Capturing Light Energy and
the Electromagnetic
Spectrum
The Big Picture: The Conversion
of Light Energy
• Light Reactions of Photosynthesis convert light energy
(from the sun) into chemical energy (ATP and NADPH)
• Chloroplasts in plant cells act as chemical factories
powered by the sun
– The thylakoids in the
chloroplast convert the light
energy into chemical
energy
• To understand the
conversion… we are
going to take a look at
some important
properties of light!
Sunlight (Contains Visible
Light)
• Sunlight is a form of electromagnetic
energy!
• Photosynthesis transforms (converts)
electromagnetic energy into chemical
energy of organic molecules (sugars)
• Sunlight is part of the electromagnetic
spectrum.
Sunlight –Contains Visible
Light
• Sunlight (contains Visible Light) is a form
of electromagnetic energy.
• Electromagnetic energy travels as a
wave, but is composed of particle-like
bundles of energy.
• Sunlight travels as a wave and contains
bundles of energy known as photons.
Sunlight– Wave Behavior
Wave Theory
• Light travels through space as a wave
• Waves have the following characteristics:
– Wavelength- the distance between the
crests of electromagnetic waves
• Energy- shorter the wavelength = the greater the
energy of each photon
– Frequency- a measure of the number of
wavelengths in a given amount of time
Sunlight Light – Particle
behavior
• Light behaves as though it consists of
particles called photons
– Each photon has a fixed quantity of energy
• Amount of energy is inversely related to
the wavelength of light
– The shorter the wavelength = the greater the
energy of each photon of that light
– Ex: photon of violet light packs almost twice
as much energy as a photon of red light
Sunlight and The
Electromagnetic Spectrum
• Electromagnetic Spectrum – the entire range of
radiation.
• Visible Light – the segment most important to life
– the narrow band from about 380nm to 750nm in wavelength
– it is detected as various colors by the human eye
• The sun radiates the full spectrum of
electromagnetic energy, but atmosphere is
selective and allows only visible light to pass
through.
Visible light = part of the spectrum we can see = drives photosynthesis
The Electromagnetic Spectrum
White light is a mixture of all wavelengths of visible light.
A prism can sort white light into its component colors.
Pigments
• Light may be either:
reflected, transmitted, or absorbed.
• Pigments- substances that absorb visible light.
• The color we see an object to be is the color
most reflected or transmitted by the pigment.
• EX: Leaves look green because the main
pigment in them (chlorophyll) absorbs violetblue and red light while it transmits/reflects
green light.
Absorption Spectrum
• Absorption Spectrum
– is a graph that plots a
pigment’s light
absorption versus
wavelength of light.
The three pigments shown
below differ in the colors of
light they absorb.
Absorption Spectrum
• The three pigments shown in the prior
graph differ in the colors of light they
absorb.
• Where a curve has a peak is where
much of the light at that wavelength is
absorbed
• Where there is a trough, much of the
light at that wavelength is reflected or
transmitted. (what we see)
Reexamine the Absorption
Spectrum
Light and Pigments
• Pigments- compounds that absorb light/ most
absorb certain colors more strongly than others.
• Several pigments are located in the membrane
of the thylakoid:
– Chlorophyll a
– Chlorophyll b
– Carotenoids
Chlorophyll a
• Absorbs violet-blue and red light
• Allows green light to be reflected/transmitted
• Is the main pigment within the
photosystems.
• Directly involved in the light reactions of
photosynthesis
Accessory Pigments
• Pigments with different absorption
spectra than chlorophyll a.
• Are clustered with chlorophyll a within
photosystems.
• Help the leaves to capture the maximum
amount of light energy
– Chlorophyll b
– Carotenoids
Chlorophyll b
• An accessory pigment
– Assists chlorophyll a in capturing light energy
• Almost identical to chlorophyll a but there is
a slight structural difference
• Absorbs blue light
Carotenoids
• Yellow, orange, and brown
• Accessory pigments
• Absorbs colors that chlorophyll a cannot
– enables plant to capture more energy
• In fall when plants lose chlorophylls their
leaves take on the rich hues of the
carotenoids
When Pigments Absorb Light
• Colors corresponding to the absorbed wavelengths
disappear from the spectrum
• When a molecule absorbs a photon of light, one of the
molecule’s electrons is elevated (excited) to where it has
more potential energy
• Each pigment absorbs only photons corresponding to
specific wavelengths, which is why each pigment has a
unique absorption spectrum
• Excited state is unstable so the excited electrons drop back
down releasing their excess energy.
• As excited electrons drop back down, energy is given off.
Photosystems:
 Cluster of pigments and proteins
embedded within the thylakoid
membrane.
 Contain accessory pigments that
surround chlorophyll a, the main pigment
of photosynthesis.
 Are able to absorb the maximum amount
of light possible due to the accessory
pigments and chlorophyll a.
Photosystems
• Photosystems are found
embedded within the
thylakoid membrane of the
chloroplast.
Photosystems: Are embedded
in the Thylaoid Membrane of
the Chloroplast.
Next time – Photosystems and
Chloroplasts!!!