Download Photosynthesis - McGraw Hill Higher Education

5
Photosynthesis
Learning Outline
5.1 Life Depends on Photosynthesis
5.2 Photosynthetic Pigments Capture Sunlight
5.3 Chloroplasts Are the Sites of Photosynthesis
5.4 Photosynthesis Occurs in Two Stages
5.5 The Light Reactions Begin Photosynthesis
A. Photosystem II Produces ATP
B. Photosystem I Produces NADPH
5.6 The Carbon Reactions Produce Carbohydrates
5.7 C3, C4, and CAM Plants Use Different Carbon Fixation
Pathways
5.8 Investigating Life: Solar-Powered Sea Slugs
Food from Plants. A farmer works his way through a rice terrace
in China. Rice grains are a food staple for much of the world’s
population.
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5.1 Life Depends on Photosynthesis
What’s the Point?
Most plants are easy to grow
(compared with animals, anyway)
because their needs are simple. Give a plant water,
essential elements in soil, carbon dioxide, and light, and
it will produce food and oxygen. These products build
the plant’s body and sustain its life. Meanwhile, animals
and other consumers eat plants. A leafy foundation
therefore supports Earth’s ecosystems.
How can plants do so much with such simple raw
materials? The answer lies in chloroplasts, microscopic
solar panels inside each green cell. This chapter explains
how chloroplasts use the sun’s energy to conjure sugar
out of thin air.
It is spring. A seed germinates, its tender roots and pale yellow stem extending
rapidly in a race against time. For now, the seedling’s sole energy source is food
stored in the seed itself. If the shoot does not reach light before its reserves run
out, the seedling will die. But if it makes it, green leaves will unfurl and catch the
light. The seedling begins to feed itself, and an independent new life begins.
The plant is an autotroph (“self feeder”), meaning it uses inorganic
substances such as water and carbon dioxide (CO2 ) to produce organic
compounds. The opposite of an autotroph is a heterotroph, which is an
organism that obtains carbon by consuming preexisting organic molecules.
You are a heterotroph, and so are all other animals, all fungi, and many
microorganisms.
Organisms that can produce their own food underlie every ecosystem on
Earth. It is not surprising, therefore, that if asked to designate the most important metabolic pathway, most biologists would not hesitate to cite photosynthesis: the process by which plants, algae, and some microorganisms harness
solar energy and convert it into chemical energy.
Photosynthesis is a series of chemical reactions that use light energy to
assemble CO2 into glucose (C6H12O6), the carbohydrate that feeds plants
(figure 5.1). The plant uses water in the process and releases oxygen gas (O2)
as a byproduct. These chemical reactions are summarized as follows:
light energy
6CO2 + 6H2O ⎯→ C6H12O6 + 6O2
Photosynthesis
Carbon dioxide and
water consumed
CO2 + H2O + light energy
Glucose and oxygen
produced
C6H12O6 + O2
Leaf cell
Chloroplasts
TEM
15 μm (false color)
Figure 5.1 Sugar from the Sun. In photosynthesis, a plant
produces glucose and O2 from simple starting materials: carbon
dioxide, water, and sunlight.
This process provides not only food for the plant but also the energy,
raw materials, and oxygen for most heterotrophs (see figure 4.2). Animals, fungi, and other consumers eat the leaves, stems, roots, flowers,
nectar, fruits, and seeds of the world’s producers. Even the waste product
of photosynthesis, O2, is essential to most life on Earth.
Because humans live on land, we are most familiar with the contribution that plants make to Earth’s terrestrial ecosystems. In
fact, however, more than half of the world’s photosynthesis occurs in the oceans, courtesy of countless algae and bacteria.
On land or in the water, Earth without photosynthesis
would not be a living world for long. If the sky were blackened by a nuclear holocaust, cataclysmic volcanic eruption,
or massive meteor impact, the light intensity reaching Earth’s
surface would decline to about a tenth of its normal level.
Photosynthetic organisms would die as they depleted their energy reserves faster than they could manufacture more food. Animals that normally ate these producers would go hungry, as would
the animals that ate them. A year or even two might pass before
enough life-giving light could penetrate the hazy atmosphere, but by
then, it would be too late. The lethal chain reaction would already be
well into motion, destroying food webs at their bases. No wonder biologists
consider photosynthesis to be Earth’s most important metabolic process.
5.1 Mastering Concepts
1. How is an autotroph different from a heterotroph?
2. What is photosynthesis?
3. Why is photosynthesis essential to life?
3
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UNIT 1 Science, Chemistry, and Cells
Short wavelength (high energy)
5.2 Photosynthetic Pigments
Capture Sunlight
Gamma rays
Visible light
400
Each minute, the sun converts more than 100 metric tons
of matter to energy, releasing much of it outward as waves
Blue
of electromagnetic radiation. After an 8-minute journey,
Cyan
500
Portion of Ultraviolet
about two billionths of this energy reaches Earth’s upper
Green
spectrum radiation
550
atmosphere. Of this, only about 1% is used for photosynthat
Yellow
thesis, yet this tiny fraction of the sun’s power ultimately
600
reaches
Orange
Infrared
Earth's
produces nearly 2 quadrillion kilograms of carbohydrates
650
radiation
Wavelength
surface
a year! Light may seem insubstantial, but it is a powerful
700
force on Earth.
Microwaves
Visible light is a small sliver of a much larger
750
electromagnetic spectrum, the range of possible frequencies of radiation
(figure 5.2). All electromagnetic radiation, including light, consists of
Radio waves
photons, discrete packets of kinetic energy. A photon’s wavelength is the
distance it moves during a complete vibration. The shorter a photon’s
Long wavelength (low energy)
wavelength, the more energy it contains. The visible light that provides
Figure 5.2 The Electromagnetic Spectrum. Sunlight reaching
the energy that powers photosynthesis is in the middle range of the elecEarth consists of ultraviolet radiation, visible light, and infrared
tromagnetic spectrum. We perceive visible light of different wavelengths
radiation, all of which is just a small part of a continuous spectrum
as distinct colors.
of electromagnetic radiation. The shorter the wavelength, the more
Plant cells contain several pigment molecules that capture light energy (see
energy associated with the radiation.
this chapter’s Burning Question). The most abundant is chlorophyll a, a green
photosynthetic pigment in plants, algae, and cyanobacteria. Photosynthetic organisms usually also have several types of accessory pigments, which are
Chlorophyll a
Sunlight
Chlorophyll b
energy-capturing pigment molecules other than chlorophyll a. Chlorophyll b
Reflected
80
Carotenoids
and carotenoids are accessory pigments in plants.
light
The photosynthetic pigments have distinct colors because they absorb
60
only some wavelengths of visible light, while transmitting or reflecting
a.
others (figure 5.3). Chlorophylls a and b absorb red and blue wavelengths;
they appear green because they reflect green light. Carotenoids, on the
40
other hand, reflect longer wavelengths of light, so they appear red, orange,
or yellow. (Carrots, tomatoes, lobster shells, and the flesh of salmon all
owe their distinctive colors to carotenoid pigments, which the animals
20
must obtain from their diets.)
Only absorbed light is useful in photosynthesis. Accessory pigments ab0
sorb wavelengths that chlorophyll a cannot, so they extend the range of light
400
500
600
700
wavelengths that a cell can harness. This is a little like the members of the
Wavelength
of
light
(nanometers)
b.
same team on a quiz show, each contributing answers from a different area
of expertise.
Figure 5.3 Everything but Green. (a) Chlorophyll molecules
Violet
450
Relative absorption (percent)
Wavelength in nanometers
X-rays
reflect green and yellow wavelengths of light and absorb the other
wavelengths. (b) Each pigment absorbs some wavelengths of light
and reflects others.
5.2 Mastering Concepts
1. What is the relationship between visible light and the
electromagnetic spectrum?
2. How does it benefit a plant to have multiple types
of pigments?
Figure It Out
If you could expose plants to just one wavelength of light at a time,
would a wavelength of 300 nm, 450 nm, or 600 nm produce the highest
photosynthetic rate?
Answer: 450 nm
Life Depends on Photosynthesis
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Photosynthetic Pigments Capture Sunlight
Chloroplasts Are the Sites of Photosynthesis
Photosynthesis Occurs in Two Stages
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Chapter 5 Photosynthesis
Leaf
5.3 Chloroplasts Are the
Sites of Photosynthesis
In plants, leaves are the main organs of photosynthesis. Their broad, flat surfaces expose abundant
surface area to sunlight. But light is just one requirement for photosynthesis. Water is essential,
too; roots absorb this vital ingredient, which moves
up stems and into the leaves. Plants also must exchange CO2 and O2 with the atmosphere through
stomata (singular: stoma), tiny openings in the epidermis of a leaf or stem. (The word stoma comes from
the Greek word for “mouth”). leaf epidermis, p. 000
a.
Most photosynthesis occurs in cells filling the leaf’s
interior (figure 5.4). Mesophyll is the collective term for
these internal cells (meso- means “middle,” and -phyll means
“leaf”). Leaf mesophyll cells contain abundant chloroplasts, the organelles of photosynthesis in plants and algae. Most photosynthetic cells
contain 40 to 200 chloroplasts, which add up to about 500,000 per square
millimeter of leaf—an impressive array of solar energy collectors.
Each chloroplast contains tremendous surface area for the reactions of
photosynthesis. Two membranes enclose the stroma, a gelatinous fluid containing ribosomes, DNA, and enzymes. (Be careful not to confuse the stroma
with a stoma, or leaf pore). Suspended in the stroma of each chloroplast are
between 10 and 100 grana (singular: granum), each composed of a stack of
10 to 20 disk-shaped thylakoids. Each thylakoid, in turn, consists of a membrane studded with photosynthetic pigments and enclosing a volume called the
thylakoid space.
Mesophyll
cells
Stoma
CO2
O2 + H2O
Mesophyll cell
Nucleus
Central
vacuole
Mitochondrion
Chloroplasts
TEM
15 μm (false color)
b.
Granum
Chloroplast
Thylakoid
Thylakoid
space
DNA
Pigment
molecules
embedded in
thylakoid
membrane
Outer
membrane
Inner
membrane
Granum
d.
Stroma
Ribosomes
c.
The Light Reactions Begin Photosynthesis
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The Carbon Reactions Produce Carbohydrates
Figure 5.4 Leaf and Chloroplast Anatomy.
Leaf mesophyll tissue consists of cells that contain
many chloroplasts.
C3, C4, and CAM Plants
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UNIT 1 Science, Chemistry, and Cells
Chlorophyll
H2 C
H3C
CH
N
N
Chloroplast
Thylakoid
CH2CH3
N
Mg
N
H3 C
CH3
CH2
CH2 CO CH O
2
3
O C
O
CH2
CH
C CH3
CH2
CH2
CH2
HC CH3
CH2
Hydrophobic
CH2
tail
CH2
HC CH3
CH2
CH2
CH2
HC CH3
CH3
Thylakoid
membrane
Proteins
Chlorophyll
Thylakoid
space
Photosystem
Figure 5.5 Thylakoid Membrane. This diagram of a photosystem
shows a complex grouping of proteins and pigments (including
chlorophyll) embedded in the chloroplast’s thylakoid membrane.
Light
CO2
H2O
Chloroplast
ATP
Light
reactions
NADPH
NADP+
Carbon
reactions
ADP
O2
Glucose
Figure 5.6 Overview of Photosynthesis. In the light reactions,
pigment molecules capture sunlight energy and transfer it to
molecules of ATP and NADPH. The carbon reactions use this energy to
build glucose out of carbon dioxide.
Life Depends on Photosynthesis
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The pigments and proteins that participate in photosynthesis are grouped into photosystems in the thylakoid membrane
(figure 5.5). One photosystem consists of chlorophyll a aggregated with other pigment molecules and the proteins that anchor the
entire complex in the membrane.
Within each photosystem are some 300 chlorophyll molecules
and 50 accessory pigments. Although all of the pigment molecules
absorb light energy, only one chlorophyll a molecule per photosystem actually uses the energy in photosynthetic reactions. The photosystem’s reaction center is this chlorophyll a molecule and its
associated proteins. All other pigment molecules in the photosystem
are called antenna pigments because they capture photon energy
and funnel it to the reaction center. If the different pigments are like
a quiz show team, then the reaction center is analogous to the one
member who announces the team’s answer to the show’s moderator.
Why does only one chlorophyll molecule out of a few hundred
actually participate in photosynthetic reactions? A single chlorophyll a molecule can absorb only a small amount of light energy.
Several pigment molecules near each other capture much more energy because they can pass the energy on to the reaction center,
freeing them to absorb other photons as they strike. Thus, the photosystem’s organization greatly enhances the efficiency of photosynthesis.
5.3 Mastering Concepts
1. Describe the relationship among the chloroplast, stroma, grana,
and thylakoids.
2. How does the reaction center chlorophyll interact with the
antenna pigments in a photosystem?
5.4 Photosynthesis Occurs in Two Stages
Inside a chloroplast, photosynthesis occurs in two stages: the light reactions
and the carbon reactions. Figure 5.6 summarizes the entire process, and sections 5.5 and 5.6 describe each part in greater detail.
The light reactions convert solar energy to chemical energy. (You can
think of the light reactions as the “photo-” part of photosynthesis.) In the
chloroplast’s thylakoid membranes, pigment molecules in two linked
photosystems capture kinetic energy from photons and store it as potential energy in the chemical bonds of two molecules: ATP and NADPH.
Recall from chapter 4 that ATP is a nucleotide that stores potential
energy in the covalent bonds between its phosphate groups. ATP forms
when a phosphate group is added to ADP (see figure 4.00). The other energy-rich product of the light reactions, NADPH, is a molecule that carries pairs
of energized electrons. In photosynthesis, these electrons come from chlorophyll
molecules. Once the light reactions are underway, chlorophyll, in turn, replaces
its “lost” electrons by splitting water molecules, yielding O2 as a waste product.
These two resources (energy and “loaded” electron carriers) set the stage
for the second part of photosynthesis. The carbon reactions use ATP and the
high-energy electrons in NADPH to reduce CO2 to glucose molecules. (These
reactions are the “-synthesis” part of photosynthesis.) The ATP and NADPH
Photosynthetic Pigments Capture Sunlight
Chloroplasts Are the Sites of Photosynthesis
Photosynthesis Occurs in Two Stages
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Chapter 5 Photosynthesis
come from the light reactions, and the CO2 comes from the atmosphere. Once
inside the leaf, CO2 diffuses into a mesophyll cell and across the chloroplast
membrane into the stroma, where the carbon reactions occur.
Overall, photosynthesis is an oxidation–reduction (redox) process. “Oxidation” means that electrons are removed from an atom or molecule; “reduction” means electrons are added. As you will see, photosynthesis strips
electrons from the oxygen atoms in H2O (i.e., the oxygen atoms are oxidized).
These electrons reduce the carbon in CO2. Because oxygen atoms attract electrons more strongly than do carbon atoms (see chapter 2), moving electrons
from oxygen to carbon requires energy. The energy source for this reaction is,
of course, light. redox reactions, p. 000
5.4 Mastering Concepts
Light
1. What happens in each of the two main stages of photosynthesis?
2. Where in the chloroplast does each stage occur?
CO2
H2O
Chloroplast
ATP
Light
reactions
The Light Reactions Begin Photosynthesis
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O2
Photosystem II
Light
energy
Electron transport chain
H+
Reaction center
chlorophyll
Photosystem I
NADP+
NADPH
6
Reaction center
chlorophyll
3
2e–
1/2
Electron transport chain
5
Pigment
molecules
2
Glucose
Light
energy
H+
Stroma 1
H2O
Carbon
reactions
ADP
5.5 The Light Reactions Begin
Photosynthesis
A plant placed in a dark closet literally
starves. Without light, the plant cannot
generate ATP or NADPH. And without
these critical energy and electron carriers,
the plant cannot feed itself. Once its
stored reserves are gone, the plant dies.
The plant’s life thus depends on the light
reactions of photosynthesis, which occur
in the membranes of chloroplasts.
We have already seen that the pigments and proteins of the chloroplast’s
thylakoid membranes are organized into
photosystems (see figure 5.5). More specifically, the thylakoid membranes contain two types of photosystems, dubbed I
and II. An electron transport chain connects the two photosystems.
Recall from chapter 4 that an electron
transport chain is a group of proteins that
shuttle electrons like a bucket brigade, releasing energy with each step. As you will
see, the electron transport chain that links
photosystems I and II stores potential energy
used in ATP synthesis. A second electron
transport chain extending from photosystem
I ends with the production of NADPH.
Figure 5.7 depicts the arrangement of
the photosystems and electron transport
chains in the thylakoid membrane. Refer
to this illustration as you work through
the rest of this section.
NADPH
NADP+
H+
O2 + 2H+
H+
H+
Thylakoid space
4
Stroma
ATP synthase
H+
ADP + P
ATP
Figure 5.7 Two Photosystems Participate in the Light Reactions. [1] Chlorophyll molecules in
photosystem II transfer light energy to electrons. [2] The electrons are stripped from water molecules, releasing
oxygen. [3] The energized electrons pass to photosystem I via an electron transport chain. Each transfer
releases energy that is used to pump hydrogen ions into the thylakoid space. [4] The resulting hydrogen
gradient is used to generate ATP. [5] In photosystem I, the electrons absorb more light energy and [6] are
passed to NADP+, creating the energy-rich NADPH.
The Carbon Reactions Produce Carbohydrates
C3, C4, and CAM Plants
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UNIT 1 Science, Chemistry, and Cells
A. Photosystem II Produces ATP
Burning Questions
Why do leaves change colors in the fall?
Most leaves are green throughout a plant’s
growing season, although there are
exceptions; some ornamental plants, for
example, have yellow or purple leaves.
The near-ubiquitous green color comes
from chlorophyll a, the most abundant
pigment in photosynthetic plant parts.
But the leaf also has other photosynthetic
pigments. Carotenoids contribute brilliant
yellow, orange, and red hues. Purple pigments,
such as anthocyanins, are not photosynthetically active, but they
do protect leaves from damage by ultraviolet radiation.
These accessory pigments are less abundant than chlorophyll,
so they usually remain invisible to the naked eye during the
growing season. As winter approaches, however, deciduous plants
prepare to shed their leaves. The chlorophyll degrades, and the now
“unmasked” accessory pigments reveal their colors for a short time
as a spectacular autumn display. These pigments soon disappear as
well, and the dead leaves turn brown.
Spring brings a flush of fresh, green leaves. The energy to
produce the foliage comes from glucose the plant produced during
the last growing season and stored as starch. The new leaves make
food throughout the spring and summer, so the tree can grow—
both above ground and below—and produce fruits and seeds.
As the days grow shorter and cooler in autumn, the cycle will
continue, and the colorful pigments will again participate in one of
nature’s great disappearing acts.
Submit your burning question to:
[email protected]
Photosynthesis begins in the cluster of pigment molecules of photosystem II.
These pigments absorb light and transfer the energy to a chlorophyll a reaction center, where it boosts two electrons to a higher energy level. The “excited” electrons, now packed with potential energy, are ejected from this
chlorophyll a molecule and grabbed by the first protein in the electron
transport chain that links the two photosystems (figure 5.7, step 1).
electron orbitals, p. 000
How does the chlorophyll a molecule replace these two electrons?
They come from water (H2O), which donates two electrons when it splits
into oxygen gas and two protons (H+). Chlorophyll a picks up the electrons, and O2 is the waste product that the plant releases to the environment
(step 2).
Meanwhile, the chloroplast uses the potential energy in the electrons to
create a proton gradient (step 3). As the electrons pass along the electron transport chain, the energy they lose drives the active transport of protons from the
stroma into the thylakoid space. The resulting proton gradient between the
stroma and the inside of the thylakoid represents a form of potential energy.
active transport, p. 000
An enzyme complex called ATP synthase transforms the gradient’s potential energy into chemical energy in the form of ATP (step 4). A channel in ATP
synthase allows protons trapped inside the thylakoid space to return to the chloroplast’s stroma. As the gradient dissipates, energy is released. The ATP synthase
enzyme uses this energy to add phosphate to ADP, generating ATP. (As described
in chapter 6, the same process also produces ATP in cellular respiration).
This mechanism is similar to using a dam to produce electricity. As water accumulates, tremendous pressure (a form of potential energy) builds on
the face of the dam. That pressure is released by diverting water through a
large pipe at the base of the dam, turning massive blades that spin an electric
generator.
B. Photosystem I Produces NADPH
Photosystem I functions much as photosystem II does. Photon energy strikes
energy-absorbing molecules of chlorophyll a, which pass the energy to the reaction center. The reactive chlorophyll molecules eject electrons to an electron
carrier molecule in a second electron transport chain (figure 5.7, step 5). The
boosted electrons in photosystem I are then replaced with electrons passing
down the first electron transport chain from photosystem II.
Unlike in photosystem II, however, the second electron transport chain
does not generate ATP, nor does it pass its electrons to yet another photosystem. Instead, the electrons reduce a molecule of NADP+ to NADPH (step 6).
This NADPH is the electron carrier that will reduce carbon dioxide in the
carbon reactions, while the ATP generated in photosystem II will provide
the energy.
5.5 Mastering Concepts
1. Describe the events in photosystem II, beginning with light
and ending with the production of ATP.
2. How do electrons pass from photosystem II to photosystem I?
3. How are the electrons from photosystem II replaced?
4. What happens in photosystem I?
Life Depends on Photosynthesis
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Photosynthetic Pigments Capture Sunlight
Chloroplasts Are the Sites of Photosynthesis
Photosynthesis Occurs in Two Stages
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Chapter 5 Photosynthesis
5.6 The Carbon Reactions Produce
Carbohydrates
Light
H2O
CO2
Chloroplast
The carbon reactions, also called the Calvin cycle, occur in the chloroplast’s
ATP
stroma. The Calvin cycle is the metabolic pathway that uses NADPH and ATP
from the light reactions to assemble CO2 molecules into three-carbon carbohyNADPH
Light
Carbon
drate molecules (figure 5.8). These products are eventually assembled into glureactions
reactions
NADP+
cose and other sugars.
ADP
The first step of the Calvin cycle is carbon fixation—the initial incorporation of carbon from CO2 into an organic compound. Specifically, CO2 combines with ribulose bisphosphate (RuBP), a five-carbon sugar with two
O2
Glucose
phosphate groups. An enzyme called rubisco catalyzes this first reaction.
The six-carbon product of the initial reaction immediately breaks down
3 CO2
into two three-carbon molecules (PGA). Further steps in the cycle convert PGA to a carbohydrate called phosphoglyceraldehyde
(PGAL). Some of the PGAL is rearranged to form additional
Rubisco
RuBP, continuing the cycle. But the cell can also use PGAL
enzyme
to build larger carbohydrates such as glucose and sucrose,
the most familiar products of photosynthesis.
Several fates await the carbohydrates produced
CARBON FIXATION
in the carbon reactions. A plant’s cells use about
P
3 P
3 P
P
half of the glucose as fuel for their own cellular
1 Carbon dioxide is added
Unstable intermediates
RuBP
respiration, the metabolic pathway described in
to RuBP, creating an
unstable molecule.
chapter 6. Roots, flowers, fruits, seeds, and other
nonphotosynthetic plant parts could not grow
without sugar shipments from green leaves and
6
P
REGENERATION
stems. Plants also combine glucose with other
PGAL SYNTHESIS
PGA
From light
OF RuBP
substances to manufacture additional comreactions
4 RuBP is regenerated
2 The unstable
pounds, including amino acids and a host of ecintermediate splits
by rearranging the
6 ATP
onomically important products such as rubber,
remaining molecules.
to form PGAL.
3 ADP
medicines, and spices.
6 NADPH
Moreover, glucose molecules are the building
6 NADP+
blocks of the cellulose wall that surrounds every
3 ATP
6 ADP + 6 P
plant cell. Wood is mostly made of cellulose. The timP
P
5
6
ber in the world’s forests therefore stores enormous
PGAL
PGAL
amounts of carbon. So do vast deposits of coal and other
fossil fuels, which are the remains of plants and other organisms that lived long ago. Burning wood or fossil fuels releases this stored carbon into the atmosphere as CO2. As the
PGAL
3 PGAL molecules
amount of CO2 in the atmosphere has increased, Earth’s average temfrom
are combined
P
other
perature has risen. global climate change, p. 000
to form glucose, 1
PGAL
turns
which
is
used
If a plant produces more glucose than it immediately needs for respiraof the
to form starch,
tion or building cell walls, it may store the excess as starch. CarbohydrateCalvin
sucrose, and other
cycle
rich tubers and grains, such as potatoes, rice, corn, and wheat, are all
organic molecules.
energy-storing plant organs. Some plants, including sugarcane and sugar beets,
Glucose
store energy as sucrose instead. Table sugar comes from these crops. In addition, people use starch (from corn kernels) and sugar (from sugarcane) to proFigure 5.8 The Calvin Cycle. ATP and NADPH from the light
duce biofuels such as ethanol. biofuels, p. 000
(
)
reactions power the Calvin cycle, simplified here. The cycle generates
a three-carbon molecule, PGAL, which is used to build glucose and
other carbohydrates.
5.6 Mastering Concepts
1. What happens in the carbon reactions?
2. What are the roles of CO2, ATP, and NADPH in the Calvin cycle?
The Light Reactions Begin Photosynthesis
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The Carbon Reactions Produce Carbohydrates
C3, C4, and CAM Plants
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UNIT 1 Science, Chemistry, and Cells
Why We Care
Weed Killers
One low-tech way to kill an unwanted
plant is to deprive it of light. Gardeners
who want to convert a lawn into a
garden, for example, might kill the grass
by covering it with layers of newspaper
or cardboard for several weeks. The
light reactions of photosynthesis cannot
occur in the dark; the plants die.
Many herbicides also stop the
light reactions. For example, a weed
killer called diuron blocks electron flow
in photosystem II. Paraquat, noted for
its use in destroying marijuana plants,
diverts electrons from photosystem I.
Other herbicides take a different approach. Accessory
pigments called carotenoids protect plants from damage caused by
free radicals. Triazole herbicides kill plants by blocking carotenoid
synthesis. No longer protected from free-radical damage, the cell’s
organelles are destroyed.
Still other weed killers exploit pathways not directly related
to photosynthesis. For instance, glyphosate (Roundup) inhibits
an enzyme that plants require for amino acid synthesis. Another
herbicide, 2,4-D, mimics a plant hormone called auxin (see
chapter 22).
C3 plant
Stoma
BundleVein
sheath
(vascular
tissue)
Mesophyll cell
cell
C4 plant
Vein Bundle(vascular sheath
cell
Mesophyll tissue)
cell
Stoma
Figure 5.9 C3 and C4 Leaf Anatomy. In C3 plants, the light
reactions and the Calvin cycle occur in mesophyll cells. In C4 plants, the
light reactions occur in mesophyll, but the inner ring of bundle-sheath
cells houses the Calvin cycle.
Life Depends on Photosynthesis
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5.7 C3, C4, and CAM Plants Use Different
Carbon Fixation Pathways
The Calvin cycle is also known as the C3 pathway because a three-carbon
molecule, PGA, is the first stable compound in the pathway. Although all plants
use the Calvin cycle, C3 plants use only this pathway to fix carbon from CO2.
About 95% of plant species are C3, including cereals, peanuts, tobacco, spinach, sugar beets, soybeans, most trees, and some lawn grasses.
C3 photosynthesis is obviously a successful adaptation, but it does have a
weakness: inefficiency. Photosynthesis has a theoretical efficiency rate of 30%,
but on cloudy days, individual plants average only from 0.1% to 3% photosynthetic efficiency.
How do plants waste so much solar energy? One contributing factor is a
metabolic pathway called photorespiration, a series of reactions that begin
when the rubisco enzyme uses O2 instead of CO2 as a substrate. The net result
of photorespiration is that the plant loses CO2 that it has already fixed, wasting
both ATP and NADPH.
Photorespiration is most likely in hot, dry climates. Plants in these habitats
therefore face a trade-off. If the stomata remain open too long, a plant may lose
water, wilt, and die. If the plant instead closes its stomata, CO2 supplies in the leaves
run low while O2 builds up. Under those conditions, photorespiration becomes
much more likely, and photosynthetic efficiency plummets. Plants may lose as
much as 30% of their fixed carbon to this pathway, which has no known benefit.
In hot climates, plants that minimize photorespiration may therefore have
a significant competitive advantage. One way to improve efficiency is to ensure
that rubisco always encounters high CO2 concentrations. The C4 and CAM
pathways are two adaptations that do just that.
C4 plants physically separate the light reactions and the carbon reactions into
different cells (figure 5.9). The light reactions occur in mesophyll cells, as does a
carbon-fixation reaction called the C4 pathway. In the C4 pathway, CO2 combines with a three-carbon molecule to form a four-carbon compound (hence the
name C4). This molecule then moves into adjacent bundle-sheath cells that surround the leaf veins. The CO2 is liberated inside these cells, where the Calvin
cycle fixes the carbon a second time by the C3 pathway. Unlike mesophyll cells,
bundle-sheath cells are not exposed directly to atmospheric O2. The rubisco in
bundle-sheath cells is therefore much more likely to bind CO2 instead of O2, reducing photorespiration. Meanwhile, at the cost of two ATP molecules, the threecarbon “ferry” returns to the mesophyll to pick up another CO2.
About 1% of plants use the C4 pathway. All are flowering plants growing
in hot, open environments, including crabgrass and crop plants such as sugarcane and corn. C4 plants are less abundant, however, in cooler, moister habitats.
In those environments, the ATP cost of ferrying each CO2 from a mesophyll
cell to a bundle-sheath cell apparently exceeds the benefits of reduced photorespiration.
Another energy- and water-saving strategy, called crassulacean acid metabolism (CAM), occurs in about 3% to 4% of plant species, including pineapple and cacti. Plants that use the CAM pathway open their stomata to fix
CO2 only at night, when the temperature drops and the humidity rises. CO2 diffuses in. Mesophyll cells incorporate the CO2 into a four-carbon compound,
which they store in large vacuoles. The stomata close during the heat of the
day, but the stored molecule moves from the vacuole to a chloroplast and releases its CO2. The chloroplast then fixes the CO2 in the Calvin cycle. The
CAM pathway reduces photorespiration by generating high CO2 concentrations inside chloroplasts.
Photosynthetic Pigments Capture Sunlight
Chloroplasts Are the Sites of Photosynthesis
Photosynthesis Occurs in Two Stages
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11
Chapter 5 Photosynthesis
All CAM plants are adapted to dry habitats. In cool environments, however, CAM plants cannot compete with C3 plants. Their stomata are only open
at night, so CAM plants have much less carbon available to their cells for
growth and reproduction.
Figure 5.10 compares and contrasts C3, C4, and CAM plants.
5.7 Mastering Concepts
1.
2.
3.
4.
Why is the Calvin cycle also called the C3 pathway?
How does photorespiration counter photosynthesis?
Describe how a C4 plant minimizes photorespiration.
How is the CAM pathway like C4 metabolism, and how is it different?
C4 plant
C3 plant
CAM plant
Example
CO2
CO2 or O2
Night
Mesophyll
cell
Mesophyll
cell
4-carbon molecule
4-carbon molecule
Mesophyll
cell
Calvin
cycle
Pathway
Bundlesheath
cell
CO2
CO2
CO2
Calvin
cycle
Calvin
cycle
Glucose
Glucose
Glucose
Day
Limitation
How plant avoids
photorespiration
Habitat
% of plant species
Figure 5.10
11
ATP cost
Reduced carbon availability
N/A
Light reactions and carbon
reactions occur in separate
cells.
Cool, moist
Hot, dry
Hot, dry
95%
1%
3–4%
CO2 is absorbed at night; light
reactions and carbon reactions
occur during the day.
C3, C4, and CAM Pathways Compared. The C4 and CAM pathways are adaptations that minimize photorespiration.
The Light Reactions Begin Photosynthesis
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Photorespiration
The Carbon Reactions Produce Carbohydrates
C3, C4, and CAM Plants
Investigating Life: Solar-Powered Sea Slugs
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12
UNIT 1 Science, Chemistry, and Cells
Investigating Life
5.8 Solar-Powered Sea Slugs
Most animals have an indirect relationship with photosynthesis: autotrophs
make food, which animals eat. But Elysia chlorotica is an unusual animal
(figure 5.11). This sea slug, which lives in salt marshes along the eastern coast
of North America, is solar-powered: it has chloroplasts in the lining of its gut.
These invertebrate animals do not inherit their solar panels from their parents; instead, they acquire the chloroplasts by eating algae. As a young sea slug
grazes, it punctures the filaments of the algae and sucks out the contents. The
animal digests most of the nutrients, but cells lining the slug’s gut absorb the
chloroplasts. The organelles stay there for the rest of the animal’s life.
Like a plant, the solar-powered sea slug can live on sunlight and air.
Head
The Question: A chloroplast requires a few thousand genes to
carry out photosynthesis, yet chloroplast DNA encodes less than
10% of the required proteins. DNA in a plant cell’s nucleus
makes up the difference. But slugs are animals, so their nuclei
presumably lack these genes. How can the chloroplasts operate inside their mollusk partners?
Digestive tract
Figure 5.11
A Slug with Solar
Panels. The sea slug
Elysia chlorotica owes its
green color to chloroplasts
harvested from algae.
Alga
Water DNA
Slug (control) ladder
The Approach: Mary E. Rumpho, of the University of
Maine, collaborated with James R. Manhart, of Texas A&M
University, to find out the answer. They considered two possibilities. Either the chloroplasts can work inside the host slug’s digestive tract without the help of supplemental genes, or the slug’s own
cells provide the necessary proteins.
The researchers searched the chloroplast’s DNA for genes essential to
photosynthesis and found that a gene that encodes part of photosystem II was
missing. Without this gene, photosynthesis is impossible. The researchers
therefore rejected the hypothesis that the chloroplasts are autonomous.
That left the second possibility, which suggested that the slug’s cells contain the DNA necessary to support the chloroplasts. The team looked for the
critical missing gene in the animal’s DNA, and they found it (figure 5.12).
Moreover, when they sequenced the gene from the slug’s genome, it was identical to the same gene in algae.
The Conclusion: At some point, a gene required for photosynthesis moved from
algae to the genome of a sea slug. The researchers speculate that cells in a slug’s gut
may have taken up DNA fragments that spilled from partially eaten algae.
This study provides convincing evidence that gene transfer can occur
not only in bacteria but among distantly related eukaryotes, too. Apparently, organisms have traded DNA throughout life’s long history. Many biologists are
therefore discarding the notion of a tidy evolutionary “tree” in favor of a messier,
but perhaps more fascinating, evolutionary thicket.
Gene encoding part
of photosystem II
Rumpho, Mary E., and seven colleagues, including James R. Manhart. 2008. Horizontal gene transfer of the
algal nuclear gene psbO to the photosynthetic sea slug Elysia chlorotica. Proceedings of the National Academy
of Sciences, vol. 105, pages 17867–17871.
Figure 5.12
Photosynthesis Gene. Both algae and the
solar-powered sea slug contain a particular gene required for
photosynthesis. This electrophoresis gel sorts DNA fragments by size
as they migrate from the top to the bottom of the gel. The “ladder”
contains DNA pieces of known size, allowing the researchers to
estimate the size of the DNA being studied.
Life Depends on Photosynthesis
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12
Photosynthetic Pigments Capture Sunlight
5.8 Mastering Concepts
1. Explain the most important finding of this study.
2. What evidence led the researchers to their conclusion?
Chloroplasts Are the Sites of Photosynthesis
Photosynthesis Occurs in Two Stages
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Chapter 5 Photosynthesis
Cell
Leaf
Chapter Summary
5.1 Life Depends on Photosynthesis
• Photosynthesis converts kinetic energy in light to potential energy in the
covalent bonds of glucose, according to the following chemical equation:
Chloroplast
light energy
6CO2 + 6H2O ⎯→ C6H12O6 + 6O2
• Autotrophs produce their own organic molecules from atmospheric CO2.
Plants, algae, and some bacteria are autotrophs. Heterotrophs rely on
organic molecules produced by other organisms.
• Food and oxygen produced in photosynthesis are critical to life in terrestrial
and aquatic habitats.
5.2 Photosynthetic Pigments Capture Sunlight
• Visible light is a small part of the electromagnetic spectrum.
• Photons move in waves. The longer the wavelength, the less kinetic
energy per photon. Visible light occurs in a spectrum of colors representing
different wavelengths.
• Chlorophyll a is the primary photosynthetic pigment in plants. Accessory
pigments absorb wavelengths of light that chlorophyll a cannot absorb,
extending the range of wavelengths useful for photosynthesis.
Stroma
Light reactions
(in thylakoid
membranes)
Light
energy
Light
energy Chlorophyll
H2O
NADP+
H+
H+
2H+
H+
5.3 Chloroplasts Are the Sites of Photosynthesis
• Plants exchange gases with the environment through pores called stomata.
• Leaf mesophyll cells contain abundant chloroplasts.
• A chloroplast includes a gelatinous matrix called the stroma. This fluid
surrounds the grana, which are composed of stacked thylakoid membranes.
Photosynthetic pigments are embedded in the thylakoid membranes, which
enclose the thylakoid space.
• A photosystem consists of antenna pigments and a reaction center.
NADPH
H+
2e–
1/2 O +
2
Granum
3 CO2
Carbon reactions
(in stroma)
H+
ADP
+
P
Rubisco
enzyme
ATP
ATP
ADP + P
NADPH
5.4 Photosynthesis Occurs in Two Stages
NADP+
• The light reactions of photosynthesis produce ATP and NADPH; these
molecules provide energy and electrons for the glucose-producing carbon
reactions.
• Photosynthesis is a redox reaction in which water is oxidized and CO2 is
reduced to glucose.
ATP
1
PGAL
5.5 The Light Reactions Begin Photosynthesis
A. Photosystem II Produces ATP
• Photosystem II captures light energy and sends electrons from reactive
chlorophyll a to an electron transport chain that joins photosystem II to
photosystem I.
• Electrons from chlorophyll are replaced with electrons from water. O2 is the
waste product.
• The energy released in the electron transport chain drives the active transport
of protons into the thylakoid space. The protons diffuse out through channels
in ATP synthase. This movement powers the production of ATP.
B. Photosystem I Produces NADPH
• Photosystem I receives electrons from the electron transport chain and uses
them to reduce NADP+, producing NADPH. Light provides the energy.
ADP + P
P
Glucose, starch,
sucrose
• In the Calvin cycle, rubisco catalyzes the reaction of CO2 with ribulose
bisphosphate (RuBP) to yield two molecules of PGA. These are converted
to PGAL, the immediate carbohydrate product of photosynthesis. PGAL
later becomes glucose.
• Plants use glucose to generate ATP, grow, nourish nonphotosynthetic plant
parts, and produce cellulose and many other biochemicals. Most store
excess glucose as starch or sucrose.
5.6 The Carbon Reactions Produce Carbohydrates
5.7 C3, C4, and CAM Plants Use Different Carbon Fixation
Pathways
• The carbon reactions use energy from ATP and electrons from NADPH in
carbon fixation reactions that incorporate CO2 into organic compounds.
• The Calvin cycle is also called the C3 pathway. Most plant species are C3
plants, which use only this pathway to fix carbon.
The Light Reactions Begin Photosynthesis
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13
The Carbon Reactions Produce Carbohydrates
C3, C4, and CAM Plants
Investigating Life: Solar-Powered Sea Slugs
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14
UNIT 1 Science, Chemistry, and Cells
• Photorespiration wastes carbon and energy when rubisco reacts with O2
instead of CO2.
• The C4 pathway reduces photorespiration by separating the light and carbon
reactions into different cells. In mesophyll cells, CO2 is fixed as a fourcarbon molecule, which moves to a bundle-sheath cell and liberates CO2 to
be fixed again in the Calvin cycle.
• In the CAM pathway, desert plants such as cacti open their stomata and
take in CO2 at night, storing the fixed carbon in vacuoles. During the day,
they split off CO2 and fix it in chloroplasts in the same cells.
9. What happens to the enzyme rubisco during photorespiration?
a. The enzyme speeds up the formation of glucose.
b. The enzyme’s active site binds to O2 instead of CO2.
c. It becomes denatured.
d. The enzyme catalyzes the breakdown of glucose.
10. A plant that only opens its stomata at night is a
a. C2 plant.
c. C4 plant.
b. C3 plant.
d. CAM plant.
5.8 Investigating Life: Solar-Powered Sea Slugs
• The sea slug Elysia chlorotica contains chloroplasts acquired from its
food, a filamentous alga. The slug’s DNA includes a gene required for
photosynthesis.
Multiple-Choice Questions
1. Where does the energy come from to drive photosynthesis?
a. A chloroplast
c. The sun
b. ATP
d. Glucose
2. Algae in a swimming pool are ____; Escherichia coli bacteria in the human intestine are ___.
a. autotrophs ... autotrophs
b. heterotrophs ... heterotrophs
c. autotrophs ... heterotrophs
d. heterotrophs ... autotrophs
3. Photosynthesis is essential to animal life because it provides ___.
a. CO2 required in respiration
b. O2
c. organic molecules
d. Both b and c are correct.
4. A plant appears green because
a. its chloroplasts use green wavelengths of light.
b. chlorophyll a absorbs red and blue light.
c. chlorophyll a absorbs ultraviolet light.
d. Both a and c are correct.
5. Only high-energy light can penetrate the ocean and reach photosynthetic
organisms in coral reefs. What color light would you predict these organisms use?
a. Red
c. Blue
b. Yellow
d. Orange
6. Which part of the chloroplast is associated with the production of carbohydrates?
a. The thylakoid
c. The thylakoid space
b. The grana
d. The stroma
7. The ATP that is produced in the light reactions is used by the cell to
a. reproduce and grow.
b. build carbohydrate molecules.
c. move electrons through the electron transport chain.
d. split water into H+ and O2.
8. Can carbon fixation occur at night?
a. Yes, because CO2 can always enter a leaf.
b. No, because a plant cell is not active at night.
c. Yes, if there is a source of ATP and NADPH.
d. No, because photorespiration occurs at night.
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14
Write It Out
1. Photosynthesis takes place in plants, algae, and some microbes. How does
it affect a meat-eating animal?
2. What color would plants be if they absorbed all wavelengths of visible
light? Why?
3. Define these terms and arrange them from smallest to largest: thylakoid
membrane; photosystem; chloroplast; electron transport chain; reaction
center.
4. Determine whether each of the following molecules is involved in the
light reactions, the carbon reactions, or both and explain how: O2, CO2,
carbohydrate, chlorophyll a, photons, NADPH, ATP, H2O.
5. One of the first investigators to explore photosynthesis was Flemish
physician and alchemist Jan van Helmont. In the early 1600s, he grew
willow trees in weighed amounts of soil, applied known amounts of
water, and noted that in 5 years the trees gained more than 45 kg, but the
soil had lost only a little weight. Because he had applied large amounts
of water, van Helmont concluded (incorrectly) that plants grew solely by
absorbing water. What is the actual source of the added biomass? Explain
your answer.
6. One of the classic experiments in photosynthesis occurred in 1771, when
Joseph Priestley found that if he placed a mouse in an enclosed container
with a lit candle, the mouse would die. But if he also added a plant to the
container, the mouse could live. Priestley concluded that plants “purify”
air, allowing animals to breathe. What is the biological basis for this
observation?
7. In 1941, biologists exposed photosynthesizing cells to water containing a
heavy oxygen isotope, designated 18O. The “labeled” isotope appears in
the O2 gas released in photosynthesis, showing that the oxygen came from
the water. Where would the 18O have ended up if the researchers had used
18
O-labeled CO2 instead of H2O?
8. Over the past decades, the CO2 concentration in the atmosphere has
increased.
a. Predict the effect of increasing carbon dioxide concentrations on
photorespiration.
b. Scientists suggest that increasing CO2 concentrations are leading to
higher average global temperatures. If temperatures are increasing,
does this change your answer to part (a)?
9. How is the CAM pathway adaptive in a desert habitat?
10. Explain why each of the following misconceptions about photosynthesis
is false:
a. Only plants are autotrophs.
b. Plants do not need cellular respiration because they carry out
photosynthesis.
c. Chlorophyll is the only photosynthetic pigment.
5/19/11
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Chapter 5 Photosynthesis
Pull It Together
PHOTOSYNTHESIS
Light
O2 + C6H12O6
CO2 + H2O
occurs in two stages
Light reactions
H2O
Light
Carbon reactions
CO2
Light
ATP
NADPH
NADP+
NADPH
NADP+
ADP
O2
is energy
source for
Glucose
ATP
is energy
source for
P
require photosynthetic
pigments such as
P
Chlorophyll
H3C
ADP
O2
ATP
NADPH
N
Light
N
N
Mg
CH2CH3
N
CH3
Glucose
CH2
CH2 CO CH O
2
3
O C
O
CH2
CH
C CH3
CH2
CH2
CH2
HC CH3
CH2
CH2
CH2
HC CH3
CH2
CH2
CH2
HC CH3
CH3
15
produce
CH
H3C
hoe96928_ch05.indd
Glucose
is an
electron
source for
P
produce
H2C
1. Where does the electron transport chain fit into this concept map?
2. What specific process in the light reactions gives rise to the waste
product, O2?
3. How would you incorporate the Calvin cycle, rubisco, C3 plants, C4
plants, and CAM plants into this concept map?
4. Where do humans and other heterotrophs fit into this concept map?
5. Build another small concept map showing the relationships among
the terms chloroplast, stroma, grana, thylakoid, photosystem, and
chlorophyll.
6. What happens to the glucose produced in photosynthesis?
CO2
H2O
produce
ATP
15
H
C
HO
absorbs specific
wavelengths of
CH2OH
C
H
OH
C
H
O
H
C
H
OH
C
OH
Enhance your study of this chapter
with practice quizzes, animations
and videos, answer keys, and
downloadable study tools.
www.mhhe.com/hoefnagels.
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