Botany 130, Lecture 13

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Photosynthesis Botany 130 Lecture 13
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The light reactions of photosynthesis occur on
then thylakoid membranes of chloroplasts. (Comp
are with respiratory electron transport, which
occurs on the inner membranes of mitochondria)
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Using the energy of light

• Light has properties of waves and particles
• It is the particle properties of light that are
  important in photosynthesis
• Light is absorbed by pigments, by far the most
  important one is chlorophyll

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Light is a small range of wavelengths of
electromagnetic radiation.
Colors are a small range of wavelengths of
light.
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Chlorophyll absorbs red light or blue light but
little green light
______________ S2 ______________
S1 ______________ S0
e-
 
e-?
Energy states of the outermost electron of
chlorophyll
e-
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When electrons are in state 1 or state 2 they are
said to be in excited states
______________ S2 ______________
S1 ______________ S0
e-
 
Energy states of the outermost electron of
chlorophyll
e-
e-
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Electrons in state 2 always fall to state 1 with
the release of heat
______________ S2 ______________
S1 ______________ S0
e-
 
Energy between S1 and S2 is always lost as heat
e-
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Electrons in state 1 can do one of three things
1. Radiationless transfer
2. Fluorescence
3. Intersystem transfer
 
________ S2 ________ S1 ________ S0
________ S2 ________ S1 ________ S0
________ S2 ________ S1 ________ S0
e-
e-
Heat
Energy lost as heat
Energy can be transferred to another molecule.
Energy lost as light
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Consider fluorescence in blue light
1. Radiationless transfer
2. Fluorescence
 
________ S2 ________ S1 ________ S0
________ S2 ________ S1 ________ S0
______________ S2 ______________
S1 ______________ S0
Heat
Energy lost as heat
Absorption of blue results in emission of red
light
Energy lost as light
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Important Pigments in Photosynthesis
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Important Pigments in Photosynthesis
Carotene
Pheophytin
Chlorophyll a
Chlorophyll b
Xanthophylls (Lutein)
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Important Pigments in Photosynthesis
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An experiment to determine what colors of
light are used in photosynthesis.
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Chlorophyll is related to heme, but has
magnesium where heme has iron. The tetrapyrole
structure is common in biology, especially in
relation to processes involving oxygen. The
long hydrophobic tail of chlorophyll (the phytol
tail) is an isoprenoid and anchors the
molecule in a protein.
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There are typically 200 antenna chlorophylls
for every reaction center chlorophyll. Only
chlorophyll a can be a reaction center
chlorophyll.
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