Bio 226: Cell and Molecular Biology

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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
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• 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

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Light Rxns 3 stages 1) Catching a photon
(primary photoevent) 2) ETS 3) ATP synthesis by
chemiosmosis
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Light Reactions 1) Primary photoevent pigment
absorbs a photon
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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
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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
frequency Only energy is transferred e- returns
to ground state
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Photosystems Pigments 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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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
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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
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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
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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
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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
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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
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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
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Photosystems Dinoflagellates absorb light with
peridininchlorophyll a-proteins mix of
proteins the carotenoid peridinin that absorb _at_
480 channel to Chl a
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Photosystems Result in very different absorption
spectra
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Photosystems PSI performs cyclic
photophosphorylation Absorbs photon transfers
energy to P700
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cyclic photophosphorylation Absorbs photon
transfers energy to P700 transfers excited e-
from P700 to fd
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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
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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
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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
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Cyclic photophosphorylation first 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 photophosphorylation first 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 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
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• Cyclic photophosphorylation
• Ferredoxin branchpoint in cyclic PS FD reduces
  PQ

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• 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)

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Cyclic photophosphorylation 3) Fe/S reduces
plastocyanin via cyt f cyt b6 reduces PQ to form
PQ-
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Cyclic photophosphorylation 4) repeat process,
Fe/S reduces plastocyanin via cyt f cyt b6
reduces PQ- to form PQH2
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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)
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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- -gt 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- -gt 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- -gt 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- -gt 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- -gt 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 photophosphorylation Limitations Only
makes ATP
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Cyclic photophosphorylation Limitations Only
makes ATP Does not supply electrons for
biosynthesis no reducing power
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Photosystems PSI performs cyclic
photophosphorylation Makes 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 -gt added to PSI
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Photosystem II Evolved to provide reducing
power Added to PSI rxn 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 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)
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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) 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 steps each 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 steps each step uses a photon 2 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 PSI Each
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 PSI Each
e- drives cyt b6/f Use 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 PSI Each
e- drives cyt b6/f Use PMF to make ATP PSII
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 ATP Light absorbed
by PS I makes reducing power
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cyclic non-cyclic Ultimate e-
source None water O2 released? No yes Termin
al e- acceptor None NADP Form in which energy
is ATP ATP temporarily captured NADPH Photos
ystems required PSI PSI PSII
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Z-scheme energetics
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• Physical organization of Z-scheme
• PS II consists of P680 (a dimer of chl a)
• 30 other chl a a few carotenoids
• gt 20 proteins
• D1 D2 bind P680 all e- carriers

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• 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

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• 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

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Physical organization of Z-scheme D1 D2 bind
P680 all e- carriers Synechoccous elongatus
associates phycobilisomes cf LHCII with PSII
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Physical organization of Z-scheme D1 D2 bind
P680 all e- carriers Synechoccous elongatus
associates phycobilisomes cf LHCII with PSII
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• Physical organization of Z-scheme
• 2 mobile carriers
• 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-scheme 2 mobile
carriers 1) plastoquinone hydrophobic molecule
like ubiquinone headgroup alternates between
quinone and quinol Carries 2 e- 2 H diffuses
within bilayer
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Physical organization of Z-scheme 2 mobile
carriers 1) plastoquinone 2) plastocyanin (PC)
peripheral membrane protein of thylakoid lumen
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Physical organization of Z-scheme 2) plastocyanin
(PC) peripheral membrane protein of thylakoid
lumen Cu is alternately oxidized
reduced carries 1 e- 1 H
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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
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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