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The starch-rich endosperm of this corn kernel
has been stained blue-black with iodine. Starch
is fuel for the emergent seedling. How might
the accumulation of starch in the corn grain
(kernel) provide a selective advantage?
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Eleven
C H A P T E R
1 1
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Respiration
Chapter Outline
CHAPTER OUTLINE
The two primary energy and carbon transformation
systems in plants are photosynthesis and respiration.
Plants retrieve energy and carbon via respiration of
sucrose and starch.
The energy exchange and carbon economy of most
organisms depends on aerobic respiration.
Cellular respiration forms ATP and carbon skeletons
used in biosynthesis.
Cellular respiration includes glycolysis, the Krebs
cycle, and the electron transport chain.
Perspective 11.1 The Potential Energy of Glucose
Glycolysis splits glucose, producing some ATP.
The Krebs cycle is fueled by pyruvate and forms
some ATP.
Perspective 11.2 Models and Respiration
ATP is formed as electrons flow to oxygen.
ATP synthesis is driven by electron flow that generates
a proton gradient.
Perspective 11.3 Botany and Politics
Fermentation can occur when O2 levels are low.
Cyanide-resistant respiration is important in some plants.
Perspective 11.4 How Does Skunk Cabbage Get So Hot?
Lipids are respired in some germinating seeds.
Learning Online
LEARNING ONLINE
Check out the hyperlink www.fgi.net/~corpalt/science/glycol.html
in chapter 11 of The Online Learning Center at www.mhhe.com/
botany to see how plants convert food to energy. This, and other
helpful information such as articles about botany from countries
around the world, is available to you when you visit the OLC.
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C H A P T E R
E L E V E N
Corn is one of the most widely planted crop in the United
States. To people who have driven across the midwestern
United States (also called the Corn Belt) during the summer, this is no surprise, for in many areas there is little except mile after mile of corn. This crop covers a combined
area about the size of Arizona and produces about 8 billion bushels of corn per year. According to one mathematically minded urbanite, that’s enough corn to bury all
of Manhattan Island about 5 meters deep.
Corn is the most efficient of all major grain crops;
that is, corn is the champion at harvesting sunlight and using the energy to convert CO2 to sugars and then storing
the fixed carbon and energy as starch and oil in grain. We
use these grains for a variety of purposes. For example, we
feed about half of our corn to livestock; another 25% of the
crop is exported. About 10% of the crop—optimally within
5 minutes of picking—appears on our dinner tables. The
rest of the crop passes into an enormous number of pe-
ripheral products, including corn syrup, cornmeal, corn
oil, cardboard, crayons, firecrackers, aspirin, wallpaper,
gasohol, pancake mix, shoe polish, ketchup, chewing
gum, marshmallows, soap, and corn whiskey.
In the previous chapter, you learned about the
most important set of chemical reactions on earth: photosynthesis. These are the reactions that convert sunlight
to chemical energy, thus providing fuel and fixed carbon
for virtually all organisms. In this chapter, you learn
about energy conversions and about energy use by
plants. Also, you will learn about carbon flow in plants
following photosynthesis and how organisms—plants
included—use the energy and carbon trapped in sugars
and other compounds to do work and make new compounds. In the process, you’ll come to appreciate one of
the themes of life: all organisms, in addition to requiring
energy and carbon to stay alive, exchange energy and
carbon with their environment.
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The Two Primary Energy and Carbon
Transformation Systems in Plants Are
Photosynthesis and Respiration.
The most important energy and carbon transformation systems in plants are photosynthesis and respiration. Photosynthesis was considered in the previous chapter; respiration is discussed in this chapter. We use the term respiration
interchangeably with the technically more correct term cellular respiration, which will be defined later in the chapter.
Recall the summary equation for the energy-requiring,
“uphill” process of photosynthesis:
CO2 H2O light energy
enzymes
chlorophyll
sugars O2
During photosynthesis, plants use light energy to fix CO2
and make sugars that provide the building blocks to make
proteins, fats and nucleic acids, fuel the activities of plants
and all other organisms. Of course, O2 is released during
photosynthesis. This O2 is used in aerobic respiration and is
therefore important to most organisms in an ecosystem.
The energy-releasing, “downhill” stage of metabolism is cellular respiration, a series of chemical reactions
that sustain life in all organisms. During respiration, energy-rich molecules such as sugars are broken down, or oxidized, to carbon dioxide and water. A summary equation
for respiration is:
enzymes
CO2 H2O ATP heat
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Compare the summary equations for photosynthesis
and respiration. Photosynthesis removes hydrogen from
water and adds it to carbon; thus, carbon is reduced, or
fixed, during photosynthesis, an anabolic pathway. Cellular respiration removes hydrogen from carbon and adds it
to oxygen, forming water; thus, carbon is oxidized during
respiration, a catabolic pathway. Although one summary
equation is essentially the reverse of the other, they proceed
by very different mechanisms. In addition, photosynthesis
occurs in chloroplasts while respiration begins in the cytosol and is completed in mitochondria.
Now examine figure 11.1, which shows how photosynthesis and respiration are integrated in nature. This diagram illustrates an important concept about energy: energy flows through a system and is ultimately converted
to heat (see chapter 3). For example, sunlight is the energy
input that sustains almost all of the life on earth (an exception is the thermal vents deep in the ocean floor where
geothermal energy powers the dark ecosystem). This is
possible only because plants use photosynthesis to convert sunlight into chemical energy and fix CO2 into sugars; thus, plants are producers in an ecosystem. Animals
stay alive by eating or consuming plants (or by eating
other animals that ate plants) and using the stored energy
to power their activities. These animals are then eaten by
other animals to fuel their activities. Ultimately, the organisms die and are decomposed by bacteria and fungi. In
the process, the energy stored temporarily by organisms
in this so-called food chain is released as heat. Organisms
may temporarily store various amounts of the energy, but
the net effect is that it flows through the system and is ultimately transformed to heat and lost from the system.
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Respiration
257
The Lore of Plants
On average, vegetables and fruits make up about 10% to
20% of our intake of calories, 7% to 20% of our intake of
protein, most of our fiber, often over 80% of many vitamins and minerals, an unimpressive 1% of our intake of
fat, and no cholesterol. That’s why fruits and vegetables
are high on the list when it comes to a good diet—little
fat, no cholesterol, and lots of important nutrients. How
do grains and cereals figure into our diet?
starch, more of the energy is lost. When this starch is fed to
a cow, only about 10% of the usable energy is stored by the
cow; the rest is lost as heat. By the time we eat a steak made
from the cow, another 90% of the usable energy has been
lost. Thus, the amount of usable energy in the steak is only
about 1% of that contained in the starch of the corn:
Energy
Nutrients
Heat
Heat
Sun
100 units of
10 units of
1 unit of
energy in a
→ energy in an
→ energy in a
producer (corn)
herbivore (cow)
carnivore (human)
Respiration by
producers,
consumers and
decomposers
Photosynthesis
by producers
These energy transformations, as predicted by the second
law of thermodynamics, are inefficient because much of the
energy is converted to heat. By inserting an extra energy
transformation between the corn plant and ourselves (i.e.,
the cow), we lose even more of the usable energy.
Look again at figure 11.1. Notice that although energy
moves through the system in a one-way flow (i.e., toward
heat), nutrients are cycled. That is, photosynthesis produces sugars and oxygen from carbon dioxide, light, and
water. The sugars and oxygen are converted back to carbon
dioxide, water and heat during cellular respiration.
Inorganic
nutrients
INQUIRY SUMMARY
FIGURE
Photosynthesis and respiration are the major energy
transformations in organisms. Photosynthesis uses
light energy to reduce CO2 to sugars, whereas respiration oxidizes sugars, producing ATP in the process. Together, photosynthesis and respiration form a system
by which energy flows through an ecosystem and is ultimately converted to heat.
11.1
Photosynthesis and respiration, the major transformations of energy by organisms. Photosynthesis provides food for virtually all
organisms on earth. Organisms use cellular respiration to extract
energy from food. Although nutrients cycle in an ecosystem, energy flows through an ecosystem. All energy is eventually converted to heat.
What happens to this heat?
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Chapter 2 provides more information on plant ecology
and ecosystems.
According to the second law of thermodynamics
(chapter 3), all of the energy transformations at every step
in the food chain are less than 100% efficient. Indeed, there
is a 90% loss of usable energy at each stage. This has
tremendous implications. For example, consider the corn
grain shown at the beginning of this unit. Most of the energy striking the plants’ leaves is reflected or converted to
heat; only a small amount of the energy is trapped by photosynthesis in sugars. When these sugars are converted to
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Plants Retrieve Energy and Carbon via
Respiration of Sucrose and Starch.
Most textbook discussions of respiration focus on glucose,
probably because the metabolism of glucose is so similar in
all organisms. However, free (unbound) glucose is usually
not abundant in plants or their cells. Thus, in plants, cellular respiration begins with the conversion of energy and
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C H A P T E R
E L E V E N
The Lore of Plants
The amount of CO2 released by running a car engine is
about 100 kilograms per 40-liter tank of gasoline. If 1 mole
of glucose (180 grams) yields 6 moles of CO2 (264 grams)
during aerobic respiration, then burning a tank of gasoline is equivalent to the respiration of about 68 kilograms
carbon storage compounds—such as starch—into glucose,
which can then be quickly moved into cellular respiration.
Such storage compounds vary from one organism to another or, in plants, even from one organ to another. For example, humans and other vertebrates store the polysaccharide glycogen. Potatoes, beans, corn, rice, yams, and
bananas store the polysaccharide starch (which, like glycogen, is a polymer of glucose); a few plants, such as
Jerusalem artichokes and onions, store polymers of fructose. Sucrose, from photosynthesis, is the common starter
molecule for respiration in most parts of a plant, but, as
suggested, respiration may begin with starch in nongreen
organs such as roots and stems. In other plants, or, more
specifically, in other plant parts, respiration may utilize
fats, organic acids, or proteins. Nevertheless, the most wellknown and probably most common sources of glucose in
plants are sucrose and starch.
Sucrose, abundant in sugarcane stems and sugar beet
roots, is common in plants and is commercially important
because humans consume so much of it as table sugar (during an average lifetime, people in the United States consume more than 4,600 pounds of table sugar; that is over 8
million kilocalories). Besides being a common energy and
carbon source in leaf cells, sucrose moves most readily
through conducting cells from leaves to growing tissues,
where respiration is active. The breakdown of sucrose, via
hydrolysis, to its component monomers, glucose and fructose, is shown by the following equation:
sucrose H2O
sucrase
glucose fructose
Enzymes that catalyze this reaction are called sucrases; they
occur in the cytosol, in the central vacuole, and sometimes
in cell walls. Hydrolysis by sucrases frees glucose and fructose for respiration. In us, as in plants, fructose can be
quickly converted to glucose.
Starch is a polymer of hundreds of glucose monomers
(chapter 3). Because of the complexity of starch, its breakdown requires several steps and several enzymes for the
complete retrieval of glucose.
enzymes
glucose
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of glucose, or the weight of an average adult human. This
is roughly the amount of glucose, made from sucrose and
starch, that is respired by 10,000 young sunflower plants
during a warm summer night.
Recall that most of a plant’s starch is made in storage organs, such as stems, roots, and seeds, from sugars originally produced in leaves.
The energy exchange and carbon economy of most
organisms depend on aerobic respiration.
Cellular respiration converts the relatively large amount of
energy stored in a molecule of glucose to the relatively
small amounts of energy in several molecules of ATP, as
discussed in chapter 3. This is one of the two main functions of respiration—ATP production. The overall equation
for the cellular respiration of glucose is as follows:
several steps
glucose oxygen
carbon dioxide water ATP
enzymes
Or using the chemical formula:
C6H12O6 O2
several steps
enzymes
CO2 H2O ATP
In our example, glucose (from sucrose or starch), on
the left side of the equation, represents the chemical energy derived from photosynthesis. On the right side of
this equation, some of the energy ends up in ATP and
some in CO2 and H2O, but most of the energy is lost as
heat (which we have not shown to keep the example simple). Molecular oxygen (O2) is shown because the equation is for aerobic respiration (i.e., with oxygen)—the
kind of respiration that occurs in plants and animals in the
presence of ample O2. The overall efficiency of the
process, in terms of converting the energy stored in glucose into ATP for work of the cell, is not high—perhaps
about 35 to 50% efficiency. Nonetheless, the total energy in
the reactants on the left side is equal to the total energy of
the products on the right side, including heat.
Remember, as you read further, that cellular respiration occurs in plants, fungi, protists bacteria, and animals,
including you. To include respiration in all known organisms, including bacteria, we can now define cellular respiration as the chemically driven flow of electrons through a
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259
Respiration
membrane coupled to ATP synthesis. If the acceptor of
electrons at the end of flow is O2, the process is called
aerobic respiration (or often, just respiration); if some
other molecule accepts the electrons, it is called anaerobic respiration. We return to these definitions throughout the chapter.
O2
Electron transport chain
H2O
(a)
ATP
Starch
Sucrose
Fructose
Some
ATP
Amino
acids
Glycolysis
Glucose
In cellular respiration, chemical energy from sugars,
such as glucose, is used to make ATP. Glucose can be
formed from the breakdown of sucrose or starch. In
plants, aerobic respiration (with oxygen) releases CO2
and requires O2. Cellular respiration has two roles: synthesis of ATP for energy requirements and formation of
carbon skeletons for subsequent biosynthesis of complex molecules.
Cellulose
NADH
ADP
+ Pi
Pyruvate
CO2
Fatty
acids
O2
Electron
transport chain
Acetyl-CoA
H2 O
ATP
Plants make all their organic molecules from the carbon in
CO2. How do animals make their organic molecules?
Some
ATP
Cellular Respiration Includes
Glycolysis, the Krebs Cycle, and the
Electron Transport Chain.
NADH
Krebs
cycle
Ubiquinol
CO2
Amino acids
(b)
FIGURE
Cellular respiration includes three main stages: glycolysis,
Krebs cycle, and the electron transport chain (fig. 11.2a ).
These are discussed in detail later but note that sugars are consumed and CO2 is released during the process as ATP is
formed. Glycolysis is a pathway; the Krebs cycle is a cycle. For
convenience a series of chemical reactions in a cell is called a
pathway or cycle depending on the nature of the steps involved. Pathways “end” in a product or products; cycles also
form products but return molecules to the first events in the
series. Each step of a pathway or cycle—that is, each chemical
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CO2
Electron
carriers
INQUIRY SUMMARY
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Krebs
cycle
ATP
In addition to ATP production, aerobic respiration also supplies the basic building blocks for the manufacture of the
other molecules in the plant. Several intermediate molecules of respiration serve as carbon skeletons for biosynthesis of amino acids, fatty acids, and nucleic acids (see
chapter 3 for information on these and other molecules that
originate from respiration). Thus, cellular respiration, not
only produces ATP but also forms carbon skeletons for
biosynthesis (which is powered by ATP): these are the two
primary roles of cellular respiration.
As you read the more detailed description of the
events in respiration, remember the same chemical reactions occur in you, and, in fact, they are going on right now
in your cells or you would be a goner.
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ATP
Organic
molecules
Cellular respiration forms ATP and carbon
skeletons used in biosynthesis.
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Glycolysis
Sugars
Chlorophyll
Nucleic acids
11.2
(a) Simplified summary of cellular respiration—glycolysis, Krebs cycle, and electron transport chain. (b) Important steps in aerobic, cellular respiration of plants. Sucrose or starch is converted to glucose,
one starting point for aerobic respiration in plants. Upon complete
aerobic respiration of a molecule of glucose, 6 molecules of CO2 are released and up to 38 molecules of ATP can be formed. Oxygen (O2) is
reduced to water in the final step. Glycolysis occurs in the cytosol; the
Krebs cycle and the electron transport chain occur in a mitochondrion.
indicates molecules flow both directions. Cellular respiration
in the presence of oxygen is also called aerobic respiration.
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E L E V E N
11.1 P e r s p e c t i v e P e r s p e c t i v e
The Potential Energy of Glucose
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reaction in the series—is catalyzed by a specific enzyme (see
Perspective 11.1, “The Potential Energy of Glucose”).
Respiration can be considered a series of “baby steps”
that begins with a sugar and progressively releases small
amounts of energy (transferred to ATP) along the way as
the sugar is broken down (oxidized) and CO2 is released. If
all the energy of glucose, for example, was released at once,
it would be very inefficient and even harmful. If you released all the energy in a tank of gasoline at once, your automobile would certainly suffer during the explosion. It is
better to release that energy in a series of baby steps—the
controlled explosions in the cylinders of your engine. The
same is true for organisms. Remember, each “baby step”—
that is, each chemical reaction—is catalyzed by a specific
enzyme that has undergone natural selection during the
evolution of the metabolic pathway or cycle.
As mentioned previously, cellular respiration involves
the breakdown of glucose (or another sugar) with the release
of CO2. When this happens, electrons are released from the
breakdown. These electrons flow to O2, and ATP is produced
as O2 is used during the formation of water. Aerobic respiration is cellular respiration in the presence of oxygen (O2). Figure 11.2b shows this in more detail than seen previously.
Note that important organic molecules, such as amino acids
and chlorophyll, are made from products of respiration.
Glycolysis is the pathway that starts cellular respiration of sugar in the cytosol (fig. 11.3). In glycolysis, glucose,
a 6-carbon sugar, is rearranged, then split in half during the
pathway. When glucose is split, the 3-carbon acid pyruvate
is formed as the product (pyruvate is the ionized or salt
form of pyruvic acid; pyruvate predominates in cells).
Pyruvate then can enter a mitochondrion where it fuels the
Krebs cycle (see fig. 11.2b). Both glycolysis and the Krebs
cycle form some NADH; the Krebs cycle also forms
ubiquinol, an electron carrier. Both processes can produce a
small amount of ATP directly, while neither requires O2 directly. The release of CO2 occurs via reactions in the mitochondria, not during glycolysis in the cytosol.
Most of the ATP from cellular respiration is made by
the electron transport chain in mitochondria (singular is mitochondrion) (fig. 11.2b). Here, energy is released as electrons
flow through the membrane to O2. The energy is used to
drive the synthesis of ATP from ADP plus Pi, like electricity
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the energy is released in small increments
during each step of respiration, the excess
heat does not damage the cell. A mole
refers to the amount of a substance.
Glucose
ATP
ADP
P
Glucose-6-P
Two steps
G
(686 kilocalories) when it is oxidized to
CO2 and H2O. However, cells recover
only a small amount of this energy during
glycolysis; the rest is lost as heat. Because
lucose contains 686 kilocalories
per mole. This means that with
the addition of some energy (i.e.,
energy of activation) to get it going, 1
mole of glucose will yield 686,000 calories
Fructose
ATP
ADP
P
2 NAD+
P
Triose-P
Triose-P
Several
steps
2 NADH
Carbon skeletons
for biosynthesis
4 ADP + Pi
4 ATP
P
2 pyruvate
FIGURE
11.3
Overview of glycolysis. Glycolysis starts with a molecule of glucose, a 6-carbon sugar, and ends with two molecules of pyruvate,
a 3-carbon acid. Two ATPs are used and four are formed in the
process, along with two molecules of the electron transport molecule, NADH. Along with energy conversions, molecules formed
during glycolysis can be drained from the pathway and used to
make other molecules such as amino acids. P is phosphate, attached to the carbon skeleton.
drives a motor. The electrons come from NADH formed during glycolysis and the Krebs cycle and also from ubiquinol,
the related electron transporter from the Krebs cycle. Electrons (e) and protons (H) come from hydrogen (H2 → 2
H 2 e) released from NADH (NADH H → NAD H2) or ubiquinol (ubiquinol → ubiquinone H2).
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Respiration
Electrons from NADH and ubiquinol are passed
through a chain of electron carrier molecules in the mitochondrial membrane to O2, the final electron acceptor. This
synthesis of ATP, via the electron transport chain, is called
oxidative phosphorylation (compare to photophosphorylation, described in chapter 10). Thus, mitochondrial membranes play an important role in coupling energy from the
oxidation of glucose to the phosphorylation of ADP to ATP:
P
Pyruvate
NAD+
Pi
NADH
CO2
CoA
Fatty acid
Acetyl-CoA
CoA
ADP Pi → ATP H2O
CoA
Glycolysis splits glucose, producing some ATP.
NADH
During glycolysis, each 6-carbon glucose molecule is split
into two, 3-carbon pyruvate molecules. The entire process
requires 10 steps, all of which occur in the cytosol (fig. 11.3).
The first step adds Pi (inorganic phosphate from phosphoric acid) to glucose by using one ATP. This glucose plus
phosphate molecule is rearranged, and a second ATP adds
another phosphate before the glucose molecule is split in
half. After glucose is split, each of the two resulting 3carbon sugars has one phosphate attached. These two
triose-phosphate molecules represent the halfway point of
glycolysis. By this point, energy (as two molecules of ATP)
has been used, but no ATP has been made.
The ATP-producing steps of glycolysis occur in the second half of the pathway. First, the reduction of (addition of
electrons to) NAD provides two molecules of NADH. Then,
four molecules of ATP are produced from ADP, and the final
product of glycolysis, pyruvate, remains. Thus, the products
of glycolysis are two molecules of NADH, two molecules of
ATP (net; four are formed, but two are consumed along the
pathway starting with glucose), and two molecules of pyruvate. Along the way, different steps form carbon skeletons
that are drained from the pathway and used in the cell to
make other, more complex, molecules. The functions of glycolysis are to produce ATP, NADH and pyruvate and also the
carbon skeletons for biosynthesis (see figs. 11.2 and 11.3).
NAD+
Ubiquinol
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NAD+
Several
steps
NADH
ATP
ADP
+ Pi
FIGURE
CO2
Carbon skeletons
for biosynthesis
11.4
Overview of the Krebs cycle. In a mitochondrion, pyruvate from
glycolysis undergoes a reaction that forms acetyl-CoA (CoA is just
a carrier; the acetyl is a two-carbon acid group), which enters the
Krebs cycle. For each pyruvate molecule that enters the mitochondria, one ATP, three NADH, and one ubiquinol can be made
along with three molecules of CO2 released (remember that twice
this number can be made per glucose because each glucose splits
into two pyruvate molecules). Also, carbon skeletons can be
drained from the cycle and used for biosynthesis of other complex
molecules.
Glycolysis and the Krebs cycle are linked by the conversion of pyruvate to acetyl CoA in mitochondria. Pyruvate is the product of glycolysis that enters a mitochondrion, but it is the acetyl group of acetyl CoA that actually
enters the Krebs cycle. The 2-carbon acetyl group is the ionized form of acetic acid, or vinegar.
The Krebs cycle, named in honor of the British scientist Sir Hans Krebs (see Perspective 11.3, “Botany and Politics”), is a cycle because, unlike a pathway, the last step regenerates the starter chemicals for the first step. In all, there
are eight enzyme-catalyzed steps of the Krebs cycle that occur in the mitochondria. The combined result of these steps
is the production of ATP, NADH, ubiquinol (ubiquinol is
the mobile product of the Krebs cycle, not FADH2 as shown
in many books), and carbon skeletons for biosynthesis. The
production of these molecules is the function of the Krebs
cycle (see figs. 11.2 and 11.4). Although CO2 is released from
the reactions in the mitochondrion, no O2 is involved yet in
The pyruvate from glycolysis can enter a mitochondrion and
fuel the Krebs cycle (also known as the citric acid cycle because the organic acid citrate is involved in the cycle as an intermediate). However, the pyruvate that is transported into
the mitochondrion is not used in the Krebs cycle directly. Instead, pyruvate first loses a molecule of carbon as CO2, and
some NADH is formed via a chemical reaction (fig. 11.4).
The remaining 2-carbon group, acetyl CoA (CoA is a carrier;
the acetyl group, with two carbon atoms, is the molecule of
interest to us) combines with a 4-carbon molecule to maintain the cycle. Note that when CO2 is released from pyruvate, NAD is reduced to NADH as protons (H) and electrons (e) are combined with NAD.
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CO2
Ubiquinone
The Krebs cycle is fueled by pyruvate
and forms some ATP.
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E L E V E N
11.2 P e r s p e c t i v e P e r s p e c t i v e
Models and Respiration
R
data, but it is easier to visualize the stages of
respiration using models. Remember there
are actually thousands of copies of each enzyme and millions of copies of each intermediate compound that participate in each
chemical reaction of respiration within a
cell. Also remember the entire oxidation of
glucose to CO2 might take less than a second; respiration is fast! One last point about
emember that the models shown in
figures 11.3, 11.4, and 11.5 are just
that, models. Just as a map is not the
territory, models are not the real molecules
or enzymes. We use models to represent the
research that leads to our understanding of
respiration and of the chemical reactions
and enzymes that make up the pathways
and cycles. We could present the original
respiration. Most of the energy derived from the steps of
the Krebs cycle is temporarily contained in the high-energy
electrons of NADH and ubiquinol. The energy in these
molecules which move within a mitochondrion—they are
mobile—is harvested via oxidative phosphorylation in the
third phase of respiration, the electron transport chain.
ATP is formed as electrons flow to oxygen.
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When compared to glycolysis and the Krebs cycle, the
electron transport chain is a bonanza for ATP synthesis.
Energy for ATP synthesis via the chain comes from
NADH and ubiquinol, which are produced by the first
two stages of respiration. The electrons of NADH and
ubiquinol cannot be used directly for ATP synthesis; instead, they start a series of oxidation-reduction reactions
that move electrons through several carriers, which are
proteins in or on the mitochondrial membrane. This
movement of electrons is called electron flow. The sequence of electron carriers is known, appropriately
enough, as the electron transport chain. Energy from
electron flow ultimately drives the synthesis of ATP—
like electricity can set a lightbulb aglow. Oxidative phosphorylation refers to the combination of the oxidationreduction reactions of electron transport that enable a cell
to use the energy in NADH and ubiquinol to convert
ADP plus Pi to ATP. This conversion is called phosphorylation because phosphate (Pi) is added to the molecule, in
this case, ADP, producing ATP.
The electron transport chain (fig. 11.5) is like a series of
tiny, successively stronger magnets; that is, each has a
higher potential than its predecessor. This means that each
component of the chain pulls electrons away from a weaker
neighbor and gives them up to a stronger one. Thus, the
strongest electron acceptor in the chain is the terminal acceptor, molecular oxygen (O2). You can also think of the elec-
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models: most textbooks show the Krebs cycle as a cycle, like a Ferris wheel. However,
the cycle does not actually “turn” but is a series of chemical reactions catalyzed by enzymes. The Krebs cycle does not turn clockwise any more than glycolysis runs vertical
rather than horizontal. Textbooks simply
show it turning to help students understand the principle.
tron transport chain as a bucket brigade, where electrons are
the buckets and the brigade of firefighters are the carriers,
passing buckets (electrons) along to the end—oxygen.
There are at least nine electron carriers in the electron
transport chain of the membrane. The electron carriers are
proteins and complexes of proteins and pigments. Some of
these carriers also accept protons and release them into the
intermembrane space. Although scientists are uncertain
how these components work together during electron and
proton transport, figure 10.14 shows a widely accepted
model for the sequence of steps in the chain.
According to the model, as electrons continue to
pass down the chain between carriers, the energy from
this electron flow drives the transport of protons from
one side of the inner mitochondrial membrane to the
other. The electrons flow to cytochrome a/a3 (cytochrome
oxidase or the terminal oxidase), which, in turn, passes
the electrons to O2, the terminal electron acceptor during
aerobic respiration. The O2 combines with electrons from
the chain and with protons from the mitochondrial matrix, producing water:
O2 4 e- 4 H → 2 H2O
Note that the transport of protons from one side of the
inner mitochondrial membrane to the other requires energy, which is provided by the movement of electrons
through the electron transport chain (fig. 11.5).
ATP synthesis is driven by electron flow
that generates a proton gradient.
How do the protons that are pumped by the electron transport chain provide energy for ATP synthesis? British scientist and Nobel Prize winner Peter Mitchell suspected
that the membrane must have a role in ATP synthesis and
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Respiration
Mitochondrion
Intermembrane
space
2 H+
2 H+
+
2H
2 H+
e−
Inner
mitochondrial
membrane
Cyt c
Cyt c 1
FeS
e−
H+
ATP synthase
complex
Cyt a
FeS
Cyt b
Cyt a 3
FMNH 2
e−
NADH + H+
Cytochrome b–c 1
complex
Ubiquinol
2 H+
NADH
dehydrogenase
complex
NAD +
Cytochrome
oxidase complex
2 H+ +
2 H+
1
2
O2
H 2O
ADP + Pi
H+
ATP
Matrix
FIGURE
11.5
Overview of the electron transport chain. Moving from the Krebs cycle, NADH and ubiquinol (and from glycolysis for NADH) donate
electrons to the chain. As electrons pass from carrier to carrier in or on the membrane, the energy of electron flow is used to concentrate
(pump) protons into the intermembrane space. As the electrons flow to the final acceptor, O2, the protons flow back through the ATP synthase complex and, in doing so, the synthesis of ATP—oxidative phosphorylation. In theory, each NADH and each ubiquinol can drive
the synthesis of three and two ATP molecules, respectively.
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developed a hypothesis to explain ATP synthesis in mitochondria (fig. 11.5). As explained, carriers of the electron
transport chain are embedded in or on the inner mitochondrial membrane. The flow of electrons through this
series of carriers is used to actively pump protons across
the membrane into the intermembrane space. Because the
membrane is not very permeable to protons, relatively few
protons leak back across the membrane. Continued electron transport pumps more and more protons into the
space between the membranes of the mitochondria. This
creates a gradient of protons across the membrane, with a
much higher concentration of protons accumulating in the
intermembrane space. According to Mitchell’s model, that
gradient of protons drives the synthesis of ATP. The high
concentration of protons functions like a battery to do
work, including making ATP.
Other scientists used electron microscopy to show that
ATP synthase, the enzyme that makes ATP, protrudes into
the matrix. Thousands of copies of the ATP synthase occur
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throughout the membrane, making the mitochondrial
membrane a powerhouse for making ATP. The proton gradient is unstable, with the ever-increasing concentration of
protons in the intermembrane space “pushing” out on the
membrane. The pressure reaches a point where it must be
relieved, like filling a water balloon to the maximum. Pressure from the gradient is relieved as protons escape through
ATP synthase. The energy from the flow of escaping protons
is used to make ATP by combining ADP and Pi (fig. 11.5).
This is like water from the bottom of a reservoir rushing out
through a drainpipe in the dam and turning a turbine that
makes electricity. The water in the reservoir represents the
high concentration of protons on one side of the membrane,
the dam represents the mitochondrial membrane, the rush
of water is the proton flow, the turbine is the ATP synthase,
and the electricity is ATP. In fact, ATP synthase can be considered a tiny, molecular turbine that spins as protons rush
through and uses the energy from the spin to make ATP
from ADP plus Pi.
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E L E V E N
11.3 P e r s p e c t i v e P e r s p e c t i v e
Botany and Politics
S
dered back to work on cancer by Hitler,
who feared dying of cancer. Warburg
stayed in Germany because he thought his
work would be destroyed if he left. Although Warburg’s pact with the Nazis incensed his colleagues outside of Germany,
his research was brilliant. For example,
Warburg’s 1935 discovery that nicotinamide is the active group of hydrogentransferring molecule, NADH, would
have earned him a second Nobel Prize, but
Hitler forbade German citizens from accepting the award. Nevertheless, Warburg
continued to study cancer, photosynthesis,
and respiration until his death in 1970.
Nazi policies also affected other German scientists. One of these scientists was
cience is a search for truth. Although
truth is not affected by politics, the
business of science often is affected.
Consider, for example, Otto Heinrich Warburg, a German biochemist. Warburg began his scientific career in the early 1900s
and in 1924 announced his discovery of
“iron oxygenase,” the oxygen carrier that
we now call cytochrome oxidase. In 1931,
Warburg was awarded a Nobel Prize for
this work. However, less than two years
later, Hitler was appointed chancellor of
Nazi Germany. Although Warburg was
half Jewish, he was unharmed by the
Nazis because he studied cancer. Indeed,
when Nazis removed Warburg from his
position in 1941, he was personally or-
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Just how powerful is this flow of electrons and gradient of protons? That is, how much ATP can be made by
the electron transport chain? Scientists have estimated
that each NADH might drive the synthesis of up to three
ATPs, while each ubiquinol might drive the synthesis of
up to two ATPs. When there is sufficient O2 present, each
molecule of glucose, if it goes all the way through aerobic
respiration, has the net potential to form as many as 38
molecules of ATP. Up to 34 of these ATPs can come from
the electron transport chain when powered by NADH and
ubiquinol; however, this impressive number can be realized only when sufficient O2 is present to act as the terminal electron acceptor. Put another way, under ideal conditions, glycolysis and the Krebs cycle can net only four
ATPs per glucose; the other 34 come from the chain. The
electron transfer chain is the clear champion of ATP production in cells when O2 is present.
Note that O2, which is essential to most life as we
know it, comes into the respiration picture only at the
very end of the three stages. In humans, think about all
the anatomical and biochemical adaptations that have
evolved to deliver that O2 to the end of the electron transport chains in our cells; you might list noses, lungs, arteries, red blood cells, and hemoglobin among these
adaptations. In plants, elaborate systems of aerenchyma
(chapter 9) have evolved as a mechanism for air to reach
submerged roots. Note also that CO2 is released from glucose during respiration, but only in mitochondria, and
only if O2 is present to receive electrons from the first two
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Hans Adolf Krebs, who assisted Warburg
in Berlin from 1926 to 1930. In 1932, using
much of what he learned from Warburg,
Krebs announced the first metabolic cycle
ever to be described. However, the next
year (1933), Krebs was fired by the Nazis
and fled from Nazi Germany to Britain.
Only four years after arriving in Britain,
Krebs announced his discovery of the “tricarboxylic acid cycle,” a pathway used by
almost all organisms to convert two-carbon
fragments to carbon dioxide, water, and energy. This pathway, which later became
known as the Krebs cycle, earned Krebs a
Nobel Prize in 1953. Krebs was knighted in
1958, making him Sir Hans Krebs. Krebs
worked in Britain until his death in 1981.
stages. What happens if there is insufficient O2 present?
For us, bad things, as you know. But what happens in
plants? We turn to that question next.
INQUIRY SUMMARY
During glycolysis, glucose is split into pyruvate, and
some ATP and NADH are formed along with carbon
skeletons for biosynthesis. In the presence of sufficient oxygen, pyruvate from glycolysis moves into
mitochondria and is converted into acetyl CoA, which
enters the Krebs cycle. Each “turn” of the Krebs cycle
uses one molecule of acetyl CoA and produces ATP,
ubiquinol, NADH, and carbon skeletons for biosynthesis. The six carbons that make up glucose are released as six molecules of CO2.
Electrons from NADH and ubiquinol are donated to the electron transport chain in which electron carriers in the mitochondrial membrane pass
electrons to O2 and pump protons to the intermembrane space. The resulting proton gradient produced
by electron transport provides the energy necessary
for making ATP. The complete aerobic respiration of
one molecule of glucose can produce as many as 38
molecules of ATP.
Which part of respiration probably evolved first? Explain
your answer.
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produces an organic product, such as lactate or alcohol,
along with small amounts of ATP. Only lactate fermentation
occurs in animals.
Although O2 is required only at the end of electron
transport during aerobic respiration, the electron transport
chain and the Krebs cycle are both inhibited when oxygen is
low or not available. For animals, certain fungi, some bacteria, and a few plants, pyruvate is converted to lactate (lactic
acid, a 3-carbon organic acid). This step, which forms lactate,
uses excess NADH produced during glycolysis (fig. 11.6)
and returns NAD to glycolysis. NAD is a reactant needed
in a critical step of the glycolysis pathway. The return of
NAD allows some ATP formation in the absence of O2 and
probably explains why fermentation has not been removed
by natural selection as plants and animals have evolved.
When lactate is formed in humans, it is most often because we exercise at a rate exceeding our ability to deliver
oxygen to our muscle cells. The accumulation of lactate is
painful, and we stop exercising until we “catch our breath,”
thereby allowing the lactate to diffuse out of the muscle
cells to the liver. If you are in shape, you probably have a
more efficient system to deliver and use the O2. If brain or
heart cells are deprived of O2 for more than a few minutes,
stroke or heart damage may occur.
In some fungi, such as yeast, and in most plants, pyruvate from glycolysis is converted to ethanol and CO2 (fig.
11.6). These anaerobic reactions are called alcoholic fermentation. Recall that fermentation to lactate (lactate fermentation) or ethanol (alcoholic fermentation) returns NAD to
a step of glycolysis and allows the pathway to continue (fig.
11.6). Note the release of CO2 during alcoholic fermentation, making the process essentially irreversible. Alcoholic
fermentation in plants and yeast leads to the buildup of
ethanol, which is toxic to the organism at levels around
10%. This is why wine has a maximum natural alcohol content of about 10%.
Unless they have adaptations (such as aerenchyma—
see chapters 4, 7, and 8) to facilitate diffusion of air (with O2)
to their roots, many plants ferment when they grow in mud
or in oxygen-poor water; this is why houseplants die when
they are overwatered or seeds do not germinate when submerged. Fermentation in plants can be lethal because of the
potential buildup of ethanol; it is also inefficient because it
produces relatively little ATP compared to aerobic respiration. In fact, fermentation of glucose to ethanol in a plant
produces just two new ATP molecules per glucose molecule. Can you estimate how many ATP molecules could be
made from fermentation of a molecule of sucrose? How
does this compare to aerobic respiration of sucrose?
Fermentation was probably the first form of energyconverting metabolism to evolve, perhaps more than three
billion years ago. Following the evolution of photosynthesis, O2 became available and aerobic respiration evolved.
This provided a selective advantage for the evolution of eukaryotic cells, and therefore plants. Thus, while fermentation seems less important to most organisms today, it was
OH
2 ADP
+
2 P
Glucose
2 ATP
Glycolysis
2 NAD +
C
O
C
O
CH 3
2 pyruvate
2 NADH
+ 2 H+
Low
O2
H
H
C
2 CO 2
H
OH
C
CH 3
2 ethanol
Sufficient
O2
O
Krebs
cycle
CH 2
2 acetaldehyde
(a) Fermentation to produce ethanol
2 ADP
+
2 P
Glucose
OH
C
O
C
O
Glycolysis
2 NAD +
O
H
2 ATP
C
O
C
OH
2 NADH
+ 2 H+
CH 3
2 pyruvate
Sufficient
O2
CH 3
Low O 2
2 lactate acid
Krebs
cycle
(b) Fermentation to produce lactate
FIGURE
11.6
Anaerobic respiration usually produces ethanol and carbon dioxide in plants and certain fungi (a); in other fungi, some bacteria
and in animals, it produces lactate (b).
Fermentation Can Occur When O2
Levels Are Low.
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The foregoing discussion of respiration focuses on the main
type of respiration in most organisms. This respiration is
called aerobic respiration because it requires oxygen as the
terminal electron acceptor. In some bacteria, other molecules have evolved as electron acceptors in a process called
anaerobic respiration because O2 is not the final electron acceptor at the end of the electron transport chain. However,
in plants and animals, including us, when O2 is insufficient,
the pathway of cellular respiration backs up to pyruvate,
and the process called fermentation occurs (fig. 11.6). Fermentation includes glycolysis and a step after pyruvate that
forms either lactate, ethanol, or (in some bacteria) other
lesser-known products of fermentation. We define fermentation as an anaerobic process (meaning no O2 required)
that involves the first stage of respiration (glycolysis) but
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E L E V E N
critical in the evolution of life. How was ATP important in
this evolution?
Fermentation is economically important. For example,
fermentation by bacteria and fungi is important for producing and flavoring cheese and yogurt. Similarly, fermentation
by yeast is important for making alcoholic beverages and
bread; wine, beer, and bread are made by different strains of
brewer’s yeast (Saccharomyces cerevisiae). The CO2 produced
during alcoholic fermentation makes bread rise and forms
the holes in the loaf, but the alcohol from fermentation evaporates when the bread is baked. Wine is usually made by
yeast that grows on grapes, although fermented honey
(mead), apples (cider), and other carbohydrate-rich substrates also are used to make wine or winelike beverages.
Likewise, although beer is usually made by fermenting barley, rice beer is common in the Orient, and corn beer is made
by the Tarahumara tribe of northern Mexico.
NADH
FMN
FeS
Ubi
Cyt b
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Terminal oxidase
inhibited by cyanide
Cyt c
Cyt a
Cyt a 3
Cyanideresistant
electron
transport
Normal
electron
transport
eLittle, if
any, ATP
eHeat
What might be the selective advantage of switching to fermentation when oxygen is low?
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Cyt c 1
e-
When O2 levels are low, the flow of carbon is backed up
to pyruvate. To cycle NAD, carbon from pyruvate
flows to ethanol or lactate in an anaerobic process
called fermentation. In plants, alcoholic fermentation
can lead to production of some ATP but the buildup of
ethanol can be toxic.
In early laboratory experiments, it was discovered that aerobic respiration can be strongly inhibited when the terminal
electron carrier, cytochrome oxidase (see figs. 11.6 and 11.7),
combines with cyanide or other poisons. However, later it
was discovered that in many plant tissues, respiration continues despite the addition of cyanide. Such continued respiration was originally called cyanide-resistant respiration
for its discovery in a botany laboratory. Certain plants,
fungi, and bacteria have cyanide-resistant respiration, but it
is rare in animals.
The reason respiration does not stop when cytochrome
oxidase is poisoned is that plant mitochondria often have an
alternative short chain of electron carriers that branches from
the main chain. Because this branching chain also ends in an
oxidase, it is aerobic; that is, oxygen is also the terminal electron acceptor in this alternative pathway or chain (fig. 11.7).
This oxidase is called the alternative oxidase, and the pathway is called the alternative respiratory pathway or chain. Little
or no oxidative phosphorylation is coupled to this alternative chain, however, so the energy from the oxidation of
NADH and ubiquinol in this pathway produces heat instead
Considerable
ATP
FeS
Alternate
oxidase
INQUIRY SUMMARY
Cyanide-Resistant Respiration Is
Important in Some Plants.
Ubiquinol
1
2
O2
2 H+
FIGURE
H 2O
H 2O
2 H+
1
2
O2
11.7
Model of electron transport chain for cyanide-resistant respiration. As in cyanide-susceptible electron transport (see fig. 11.5),
the final carrier of cyanide-resistant electron transport is an oxidase, because the terminal electron acceptor is oxygen.
of ATP. Although cyanide resistance was discovered over 20
years ago, the alternative oxidase has proven difficult to
study, and relatively little is known about it.
In nature, alternative respiration is active in sugarrich cells, where glycolysis and the Krebs cycle occur unusually fast. This observation provides indirect evidence
that the main electron transport chain becomes saturated
and the alternative pathway takes up the overflow of electrons. Both the normal and the alternative pathways may
operate simultaneously and naturally. Thus, the main benefit of this alternative path of electron flow for many plants
is probably that it dissipates excess energy from respiration.
In some tissues, such as leaves and roots of spinach or pea,
up to 40% of total respiration may occur via the alternative
pathway. Remember that the application of cyanide to
plant mitochondria in a laboratory led to the discovery but
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267
11.4 P e r s p e c t i v e P e r s p e c t i v e
How Does Skunk Cabbage Get So Hot?
S
kunk cabbage spends the spring and
summer packing starch into its fleshy
rhizome. The rhizome then acts as a
furnace during late winter by providing the
energy needed to heat the flowering shoot
and melt the snow around it as it grows up
through the snow. Skunk cabbage burns
this fuel and uses oxygen at a rate comparable to that of a hummingbird. Heat is conserved by a thick, spongy structure that acts
as an insulator around the flowering shoot.
The overall effect is that the flowers develop
in a near-tropical climate even though the
plant is buried in snow.
These plants have an alternate form
of respiration in their mitochondria. This
alternate form of respiration generates so
much heat that it is costly to the plant;
moreover, it makes little or no ATP. There
is no evidence that getting such an early
jump on spring gives skunk cabbage an
advantage over other plants. Why, then,
does skunk cabbage get so hot? One possible explanation is an evolutionary one;
that is, heat-generating, alternate respiration was passed down unchanged from
the ancestors of skunk cabbage.
To study this hypothesis, botanists
have compared skunk cabbage with its
Skunk cabbage.
tropical relatives, which include many
well-known houseplants, such as caladiums, philodendrons, voodoo lilies, and
dumbcanes. These plants, as well as
skunk cabbage, belong to the Arum family (Araceae), most of whose members
still live in the tropics. When some aroids
heat up, they vaporize foul odors that diffuse into the air around the flowering
shoot. These odors include such chemicals as putrescine, cadaverine, and ska-
that the alternative pathway operates naturally in plants,
independent of cyanide.
In some plants, such as the aroid lilies, the amount of
heat generated from an alternative pathway of respiration
is remarkable. For example, in the eastern skunk cabbage
(Symplocarpus foetidus), this heat can raise temperatures in
some parts of the plant more than 20°C above the air temperature. Such extreme temperatures are important in the
ecology of these plants (see Perspective 11.4, “How Does
Skunk Cabbage Get So Hot?”).
How might the alternative pathway be critical to alpine plants?
Lipids Are Respired in Some
Germinating Seeds.
Lipids, which are made mostly of fatty acids (see chapter 3),
are important energy and carbon storage compounds in
oil-containing seeds. When such seeds germinate, their
fatty acids are metabolized in organelles called glyoxysomes
that allow the plant to utilize the energy and carbon stored
in the fat, usually an oil such as in coconut, soybean, corn,
or safflower. In germinating seeds, fatty acids are first removed from glycerol in stored fats (oils), then snipped into
two-carbon pieces that form acetyl CoA (fig. 11.8; recall that
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Some plants contain an alternative pathway of respiration
that was discovered by an insensitivity to cyanide that
normally is lethal to cells. The alternative pathway may
be important for dissipation of excess energy in a plant. In
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What kinds of information and experiments
would help you answer this question?
others, the alternative pathway may generate heat important in the growth and reproduction of the plant.
INQUIRY SUMMARY
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tole. They smell like dung or decaying
flesh, which attracts flies and other insects seeking suitable places for laying
their eggs. Instead of finding places to lay
their eggs, however, the flies and insects
are often trapped temporarily by the
plant and become covered with pollen.
After they are released, they may get
trapped again and leave pollen in another
flowering shoot. In this way, they transfer
pollen from one flower to another.
Unlike its tropical relatives, skunk
cabbage does not seem to benefit from its
vaporized fragrance by tempting pollinators. For one thing, few insects are active
when skunk cabbage is hot. Also, although skunk cabbage smells a lot like its
tropical relatives, it has no mechanism for
trapping insects.
The heat-generating ability of
skunk cabbage and some of its relatives
has intrigued botanists for decades. Is the
heat-generating process an adaptation for
surviving cold weather, or is it merely an
evolutionary remnant of a feature of some
tropical plants?
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268
E L E V E N
cules in a plant. The most common kind of cellular respiration in plants is called aerobic respiration, which requires O2
to produce ATP and release CO2. Aerobic respiration functions to form ATP and carbon skeletons for biosynthesis.
Triacylglyceride (oil)
Glycerol
Fatty acids
Aerobic respiration of sugars occurs in three stages:
1. During glycolysis, glucose is broken down into pyruvate. This occurs exclusively in the cytosol and converts some of the energy stored in glucose to ATP
and NADH.
2. The Krebs cycle occurs in the mitochondria. First,
pyruvate is converted into acetyl CoA, which, in turn,
enters the Krebs cycle—a series of reactions that produce ATP, NADH, ubiquinol, and release CO2.
3. The electron transport chain receives energy-rich electrons from NADH and ubiquinol. Electrons flow
through a series of carriers that release energy at each
step. The terminal electron acceptor is O2, which is reduced to H2O. The energy of electron flow is used to
pump protons that accumulate, then stream back
through ATP synthase complexes, forming ATP. This
synthesis of ATP from the energy of electron transport
is called oxidative phosphorylation.
NADH
Acetyl-CoA
Sucrase
FIGURE
Krebs cycle
11.8
Respiration of lipids. Triacylglycerides are hydrolyzed into glycerol and fatty acids. Fatty acids are degraded into acetyl groups
that are attached to coenzyme A, which can be used in respiration
or for carbohydrate synthesis. NAD is also reduced.
CoA serves as a carrier of the 2-carbon acetyl group). This
reaction is repeated for every pair of carbon atoms until all
of the fatty acid is converted into acetyl-CoA molecules.
Thus, a molecule of stearic acid, which has 18 carbons,
yields nine molecules of acetyl CoA.
Acetyl CoA from lipids, which is the same molecule
described previously that is formed from pyruvate, can be
used in the Krebs cycle and generate ATP. However, in germinating seeds, most of the acetyl CoA from the fatty acids
is converted to sucrose and exported to developing parts of
the germinated seed. In this way, the stored lipid acts as a
reservoir of carbon and energy for the new seedling—the
shoot and root—to use until it becomes photosynthetic. Because fatty acids are not soluble in water, they are converted
to sucrose that flows readily through the vascular tissue of
the young seedling. What might be the selective advantage
of storing carbon and energy in oil rather than starch?
Fermentation occurs in low levels or the absence of
oxygen and produces limited amounts of ATP. An alternative pathway that is not inhibited by cyanide may provide
some selective advantage to certain plants. Lipids are
stored in seeds as a source of energy and carbon. Upon germination, most of the carbon in the oil is used to make sucrose for export from the germinating seed.
Writing to Learn Botany @
What are the ecological and evolutionary advantages and
disadvantages of aerobic versus anaerobic respiration?
Thought Questions
Summary
SUMMARY
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Photosynthesis and respiration are the primary energy
transformations in organisms. Together, they comprise a
system by which energy and carbon flow through an
ecosystem and energy is ultimately converted to heat. Unlike energy, nutrients cycle in an ecosystem.
Cellular respiration harvests energy from organic
molecules and converts the energy to ATP. Respiration also
forms carbon skeletons for biosynthesis of complex mole-
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THOUGHT QUESTIONS
1. What is aerobic respiration? How does it depend on
oxygen?
2. What is CO2? How is carbon passed between organisms in an ecosystem?
3. Which contains more energy for use by a cell, a molecule
of ATP or a molecule of glucose? How do you know?
4. Does the Krebs cycle turn clockwise? Explain.
5. What is anaerobic respiration and fermentation? What
are the most important cellular roles of fermentation?
6. Suggest reasons why the absence of oxygen inhibits
both electron transport and the Krebs cycle but not
glycolysis.
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7. Without distillation, wine and other fermentation
products do not exceed about 10% to 12% alcohol.
Suggest reasons why fermentation does not continue
after this concentration of alcohol is reached.
8. How are lipids broken down in plant cells?
9. Where might an organism live if it could respire only
anaerobically?
10. What is the importance of compartmentation of the
reactions within organelles during respiration?
269
Wivagg, D. 1987. Research reviews: How many ATPs per
glucose molecule? American Biology Teacher 49:113–14.
Books
Dey, P. M., and J. B. Harbourne, eds. 1998. Plant Biochemistry. NY: Academic Press.
Hopkins, W. G. 1999. Introduction to Plant Physiology, 2d ed.
New York: John Wiley and Sons.
Salisbury, F. B., and C. W. Ross. 1992. Plant Physiology, 4th
ed. Belmont, CA: Wadsworth.
Taiz, L., and E. Zeiger. 1998. Plant Physiology, 2d ed. Sunderland, MA: Sinauer.
Suggested Readings
SUGGESTED READINGS
Articles
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Amthor, J. S. 1991. Respiration in a future, higher-CO2
world: Opinion. Plant, Cell and Environment
14:13–20.
Cammack, R. 1987. FADH2 as a “product” of the citric acid
cycle. Trends in Biochemical Sciences 12:377.
McIntosh, L. 1994. Molecular biology of the alternative oxidase. Science 105:781–86.
Moller, I., and A. Rasmusson. 1998. The role of NADP in the
mitochondrial matrix. Trends in Plant Science 3:21–27.
Ryan, M. 1991. Effects of climate change on plant respiration. Ecological Applications 1:157–67.
Storey, R. D. 1991. Textbook errors and misconceptions in
biology: Cell metabolism. American Biology Teacher
53:339–43.
Storey, R. D. 1992. Textbook errors and misconceptions in
biology: Cell energetics. American Biology Teacher
54:161–66.
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On the Internet
ON THE INTERNET
Visit the textbook’s accompanying web site at
http://www.mhhe.com/botany to find live Internet links for each
of the topics listed below:
Glycolysis
Krebs Cycle
Hans Krebs
Electron transport chain
Aerobic respiration
Anaerobic respiration
Fermentation
Yeast and alcohol
Respiration and the germination of seeds
Respiration and heat generation by plants
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