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Where It Starts: Photosynthesis
Chapter 5
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Photosynthesis
Metabolic Pathways
Converts light energy to chemical energy.
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Photoautotrophs
Organisms that can perform photosynthesis
Cyanobacteria (prokaryotic-no chloroplast)
Plants
Algae
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Photosynthesis Equation
12H2O + 6CO2 6O2 + C6H12O6 + 6H2O
water carbon dioxide
oxygen glucose water
LIGHT ENERGY
chlorophyll
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(see next slide)
upper leaf surface photosynthetic cells
Cutaway section of leaf
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Chloroplast
two outer membranes
Thylakoids(inner membrane system)
stroma
Photosynthetic organelle in plants and algae
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Wavelength of light (nanometers)
Different Types of Energy
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Visible Light Spectrum
Composed of different colors
Violet (380 nm) to red (750 nm)
Longer wavelengths, lower energy
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Pigments
Chemicals that interact with visible light
Absorbed colors/wavelength (not seen)
Reflect colors/wavelength (color seen)
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Variety of Plant Pigment
Photosynthetic Pigments
Chlorophylls
Accessory Pigments
Carotenoids
Anthocyanins
Phycobilins
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Photosynthesis….
a Two-Step Process
1. Light-dependent reactions
2. Light-independent reaction
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Light Dependant Reactions
Pigments
Electron transport chain
ATP Production
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Photosystems
Capture light energy
Two types (I and II)
Composed of….
– Antenna pigments (accessory pigments)
– Reaction center (chlorophyll)
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Electron Transfer Chains
Next to photosystems
Accepts electrons from reaction center
Electrons pass along chain
ATP generated.
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Thylakoid Membrane Section
stroma
thylakoid
membrane
thylakoid
compartment
PHOTOSYSTEM II
reaction center
PHOTOSYSTEM I
reaction center
electron
transfer chain
electron
transfer chain
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Light-Dependent Reactions
Two variants
1. Noncyclic pathway
2. Cyclic pathway
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Noncyclic Electron Flow
Two-step pathway
Uses both photosystems (I and II)
Produces ATP and NADPH
Split water
Release oxygen
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Photosystem I
e–
H2O
NADP+
1/2 O2 + 2H+
Fig. 5-6a, p.76
en
erg
y
light
light
Photosystem II
e–
a
Non Cyclic Electron Flow
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ATP Synthesis
Noncyclic Pathway
H+ concentrated in thylakoid
H+ Passive transport through ATP synthase
ATP produced
Chemiosmosis
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NADPH
NADP + + H+
thylakoid
compartment
thylakoid
membrane
stromaATPADP + Pi
H+
H+
H+H+
H+H+
H+
H+H+
H+
H+
Photosystem IsunlightPhotosystem II
Light-
Harvesting
Complex
Fig. 5-7, p.77
H+
e– e–e– e–e– e–
H+
e–
O2
H2O
cross-section
through a disk-
shaped fold in the
thylakoid
membrane
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Non Cyclic Electron Flow:
Summary
Location: Thylakoid
Membranes
Light
Photosystem I and II with
Chlorophyll
Water
Electron Transport Chains
ADP
NADP+
Oxygen
ATP
NADPH
Reactants Products
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Cyclic Electron Flow
Photosystem I only
Electrons
– Donated by chlorophyll a
– Passed to electron transfer chain
– Passed back to photosystem I
Electron flow drives ATP formation
No NADPH is formed
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Fig. 5-6b, p.76
Photosystem Ib
en
erg
y
light
e–
Cyclic Electrton Flow
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Cyclic Electron Flow: Summary
Location: Thylakoid
Membranes
Light
Photosystem I with
Chlorophyll
Electron Transport
Chain
ADP
ATP
Reactants Products
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Photosynthesis Equation
12H2O + 6CO2 6O2 + C6H12O6 + 6H2O
water carbon dioxide
oxygen glucose water
LIGHT ENERGY
chlorophyll
LR LR LR
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Light Independent reaction
Synthesis of glucose
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Fixes carbon dioxide
Synthesizes sugar
Independent of light
Take place in the stroma
Calvin-Benson cycle
Light-Independent reaction
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Fig. 5-8, p.78
THESE
REACTIONS
PROCEED IN THE
CHLOROPLAST’S
STROMA
Calvin-Benson
cycle
12 PGAL
glucose1
ATP
ATP
NADPH
6 RuBP12 PGA
6CO2
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Light Independent reaction
Reactants
– Carbon dioxide
– ATP
– NADPH
– RuBP
Products
– Glucose
– ADP
– NADP+
– RuBP
Reaction pathway is cyclic and RuBP
(ribulose bisphosphate) is used and produced
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Photosynthesis Equation
12H2O + 6CO2 6O2 + C6H12O6 + 6H2O
water carbon dioxide
oxygen glucose water
LIGHT ENERGY
chlorophyll
LIR LIR
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sunlight
energy
H2O
(water)
ATP
NADPH
O2H2O (metabolic water)
light-
dependent
reaction
light-
independent
reaction
glucose
NADP+
ADP + Pi
CO2
(carbon dioxide)
Photosynthesis (Summary)
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The standard photysynthesis pathway
The first stable intermediate is a three-carbon
PGA
Because the first intermediate has three carbons,
the pathway is called the C3 pathway
The C3 Pathway
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Fig. 5-8, p.78
Leaves of basswood,
a typical C3 plant. Far right,
basswood leaf cross section.
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upper
epidermis
palisade
mesophyll
spongy
mesophyll
lower
epidermis
stoma vein air space
Fig. 5-8, p.78
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Photorespiration in C3 Plants
On hot, dry days stomata close
Inside leaf
– Oxygen levels rise
– Carbon dioxide levels drop
RuBP bonds to oxygen instead of carbon dioxide
Only one PGAL forms instead of two glucose
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upper leaf surface photosynthetic cells
Cutaway section of leafstoma
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C4 Plants
Carbon dioxide is fixed twice
– Carbon dioxide is stored as a four carbon compound
– Carbon dioxide is released from the compound for use in Calvin-Benson cycle
Evolutionary defense against photorespiration
Corn and Crabgrass are examples
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Fig. 5-9, p.79
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upper
epidermis
mesophyll
cell
bundle-
sheath cell
lower
epidermis
Corn leaf, cross-sectionFig. 5-9a, p.79
vein stoma
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Fig. 5-9b, p.79
C4
cycle
stomata closed,
no CO2 uptake
oxaloacetatemesophyll
cell
bundle-
sheath
cell
Calvin-
Benson
cycle
CO2
RuBP PGA
sugar
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CAM Plants
Carbon is fixed twice (in same cells)
Night
– Stomates open for gas exchange.
– Carbon dioxide is fixed by repeated turns of a type of
C4 cycle
Day
– Carbon dioxide is released and fixed in Calvin-Benson
cycle
Cacti and other fleshy plants
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Fig. 5-10, p.79
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Fig. 5-10, p.79
C4
cycle
CO2 uptake at
night only
mesophyll
cellruns
during
day
Calvin-
Benson
cycle
sugar
runs
at
night
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Summary of Photosynthesis
12H2O
sunlight
Calvin-
Benson
cycle
6O2
Light
Dependent
Reactions
Light
Independent
Reactions
NADP+ADP + Pi
6 RuBP 12 PGAL
P
end products (e.g., sucrose, starch, cellulose)
phosphorylated glucose
6H2O
6CO2
ATP NADPH
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Photoautotrophs
– Carbon source is carbon dioxide
– Energy source is sunlight
Heterotrophs
– Get carbon and energy by eating autotrophs or one
another
Carbon and Energy Sources
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Linked Processes
Photosynthesis
Energy-storing
pathway
Releases oxygen
Requires carbon
dioxide
Aerobic Respiration
Energy-releasing
pathway
Requires oxygen
Releases carbon
dioxide