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3.2 Light Energy and
Photosynthetic Pigments
Pages 147- 154
Photosynthesis…..
 In plants and
algae, all the
reactions of
photosynthesis
take place
within the
chloroplasts.
The 3 Stages of Photosynthesis
Stage 1: Capturing of light energy
Stage 2: Using the light energy to make ATP
and reduced NADP+
Stage 3: Synthesis of organic compounds
from CO2
The first two stages involve a series of
reactions that are directly energized by
light, so we call them LIGHT DEPENDENT
REACTIONS
They occur on the thylakoid membranes of
chloroplasts.
The third stage occurs in the stroma and is
LIGHT INDEPENDENT
Light (Electromagnetic Radiation)
Light behaves as if it were composed of
“units” or “packets” of energy that travels
in waves. These packets are PHOTONS.
Photons are characterized by a
wavelength that is inversely proportional to
their energy.
Therefore, photons with short wavelengths
have high energy and those with long
wavelengths have low energy.
The wavelength of light determines its
colour. For example, the wavelength of red
light is about 700nm and the wavelength
of blue light is about 470nm.
700nm
RED
BLUE
470nm
The Electromagnetic Spectrum
Figure 3, p 148
Most of the photons in the electromagnetic
spectrum are invisible to humans.
Visible light ranges from 750 nm (red light)
to 380 nm (violet light). The colours visible
are:
red-orange-yellow-green-blue-indigo-violet
(ROY G BIV)
Red light has a wavelength of 750 nm and
thus has the longest wavelength (of visible
light) with the lowest energy.
Violet light has wavelengths of 380 nm
and thus has the shortest wavelength (of
visible light) with the highest energy.
Wavelengths in Photosynthesis
The best wavelengths for promoting
photosynthesis are RED and BLUE.
This was discovered by T.W. Engelmann
in 1881. He put green algae (Spirogyra)
under a microscope in a liquid medium
and illuminated it with visible light. He
then put motile bacteria in the medium.
After a few minutes the bacteria tended to
congregated around the sections
illuminated with blue and red light. He
concluded that these sections must be
putting out the greatest concentration of
oxygen.
Video Clip: https://www.youtube.com/watch?v=Rt37Hyn4Qv4
Importance of Pigments
Substances can do one of two things
when they are struck by a particular colour
of light:
absorb that wavelength – allows energy to
be absorbed and possibly used
reflect that wavelength – energy is not
absorbed and that colour will be seen
The Importance of Pigments
Pigments are molecules that absorb light.
Ex. Chlorophyll
When a photon of light strikes a
photosynthetic pigment, an electron in an
atom contained with the molecule
becomes excited.
Electrons of the excited molecule are
energized and move further from the
nucleus of the atom.
The excited (energized) molecule can
pass the energy to another molecule or
release it in the form of light or heat.
Chlorophyll a is the main photosynthetic
pigment in all organisms except bacteria. It
absorbs blue-green light and red light.
It REFLECTS green light!
It is the only pigment that can transfer the
energy of light to the carbon fixation
reactions of photosynthesis.
 Other pigments, called accessory pigments absorb
slightly different wavelengths of light:
Ex. Chlorophyll b REFLECTS yellow-green
light.
It absorbs blue-green and red light
Carotenoids REFLECT yellow-orange
light
It absorbs blue-green light.
The combination of all of the pigments
increases the range of colours (of light) that
plants can use in photosynthesis.
Chlorophyll
 Chlorophyll a and b
have 2 basic parts.
i) A long hydrocarbon tail
(embedded in the lipid
bilayer of the thylakoid
membrane)
ii) A hydrophilic porphyrin
ring that has alternating
single and double
bonds
When light is absorbed by chlorophyll, it
excites electrons from magnesium within
the porphyrin ring to begin the process of
photosynthesis.
Carotenoids
 Carotenoids do not participate directly in
photosynthetic reactions but are able to
pass their energy to chlorophyll a.
 Carotenoids also have two basic parts:
i) Carbon rings
ii) Hydrocarbon chains containing
alternating single and double bonds
Carbon Ring
Hydrocarbon Chain
When light is
absorbed by
carotenoids, it excites
electrons in the
hydrocarbon chain