61 Capturing the Energy in Light

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6-1 Capturing the Energy in Light
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• Biochemical Pathways series of chemical
  reactions
• Photosynthesis
• 6CO2 6H2O light energy ? C6H12O6 6O2
• Respiration
• C6H12O6 6O2 ?
• 6CO2 6H2O ATP

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• Autotrophs

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• Two parts of Photosynthesis
• Light reactions light absorption, electron
  transport chain, chemiosmosis
• Calvin Cycle

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Chloroplast

• Surrounded by two membranes
• Thylakoids flattened sacs that contain
  chlorophyll
• Grana stacks of thylakoids
• Stroma solution surrounding thylakoids

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Visible Spectrum
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Chloroplast Pigments

• Chlorophyll a captures light energy (appears
  green), carries on photosynthesis
• Accessory pigments capture light energy and
  transfer it to chlorophyll a
• Chlorophyll b appears green
• Carotenoids appear yellow, orange, brown

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Light Reactions

• Pigment clusters (Photosystems I and II) in
  thylakoids absorb light (electrons become excited
  and jump to higher energy levels)
• Energy is passed along chlorophyll a molecules
• Excited electrons move from specialized
  chlorophyll a molecules to a primary electron
  acceptor in the thylakoid membrane
• (chlorophyll a is oxidized loses electrons/the
  primary electron acceptor is reduced gains
  electrons)

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• Electrons are transferred along the electron
  transport chain in the thylakoid membrane
• These electrons supply energy to pump protons
  from the stroma into the thylakoid
• Electrons from Photosystem I join with NADP
  forming NADPH in the stroma
• Electrons lost from Photosystem I are replaced by
  electrons from Photosystem II
• Electrons lost from Photosystem II are replaced
  by electrons from water
• 2H2O ? 4H 4e- O2

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• A concentration gradient of protons in the
  thylakoids results from the electron transport
  chain
• Protons diffuse through a specialized channel
  (ATP synthase) in the thylakoid membrane
• This results in the formation of ATP from ADP and
  phosphate

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