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Chapter 11
Phototrophic
Energy
Metabolism:
Photosynthesis
Lectures by
Kathleen Fitzpatrick
Simon Fraser University
© 2012 Pearson Education, Inc.
Phototrophic Energy Metabolism:
Photosynthesis
• Most chemotrophs depend on an external
source of organic substrates for survival
• Photosynthetic organisms produce the
chemical energy and organic carbon required
by chemotrophs
• They use solar energy to reduce CO2 to
produce carbohydrates, fats, and proteins
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Important terminology
• Photosynthesis: the conversion of light
energy to chemical energy and its
subsequent use in synthesis of organic
molecules
• Phototrophs: organisms that convert solar
energy into chemical energy as ATP and
reduced coenzymes
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Types of phototrophs
• Photoheterotrophs: organisms that acquire
energy from sunlight but depend on organic
sources of reduced carbon
• Photoautotrophs: organisms that use solar
energy to synthesize energy-rich organic
molecules using starting materials such as
CO2 and H2O
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An Overview of Photosynthesis
• Photosynthesis involves two major
biochemical processes
- Energy transduction reactions: light energy
is captured and converted into chemical
energy
- Carbon assimilation reactions: (carbon
fixation reactions) carbohydrates are formed
from CO2 and H2O
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Figure 11-1
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BioFlix: Photosynthesis
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The Energy Transduction Reactions
Convert Solar Energy to Chemical
Energy
• Light energy is captured by green pigment
molecules called chlorophylls, present in the green
leaves of plants and the cells of algae and
photosynthetic bacteria
• Light absorption by a chlorophyll molecule excites
one of its electrons, which is then ejected from the
molecule and enters an electron transport system
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Unidirectional proton pumping
• The photosynthetic ETS is coupled to
unidirectional proton pumping
• The electrochemical gradient produced is
used to generate ATP through
photophosphorylation
• This is similar to oxidative phosphorylation in
mitochondria
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Reduction of carbon
• Photoautotrophs use NADPH2 to reduce carbon
for incorporation into organic molecules
• Oxygenic phototrophs (plants, algae,
cyanobacteria) use water as the donor of two
electrons
• Anoxygenic phototrophs (green and purple
photosynthetic baceria) use compounds such as
sulfide, thiosulfate, or succinate as donors
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Photoreduction
• Oxygenic phototrophs release oxygen as
water is oxidized
• Anoxygenic phototrophs release oxidized
forms of the electron donors
• In both types of organisms the light
dependent generation of NADPH is called
photoreduction
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The Carbon Assimilation Reactions Fix
Carbon by Reducing Carbon Dioxide
• Most of the energy accumulated by the
generation of ATP and NADPH is used for
carbon dioxide fixation and reduction
• “H2A” is a suitable electron donor, and “A” is
the oxidized form of the donor
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Oxygenic phototrophs
• For oxygenic phototrophs, in which water is
the electron donor, we can summarize the
reaction as follows:
• The intermediate product of carbon fixation is
a triose (3-carbon sugar) rather than the
hexose shown in the equation
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Production of sugars
• The intermediates of photosynthesis are used
for biosynthesis of a variety of products,
including glucose, sucrose, and starch
• Sucrose is the major transport carbohydrate
in most plants
• Starch is the major storage carbohydrate in
most plants
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The Chloroplast Is the Photosynthetic
Organelle in Eukaryotic Cells
• The most familiar oxygenic phototrophs are
the green plants, in which the photosynthetic
organelle is the chloroplast
• Chloroplasts are large and a mature leaf may
contain 20-100
• The shape varies from simple flattened
spheres to ribbon-shaped
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Figure 11-2A
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Figure 11-2B
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Not all plant cells contain chloroplasts
• Newly differentiated plant cells have smaller
organelles called proplastids, which may
develop into any of several types of plastids
depending on the function of the cell
• Amyloplasts are specialized for storing starch
• Chromoplasts give flowers and fruits their
distinctive colors
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Chloroplasts Are Composed of Three
Membrane Systems
• A chloroplast has both an outer membrane
and an inner membrane
• These are usually separated by an
intermembrane space
• The inner membrane encloses the stroma, a
gel-like matrix full of enzymes for C, N, and S
reduction and assimilation
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Figure 11-3A,B
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Figure 11-3C
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The outer membrane is freely
permeable
• The outer membrane contains porins similar to
those in the mitochondrial outer membrane
• These allow passage of solutes with molecular
weights up to 5000
• In the inner membrane transport proteins control
the flow of metabolites between the stroma and
intermembrane space
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Thylakoids
• Chloroplasts have a third membrane system,
called thylakoids
• These are flat, saclike structures in the
stroma, arranged in stacks called grana
• Grana are interconnected by stroma
thylakoids
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Thylakoid lumen
• Grana and stroma thylakoids enclose a single
continuous compartment called the thylakoid
lumen
• The semipermeable barrier of the thylakoid
membrane allows for creation of an
electrochemical proton gradient between the
lumen and stroma
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Figure 11-3C,D
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Video: Chloroplast structure
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Organisms without chloroplasts
• Photosynthetic bacteria have no chloroplasts
• In some of them, such as the cyanobacteria, the
plasma membrane folds inward to form
photosynthetic membranes
• To some extent, cyanobacteria appear to be
free-living chloroplasts, a resemblance that has
contributed to the endosymbiont theory
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Figure 11-4
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Figure 11A-1
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Photosynthetic Energy
Transduction I: Light Harvesting
• The first stage of photosynthetic energy
transduction is the capture of solar energy
• Light behaves as a stream of particles called
photons, each carrying a quantum (indivisible
packet) of energy
• When a photon is absorbed by a pigment such as
chlorophyll, the energy of the photon is transferred
to an electron
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Photoexcitation
• The energy transferred from a photon energizes
the electron from its ground state in a low-energy
orbital, to an excited state in a high-energy orbital
• This first step of photosynthesis is called
photoexcitation
• Different pigments have different absorption
spectra, to describe the wavelengths absorbed
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Figure 11-5
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Photoexcited electrons are unstable
• A photoexcited electron in a pigment molecule is
unstable and must either return to the ground
state or transfer to a stable high-energy orbital
• If it returns to the ground state, the energy is lost
as heat or light
• The energy can also be transferred to an
electron in an adjacent molecule, in a process
called resonance energy transfer
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Transfer of the photoexcited electron
• If the excited electron is transferred to another
molecule, it is called photochemical reduction
• Photochemical reduction is essential for
converting light energy into chemical energy
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Chlorophyll Is Life’s Primary Link to
Sunlight
• Chlorophyll is found in nearly all photosynthetic
organisms
• Chlorophyll a and b each have a central porphyrin
ring, which absorbs visible light
• Their strongly hydrophobic phytol side chains
anchor the chlorophylls in the thylakoid
membranes
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Chlorophyll a and b
• The Mg2+ in chlorophyll a and b affects the
electron distribution in the porphyrin ring and
ensures that high-energy orbitals are available
• Chlorophyll a has a broad absorption spectrum
with maxima at about 420 and 660 nm
• Chlorophyll b has a formyl group in place of a
methyl group, which shifts the maxima toward the
center of the spectrum
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Figure 11-6
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3-D Structure: Chlorophylla
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Other types of chlorophyll
• All plants and green algae contain both
chlorophyll a and b
• Brown algae, diatoms, and dinoflagellates
supplement chlorophyll a with chlorophyll c
• Red algae have chlorophyll d
• Red algae and cyanobacteria have phycobilin
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Bacteriochlorophyll
• Bacteriochlorophyll is a subfamily restricted to
anoxygenic phototrophs
• It is characterized by a saturated site not found in
other chlorophylls
• The absorption maxima of bacteriochlorophylls
are shifted toward the near-ultraviolet and the farred regions
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Accessory Pigments Further Expand
Access to Solar Energy
• Most photosynthetic organisms also contain
accessory pigments, which absorb photons that
cannot be captured by chlorophyll
• The energy is transferred to a chlorophyll molecule
by resonance energy transfer
• Two types of accessory pigments are carotenoids
and phycobilins
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Carotenoids
• Two carotenoids that are abundant in the
thylakoid membranes of most plants and green
algae are -carotene and lutein
• When not masked by chlorophyll these pigments
confer an orange or yellow tint to leaves
• They absorb photons from a broad range of the
blue region of the spectrum
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Phycobilins
• Phycobilins are found only in red algae and
cyanobacteria; two common examples are
– Phycoerythrin, which allows absorption of light
that penetrates the ocean’s surface water
– Phycocyanin, which is characteristic of
cyanobacteria near the surface of a lake, or on
land
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Light-Gathering Molecules Are Organized
into Photosystems and Light-Harvesting
Complexes
• Functional units, photosystems, contain
–Chlorophyll
– Accessory proteins
–Chlorophyll-binding proteins that stabilize the
chlorophyll in a photosystem
–Other proteins that bind components of the
electron transport system
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Photosystems and Light-Harvesting
Complexes
• Most pigments of a photosystem serve as
light-gathering antenna pigments
• These absorb photons and pass the energy
to a neighboring chlorophyll or accessory
protein by resonance energy transfer
• Some antenna pigments are “wired” together
by quantum mechanical probability effects
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The reaction center
• The events that drive electron flow and proton
pumping do not begin until the energy
reaches the reaction center of a photosystem
• Here, two chlorophyll a molecules called the
special pair are found
• These molecules catalyze the conversion of
solar energy into chemical energy
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Figure 11-7
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Figure 11B-1
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The light-harvesting complex
• Each photosystem is associated with a lightharvesting complex (LHC), which collects
light energy and can move in response to
changing light conditions
• The LHC does not contain a reaction center
• Instead it passes the collected energy to a
nearby photosystem by resonance energy
transfer
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The light-harvesting complex (continued)
• Plants and green algae have LHCs composed of
80-250 chlorophyll a and b molecules along with
carotenoids and pigment-binding proteins
• Red algae have a phycobilisome, which
contains phycobilins
• Together a photosystem and the associated
LHCs are referred to as a photosystem
complex
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Oxygenic Phototrophs Have Two
Types of Photosystems
• In the 1940s Emerson and colleague discovered
that two separate photosystems are involved in
oxygenic photosynthesis
• They found that photosynthesis driven by a
combination of wavelengths exceeded the sum
of activities with either wavelength alone
• This synergistic phenomenon was called the
Emerson enhancement effect
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Photosystems I and II
• Photosystem I (PSI) has an absorption
maximum of 700nm, whereas photosystem II
(PSII) has an absorption maximum of 680 nm
• Each electron that passes from water to NADP+
must be photoexcited once for each photosystem
• With illumination of 690nm and above,
photosynthesis is severely impaired
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Photosystems I and II (continued)
• Each electron is first excited by PSII and then
by PSI
• The special pair of chlorophyll a molecules in
the reaction center of each photosystem are
designated P680 for PSII and P700 for PSI
• The granal and stromal thylakoid membranes
have differing amounts of the two
photosystems, which can move to respond to
changing light conditions
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Photosynthetic Energy
Transduction II: NADPH Synthesis
• The second stage of photosynthesis uses a series
of electron carriers to transport electrons from
chlorophyll to the coenzyme nicotine adenine
dinucleotide phosphate (NADP+)
• It forms NADPH when reduced
• This is called photoreduction and involves a
chloroplast electron transport system (ETS)
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Similarity to mitochondrial electron
transport
• The chloroplast electron transport system is
similar to that of mitochondria
• The complete pathway includes several
components
• Many of the molecules are similar to those of
the mitochondrial ETS—cytochromes, ironsulfur proteins, etc.
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Go and Eo
• Recall that Go (standard free energy) and Eo
(standard reduction potential) are opposite in
sign
• This means that electrons will spontaneously
flow toward a compound with a higher reduction
potential
• Absorption of light by each photosystem boosts
electrons to the top of an ETS
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Electron flow
• As electrons flow from PSII to PSI, a portion of
their energy is conserved in a proton gradient
across the thylakoid membrane
• From PSI the electrons flow to ferredoxin and
then to NADP+
• NADP+ is the coenzyme mainly used for anabolic
pathways, whereas NAD+ is usually involved in
catabolic pathways
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Figure 11-8
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Figure 11-8A
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Figure 11-8B
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Figure 11-8C
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Figure 11-8D
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Photosystem II Transfers Electrons
from Water to a Plastoquinone
• Photosystem II uses electrons from water to
reduce a plastoquinone (QB) to plastoquinol
(QBH2)
• PSII is associated with light-harvesting
complex II (LHCII), which contains about 250
chlorophyll and many carotenoid molecules
• Energy captured by antenna pigments of PSII or
LHCII is funneled to the reaction center
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Figure 11-9
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Figure 11-9A
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Figure 11-9B
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Figure 11-9C
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Figure 11-9D
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Photosystem II (continued)
• Captured energy in the reaction center lowers
the reduction potential of a P680 molecule
making it a better electron donor
• A photoexcited electron is passed to pheophytin
(Ph), a chlorophyll a molecule with two protons
in place of the Mg2+
• The charge separation between P680+ and Ph–
prevents the electron from returning to its
ground state
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Photosystem II (continued)
• Solar energy has been harvested and converted
into electrochemical potential energy in the form
of the charge separation
• The electron is passed to QA, a plastoquinone
(similar to coenzyme Q) tightly bound to protein
D2
• QB receives two electrons from QA and picks up
two protons from the stroma to form QBH2
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Photosystem II (continued)
• QBH2 enters a mobile pool of QBH2 inside the
photosynthetic membrane, where it passes two
electrons and two protons to the cytochrome b6 /f
complex
• Formation of one mobile plastoquinone molecule
depends on two sequential photoreactions
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Photosystem II (continued)
• To replace the electron lost to plastoquinone,
oxidized P680+ is reduced by an electron from
water
• PSII includes an oxygen-evolving complex
that catalyzes the splitting and oxidation of
water, producing O2, electrons, and protons
• Two water molecules donate four electrons one
at a time to four molecules of P680+
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Photosystem II (continued)
• .
• The protons accumulating in the lumen
contribute to an electrochemical proton gradient
across the thylakoid membrane, and the O2
diffuses out of the chloroplast
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Photosystem II summarized
• The net reaction catalyzed by four photoexcitations
at PSII can be summarized as
• The light-dependent oxidation of water is called
water photolysis
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The Cytochrome b6 /f Complex
Transfers Electrons from a
Plastoquinone to Plastocyanin
• Electrons carried by QBH2 flow through an ETS
coupled to unidirectional proton pumping into the
lumen
• This happens by way of the cytochrome b6 /f
complex, which is composed of seven different
integral transmembrane proteins including two
cytochromes and an iron-sulfur protein
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The Cytochrome b6 /f complex
• QBH2 donates two electrons via cytochrome b6
and the iron-sulfur protein to cytochrome f
• Each oxidation of QBH2 releases two protons
into the thylakoid lumen
• Additional protons can be pumped into the
lumen by the Q cycle
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The Q cycle
• .
• Because half the electrons are recycled back
to QB, the Q cycle would double the number
of protons translocated to the lumen
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Electrons are passed to plastocyanin
• Reduced cytochrome f donates electrons to a
copper-containing protein called plastocyanin
(PC), which is a mobile electron carrier
• PC, a peripheral membrane on the lumenal side
of the thylakoid membrane, carries electrons one
at a time to PSI
• .
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Photosystem I Transfers Electrons
from Plastocyanin to Ferredoxin
• PSI transfers photoexcited electrons from reduced
plastocyanin to the protein ferredoxin, the
immediate electron donor to NADP+
• The PSI reaction center includes a chlorophyll a
molecule called Ao (instead of pheophytin), as well
as phylloquinone and three Fe-Su centers that
form an ETS from Ao to ferredoxin
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Light-harvesting complex I
• PSI in plants and green algae is associated
with light-harvesting complex I (LHCI), with
fewer antennae molecules than LHCII
• Energy is funneled to a reaction center with a
special pair of chlorophyll a molecules, P700
• The energy absorbed by PSI lowers the
reduction potential of the P700s so that a
photoexcited electron is rapidly passed to Ao
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Charge separation
• The charge separation between P700+ and
reduced Ao prevents the electron from returning to
the ground state
• The electron lost by P700 is replaced by an
incoming electron from reduced plastocyanin
• From Ao electrons flow exergonically through the
ETS to ferredoxin, the final electron acceptor for
PSI
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Ferredoxin
• Ferredoxin (Fd) is a mobile iron-sulfur
protein found in the stroma
• Overall the net reaction catalyzed by PSI can
be summarized as follows
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Ferredoxin-NADP+ Reductase
Catalyzes the Reduction of NADP+
• The final step in photoreduction is the transfer
of electrons from ferredoxin to NADP+,
producing the NADPH needed for carbon
reduction and assimilation
• The enzyme responsible is ferredoxinNADP+ reductase (FNR)
• .
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Noncyclic electron flow
• The components of the chloroplast ETS
provide a continuous unidirectional flow of
electrons from water to NADP+
• This is called noncyclic electron flow, and
the net result is
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Photosynthetic Energy Transduction
III: ATP Synthesis
• In the final stage of photosynthetic energy
transduction the potential energy stored in a
proton gradient is used to synthesize ATP
• This process is called photophosphorylation
• The thylakoid membrane is virtually impermeable
to protons, so a substantial proton gradient can
develop
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Pmf in chloroplasts
• In chloroplasts, the pH is more important than
Vm and contributes about 80% of the proton
motive force
• Light-induced proton pumping causes the pH in
the lumen to drop to 6, while the stromal pH rises
to about 8 due to proton depletion
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The ATP Synthase Complex Couples
Transport of Protons Across the Thylakoid
Membrane to ATP Synthesis
• The movement of protons back across the
membrane to regions of lower concentration
drives the synthesis of ATP by an ATP synthase
• The ATP synthase complex found in chloroplasts
is called the CF0CF1 complex, very similar to the
F0F1 complex of mitochondria
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The CF0CF1 complex
• CF1 is a hydrophylic group of polypeptides
protruding from the stromal side of the thylakoid
membrane, and containing three catalytic sites
for ATP synthesis
• CF0 is a hydrophobic assembly of polypeptides
anchored to the thylakoid membrane
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Components of CF0
• Subunits I and II form a stalk that connects CF0
and CF1
• Subunit IV is the proton translocator, through
which protons flow back to the stroma
• Subunit III is a ring of polypeptides next to
subunit IV, the rotation of which is coupled to
ATP synthesis, similar to mitochondria
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Four protons per ATP
• Recent evidence suggests that four protons are
translocated for every ATP generated
• There are 14 copies of subunit III, and 3 ATP
are generated by one complete rotation, leading
to estimates of more than four protons per ATP
• .
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Cyclic Photophosphorylation Allows a
Photosynthetic Cell to Balance NADPH and
ATP Synthesis
• When NADPH consumption is low, or more ATP
is needed, cyclic electron flow can divert the
reducing power of PSI into ATP synthesis rather
than NADP+ reduction
• This is called cyclic photophosphorylation
• No water is oxidized nor O2 released, because
PSII is not involved
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Figure 11-10
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A Summary of the Complete
Energy Transduction System
• The component parts of the complete system
can be summarized as follows:
• 1. Photosystem II complex
- Assembly of chlorophyll, pigments, proteins
- Oxidization of water, evolving O2
- P680 becomes photoexcited, enabling it to
reduce plastoquinone
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The complete energy transduction
system (continued)
• 2. Cytochrome b6 /f complex
– Accepts electrons from plastoquinone (noncyclic)
or ferredoxin (cyclic)
– Pumps protons unidirectionally into thylakoid
lumen
• 3. Photosystem I complex
– Assembly of chlorophyll, pigments, proteins
– P700 becomes photoexcited, enabling it to reduce
ferredoxin (stromal protein)
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The complete energy transduction
system (continued)
• 4. Ferredoxin-NADP+reductase
- Enzyme on stromal side of thylakoid
membrane
- Catalyzes transfer of electrons from two
reduced ferredoxins to NADP+
- NADPH product essential reducing agent in
many anabolic pathways
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The complete energy transduction
system (continued)
• 5. ATP synthase complex (CF0CF1)
- CFoCF1 proton channel and ATP synthase
- Uses energy from exergonic flow of protons to
synthesize ATP in the stroma
- ATP essential for carbon fixation/assimilation
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Photosynthetic Carbon
Assimilation I: The Calvin Cycle
• The main pathway for movement of inorganic
carbon into the biosphere is the Calvin cycle
• In plants and algae, the cycle is confined to
the chloroplast stroma, where ATP and
NADPH accumulate
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Entry of CO2 into plants
• In plants, CO2 enters the leaves through
special pores called stomata
• Once inside a leaf, CO2 diffuses into
mesophyll cells and usually travels into the
stroma
• The stroma is the site of carbon fixation
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Three stages of the Calvin cycle
• 1. The carboxylation of ribulose-1,5bisphosphate, and generation of two 3phosphoglycerate molecules
• 2. Reduction of 3-phosphoglycerate into
glyceraldehyde-3-phosphate
• 3. Regeneration of the original acceptor to allow
continued carbon assimilation
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Figure 11-11
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Activity: The Calvin cycle – Part 1
Activity: The Calvin cycle – Part 2
Activity: The Calvin cycle – Part 3
Activity: The Calvin cycle – Part 4
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Carbon Dioxide Enters the Calvin
Cycle by Carboxylation of Ribulose1,5-Bisphosphate
• The first stage begins with the covalent attachment
of CO2 to ribulose-1,5-bisphosphate (CC-1)
• This leads to production of two 3-carbon molecules,
3-phosphoglycerate
• Ribulose-1,5-bisphosphate
carboxylase/oxygenase(“rubisco”) is the most
abundant protein on the planet
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The overall reaction
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Figure 11-12
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3-Phosphoglycerate Is Reduced to
Form Glyceraldehyde-3-Phosphate
• The reduction of 3-phosphoglycerate to form
glyceraldehyde-3-phosphate is essentially the
reverse of the oxidative sequence of glycolysis
• The coenzyme is NADPH instead of NADH
• Phosphoglycerokinase catalyzes reaction CC-2
and glyceraldehyde-3-phosphate dehydrogenase
catalyzes CC-3
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Energy is consumed in the first
stages of carbon fixation
• For every CO2 molecule fixed by rubisco two
ATP molecules must be hydrolyzed and two
NADPH molecules are oxidized
• .
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Regeneration of Ribulose-1,5Bisphosphate Allows Continuous
Carbon Assimilation
• One of six triose phosphate molecules generated is
used for biosynthesis of organic molecules
• The remaining five are used to regenerate three
molecules of the (five-carbon) acceptor ribulose1,5-bisphosphate (CC-4)
• The reactions are catalyzed by aldolases,
transketolases, phosphatases, and isomerases
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Regeneration of ribulose-1,5bisphosphate requires energy
• Three molecules of ribulose-5-phosphate are
converted to ribulose-1,5-bisphosphate by
phosphoribulokinase (PRK)
• Regeneration of the three ribulose-1,5bisphosphate consumes three more ATPs
• .
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The Complete Calvin Cycle and Its
Relation to Photosynthetic Energy
Transduction
• .
• The Calvin cycle uses 9 ATP molecules and 6
NADPH for every 3-carbon carbohydrate
synthesized
• Activity of the cyclic pathway of PSI and/or the Q
cycle are needed to produce the ATP
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Overall reaction including energy
transduction and carbon assimilation
• .
• The absorption of 26 photons is accounted for by
- 12 photoexcitation events at PSI and 12 at PSII
during noncyclic electron flow
- 2 photoexcitation events at PSI during cyclic flow
• If the Q cycle is operating, fewer photons will be
needed
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Combining photosynthetic energy
transduction with the Calvin cycle
• .
• .
• This reaction is almost identical to the net
photosynthetic reaction
• .
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Maximum efficiency of photosynthetic
energy transduction and carbon
assimilation
• Assuming wavelength = 670nm, 26 moles of
photons are 1118 kcal of energy
• Glyceraldehyde differs in free energy from CO2
and H2O by 343 kcal/mol
• So the efficiency of energy transduction is about
31%, greater than most man-made energy
transducing machinery
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Regulation of the Calvin Cycle
• In the dark, phototrophs must meet a steady
demand for energy and carbon using the
surplus accumulated when light is available
• Several regulatory systems are used to
ensure that the Calvin cycle does not operate
unless light is available
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The Calvin Cycle Is Highly Regulated
to Ensure Maximum Efficiency
• The first level of control is regulation of key
enzymes in the Calvin cycle
• These enzymes are not synthesized in tissues
that are not exposed to light
• Also, reduced ferredoxin, ATP, and NADPH, act
as signals to activate Cavin cycle enzymes
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Rubisco and other enzymes are points
for metabolic control
• Rubisco is an obvious control point as it
catalyzes the carboxylation reaction of the
Calvin cycle
• Sedoheptulose bisphosphatase and PRK, with
roles in regenerating the acceptor molecule, are
also regulated
• All are stimulated by high pH and high [Mg2+]
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Rubisco and other enzymes are points
for metabolic control (continued)
• All three of the regulated enzymes
- Are unique to the Calvin cycle
- Catalyze reactions that are essentially
irreversible
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Regulation based on ferredoxin
• In the light, electrons donated by water are used
to reduce ferredoxin, then transferred to
thioredoxin (enzyme: ferredoxin-thioredoxin
reductase)
• Glyceraldehyde-3-phosphate dehydrogenase,
sedoheptulose bisphosphatase, and PRK are
activated by the conformational change caused
by thioredoxin, which is not available in the dark
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Figure 11-13
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Early glycolysis is inhibited in the light
• The same mechanisms that activate enzymes of
the Calvin cycle, inactivate enzymes of
degradative pathways
• When the Calvin cycle is operating in the light,
phosphofructokinase, an important control point
in glycolysis, is inhibited
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Rubisco Activase Regulates Carbon
Fixation by Rubisco
• Rubisco activase removes inhibitory sugarphosphate compounds from the rubisco
active site
• Rubisco activase has ATPase activity, which
is sensitive to the ADP/ATP ratio
• In the dark, accumulated ADP inhibits rubisco
activase, leaving rubisco inactive
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Photosynthetic Carbon Assimilation
II: Carbohydrate Synthesis
• The most abundant protein in the chloroplast is a
phosphate translocator, which catalyzes the exchange
of triose phosphates in the stroma for Pi in the cytosol
• This antiport system only exports triose phosphates if Pi
for making new triose phosphates returns to the stroma
• The triose phosphates that remain in the stroma are
used for starch synthesis
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Glucose-1-Phosphate Is Synthesized
from Triose Phosphates
• Two triose phosphates undergo a condensation
reaction catalyzed by aldolase, to generate
fructose-1,6-bisphosphate
• This is dephosphorylated by fructose-1,6bisphosphatase to form fructose-6-phosphate (S-1)
• This is catalyzed in both stroma and cytosol by
distinct forms of the enzyme, called isoenzymes
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S-2 and S-3
• Fructose-6-phosphate can be converted to
glucose-6-phosphate (S-2)
• This is then converted to glucose-1phosphate (S-3)
• There are separate stromal and cytosolic
isoenzymes for these reactions too, as the
hexoses involved cannot be transported
between the cytosol and stroma
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Figure 11-14
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The Biosynthesis of Sucrose Occurs
in the Cytosol
• Sucrose synthesis is located in the cytosol of a
photosynthetic cell
• Triose phosphates exported from the stroma that are
not used in other metabolic pathways are converted
to glucose-1-phosphate
• Glucose is then produced by reaction with UTP
(uridine triphosphate) to produce UDP-glucose (S-4c)
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Biosynthesis of Sucrose (continued)
• The glucose of UDP-glucose is transferred to
fructose-6-phosphate to form sucrose-6phosphate (S-5c)
• The hydrolytic removal of the phosphate group
generates free sucrose (S-6c)
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Control of sucrose biosynthesis
• Sucrose synthesis is controlled to prevent
conflict with degradation pathways
• Cytosolic fructose-1,6-bisphosphatase is
inhibited by fructose-2,6-bisphosphate, a
regulator of glycolysis and gluconeogenesis
• Sucrose phosphate synthase is stimulated by
glucose-6-phosphate and inhibited by
sucrose-6-phosphate, UDP, and Pi
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The Biosynthesis of Starch Occurs in
the Chloroplast Stroma
• Starch synthesis is confined to plastids,
where triose phosphates are converted to
glucose-1-phosphate, which is then used for
starch synthesis
• Glucose-1-phosphate reacts with ATP to
generate ADP-glucose (S-4s)
• The activated glucose is added to a growing
starch chain by starch synthase (S-5s)
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Control of starch biosynthesis
• Starch synthesis is regulated to prevent
conflict with degradation pathways
• For example, ADP-glucose phosphorylase is
stimulated by glyceraldehyde-3-phosphate
and inhibited by Pi
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Photosynthesis Also Produces
Reduced Nitrogen and Sulfur
Compounds
• ATP and NADPH generated by photosynthetic
energy transduction are consumed by a variety of
other anabolic pathways
– For example, the reduction of nitrite (NO2–) to
ammonia (NH3)
– For example, the reduction of sulfate (SO42–) to
sulfide (S2–)
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Rubisco’s Oxygenase Activity
Decreases Photosynthetic Efficiency
• The primary reaction catalyzed by rubisco is the
addition of CO2 and H2O to ribulose-1,5bisphosphate, forming two 3-phosphoglycerate
• However, in addition to this function as a
carboxylase, rubisco can act as an oxygenase
• In this way rubisco can add molecular oxygen
rather than CO2
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Rubisco’s Oxygenase Activity
• The result is phosphoglycolate, which cannot
be used in the Calvin cycle, and thus appears
wasteful
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The Glycolate Pathway Returns
Reduced Carbon from
Phosphoglycolate to the Calvin Cycle
• Phosphoglycolate is channeled into the glycolate
pathway, which returns about 75% of it to the Calvin
cycle as 3-phosphoglycerate
• The pathway is also called photorespiration because
of its light-dependent uptake of O2 and release of CO2
• Several steps of the pathway take place in a leaf
peroxisome
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The glycolate pathway
• Phosphoglycolate is rapidly dephosphorylated by
a phosphatase in the stroma (GP-1)
• The resulting glycolate diffuses to a leaf
peroxisome where an oxidase converts it to
glyoxylate (GP-2)
• This is accompanied by O2 uptake and H2O2
generation, which is immediately degraded by
catalase
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The glycolate pathway (continued)
• An aminotransferase transfers an amino group to
glyoxylate, forming glycine (GP-3)
• Glycine diffuses from the peroxisome to a
mitochondrion where a decarboxylase and a
hydroxymethyl transferase convert two glycines
to one serine, along with formation of NADH and
release of CO2 and NH3 (GP-4)
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The glycolate pathway (continued)
• Serine diffuses back to the peroxisome, where
another aminotransferase removes an amino
group to form hydroxypyruvate (GP-5)
• A reductase reduces it to glycerate (GP-6)
• Glycerate diffuses to the chloroplast and is
phosphorylated by glycerate kinase, to form 3phosphoglycerate (GP-7)
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The value of the glycolate pathway
• 75% of the phosphoglycolate molecules
produced are recovered as 3-phosphoglycerate
and are thus not wasted
• The glycolate pathway prevents a toxic buildup of
phosphoglycolate
• The pathway is metabolically expensive, but still
represents a net gain for the plant
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Figure 11-15
© 2012 Pearson Education, Inc.
C4 Plants Minimize Photorespiration by
Confining Rubisco to Cells Containing
High Concentrations of CO2
• Plants in hot, arid environments are particularly
affected by rubisco’s oxygenase activity
– For example, the CO2 solubility is more affected by
temperature than that of O2, creating a problem of
CO2:O2 balance
– For example, when stomata are closed [CO2]
declines whereas photolysis continues to produce O2
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Adaptive strategies
• In some cases, problems of energy and carbon
drain necessitate adaptive strategies to allow a
plant to overcome the problem
• One general approach is to confine rubisco to
those cells with a high [CO2], to minimize the
oxygenase activity
• In some plants the Hatch–Slack cycle is used
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The Hatch–Slack cycle
• The Hatch–Slack cycle is a short carboxylation/
decarboxylation pathway, with oxaloacetate as
the intermediate of carbon fixation
• Plants that use this strategy are called C4
plants (oxaloacetate is a 4-carbon compound)
• C3 plants, have the 3-carbon compound 3phosphoglycerate as the first detectable product
of carbon fixation
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Leaf structure of C4 plants
• C4 plants have two types of photosynthetic cells in
their leaves
– Mesophyll cells: CO2 fixation here uses an
enzyme other than rubisco; these cells are
exposed to CO2 and O2
– Bundle sheath cells: These are relatively isolated
from the atmosphere, and the entire Calvin cycle is
confined to these cells
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Figure 11-16
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The Hatch–Slack Cycle in a C4 Leaf
• The Hatch–Slack cycle begins with carboxylation
of PEP to form oxaloacetate (HS-1)
• Carboxylation is catalyzed by PEP carboxylase,
which is abundant in mesophyll cells
• Oxaloacetate is rapidly converted to malate by
NADPH-dependent malate dehydrogenase (HS-2)
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The Hatch–Slack Cycle in a C4 Leaf
(continued)
• Malate moves to bundle sheath cells, where
decarboxylation by NADP+ malic enzyme
releases CO2 (HS-3), which is refixed and
reduced by the Calvin cycle
• The pyruvate produced in HS-3 diffuses into
mesophyll cells, where it is phosphorylated, to
regenerate PEP (HS-4), at the expense of one
ATP
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Figure 11-17
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Figure 11-17A
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Figure 11-17B
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Carbon assimilation in a C4 plant
• Carbon assimilation in a C4 plant uses 5 ATP
rather than the 3 used in C3 plants
• When temperatures exceed about 30oC, the
efficiency of a C4 plant may be twice that of a
C3 plant
• C4 plants are also less affected by conditions of
low [CO2]
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CAM Plants Minimize Photorespiration
and Water Loss by Opening Stomata
Only at Night
• Crassulacean acid metabolism (CAM) plants
open stomata only at night to minimize water loss
• CO2 enters mesophyll cells and goes through the
first two steps of the Hatch–Slack cycle, to
produce malate
• The accumulated malate is stored in vacuoles
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CAM plants (continued)
• During the day the stomata are closed and the
malate diffuses to the cytosol where the Hatch–
Slack cycle continues
• CO2 released diffuses to chloroplast stroma,
where it is refixed and reduced in the Calvin
cycle
• CAM plants may assimilate over 25 times as
much carbon as a C3 plant does
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