PHOTOSYNTHESIS
Photosynthesis
An anabolic, endergonic, carbon
dioxide (CO2) requiring process
that uses light energy (photons)
and water (H2O) to produce
organic macromolecules (glucose).
SUN
photons
6CO2 + 6H2O  C6H12O6 + 6O2
glucose
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Question:
Where does
photosynthesis
take place?
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Plants
• Autotrophs – produce their own food
(glucose)
• Process called photosynthesis
• Mainly occurs in the leaves:
a. stoma - pores
b.mesophyll cells
Mesophyll
Cell
Chloroplast
Stoma
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Stomata (stoma)
Pores in a plant’s cuticle through
which water and gases are
exchanged between the plant and
the atmosphere.
Oxygen
(O2)
Carbon Dioxide
(CO2)
Guard Cell
Guard Cell
Found on the underside of leaves
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Mesophyll Cell of Leaf
Nucleus
Cell Wall
Chloroplast
Central Vacuole
Photosynthesis occurs in these cells!
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Chloroplast
Organelle where photosynthesis
takes place.
Stroma
Outer Membrane
Inner Membrane
Thylakoid
Granum
Thylakoid stacks are connected together
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Thylakoid
Thylakoid Membrane
Granum
Thylakoid Space
Grana make up the inner membrane
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Question:
Why are
plants
green?
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Chlorophyll Molecules
• Located in the thylakoid membranes
• Chlorophyll have Mg+ in the center
• Chlorophyll pigments harvest energy
(photons) by absorbing certain
wavelengths (blue-420 nm and red660 nm are most important)
• Plants are green because the green
wavelength is reflected, not absorbed.
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Wavelength of Light (nm)
400
Short wave
(more energy)
500
600
700
Long wave
(less energy)
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Absorption of Light by
Chlorophyll
Absorption
violet
blue
green
yellow
wavelength
orange
red
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Question:
During the fall,
what causes the
leaves to change
colors?
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Fall Colors
• In addition to the chlorophyll
pigments, there are other pigments
present
• During the fall, the green
chlorophyll pigments are greatly
reduced revealing the other pigments
• Carotenoids are pigments that are
either red, orange, or yellow
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Redox Reaction
The transfer of
electrons from
another
Two types:
1. Oxidation is
2. Reduction is
one or more
one reactant to
the loss of ethe gain of e-
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Oxidation Reaction
The loss of electrons from a
substance or the gain of
oxygen.
Oxidation
6CO2 + 6H2O 
C6H12O6 + 6O2
glucose
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Reduction Reaction
The gain of electrons to a
substance or the loss of
oxygen.
Reduction
6CO2 + 6H2O  C6H12O6 + 6O2
glucose
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Two Parts of Photosynthesis
Two reactions make up
photosynthesis:
1.Light Reaction or Light
Dependent Reaction Produces energy from solar
power (photons) in the form of
ATP and NADPH.
SUN
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Two Parts of Photosynthesis
2. Calvin Cycle or Light
Independent Reaction
• Also called Carbon Fixation
or C3 Fixation
• Uses energy (ATP and
NADPH) from light reaction
to make sugar (glucose).
ATP
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Light Reaction (Electron Flow)
• Occurs in the Thylakoid
membranes (inner membrane)
• During the light reaction,
there are two possible routes
for electron flow
A.Cyclic Electron Flow
B. Noncyclic Electron Flow
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Cyclic Electron Flow
•
•
•
•
Occurs in the thylakoid membrane
Uses Photosystem I only
P700 reaction center- chlorophyll a
Uses Electron Transport Chain
(ETC)
• Generates ATP only
ADP +
P
ATP
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Cyclic Electron Flow
Primary
Electron
Accepto
r
SUN
e-
e-
ePhotons
P700
ATP
produced
by ETC
e-
Accessory
Pigments
Photosystem I
Pigments absorb photons, excite electrons, which produce
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ATP
Noncyclic Electron Flow
• Occurs in the thylakoid membrane
• Uses PS II and PS I
• P680 reaction center (PSII) chlorophyll a
• P700 reaction center (PS I) chlorophyll a
• Uses Electron Transport Chain (ETC)
• Generates O2, ATP and NADPH
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Noncyclic Electron Flow
Primary
Electron
Acceptor
Primary
Electron
Acceptor
SUN
2e-
2e-
Photon
H2O
1/2O2 + 2H+
Enzyme
Reaction
2e-
ETC
2e-
2e-
P700
NADPH
ATP
P680
Photon
Photosystem I
Photosystem II
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B. Noncyclic Electron Flow

• ADP +
• NADP
P ++ H
ATP

NADPH
(Reduced
)
• Oxygen comes from the splitting
(Reduced)
of H2O, not CO2
H 2O 
(Oxidized)
1/2 O2 + 2H+
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Chemiosmosis
• Powers ATP synthesis.
• Located in the thylakoid membranes.
• Uses ETC and ATP synthase (enzyme)
to make ATP.
• Photophosphorylation: addition of
phosphate to ADP to make ATP.
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Chemiosmosis
SUN
H+ H+
Thylakoid
(Proton Pumping)
E
T
PS II
PS I
C
H+
H+ H+
H+ H+
H+
ADP + P
H+
H+
high H+
concentration
ATP Synthase
ATP
Thylakoid
Space
low H+
concentration
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Calvin Cycle
• Carbon Fixation (light
independent rxn).
• C3 plants (80% of plants on earth).
• Occurs in the stroma.
• Uses ATP and NADPH from light
rxn.
• Uses CO2.
• To produce glucose: it takes 6
turns and uses 18 ATP and 12
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Chloroplast
Stroma
Outer Membrane
Inner Membrane
Thylakoid
Granum
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Calvin Cycle (C3 fixation)
(36C)
6C-C-C-C-C-C
(6C)
6CO2
(unstable)
(30C)
6C-C-C-C-C
RuBP
(30C)
glucose
6C-C-C 12PGA
(36C)
6ATP
6ATP
6NADPH
6NADPH
6C-C-C
6ATP
C3
6C-C-C
(36C)
6C-C-C 12G3P
(6C)
C-C-C-C-C-C
Glucose 30
Calvin Cycle
• Remember: C3 = Calvin Cycle
C3
Glucose
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Photorespiration
• Occurs on hot, dry, bright days.
• Stomates close.
• Fixation of O2 instead of CO2.
• Produces 2-C molecules instead of 3-C
sugar molecules.
• Produces no sugar molecules or no
ATP.
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Photorespiration
• Because of photorespiration: Plants
have special adaptations to limit the
effect of photorespiration.
1. C4 plants
2. CAM plants
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C4 Plants
• Hot, moist environments.
• 15% of plants (grasses, corn,
sugarcane).
• Divides photosynthesis spatially.
• Light rxn - mesophyll cells.
• Calvin cycle - bundle sheath cells.
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C4 Plants
Malate
C-C-C-C
Malate
C-C-C-C
Transported
CO2
CO2
C3
glucose
C-C-C
PEP
ATP
Mesophyll Cell
Vascular
Tissue
C-C-C
Pyruvic Acid
Bundle Sheath Cell
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CAM Plants
• Hot, dry environments.
• 5% of plants (cactus and ice plants).
• Stomates closed during day.
• Stomates open during the night.
• Light rxn - occurs during the day.
• Calvin Cycle - occurs when CO2 is
present.
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CAM Plants
Night (Stomates Open)
Day (Stomates Closed)
Vacuole
CO2
C-C-C-C
Malate
C-C-C-C
Malate
C-C-C-C
Malate
CO2
C3
C-C-C
PEP
ATP
C-C-C
Pyruvic acid
glucose
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Question:
Why would CAM
plants close
their stomates
during the day?
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