Light Energy and Photosynthetic
Pigments
Photosynthesis…..
• In plants and
algae, all the
reactions of
photosynthesis
take place within
the chloroplasts.
The 3 Stages of Photosynthesis
(they are distinct but connected)
Stage 1: Capturing of light energy
Stage 2: Using the light energy to make ATP
and reduced NADP+
Stage 3: Synthesis of organic compounds such as
glucose from CO2
The Light Reactions (Step 1 & 2)
• The first two stages involve a series of
reactions that are directly energized by light,
so we call them LIGHT REACTIONS.
• They occur on the thylakoid membranes of
chloroplasts.
• The energy absorbed by the thylakoid is
eventually transferred to carbohydrate
molecules in the last stage
Carbon Fixation (Step 3)
• The third stage occurs in the stroma.
• The reactions in the third stage result in
carbon fixation – the incorporation of the
carbon of CO2 into organic compounds, such
as glucose
• These reactions require ATP and NADPH
• Carbon fixation occurs during a sequence of
enzyme catalyzed reactions called the Calvin
cycle
Light Reactions:
• Require light energy
• take the H from H20 and reduce NADP+ to
NADPH + H +
• Release O2 gas
• Convert ADP + Pi to ATP
Carbon Fixation
• Incorporate the C from CO2 into organic
compounds such as glucose
• Require ATP (from light reactions)
• Require NADPH (from light reactions)
Note: Step 3 was once called the “dark reactions” but it has
now been shown that many enzymes that catalyze these
reactions are activated by light
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.
• The best wavelengths for promoting
photosynthesis are RED and BLUE.
The Importance of Pigments
• Pigments are molecules that absorb light.
Example: Chlorophyll
• When a photon of light strikes a
photosynthetic pigment, an electron in the
atom 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.
These accessory pigments can pass on their
energy to chlorophyll a.
Example 1: Chlorophyll b REFLECTS yellow-green light. It
absorbs blue-green and red light
Example 2: Carotenoids REFLECT yellow-orange light. They
absorb 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) acts as an
anchor
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