PHOTOSYNTHESIS

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PHOTOSYNTHESIS
LIGHT REACTION
DARK REACTION
Photons to energy
CO2 fixation
Z-scheme
Calvin Cycle
Photophosphorylation
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What is Photosynthesis?
Ans
A process in which electrons from excited
chlorophyll molecules are passed to
acceptors that convert electronic energy to
chemical energy ATP and NADPH
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Chloroplasts
Anatomy is Essential
to Photosynthesis
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Absorption of Light
Absorption Intensity
300
Wavelength, nm
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ABSORPTION OF LIGHT ENERGY (Photons)
Photon
Allowable Transitions
E hv
Depends on frequency of wave length of light
h Plancks constant
6.626 x 10-34 J sec
v frequency of light energy in waves
per sec
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Rules of the Game

• Electromagnetic radiation is transmitted in
  quanta (discrete energy packages)
• A molecule can absorb photons of only certain
  wavelengths
• Absorbed energy must match the energy difference
  between two states
• Energy of absorbed photon is transferred to
  photosynthetic reaction system

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Dissipation of Excitation Energy of Chlorophyll
Energy
Chlorophyll can absorb energy in 2 energy states,
Blue or short wave and Red or first energy state
Second Excited State
Internal
First Excited State
Photooxidation (transfer to a second molecule)
Ground State
Red
Blue
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Options to Absorption
1. Fluorescence
Absorb low ?, emit high (visible)
2. Resonance Transfer Excite neighbor
(trampoline)
3. Electron Transfer Pass excited electron
2 and 3 are used in the light reaction
of PHOTOSYNTHESIS
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LIGHT HARVESTING COMPLEXES
Capture Light Energy
Multi-subunit Protein Complexes in the
thylakoid membranes
Two Components
Antenna molecules that absorb and direct energy
by resonance transfer,
Reaction centers that split water molecules and
conduct electron transfer
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The Hill Reaction
Leaf extracts rich in chloroplasts, when
radiated, evolve oxygen and acquire the capacity
to reduce hydrogen acceptors. Robert Hill,
1937
light
2H2O 2A
2AH2 O2
Hill used 2,6-dichlorophenolindophenol
light
A BLUE
AH2 COLORLESS
Hill separated the light from the dark reaction.
Hill showed that light induced an electon flow
that drove an unfavorable energy reaction.
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3H
Stroma
3H
Thylacoid lumen
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Stroma
3H
Matrix
Thylacoid lumen
3H
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NADP
NADPH
(P700)
PS-I
Z Scheme of the Light Reaction
(P680)
PS-II
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What is a Photosynthetic Reaction Center?

• Only purple bacterial centers are known
• A transmembrane protein complex with 3 protein
  components H, L, M
• Typically, 2 chlorophyll, 2 pheophytin, one Fe2,
  and 2 ubiquinone or 1 ubiquinoine and one
  menaquinone
• Nearly perfect 2-fold symmetry

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Photosynthetic Reaction Center (Rhodobacter
sphaeroides)
Rps. viridis
L
M
H
Cytoplasm