Photosynthesis I: The Light Reactions

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• Photosynthesis I The Light Reactions
• Protection and repair of light reaction
  components
• Plants usually compete with
• each other for access to light,
• but sometimes the plant needs
• to limit the amount of light it
• is absorbing and/or using to Figure 7.35
• do electron transport.
• Some nasty bad reactive
• species can be formed,
• especially if there is too much light.
• Plants have ways of limiting damage
• caused by light, and ways of repairing
• any damage that gets by its defenses.

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• Photosynthesis I The Light Reactions
• A. Whats the big deal? Why do chloroplasts
  need protection from too much light?
• 1. The photosystems are easily damaged by
  excess light.
• a. PSII can be easily
• damaged as its D1
• polypeptide gets Figure 7.25A
• oxidized.
• b. PSI is particularly
• vulnerable to reactive
• species.
• 2. Heat--excess light energy
• has to be dissipated by
• conversion to heat.

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• Photosynthesis I The Light Reactions
• 3. Photoinhibition--reduction in
  photosynthetic ability
• due to damage by excess light.
• a. Dynamic photoinhibition is causes by
  moderate
• excesses, where decrease in efficiency is a
  sign that
• energy is being lost as heat.
• Reversible.
• b. Chronic photoinhibition Figure 9.15
• results in a lowering of
• maximum rate, and reflects
• actual damage to the
• photosystems, mostly PSII.
• Effects can last for weeks
• or months.

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• Photosynthesis I The Light Reactions
• B. Protection--lines of defense
• 1. Photoprotection--Carotenoids protect cells
  by dissipating excess energy from
  chlorophyll.
• a. Chlorophyll excited by blue-light has
  enough energy to react with oxygen to form
  superoxide anion (O2) or other reactive
  species, which can damage DNA, proteins, or
  membranes.
• b. Instead, this energy is quickly
  transferred to carotenoids, which have a
  lower-energy excited state that cannot react
  in that way. Energy lost as heat.
• c. Carotenoids are essential to plants in
  this way.

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• Photosynthesis I The Light Reactions
• 2. Non-photochemical quenching
• a. When light levels get too high, a
  conversion happens, turning violaxanthin, a
  type of xanthophyll (another class of
  carotenoids), into zeaxanthin.
• b. Zeaxanthin binds to proteins of
  the antenna complex, and causes the
  pigments there to lose their Figure
  7.36 energy as heat more frequently than
  they would normally.

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• Photosynthesis I The Light Reactions
• 3. Scavengers--Reduced ferredoxin is a strong
  enough agent to generate superoxide or other
  reactive species. If a reactive species gets
  made, how does the cell deal with it before it
  can cause damage?
• a. Superoxide dismutase
• b. Ascorbate peroxidase

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• Photosynthesis I The Light Reactions
• 4. Made in the shade--chloroplasts move to the
  cells periphery when there is excess light,
  shade each other.
• figure 9.5

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• Photosynthesis I The Light Reactions
• 5. Heat dissipation.
• Radiation (red and IR
• photons are emitted
• figure 9.14
• Sensible heat loss
• Release of gaseous isoprene
• Water evaporation

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• Photosynthesis I The Light Reactions
• C. Repair
• 1. Anything that gets damaged by reactive
  species will need to be replaced. Reactive
  species can cause breaks in the DNA chains, as
  well as damage to the bases themselves.
• 2. The most susceptible part of the system the
  D1 polypeptide of PSII, has to be
  re-synthesized and replaced if damaged.