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Chapter 10
Photosynthesis
PowerPoint Lectures for
Biology, Seventh Edition
Neil Campbell and Jane Reece
Lectures by Chris Romero
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• Overview: The Process That Feeds the
Biosphere
• Photosynthesis
– Is the process that converts solar energy into
chemical energy
• Plants and other autotrophs
– Are the producers of the biosphere
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• Plants are photoautotrophs
– They use the energy of sunlight to make
organic molecules from water and carbon
dioxide
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 their organic material from other
organisms
– Are the consumers of the biosphere
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Chloroplasts: The Sites of Photosynthesis in Plants
• Concept 10.1: Photosynthesis converts light
energy to the chemical energy of food
• The leaves of plants
– Are the major sites of photosynthesis
Leaf cross section
Vein
Mesophyll
Stomata
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Figure 10.3
CO2
O2
• Chloroplasts
–
Are the organelles in which photosynthesis occurs
–
Contain thylakoids and grana
Mesophyll
Chloroplast
5 µm
Outer
membrane
Stroma Granum
Thylakoid Thylakoid
space
Intermembrane
space
Inner
membrane
1 µm
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Tracking Atoms Through Photosynthesis:
Scientific Inquiry
• Photosynthesis is summarized as
6 CO2 + 12 H2O + Light energy  C6H12O6 + 6 O2 + 6 H2 O
• Chloroplasts split water into
– Hydrogen and oxygen, incorporating the
electrons of hydrogen into sugar molecules
Reactants:
Products:
12
H2O
6 CO2
C6H12O
6
Figure 10.4
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6
H2O
6
O2
Photosynthesis as a Redox Process
• Photosynthesis is a redox process
– Water is oxidized, carbon dioxide is reduced
• Photosynthesis consists of two processes
– The light reactions
– The Calvin cycle
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• The light reactions
– Occur in the grana
– Split water, release oxygen, produce ATP, and
form NADPH
• The Calvin cycle
– Occurs in the stroma
– Forms sugar from carbon dioxide, using ATP
for energy and NADPH for reducing power
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• An 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)
• Concept 10.2: The light reactions convert solar
energy to the chemical energy of ATP and
NADPH
• Light
– Is a form of electromagnetic energy, which
travels in waves
• Wavelength
– Is the distance between the crests of waves
– Determines the type of electromagnetic energy
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• The electromagnetic spectrum
– Is the entire range of electromagnetic energy, 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
• The visible light spectrum
– Includes the colors of light we can see
– Includes the wavelengths that drive
photosynthesis
• Pigments
– Are 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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• The absorption spectra of chloroplast pigments
– Provide clues to the relative effectiveness of
different wavelengths for driving
photosynthesis
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• The absorption spectra of three 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
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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• 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.
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• Chlorophyll a
–
Is the main photosynthetic pigment (blue-green)
• Chlorophyll b
–
•
Is an accessory pigment (yellow-green)
Other accessory pigments
(Carotinoids)
–
CH2
CH3
CH H
C
C
C
C
C
C CH2 CH3
H3C C
C N
N C
H C
C H
Mg
N C
H3C C N
C CH3
C
C
C
C
C
H C
H
CH2 H C
C
CH2
O O
C O O
Absorb different
wavelengths of light
and pass the energy
to chlorophyll a
O
CH2
Figure 10.10
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CH3
CHO
in chlorophyll a
in chlorophyll b
Porphyrin ring:
Light-absorbing
“head” of molecule
note magnesium
atom at center
CH3
Hydrocarbon tail:
interacts with hydrophobic
regions of proteins inside
thylakoid membranes of
chloroplasts: H atoms not
shown
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
Chlorophyll
molecule
Figure 10.11 A
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Ground
state
• If an isolated solution of chlorophyll is
illuminated
– It will fluoresce, giving off light and heat
Figure 10.11 B
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A Photosystem: A Reaction Center Associated with LightHarvesting Complexes
• A photosystem
– Is composed of a reaction center surrounded by a
number of light-harvesting complexes
Thylakoid
Photosystem
Photon
Thylakoid membrane
Light-harvesting
complexes
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STROMA
Primary election
acceptor
e–
Transfer
of energy
Figure 10.12
Reaction
center
Special
chlorophyll a
molecules
Pigment
molecules
THYLAKOID SPACE
(INTERIOR OF THYLAKOID)
• The light-harvesting complexes
– Consist of pigment molecules bound to
particular proteins
– Funnel the energy of photons of light to the
reaction center
• When a reaction-center chlorophyll molecule
absorbs energy
– One of its electrons gets bumped up to a
primary electron acceptor
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• The thylakoid membrane
– Is populated by two types of photosystems, I and II
– Light drives the synthesis of NADH and ATP
– Two possible pathways of electrons can occur:
• Noncyclic electron flow
– Is the primary pathway of energy
transformation in the light reactions
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• Produces NADPH, ATP, and oxygen
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)
• A mechanical analogy for the light reactions
e–
ATP
e–
e–
NADPH
e–
e–
e–
Mill
makes
ATP
e–
Figure 10.14
Photosystem II
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Photosystem I
• Under certain conditions
– Photoexcited electrons take an alternative path
• In cyclic electron flow
– Only photosystem I is used
– Only ATP is produced
Primary
acceptor
Primary
acceptor
Fd
Fd
Pq
NADP+
reductase
Cytochrome
complex
Pc
Figure 10.15
Photosystem II
ATP
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Photosystem I
NADP+
NADPH
A Comparison of Chemiosmosis in Chloroplasts
and Mitochondria
• Chloroplasts and mitochondria
– Generate ATP by the same basic mechanism:
chemiosmosis
– But use different sources of energy to
accomplish this (Mitochondria: electrons come
from organic compounds. Chloroplast:
electrons come from sunlight)
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• The spatial organization of chemiosmosis
– Differs in chloroplasts and mitochondria
Key
Higher [H+]
Lower [H+]
Chloroplast
Mitochondrion
CHLOROPLAST
STRUCTURE
MITOCHONDRION
STRUCTURE
Intermembrance
space
Membrance
H+ Diffusion
Electron
transport
chain
ATP
Synthase
Matrix
Figure 10.16
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ADP+
Thylakoid
space
Stroma
P
H+
ATP
• In both organelles
– Redox reactions of electron transport chains
generate a H+ gradient across a membrane
• ATP synthase
– Uses this proton-motive force to make ATP
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• The light reactions and chemiosmosis: the
organization of the thylakoid membrane
H2O
CO2
LIGHT
NADP+
ADP
LIGHT
REACTOR
CALVIN
CYCLE
ATP
NADPH
STROMA
(Low H+ concentration)
O2
[CH2O] (sugar)
Cytochrome
Photosystem II
complex
Photosystem I
NADP+
reductase
Light
2 H+
Fd
3
NADP+ + 2H+
NADPH + H+
Pq
Pc
2
H2O
THYLAKOID SPACE
1
(High H+ concentration)
1⁄
2
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+
• Concept 10.3: The Calvin cycle uses ATP and
NADPH to convert CO2 to sugar
• The Calvin cycle
– Is similar to the citric acid cycle
– Occurs in the stroma
• The Calvin cycle has three phases
– Carbon fixation
– Reduction
– Regeneration of the CO2 acceptor
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• The Calvin cycle
Light
H2 O
CO2
Input
3 (Entering one
CO2 at a time)
NADP+
ADP
CALVIN
CYCLE
LIGHT
REACTION
ATP
Phase 1: Carbon fixation
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
Phase 3:
Regeneration of
the CO2 acceptor
(RuBP)
6 P
P
1,3-Bisphoglycerate
6 NADPH
6 NADPH+
6 P
P
5
(G3P)
6
P
Glyceraldehyde-3-phosphate
(G3P)
P
1
Figure 10.18
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G3P
(a sugar)
Output
Glucose and
other organic
compounds
Phase 2:
Reduction
• Concept 10.4: Alternative mechanisms of
carbon fixation have evolved in hot, arid
climates
• On hot, dry days, many plants close their
stomata
– Conserving water but limiting access to CO2
– Causing oxygen to build up
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Photorespiration: An Evolutionary Relic?
• In photorespiration
– O2 substitutes for CO2 in the active site of the
enzyme rubisco
– The photosynthetic rate is reduced
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C4 Plants
• C4 plants minimize the cost of photorespiration
– By incorporating CO2 into four carbon
compounds in mesophyll cells
• These four carbon compounds
– Are exported to bundle sheath cells, where
they release CO2 used in the 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
• CAM plants
– Open their stomata at night, incorporating CO2
into organic acids
• During the day, the stomata close
– And the CO2 is released from the organic acids
for use in the Calvin cycle
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• The 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.
The Importance of Photosynthesis: A Review
• A review of photosynthesis
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 energy and building material for
ecosystems
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