Photosynthesis Light & Dark Reactions

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Photosynthesis Light & Dark Reactions Powered By Docstoc
					Photosynthesis
   sorry– no guacamole

Lehninger Principles of Biochemistry
          Chapters 19, 20
       By Kathleen Quigley
        Photophosphorylation

Solar energy
captured & converted
to chemical energy


O2 formed used by
aerobic heterotrophs
& converted back to
CO2; cyclic
                 Equations

General: CO2     • Oxidation reduction
+ H2O + light    • Also forms 2NADPH & 2H+ from
 O2 + CH2O        2NADP



Hill Reaction:   • Illuminated leaf extracts containing chloroplasts
2H2O + 2A       •
                     evolved O2 & reduce a nonbiological electron acceptor
                     Hill reagant (2,6-dichlorophenolindophenol) blue
  2H2 + O2           when oxidized, colorless when reduced
       2 Reactions
• Light reactions:
  – Pigments absorb light energy
    & conserve it as ATP and
    NADPH; O2 formed
• Dark (carbon fixation)
  reactions:
  – Reduce CO2 to trioses and
    carbohydrates; hydrolyze ATP
                   Where?
• Chloroplast
• Thlakoid
  membrane-
  pigments &
  enzymes for light
  reactions
• Stroma- enzymes
  for dark reactions
               Light Absorption




Visible light= 400-700nm; short wavelength= high energy
• Photon absorbed,
  electron bumped
  up to excited state
• Chlorophyll is most
  important pigment
  – Similar to
    porphyrin
  – Highly excitable
• Accessory
  pigments extend
  range of light
  absorption
  Action Spectra show which
wavelengths are most effective in
        photosynthesis
              Photosystems
• Functional
  arrangements of
  pigments in the
  thylakoid
  membrane
• Light-harvesting
  molecules & a
  photo reaction
  center
• Electric charge
  separation
  initiates the redox
  reaction
• “exciton transfer”
• Reaction center=
  special pair of
  chlorophyll a
  molecules
Electron Flow is light-driven
  Plants use both reaction centers in
          tandem: Z Scheme
• Plastocyanin & b6f
• Electrons carried
  from H2O to
  NADP+
• 2 photons
  absorbed= 1
  electron passed
    Herbicides function by blocking
     electron transfer through b6f
• PQA binding site is
  targeted
• Photosystem II
  inhibited!
• No proton
  gradient!
• No ATP generated!
  Need separation between PSI & PSII
• PSI requires less energy
  than PSII for excitation
• PSII in tightly
  appressed membrane
  between light-
  harvesting complexes
• PSI in no-opressed
  membranes so it has
  access to stroma
    Cyanobacteria: a special case
• b6f used for both
  oxidative
  phosphorylation &
  photophosphorylation
• Electrons carried from
  plastoquinone or
  ubiquinone to Cyt C6
              Splitting Water

                                         •2H2O 4H+
                                         + 4e- + O2
                                         •4 photons
                                         required



• Source of electrons passed to NADPH
• “oxygen evolving complex”
• e- passed one at a time to Tyr residue of P680
    Ultimate Goal: ATP synthesis!
• Light energy
  transformed to P
  bond energy
• pH gradient creates
  force to generate ATP
• 12 H+ move from
  stroma to thylakoid
  per 4e- passed along
    Similar to ATP
synthase orientation in
 mitochondria for ETC
   Water is not the only H+ donor
• Obligate anaerobes
  – Use lactate or H2S
  – H2S used first & oxygenic photosynthesis evolved
    later

				
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