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Photosynthesis
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Photosynthesis Overview
– Energy for all life on Earth ultimately comes from photosynthesis
6CO2 + 12H2O C6H12O6 + 6H2O + 6O2
– Oxygenic photosynthesis is carried out by
– Cyanobacteria
– 7 groups of algae
– All land plants – chloroplasts
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Chloroplast
– Thylakoid membrane – internal membrane
– Contains chlorophyll and other photosynthetic
pigments
– Pigments clustered into photosystems
– Grana – stacks of flattened sacs of thylakoid
membrane
– Stroma lamella – connect grana
– Stroma – semiliquid surrounding thylakoid
membranes
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Vascular bundle Stoma
Cuticle
Epidermis
Mesophyll
Chloroplast
Inner membrane
Outer membrane
Cell wall
1.58 mm
Vacuole
Courtesy Dr. Kenneth Miller, Brown University
Stages
– Light-dependent reactions
– Require light
1.Capture energy from sunlight
2.Make ATP and reduce NADP+ to NADPH
– Carbon fixation reactions or light-independent
reactions
– Does not require light
3.Use ATP and NADPH to synthesize organic
molecules from CO2
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O2
Stroma
Photosystem
Thylakoid
NADP+ADP + Pi
CO2
Sunlight
Photosystem
Light-Dependent
Reactions
Calvin
Cycle
Organic
molecules
O2
ATP NADPH
H2O
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Pigments
– Molecules that absorb light energy in the visible
range
– Light is a form of energy
– Photon – particle of light
– Acts as a discrete bundle of energy
– Energy content of a photon is inversely
proportional to the wavelength of the light
– Photoelectric effect – removal of an electron
from a molecule by light
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400 nm
Visible light
430 nm 500 nm 560 nm 600 nm 650 nm 740 nm
1 nm0.001 nm 10 nm 1000 nm
Increasing wavelength
Increasing energy
0.01 cm 1 cm 1 m
Radio wavesInfraredX-raysGamma rays
100 m
UV
light
Absorption spectrum
– When a photon strikes a molecule, its energy is
either
– Lost as heat
– Absorbed by the electrons of the molecule
– Boosts electrons into higher energy level
– Absorption spectrum – range and efficiency of
photons a molecule is capable of absorbing
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Wavelength (nm)
400 450 500 550 600 650 700
Lig
ht
Ab
so
rbti
on
low
highcarotenoidschlorophyll a
chlorophyll b
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– Organisms have evolved a variety of different
pigments
– Only two general types are used in green plant
photosynthesis
– Chlorophylls
– Carotenoids
– In some organisms, other molecules also absorb
light energy
11Pigments in Photosynthesis
Chlorophylls
– Chlorophyll a
– Main pigment in plants and cyanobacteria
– Only pigment that can act directly to convert
light energy to chemical energy
– Absorbs violet-blue and red light
– Chlorophyll b
– Accessory pigment or secondary pigment
absorbing light wavelengths that chlorophyll a
does not absorb
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Photosystem Organization
– Antenna complex
– Hundreds of accessory pigment molecules
– Gather photons and feed the captured light energy to the reaction center
– Reaction center
– 1 or more chlorophyll a molecules
– Passes excited electrons out of the
photosystem
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e–Photon
Photosystem
Thylakoid membrane
Chlorophyll
molecule
Electron
acceptor
Reaction center
chlorophyll
Thylakoid membrane
Electron
donor e–
Reaction center
– Transmembrane protein–pigment complex
– When a chlorophyll in the reaction center absorbs a photon of light, an electron is excited to a higher energy level
– Light-energized electron can be transferred to the primary electron acceptor, reducing it
– Oxidized chlorophyll then fills its electron “hole” by oxidizing a donor molecule
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Light
e–
–+–+
Excited
chlorophyll
molecule
Electron
donorElectron
acceptor
Chlorophyll
reduced
Chlorophyll
oxidizedDonor
oxidizedAcceptor
reduced
e–
e– e–
e–
e–
e–
e–
Light-Dependent Reactions
1. Primary photoevent
– Photon of light is captured by a pigment molecule
2. Charge separation
– Energy is transferred to the reaction center; an excited electron is transferred to an acceptor molecule
3. Electron transport
– Electrons move through carriers to reduce NADP+
4. Chemiosmosis
– Produces ATP
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Captu
re o
f lig
ht
energ
y
Cyclic photophosphorylation
– In sulfur bacteria, only one photosystem is used
– Generates ATP via electron transport
– Anoxygenic photosynthesis
– Excited electron passed to electron transport
chain
– Generates a proton gradient for ATP synthesis
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En
erg
y o
f ele
ctr
on
s
High
Low
e–
Photon
Photosystem
Excited reaction center
Electron
acceptor
Electron
acceptor
Reaction
center (P870)
b-c1
complex ATPe–
e–
Chloroplasts have two
connected photosystems
– Oxygenic photosynthesis
– Photosystem I (P700)
– Functions like sulfur bacteria
– Photosystem II (P680)
– Can generate an oxidation potential high
enough to oxidize water
– Working together, the two photosystems carry out
a noncyclic transfer of electrons that is used to
generate both ATP and NADPH
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– Photosystem I transfers electrons ultimately to
NADP+, producing NADPH
– Electrons lost from photosystem I are replaced by
electrons from photosystem II
– Photosystem II oxidizes water to replace the
electrons transferred to photosystem I
– 2 photosystems connected by cytochrome/ b6-f
complex
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Noncyclic
photophosphorylation
– Plants use photosystems II and I in series to
produce both ATP and NADPH
– Path of electrons not a circle
– Photosystems replenished with electrons
obtained by splitting water
– Z diagram
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En
erg
y o
f ele
ctr
on
s
Photon
Excited reaction center
Excited reaction center
Plastoquinone
Plastocyanin
Ferredoxin
Photosystem II
Photosystem I
Photon
b6-f
complex
3. A pair of chlorophylls in the reaction
center absorb two photons. This
excites two electrons that are passed to
NADP+, reducing it to NADPH. Electron
transport from photosystem II replaces
these electrons.
H2O
H+PC
Fd
2H+ + 1/2O2
NADP+ + H+
2
2
2
2
2
1. A pair of chlorophylls in the reaction center absorb
two photons of light. This excites two electrons that
are transferred to plastoquinone (PQ). Loss of
electrons from the reaction center produces an
oxidation potential capable of oxidizing water.
Reaction
center
Proton gradient formed
for ATP synthesis
Reaction
center
e–
e–
PQ
e–
NADP
reductase
NADPHe–
2. The electrons pass through the b6-f
complex, which uses the energy
released to pump protons across
the thylakoid membrane. The proton
gradient is used to produce ATP by
chemiosmosis.
e–
Photosystem II
– Core of 10 transmembrane protein subunits with electron transfer components and two P680 chlorophyll molecules
– Reaction center contains four manganese atoms
– Essential for the oxidation of water
– b6-f complex
– Proton pump embedded in thylakoid membrane
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Photosystem I
– Reaction center consists of a core
transmembrane complex consisting of 12 to 14
protein subunits with two bound P700 chlorophyll
molecules
– Photosystem I accepts an electron from
plastocyanin into the “hole” created by the exit of
a light-energized electron
– Passes electrons to NADP+ to form NADPH
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Photosystem II Photosystem Ib6-f complex
Stroma
Plastoquinone
Proton
gradientPlastocyanin Ferredoxin
H+
H+H+
H+
NADPH
ATPADP
+ NADP+
NADPHNADPATPADP + Pi
Calvin
Cycle
PhotonPhoton
H2O
e–e–
e–
Fd
PC
PQ
1. Photosystem II
absorbs photons,
exciting electrons
that are passed to
plastoquinone (PQ).
Electrons lost from
photosystem II are
replaced by the
oxidation of water,
producing O2
2. The b6-f complex
receives electrons
from PQ and passes
them to plastocyanin
(PC). This provides
energy for the b6-f
complex to pump
protons into the
thylakoid.
3. Photosystem I absorbs
photons, exciting
electrons that are
passed through a
carrier to reduce
NADP+ to NADPH.
These electrons are
replaced by electron
transport from
photosystem II.
4. ATP synthase uses
the proton gradient
to synthesize ATP
from ADP and Pi
enzyme acts as a
channel for protons
to diffuse back into
the stroma using this
energy to drive the
synthesis of ATP.
NADP
reductase
ATP
synthase
1/2O2 2H+
Water-splitting
enzyme
Thylakoid
space
Antenna
complexThylakoid
membrane
Light-Dependent
Reactions
H+
H+
e–22 22
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Chemiosmosis
– Electrochemical gradient can be used to synthesize ATP
– Chloroplast has ATP synthase enzymes in the thylakoid membrane
– Allows protons back into stroma
– Stroma also contains enzymes that catalyze the reactions of carbon fixation –the Calvin cycle reactions
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Production of additional
ATP
– Noncyclic photophosphorylation generates
– NADPH
– ATP
– Building organic molecules takes more energy than that alone
– Cyclic photophosphorylation used to produce additional ATP
– Short-circuit photosystem I to make a larger proton gradient to make more ATP
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Carbon Fixation – Calvin
Cycle
– To build carbohydrates cells use
– Energy
– ATP from light-dependent reactions
– Cyclic and noncyclic photophosphorylation
– Drives endergonic reaction
– Reduction potential
– NADPH from photosystem I
– Source of protons and energetic electrons
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Calvin cycle
– Named after Melvin Calvin (1911–1997)
– Also called C3 photosynthesis
– Key step is attachment of CO2 to RuBP to form
PGA
– Uses enzyme ribulose bisphosphate
carboxylase/oxygenase or rubisco
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3 phases
1. Carbon fixation
– RuBP + CO2 → PGA
2. Reduction
– PGA is reduced to G3P
3. Regeneration of RuBP
– PGA is used to regenerate RuBP
– 3 turns incorporate enough carbon to produce a
new G3P
– 6 turns incorporate enough carbon for 1 glucose
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4 Pi
12 NADP+
12
12 ADP
NADPHNADP+ADP+ Pi
Light-Dependent
Reactions
Calvin
Cycle
6 molecules of12 molecules of
12 molecules of
1,3-bisphosphoglycerate (3C)
12 molecules of
Glyceraldehyde 3-phosphate (3C) (G3P)
10 molecules of
Glyceraldehyde 3-phosphate (3C) (G3P)
Stroma of chloroplast6 molecules of
Carbon
dioxide (CO2)
12 ATP
6 ADP
6 ATP
Rubisco
Calvin Cycle
Pi
Ribulose 1,5-bisphosphate (5C) (RuBP)3-phosphoglycerate (3C) (PGA)
Glyceraldehyde 3-phosphate (3C)
2 molecules of
Glucose and
other sugars
12 NADPH
ATP
Output of Calvin cycle
– Glucose is not a direct product of the Calvin cycle
– G3P is a 3 carbon sugar
– Used to form sucrose
– Major transport sugar in plants
– Disaccharide made of fructose and glucose
– Used to make starch
– Insoluble glucose polymer
– Stored for later use
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