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Chapter 7
Photosynthesis
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7.1 Photosynthetic Organisms
• All life on Earth depends on solar energy
• Photosynthetic organisms (algae, plants, and
cyanobacteria) transform solar energy into the chemical
energy of carbohydrates
 Called autotrophs because they produce their own
food.
• Photosynthesis:
 A process that captures solar energy
 solar energy  chemical energy
 Energy ends up stored in a carbohydrate
• Photosynthesizers produce food energy
 Feed themselves as well as heterotrophs
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Photosynthetic Organisms
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
mosses
trees
kelp
Euglena
garden plants
cyanobacteria
diatoms
(Moss): © Steven P. Lynch; (Trees): © Digital Vision/PunchStock; (Kelp): © Chuck Davis/Stone/Getty Images; (Cyanobacteria): © Sherman Thomas/Visuals Unlimited; (Diatoms): © Ed Reschke/Peter Arnold;
(Euglena): © T.E. Adams/Visuals Unlimited; (Sunflower): © Royalty-Free/Corbis
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Photosynthetic Organisms
• Photosynthesis takes place in the green portions of
plants
 Cells containing chloroplasts are specialized to carry out
photosynthesis
• The raw materials for photosynthesis are carbon dioxide
and water
 Roots absorb water that moves up vascular tissue
 Carbon dioxide enters a leaf through small openings called
stomata and diffuses into chloroplasts in mesophyll cells
 In stroma, CO2 is combined with H2O to form C6H12O6 (sugar)
 Energy supplied by light
• Chlorophyll and other pigments absorb solar energy and energize
electrons prior to reduction of CO2 to a carbohydrate
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Leaves and Photosynthesis
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
cuticle
upper
epidermis
Leaf cross section
mesophyll
lower
epidermis
CO2
O2
leaf vein
outer membrane
stoma
inner membrane
stroma
stroma
granum
Chloroplast
37,000
thylakoid space
thylakoid membrane
Grana
independent thylakoid
in a granum
overlapping thylakoid
in a granum
© Dr. George Chapman/Visuals Unlimited
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7.2 The Process of Photosynthesis
• Light Reactions – take place on thylakoid
membrane




Energy-capturing reactions
Chlorophyll absorbs solar energy
This energizes electrons
Electrons move down an electron transport chain
• Pumps H+ into thylakoids
• Used to make ATP out of ADP and NADPH out of NADP
• Calvin Cycle Reactions (Dark Reactions) –
take place in the stroma
 CO2 is reduced to a carbohydrate
 Use ATP and NADPH to produce carbohydrate
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Overview of Photosynthesis
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
CO2
H2O
solar
energy
ADP + P
NADP+
Light
reactions
Calvin
cycle
reactions
NADPH
ATP
stroma
thylakoid
membrane
O2
CH2O
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7.3 Plants as Solar Energy
Converters
• Pigments:
 Chemicals that absorb certain wavelengths of light
 Wavelengths that are not absorbed are
reflected/transmitted
• Absorption Spectrum
 Pigments found in chlorophyll absorb various portions
of visible light
 Graph showing relative absorption of the various
colors of the rainbow
 Chlorophyll is green because it absorbs much of the
reds and blues of white light
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Photosynthetic Pigments
and Photosynthesis
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Increasing wavelength
chlorophyll a
chlorophyll b
carotenoids
Gamma
rays
X rays
UV
Micro- Radio
Infrared waves waves
visible light
500
600
Wavelengths (nm)
a. The electromagnetic spectrum includes visible light.
750
Relative Absorption
Increasing energy
380
500
600
750
Wavelengths (nm)
b. Absorption spectrum of photosynthetic pigments.
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Plants as Solar Energy
Converters
• Noncyclic pathway




Takes place in the thylakoid membrane
Uses two photosystems, PS I and PS II
PS II captures light energy
Causes an electron to be ejected from the reaction center
(chlorophyll a)
• Electron travels down electron transport chain to PS I
• Replaced with an electron from water, which is split to form
O2 and H+
• This causes H+ to accumulate in thylakoid chambers
• The H+ gradient is used to produce ATP
 PS I captures light energy and ejects an electron
• The electron is transferred permanently to a molecule of
NADP+
• Causes NADPH production
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Noncyclic Electron Pathway
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
H2 O
CO2
solar
energy
ADP+ P
NADP+
Light
reactions
sun
Calvin
cycle
sun
NADPH
ATP
thylakoid
membrane
energy level
electron
acceptor
electron
acceptor
O
CH2O
e–
e–
e–
e–
NADP+
H+
ATP
e–
NADPH
e–
reaction center
reaction center
pigment
complex
pigment
complex
Photosystem I
e–
Photosystem II
CO2
H2O
CH2O
Calvin cycle
reactions
2H+
1
– O2
2
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Plants as Solar Energy
Converters
• PS II:
 Consists of a pigment complex and electron acceptors
 Receives electrons from the splitting of water
 Oxygen is released as a gas
• Electron transport chain:
 Consists of cytochrome complexes and plastoquinone
 Carries electrons between PS II and PS I
 Also pumps H+ from the stroma into the thylakoid space
• PS I:
 Has a pigment complex and electron acceptors
 Adjacent to the enzyme that reduces NADP+ to NADPH
• ATP synthase complex:
 Has a channel for H+ flow
 H+ flow through the channel drives ATP synthase to join ADP
and Pi
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Organization of a Thylakoid
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
H2O
CO2
solar
energy
ADP +
P
NADP +
Light
reactions
Calvin
cycle
reactions
NADPH
ATP
thylakoid membrane
thylakoid
membrane
thylakoid
thylakoid space
O2
CH2O
granum
photosystem II
electron transport
chain
H+
stroma
photosystem I
H+
NADP
reductase
Pq
ee-
e-
NADP+
NADPH
e-
eH+
H+
H2 O
2
H+
+
1
2
H+
H+
H+
H+
H+
H+
H+
O2
ATP synthase
H+
H+
H+
H+
Thylakoid
space
H+
ATP
H+
H+
chemiosmosis
P
Stroma
+ ADP
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Plants as Solar Energy
Converters
• The thylakoid space acts as a reservoir for hydrogen ions
(H+)
• Each time water is oxidized, two H+ remain in the thylakoid
space
• Transfer of electrons in the electron transport chain yields
energy
 Used to pump H+ across the thylakoid membrane
 Protons move from stroma into the thylakoid space
• Flow of H+ back across the thylakoid membrane
 Energizes ATP synthase, which
 Enzymatically produces ATP from ADP + Pi
• This method of producing ATP is called chemiosmosis
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Tropical Rain Forest Destruction
and Climate Change
• Tropical rain forests can exist where
 Temperatures are above 26º C
 Rainfall is heavy (100-200 cm) and regular
• Most plants are woody; many vines and epiphytes; little
or no undergrowth
• Contribute greatly to CO2 uptake, slowing global
warming
 Development has reduced them from 14% to 6% of Earth’s
surface
 Deforestation accounts for 20-30% of atmospheric CO2, but also
removes a CO2 sink
 Increasing temperatures also reduce productivity
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Tropical Rain Forest Destruction
and Climate Change
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Mean Global Temperature Change (0c)
5.5
4.5
3.5
maximum likely increase
most probable temperature
increase for 2 × CO2
2.5
1.5
Minimum likely increase
0.5
–0.5
1860
1940
2020
Year
2060
2100
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7.4 Plants as Carbon Dioxide
Fixers
• A cyclical series of reactions
• Utilizes atmospheric carbon dioxide to
produce carbohydrates
• Known as C3 photosynthesis
• Involves three stages:
• Carbon dioxide fixation
• Carbon dioxide reduction
• RuBP regeneration
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Plants as Carbon Dioxide
Fixers
• CO2 is attached to 5-carbon RuBP carboxylase
 Results in a 6-carbon molecule
 This splits into two 3-carbon molecules (3PG)
 Reaction is accelerated by RuBP carboxylase
(Rubisco)
• CO2 is now “fixed” because it is part of a
carbohydrate
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The Calvin Cycle Reactions
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
H2O
CO2
solar
energy
ADP +
P
NADP +
Calvin
cycle
Light
reactions
NADPH
ATP
Metabolites of the Calvin Cycle
stroma
O2
CH2O
3CO2
intermediate
3 C6
3 RuBP
C5
3 ADP + 3
ribulose-1,5-bisphosphate
3PG
3-phosphoglycerate
BPG
1,3-bisphosphoglycerate
G3P
glyceraldehyde-3-phosphate
6 3PG
C3
CO2
fixation
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ATP
CO2
reduction
Calvin cycle
P
RuBP
6 ADP + 6 P
These ATP and
NADPH molecules
were produced by
the light reactions.
regeneration
of RuBP
These ATP
molecules were
produced by the
light reactions.
6 BPG
C3
3
ATP
6 NADPH
5 G3P
C3
6 G3P
C3
6 NADP+
net gain of one G3P
Other organic molecules
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Glucose
Plants as Carbon Dioxide
Fixers
• Importance of the Calvin Cycle:
 G3P (glyceraldehyde-3-phosphate) can be converted
to many other molecules
 The hydrocarbon skeleton of G3P can form
• Fatty acids and glycerol to make plant oils
• Glucose phosphate (simple sugar)
• Fructose (which with glucose = sucrose)
• Starch and cellulose
• Amino acids
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Fate of G3P
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
G3P
fatty acid
synthesis
glucose
phosphate
amino acid
synthesis
+
fructose
phosphate
Sucrose (in leaves,
fruits, and seeds)
Starch (in roots
and seeds)
Cellulose (in trunks,
roots, and branches)
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© Herman Eisenbeiss/Photo Researchers, Inc.
7.5 Other Types of
Photosynthesis
• In hot, dry climates




Stomata must close to avoid wilting
CO2 decreases and O2 increases
O2 starts combining with RuBP, leading to the production of CO2
This is called photorespiration
• C4 plants solve the problem of photorespiration
 Fix CO2 to PEP (a C3 molecule)
 The result is oxaloacetate, a C4 molecule
 In hot & dry climates
• C4 plants avoid photorespiration
• Net productivity is about 2-3 times greater than C3 plants
 In cool, moist environments, C4 plants can’t compete with C3
plants
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Chloroplast Distribution in C4 vs. C3
Plants
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
C3 Plant
C4 Plant
mesophyll
cells
bundle sheath
cell
vein
stoma
bundle sheath
cell
vein
stoma
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Other Types of
Photosynthesis
• CAM Photosynthesis
 Crassulacean-Acid Metabolism
 CAM plants partition carbon fixation by time
• During the night
– CAM plants fix CO2
– Form C4 molecules, which are
– Stored in large vacuoles
• During daylight
– NADPH and ATP are available
– Stomata are closed for water conservation
– C4 molecules release CO2 to Calvin cycle
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Other Types of Photosynthesis
• Each method of photosynthesis has advantages and
disadvantages
 Depends on the climate
• C4 plants most adapted to:
 High light intensities
 High temperatures
 Limited rainfall
• C3 plants better adapted to
 Cold (below 25°C)
 High moisture
• CAM plants are better adapted to extreme aridity
 CAM occurs in 23 families of flowering plants
 Also found among nonflowering plants
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