The Dual Nature of Light
Wave and Particle
Light as a particle
Particles or packets of light are known
as photons
Brightness or intensity depends on the
number of photons absorbed/unit area/unit
time
Photon carries fixed amount of energy
Determines how fast it vibrates
high energy = fast
low energy = slow
Light as a wave
The distance moved by a photon during
one of it vibrations is referred to as its
wavelength
abbreviations used in class:
 = lambda = wavelength
unit = nanometers (nm)
One nanometer = 10-9 meter
 = nu = my shorthand for light
Long wavelength
Low energy
Short wavelength
High energy
Effective wavelengths for
photosynthesis
A prism separates visible
light into a color
spectrum
The visible spectrum
ROYGBIV
Between 380 and 750 nm
Why only visible light? Sunlight has
three components
4% UV (ultraviolet radiation) TOO STRONG!
ionizing radiation
breaks weak chemical bonds
causes DNA damage and sunburn
absorbed by O2 and O3 (ozone), glass, plastic
52% IR (infrared radiation) TOO WEAK!
low energy
most energy of IR converted to heat
44% visible light JUST RIGHT!
suitable energy for life - photosynthesis
absorbed by pigments
What Happens to Light?
Reflected
Chlorophyll is green because it reflects the green
wavelengths.
Sky is blue because water molecules reflect blue
wavelengths.
Transmitted
Glass permits most visible wavelengths to pass
through.
Absorbed
Chlorophyll absorbs all colors except green,
especially red and blue regions.
Pigments
Molecules that absorb light
Black absorbs all wavelengths
White absorbs no wavelengths
Color that we see is the reflected
wavelengths
Quality of Light
Light quality (availability of
different wave-lengths)
can limit rate of photosynthesis
Blue and red wavelengths
are absorbed by
chlorophyll
Green wavelengths are
reflected or transmitted
Therefore most plants
look green to us
Usable wavelengths are called PAR –
photosynthetically active radiation
Chlorophyll
Structure
tetrapyrrolic rings
Mg+2 atom
phytol chain (tail)
Forms
chl a
grass green
absorbs in red and violet blue regions
chl b, c, and d
bluish green
Chlorophyll
Absorption spectrum: absorbs all
wavelengths except green
Absorbs most strongly in red and blue
region of spectrum
Action spectrum: doesn’t match
absorption spectrum
Chl a is primary photosynthetic pigment
But there must be others
Accessory Pigments
Functions
Extend range of photosynthesis by
absorbing wavelengths not picked up by
chl a
Carotenoids also protect against photooxidation
Oxygen radicals formed by excited electrons kill
cells (basis for some herbicides)
Accessory Pigment Types
Chlorophylls
Chl a is the main photosynthetic pigment
Grassy green
absorbs around 420 and 660 nm
Chl b
bluish green
absorbs 453 and 642 nm
about half as abundant as chl a
Chl c and d
Accessory Pigment Types
Carotenoids
fat soluble
absorb 460 and 550 nm
Blue, bluegreen, violet
Structure - see plant chem. Notes
Example: -carotene
reddish yellow, orange
precursor to vitamin A (split to get 2 vit A)
needed to produce retinal pigment for vision
carrots, tomatoes, bananas, squash, fall leaves
Accessory Pigment Types
Carotenoids are also found in animals
egg yolks
flamingo feathers
squid ink
corals
fish
amphibians
lots of colors
proteins attached to carotenoids
remove protein => see red or orange
this is what happens when you cook shrimp
or lobster
Absorption
spectra
Accessory Pigment Types
Xanthophylls and fucoxanthin (brown algae)
yellowish or red
absorb blue light (some plants grow towards blue
light)
not as efficient as -carotene
Phycoerythrin and phycocyanin found in
cyanobacteria and red algae
Remember: these bacteria as well as the algae
contribute to production in aquatic systems
Pigments
Light reaching the
surface of the earth
Peak irradiation occurs
at approx 500 nm
Light intensity drops off
at higher & lower
wavelengths
Pigments
If we combine all of
these, including
chlorophyll and
carotenoids, there is
good light absorption across the
entire visible
spectrum!
Absorption of Light by Water
Absorption of Light by Water
Red & blue wavelengths
preferentially absorbed by water
Wavelengths 500 – 600 nm
absorbed least by water
correspond with absorption spectra of phycobilins.
Ecological Significance of
Phycobilins
Organisms with Phycobilin pigments are
better adapted for life in an aquatic
environment and can exploit greater depths in
lakes and oceans!
Therefore, red algae and the cyanobacteria
can survive at greater depths than any other
algae or vascular plants.
Why Do Leaves Change
Color in the Fall?
Chlorophyll synthesis shuts down
Chlorophyll molecules break down
“True” colors of leaves show through
Accessory pigments
Carotenoids - orangeIn chromoplasts
Xanthophylls - yellow
Anthocyanins - red, purple
In central vacuole
Quantity of Light
Amount of light reaching thyllakoid
membranes is limited by
Location of chloroplast in leaf
Incident angle of sunlight as it strikes leaf
Self shading by other leaves
Shading by competitors
See discussion of Chazdon 1985 and Tanaka et
al. 2001.
Global Light Availability
Tropical latitudes - day
and night equal
Polar latitudes continuously light at
midsummer, continuously
dark at midwinter
Maximum sunlight energy
greater in tropics than
polar regions
Global Light Availability
Maximum sunlight energy
greater at high altitudes
than at sea level
Damaging UV-B radiation
greater in tropics than
polar regions, high
elevations vs. low
elevations
Biochemical protection:
Flavonoids to absorb
UV-B
Increased levels of
antioxidant enzymes
and
DNA repair enzymes