Photosynthesis 1) Light rxns use light to pump H+ use ∆ pH to make ATP by chemiosmosis 2) Light-independent (dark) rxns use ATP & NADPH from light rxns to make organics only link: each provides substrates needed by the other

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 Rxns 3 stages 1) Catching a photon (primary photoevent) 2) ETS 3) ATP synthesis by chemiosmosis

Light Reactions 1) Primary photoevent: pigment absorbs a photon

4 fates for excited e-: 1) fluorescence 2) transfer to another molecule 3) Returns to ground state dumping energy as heat 4) 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 frequency Only energy is transferred e- returns to ground state

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

Photosystems Pigments are bound to proteins arranged in thylakoids in photosystems arrays that channel energy absorbed by any pigment to rxn center chls Need 2500 chlorophyll to make 1 O2

Photosystems Arrays that channel energy absorbed by any pigment to rxn center chls 2 photosystems : PSI & PSII PSI rxn center chl a dimer absorbs 700 nm = P700

Photosystems Arrays that channel energy absorbed by any pigment to rxn center chls 2 photosystems : PSI & PSII PSI rxn center chl a dimer absorbs 700 nm = P700 PSII rxn center chl a dimer absorbs 680 nm = P680

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

Photosystems Each may have associated LHC (light harvesting complex) (LHC can diffuse within membrane) PSI has LHCI: ~100 chl a, a few chl b & carotenoids PSII has LHCII: ~250 chl a, many chl b & carotenoids Proteins of LHCI & LHCII also differ

Photosystems Cyanobacteria & red algae associate phycobilisomes cf LHCII with PSII = proteins that absorb light & pass energy to rxn center chl Absorb 500-650 nm PE= phycoerythrin: Absorbs 500 & 570 PC= phycocyanin Absorbs 620 AP = allophycocyanin Absorbs 650

Photosystems green 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

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

Photosystems Result in very different absorption spectra

Photosystems PSI performs cyclic photophosphorylation Absorbs photon & transfers energy to P700

cyclic photophosphorylation Absorbs photon & transfers energy to P700 transfers excited e- from P700 to fd

cyclic photophosphorylation Absorbs photon & transfers energy to P700 transfers excited e- from P700 to fd fd returns e- to P700 via PQ, cyt b6/f & PC

cyclic photophosphorylation Absorbs photon & transfers energy to P700 transfers excited e- from P700 to fd fd returns e- to P700 via PQ, cyt b6/f & PC Cyt b6/f pumps H+

Cyclic Photophosphorylation Transfers excited e- from P700 to fd Fd returns e- to P700 via cyt b6-f & PC Cyt b6-f pumps H+ Use PMF to make ATP

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

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

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

Cyclic photophosphorylation Ferredoxin = branchpoint: in cyclic PS FD reduces PQ

Cyclic photophosphorylation Ferredoxin reduces PQ PQH2 diffuses to cyt b6/f 2) PQH2 reduces cyt b6 and Fe/S, releases H+ in lumen since H+ came from stroma, transports 2 H+ across membrane (Q cycle)

Cyclic photophosphorylation 3) Fe/S reduces plastocyanin via cyt f cyt b6 reduces PQ to form PQ-

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

Cyclic photophosphorylation 4) repeat process, Fe/S reduces plastocyanin via cyt f cyt b6 reduces PQ- to form PQH2 Pump 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 photophosphorylation Limitations Only makes ATP

Cyclic photophosphorylation Limitations Only makes ATP Does not supply electrons for biosynthesis = no reducing power

Photosystems PSI performs cyclic photophosphorylation Makes 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 II Evolved to provide reducing power Added to PSI rxn 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 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) Use e- from H2O to reduce NADP+ (the e- carrier used for catabolic reactions)

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) 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 steps each 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 steps each step uses a photon 2 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 PSI Each e- drives cyt b6/f

PSI and PSII work together in the “Z-scheme” 2) PSII gives excited e- to ETS ending at PSI Each e- drives cyt b6/f Use PMF to make ATP

PSI and PSII work together in the “Z-scheme” 2) PSII gives excited e- to ETS ending at PSI Each e- drives cyt b6/f Use PMF to make ATP PSII replaces e- from H2O forming O2

PSI and PSII work together in the “Z-scheme” Light absorbed by PS II makes ATP Light absorbed by PS I makes reducing power

Ultimate e- source None water O2 released? No yes cyclic non-cyclic Ultimate e- source None water O2 released? No yes Terminal e- acceptor None NADP+ Form in which energy is ATP ATP & temporarily captured NADPH Photosystems required PSI PSI & PSII

Z-scheme energetics

Physical organization of Z-scheme PS 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-scheme PSII has 2 groups of closely associated proteins 1) OEC (oxygen evolving complex) on lumen side, near rxn center Ca2+, Cl- & 4 Mn2+

Physical organization of Z-scheme PSII 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 D1 & D2 bind P680 & all e- carriers Synechoccous elongatus associates phycobilisomes cf LHCII with PSII

Physical organization of Z-scheme D1 & D2 bind P680 & all e- carriers Synechoccous elongatus associates phycobilisomes cf LHCII with PSII

Physical organization of Z-scheme 2 mobile carriers 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-scheme 2 mobile carriers 1) plastoquinone : hydrophobic molecule like ubiquinone “headgroup” alternates between quinone and quinol Carries 2 e- & 2 H+ diffuses within bilayer

Physical organization of Z-scheme 2 mobile carriers 1) plastoquinone 2) plastocyanin (PC) : peripheral membrane protein of thylakoid lumen

Physical organization of Z-scheme 2) plastocyanin (PC) : peripheral membrane protein of thylakoid lumen Cu is alternately oxidized & reduced carries 1 e- & 1 H+

Physical organization of Z-scheme 3 protein complexes (visible in EM of thylakoid) 1) PSI 2) PSII 3) cytochrome b6/f 2 cytochromes & an Fe/S protein

Physical organization of Z-scheme 2 mobile carriers 1) plastoquinone 2) plastocyanin (PC) 3 protein complexes 1) PSI 2) PSII 3) cytochrome b6/f ATP synthase (CF0-CF1 ATPase) is also visible in E/M