Photosynthesis sorry no guacamole

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Photosynthesissorry no guacamole

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

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Photophosphorylation
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Equations
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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

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Where?

• Chloroplast
• Thlakoid membrane- pigments enzymes for light
  reactions
• Stroma- enzymes for dark reactions

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Light Absorption
Visible light 400-700nm short wavelength high
energy
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• 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

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Action Spectra show which wavelengths are most
effective in photosynthesis
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Photosystems

• Functional arrangements of pigments in the
  thylakoid membrane
• Light-harvesting molecules a photo reaction
  center

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• Electric charge separation initiates the redox
  reaction
• exciton transfer
• Reaction center special pair of chlorophyll a
  molecules

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Electron Flow is light-driven
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Plants use both reaction centers in tandem Z
Scheme

• Plastocyanin b6f
• Electrons carried from H2O to NADP
• 2 photons absorbed 1 electron passed

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Herbicides function by blocking electron transfer
through b6f

• PQA binding site is targeted
• Photosystem II inhibited!
• No proton gradient!
• No ATP generated!

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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

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Cyanobacteria a special case

• b6f used for both oxidative phosphorylation
  photophosphorylation
• Electrons carried from plastoquinone or
  ubiquinone to Cyt C6

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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

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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

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Water is not the only H donor

• Obligate anaerobes
• Use lactate or H2S
• H2S used first oxygenic photosynthesis evolved
  later