Phototrophic Energy Metabolism: Photosynthesis I

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Phototrophic Energy Metabolism Photosynthesis I

• Reading Becker, ch. 15, pp. 445-455

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Chemotrophs Rely on Phototrophs

• Aerobic Respiration (chemotrophic energy
  metabolism)
• C6H12O6 6O2 ? 6CO2 6H2O
• Cs of glucose are released to atmosphere as
  CO2
• O2 is reduced to H2O
• Photosynthesis (phototrophic energy metabolism)
• Light 3CO2 6H2O ? C3H6O3 3O2 3H2O
• Energy from light drives synthesis of 3C sugars
  from CO2 and H2O (carbon fixation)
• Sugars and other resulting organic molecules
  can be comsumed by chemotrophs
• O2 released to atmosphere to be consumed by
  chemotrophs

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The 2 Major Processes of Photosynthesis

• Energy Transduction Reactions (aka Light
  Reactions)
• Light energy ? Chemical energy
• (ATP and NADPH)
• Carbon Assimilation Reactions (aka Dark
  Reactions)
• Carbon fixation
• Oxidized carbons from CO2 are covalently
  attached to organic acceptor molecules
• Subsequent reduction and rearrangement
  generates small organic molecules (sugars, etc.)

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The Chloroplast

• Site of photosynthesis in cells of green plants
  and in certain algae
• One of largest organelles in plant cells ( 10
  mm)
• Bounded by 2 membranes
• inner and outer chloroplast membranes
• 2 membranes enclose the intermembrane space
• Central enclosed compartment is the stroma
• Stroma contains chloroplast DNA, RNA, ribosomes
• Contain an internal membrane system, the
  thylakoids

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The Chloroplast
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Chloroplast Membranes

• Outer Membrane
• Not an efficient permeability barrier due to
  presence of porins
• Composition of intermembrane space highly
  similar to that of cytoplasm
• Inner Membrane
• Significiant permeability barrier
• Contains transport molecules for transport of
  critical molecules across the inner membrane
• H2O, CO2, and O2 can diffuse readily
• The inner chloroplast membrane does not
  contain the components of the electron
  transport chain!

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Chloroplast Membranes

• Thylakoids
• Membrane-bound sacs organized into stacks called
  grana within the stroma
• Grana interconnected by stroma thylakoids
• Interior of each thylakoid is the lumen
• Contain all the photosynthetic machinery

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Thylakoids
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Thylakoids
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Chlorophyll

• Green pigment molecule associated with thylakoid
  membrane
• Absorbs light

Porphyrin Ring
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Accessory Pigments

• Present in the thylakoid membranes
• Absorb light of different wavelengths than the
  chlorophylls
• Allows for collection of energy from a broader
  spectrum of light than achieved with chlorophylls
  alone
• Carotenoids (e.g., b-carotene)
• present in photosynthetic plants and algae
• Phycobilins (e.g., phycoerythrin, phycocyanin)
• present only in photosynthetic algae

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Photoexcitation

• Absorbed light excites an electron within the
  pigment molecule
• What happens to the excited electron?
• 1) Returns to ground state, releasing energy as
  heat and fluorescence
• OR
• 2) Transfers energy to second electron within
  an adjacent pigment molecule (resonance energy
  transfer)
• OR
• 3) Transfer of entire excited electron to
  another molecule (photochemical reduction)

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Pigments Are Arranged Into Photosystems

• Within thylakoid membrane, pigment molecules are
  arranged into photosystems
• Antenna pigments
• Absorb light and pass energy to adjacent
  pigments within photosystem via resonance
  energy transfer
• Reaction center
• Contains 2 chlorophyll molecules
• Receive energy from antenna pigments

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Photochemical Reduction

• Excitation of chlorophyll in the reaction center
  causes chlorophyll to donate an electron to
  quinone, a carrier molecule present in the
  thylakoid membrane
• This is referred to as photochemical reduction,
  because quinone is reduced in the process
• The oxidized chlorophyll accepts an electron from
  an adjacent electron donor (depicted here in
  orange)

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Light-Harvesting Complexes (LHC)

• Present within thylakoid membranes
• Comprised of pigment molecules
• Lack a reaction center
• Pass energy from absorbed light on to adjacent
  photosystem via resonance energy transfer
• Photosystem associated LHCs photosystem
  complex