Chapter 9Photosynthesis: Capturing Energy

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• Chapter 9 Photosynthesis Capturing Energy

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Photosynthesis in nature

• Autotrophs biotic producers photoautotrophs
  chemoautotrophs obtains organic food without
  eating other organisms
• Heterotrophs biotic consumers
  obtains organic food by eating other organisms or
  their by-products (includes decomposers)

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The chloroplast

• Sites of photosynthesis
• Pigment chlorophyll
• Plant cell mesophyll
• Gas exchange stomata
• Double membrane
• Thylakoids, grana, stroma

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Leaf Structure
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Outer membrane
Inner membrane
Stroma
1 µm
Granum (stack of thylakoids)
Thylakoid membrane
Thylakoid lumen
Intermembrane space
(c) In the chloroplast, pigments necessary for
the light-capturing reactions of photosynthesis
are part of thylakoid membranes, whereas the
enzymes for the synthesis of carbohydrate
molecules are in the stroma.
Fig. 9-4c, p. 194
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Electromagnetic Spectrum
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One wavelength
Longer wavelength
760 nm
TV and radio waves
Red
700 nm
Micro- waves
Orange
Infrared
Color spectrum of visible light
600 nm
Visible
Yellow
UV
Green
X-rays
500 nm
Blue
Gamma rays
Violet
400 nm
380 nm
Electromagnetic spectrum
Shorter wavelength
Fig. 9-1, p. 192
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Photosynthesis an overview

• Redox process
• H2O is split, e- (along w/ H) are transferred to
  CO2, reducing it to sugar
• 2 major steps
  light reactions (photo) v NADP
  (electron acceptor) to NADPH
  vPhotophosphorylation ADP ---gt ATP
  Calvin cycle (synthesis) v Carbon fixation
  carbon into organics

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Photosystems

• Light harvesting units of the thylakoid membrane
• Composed mainly of protein and pigment antenna
  complexes
• Antenna pigment molecules are struck by photons
• Energy is passed to reaction centers (redox
  location)
• Excited e- from chlorophyll is trapped by a
  primary e- acceptor

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Noncyclic electron flow

• Photosystem II (P680) v
  photons excite chlorophyll e- to an acceptor
  v e- are replaced
  by splitting of H2O (release of O2)
  v e-s travel to Photosystem
  I down an electron transport chain
  (PqcytochromesPc) v as e-
  fall, ADP ---gt ATP (noncyclic photophosphorylation
  )
• Photosystem I (P700) v
  fallen e- replace excited e- to primary e-
  acceptor v 2nd ETC ( FdNADP
  reductase) transfers e- to NADP ---gt NADPH
  (...to Calvin cycle)
• These photosystems produce equal amounts of ATP
  and NADPH

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Noncyclic Electron Transport

• Light-dependent reactions
• form ATP and NADPH

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The Calvin cycle

• 3 molecules of CO2 are fixed into
  glyceraldehyde 3-phosphate (G3P)
• Phases 1- Carbon fixation each CO2 is
  attached to RuBP (rubisco enzyme) 2-
  Reduction electrons from NADPH reduces to G3P
  ATP used up 3- Regeneration G3P
  rearranged to RuBP ATP used cycle continues

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The Calvin Cycle
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Calvin Cycle, net synthesis

• For each G3P (and for 3 CO2). Consumption of
  9 ATPs 6 NADPH (light reactions regenerate
  these molecules)
• G3P can then be used by the plant to make glucose
  and other organic compounds

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Cyclic electron flow

• Alternative cycle when ATP is deficient
• Photosystem I used but not II produces ATP but
  no NADPH
• Why? The Calvin cycle consumes more ATP than
  NADPH.
• Cyclic photophosphorylation

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Cyclic Electron Transport

• Electrons from photosystem I
• return to photosystem I
• ATP produced by chemiosmosis
• No NADPH or oxygen generated

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Alternative carbon fixation methods, I

• Photorespiration hot/dry days stomata close
  CO2 decrease, O2 increase in leaves O2 added to
  rubisco no ATP or food generated
• Two Solutions..
• 1- C4 plants 2 photosynthetic cells,
  bundle-sheath mesophyll PEP carboxylase
  (instead of rubisco) fixes CO2 in mesophyll new
  4C molecule releases CO2 (grasses)

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C3 and C4 Plants
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Alternative carbon fixation methods, II

• 2- CAM plants open stomata during night, close
  during day (crassulacean acid metabolism) cacti,
  pineapples, etc.

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A CAM Plant
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A review of photosynthesis