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• Overview: The Process That Feeds the Biosphere
• Photosynthesis
– Is the process that converts solar energy into chemical energy
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• Plants and other autotrophs
– Are the producers of the biosphere
• Plants are photoautotrophs
– They use the energy of sunlight to make organic molecules from water and carbon dioxide
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• Photosynthesis
– Occurs in plants, algae, certain other protists, and some prokaryotes
These organisms use light energy to drive the synthesis of organic molecules from carbon dioxideand (in most cases) water. They feed not onlythemselves, but the entire living world. (a) Onland, plants are the predominant producers offood. In aquatic environments, photosyntheticorganisms include (b) multicellular algae, suchas this kelp; (c) some unicellular protists, suchas Euglena; (d) the prokaryotes calledcyanobacteria; and (e) other photosyntheticprokaryotes, such as these purple sulfurbacteria, which produce sulfur (sphericalglobules) (c, d, e: LMs).
(a) Plants
(b) Multicellular algae
(c) Unicellular protist 10 m
40 m(d) Cyanobacteria
1.5 m(e) Pruple sulfurbacteria
Figure 10.2
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• Heterotrophs
– Obtain their organic material from other organisms
– Are the consumers of the biosphere
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• Concept 10.1: Photosynthesis converts light energy to the chemical energy of food
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Chloroplasts: The Sites of Photosynthesis in Plants
• The leaves of plants
– Are the major sites of photosynthesis
Vein
Leaf cross section
Figure 10.3
Mesophyll
CO2 O2Stomata
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• Chloroplasts
– Are the organelles in which photosynthesis occurs
– Contain thylakoids and grana
Chloroplast
Mesophyll
5 µm
Outermembrane
Intermembranespace
Innermembrane
Thylakoidspace
Thylakoid
GranumStroma
1 µm
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Tracking Atoms Through Photosynthesis: Scientific Inquiry
• Photosynthesis is summarized as
6 CO2 + 12 H2O + Light energy C6H12O6 + 6 O2 + 6 H2 O
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Photosynthesis as a Redox Process
• Photosynthesis is a redox process
– Water is oxidized, carbon dioxide is reduced
– This is the reverse flow of electrons as in cellular respiration
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The Two Stages of Photosynthesis: A Preview
• Photosynthesis consists of two processes
– The light reactions
– The Calvin cycle
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• The light reactions
– Occur in the grana
– Split water, release oxygen, produce ATP, and form NADPH
• The ATP is produced by photophosphorylation – generating ATP using chemiosmosis to power the addition of phosphate to ADP
– The third way of generating ATP we’ve seen
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• The Calvin cycle
– Occurs in the stroma
– Forms sugar from carbon dioxide, using ATP for energy and NADPH for reducing power
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• An overview of photosynthesis
H2O CO2
Light
LIGHT REACTIONS
CALVINCYCLE
Chloroplast
[CH2O](sugar)
NADPH
NADP
ADP
+ P
O2Figure 10.5
ATP
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• Concept 10.2: The light reactions convert solar energy to the chemical energy of ATP and NADPH
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• The absorption spectra of chloroplast pigments
– Provide clues to the relative effectiveness of different wavelengths for driving photosynthesis
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• The absorption spectra of three types of pigments in chloroplasts Three different experiments helped reveal which wavelengths of light are photosynthetically important. The results are shown below.
EXPERIMENT
RESULTSA
bso
rptio
n o
f lig
ht
by
chlo
rop
last
pig
me
nts
Chlorophyll a
(a) Absorption spectra. The three curves show the wavelengths of light best absorbed by three types of chloroplast pigments.
Wavelength of light (nm)
Chlorophyll b
Carotenoids
Figure 10.9
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• Chlorophyll a
– Is the main photosynthetic pigment
• Chlorophyll b
– Is an accessory pigment C
CH
CH2
CC
CC
C
CNNC
H3C
C
CC
C C
C
C
C
N
CC
C
C N
MgH
H3C
H
C CH2CH3
H
CH3C
HHCH2
CH2
CH2
H CH3
C O
O
O
O
O
CH3
CH3
CHO
in chlorophyll a
in chlorophyll b
Porphyrin ring:Light-absorbing“head” of moleculenote magnesiumatom at center
Hydrocarbon tail:interacts with hydrophobicregions of proteins insidethylakoid membranes ofchloroplasts: H atoms notshown
Figure 10.10
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• Other accessory pigments
– Absorb different wavelengths of light and pass the energy to chlorophyll a
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Excitation of Chlorophyll by Light
• When a pigment absorbs light
– It goes from a ground state to an excited state, which is unstable
Excitedstate
Ene
rgy
of e
lect
ion
Heat
Photon(fluorescence)
Chlorophyllmolecule
GroundstatePhoton
e–
Figure 10.11 A
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A Photosystem: A Reaction Center Associated with Light-Harvesting Complexes
• A photosystem
– Is composed of a reaction center surrounded by a number of light-harvesting complexes
Primary electionacceptor
Photon
Thylakoid
Light-harvestingcomplexes
Reactioncenter
Photosystem
STROMA
Thy
lako
id m
embr
ane
Transferof energy
Specialchlorophyll amolecules
Pigmentmolecules
THYLAKOID SPACE(INTERIOR OF THYLAKOID)Figure 10.12
e–
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• The light-harvesting complexes
– Consist of pigment molecules bound to particular proteins
– Act as an antenna for the reaction center
– Funnel the energy of photons of light to the reaction center
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• Reaction center –
– Is a protein complex, including two special chlorophyll molecules and the primary electron acceptor
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First step of light reactions
• When a reaction-center chlorophyll molecule absorbs energy
– One of its electrons gets bumped up to a primary electron acceptor
– This is a redox rxn
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• The thylakoid membrane
– Is populated by two types of photosystems, I and II
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Noncyclic Electron Flow
• Noncyclic electron flow
– Is the primary pathway of energy transformation in the light reactions
– Produces ATP and NADPH which will provide chemical energy and reducing power to the sugar making Calvin cycle
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• Produces NADPH, ATP, and oxygen
Figure 10.13Photosystem II
(PS II)
Photosystem-I(PS I)
ATP
NADPH
NADP+
ADP
CALVINCYCLE
CO2H2O
O2 [CH2O] (sugar)
LIGHTREACTIONS
Light
Primaryacceptor
Pq
Cytochromecomplex
PC
e
P680
e–
e–
O2
+
H2O2 H+
Light
ATP
Primaryacceptor
Fd
ee–
NADP+
reductase
ElectronTransportchain
Electron transport chain
P700
Light
NADPH
NADP+
+ 2 H+
+ H+
1
5
7
2
3
4
6
8
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Cyclic Electron Flow
• Under certain conditions
– Photoexcited electrons take an alternative path
– This is because the Calvin cycle needs more ATP than NADPH.
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• In cyclic electron flow
– Only photosystem I is used
– Only ATP is produced
Primaryacceptor
Pq
Fd
Cytochromecomplex
Pc
Primaryacceptor
Fd
NADP+
reductaseNADPH
ATPFigure 10.15
Photosystem II Photosystem I
NADP+
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A Comparison of Chemiosmosis in Chloroplasts and Mitochondria
• Chloroplasts and mitochondria
– Generate ATP by the same basic mechanism: chemiosmosis
– But use different sources of energy to accomplish this
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• In both organelles
– Redox reactions of electron transport chains generate a H+ gradient across a membrane
• ATP synthase
– Uses this proton-motive force to make ATP
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• The light reactions and chemiosmosis: the organization of the thylakoid membrane
LIGHTREACTOR
NADP+
ADP
ATP
NADPH
CALVINCYCLE
[CH2O] (sugar)STROMA(Low H+ concentration)
Photosystem II
LIGHT
H2O CO2
Cytochromecomplex
O2
H2O O21
1⁄2
2
Photosystem ILight
THYLAKOID SPACE(High H+ concentration)
STROMA(Low H+ concentration)
Thylakoidmembrane
ATPsynthase
PqPc
Fd
NADP+
reductase
NADPH + H+
NADP+ + 2H+
ToCalvincycle
ADP
PATP
3
H+
2 H++2 H+
2 H+
Figure 10.17
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• Concept 10.3: The Calvin cycle uses ATP and NADPH to convert CO2 to sugar
• The Calvin cycle
– Is similar to the citric acid cycle
– Occurs in the stroma
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• The Calvin cycle has three phases
– Carbon fixation
– Reduction
– Regeneration of the CO2 acceptor
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• The Calvin cycle
(G3P)
Input(Entering one
at a time)CO2
3
Rubisco
Short-livedintermediate
3 P P
3 P P
Ribulose bisphosphate(RuBP)
P
3-Phosphoglycerate
P6 P
6
1,3-Bisphoglycerate
6 NADPH
6 NADPH+
6 P
P6
Glyceraldehyde-3-phosphate(G3P)
6 ATP
3 ATP
3 ADP CALVINCYCLE
P5
P1
G3P(a sugar)Output
LightH2O CO2
LIGHTREACTION
ATP
NADPH
NADP+
ADP
[CH2O] (sugar)
CALVINCYCLE
Figure 10.18
O2
6 ADP
Glucose andother organiccompounds
Phase 1: Carbon fixation
Phase 2:Reduction
Phase 3:Regeneration ofthe CO2 acceptor(RuBP)
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• Concept 10.4: Alternative mechanisms of carbon fixation have evolved in hot, arid climates
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• On hot, dry days, plants close their stomata
– Conserving water but limiting access to CO2
– Causing oxygen to build up
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Photorespiration: An Evolutionary Relic?
• In photorespiration
– O2 substitutes for CO2 in the active site of the enzyme rubisco
– The photosynthetic rate is reduced
– No sugar is made, ATP is used, not made
– Hold over from earlier time
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C4 Plants
• C4 plants minimize the cost of photorespiration
– By incorporating CO2 into four carbon compounds in mesophyll cells
– These four carbon compounds
• Are exported to bundle sheath cells, where they release CO2 used in the Calvin cycle
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• C4 leaf anatomy and the C4 pathway
CO2
Mesophyll cell
Bundle-sheathcell
Vein(vascular tissue)
Photosyntheticcells of C4 plantleaf
Stoma
Mesophyllcell
C4 leaf anatomy
PEP carboxylase
Oxaloacetate (4 C) PEP (3 C)
Malate (4 C)
ADP
ATP
Bundle-Sheathcell CO2
Pyruate (3 C)
CALVINCYCLE
Sugar
Vasculartissue
Figure 10.19
CO2
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CAM Plants
• CAM plants
– Open their stomata at night, incorporating CO2 into organic acids
– During the day, the stomata close
– And the CO2 is released from the organic acids for use in the Calvin cycle
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• The CAM pathway is similar to the C4 pathway
Spatial separation of steps. In C4 plants, carbon fixation and the Calvin cycle occur in differenttypes of cells.
(a) Temporal separation of steps. In CAM plants, carbon fixation and the Calvin cycle occur in the same cellsat different times.
(b)
PineappleSugarcane
Bundle-sheath cell
Mesophyll Cell
Organic acid
CALVINCYCLE
Sugar
CO2 CO2
Organic acid
CALVINCYCLE
Sugar
C4 CAM
CO2 incorporatedinto four-carbonorganic acids(carbon fixation)
Night
Day
1
2 Organic acidsrelease CO2 toCalvin cycle
Figure 10.20
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The Importance of Photosynthesis: A Review
• A review of photosynthesis
Light reactions:• Are carried out by molecules in the thylakoid membranes• Convert light energy to the chemical energy of ATP and NADPH• Split H2O and release O2 to the atmosphere
Calvin cycle reactions:• Take place in the stroma• Use ATP and NADPH to convert CO2 to the sugar G3P• Return ADP, inorganic phosphate, and NADP+ to the light reactions
O2
CO2H2O
Light
Light reaction Calvin cycle
NADP+
ADP
ATP
NADPH
+ P 1
RuBP 3-Phosphoglycerate
Amino acidsFatty acids
Starch(storage)
Sucrose (export)
G3P
Photosystem IIElectron transport chain
Photosystem I
Chloroplast
Figure 10.21
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• Organic compounds produced by photosynthesis
– Provide the energy and building material for ecosystems