Overall scheme of photosynthesis and the separation of light

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YIELD ENERGY
CONSUME ENERGY
Overall scheme of photosynthesis and the
separation of light and C-assimilation
reactions CO2 H2O ? O2 (CH2O) Photosynthesis
occurs in some bacteria, algae, vascular
plants Light reactions 2H2O 2NADP ? 2 NADPH
2H O2
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O2 is an excellent electron acceptor, but H2O
is a very poor electron donor H2O ? ½ O2 2H
2e- E(V) -0.82
Light energy is used to drive the creation of a
good electron donor (NADPH), in addition to
the synthesis of ATP
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• Photosynthesis takes place in chloroplasts
• Two membranes outer is porous, inner is not
• Inner compartment has an aqueous region
  (stroma)
• Stroma contains flattened vesicles
  (thylakoids), stacked in grana,
• containing membrane-associated ATP synthesis
  machinery
• Light reactions occur in thylakoid membrane
• carbon assimilation reactions occur in
  stroma
• Chloroplasts produce ATP during daylight
• mitochondria produce ATP during darkness

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Absorption of visible light by chromophores
chlorophylls a and b, together with accessory
pigments, allow most of the suns energy to be
harvested. Absorption in the red and blue areas
by plants gives them their green color only
green light is left to reflect or
transmit Absorption of 1 mol of photons yields
170-300 kJ ATP synthesis 50 kJ/mol
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Structures of the important chromophores in
photosynthesis. Primary pigments are
chlorophylls a, b (plants), bacteriochlorophyll
(bacteria), and phycobilins (algae,
cyanobacteria). Polyene structure in 5-membered
ring system (larger than heme). B-carotene and
lutein are accessory pigments in plants (extend
absorption range)
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Light-harvesting complex from plants
chromophores are bound to protein and are in a
fixed geometric orientation an LHC trimer
has 36 chlorophylls and 6 luteins
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Thylakoid membranes are functionally arranged
into photosystems with 50-200 chromophores
per system Light is transduced to chemical
energy only in the photochemical reaction
center a special pair of two chlorophyll a
molecules The function of most of the
chromophores in a photosystem is to harvest
light photons and to transfer, by exciton
transfer, the energy successively to the
reaction center.
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Thylakoid membranes are functionally arranged
into photosystems with 50-200 chromophores
per system Light is transduced to chemical
energy only in the photochemical reaction
center a special pair of two chlorophyll a
molecules The function of most of the
chromophores in a photosystem is to harvest
light photons and to transfer, by exciton
transfer, the energy successively to the
reaction center.
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There are several classes of photosynthetic
reaction centers I. Single reaction centers are
the Type I Fe-S (green sulfur bacteria) and the
Type II Pheophytin-Quinone (purple bacteria) II.
Double reaction centers (cyanobacteria, algae,
vascular plants)
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• Purple bacteria (left) Light photons excite the
  P870 chromophore,
• which passes an electron to pheophytin.
• The electron is then passed to Q, to the
  cytochrome bc1 complex,
• cytochrome 2, and back to P870. Electron flow
  is cyclic.
• A proton gradient is generated by the downhill
  flow of electrons
• to drive ATP synthesis

Cyt bc1 complex similar to complex III of
oxidative phosphorylation ATP synthase very
similar to that found in mitochondria
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Xray structure of purple bacterium reaction
center three integral membrane subunits plus
cytochrome c (yellow) outside the membrane
Space-filling models are the chromophores in
the membrane and in the cytochrome c
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• Structure of purple bacterium photosynthetic
• reaction center together with rapid reaction
• kinetics allows assignment of the
• sequence of events in electron transfer
• Light excites the special Chl2 pair
• The electron is rapidly passed to pheophytin
• The electron rapidly moves to quinone A
• There is a slow transfer, through nonheme
• iron, to quinone B, which is diffusible
• The electron hole in Chl2 is filled by
• transfer from a heme of cytochrome c,
• after cycling through cytochrome bc1
• The Chl2 pair is an excellent electron
  donor
• (E -1.0 V) reactions are highly
  favored
• thermodynamically fixing of chromophore
• geometry by protein ensures efficiency

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• Green-sulfur bacteria (right) Also features
  P840, cytochrome bc1 and cytochrome c,
• and also couples electron flow to ATP synthesis
  via a proton gradient
• Unique feature of green-sulfur bacteria Some
  electrons from P840 are passed to
• the redox protein ferredoxin, thru a
  reductase, to produce NADH
• Coupled reaction H2S is oxidized to S, then to
  SO42-, to provide electrons to P840

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• In plant chloroplasts, photosynthesis
• requires two reaction centers
• The overall Z scheme can operate
• in cyclic or noncyclic modes
• H2O is oxidized and NADP
• is ultimately reduced to NADPH
• Two photon absorptions are needed
• to drive the reduction, in PSI/PSII
• Oxygen is evolved in a specific
• complex (water-splitting complex)
• A proton gradient again drives
• ATP synthesis

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• PSII resembles the purple bacteria
• reaction center, and PSI resembles
• the green-sulfur bacterial center
• The oxidation of H2O by plants is
• analogous to the H2S oxidation
• by green-sulfur bacteria both
• reactions replace activated electrons
• Oxygenic photosynthesis if O2 is
• produced distinguish anoxygenic
• Plastocyanin is a diffusible small
• protein (one-electron carrier) that
• shuttles between PSII and PSI
• same role as cytochrome c in
• mitochondrial electron transport
• All organisms that evolve O2 have
• two coupled photosystems

• Overall Z-scheme -- 2 H2O 2 NADP 8 hv ? O2
  2 NADPH 2 H
• One electron transferred per 2 photons absorbed
  (one each in PSI and PSII)
• Forming one O2 requires 4 electron transfers (8
  photons)

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• PSII mechanism. D1-D2 dimer, but
• electron flow thru only D1.
• Mn4 cluster is the active site for the
• water-splitting enzyme
• Electrons removed from water replace
• those excited by light in P680
• After excitations, electrons flow thru
• carriers to quinone PQB and
• eventually to cytochrome b6f

• PSI mechanism. Excited electron
• from P700 (Chl)2 pair is
• transferred thru a long chain
• eventually to ferredoxin
• FerredoxinNADP oxidoreductase
• transfers electrons to NADP,
• to yield NADPH and oxidized
• ferredoxin
• Plastocyanin resupplies P700
• with electrons

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PROTEINS OMITTED
ALL THE GREEN/YELLOW IS CHROMOPHORES
Crystal structure of PSI with associated antennae
chlorophylls chlorophylls are green,
carotenoids are yellow and Fe-S centers are
red/orange Role of protein Hold chromophores in
fixed, rigid orientations to maximize exciton
transfer efficiencies to P700
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Connection between the PSII and PSI centers
Plastoquinol formed in PSII is oxidized by the
cytochrome b6f complex Sets up a Q-cycle that
generates a proton gradient for driving ATP
synthesis Analogous function to Complex III in
the mitochondrial transport chain Protons are
pumped from the stroma to the lumen
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Water-splitting activity in the oxygen-evolving
complex
The ultimate source of electrons is
water Splitting water requires the energy in four
photons 2H2O ? 4 H 4 e- O2 Each photon
into P680 causes the ejection of an electron
which is replaced by funneling from the Mn
complex through protein. After four electrons
have been ejected, the Mn complex has a charge of
4 The Mn4 complex then accepts four electrons
from 2 waters to generate oxygen P680 can
accept only one electron at a time, requiring
evolution of this device Released protons go into
the thylakoid lumen ? this complex is also a
proton pump
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Stoichiometry 2 H2O 8hv 2 NADP 3 ADP
3 Pi ? O2 3 ATP 2 NADPH
About 12 H transferred result in 3 ATP
An ATP synthase in chloroplasts harnesses the
proton gradient generated by the cytochrome
b6f complex, to synthesize ATP
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Comparative topologies of oxidative
phosphorylation and photophosphorylation
Inside (matrix) of mitochondria is equivalent to
the outside (stroma) of the thylakoid membrane
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Cyanobacteria no mitochondria or
chloroplasts, but do both oxidative
phosphorylation and photophosphorylation Solubl
e electron carrier, proton gradient generation,
and ATP synthase are used in
common Endosymbiosis evolutionary progenitor
of modern plant cells engulfed a
photosynthetic cyanobacterium cyanobacteria
possess both PSI and PSII
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• An alternative path of light-induced
• electron flow allows control of
• the amounts of ATP and NADPH
• that are made
• This is a cyclic pathway electrons
• are partitioned at ferredoxin
• some return to the
• cytochrome b6f complex and
• do not reduce NADP to NADPH
• Only PSI is involved in this pathway
• No O2 or NADPH is made, but
• protons are pumped and ATP is
• synthesized
• The cyclic photophosphorylation
• resembles that found in the green
• sulfur bacteria

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Physical organization of photosynthetic complexes

• Photosystems I and II each have their own antenna
  system
• Photosystems I and II are physically segregated
• PSI can be excited with higher wavelength light
  need to prevent
• exciton transfer from PSII (higher energy
  photon) to PSI
• PSI has more access to the stroma, which contains
  NADP and ADP