photosynthesis converts light to chemical energy 6 co 2 + 6 h 2 o + light energy c 6 h 12 o 6 + 6 o...
TRANSCRIPT
Photosynthesis Converts light to chemical energy
6 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 chloroplasts
very large organelles: 5-10 µm long, 2-4 µm wide
Important structural features of chloroplasts3 membranes
1) outer envelopepermeable to molecules up to 10 kDa due to porins
Important structural features of chloroplasts3 membranes
1) outer envelope2) inner envelope
impermeable: all import/export is via transporters
Important structural features of chloroplasts
1) outer envelope
2) inner envelope
3) thylakoids:
Stromal membranes
Important structural features of chloroplasts
3) thylakoids: Stromal membranes
a) grana: stacks of closely appressed membranes
b) stromal lamellae: single thylakoids linking grana
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
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
Light Rxns3 stages
1) Catching a photon (primary photoevent)
Light Rxns3 stages
1) Catching a photon (primary photoevent)2) ETS
Light Rxns3 stages
1) 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: it can absorb
PigmentsChlorophyll a is most abundant pigmentchlorophyll a looks green-> absorbs all but greenReflects green
Accessory Pigments absorb which chlorophyll a misses chlorophyll b is an importantaccessory pigment
Accessory Pigments absorb which chlorophyll a misseschlorophyll b is an important accessory pigmentothers include xanthophylls & carotenoids
Accessory Pigments action spectrum shows use of accessory pigments used for photosynthesis
Accessory Pigments action spectrum shows use of accessory pigments used for photosynthesisplants use entire visible spectrum absorbed by chlorophyll work best
Light Reactions1) Primary photoevent: pigment absorbs a photon
Light Reactions1) Primary photoevent: pigment absorbs a photon
e- 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 resonance
excited e- vibrates and induces adjacent e- to vibrate at same frequency
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
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
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 & PSII
PSI rxn center chl a dimer absorbs 700 nm = P700
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
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 photophosphorylation1) Ferredoxin = branchpoint: in cyclic PS FD reduces PQ
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)
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 photophosphorylation
energetics:
light adds its energy to e-
-> excited state
Eo' P700 = +0.48 V
Eo' P700* = -1.3 V
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
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
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
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
Cyclic photophosphorylation
e- left in excited state
returns in ground state
Energy 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 II
Evolved to provide reducing power
-> added to PSI
Photosystem IIEvolved to provide reducing powerAdded to PSIrxn center absorbs 680 nm (cf 700 nm)
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)
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 photophosphorylation”General 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- source None waterO2 released? No yesTerminal e- acceptor None NADP+Form in which energy is ATP ATP &temporarily captured NADPHPhotosystems required PSI PSI & 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 proteins
1) OEC (oxygen evolving complex) • on lumen side, near rxn center• Ca2+, Cl- & 4 Mn2+
2) variable numbers of LHCII complexes
Physical organization of Z-scheme
2 mobile carriers
1) plastoquinone : lipid similar
to ubiquinone
Physical organization of Z-scheme
2 mobile carriers
1) plastoquinone : lipid
similar to ubiquinone
“headgroup” alternates
between quinone & quinol
Physical organization of Z-scheme
2 mobile carriers
1) plastoquinone : lipid
similar to ubiquinone
“headgroup” alternates
between quinone & quinol
Carries 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 carriers
1) plastoquinone 2) plastocyanin (PC) : peripheral membrane protein of thylakoid lumen
Physical organization of Z-scheme2) plastocyanin (PC) : peripheral membrane protein of thylakoid lumen
Cu 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 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