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Chapter 08
Lecture and Animation Outline
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Photosynthesis
Chapter 8
2
3
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
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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5
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Vascular bundle Stoma
Cuticle
Epidermis
Mesophyll
Chloroplast
Inner membraneOuter membrane
Cell wall
1.58 mm
Vacuole
Courtesy Dr. Kenneth Miller, Brown University
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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
PhotosystemPhotosystem
Light-DependentReactions
CalvinCycle
Organicmolecules
O2
ATP NADPH
H2O
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8
Discovery of Photosynthesis
• Jan Baptista van Helmont (1580–1644)– Demonstrated that the substance of the plant
was not produced only from the soil
• Joseph Priestly (1733–1804)– Living vegetation adds something to the air
• Jan Ingenhousz (1730–1799)– Proposed plants carry out a process that uses
sunlight to split carbon dioxide into carbon and oxygen (O2 gas)
• F.F. Blackman (1866–1947)– Came to the startling
conclusion that photosynthesis is in fact a multistage process, only one portion of which uses light directly
– Light versus dark reactions
– Enzymes involved
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Maximum rate
Temperature limited
Excess CO2; 20ºC
CO2 limited
Light Intensity (foot-candles)500 1000 1500 2000 2500
Inc
rea
se
d R
ate
of
Ph
oto
sy
nth
es
is
0
Excess CO2; 35ºC
Insufficient CO2 (0.01%); 20ºC
Ligh
t lim
ited
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• C. B. van Niel (1897–1985)– Found purple sulfur bacteria do not release O2
but accumulate sulfur– Proposed general formula for photosynthesis
• CO2 + 2 H2A + light energy → (CH2O) + H2O + 2 A
– Later researchers found O2 produced comes from water
• Robin Hill (1899–1991)– Demonstrated Niel was right that light energy
could be harvested and used in a reduction reaction
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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
12
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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
UVlight
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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 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 achlorophyll 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
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Pigments 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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• Structure of chlorophyll
• porphyrin ring– Complex ring structure
with alternating double and single bonds
– Magnesium ion at the center of the ring
• Photons excite electrons in the ring
• Electrons are shuttled away from the ring
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H2C CH
CH2CH3
H
H
H
CO
CHCCH3
CHCH3
CH2
CH2
CH2
CHCH3
CH2
CH2
CH2
CHCH3
CH3
O
CO2CH3
O
N N
N N
Mg
H
HChlorophyll a: = CH3
Chlorophyll b: = CHO
R
R
R
H
Porphyrinhead
H3C
H3CCH3
CH2
CH2
CH2
CH2
CH2
CH2
Hydrocarbontail
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• Action spectrum– Relative effectiveness of different
wavelengths of light in promoting photosynthesis
– Corresponds to the absorption spectrum for chlorophylls
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Lig
ht
Ab
so
rbti
on
low
high Oxygen-seeking bacteria
Filament of green algae
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• Carotenoids– Carbon rings linked to
chains with alternating single and double bonds
– Can absorb photons with a wide range of energies
– Also scavenge free radicals – antioxidant
• Protective role
• Phycobiloproteins– Important in low-light
ocean areas19
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Oak leafin summer
Oak leafin autumn
© Eric Soder/pixsource.com
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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
Antenna complex
• Also called light-harvesting complex
• Captures photons from sunlight and channels them to the reaction center chlorophylls
• In chloroplasts, light-harvesting complexes consist of a web of chlorophyll molecules linked together and held tightly in the thylakoid membrane by a matrix of proteins
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22
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e–Photon
Photosystem
Thylakoid membrane
Chlorophyllmolecule
Electronacceptor
Reaction centerchlorophyll
Thylakoid membrane
Electrondonor 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–
–+–+
Excitedchlorophyllmolecule
Electrondonor
Electronacceptor
Chlorophyllreduced
Chlorophylloxidized
Donoroxidized
Acceptorreduced
e–
e– e–
e–
e–
e–
e–
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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
Cap
ture
of
light
ene
rgy
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• 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
Cyclic photophosphorylation
27
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En
erg
y o
f el
ectr
on
s
High
Low
e–
Photon
Photosystem
Excited reaction center
Electronacceptor
Electronacceptor
Reactioncenter (P870)
b-c1
complex ATPe–
e–
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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
31
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En
erg
y o
f el
ectr
on
s
Photon
Excited reaction center
Excited reaction center
Plastoquinone
Plastocyanin
Ferredoxin
Photosystem II
Photosystem I
Photon
b6-fcomplex
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.
Reactioncenter
Proton gradient formedfor ATP synthesis
Reactioncenter
e–
e–
PQ
e–
NADPreductase
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–
Two photosystems
32
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Rat
e o
f P
ho
tosy
nth
esis
low
high
Far-red light on Both lights onRed light onOff Off
Time
Off
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Photosystem II
• Resembles the reaction center of purple bacteria• Core of 10 transmembrane protein subunits with
electron transfer components and two P680 chlorophyll molecules
• Reaction center differs from purple bacteria in that it also 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
35
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Photosystem II Photosystem Ib6-f complex
Stroma
PlastoquinoneProton
gradientPlastocyanin Ferredoxin
H+
H+H+
H+
NADPH
ATPADP
+ NADP+
NADPHNADPATPADP + Pi
CalvinCycle
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.
NADPreductase
ATPsynthase
1/2O2 2H+
Water-splittingenzyme
Thylakoidspace
AntennacomplexThylakoid
membrane
Light-DependentReactions
H+
H+
e–22 22
22
22
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 ATP37
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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
41
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4 Pi
12 NADP+
12
12 ADP
NADPHNADP+ADP+ Pi
Light-DependentReactions
CalvinCycle
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
Carbondioxide (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 andother sugars
12 NADPH
ATP
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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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Energy cycle
• Photosynthesis uses the products of respiration as starting substrates
• Respiration uses the products of photosynthesis as starting substrates
• Production of glucose from G3P even uses part of the ancient glycolytic pathway, run in reverse
• Principal proteins involved in electron transport and ATP production in plants are evolutionarily related to those in mitochondria
45
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O2
Heat
ATP NADPH NADH
ATP
Sunlight
Pyruvate
CO2
Glucose
ADP + Pi NAD+NADP+
H2O
Photo-system
II
Photo-system
I
ElectronTransportSystem
ADP + Pi
ADP + Pi
ATP
ATP
CalvinCycle
KrebsCycle
46
Photorespiration
• Rubisco has 2 enzymatic activities– Carboxylation
• Addition of CO2 to RuBP
• Favored under normal conditions
– Photorespiration• Oxidation of RuBP by the addition of O2
• Favored when stoma are closed in hot conditions
• Creates low-CO2 and high-O2
• CO2 and O2 compete for the active site on RuBP
47
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Heat
Stomata
O2O2
CO2 CO2
Under hot, arid conditions, leaves lose water byevaporation through openings in the leavescalled stomata.
The stomata close to conserve water but as aresult, O2 builds up inside the leaves, and CO2
cannot enter the leaves.
Leafepidermis
H2OH2O
48
Types of photosynthesis
• C3
– Plants that fix carbon using only C3 photosynthesis (the Calvin cycle)
• C4 and CAM
– Add CO2 to PEP to form 4 carbon molecule
– Use PEP carboxylase
– Greater affinity for CO2, no oxidase activity
– C4 – spatial solution
– CAM – temporal solution
49
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CO2
RuBP
3PG(C3)
a. C4 pathway
Bundle-sheath cellMesophyll cell
Stoma Vein
G3P
b. C4 pathwayStoma Vein
Mesophyll cell
G3P
CO2
CO2
C4
Bundle-sheath cell
Mesophyllcell
Bundle-sheathcell
CalvinCycle
Mesophyllcell
CalvinCycle
a: © John Shaw/Photo Researchers, Inc. b: © Joseph Nettis/National Audubon Society Collection/Photo Researchers, Inc.
50
C4 plants
• Corn, sugarcane, sorghum, and a number of other grasses
• Initially fix carbon using PEP carboxylase in mesophyll cells
• Produces oxaloacetate, converted to malate, transported to bundle-sheath cells
• Within the bundle-sheath cells, malate is decarboxylated to produce pyruvate and CO2
• Carbon fixation then by rubisco and the Calvin cycle
51
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Oxaloacetate
Pyruvate Malate
Glucose
MalatePyruvate
+ Pi
Mesophyllcell
Phosphoenolpyruvate (PEP)
Bundle-sheathcell
CalvinCycle
AMP +PPi
ATP
CO2
CO2
• C4 pathway, although it overcomes the problems of photorespiration, does have a cost
• To produce a single glucose requires 12 additional ATP compared with the Calvin cycle alone
• C4 photosynthesis is advantageous in hot dry climates where photorespiration would remove more than half of the carbon fixed by the usual C3 pathway alone
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CAM plants
• Many succulent (water-storing) plants, such as cacti, pineapples, and some members of about two dozen other plant groups
• Stomata open during the night and close during the day– Reverse of that in most plants
• Fix CO2 using PEP carboxylase during the night and store in vacuole
• When stomata closed during the day, organic acids are decarboxylated to yield high levels of CO2
• High levels of CO2 drive the Calvin cycle and minimize photorespiration
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55
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night
day
CO2
CO2
C4
G3P
CalvinCycle
(inset): © 2011 Jessica Solomatenko/Getty Images RF
Compare C4 and CAM
• Both use both C3 and C4 pathways
• C4 – two pathways occur in different cells
• CAM – C4 pathway at night and the C3 pathway during the day
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