Machine that sends light through pigments measures


Absorption spectrum

Graph plotting light absorption vs. wavelength
Figure 10.8
White
light
Refracting
prism
Chlorophyll
solution
Photoelectric
tube
Galvanometer
Slit moves to
pass light
of selected
wavelength
Green
light
The high transmittance
(low absorption)
reading indicates that
chlorophyll absorbs
very little green light.
The low transmittance
(high absorption) reading
chlorophyll absorbs most blue light.
Blue
light
1
2
3
4
0
100
0
100


Absorption spectra of 3 types of pigments in
chloroplasts
Three different experiments helped reveal which wavelengths of light are photosynthetically
important. The results are shown below.
EXPERIMENT
RESULTS
Absorption of light by
chloroplast pigments
Chlorophyll a
(a) Absorption spectra. The three curves show the wavelengths of light best absorbed by
three types of chloroplast pigments.
Wavelength of light (nm)
Chlorophyll b
Carotenoids
Figure 10.9


Action spectrum of a pigment

Profiles relative effectiveness of different
wavelengths of radiation in driving
photosynthesis
Rate of photosynthesis (measured by O
2
release)
Action spectrum. (b)


Action spectrum for photosynthesis

first demonstrated by Theodor W. Engelmann
400
500 600 700
Aerobic bacteria
Filament
of alga
Engelmann‘s experiment.
(c)


Chlorophyll a (blues)

Main photosynthetic pigment

Chlorophyll b (yellows)

Accessory pigment
C
CH
CH
2
C
C
C
C
C
C N N C
H
3
C
C
C
C
C C
C
C
C
N
C
C
C
C
N
Mg H
H
3
C
H
C CH
2
CH
3
H
CH
3
C
H
H
CH
2
CH
2
CH
2
H
CH
3
C O
O
O
O
O
CH
3
CH
3
CHO
in chlorophyll a
in chlorophyll b
Porphyrin ring:
Light-absorbing
“head” of molecule
note magnesium
atom at center
Hydrocarbon tail:
interacts with hydrophobic
regions of proteins inside
thylakoid membranes of
chloroplasts: H atoms not
shown
Figure 10.10


Other accessory pigments

Absorb different wavelengths of light & pass the
energy to chlorophyll a

Carotenoids – yellows & oranges

Excitation of Chlorophyll by Light 
When a pigment absorbs light

Goes from ground state to excited state,
which is unstable
Excited
state
Energy of election
Heat
Photon
(fluorescence)
Chlorophyll
molecule
Ground
state
Photon
e
–
Figure 10.11 A