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Chapter 7
Photosynthesis: Using Light to Make Food
PowerPoint Lectures
Campbell Biology: Concepts & Connections, Eighth Edition
REECE • TAYLOR • SIMON • DICKEY • HOGAN
© 2015 Pearson Education, Inc.
Lecture by Edward J. Zalisko
Introduction
• Photosynthesis
• removes CO2 from the atmosphere and
• stores it in plant matter.
• The burning of sugar in the cellular respiration of
almost all organisms releases CO2 back to the
environment.
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Figure 7.0-2
Chapter 7: Big Ideas
An Introduction to
Photosynthesis
The Light Reactions:
Converting Solar Energy
to Chemical Energy
The Calvin Cycle:
Reducing CO2 to Sugar
The Global Significance
of Photosynthesis
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AN INTRODUCTION TO PHOTOSYNTHESIS
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7.1 Photosynthesis fuels the biosphere
• Plants are autotrophs, which
• sustain themselves,
• do not usually consume organic molecules derived
from other organisms, and
• make their own food through the process of
photosynthesis, in which they convert CO2 and
H2O to sugars and other organic molecules.
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7.1 Photosynthesis fuels 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 Photosynthesis fuels the biosphere
• Photoautotrophs
•
•
•
•
feed us,
clothe us (think cotton),
house us (think wood), and
provide energy for warmth, light, transport, and
manufacturing.
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Figure 7.1
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7.1 Photosynthesis fuels the biosphere
• Photosynthesis in plants takes place in
chloroplasts.
• Photosynthetic bacteria have infolded regions of
the plasma membrane containing clusters of
pigments and enzymes.
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7.2 Photosynthesis occurs in chloroplasts in
plant cells
• 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 7.2-0
Leaf Cross Section
Mesophyll
Leaf
Vein
Mesophyll Cell
CO2
O2
Stoma
Chloroplast
Inner and outer
membranes
Granum
Thylakoid
Thylakoid space
Stroma
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7.2 Photosynthesis occurs in chloroplasts in
plant cells
• Chloroplasts consist of an envelope of two
membranes, which encloses an inner compartment
filled with a thick fluid called stroma that contains
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 in the
generation of ATP.
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7.2 Photosynthesis occurs in chloroplasts in
plant cells
• 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
Mesophyll Cell
Chloroplast
Inner and outer
membranes
Granum
Thylakoid
Thylakoid space
Stroma
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7.3 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 7.3
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7.3 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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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 (H+) 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
6 CO2 + 6 H2O
C6H12O6
+ 6 O2
Becomes oxidized
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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
C6H12O6
+ 6 O2
6 CO2 + 6 H2O
Becomes reduced
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7.5 The two stages of photosynthesis are
linked by ATP and NADPH
• Photosynthesis occurs in two 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
“reducing power” to the Calvin cycle.
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7.5 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 energyrich products of the light reactions.
• During the Calvin cycle, CO2 is incorporated into organic
compounds in a process called carbon fixation.
• After carbon fixation, the carbon compounds are reduced
to sugars.
• The Calvin cycle is often called the dark reactions, or
light-independent reactions, because none of the steps
requires light directly.
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Figure 7.5-3
H2O
CO2
Light
NADP +
ADP
+ P
Calvin
Cycle
(in stroma)
Light
Reactions
(in thylakoids)
ATP
NADPH
Chloroplast
O2
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Sugar
THE LIGHT REACTIONS:
CONVERTING SOLAR ENERGY TO
CHEMICAL ENERGY
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7.9 VISUALIZING THE CONCEPT: The light
reactions take place within the thylakoid
membranes
• A thylakoid membrane includes numerous copies
of
• the two photosystems and
• the electron transport chain.
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7.9 VISUALIZING THE CONCEPT: The light
reactions take place within the thylakoid
membranes
• Light energy absorbed by the two photosystems
drives the flow of electrons from water to NADPH.
• The electron transport chain helps to produce the
concentration gradient of H+ across the thylakoid
membrane, which drives H+ through ATP synthase,
producing ATP.
• Because the initial energy input is light (“photo”),
this chemiosmotic production of ATP is called
photophosphorylation.
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Figure 7.9-0
Thylakoid
sac
Chloroplast
Light
H+
Photosystem
II
Electron Light
Photosystem
transport chain
H2O
1
2
H+
H+
H+
H+
O2 + 2 H+
NADPH
I
H+
H+
NADP+ + H+
H+
H+
H+
To
Calvin
Cycle
H+
H+
H+
THYLAKOID SPACE
H+
H+
H+
H+
Thylakoid membrane
H+
H+
H+
H+
ATP synthase
STROMA
H+
H+
H+
ADP + P
ATP
H+
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H+
THE CALVIN CYCLE:
REDUCING CO2 TO 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.
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7.10 ATP and NADPH power sugar synthesis
in the Calvin cycle
• The Calvin cycle uses these three ingredients to
produce an energy-rich, three-carbon sugar called
glyceraldehyde 3-phosphate (G3P).
• A plant cell uses G3P to make
• glucose,
• the disaccharide sucrose, and
• other organic molecules as needed.
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7.10 ATP and NADPH power sugar synthesis
in the Calvin cycle
• The steps of the Calvin cycle include
•
•
•
•
carbon fixation,
reduction,
release of one molecule of G3P, and
regeneration of the starting molecule, ribulose
bisphosphate (RuBP).
• Photosynthesis is an emergent property of the
structural organization of a chloroplast, which
integrates the two stages of photosynthesis.
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Figure 7.10-0
CO2
H2O
Light
NADP+
ADP
+ P
Light
Reactions
Input
Calvin
Cycle
3
CO2
ATP
Step 1
Carbon fixation
NADPH
Chloroplast
O2
Rubisco
Sugar
3 P
6
P
P
3-PGA
RuBP
Step 4
Regeneration of RuBP
3 ADP
6 ATP
P
6 ADP + P
CALVIN
CYCLE
3 ATP
6 NADPH
6 NADP+
5
6
P
G3P
G3P
Step 2
Reduction
Step 3
Release of one
molecule of G3P
1
P
G3P
Output
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P
Glucose and other
compounds
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 this prevents
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
• An alternate mode of carbon fixation has evolved
that
• minimizes photorespiration and
• optimizes 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, cacti, aloe, and jade.
• 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 7.11-0
CO2
Mesophyll
cell
Bundlesheath
cell
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CO2
4-C compound
4-C compound
CO2
CO2
Calvin
Cycle
Calvin
Cycle
Sugar
Sugar
C4 plant
Sugarcane
Night
Day
CAM plant
Pineapple
Figure 7.11-1
CO2
Mesophyll
cell
Bundlesheath
cell
4-C compound
4-C compound
CO2
CO2
Calvin
Cycle
Calvin
Cycle
Sugar
Sugar
C4 plant
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CO2
Night
Day
CAM plant
THE GLOBAL SIGNIFICANCE
OF PHOTOSYNTHESIS
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7.12 Photosynthesis makes sugar from CO2
and H2O, providing food and O2 for almost all
living organisms
• Two photosystems in the thylakoid membranes
capture solar energy, energizing electrons in
chlorophyll molecules.
• Simultaneously,
• water is split,
• O2 is released, and
• electrons are funneled to the photosystems.
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7.12 Photosynthesis makes sugar from CO2
and H2O, providing food and O2 for almost all
living organisms
• The photoexcited electrons are transferred through
an electron transport chain, where energy is
harvested to make ATP by the process of
chemiosmosis, and finally to NADP+, reducing it to
the high-energy compound NADPH.
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Figure 7.12
H2O
Chloroplast
CO2
Light
NADP+
ADP
Light
Reactions
+ P
RuBP
Photosystem II
Electron
transport chain
Thylakoids
Photosystem I
ATP
NADPH
O2
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Calvin
Cycle 3-PGA
(in stroma)
Stroma
G3P
Sugars
Cellular
respiration
Cellulose
Starch
Other organic
compounds
7.12 Photosynthesis makes sugar from CO2
and H2O, providing food and O2 for almost all
living organisms
• About 50% of the carbohydrates made by
photosynthesis is 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.
• Many glucose molecules are linked together to
make cellulose, the main component of cell walls.
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7.12 Photosynthesis makes sugar from CO2
and H2O, providing food and O2 for almost all
living organisms
• Most plants make much more food each day than
they need. They store the excess in
•
•
•
•
roots,
tubers,
seeds, and
fruits.
• Plants (and other photosynthesizers) are the
ultimate source of food for virtually all other
organisms.
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7.13 SCIENTIFIC THINKING: Rising
atmospheric levels of carbon dioxide and
global climate change will affect plants in
various ways
• The greenhouse effect operates on a global scale.
• Solar radiation passes through the atmosphere and
warms Earth’s surface.
• Heat radiating from the warmed planet is absorbed
by greenhouse gases, such as CO2, water vapor,
and methane, which then reflect some of the heat
back to Earth.
• Without this natural heating effect, the average air
temperature would be a frigid –18C (–0.4F), and
most life as we know it could not exist.
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7.13 SCIENTIFIC THINKING: Rising
atmospheric levels of carbon dioxide and
global climate change will affect plants in
various ways
• Increasing concentrations of greenhouse gases
have been linked to global climate change, of
which one major aspect is global warming.
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7.13 SCIENTIFIC THINKING: Rising
atmospheric levels of carbon dioxide and
global climate change will affect plants in
various ways
• The predicted consequences of global climate
change include
•
•
•
•
•
•
melting of polar ice,
rising sea levels,
extreme weather patterns,
droughts,
increased extinction rates, and
the spread of tropical diseases.
• Many of these effects are already being
documented.
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7.13 SCIENTIFIC THINKING: Rising
atmospheric levels of carbon dioxide and
global climate change will affect plants in
various ways
• How may global climate change affect plants?
• Increasing CO2 levels can increase plant
productivity.
• Research has documented such an increase,
although results often indicate that the growth rates
of weeds, such as poison ivy, increase more than
those of crop plants and trees.
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7.13 SCIENTIFIC THINKING: Rising
atmospheric levels of carbon dioxide and
global climate change will affect plants in
various ways
• How do scientists study the effects of increasing
CO2 on plants?
• Many experiments are done in small growth
chambers in which variables can be carefully
controlled.
• Other scientists are turning to long-term field
studies that include large-scale manipulations of
CO2 levels.
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7.13 SCIENTIFIC THINKING: Rising
atmospheric levels of carbon dioxide and
global climate change will affect plants in
various ways
• In the Free-Air CO2 Enrichment (FACE)
experiment set up in Duke University’s
experimental forest, scientists monitored the
effects of elevated CO2 levels on an intact forest
ecosystem over a period of 15 years, using six
study sites, each 30 m in diameter and ringed by
16 towers.
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Figure 7.13a
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7.13 SCIENTIFIC THINKING: Rising
atmospheric levels of carbon dioxide and
global climate change will affect plants in
various ways
• Another study compared the growth of poison ivy
in experimental and control plots. The results
showed that poison ivy in the elevated CO2 plots
• showed an average annual growth increase of
149% compared to control plots and
• produced a more potent form of poison ivy’s
allergenic compound.
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Mean plant dry biomass (g)
Figure 7.13b
10
9
8
7
6
5
4
3
2
1
0
Key
Control plots
Elevated CO2
plots
1999 2000 2001 2002 2003 2004
Year
Source: Adaptation of Figure 1A from “Biomass and Toxicity Responses of Poison Ivy
(Toxicodendron Radicans) to Elevated Atmospheric CO2” by Jacqueline E. Mohan, et al.,
from PNAS, June 2006, Volume 103(24). Copyright © 2006 by National Academy of
Sciences. Reprinted with permission.
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7.14 Scientific research and international
treaties have helped slow the depletion of
Earth’s ozone layer
• Solar radiation converts O2 high in the atmosphere
to ozone (O3), which shields organisms from
damaging UV radiation.
• A gigantic hole in the ozone layer has appeared
every spring over Antarctica since the late 1970s
and continues today.
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Figure 7.14a
Southern
tip of
South
America
Antarctica
September 2012
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7.14 Scientific research and international
treaties have helped slow the depletion of
Earth’s ozone layer
• Industrial chemicals called CFCs have caused
dangerous thinning of the ozone layer.
• This destruction of the ozone layer was
documented by scientists from the British Antarctic
Survey, NASA, and National Oceanic and
Atmospheric Administration.
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Figure 7.14b
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7.14 Scientific research and international
treaties have helped slow the depletion of
Earth’s ozone layer
• In response to these scientific findings, the first
treaty to address Earth’s environment was signed
in 1987.
• As research continued to establish the extent and
danger of ozone depletion, these agreements were
strengthened in 1990, and now nearly 200 nations
participate in the protocol.
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7.14 Scientific research and international
treaties have helped slow the depletion of
Earth’s ozone layer
• In 1995, Molina and Rowland shared a Nobel Prize
for their work in determining how CFCs were
damaging the atmosphere.
• Global emissions of CFCs are near zero now, but
because these compounds are so stable, recovery
of the ozone layer is not expected until around
2060.
• Meanwhile, unblocked UV radiation is predicted to
increase skin cancer and cataracts and damage
crops and phytoplankton in the oceans.
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