Photosynthesis
Professor Andrea Garrison
Biology 11
Illustrations ©2010 Pearson Education, Inc.
Photosynthesis
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Photosynthesis
• Converts solar energy to molecular bond
energy
• Occurs in chloroplasts of eukaryotes
• Requires
– Visible light for energy
– Chlorophyll to trap the light
– CO2 for carbon to build sugars
– H2O for electrons to make bonds
– NADPH to carry electrons
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Photosynthesis
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Photosynthesis
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Photosynthesis
• Occurs in two stages
– Light reactions
• Occur on thylakoid
membranes
• Require light
• Supply ATP and NADPH
for Calvin Cycle
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Photosynthesis
• Occurs in two stages
– Light reactions
• Occur on thylakoid
membranes
• Require light
• Supply ATP and NADPH
for Calvin Cycle
– Calvin Cycle
• Need no light, but will
occur in light
• Use products of light
reactions
• Produce carbohydrates
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Light Reactions
• Use visible spectrum of sunlight
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Engelmann’s Experiment
• What colors of light drive P/S?
• O2 loving bacteria cluster around algae producing most O2
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Light used by chlorophyll
• Chlorophyll absorbs
light in red/orange
wavelengths and
blue/violet wavelengths
• Chlorophyll does not
absorb green light
– Plants look green
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Accessory Pigments
• Carotenoids
– Absorb blue-green light
– Some pass the energy to
chlorophyll
– Some absorb excessive
light energy which might
damage chlorophyll
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Light Reactions
• Pigments harvest
light energy through
excited e- s
• Photon strikes e– Excited e- jumps to
higher energy shell
– e- falls back down
• Releases energy
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Light Reactions
• Sometimes an electron
can get so excited it flies
off the molecule and
binds to an e- acceptor,
then moves down an etransport chain
Photosynthesis
Energy
released
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Light Reactions
• Two photosystems
– Involve both excited e-s falling back to their
original energy state and excited e-s leaving the
molecule and moving down electron transport
chains
– Trap solar energy as excited e-s and use the energy
to make ATP and NADPH for Calvin Cycle
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Photosystems
•Cluster of pigment molecules
•gather light energy in excited electrons
•pass that energy between molecules and eventually to reaction center
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NADPH-Producing Photosystem
• Photon hits photosystem
pigment molecule
• Energy eventually passed to
reaction center
• e- s leave reaction center,
trapped by primary eacceptor
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NADPH-Producing Photosystem
• Photon hits photosystem
pigment molecule
• Energy eventually passed to
reaction center
• e- s leave reaction center,
trapped by primary eacceptor
• e- s passed to NADP →
NADPH
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Water-Splitting Photosystem
• Photon hits photosystem
pigment molecule
• Energy eventually passed
to reaction center
• e- s leave reaction center,
trapped by primary eacceptor
• Water split to release e-s
to replace those lost from
reaction center
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Water-Splitting Photosystem
• Photon hits photosystem
pigment molecule
• Energy eventually passed to
reaction center
• e- s leave reaction center,
trapped by primary eacceptor
• Water split to release e-s to
replace those lost from
reaction center
• e- s passed down etransport to make ATP
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Summary of Light Reactions
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Calvin Cycle
• Uses ATP for energy and NADPH for e-s to
make bonds of sugar
• Uses CO2 for atoms of sugar
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Calvin Cycle
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Calvin Cycle
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Calvin Cycle
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Calvin Cycle
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