Electron Transport System

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PhotosynthesisOverview of photosynthesis,
conversion of light energy to redox energy, and
the Z scheme
Bioc 460 Spring 2008 - Lecture 31 (Miesfeld)
Paraquat
Agricultural crops are the primary means for
converting solar energy into chemical energy for
life on earth
Solar power can provide a sustainable energy
source for some areas in the world
Paraquat is an inhibitor of PSI that was used to
kill marijuana plants
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Key Concepts in Photosynthesis

• The energy from sunlight is used to initiate
  photooxidation reactions in light-absorbing
  pigments that convert light energy into chemical
  energy.
• The light reactions of photosynthesis are similar
  to the electron transport system in that they
  require a proton impermeable membrane and a
  series of linked redox reactions to generate
  proton motive force used for ATP synthesis.
• Photooxidation uses the oxidation of H2O to
  produce O2 in a process that provides electrons
  for photophosphorylation and the reduction of
  NADP to produce NADPH.
• Chemical energy in the form of ATP and NADPH is
  used to convert CO2 to glyceraldehyde-3-P using
  enzymes in the Calvin Cycle pathway. Plants use
  sunlight during the day and aerobic respiration
  at night.

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• The photosynthetic electron transport system is
  often referred to as the light reactions of
  photosynthesis, whereas, the Calvin cycle has
  been called the dark reactions.
• However, the term "dark reactions" can be
  misleading because the Calvin cycle is most
  active in the light when ATP and NADPH levels are
  high.

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Net reaction of photosynthesis and carbon
fixation
H2O CO2 -(light energy)? (CH2O) O2

• The O2 generation is the result of H2O oxidation,
  whereas, the CO2 is used to synthesize
  carbohydrate (CH2O)
• It takes 2 H2O to make an O2 and six CO2
  molecules are required for the synthesis of each
  molecule of glucose, therefore
• ?Gº' for this reaction is 2868 kJ/mol!
• This is overcome by the energy potential stored
  in the products of photosynthetic electron
  transport, namely, ATP and NADPH.

6 H2O 6 CO2 -(light energy)? C6H12O6 6 O2
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The Biosphere Experiment, then and now!
Joseph Priestly - 1772

• Biosphere 2 did not actually work very well
  because CO2 levels built up inside the sealed
  environment and periodic CO2 removal was
  required. The first Biospherian came out after
  1 year.

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1. What do the photosynthetic electron transport
system and Calvin cycle accomplish for the cell?

• The photosynthetic electron transport system
  converts light energy into redox energy which is
  used to generate ATP by chemiosmosis and reduce
  NADP to form NADPH.
• Calvin cycle enzymes use energy available from
  ATP and NADPH to reduce CO2 to form
  glyceraldehyde-3-P, a three carbon carbohydrate
  used to synthesize glucose.
• Photosynthetic cells use the carbohydrate
  produced by the Calvin cycle as a chemical energy
  source for mitochondrial respiration in the dark.
  
• Photosynthetic organisms are autotrophs because
  they derive energy from light rather than from
  organic materials (as food).

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2. What are the overall net reactions of
photosynthetic electron transport system and the
Calvin cycle?

• Photosynthetic electron transport system
• (production of ATP and O2)
• 2 H2O 8 photons 2 NADP 3 ADP 3 Pi ?
• O2 2 NADPH 3 ATP
• Calvin cycle
• (six turns of cycle yields glucose)
• 6 CO2 12 NADPH 18 ATP 12 H2O ?
• Glucose 12 NADP 18 ADP 18 Pi

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3. What are the key enzymes in the photosynthetic
electron transport system and the Calvin cycle?

• Protein components of the photosynthetic electron
  transport system three protein complexes are
  required for the oxidation of H2O and reduction
  of NADP photosystem II (P680 reaction center),
  cytochrome b6f (proton pump) and photosystem I
  (P700 reaction center).
• Chloroplast ATP synthase enzyme responsible for
  the process of photophosphorylation which
  converts proton-motive force into net ATP
  synthesis this enzyme is very similar to
  mitochondrial ATP synthase.
• Ribulose-1,5-bisphosphate carboxylase/oxygenase
  (rubisco) - is responsible for CO2 fixation in
  the first step of the Calvin cycle. Rubisco
  activity is maximal in the light when stromal pH
  is 8 and Mg2 levels are elevated due to proton
  pumping.

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4. What are examples of the photosynthetic
electron transport system and Calvin Cycle in
real life?

• DCMU (dichlorophenyl dimethylurea) is a broad
  spectrum herbicide that functions by blocking
  electron flow through photosystem II and is used
  to reduce weeds in non-crop areas.
• Another herbicide, paraquat, prevents reduction
  of NADP by accepting electrons from intermediate
  reductants in photosystem I.

PSI
PSII
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Overview of Photosynthesis
The photosynthetic electron transport system and
photophosphorylation work togethe to generate ATP
and NADPH for sugar synthesis by the Calvin Cycle
4e-
Light energy is used to oxidize 2 H2O which
releases 4e- and 4H
Redox reactions are used to translocate an
additional 8H
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Pay attention to the compartmentalization inside
and outside of the chloroplast.The product of
the carbon fixation is glyceraldehyde-3P (GAP)
which is converted to hexose sugars for use as
chemical energy at night.
What pathway has GAP as a central intermediate,
does this make sense here?
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Peter Mitchells chemiosmotic theory was actually
first proven using photosynthetic systems in
which an artificial proton gradient was
established using buffered solutions at different
pH values.
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2
3
What would happen to the ATP if the buffer was
now switched back to pH4?
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5
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The chloroplast ATP synthase is structurally and
functionally similar to the mitochondrial ATP
synthase except that 4 H are required for every
ATP synthesized based on experiments showing that
3 ATP are synthesized for every O2 generated (12
H transported/O2 generated). This difference
(3 H/ATP in mitochondria versus 4 H/ATP in
chloroplasts) could be due to differences in the
"gear ratio" of the complex, or uncertainties in
the experimental measurements.
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Chloroplast Structure
Inner envelope
Stroma thylakoids
Outer envelope
Stroma
Granal thylakoids
Buchanon et al., Fig.12.1
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Light energy is absorbed by numerous accessory
pigments which can transfer the absorbed energy
to nearby reaction centers containing specialized
chlorphyll molecules. These accessory pigments
function as light harvesting antenna.
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Chlorophylls are the primary light gathering
pigments They have a heterocyclic ring system
that constitutes an extended polyene structure,
which typically has strong absorption in visible
light.
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Plant pigments cover a broad absorption spectra
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Organization of Photosynthetic pigments

• Light absorbing pigments are organized in
  functional arrays called Photosystems.
• Light harvesting or antenna pigment molecules
  are specialized to absorb light and transfer the
  energy to neighboring pigment molecules.
• Photochemical reaction center pigment molecules
  are specialized to transduce light energy into
  chemical energy.
• Several hundred light harvesting pigment
  molecules funnel light to one reaction center
  molecule.

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Light harvesting pigment molecules transfer
energy to nearby molecules which eventually
results in photooxidation
Top view of antenna chlorophylls. Note that both
PSI and PSII are reaction centers.
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The Z Scheme of Photosynthetic Electron Transport

• The photosynthetic electron transport system in
  plants consists of two linked electron circuits,
  each requiring an input of energy from light
  absorption at PSII and PSI reaction center
  complexes to initiate electron flow. The Z scheme
  energy diagram showing how photon absorption by
  the PS II reaction center complex results in
  electron flow from H2O to plastocyanin, providing
  energy to translocate H across the thylakoid
  membrane. A second photon absorption event at PS
  I drives electron transport from plastocyanin to
  NADP.
• Electron flow through protein-linked redox
  reactions involves numerous electron carriers,
  including Fe-S centers, the hydrophobic molecule
  plastoquinone (Q) which is reduced to form
  plastoquinol (QH2) and analogous to
  ubiquinone/ubiquinol in the mitochondrial
  electron transport system. Plastocyanin (PC) has
  the same job in photosynthetic electron transport
  as does cytochrome c in mitochondrial electron
  transport.

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Z Scheme of Photosynthetic Electron Transport
Light energy absorbance kicks electrons into
higher energy states which is used to convert
solar energy into redox energy when the electrons
are passed one at a time to electron carriers
with increasingly positive standard reduction
potentials. The redox energy is captured in the
form of chemical energy (ATP, NADPH.
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The oxidation of H2O to produce one mole of O2
requires the absorbance of 8 photons to move the
4 e- through the entire photosynthetic electron
transport system.
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Energy input at both the PSII and PSI reaction
centers
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Overview of Photosynthesis

• Light excitation of Photosystems I and II results
  in oxygen evolution from the splitting H2O, and
  the generation of chemical energy in the form of
  ATP and NADPH. Plants use this chemical energy
  (ATP and NADPH) to convert CO2 into sugars via
  the Calvin cycle, which takes place in the
  stroma.

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Photosystem II (PSII)
Photosystem II contains chlorophylls a and b and
absorbs light at 680nm. This is a large protein
complex that is located in the thylakoid
membrane.
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Functional organization of the PSII complex
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In the chloroplast PSII reaction center, the
electron acceptor is a molecule called pheophytin
which becomes negatively charge as denoted by
Pheo-. Importantly, the oxidized chlorophyll
molecule (now positively charged, Chl) returns
to the ground state by accepting an electron
through a coupled redox reaction involving the
oxidation of H2O. This process of O2 evolution
takes place in the manganese center present in
the thylakoid membrane and is ultimately the
source of electrons needed for the photosynthetic
electron transport system.
Light absorption kicks an e- into a higher
orbital, what happens next is either energy
transfer, photooxidation, or fluorescence.
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The electron that was transferred from the P680
chlorophyll reaction center needs to be replaced,
this replacement electron comes from the
oxidation of H2O within the oxygen evolving
complex. The tricky part is that the oxidation
of H2O releases 4 e-, however, photooxidation
only transfers one e- at a time to
pheophytin. Therefore, the Mn atoms must be able
to store electrons and release them one at a
time.
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Functional organization of the cytochrome b6f
complex
The same deal here as in complex III of ETS, we
need to convert the 2 e- carrier PQBH2 into a 1
e- carrier in PC. The answer is the Q cycle, duh!
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The light-driven Q cycle is responsible for
translocation of 8 H from the stroma to the lumen
ATP synthase complex
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Photosystem I (PSI)
The final stage of photosynthesis the absorption
of light energy by PS I is at a maximum of 700
nm. Again 4 photons are absorbed, but in this
case, the energy is used to generate reduced
ferredoxin, which is a powerful reductant.
Structure of PS I complex showing Fe-S clusters
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Functional organization of the PSI complex
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Ferrodoxin NADP reductase plays a crucial role
in converting redox energy into a useable form
for the Calvin Cycle by generating NADPH. Since
e- arrive in PSI one at a time, the FAD coenzyme
must store on e- in a semiquinone chemical
structure.
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Paraquat has a higher reduction potential than
QKA and steals electrons from PSI. This not
only inhibits NADPH production, but it also
results in the generation of dangerous reactive
oxygen species (ROS) that cause cell damage as
potent oxidants.
Paraquat was once used extensively as an aerial
herbicide to destroy illegal fields of marijuana
and coca plants in North and South America.
However its use was discontinued because smoking
paraquat-contaminated plants causes lung damage.