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

1) outer envelope

2) inner envelope

3) thylakoids: stromal membranes: most fluid known•PSI & ATP synthase are on outside•PSII is on inside of grana

Light Rxns3 stages

1) Catching a photon (primary photoevent)2) ETS3) ATP synthesis by chemiosmosis

Light Reactions1) Primary photoevent: pigment absorbs a photon

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 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

PhotosystemsCyanobacteria & red algae associate phycobilisomes cf LHCII with PSII = proteins that absorb light & pass energy to rxn center chlAbsorb 500-650 nmPE= phycoerythrin:Absorbs 500 & 570PC= phycocyaninAbsorbs 620AP = allophycocyaninAbsorbs 650

Photosystemsgreen sulfur bacteria absorb light with chlorosomes = mix of proteins, carotenoids and Bact Chl c that channel light to Bact Chl a (795 nm)then rxn center p840

PhotosystemsDinoflagellates absorb light with peridinin–chlorophyll a-proteins = mix of proteins & the carotenoid peridinin that absorb @ 480 & channel to Chl a

PhotosystemsResult in very different absorption spectra

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-schemeD1 & D2 bind P680 & all e- carriersSynechoccous elongatus associates phycobilisomes cf LHCII with PSII

Physical organization of Z-schemeD1 & D2 bind P680 & all e- carriersSynechoccous elongatus associates phycobilisomes cf LHCII with PSII

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