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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.
Leaf cross section
Stomata
Vein
CO2 O2
Figure 7.2-1
Leaf cross section
Stomata
Vein
CO2 O2
Interior cellL
M
StromaGranum Thylakoid
ChloroplastOutermembrane
Innermembrane
TE
M
Figure 7.2-2
Leaf cross section
Stomata
Vein
CO2 O2
Figure 7.2a
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.
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
Light
H2O
O2
Chloroplast
Lightreactions
NADPH
ATP
Figure 7.3-1
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.
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.
Chloroplast
Figure 7.9-1
Chloroplast
Thylakoidmembrane
Pigment molecules
Figure 7.9-2
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
Primaryelectronacceptor
Water-splittingphotosystem
Light
H2O
2 H
Reaction-centerchlorophyll
2e–
2e–
O2++ 1
2
Figure 7.10-1
Primaryelectronacceptor
Water-splittingphotosystem
Light
H2O
2 H
Reaction-centerchlorophyll
2e–
2e–
O2++
Electron transport chain
Energyto make
ATP
1
2
Figure 7.10-2
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
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
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
Water-splittingphotosystem
e–
Ph
oto
n
ATP
NADPH
e–
e–
e–
e–
e–
Ph
oto
n
e–
NADPH-producingphotosystem
Figure 7.12
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
ATP
NADPH
Calvincycle
ADP P
NADP
P
P P
P
G3P sugar
Three-carbon moleculeRuBP sugar
CO2 (from air)
Figure 7.13-2
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
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
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
Sugar
C4 Pathway(example: sugarcane)
C4 plant
Calvincycle
Celltype 1
Four-carboncompound
Celltype 2
CO2
CO2
Figure 7.14a
CAM plant
Sugar
Calvincycle
Day
Four-carboncompound
Night
CAM Pathway(example: pineapple)
CO2
CO2
Figure 7.14b
Carbon dioxide
6 O26 CO2 6 H2O C6H12O6
Water GlucosePhoto-synthesis Oxygen gas
Light energy
Figure 7.UN1
Light
reactions
CO2
O2
H2O
(C6H12O6)
NADPH
Light
Sugar
ATP
ADPP
NADP
Calvin
cycle
Figure 7.UN2
Light
reactions
CO2
O2
H2O
(C6H12O6)
NADPH
Light
Sugar
ATP
ADP
P
NADP
Calvin
cycle
Figure 7.UN3
Carbon dioxide
6 O26 CO2 6 H2O C6H12O6
Water GlucosePhotosynthesis
Oxygen gas
Light energy
Figure 7.UN4
Light H2O
O2
Chloroplast
Lightreactions
NADPH
ATP
Calvincycle
CO2
NADP+
ADPP
Sugar
(C6H12O6)
Stack ofthylakoids Stroma
Figure 7.UN5
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
NADPH
Calvincycle
ADP P
NADP
P
ATP
G3P
CO2
Glucose andother compounds
Figure 7.UN7