Photosynthesis
Chapter 10
A. P. Biology
Liberty Senior High School
Mr. Knowles
Question:
Does eating carrots
really improve your
vision?
• Photosynthesis
– Occurs in plants, algae, certain other protists, and
some prokaryotes (photoautotrophs).
These organisms use light energy to drive
the
synthesis of organic molecules from
carbon dioxide
and (in most cases) water. They feed not
only
themselves, but the entire living world.
(a) On
land, plants are the predominant
producers of
food. In aquatic environments,
photosynthetic
organisms include (b) multicellular algae,
such
as this kelp; (c) some unicellular protists,
such
as Euglena; (d) the prokaryotes called
cyanobacteria; and (e) other
photosynthetic
prokaryotes, such as these purple sulfur
bacteria, which produce sulfur (spherical
globules) (c, d, e: LMs).
(a) Plants
(c) Unicellular protist
10 m
(e) Pruple sulfur
bacteria
Figure 10.2
(b) Multicellular algae
(d) Cyanobacteria
40 m
1.5 m
• Plants are photoautotrophs
– They use the energy of sunlight to make organic
molecules from water and carbon dioxide
Figure 10.1
Chloroplasts: The Sites of
Photosynthesis in Plants
• Leaves are the major sites of photosynthesis
Leaf cross section
Vein
Mesophyll
Stomata
Figure 10.3
CO2
O2
• Chloroplasts
– Are the organelles in which photosynthesis
occurs; contain thylakoids and grana
Mesophyll
Chloroplast
5 µm
Outer
membrane
Thylakoid
Stroma
Granum
Intermembrane
space
Thylakoid
space
Inner
membrane
1 µm
Chloroplasts
Photosynthesis
• < 1.0 % of sun’s energy that strikes Earth is
converted into 150 billion metric tons of
sugar.
• In cellular respiration, electrons are
transferred from sugar to oxygen; they lose
potential energy to make ATP.
• In photosynthesis, electrons from water
gain potential energy as they are
transferred to carbon dioxide to make
sugar; the energy is provided by light.
Free Energy  G
C6H12O6 + O2
CO2 + H2O
Rxn Time
Historical Perspective
• 17th Century: van Helmont measured
willow tree growth; compared mass
of tree to mass of soil.
• Late 18th Century: J. Priestly
“restores” air in a vacuum with mint.
• 18th Century: Ingenhousz found air
only restored in the presence of
sunlight; hypothesized CO2  C + O2
Role of Water
• 1930’s: Van Niel studied purple
sulfur bacteria that performed
photosynthesis w/o water:
CO2 + 2 H2S + Light Energy
(CH2O) + H2O +2 S
• The H2S serves an electron
donor.
• H2O serves as donor in green
plants.
Role of Water
• In the 1950’s: used
( an isotope
of oxygen ) in water to trace
oxygen in the reaction.
• 6 CO2 + 12 H218O + Light Energy
 C6H12O6 + 6 18O2 + 6 H2O
18O
The Role of Light
• In 1900’s, F. F. Blackman determined that
photosynthesis occurred in two steps.
• Initial set of reactions were dependent on
light- “Light Reactions” or Light-Requiring
Reactions.
• A second set of reactions were independent of
light-”Dark Reactions” or Non-Light
Requiring Reactions- affected by temperature.
• Dark Reactions use enzymes.
The Splitting of Water
• Chloroplasts split water into
– Hydrogen and oxygen, incorporating the electrons of
hydrogen into sugar molecules
Reactants:
Products:
Figure 10.4
12 H2O
6 CO2
C6H12O6
6 H2O
6 O2
The Biophysics of Light
• Light consists of units of energyPhotons.
• Not all photons have the same
amount of energy.
• Photons travel in waves; higher the
energy the shorter the wavelength
(measured in nm, lambda).
• The electromagnetic spectrum
– Is the entire range of electromagnetic energy, or radiation
10–5
nm
10–3
Gamma
rays
103
1 nm
nm
X-rays
UV
106
nm
Infrared
1m
106 nm
nm
Microwaves
103 m
Radio
waves
Visible light
380
450
500
Shorter wavelength
Higher energy
Figure 10.6
550
600
650
700
Longer wavelength
Lower energy
750 nm
– Reflect light, which include the colors we see
Light
Reflected
Light
Chloroplast
Absorbed
light
Granum
Transmitted
light
Figure 10.7
Absorption Spectrum
• The chemical nature of the molecule
light hits determines if the energy is
absorbed.
• Each molecule has a characteristic
range of photons – absorption
spectrum – it can absorb.
• Spectrophotometer
– Is a machine that sends light through pigments
and measures the fraction of light transmitted
at each wavelength.
• An absorption spectrum (with a spectrophotometer)
– Is a graph plotting light absorption versus wavelength
Refracting
prism
White
light
Chlorophyll
solution
Photoelectric
tube
Galvanometer
2
3
1
0
100
4
Slit moves to
pass light
of selected
wavelength
Green
light
The high transmittance
(low absorption)
reading indicates that
chlorophyll absorbs
very little green light.
0
Blue
light
Figure 10.8
100
The low transmittance
(high absorption) reading
chlorophyll absorbs most blue light.
Energy Absorption
• Pigments are molecules that absorb visible
light well.
• Two kinds of photosynthetic pigments:
Carotenoids
• Carotenoids- yellow and orange
pigments that are highly efficient at
absorbing a broad range of energies.
Ex. Beta-carotene in carrots.
• Ex. Xanthophylls are an oxygenated
form of carotene. These are
accessory pigments.
• Other carotenoids are found in
petals, birds and shrimp
• Accessory pigments
– Absorb different wavelengths of light and pass the
energy to chlorophyll a
Beta-Carotene
Chlorophylls
• Cholorophyllschlorophyll a and b- absorb
narrow ranges of spectrum
but highly efficient.
Chlorophyll a is the major
photosynthetic pigment.
• Chlorophyll a
– Is the main
photosynthetic
pigment
• Chlorophyll b
– Is an accessory
pigment
CH3
in chlorophyll a
CHO
in chlorophyll b
CH2
CH
C
C
H
C
C
N
N
C
C
N
C
C
C
C
H
N
H
H
CH2
C
O
O
C
C
C
C
C
O
H
C
C
CH2
CH2
C
Mg
C
H3C
C
C
C
H3C
CH3
H
CH3
Porphyrin ring:
Light-absorbing
“head” of molecule
note magnesium
atom at center
CH3
O
O
CH3
CH2
Hydrocarbon tail:
interacts with hydrophobic
regions of proteins inside
thylakoid membranes of
chloroplasts: H atoms not
shown
Figure 10.10
• The absorption spectra of three types of pigments in
chloroplasts
EXPERIMENT
Three different experiments helped reveal which wavelengths of light are photosynthetically important. The
results are shown below.
RESULTS
Absorption of light by
chloroplast pigments
Chlorophyll a
Chlorophyll b
Carotenoids
Wavelength of light (nm)
(a) Absorption spectra. The three curves show the wavelengths of light best absorbed by
three types of chloroplast pigments.
Figure 10.9
• The action spectrum of a pigment
Rate of photosynthesis
(measured by O2 release)
– Profiles the relative effectiveness of different
wavelengths of radiation in driving photosynthesis
(b) Action spectrum. This graph plots the rate of photosynthesis versus wavelength.
The resulting action spectrum resembles the absorption spectrum for chlorophyll
a but does not match exactly (see part a). This is partly due to the absorption of light
by accessory pigments such as chlorophyll b and carotenoids.
• The action spectrum for photosynthesis
– Was first demonstrated by Theodor W. Engelmann
Aerobic bacteria
Filament
of alga
400
500
600
700
(c)
Engelmann‘s experiment. In 1883, Theodor W. Engelmann illuminated a filamentous alga with light that had been passed through a
prism, exposing different segments of the alga to different wavelengths. He used aerobic bacteria, which concentrate near an oxygen
source, to determine which segments of the alga were releasing the most O2 and thus photosynthesizing most.
Bacteria congregated in greatest numbers around the parts of the alga illuminated with violet-blue or red light. Notice the close match
of the bacterial distribution to the action spectrum in part b.
CONCLUSION
Light in the violet-blue and red portions of the spectrum are most effective in driving photosynthesis.
Excitation of Chlorophyll by Light
• When a pigment absorbs light
– It goes from a ground state to an excited state, which
is unstable
e–
Excited
state
Heat
Photon
(fluorescence)
Photon
Figure 10.11 A
Chlorophyll
molecule
Ground
state
• If an isolated solution of chlorophyll is
illuminated
– It will fluoresce, giving off light and heat
Figure 10.11 B