Chapter 7 Photosynthesis: The light reactions

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Chapter 7 Photosynthesis The light reactions

• ? synthesis using light
• ? is the only process of biological system that
  can harvest
• sunny energy.
• ? a deceptively simple equation
• 6 CO2 6 H2O ? C6H12O6 6 O2
• ? stored energy can be used to power other
  cellular processes in the
• plant and provide the energy source for
  all forms of life.
• ? Topics
• the characteristics of light,
• the structure of the photosynthetic
  apparatus,
• the processes of chlorophyll excitation,
• the synthesis of ATP and NADPH.

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Light a wave and a particle

• c (the speed of wave) ??
• E (the energy of photon) h?
• h Plancks constant (6.62 10-34 Js)
• Quantum (quanta)
• each photon contains an amount of energy
• discontinuous

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Electromagnetic spectrum

• Sunlight like a rain of photons of different
  frequencies.

UV-A 320 400 nm UV-B 280 320 nm UV-C ?
290 nm
Light emitting diode (LED) blue 470 nm red
660 nm
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Absorption spectrum the amount of light energy
taken up or absorbed by a molecule or substance
as a function of the light wavelength of the
light.
green
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Action spectrum??the magnitude of a response of
a biological system to light, as a function of
wavelength
Light-induced phenomenon
Carotenoid absorption 400 500 nm
? the pigment responsible for the particular
light-induced phenomenon
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O2 evolved
Late 1800s
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Photosynthetic pigments
accessory pigments
porphyrin-like group
phytol chain
cyanobacteria red algae
xanthophyll
Chl a, b, c, d
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2. Chlorophyll a in nonpolar solvent 3.
Chlorophyll b in nonpolar solvent 4.
Phycoerythrobilin in an aqueous buffer 5.
ß-carotene in nonpolar solvent
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Light absorption and emission by chlorophyll
Chl h? ? Chl (10-9s), discontinuous
1. heat loss 2. fluorescence 3. energy
transfer 4. photochemistry.
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Light energy Chemical energy
pigments, e- transfer proteins
more efficiency
Core/peripheral
Chl b Chl a

• Antenna?? complex collect light and physical
  resonance transfer, no chemical reactions high
  efficiency of energy transfer.
• Reaction center the chemical oxidation and
  reduction reactions leading to long-term energy
  storage take place.

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The antenna funnels?? energy to the reaction
center fluorescene resonance energy transfer
p. 138
?max of Chl b 650 nm Chl a 670 nm
red shif
A physical phenomenon directionality and
irreversibility
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1932 Emerson and Arnold ? oxygen production vs.
flash energy a suspension of Chlorella
pyrenoidosa 10-5 s flash, 0.1 s apart
varied light intensity??, slope
saturation ? many chlorophyll molecules
involve in energy conversion during
photosynthesis
Quantum yield (?) No. of photochemical
products / Total No. of quanta absorbed ? 0.1
for O2 production Quantum requirement
? 10 for O2 evolution
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Quantum efficiency????? 1 engage in
photochemistry / absorbed photons Energy
efficiency ? 0.25 the stored energy of
chemical compound / the energy of absorbed
photons CO2 H2O light (CH2O) O2
?G 467 kJ/mol red light
467103/10 17.1104 blue light 467103/10
23.95104
E h? O2/10 photons
EE
low
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Hill reaction (1937)

• ? an artificial electron acceptors in isolated
  chloroplast thylakoid
• No CO2 condition, still O2 production
• 4 Fe3 2 H2O ? 4 Fe2 O2 4 H
• ? CO2 2 H2S ? (CH2O) 2 S H2O
• H218O CO2 ? (CH2O) 18O2
• (1) CO2 2 H2O ? (CH2O) O2 H2O
• (2) CO2 H2O ? (CH2O) O2

In eukaryotic photosynthetic organisms, the
reaction centers and most of the antenna
complexes are integral within the membrane of
chloroplast in photosynthetic prokaryotes, the
site of photosynthesis is the plasma membrane or
membranes derived from it. P. 132