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TRANSCRIPT
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© 2010 Pearson Education, Inc.
THE BASICS OF PHOTOSYNTHESIS• Photosynthesis:
– Is used by plants, some protists, and some bacteria
– Transforms light energy into chemical energy
– Uses carbon dioxide and water as starting materials
• The chemical energy produced via photosynthesis is stored in the bonds of sugar molecules.
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Chloroplasts: Sites of Photosynthesis• Chloroplasts are:
– The site of photosynthesis
– Found mostly in the interior cells of leaves
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• Inside chloroplasts are membranous sacs called thylakoids, which are suspended in a thick fluid, called stroma.
• Thylakoids are concentrated in stacks called grana.
• The green color of chloroplasts is from chlorophyll, a light-absorbing pigment.
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• Stomata are tiny pores in leaves where carbon dioxide enters and oxygen exits.
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Leaf cross section
Stomata
Vein
CO2 O2
Figure 7.2-1
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Leaf cross section
Stomata
Vein
CO2 O2
Interior cellL
M
StromaGranum Thylakoid
ChloroplastOutermembrane
Innermembrane
TE
M
Figure 7.2-2
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Leaf cross section
Stomata
Vein
CO2 O2
Figure 7.2a
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StromaGranum Thylakoid
Chloroplast Outermembrane
Innermembrane
TE
M
Figure 7.2b
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The Overall Equation for Photosynthesis• In the overall equation for photosynthesis, notice that:
– The reactants of photosynthesis are the waste products of cellular respiration.
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Carbon dioxide
6 O26 CO2 6 H2O C6H12O6
Water GlucosePhoto-synthesis Oxygen gas
Light energy
Figure 7.UN1
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• In photosynthesis:
– Sunlight provides the energy
– Electrons are boosted “uphill” and added to carbon dioxide
– Sugar is produced
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• During photosynthesis, water is split into:
– Hydrogen
– Oxygen
• Hydrogen is transferred along with electrons and added to carbon dioxide to produce sugar.
• Oxygen escapes through stomata into the atmosphere.
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A Photosynthesis Road Map• Photosynthesis occurs in two stages:
– The light reactions convert solar energy to chemical energy
– The Calvin cycle uses the products of the light reactions to make sugar from carbon dioxide
Blast Animation: Photosynthesis: Light-Independent Reaction
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Light
H2O
O2
Chloroplast
Lightreactions
NADPH
ATP
Figure 7.3-1
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Light
H2O
O2
Chloroplast
Lightreactions
NADPH
ATP
Calvincycle
CO2
NADP+
ADPP
Sugar
(C6H12O6)
Figure 7.3-2
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How Photosystems Harvest Light Energy• Light behaves as photons, discrete packets of energy.
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• Chlorophyll molecules absorb photons.
– Electrons in the pigment gain energy.
– As the electrons fall back to their ground state, energy is released as heat or light.
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Light
Photon
Heat
Light (fluorescence)
Ground stateChlorophyllmolecule
Excited state
e–
(a) Absorption of a photonFigure 7.8a
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• A photosystem is a group of chlorophyll and other molecules that function as a light-gathering antenna.
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Chloroplast
Figure 7.9-1
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Chloroplast
Thylakoidmembrane
Pigment molecules
Figure 7.9-2
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Chloroplast
Thylakoidmembrane
Pigment molecules
Antenna pigmentmolecules
Primaryelectronacceptor
Reaction-centerchlorophyll a
Photon
Electrontransfer
Reactioncenter
PhotosystemTransfer of energy
Figure 7.9-3
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How the Light Reactions Generate ATP and NADPH
• Two types of photosystems cooperate in the light reactions:
– The water-splitting photosystem
– The NADPH-producing photosystem
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Primaryelectronacceptor
Water-splittingphotosystem
Light
H2O
2 H
Reaction-centerchlorophyll
2e–
2e–
O2++ 1
2
Figure 7.10-1
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Primaryelectronacceptor
Water-splittingphotosystem
Light
H2O
2 H
Reaction-centerchlorophyll
2e–
2e–
O2++
Electron transport chain
Energyto make
ATP
1
2
Figure 7.10-2
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Primaryelectronacceptor
Water-splittingphotosystem
Light
H2O
2 H
Reaction-centerchlorophyll
2e–
2e–
O2++
Electron transport chain
Energyto make
ATP
Primaryelectronacceptor
2e–
Light
NADPH-producingphotosystem
Reaction-centerchlorophyll
2e–
NADPH
NADP
1
2
Figure 7.10-3
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• The light reactions are located in the thylakoid membrane.
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• An electron transport chain:
– Connects the two photosystems
– Releases energy that the chloroplast uses to make ATP
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Light
H2O
Thylakoidmembrane
2e–
O2
ATP
NADP
Light
Stroma
Inside thylakoid
Photosystem PhotosystemElectron transport chain
NADPH
ADP P
H+
ATPsynthase
To Calvin cycle
H
Electron flow
H+H
H
H
H
1
2
Figure 7.11
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Light
H2O
Thylakoidmembrane
2e–
O2
ATP
NADP
Light
Stroma
Inside thylakoid
Photosystem PhotosystemElectron transport chain
NADPH
ADP P
ATPsynthase
To Calvin cycle
Electron flow
H
1
2
HH
H
H H
H
Figure 7.11a
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Water-splittingphotosystem
e–
Ph
oto
n
ATP
NADPH
e–
e–
e–
e–
e–
Ph
oto
n
e–
NADPH-producingphotosystem
Figure 7.12
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THE CALVIN CYCLE: MAKING SUGAR FROM CARBON DIOXIDE
• The Calvin cycle:
– Functions like a sugar factory within the stroma of a chloroplast
– Regenerates the starting material with each turn
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Calvincycle
P P
P
Three-carbon moleculeRuBP sugar
CO2 (from air)
Figure 7.13-1
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ATP
NADPH
Calvincycle
ADP P
NADP
P
P P
P
G3P sugar
Three-carbon moleculeRuBP sugar
CO2 (from air)
Figure 7.13-2
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ATP
NADPH
ADP P
NADP
P
P
P
P P
P
G3P sugar
Three-carbon molecule
G3P sugar
G3P sugar
RuBP sugar
CO2 (from air)
Calvincycle
Glucose (and other organic compounds)
Figure 7.13-3
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ATP
NADPH
Calvincycle
ADP P
NADP
P
P
P
P P
P
ADP P
ATP
G3P sugar
Three-carbon molecule
G3P sugar
G3P sugar
RuBP sugar
CO2 (from air)
Glucose (and other organic compounds)
Figure 7.13-4
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Evolution Connection:Solar-Driven Evolution
• C3 plants:
– Use CO2 directly from the air
– Are very common and widely distributed
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• C4 plants:
– Close their stomata to save water during hot and dry weather
– Can still carry out photosynthesis
• CAM plants:
– Are adapted to very dry climates
– Open their stomata only at night to conserve water
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Sugar
C4 Pathway(example: sugarcane)
C4 plant
Calvincycle
Celltype 1
Four-carboncompound
Celltype 2
CO2
CO2
Figure 7.14a
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CAM plant
Sugar
Calvincycle
Day
Four-carboncompound
Night
CAM Pathway(example: pineapple)
CO2
CO2
Figure 7.14b
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Carbon dioxide
6 O26 CO2 6 H2O C6H12O6
Water GlucosePhoto-synthesis Oxygen gas
Light energy
Figure 7.UN1
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Light
reactions
CO2
O2
H2O
(C6H12O6)
NADPH
Light
Sugar
ATP
ADPP
NADP
Calvin
cycle
Figure 7.UN2
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Light
reactions
CO2
O2
H2O
(C6H12O6)
NADPH
Light
Sugar
ATP
ADP
P
NADP
Calvin
cycle
Figure 7.UN3
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Carbon dioxide
6 O26 CO2 6 H2O C6H12O6
Water GlucosePhotosynthesis
Oxygen gas
Light energy
Figure 7.UN4
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Light H2O
O2
Chloroplast
Lightreactions
NADPH
ATP
Calvincycle
CO2
NADP+
ADPP
Sugar
(C6H12O6)
Stack ofthylakoids Stroma
Figure 7.UN5
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acceptor
Water-splittingphotosystem
Photon
H2O
2 H
Chlorophyll
2e–
O2++
Electron transport chain
ATP
NADPH-producingphotosystem
Chlorophyll
NADPH
NADP+
e–
2e–
2e–
2e–
e–
acceptorADP
Photon
21
Figure 7.UN6
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NADPH
Calvincycle
ADP P
NADP
P
ATP
G3P
CO2
Glucose andother compounds
Figure 7.UN7