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Chapter Introduction
Autotrophy and Photosynthesis
4.1 What Are Autotrophs?
4.2 Overview of Photosynthesis
4.3 The Light Reactions
4.4 The Calvin Cycle
Photosynthesis and the Environment
4.5 Rate of Photosynthesis
4.6 Photorespiration and Special Adaptations
4.7 Photosynthesis and the Atmosphere
Chemoautotrophy
4.8 Varieties of Chemoautotrophs
4.9 Chemoautotrophs and the Environment
Chapter Highlights
Chapter Animations
Learning Outcomes
By the end of this chapter you will be able to:
A State the importance of photosynthesis and
identify the plant structures that are involved
in photosynthesis.
B Identify the steps by which light energy is converted
to chemical energy during the light reactions.
C Discuss the importance of the Calvin cycle.
D Describe how environmental factors affect the rate of
photosynthesis and photorespiration.
E Describe the effects of photosynthesis on the
atmosphere.
F Discuss varieties of chemoautotrophs and their
distribution in the environment.
Autotrophy: Collecting Energy from
the Nonliving Environment
 How do organisms
obtain energy from
their environment?
 How could animals survive
without plants?
This photo shows a group of fungi.
Autotrophy: Collecting Energy from
the Nonliving Environment
• Photosynthesis can be
compared to a “living
bridge”—connecting the
Sun with the organisms on
Earth by providing the
energy needed for life.
• Some bacteria obtain energy
from minerals such as iron or
sulfur instead of sunlight.
This photo shows a group of fungi.
Autotrophy and Photosynthesis
4.1 What are Autotrophs?
• Photosynthesis, performed by plants, some
bacteria, and other small organisms called algae,
uses the energy of sunlight to convert carbon
dioxide to sugars.
• Autotrophs such as plants that depend on
photosynthesis for both energy and carbon
compounds are known as photoautotrophs.
• We depend on photosynthesis for agricultural
production and the products of ancient
photosynthesis—petroleum and coal.
Autotrophy and Photosynthesis
4.1 What are Autotrophs? (cont.)
• In environments where photoautotrophs cannot
survive, bacteria called chemoautotrophs obtain
energy by oxidizing inorganic substances such as
iron, sulfur, or other minerals.
In the upper geyser basin at
Yellowstone National Park,
note the steam rising from the
water. The water is too hot to
support photoautotrophs.
Autotrophy and Photosynthesis
4.1 What are Autotrophs? (cont.)
Autotrophy and Photosynthesis
4.2 Overview of Photosynthesis
• Photoautotrophs have adapted to take advantage
of sunlight.
• Light consists of a vibrating electric and magnetic
field like a wave.
• The length of the waves determines the light’s
color and energy; the shorter the wave, the greater
its energy.
Autotrophy and Photosynthesis
4.2 Overview of Photosynthesis (cont.)
Energy that radiates from the Sun forms a continuous series of waves called a
spectrum. The range of wavelengths that animals can detect with their eyes—
visible light—is roughly the same range plants use in photosynthesis. Shorter
wavelengths (blue light) have more energy than longer wavelengths (red light).
Autotrophy and Photosynthesis
4.2 Overview of Photosynthesis (cont.)
• Photoautotrophic cells contain light-absorbing
substances, or pigments, that absorb visible light.
• The light-absorbing pigments are embedded in
membranes within cells, called thylakoids, that form
closed sacs.
• In the cells of plants and algae, each thylakoid sac is
part of an organized structure called a chloroplast.
Autotrophy and Photosynthesis
4.2 Overview of Photosynthesis (cont.)
Electron micrograph of a
chloroplast in a leaf of corn,
Zea mays, x24,000. The
darker areas are stacks of
thylakoids called grana; the
drawing shows the structure
of one enlarged granum.
Photosynthetic pigments are
embedded in the thylakoid
membranes; DNA, RNA,
and Calvin-cycle enzymes
are in the stroma.
Autotrophy and Photosynthesis
4.2 Overview of Photosynthesis (cont.)
• Most photosynthesis
depends on the green
pigment chlorophyll,
found in the thylakoids.
• Plants contain two
forms of chlorophyll,
a and b.
The structure of chlorophyll a. Chlorophyll b differs
only in having a CHO—group in place of the
circled CH3—. The part of the molecule shown in
green absorbs light; the hydrophobic
tail helps to keep the molecule anchored in the
lipid-rich thylakoid membrane.
Autotrophy and Photosynthesis
4.2 Overview of Photosynthesis (cont.)
• Chlorophylls a and b absorb light in the violet/blue
and orange/red ranges, but not in the green range.
Autotrophy and Photosynthesis
4.2 Overview of Photosynthesis (cont.)
• The green light that is not absorbed gives leaves
their color.
• Other accessory pigments absorb additional
wavelengths of light.
• As the chlorophyll
content of leaves
declines in the
fall, the accessory
pigments become
more visible.
Autotrophy and Photosynthesis
4.2 Overview of Photosynthesis (cont.)
• The process of photosynthesis involves three energy
conversions:
1. absorption of light energy
2. conversion of light energy into chemical energy
3. storage of chemical energy in the form of sugars
Autotrophy and Photosynthesis
4.2 Overview of Photosynthesis (cont.)
• In the light reactions, pigment molecules in the
thylakoids absorb light and convert it to chemical
energy.
• The energy produced by light reactions is used to
make 3-carbon sugars from carbon dioxide in a
series of reactions known as the Calvin cycle.
Autotrophy and Photosynthesis
4.2 Overview of Photosynthesis (cont.)
Solar energy is converted to
chemical energy in the thylakoid
membranes.
Enzymes of the Calvin cycle
use this energy to reduce
carbon dioxide, forming sugars.
Autotrophy and Photosynthesis
4.2 Overview of Photosynthesis (cont.)
• The following equation summarizes the overall
reactions of photosynthesis:
Autotrophy and Photosynthesis
4.3 The Light Reactions
• During light reactions, chlorophyll and other
pigments in the thylakoid:
– absorb light energy
– water molecules are split into hydrogen and
oxygen
– light energy is converted to chemical energy which
powers sugar production in the Calvin cycle
The light reactions of photosynthesis
Click the image to view an animated version.
Autotrophy and Photosynthesis
4.3 The Light Reactions (cont.)
• The light-absorbing pigments form two types of
clusters, called photosystems (PS) I and II.
• The chlorophyll and other
pigments in each photosystem
absorb light energy and
transfer it from one molecule
to the next.
• All this energy is funneled to a
specific chlorophyll a molecule
called the reaction center.
Autotrophy and Photosynthesis
4.3 The Light Reactions (cont.)
• Some of the electrons from the reaction-center
molecules jump to other molecules, known as
electron carriers, forming an electron transport
system between the two photosystems.
• Electrons from PSII move through the system to
replace electrons lost from PSI.
• PSII receives replacements for these electrons
from an enzyme near its reaction center that
oxidizes water.
Autotrophy and Photosynthesis
4.3 The Light Reactions (cont.)
• Some photosynthetic bacteria obtain electrons from
hydrogen sulfide gas (H2S) instead of water,
resulting in solid sulfur as a by-product.
The photosynthetic bacterium
Chromatium oxidizes hydrogen
sulfide gas instead of water,
producing the yellow sulfur
globules visible in its cells.
Autotrophy and Photosynthesis
4.3 The Light Reactions (cont.)
• When electrons from water reach PSI, they receive
an energy boost giving them enough energy to
reduce a molecule known as NADP+ (nicotinamide
adenine dinucleotide phosphate).
• The electrons, along with protons from water,
combine with NADP+ to convert it to its reduced
form, NADPH, ending the electron flow in the
light reactions.
• NADPH provides the protons and electrons needed
to reduce carbon dioxide in the Calvin cycle.
Autotrophy and Photosynthesis
4.3 The Light Reactions (cont.)
• The solar energy that the electrons receive from PSII
powers the active transport of protons across the
thylakoid membrane.
• The concentrated protons inside the thylakoid then
diffuse out, transferring energy to the enzyme
complex ATP synthetase.
• The enzyme uses this energy to synthesize ATP from
ADP and phosphate.
Autotrophy and Photosynthesis
4.3 The Light Reactions (cont.)
• There are two reasons that photosynthesis does not
stop with the synthesis of ATP and NADPH:
1. ATP and NADPH are not particularly stable
compounds.
2. The light reactions do not produce any new
carbon compounds that the organism can
use to grow.
Autotrophy and Photosynthesis
4.4 The Calvin Cycle
• The Calvin cycle conserves the chemical energy
produced in the light reactions in the form of sugars
that the organism can use for growth.
– The Calvin cycle occurs in the stroma of a
chloroplast.
– The Calvin cycle completes the process of
photosynthesis.
Reduction of carbon dioxide to sugars in the Calvin cycle
Click the image to view an animated version.
Autotrophy and Photosynthesis
4.4 The Calvin Cycle (cont.)
• The Calvin cycle includes the following steps:
1. A molecule of carbon dioxide combines with the
5-carbon sugar-phosphate, ribulose bisphosphate
(RuBP), producing an unstable 6-carbon
molecule that immediately splits into two
molecules of the 3-carbon acid, phosphoglyceric
acid (PGA).
2. Two enzymatic steps reduce each molecule of
PGA to the 3-carbon sugar-phosphate,
phosphoglyceraldehyde (PGAL), requiring one
molecule each of ATP and NADPH.
Autotrophy and Photosynthesis
4.4 The Calvin Cycle (cont.)
3. A series of enzymatic reactions, combines and
rearranges molecules of PGAL, eventually
producing a 5-carbon sugar-phosphate.
4. Using an ATP molecule to add a second
phosphate group to the 5-carbon sugarphosphate, a molecule of the starting material,
RuBP, is created, completing the cycle.
Autotrophy and Photosynthesis
4.4 The Calvin Cycle (cont.)
• Three turns of the Calvin cycle, results in the
formation of six molecules of PGAL, one of which is
available for the organism to use for maintenance
and growth.
• Sugar-phosphates such as PGAL are removed from
the Calvin cycle for use in other cellular functions.
• Plants that use only the Calvin cycle to fix carbon
dioxide are called C3 plants.
Autotrophy and Photosynthesis
4.4 The Calvin Cycle (cont.)
Plants use the sugars
produced in photosynthesis
to supply energy and carbon
skeletons for growth and
other cell work. Much of this
sugar is converted to
sucrose or starch.
Photosynthesis and the Environment
4.5 Rate of Photosynthesis
• Environmental conditions such as light intensity,
temperature, and the concentrations of carbon
dioxide and oxygen all affect the rate of
photosynthesis.
• Environmental effects on organisms are usually
described in terms of how they affect the rate, or
activity per unit of time, of a biological process.
Photosynthesis and the Environment
4.5 Rate of Photosynthesis (cont.)
• The rate of photosynthesis levels off before the light
reaches the intensity of full sunlight.
As light intensity increases,
the rate of photosynthesis
increases and then reaches
a maximum rate. Data are
generalized to show trends
in C3 plants.
Photosynthesis and the Environment
4.5 Rate of Photosynthesis (cont.)
• In very bright light, chlorophyll accumulates energy
faster than it can transfer that energy to the electron
transport system.
• As extra energy passes to oxygen molecules, the
oxygen may react with water to form hydroxyl ions
(OH–) or hydrogen peroxide (H2O2) and a decline in
photosynthesis, called photoinhibition, may occur.
Photosynthesis and the Environment
4.5 Rate of Photosynthesis (cont.)
• Temperature affects photosynthesis differently
from light intensity.
As temperature increases, the
rate of photosynthesis also
increases, and then declines.
Data are generalized to show
trends in C3 plants that grow
best between 20˚C and 30˚C.
Photosynthesis and the Environment
4.5 Rate of Photosynthesis (cont.)
• An increase in carbon dioxide concentration
increases the rate of photosynthesis to a maximum
point, after which the rate levels off.
• Above the carbon dioxide saturation point, further
increases in carbon dioxide concentration have no
effect on photosynthesis.
Photosynthesis and the Environment
4.5 Rate of Photosynthesis (cont.)
• The effects of light, temperature, and carbon dioxide
all interact with each other.
• The factors in shortest supply have the most effect
on the rate of photosynthesis.
• Limiting factors are environmental factors such as
food, temperature, water, or sunlight that restrict
growth, metabolism, or population size.
Photosynthesis and the Environment
4.5 Rate of Photosynthesis (cont.)
At high light intensity, the rate
of photosynthesis is greater at
25˚C than at 15˚C. Thus,
temperature can be a limiting
factor when further increases
in light intensity no longer
stimulate photosynthesis.
Data are generalized for
typical C3 plants.
Photosynthesis and the Environment
4.6 Photorespiration and Special Adaptations
• Normal atmospheric concentrations of oxygen (about
21%) can inhibit photosynthesis by up to 50%.
Increasing concentrations of
oxygen inhibit the rate of
photosynthesis in C3 plants.
Photosynthesis and the Environment
4.6 Photorespiration and Special Adaptations
• The enzyme rubisco incorporates carbon dioxide
into sugars in the Calvin cycle.
• The molecular structures
of oxygen and carbon
dioxide are held together
by double bonds that
keep the atoms about
the same distance apart,
allowing rubisco to bind
to either molecule.
(cont.)
Photosynthesis and the Environment
4.6 Photorespiration and Special Adaptations
(cont.)
• When carbon dioxide binds to rubisco and
combines with RuBP, two molecules of PGA form.
• When oxygen binds to rubisco and combines with
RuBP, one molecule of PGA, and one molecule of
the 2-carbon acid glycolate form.
• In a pathway called photorespiration, glycolate is
transported out of the chloroplast and partly broken
down to carbon dioxide, resulting in a loss of fixed
carbon atoms.
Photosynthesis and the Environment
4.6 Photorespiration and Special Adaptations
(cont.)
Photorespiration
occurs simultaneously
with photosynthesis
and results in the loss
of previously fixed
carbon dioxide. Both
processes depend on
the enzyme rubisco,
which can react with
either carbon dioxide or
oxygen. High carbon
dioxide levels favor
photosynthesis over
photorespiration. High
oxygen levels promote
photorespiration.
Photosynthesis and the Environment
4.6 Photorespiration and Special Adaptations
(cont.)
• C4 plants have adapted to hot, dry, conditions by
having two systems of carbon dioxide fixation that
occur in different parts of the leaves.
• Surrounding each vein
in the leaves is a layer
of tightly packed cells,
the bundle sheath.
Photosynthesis and the Environment
4.6 Photorespiration and Special Adaptations
• The mesophyll cells fix carbon dioxide by
combining it with a 3-carbon acid.
(cont.)
• The resulting 4-carbon acid is rearranged and then
transported to the bundle-sheath cells
• There, carbon dioxide is released from the 4-carbon
acid and refixed by rubisco, forming PGA by way of
the Calvin cycle.
• Many C4 plants can be about twice as efficient as C3
plants in converting light energy to sugars.
Photosynthesis and the Environment
4.6 Photorespiration and Special Adaptations
(cont.)
Carbon dioxide first combines
with a 3-carbon acid in the
outside mesophyll cells. The
resulting 4-carbon acid is then
transported into the bundlesheath cells, where carbon
dioxide is released to the Calvin
cycle and refixed by rubisco.
Photosynthesis and the Environment
4.6 Photorespiration and Special Adaptations
(cont.)
• Another specialization for photosynthesis found in
some desert plants is called CAM, for crassulacean
acid metabolism.
• CAM plants open their stomates at night and
incorporate carbon dioxide into organic acids.
• During the day, the stomates close, conserving
water and enzymes then break down the organic
acids, releasing carbon dioxide that enters the
Calvin cycle.
Photosynthesis and the Environment
4.6 Photorespiration and Special Adaptations
(cont.)
• The CAM system is not very efficient and while
CAM plants can survive intense heat, they usually
grow very slowly.
The jade plant (Crassula) is
an example of a CAM plant.
Photosynthesis and the Environment
4.7 Photosynthesis and the Atmosphere
• Photosynthesis supplies oxygen gas to Earth’s
atmosphere and food to Earth’s organisms.
• Most organisms, including plants, use oxygen and
release carbon dioxide.
• Photoautotrophs use the carbon dioxide again in
photosynthesis, completing the cycle.
Summary of carbon, oxygen, and energy cycles in the biosphere
Click the image to view an animated version.
Photosynthesis and the Environment
4.7 Photosynthesis and the Atmosphere (cont.)
• Photosynthesis produces enormous amounts
of oxygen.
• Photosynthesis is the largest single biochemical
process on Earth, therefore any disruption of that
may have dramatic effects.
• The carbon dioxide content of the atmosphere has
been increasing steadily at least since 1800.
Photosynthesis and the Environment
4.7 Photosynthesis and the Atmosphere (cont.)
• Preserving the balance of oxygen and carbon
dioxide in the atmosphere may be vital to the future
of all life on Earth.
• The C4 and CAM pathways are examples of the
adaptation of photosynthesis to an oxygen-rich
atmosphere.
• Increasing levels of carbon dioxide in the
atmosphere favors C3 plants in their competition
with C4 plants.
Chemoautotrophy
4.8 Varieties of Chemoautotrophs
• Chemoautotrophs are bacteria that obtain energy
from the oxidation of some substance in the
environment, usually an inorganic mineral such
as iron or sulfur.
• Chemoautotrophy generally does not provide as
much energy as photosynthesis or heterotrophy.
• Chemoautotrophs grow best in environments
where other organisms cannot survive and light
and organic compounds are in short supply.
Chemoautotrophy
4.8 Varieties of Chemoautotrophs
(cont.)
• Three questions are important in studying a
chemoautotroph:
1. What is its source of energy?
2. What is its source of carbon?
3. What is its source of electrons for
reducing carbon?
Chemoautotrophy
4.8 Varieties of Chemoautotrophs
(cont.)
• While chemoautotrophs fix carbon dioxide, usually
with the Calvin cycle, their sources of energy and
electrons vary greatly.
Chemoautotrophy
4.8 Varieties of Chemoautotrophs
(cont.)
• Like photoautotrophs, chemoautotrophs have
electron transport systems.
• Many chemoautotrophs can adapt to changing
environments by switching electron donors or living
heterotrophically when food is plentiful.
• Some chemoautotrophs use different energy
sources depending on what is available, while
others avoid competition by using resources that
other organisms ignore.
Chemoautotrophy
4.9 Chemoautotrophs and the Environment
• Chemoautotrophs are not very important in the sunlit, organic carbon–rich environments familiar to us.
• Chemoautotrophs are so common underground
and deep in the ocean that they may make up
the majority of life on Earth.
Chemoautotrophy
4.9 Chemoautotrophs and the Environment
(cont.)
• The oxidized end products of chemoautotrophs form
important deposits of oxidized mineral ores,
especially iron and sulfur.
• When the same bacteria attack coal that is rich in
iron sulfide, or pyrite (FeS2), they oxidize the sulfur
in the pyrite, forming sulfuric acid (H2SO4).
Chemoautotrophy
4.9 Chemoautotrophs and the Environment
• Sulfuric acid and dissolved metals may wash
into local streams, killing many of the organisms
living there.
Chemoautotrophic bacteria oxidize
copper ions in the acidic waste water
that drains from copper mines
upstream, coloring the water brown
in the Queen River in Queenstown,
Tasmania (Australia).
(cont.)
Chemoautotrophy
4.9 Chemoautotrophs and the Environment
(cont.)
• Nitrogen-oxidizing bacteria contribute to plant growth
by oxidizing ammonium ions (NH4+) to nitrite ions
(NO2–), and nitrite to nitrate ions (NO3–).
• Most plants absorb and use nitrate more effectively
than ammonium ions.
Chemoautotrophy
4.9 Chemoautotrophs and the Environment
• Around deep underwater
volcanic vents, dissolved
minerals and gases
support a variety of
chemoautotrophs which
are the primary producers
that support heterotrophs
such as tube worms.
(cont.)
Chemoautotrophy
4.9 Chemoautotrophs and the Environment
(cont.)
• Hydrogen-oxidizing bacteria support heterotrophic
bacteria and fungi in the pores of rocks as deep as
2,800 m beneath the surface of the earth.
Summary
• Photosynthesis transforms sunlight into chemical energy. It
also releases the oxygen that many organisms consume.
• The cellular processes of photosynthesis include the
production of ATP and NADPH.
• NADPH and ATP are used to fix carbon dioxide into sugars
during the Calvin cycle.
• In eukaryotes, photosynthesis depends on the structure of the
chloroplast.
• Environmental factors that directly influence the rate of
photosynthesis in plants include light intensity, temperature,
and the concentrations of carbon dioxide and oxygen.
Summary (cont.)
• An increase of light intensity and carbon dioxide concentration
tends to increase the rate of photosynthesis.
• High temperatures usually cause a decline in the rate of
reactions. High concentrations of oxygen inhibit the rate of
photosynthesis by fueling photorespiration.
• Environmental factors do not act individually but instead
interact as limiting factors.
• C4 and CAM plants have evolved specializations that enable
them to reduce photorespiration and water loss in hot, dry
climates. In CAM plants, the stomates open at night and close
during the day, greatly reducing transpiration.
Summary (cont.)
• Chemoautotrophs are bacteria that fix carbon, often by the
Calvin cycle, and obtain energy by oxidizing substances in the
environment, especially inorganic minerals.
• Chemoautotrophs grow best in environments where other
organisms cannot survive and light and organic compounds
are in short supply.
• Many chemoautotrophs are able to use more than one
electron source.
• Chemoautotrophs support communities of organisms around
underwater volcanic vents and deep in the earth.
Reviewing Key Terms
Match the term on the left with the correct description.
___
photoautotroph
c
___
chemoautotroph
e
___
photoinhibition
b
___
photorespiration
f
a. an adaptation for
photosynthesis in arid
conditions
b. occurs when a chloroplast has
absorbed too much light energy
___
CAM
a
c. an organism that derives its
energy from light
___
NADP+
d
d. a hydrogen carrier in
photosynthesis
e. an organism that derives its
energy from the oxidation of
inorganic compounds
f.
a pathway in plants that
consumes oxygen
Reviewing Ideas
1. How does the principle of limiting factors apply
to photosynthesis?
The effects of light, temperature, and carbon
dioxide all interact with each other. Any of these
factors may be at an ideal level when another is
far below optimum. In this case, the factors in
shortest supply have the most effect on the rate
of photosynthesis.
Reviewing Ideas
2. Why does the rate of photosynthesis level off at
a certain light intensity?
At the light saturation point, the light reactions are
saturated with light energy and are proceeding as
fast as possible. In light that is more intense than
the saturation point, chlorophyll accumulates
energy faster than it can transfer that energy to
the electron transport system.
Using Concepts
3. How is photosynthesis linked to atmospheric
homeostasis?
Each year plants use as much as 140 billion metric
tons of carbon dioxide and 110 billion metric tons of
water in photosynthesis. They produce more than
90 billion metric tons each of organic matter and
oxygen gas. Because photosynthesis is the largest
single biochemical process on Earth, any disruption
of that process may have dramatic effects.
Using Concepts
4. Acidic mine runoff pollutes many rivers and
streams around the world. How is this
problem linked to chemoautotrophs?
Metal-oxidizing bacteria attack coal that is rich in
iron sulfide, or pyrite (FeS2), oxidizing the sulfur in
the pyrite which forms sulfuric acid (H2SO4). This
strong acid dissolves aluminum and iron from the
coal and rock. The acid and dissolved metals wash
into local streams, killing many of the organisms
living there.
Synthesize
5. How do chemoautotrophs contribute to
agricultural production?
Inorganic nitrogen ions are important plant nutrients.
The supply of nitrogen in soil is often a limiting
nutrient in plant growth. Chemoautotrophic bacteria
contribute to plant growth by oxidizing ammonium
ions (NH4+) to nitrite ions (NO2–), and nitrite to
nitrate ions (NO3–). Most plants absorb and use
nitrate more effectively than ammonium ions.
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Chapter Animations
The light reactions of photosynthesis
Reduction of carbon dioxide to sugars
in the Calvin cycle
Summary of carbon, oxygen, and energy
cycles in the biosphere
End of Custom Shows
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