II' The light reactions of photosynthesis

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• II. The light reactions of photosynthesis
• Objectives are to understand
• Variation in energy of different forms of light
• Light absorption by photosynthetic pigments
• Energy transduction - conversion of light energy
• to chemical energy as ATP and NADPH

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• Why did we calculate the energy of a photon or
  mole of photons?
• This is the energy that is absorbed by plants and
  used to power photosynthesis!
• It is the energy of each photon, the quantum
  energy,that that slams into the photosynthetic
  pigments and excites - raises the energy state -
  of electrons.
• We need to know the input of energy to understand
  the energetics of photosynthesis.
• e.g. How efficient is photosynthesis?
• efficiency energy output/energy input

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Light absorption by photosynthetic pigments
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Fig 7.15
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grana lamellae
Fig 7.16
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• Light absorption by photosynthetic pigments
• Much of the light energy reaching Earths
  surface is in the visible
• portion of the EMR spectrum.
• Chlorophyll absorbs strongly in this region of
  the spectrum.

Fig.7.3
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• Photosynthetic pigments of higher plants
• chlorophylls (a b) 2. carotenoids

Fig. 7.6
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The interaction of photosynthetic
pigments Antennae transfer light
energy to reaction centers
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• 3) Energy transduction - conversion of light
  energy
• to chemical energy as ATP and NADPH
• a. Absorption and action spectra
• b. Key experiments in understanding the light
  reactions
• c. The Z scheme of electron transfer and energy
  capture.
• d. Putting it all together - organization of the
  light harvesting antennas and photochemical
  reaction centers.

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a. absorption and action spectra
How light absorption characteristics are
measured. Measuring an absorption spectrum using
a spectrophotometer
Fig. 7.4 (blue or green or red, etc.)
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2 chl a3 chl b5 beta carotene
Fig. 7.7
High
Low
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Action spectra describe the relationship of the
effect (e.g. O2 production) of light absorption
to wavelength.
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An early action spectrum using a
bioassay. Engelmann, 1800s
Fig. 7.9
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Pond scum or a beautiful green alga?
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b. Key experiments in understanding the light
reactions Emerson-Arnold expt. 1932
O2 production depended on amount of
light. Highest efficiency at low light -
quantum yield O2 production saturated at high
light. One O2 was produced per 2500
chlorophyll molecules.
Fig. 7.11
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Quantum yield is the term given to describe the
maximum yield of O2 per photons absorbed by the
leaf (or extracted chloroplast preparations). It
is equal to the slope of the photosynthetic light
response curve at low light levels. The quantum
yield is an efficiency term Efficiency output
(O2 production) input (light absorbed)
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The red drop experiments - Emerson
again. Observation Quantum yield dropped off
sharply beyond 680nm. Why was efficiency reduced
greatly in the far red portion of the spectrum
(beyond 680nm or so)?
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Red drop experiments suggested that the energy in
light particles beyond the red portion of the
spectrum was insufficient to drive photosynthesis.
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Emerson enhancement effect Far red and red
light separately gave same rate of O2
production. Both given together gave much greater
O2 production. What could explain this behavior?
Fig. 7.13
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Emersons enhancement effect experiments
suggested the existence of two interacting
photosystems with different wavelength optima.
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c. The Z scheme of electron transfer and energy
capture.
Fig. 7.14