Download Light-Dependent Reactions

AP Bio
Photosynthesis
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Photosynthesis
• Uses light energy as energy source
• Uses CO2 as a carbon source
• Synthesizes energy rich organic molecules
(sugar and others)
• Supplies energy, stored in sugars, to most
living organisms
• Occurs in the chloroplasts of autotrophs
(green plants)
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Photosynthesis Phacts
• On a global scale, photosynthesis makes
about 160 billion metric tons of
carbohydrates a year.
• Sucrose is the transport form of
carbohydrates in most plants.
• Plants consume about 50% of the
photosynthate as fuel for cellular
respiration.
• Extra sugars are synthesized into starch
and stored in cells of roots, tubers, seeds,
and fruits.
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Summary Equation
One Overall Equation:
6 CO2 + 12 H2O + light energy 
C6H12O6 + 6 O2 + 6 H2O
In Two Stages:
a) Light Dependent Reactions
b) Light Independent Reactions
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Chloroplast Structure
Thylakoids
Stroma
Granum
Inner & Outer
Membranes
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Organization of Photosynthesis
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Leaf Structure
Vein
Upper Epidermis
Mesophyll
Cells
Stoma
Lower
Epidermis
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The Light-Dependent Reactions
1. Occur in thylakoid membranes
2. O2 released by splitting water
3. Light energy boosts P.E. of electrons
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Photosynthetic Pigments
•Chlorophyll a - main photosynthetic
pigment in all organisms except bacteria
•Accessory pigments absorb slightly
different wavelengths of light
• Chlorophyll b
• Carotenoids
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Absorption Spectra
The
combination
of all of the
pigments
increases
the range of
colors that
plants can
use in
photosynthe
sis.
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The Photosystems
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Photosystem II
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The Light-Dependent Reactions
1. Occur in thylakoid membranes
•
Photosystems
2. O2 released by splitting water
•
Photolysis
3. Light energy boosts P.E. of electrons
•
Photoexcitation
4. ATP is generated
•
Photophosphorylation
Chemiosmosis 1
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Chemiosmosis 2
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http://www.cst.cmich.edu/users/baile1re/bio
101fall/enzphoto/photoanima.htm#
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http://www.stolaf.edu/people/giannini/flash
animat/metabolism/photosynthesis.swf
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Light-Dependent Reactions
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Overview of Photosynthesis
Light-Dependent
Reactions
H2O
(in thylakoids)
Depleted
Carriers
Energized
Carriers
(ADP, NADP+)
Glucose
O2
(ATP, NADPH)
Light-Independent
Reactions
(in stroma)
CO2+H2O
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Rubisco (RuBP carboxylase)
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The Light-Independent Reactions
1. Occur in the stroma
2. Does not require light, but does require
the products of the light reactions (ATP
and NADPH)
3. Calvin Cycle (Melvin Calvin)
•
3 Phases
Assimilation of atmospheric carbon dioxide into
carbohydrates.
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The C3 Cycle
of Carbon
Fixation
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A Summary of Photosynthesis
Light-dependent
reactions occur
in thylakoids
Lightindependent
reactions (c3
cycle) occur in
stroma
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C3 Plants & the Pathway
C3 plants use the C3
pathway
In a C3 plant, most
chloroplasts are in
mesophyll cells.
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C3 Plants
Much photorespiration
occurs under hot, dry
conditions
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C4 Plants & the C4 Pathway
C4 plants use the C4
pathway
In a C4 plant, both
mesophyll & bundlesheath cells contain
chloroplasts.
Mesophyll cell in a C4 plant
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CO2 is captured
with a highly
specific enzyme.
C4 Plants & the
C4 Pathway
Much glucose
synthesis occurs.
Almost no
photorespiration
occurs under hot,
dry conditions.
Bundle-sheath cell in a C4 plant
Relationship of
Photosynthesis & Respiration
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