BIOL 204 Lecture Notes - Lecture 39: P680, Light-Harvesting Complex, Pheophytin
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BIOL204 - Photosystems
Chlorophylls and Carotenoids Cooperate in Light Absorption
- Chlorophylls are major photosynthetic pigments in plants and there are 2
dominant types: a and b
- Carotenoids are the 2nd major group of pigments involved in photosynthesis
- Only chlorophyll A becomes oxidized during photosynthesis
o Carotenoids and chlorophyll B are accessory pigments
- Absorption spectrum is a plot of the absorption of light as a function of
wavelength
o Chlorophyll A absorbs strongly blue and red but does not absorb
green or yellow light
o Accessory pigments expand the wavelengths of light to be used
- Action spectrum is a plot of the effectiveness of light of particular
wavelengths in driving photosynthesis – determined by suspension of
chloroplasts and measuring O2 released by photosynthesis
o If an action spectrum matches the absorption spectrum it means they
are linked
- Theodor Engelmann – produced action spectrum 1883, used only a light
microscope and glass prism to determine which wavelengths of light were
most effective for photosynthesis
o Placed green algae on a slide with bacteria that required oxygen to
survive, noticed clusters around specific colors (blue and violet)
Photosynthetic Pigments Are Organized into Photosystems
- pigment proteins are bound to thylakoid membranes, organized into
photosystems
- each photosystem is composed of a large antenna complex (called light-
harvesting complex) of pigment proteins and surrounds a reaction center
(holds specialized cholorphyll A molecules and primary electron acceptor)
- 2 kinds of photosystems:
o PSI – specialized chlorophyll A called P700
o PSII – specialized chlorophyll A called P680 (absorbs wavelengths
680nm)
- Function of photosystem: trap photons of light and oxidize a reaction center
chlorophyll with electron being transferred to primary electron acceptor
Photosynthetic Electron Transport
- Electron carriers of the photosynthetic system are: cytochromes, quinones
and iron-sulphur centres
- Photosystem II light harvesting sequence:
o 1) antenna complex absorbs photons, causes electron in P680 to be
raised to excited state*
o 2) P680* is oxidized to P680+ by primary electron acceptor
(pheophytin) and initiates electron transport by donation to
platoquinone (PQ)
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