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Chapter 8
Light and Pigments
Need To Know
• How photosystems convert light energy into
chemical energy. (There will be more on this
in the next couple of days.)
Figure 8.5-1
H2O
CO2
carbon fixation
Light
NADP
ADP
 Pi
Light
Reactions
Calvin
Cycle
ATP
NADPH
Chloroplast
O2
Two Stages of Photosynthesis
[CH2O]
(sugar)
Light travels in waves
Visible light
380
450
500
Shorter wavelength
Higher energy
550
600
650
700
750 nm
Longer wavelength
Lower energy
Figure 8.7
Light
Reflected
light
Chloroplast
Absorbed
light
Transmitted
light
Absorption of light
by chloroplast
pigments
An absorption spectrum is a graph plotting a
pigment’s light absorption versus wavelength.
Chlorophyll a
Chlorophyll b
Carotenoids
500
600
Wavelength of light (nm)
(a) Absorption spectra
400
700
400
(b) Action spectrum
Rate of
photosynthesis
(measured by O2
release)
Figure 8.9b
500
600
700
Setup for Theodor W. Engelmann’s
1883 experiment.
O2 is a waste product of the
light reaction of photosynthesis
Aerobic bacteria
Filament
of alga
500
400
600
Engelmann’s
(c)
Engelmann’s
experiment
experiment
- 1883
- 1883
700
Absorption of light
by chloroplast
pigments
Results
Chlorophyll a
Chlorophyll b
Carotenoids
500
600
Wavelength of light (nm)
400
700
(a) Absorption spectra
Rate of
photosynthesis
(measured by O2
release)
Figure 8.9
400
(b) Action spectrum
500
600
700
Aerobic bacteria
Filament
of alga
500
400
(c) Engelmann’s experiment
600
700
Figure 8.10
CH3
CH3 in chlorophyll a
CHO in chlorophyll b
Porphyrin ring:
light-absorbing
“head” of molecule;
note magnesium
atom at center
A slight structural difference
between chlorophyll a and
chlorophyll b causes them to
absorb slightly different
wavelengths.
Hydrocarbon tail:
interacts with hydrophobic
regions of proteins inside
thylakoid membranes of
chloroplasts; H atoms not
shown
Figure 8.11
−
Energy of electron
e
Excited
state
Heat
Photon
(fluorescence)
Photon
Photon
Chlorophyll
molecule
Ground
state
(a) Excitation of isolated chlorophyll molecule
(b) Fluorescence