Photosynthesis Overview
6 CO2 (g) + 6 H2O(I) + light energy  C6 H12 O6 (aq) + 6 O2 (g)
Light
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Only 5% of the light that hits earth's surface is
converted to organic compounds by photosynthesis.
Light travels in wave packets called photons
Light from the sun is a mixture of photons of
different energies (called wavelengths, measured
in nanometers).
Photons are captured in plants by plastids (ex.
Chloroplasts).
Chloroplasts
Functions:
 Photosynthesis factories for plants and algae
 Store excess glucose as starch
Chloroplasts
Structure:
 2 membranes (like mitochondria)
 Interior space filled with stroma (protein-rich fluid)
 In the stroma are thylakoids (membrane bound sacs)
 The thylakoid membrane contains light gathering
(photosynthetic) pigments
Thylakoids
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Grana: stack of thylakoids (~60 grana per
chloroplast with 30-50 thylakoids each)
 Lamellae: unstacked thylakoids that link grana
together.
 Lumen: interior of the thylakoid, water- filled
Pigments
Green plants contain chloroplasts with the
pigments chlorophyll a and b
 both absorb blue and red light and reflect
green light, so we assume blue and red light is
used in photosynthesis
 Chloropyhll a is used to transfer energy from
light into chemical energy.
 Chlorophyll b is an accessory pigment. It
absorbs photons that a misses.
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Accessory Pigments
In addition to chlorophyll b there are other
accessory pigments. Their job is to absorb light
that can damage chlorophyll and lose it as
heat instead.
 Carotenoids: appear yellow/orange (ie. Carrots).
 Betacarotene protects our eyes
 Xanthophylls: reflect yellow light, also found in
the thylakoid
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Photosystems
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Photosynthetic pigments are embedded in the
thylakoid membrane clustered in groups called
photosystems.
A photosystem contains a chlorophyll a molecule
called a reaction centre, surrounded by accessory
pigments called the antenna complex.
The antenna complex harvests the light and
passes it to the reaction centre.
Photosystem Diagram
PI (P700) and PII (P680)
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Two types of photosystems exist:
Photosystem I (PI) and Photosystem II (PII).
They have identical reaction centres, but they
absorb light at slightly different wavelengths
because of the proteins the reaction centre is
associated with.
The reaction center in PI is called P700;
absorption peaks at 700nm.
The reaction centre in PII is called P680,
absorption peaks at 680nm.
Photosystem Diagram