Transcript Chapter 7 Slides

Chapter 7
Photosynthesis: Using Light to
Make Food
PowerPoint Lectures for
Campbell Biology: Concepts & Connections, Seventh Edition
Reece, Taylor, Simon, and Dickey
© 2012 Pearson Education, Inc.
Lecture by Edward J. Zalisko
Introduction
 Plants, algae, and certain prokaryotes
– convert light energy to chemical energy and
– store the chemical energy in sugar, made from
– carbon dioxide and
– water.
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Introduction
 Algae farms can be used to produce
– oils for biodiesel or
– carbohydrates to generate ethanol.
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Figure 7.0_1
Chapter 7: Big Ideas
An Overview of
Photosynthesis
The Calvin Cycle:
Reducing CO2 to Sugar
The Light Reactions:
Converting Solar Energy to
Chemical Energy
Photosynthesis Reviewed
and Extended
Figure 7.0_2
AN OVERVIEW
OF PHOTOSYNTHESIS
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7.1 Autotrophs are the producers of the biosphere
 Autotrophs
– make their own food through the process of
photosynthesis,
– sustain themselves, and
– do not usually consume organic molecules derived from
other organisms.
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7.1 Autotrophs are the producers of the biosphere
 Photoautotrophs use the energy of light to produce
organic molecules.
 Chemoautotrophs are prokaryotes that use
inorganic chemicals as their energy source.
 Heterotrophs are consumers that feed on
– plants or
– animals, or
– decompose organic material.
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7.1 Autotrophs are the producers of the biosphere
 Photosynthesis in plants
– takes place in chloroplasts,
– converts carbon dioxide and water into organic
molecules, and
– releases oxygen.
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Figure 7.1A-D
Figure 7.1A
Figure 7.1B
Figure 7.1C
Figure 7.1D
7.2 Photosynthesis occurs in chloroplasts in plant
cells
 Chloroplasts are the major sites of photosynthesis
in green plants.
 Chlorophyll
– is an important light-absorbing pigment in chloroplasts,
– is responsible for the green color of plants, and
– plays a central role in converting solar energy to
chemical energy.
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7.2 Photosynthesis occurs in chloroplasts in plant
cells
 Chloroplasts are concentrated in the cells of the
mesophyll, the green tissue in the interior of the
leaf.
 Stomata are tiny pores in the leaf that allow
– carbon dioxide to enter and
– oxygen to exit.
 Veins in the leaf deliver water absorbed by roots.
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Figure 7.2
Leaf
Leaf Cross Section
Mesophyll
Vein
CO2
O2
Stoma
Mesophyll Cell
Chloroplast
Inner and outer
membranes
Granum
Thylakoid
Thylakoid space
Stroma
Figure 7.2_1
Leaf Cross
Section
Leaf
Mesophyll
Vein
Mesophyll Cell
CO2
O2
Stoma
Chloroplast
7.2 Photosynthesis occurs in chloroplasts in plant
cells
 Chloroplasts consist of an envelope of two
membranes, which
– enclose an inner compartment filled with a thick fluid
called stroma and
– contain a system of interconnected membranous sacs
called thylakoids.
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7.2 Photosynthesis occurs in chloroplasts in plant
cells
 Thylakoids
– are often concentrated in stacks called grana and
– have an internal compartment called the thylakoid space,
which has functions analogous to the intermembrane
space of a mitochondrion.
– Thylakoid membranes also house much of the machinery
that converts light energy to chemical energy.
 Chlorophyll molecules
– are built into the thylakoid membrane and
– capture light energy.
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Figure 7.2_2
Chloroplast
Inner and outer
membranes
Granum
Thylakoid
Thylakoid space
Stroma
Figure 7.2_3
Mesophyll Cell
Chloroplast
Figure 7.2_4
Granum
Stroma
7.3 SCIENTIFIC DISCOVERY: Scientists traced
the process of photosynthesis using isotopes
 Scientists have known since the 1800s that plants
produce O2. But does this oxygen come from
carbon dioxide or water?
– For many years, it was assumed that oxygen was
extracted from CO2 taken into the plant.
– However, later research using a heavy isotope of
oxygen, 18O, confirmed that oxygen produced by
photosynthesis comes from H2O.
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Figure 7.3A
7.3 SCIENTIFIC DISCOVERY: Scientists traced
the process of photosynthesis using isotopes
 Experiment 1: 6 CO2  12 H2O → C6H12O6  6 H2O  6 O2
 Experiment 2: 6 CO2  12 H2O → C6H12O6  6 H2O  6 O2
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Figure 7.3B
Reactants:
Products:
7.4 Photosynthesis is a redox process, as is
cellular respiration
 Photosynthesis, like respiration, is a redox
(oxidation-reduction) process.
– CO2 becomes reduced to sugar as electrons along with
hydrogen ions from water are added to it.
– Water molecules are oxidized when they lose electrons
along with hydrogen ions.
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Figure 7.4A
Becomes reduced
Becomes oxidized
7.4 Photosynthesis is a redox process, as is
cellular respiration
 Cellular respiration uses redox reactions to harvest
the chemical energy stored in a glucose molecule.
– This is accomplished by oxidizing the sugar and
reducing O2 to H2O.
– The electrons lose potential as they travel down the
electron transport chain to O2.
– In contrast, the food-producing redox reactions of
photosynthesis require energy.
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7.4 Photosynthesis is a redox process, as is
cellular respiration
 In photosynthesis,
– light energy is captured by chlorophyll molecules to
boost the energy of electrons,
– light energy is converted to chemical energy, and
– chemical energy is stored in the chemical bonds of
sugars.
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Figure 7.4B
Becomes oxidized
Becomes reduced
7.5 Overview: The two stages of photosynthesis
are linked by ATP and NADPH
 Photosynthesis occurs in two metabolic stages.
1. The light reactions occur in the thylakoid membranes.
In these reactions
– water is split, providing a source of electrons and giving off
oxygen as a by-product,
– ATP is generated from ADP and a phosphate group, and
– light energy is absorbed by the chlorophyll molecules to drive
the transfer of electrons and H+ from water to the electron
acceptor NADP+ reducing it to NADPH.
– NADPH produced by the light reactions provides the
electrons for reducing carbon in the Calvin cycle.
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7.5 Overview: The two stages of photosynthesis
are linked by ATP and NADPH
2. The second stage is the Calvin cycle, which occurs in
the stroma of the chloroplast.
– The Calvin cycle is a cyclic series of reactions that
assembles sugar molecules using CO2 and the energy-rich
products of the light reactions.
– During the Calvin cycle, CO2 is incorporated into organic
compounds in a process called carbon fixation.
– After carbon fixation, enzymes of the cycle make sugars by
further reducing the carbon compounds.
– The Calvin cycle is often called the dark reactions or lightindependent reactions, because none of the steps requires
light directly.
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Figure 7.5_s1
H2O
Light
NADP+
ADP
P
Light
Reactions
(in thylakoids)
Chloroplast
Figure 7.5_s2
H2O
Light
NADP+
ADP
P
Light
Reactions
(in thylakoids)
ATP
NADPH
Chloroplast
O2
Figure 7.5_s3
H2O
CO2
Light
NADP+
ADP
P
Calvin
Cycle
(in stroma)
Light
Reactions
(in thylakoids)
ATP
NADPH
Chloroplast
O2
Sugar
THE LIGHT REACTIONS:
CONVERTING SOLAR ENERGY
TO CHEMICAL ENERGY
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7.6 Visible radiation absorbed by pigments
drives the light reactions
 Sunlight contains energy called electromagnetic
energy or electromagnetic radiation.
– Visible light is only a small part of the electromagnetic
spectrum, the full range of electromagnetic
wavelengths.
– Electromagnetic energy travels in waves, and the
wavelength is the distance between the crests of two
adjacent waves.
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7.6 Visible radiation absorbed by pigments
drives the light reactions
 Light behaves as discrete packets of energy called
photons.
– A photon is a fixed quantity of light energy.
– The shorter the wavelength, the greater the energy.
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Figure 7.6A
Increasing energy
105 nm 103 nm
Gamma
rays
X-rays
103 nm
1 nm
UV
106 nm
Infrared
103 m
1m
Microwaves
Radio
waves
Visible light
380 400
500
600
Wavelength (nm)
700
650
nm
750
7.6 Visible radiation absorbed by pigments
drives the light reactions
 Pigments
– absorb light and
– are built into the thylakoid membrane.
 Plant pigments
– absorb some wavelengths of light and
– reflect or transmit other wavelengths.
 We see the color of the wavelengths that are
transmitted. For example, chlorophyll transmits
green wavelengths.
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Animation: Light and Pigments
Right click on animation / Click play
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Figure 7.6B
Light
Reflected
light
Chloroplast
Absorbed
light
Thylakoid
Transmitted
light
Figure 7.6B_1
7.6 Visible radiation absorbed by pigments
drives the light reactions
 Chloroplasts contain several different pigments,
which absorb light of different wavelengths.
– Chlorophyll a absorbs blue-violet and red light and
reflects green.
– Chlorophyll b absorbs blue and orange and reflects
yellow-green.
– Carotenoids
– broaden the spectrum of colors that can drive photosynthesis
and
– provide photoprotection, absorbing and dissipating excessive
light energy that would otherwise damage chlorophyll or
interact with oxygen to form reactive oxidative molecules.
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7.7 Photosystems capture solar energy
 Pigments in chloroplasts absorb photons (capturing
solar power), which
– increases the potential energy of the pigment’s
electrons and
– sends the electrons into an unstable state.
– These unstable electrons
– drop back down to their “ground state,” and as they do,
– release their excess energy as heat.
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Figure 7.7A
Excited state
Photon
of light
Heat
Photon
(fluorescence)
Ground state
Chlorophyll
molecule
Figure 7.7A_1
Figure 7.7A_2
Excited state
Photon
of light
Heat
Photon
(fluorescence)
Ground state
Chlorophyll
molecule
7.7 Photosystems capture solar energy
 Within a thylakoid membrane, chlorophyll and other
pigment molecules
– absorb photons and
– transfer the energy to other pigment molecules.
 In the thylakoid membrane, chlorophyll molecules
are organized along with other pigments and
proteins into photosystems.
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7.7 Photosystems capture solar energy
 A photosystem consists of a number of lightharvesting complexes surrounding a reactioncenter complex.
 A light-harvesting complex contains various
pigment molecules bound to proteins.
 Collectively, the light-harvesting complexes
function as a light-gathering antenna.
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Figure 7.7B
Photosystem
Light
Light-harvesting Reaction-center
complexes
complex
Thylakoid membrane
Primary electron
acceptor
Transfer
of energy
Pair of
chlorophyll a molecules
Pigment
molecules
7.7 Photosystems capture solar energy
 The light energy is passed from molecule to
molecule within the photosystem.
– Finally it reaches the reaction center where a primary
electron acceptor accepts these electrons and
consequently becomes reduced.
– This solar-powered transfer of an electron from the
reaction-center pigment to the primary electron acceptor
is the first step in the transformation of light energy to
chemical energy in the light reactions.
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7.7 Photosystems capture solar energy
 Two types of photosystems (photosystem I and
photosystem II) cooperate in the light reactions.
 Each type of photosystem has a characteristic
reaction center.
– Photosystem II, which functions first, is called P680
because its pigment absorbs light with a wavelength of
680 nm.
– Photosystem I, which functions second, is called P700
because it absorbs light with a wavelength of 700 nm.
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7.8 Two photosystems connected by an electron
transport chain generate ATP and NADPH
 In the light reactions, light energy is transformed
into the chemical energy of ATP and NADPH.
 To accomplish this, electrons are
– removed from water,
– passed from photosystem II to photosystem I, and
– accepted by NADP+, reducing it to NADPH.
 Between the two photosystems, the electrons
– move down an electron transport chain and
– provide energy for the synthesis of ATP.
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Figure 7.8A
Light
Photosystem II
Stroma
Electron transport chain
Provides energy for
synthesis of ATP
by chemiosmosis
NADP  H
Light
Photosystem I
1
Primary
acceptor
Thylakoid membrane
Primary
acceptor
2
4
P700
P680
Thylakoid
space
3
H2O
1
2
5
O2  2 H
6
NADPH
Figure 7.8A_1
Light
Photosystem II
Stroma
Electron transport chain
Provides energy for
synthesis of ATP
by chemiosmosis
1
Thylakoid membrane
Primary
acceptor
2
4
P680
Thylakoid
space
3
H2O
1
2
O2  2 H
Figure 7.8A_2
Electron transport chain
Provides energy for
synthesis of ATP
by chemiosmosis
NADP  H+
Light
Photosystem I
Primary
acceptor
4
5
P700
6
NADPH
Figure 7.8B
ATP
NADPH
Mill
makes
ATP
Photosystem II
Photosystem I
7.8 Two photosystems connected by an electron
transport chain generate ATP and NADPH
 The products of the light reactions are
– NADPH,
– ATP, and
– oxygen.
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7.9 Chemiosmosis powers ATP synthesis in the
light reactions
 Interestingly, chemiosmosis is the mechanism that
– is involved in oxidative phosphorylation in
mitochondria and
– generates ATP in chloroplasts.
 ATP is generated because the electron transport
chain produces a concentration gradient of
hydrogen ions across a membrane.
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7.9 Chemiosmosis powers ATP synthesis in the
light reactions
 In photophosphorylation, using the initial energy
input from light,
– the electron transport chain pumps H+ into the thylakoid
space, and
– the resulting concentration gradient drives H+ back
through ATP synthase, producing ATP.
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Figure 7.9
Chloroplast
To Calvin
Cycle
Light
Light
Stroma
(low H+
concentration)
H+
ADP
H+
NADP+
P
NADPH
H+
H+
H+
Thylakoid
membrane
H2O
Thylakoid space
(high H+
concentration)
1 O + 2 +
H
2 2
Photosystem II
H+
+
H
H+
Electron
transport chain
H+
H+
H+
H+
H+
H+
Photosystem I
H+
H+
H+
H+
ATP synthase
ATP
Figure 7.9_1
To Calvin
Cycle
ADP
Light
Light
H+
NADP+
NADPH
H+
H+
H2O
1
2
O2
2 H+
Photosystem II
H+
H+
+
H
H+
H+
H+
H+
Electron
transport chain
P
H+
H+
Photosystem I
H+
H+
H+
H+
H+
H+
ATP synthase
ATP
7.9 Chemiosmosis powers ATP synthesis in the
light reactions
 How does photophosphorylation compare with
oxidative phosphorylation?
– Mitochondria use oxidative phosphorylation to transfer
chemical energy from food into the chemical energy of
ATP.
– Chloroplasts use photophosphorylation to transfer light
energy into the chemical energy of ATP.
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THE CALVIN CYCLE:
REDUCING CO2TO SUGAR
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7.10 ATP and NADPH power sugar synthesis in
the Calvin cycle
 The Calvin cycle makes sugar within a chloroplast.
 To produce sugar, the necessary ingredients are
– atmospheric CO2 and
– ATP and NADPH generated by the light reactions.
 The Calvin cycle uses these three ingredients to
produce an energy-rich, three-carbon sugar called
glyceraldehyde-3-phosphate (G3P).
 A plant cell may then use G3P to make glucose
and other organic molecules.
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Figure 7.10A
Input
CO2
ATP
NADPH
Calvin
Cycle
Output:
G3P
7.10 ATP and NADPH power sugar synthesis in
the Calvin cycle
 The steps of the Calvin cycle include
– carbon fixation,
– reduction,
– release of G3P, and
– regeneration of the starting molecule ribulose
bisphosphate (RuBP).
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Figure 7.10B_s1
Step
1
Carbon fixation
Input:
3
CO2
Rubisco
1
3 P
6
P
RuBP
P
3-PGA
Calvin
Cycle
Figure 7.10B_s2
Step
1
Carbon fixation
Input:
3
CO2
Rubisco
1
3 P
Step 2 Reduction
6
P
RuBP
P
3-PGA
6
ATP
6 ADP
Calvin
Cycle
2
6 NADPH
6 NADP
P
6
G3P
P
Figure 7.10B_s3
Step
1
Carbon fixation
Input:
3
CO2
Rubisco
1
3 P
6
P
RuBP
Step 2 Reduction
P
3-PGA
6
ATP
6 ADP
Calvin
Cycle
Step 3 Release of one
molecule of G3P
2
6 NADPH
6 NADP
5
G3P
P
6
P
G3P
3
Output: 1
P
G3P
Glucose
and other
compounds
P
Figure 7.10B_s4
Step
1
Carbon fixation
Input:
3
CO2
Rubisco
1
3 P
6
P
RuBP
Step 2 Reduction
P
3-PGA
6
ATP
3 ADP
6 ADP
Step 3 Release of one
molecule of G3P
Calvin
Cycle
4
3 ATP
4
Regeneration of RuBP
6 NADPH
6 NADP
5
P
6
P
G3P
Step
2
G3P
3
Output: 1
P
G3P
Glucose
and other
compounds
P
7.11 EVOLUTION CONNECTION: Other
methods of carbon fixation have evolved in
hot, dry climates
 Most plants use CO2 directly from the air, and
carbon fixation occurs when the enzyme rubisco
adds CO2 to RuBP.
 Such plants are called C3 plants because the first
product of carbon fixation is a three-carbon
compound, 3-PGA.
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7.11 EVOLUTION CONNECTION: Other
methods of carbon fixation have evolved in
hot, dry climates
 In hot and dry weather, C3 plants
– close their stomata to reduce water loss but
– prevent CO2 from entering the leaf and O2 from leaving.
– As O2 builds up in a leaf, rubisco adds O2 instead of
CO2 to RuBP, and a two-carbon product of this reaction
is then broken down in the cell.
– This process is called photorespiration because it
occurs in the light, consumes O2, and releases CO2.
– But unlike cellular respiration, it uses ATP instead of
producing it.
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7.11 EVOLUTION CONNECTION: Other
methods of carbon fixation have evolved in
hot, dry climates
 C4 plants have evolved a means of
– carbon fixation that saves water during photosynthesis
while
– optimizing the Calvin cycle.
 C4 plants are so named because they first fix CO2
into a four-carbon compound.
 When the weather is hot and dry, C4 plants keep
their stomata mostly closed, thus conserving water.
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7.11 EVOLUTION CONNECTION: Other
methods of carbon fixation have evolved in
hot, dry climates
 Another adaptation to hot and dry environments
has evolved in the CAM plants, such as
pineapples and cacti.
 CAM plants conserve water by opening their
stomata and admitting CO2 only at night.
 CO2 is fixed into a four-carbon compound,
– which banks CO2 at night and
– releases it to the Calvin cycle during the day.
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Figure 7.11
Mesophyll
cell
Bundlesheath
cell
CO2
4-C compound
4-C compound
CO2
CO2
Calvin
Cycle
Calvin
Cycle
3-C sugar
C4 plant
Sugarcane
CO2
Night
3-C sugar
Day
CAM plant
Pineapple
Figure 7.11_1
Mesophyll
cell
Bundlesheath
cell
CO2
CO2
Night
4-C compound
4-C compound
CO2
CO2
Calvin
Cycle
Calvin
Cycle
3-C sugar
C4 plant
3-C sugar
Day
CAM plant
Figure 7.11_2
Sugarcane
Figure 7.11_3
Pineapple
PHOTOSYNTHESIS REVIEWED
AND EXTENDED
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7.12 Review: Photosynthesis uses light energy, carbon
dioxide, and water to make organic molecules
 Most of the living world depends on the foodmaking machinery of photosynthesis.
 The chloroplast
– integrates the two stages of photosynthesis and
– makes sugar from CO2.
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7.12 Review: Photosynthesis uses light energy, carbon
dioxide, and water to make organic molecules
 About half of the carbohydrates made by
photosynthesis are consumed as fuel for cellular
respiration in the mitochondria of plant cells.
 Sugars also serve as the starting material for making
other organic molecules, such as proteins, lipids, and
cellulose.
 Excess food made by plants is stockpiled as starch
in roots, tubers, seeds, and fruits.
© 2012 Pearson Education, Inc.
Figure 7.12
H2O
CO2
Chloroplast
Light
NADP
Light
Reactions
ADP
P
RuBP
Calvin
Cycle 3-PGA
(in stroma)
Photosystem II
Electron
transport chain
Thylakoids
Photosystem I
ATP
NADPH
O2
Stroma
G3P
Sugars
Cellular
respiration
Cellulose
Starch
Other organic
compounds
7.13 CONNECTION: Photosynthesis may
moderate global climate change
 The greenhouse effect operates on a global
scale.
– Solar radiation includes visible light that penetrates the
Earth’s atmosphere and warms the planet’s surface.
– Heat radiating from the warmed planet is absorbed by
gases in the atmosphere, which then reflects some of
the heat back to Earth.
– Without the warming of the greenhouse effect, the Earth
would be much colder and most life as we know it could
not exist.
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Figure 7.13A
Figure 7.13B
Some heat
energy escapes
into space
Sunlight
Atmosphere
Radiant heat
trapped by CO2
and other gases
7.13 CONNECTION: Photosynthesis may
moderate global climate change
 The gases in the atmosphere that absorb heat
radiation are called greenhouse gases. These
include
– water vapor,
– carbon dioxide, and
– methane.
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7.13 CONNECTION: Photosynthesis may
moderate global climate change
 Increasing concentrations of greenhouse gases
have been linked to global climate change (also
called global warming), a slow but steady rise in
Earth’s surface temperature.
 Since 1850, the atmospheric concentration of CO2
has increased by about 40%, mostly due to the
combustion of fossil fuels including
– coal,
– oil, and
– gasoline.
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7.13 CONNECTION: Photosynthesis may
moderate global climate change
 The predicted consequences of continued warming
include
– melting of polar ice,
– rising sea levels,
– extreme weather patterns,
– droughts,
– increased extinction rates, and
– the spread of tropical diseases.
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7.13 CONNECTION: Photosynthesis may
moderate global climate change
 Widespread deforestation has aggravated the
global warming problem by reducing an effective
CO2 sink.
 Global warming caused by increasing CO2 levels
may be reduced by
– limiting deforestation,
– reducing fossil fuel consumption, and
– growing biofuel crops that remove CO2 from the
atmosphere.
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7.14 SCIENTIFIC DISCOVERY: Scientific study
of Earth’s ozone layer has global significance
 Solar radiation converts O2 high in the atmosphere
to ozone (O3), which shields organisms from
damaging UV radiation.
 Industrial chemicals called CFCs have caused
dangerous thinning of the ozone layer, but
international restrictions on CFC use are allowing a
slow recovery.
© 2012 Pearson Education, Inc.
Figure 7.14A
Southern
tip of
South
America
Antarctica
Figure 7.14B
You should now be able to
1. Define autotrophs, heterotrophs, producers, and
photoautotrophs.
2. Describe the structure of chloroplasts and their
location in a leaf.
3. Explain how plants produce oxygen.
4. Describe the role of redox reactions in
photosynthesis and cellular respiration.
5. Compare the reactants and products of the light
reactions and the Calvin cycle.
© 2012 Pearson Education, Inc.
You should now be able to
6. Describe the properties and functions of the
different photosynthetic pigments.
7. Explain how photosystems capture solar energy.
8. Explain how the electron transport chain and
chemiosmosis generate ATP, NADPH, and
oxygen in the light reactions.
9. Compare photophosphorylation and oxidative
phosphorylation.
10. Describe the reactants and products of the
Calvin cycle.
© 2012 Pearson Education, Inc.
You should now be able to
11. Compare the mechanisms that C3, C4, and CAM
plants use to obtain and use carbon dioxide.
12. Review the overall process of the light reactions
and the Calvin cycle, noting the products,
reactants, and locations of every major step.
13. Describe the greenhouse effect.
14. Explain how the ozone layer forms, how human
activities have damaged it, and the
consequences of the destruction of the ozone
layer.
© 2012 Pearson Education, Inc.
Figure 7.UN01
Light
energy
6 CO2
Carbon dioxide
6
H2O
Water
C6H12O6
Photosynthesis Glucose
6
O2
Oxygen gas
Figure 7.UN02
Light
CO2
H 2O
NADP
Stroma
ADP
Thylakoids
P
Light
Reactions
Calvin
Cycle
ATP
NADPH
Chloroplast
O2
Sugar
Figure 7.UN03
Mitochondrion
Chloroplast
Intermembrane
space
H
c.
Membrane
Matrix
d.
a.
b.
e.
Figure 7.UN04
Photosynthesis
converts
includes both
(a)
(c)
(b)
to
in which
chemical
energy
light-excited
electrons of
chlorophyll
H2O is split
in which
CO2 is fixed to
RuBP
and then
and
are passed
down
(d)
reduce
NADP to
(h)
using
(f)
to produce
(e)
producing
(g)
by
chemiosmosis
sugar
(G3P)