Phototrophic Energy Metabolism: Photosynthesis

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Chapter 11 Phototrophic Energy Metabolism
Photosynthesis
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Photoheterotrophs vs. Photoautotrophs
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Photosynthesis Overview

• Energy Transduction Reactions (aka the light
  reactions)
• Light energy is captured by chlorophyll
• Reducing power is generated in the form of NADPH
• Carbon Assimilation Reactions (aka the dark
  reactions, the Calvin cycle)
• Carbon dioxide is fixed and reduced

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The Chloroplast
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Cyanobacterium
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Light

• Electromagnetic radiation with both wave and
  particle properties
• Photon discreet particle of light
• Quantum packet of energy carried by the photon
• Energy carried by photon is inversely
  proportional to the wavelength
• Pigment light absorbing molecule
• Photoexcitation photon energy is used to excite
  an electron

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Chlorophyll
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Photosystems

• Combination of chlorophyll molecules, accessory
  pigments, and associated proteins (all arranged
  into a functional unit)
• Localized to thylakoid or bacterial membranes

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Summary of Photosystems

• Electrons move from water to NADPH
• Called non-cyclic electron flow
• 4 photons absorbed in PSII, 8 in PSI, 8 total
• 2 NADPH molecules generated
• 12 protons pumped

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ATP Synthesis

• Photophosphorylation
• Very similar to ATP synthesis in cellular
  respiration
• Approximately 1 ATP for every four protons

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Cyclic Electron Flow
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The Calvin Cycle

• Named after Melvin Calvin
• Won a Nobel prize in 1961
• Elucidated through the use of radiolabeled
  isotopes
• Occurs in the stroma of chloroplasts
• Split into 3 stages
• Carboxylation of Ribulose-1,5-Bisphosphate
• Reduction of 3-phosphoglycerate
• Regeneration of acceptor molecule

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Carbohydrate Synthesis
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The Glycolate Pathway

• Remember Rubisco normally has carboxylase
  activity
• It also has oxygenase activity
• Produces 2 carbon molecule phosphoglycolate
• Phosphoglycolate cannot be used in Calvin Cycle
• Why would evolution favor this?
• How do plants deal with it?

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C4 Plants

• Carbon is fixed into 4 carbon molecule instead of
  3
• Plants adapted to hot, arid environments with
  lots of sunshine (tropical grasses, maize,
  sugarcane, etc)
• Strategy Isolate rubisco in cells that have
  high CO2 levels

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The Hatch-Slack Cycle
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CAM plants

• Crassulacean Acid Metabolism
• Cacti, succulents, orchids, etc. (where water is
  limited)
• Strategy Segregate reactions by time (rather
  than space as in C4 plants)
• Only open stomata at night