Ch. 10
• Photosynthesis
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• Feeds the Biosphere
• Converts solar E into chemical E
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• Plants and other autotrophs
Producers of the biosphere
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• Photoautotrophs
– Use E of sunlight to make organic molecules
from water and CO2
Figure 10.1
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Photosynthesis
• Occurs in plants, algae, certain other protists,
and some prokaryotes
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
(a) Plants
bacteria, which produce sulfur (spherical
globules) (c, d, e: LMs).
(c) Unicellular protist 10 m
(e) Pruple sulfur
bacteria
Figure 10.2
(b) Multicellular algae
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(d) Cyanobacteria
40 m
1.5 m
Heterotrophs
• Obtain organic material f/ other organisms
• Consumers of the biosphere
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• Photosynthesis converts light E to the chemical
E of food
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Chloroplasts: Site of Photosynthesis (plants)
• Leaf
– Major site of photosynthesis
Leaf cross section
Vein
Mesophyll
Stomata
Figure 10.3
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CO2
O2
• Chloroplasts
–
Contain thylakoids and grana
Mesophyll
Chloroplast
5 µm
Outer
membrane
Stroma
Thylakoid Thylakoid
Granum
space
Intermembrane
space
Inner
membrane
1 µm
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Photosynthesis summary reaction
6 CO2 + 12 H2O + Light energy  C6H12O6 + 6 O2 + 6 H2 O
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Chloroplasts split water into
• H2 and O2, incorporating the e- of H2 into sugar
molecules
Reactants:
Products:
12 H2O
6 CO2
C6H12O6
Figure 10.4
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6
H2O
6
O2
Photosynthesis as a Redox Process
• Water is oxidized, CO2 is reduced
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The Two Stages of Photosynthesis: A Preview
• Light reactions
• Calvin cycle
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• Light reactions
– Occurs on thylakoid membranes
– Converts solar E to chemical E
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• Calvin cycle
– Occurs in the stroma
– Forms sugar from carbon dioxide, using ATP
for energy and NADPH for reducing power
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• Overview of photosynthesis
H2O
CO2
Light
NADP 
ADP
+ P
LIGHT
REACTIONS
CALVIN
CYCLE
ATP
NADPH
Chloroplast
Figure 10.5
O2
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[CH2O]
(sugar)
• Light reactions convert solar E to the chemical
E of ATP and NADPH
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The Nature of Sunlight
• Form of electromagnetic E, travels in waves
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• Wavelength (l)
– Distance between the crests of waves
– Determines the type of electromagnetic E
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• Electromagnetic spectrum
– Entire range of electromagnetic E, or radiation
10–5 nm
10–3 nm
Gamma
rays
X-rays
UV
1m
106 nm
106 nm
103 nm
1 nm
Infrared
Microwaves
103 m
Radio
waves
Visible light
380
450
500
550
Shorter wavelength
Figure 10.6
Higher energy
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600
650
700
Longer wavelength
Lower energy
750 nm
• Visible light spectrum
– Colors of light we can see
– l’s that drive photosynthesis
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Photosynthetic Pigments: The Light Receptors
• Substances that absorb visible light
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– Reflect light, which include the colors we see
Light
Reflected
Light
Chloroplast
Absorbed
light
Granum
Transmitted
light
Figure 10.7
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• Spectrophotometer
– Machine that sends light through pigments and
measures the fraction of light transmitted at
each l
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• Absorption spectrum
– A graph plotting light absorption versus l
Refracting Chlorophyll
prism
solution
White
light
2
Photoelectric
tube
Galvanometer
3
1
0
100
4
Slit moves to Green
pass light
light
of selected
wavelength
The high transmittance
(low absorption)
reading indicates that
chlorophyll absorbs
very little green light.
0
Figure 10.8
Blue
light
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100
The low transmittance
(high absorption) reading
chlorophyll absorbs most blue light.
• Absorption spectra of chloroplast pigments
– Clues to the relative effectiveness of different l
f/ driving photosynthesis
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• Absorption spectra of 3 types of pigments
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
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
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• Action spectrum of a pigment
Rate of photosynthesis
(measured by O2 release)
– Effectiveness of different l 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.
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• First demonstrated by Theodor W. Engelmann
Aerobic bacteria
Filament
of alga
400
(c)
500
600
700
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 O 2 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.
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• Chlorophyll a
–
Main photosynthetic pigment
• Chlorophyll b
in chlorophyll a
CHO
in chlorophyll b
CH2
CH
–
CH3
Accessory pigment
C
H3C
C
H
C
C
C
C
C
N
C
N
C
Mg
N
C
C
C
H
C
N
C
H3C
CH3
H
CH2
H
H
C
C
C
O
C
H
C
CH3
CH3
Porphyrin ring:
Light-absorbing
“head” of molecule
note magnesium
atom at center
C
O
O
CH2
C
C
CH2
C
O
O
CH3
CH2
Figure 10.10
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Hydrocarbon tail:
interacts with hydrophobic
regions of proteins inside
thylakoid membranes of
chloroplasts: H atoms not
shown
• Other accessory pigments
– Absorb different ls of light and pass the E to
chlorophyll a
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Excitation of Chlorophyll by Light
• When a pigment absorbs light it goes f/ a
ground state to an excited state (unstable)
e–
Excited
state
Heat
Photon
(fluorescence)
Photon
Figure 10.11 A
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Chlorophyll
molecule
Ground
state
• Isolated chlorophyll fluoresce
Figure 10.11 B
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A Photosystem
• Composed of a reaction center surrounded by a
number of light-harvesting complexes
Thylakoid
Photosystem
Photon
STROMA
Thylakoid membrane
Light-harvesting
complexes
Reaction
center
Primary election
acceptor
e–
Transfer
of energy
Figure 10.12
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Special
chlorophyll a
molecules
Pigment
molecules
THYLAKOID SPACE
(INTERIOR OF THYLAKOID)
• Light-harvesting complexes
– Pigment molecules bound to proteins
– Funnel the E of photons of light to the reaction
center
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• When a reaction-center chlorophyll molecule
absorbs E
– Electrons gets bumped up to a primary eacceptor
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• Thylakoid membrane
– 2 types of photosystems, I and II
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Noncyclic Electron Flow
• Primary pathway
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• Produces NADPH, ATP, and O2
H2O
CO2
Light
NADP+
ADP
CALVIN
CYCLE
LIGHT
REACTIONS
ATP
NADPH
O2
[CH2O] (sugar)
Primary
acceptor
Primary
acceptor
Fd
2
2 H+
+
O2
Pq
e
H2O
e
NADP+
NADP+
+ 2 H+
reductase
3
NADPH
PC
e–
5
+ H+
P700
P680
Light
6
ATP
Figure 10.13
8
e–
Cytochrome
complex
e–
Light
1
7
4
Photosystem II
(PS II)
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Photosystem-I
(PS I)
e–
ATP
e–
e–
NADPH
e–
e–
e–
Mill
makes
ATP
e–
Figure 10.14
Photosystem II
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Photosystem I
• Cyclic e- flow
– Only photosystem I is used
– Only ATP is produced
Primary
acceptor
Primary
acceptor
Fd
Fd
Pq
NADP+
reductase
Cytochrome
complex
NADPH
Pc
Figure 10.15
Photosystem II
ATP
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NADP+
Photosystem I
Chemiosmosis in Chloroplasts v. Mitochondria
• Chloroplasts and mitochondria
– Generate ATP by the same basic mechanism:
chemiosmosis
– Use different sources of E to accomplish this
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• Spatial organization of chemiosmosis
Key
Higher [H+]
Lower [H+]
Chloroplast
Mitochondrion
MITOCHONDRION
STRUCTURE
CHLOROPLAST
STRUCTURE
H+
Intermembrance
space
Membrance
Diffusion
Thylakoid
space
Electron
transport
chain
ATP
Synthase
Matrix
Figure 10.16
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ADP+
Stroma
P
H+
ATP
• In both
– Redox reactions of e- transport chains
generate a H+ gradient across membrane
• ATP synthase
– Uses proton-motive force to form ATP
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• Light reactions and chemiosmosis
H2O
CO2
LIGHT
NADP+
ADP
CALVIN
CYCLE
LIGHT
REACTOR
ATP
NADPH
STROMA
(Low H+ concentration)
O2
[CH2O] (sugar)
Photosystem II
Cytochrome
complex
Photosystem I
NADP+
reductase
Light
2 H+
3
NADP+ + 2H+
Fd
NADPH
+ H+
Pq
Pc
2
H2O
THYLAKOID SPACE
(High H+ concentration)
1⁄
2
1
O2
+2 H+
2 H+
To
Calvin
cycle
STROMA
(Low H+ concentration)
Thylakoid
membrane
ATP
synthase
ADP
ATP
P
Figure 10.17
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H+
Calvin cycle
• Uses ATP and NADPH to convert CO2 to sugar
• Similar to the citric acid cycle
• Occurs in the stroma
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• 3 phases
– Carbon fixation
– Reduction
– Regeneration of CO2 acceptor
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• Calvin cycle
Light
H2 O
CO2
Input
3 (Entering one
CO2 at a time)
NADP+
ADP
CALVIN
CYCLE
LIGHT
REACTION
Phase 1: Carbon
fixation
ATP
NADPH
O2
Rubisco
[CH2O] (sugar)
3 P
3 P
P
Short-lived
intermediate
P
Ribulose bisphosphate
(RuBP)
P
6
3-Phosphoglycerate
6
ATP
6 ADP
CALVIN
CYCLE
3 ADP
3
ATP
6 P
P
1,3-Bisphoglycerate
6 NADPH
Phase 3:
Regeneration of 5
the CO2 acceptor
(RuBP)
6 NADPH+
6 P
P
(G3P)
6
1
Figure 10.18
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P
Glyceraldehyde-3-phosphate
(G3P)
P
G3P
(a sugar)
Output
Glucose and
other organic
compounds
Phase 2:
Reduction
• Alternative mechanisms of carbon fixation have
evolved in hot, arid climates
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• On hot, dry days, plants close their stomata
– Conserving water but limiting access to CO2
– Causing O2 to build up
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Photorespiration: An Evolutionary Relic?
• O2 substitutes for CO2 in the active site of the
enzyme rubisco
• Photosynthetic rate is reduced
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C4 Plants
• Minimize photorespiration
– Incorporate CO2 into four carbon compounds
in mesophyll cells
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• 4 carbon compounds in bundle sheath cells
 release CO2  CO2 Calvin cycle
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• C4 leaf anatomy and the C4 pathway
Mesophyll
cell
Mesophyll cell
Photosynthetic
cells of C4 plant
leaf
CO
CO
2 2
PEP carboxylase
Bundlesheath
cell
PEP (3 C)
ADP
Oxaloacetate (4 C)
Vein
(vascular tissue)
Malate (4 C)
ATP
C4 leaf anatomy
BundleSheath
cell
Pyruate (3 C)
CO2
Stoma
CALVIN
CYCLE
Sugar
Vascular
tissue
Figure 10.19
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CAM Plants
• Open their stomata at night, CO2  organic
acids
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• During the day, stomata close
– CO2 is released from the organic acids for use
in the Calvin cycle
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• CAM pathway is similar to the C4 pathway
Pineapple
Sugarcane
C4
Mesophyll Cell
Organic acid
Bundlesheath
cell
(a) Spatial separation
of steps. In C4
plants, carbon fixation
and the Calvin cycle
occur in different
Figure 10.20 types of cells.
CAM
CO2
CALVIN
CYCLE
CO2
1 CO2 incorporated Organic acid
into four-carbon
organic acids
(carbon fixation)
2 Organic acids
release CO2 to
Calvin cycle
Sugar
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CALVIN
CYCLE
Sugar
Night
Day
(b) Temporal separation
of steps. In CAM
plants, carbon fixation
and the Calvin cycle
occur in the same cells
at different times.
• Review
Light reaction
Calvin cycle
H2O
CO2
Light
NADP+
ADP
+P1
RuBP
3-Phosphoglycerate
Photosystem II
Electron transport chain
Photosystem I
ATP
NADPH
G3P
Starch
(storage)
Amino acids
Fatty acids
Chloroplast
Figure 10.21
O2
Light reactions:
• Are carried out by molecules in the
thylakoid membranes
• Convert light energy to the chemical
energy of ATP and NADPH
• Split H2O and release O2 to the
atmosphere
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Sucrose (export)
Calvin cycle reactions:
• Take place in the stroma
• Use ATP and NADPH to convert
CO2 to the sugar G3P
• Return ADP, inorganic phosphate,
and
NADP+ to the light reactions
• Organic compounds produced by
photosynthesis
– Provide the E and building material for
ecosystems
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