Biology 211
Chapter 10:Photosynthesis: Energy from the Sun
Early ExperimentsVan Helmont
Priestly
Isotopic tracer- to determine the source of the Oxygen
Full Equation for Photosynthesis:
Two Pathways of Photosynthesis
Light Reactions:
Light Energy is captured and converted to chemical bond energy
Water is split and oxygen is evolved
Calvin Benson Cycle:
Use the chemical bond energy and reducing power to reduce CO2 into sugar
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Converting Light Energy into Chemical Energy
To use light in any photobiological process it must be absorbed.
Pigments:
chemicals that absorb light
electron in the pigment goes from ground state to excited state
Four things that can happen to the excited electron:
1) Drops to ground state and releases heat
2) Drops to ground state and fluoresces
3) Pass the energy to another electron
4) Can be a reducing agent (pass along the excited electron)
Absorbed wavelengths correlate with Biological activity:
Absorption spectrum:
Action spectrum:
Pigments involved in photosynthesis in eukaryotes:
Chlorophyll a --------------Absorbs red and blue
Accessory Pigments- pass energy to the chlorophyll a
Chlorophyll b ----- Absorbs red and blue
Carotenoids -------Absorbs blue, blue-green
Phycobilins -------Absorbs yellow-green, yellow, orange
NEW Chlorophyll!!!!
Antenna systems- light-harvesting complexes
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Light Reactions: Electron Transport and Production of ATP
Electron transport:
Noncyclic electron transport/ The Z Scheme
End-products of Non-cyclic Electron Transport:
Oxygen
NADPH
ATP
Cyclic Electron Transport
End-products of Cyclic Electron Transport:
ATP
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Calvin-Benson Cycle: Making carbohydrates
Three parts to the Calving Benson Cycle
1) Fixation of carbon dioxide ( via RUBISCO enzyme)
2) Reduction of 3PG to form a three-carbon sugar, glyceraldehydes-3-phospate
3) Regeneration of RuBP
Photorespiration:
RUBISCO—a confused enzyme
RUBISCO ---- ribulose bisphosphate carboxylase/oxygenase
Fixing oxygen undoes photosynthesis, breaks down sugars
When is RUBISCO an Oxygenase?
When oxygen levels are high:
When plant closes stomates to avoid water loss:
High temperatures
Dry weather
Avoiding Photorespiration: use PEP Carboxylase
PEP Carboxylase:
does not “fix” oxygen
has a higher affinity for carbon dioxide than RUBISCO
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Two strategies to avoid Photorespiration:
C4 Plants: Physically separate the oxygen from the RUBISCO
CAM Plants: Temporally separate the oxygen from the RUBISCO
Both use PEP Carboxylase
Both strategies require an extra step and the use of extra ATP
Connection between Photosynthesis and Other Metabolic Pathways in Plants
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