + 0.82 V for H2O)
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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)
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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
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PSI and PSII work together in the “Z-scheme” - a.k.a. “non-cyclic photophosphorylation”General idea: ∆ redox potential from H2O to NADP+ is so great that must boost energy of H2O e- in 2 steps
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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
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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
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PSI and PSII work together in the “Z-scheme” 1) PSI reduces NADP+
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PSI and PSII work together in the “Z-scheme” 1) PSI reduces NADP+
e- are replaced by PSII
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PSI and PSII work together in the “Z-scheme” 2) PSII gives excited e- to ETS ending at PSI
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PSI and PSII work together in the “Z-scheme” 2) PSII gives excited e- to ETS ending at PSIEach e- drives cyt b6/f
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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
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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
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PSI and PSII work together in the “Z-scheme” Light absorbed by PS II makes ATPLight absorbed by PS I makes reducing power
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cyclic non-cyclicUltimate e- source None waterO2 released? No yesTerminal e- acceptor None NADP+Form in which energy is ATP ATP &temporarily captured NADPHPhotosystems required PSI PSI & PSII
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Z-scheme energetics
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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
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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+
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Physical organization of Z-schemePSII also has two groups of closely associated proteins
1) OEC (oxygen evolving complex) • on lumen side, near rxn center• Ca2+, Cl- & 4 Mn2+
2) variable numbers of LHCII complexes
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Physical organization of Z-scheme
2 mobile carriers
1) plastoquinone : lipid similar
to ubiquinone
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Physical organization of Z-scheme
2 mobile carriers
1) plastoquinone : lipid
similar to ubiquinone
“headgroup” alternates
between quinone & quinol
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Physical organization of Z-scheme
2 mobile carriers
1) plastoquinone : lipid
similar to ubiquinone
“headgroup” alternates
between quinone & quinol
Carries 2 e- & 2 H+
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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
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Physical organization of Z-scheme2 mobile carriers
1) plastoquinone 2) plastocyanin (PC) : peripheral membrane protein of thylakoid lumen
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Physical organization of Z-scheme2) plastocyanin (PC) : peripheral membrane protein of thylakoid lumen
Cu is alternately oxidized & reducedcarries 1 e- & 1 H+
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Physical organization of Z-scheme3 protein complexes (visible in EM of thylakoid)
1) PSI2) PSII3) cytochrome b6/f
2 cytochromes & an Fe/S protein
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Physical organization of Z-scheme2 mobile carriers
1) plastoquinone 2) plastocyanin (PC)
3 protein complexes 1) PSI2) PSII3) cytochrome b6/f
ATP synthase (CF0-CF1 ATPase) is also visible in E/M