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Energy for Cells
CHAPTER
7
OUTLINE
7.1 Cellular Respiration
• The breakdown of glucose to CO2 and H2O
during cellular respiration drives the synthesis
of ATP. 98
• The complete breakdown of glucose requires
four phases: three metabolic pathways and one
individual enzymatic reaction. 99
7.2 Outside the Mitochondria: Glycolysis
• Glycolysis is a metabolic pathway that
partially breaks down glucose outside the
mitochondria. 101
Anaerobic cellular
respiration makes
muscles ache.
7.3 Inside the Mitochondria
Carbon monoxide and
cyanide kill by stopping
cellular respiration.
• The preparatory reaction and the citric acid
cycle, which occur inside the mitochondria,
continue the breakdown of glucose products
until carbon dioxide and water result. 102
• The electron transport chain, which receives
electrons from NADH and FADH2, produces
most of the ATP during cellular respiration. 104
• Other nutrients in addition to glucose can be
broken down to drive ATP synthesis. 106
7.4 Fermentation
• Fermentation is a metabolic pathway that
partially breaks down glucose under anaerobic
conditions. 107
If a diet sounds too good
to be true, it probably is.
In American society, we think a lot about weight loss.
Everywhere we look, advertisements claim that some
product will rev up our slow metabolism and cure our
weight problems for good. In the meantime, our society
has more obese individuals than ever. A “miracle cure” for
being heavy often comes in the form of a very odd diet or
a nutritional supplement. Have you ever considered the
science behind how these products claim to work? When
97
you do, you will see that many of these claims
simply can’t be verified. Yet millions of people spend
a fortune hoping for a “miracle,” and many of them
suffer health problems as a result of fad diets or the
use of nutritional supplements.
Numerous nutritional supplements are supposed to
allow you to eat everything you want, with no need
to exercise. It sounds nice, but the reality is that our
cells don’t work that way! In order to acquire the
energy you need to live, cells break down glucose in
a process called cellular respiration. Both the glucose
and the oxygen needed for cellular respiration are
provided by the process of photosynthesis in plants.
In this chapter, you will learn how cells use glucose
to produce the ATP they need. Understanding
this process will then help you make informed
decisions about weight loss, diets, and nutritional
supplements.
oxy
ge
n
Cellular Respiration
7.1
Whether you go skiing, take an aerobics class, or just hang out, ATP molecules
provide the energy needed for your muscles to contract. ATP molecules are
produced during cellular respiration, a process that requires the participation
of mitochondria. Cellular respiration is aptly named because just as you take
in oxygen (O2) and give off carbon dioxide (CO2) during breathing, so do the
mitochondria in your cells (Fig. 7.1). In fact, cellular respiration, which occurs
in all cells of the body, is the reason you breathe.
Oxidation of substrates is a fundamental part of cellular respiration. In
living things, oxidation doesn’t occur by the addition of oxygen (O2). Instead,
oxidation is the removal of hydrogen atoms from a molecule. As cellular
respiration occurs, hydrogen atoms are removed from glucose (and glucose
products) and transferred to oxygen atoms, forming carbon dioxide (CO2) and
water (H2O):
Oxidation
C6H12O6
glucose
+
6 O2
6 CO2
CO2
gl u c o s e
matrix
intermembrane
space
Glucose from our food and the oxygen we breathe are
requirements for cellular respiration, a process completed
within the mitochondria.
98
+
energy
The breakdown of glucose releases a lot of energy. If you mistakenly
burn sugar in a skillet, the energy escapes into the atmosphere as heat. A cell is
more sophisticated than that. In a cell, glucose is broken down slowly—not all
at once—and the energy given off isn’t all lost as heat. Hydrogen atoms
are removed bit by bit, and this allows energy to be captured and
used to make ATP molecules.
double membrane
Cellular respiration.
6 H2O
Reduction
H2O
Figure 7.1
+
cristae
outer membrane
inner membrane
Chapter 7 Energy for Cells
99
Phases of Complete Glucose Breakdown
The enzymes that carry out oxidation during cellular respiration are assisted by nonprotein helpers called coenzymes. As glucose is oxidized, the coenzymes NAD⫹
and FAD1 receive hydrogen atoms (Hⴙ ⫹ eⴚ) and become NADH and FADH2,
respectively (Fig. 7.2).
During these phases, notice where CO2 and H2O are produced.
•
Glycolysis, which occurs in the cytoplasm outside the mitochondria, is the
breakdown of glucose to 2 molecules of pyruvate. Oxidation results in NADH,
and there is enough energy left over for a net gain of 2 ATP molecules.
•
The preparatory (prep) reaction takes place in the matrix of
mitochondria. Pyruvate is broken down to a 2-carbon acetyl
group carried by coenzyme A (CoA). Oxidation of
pyruvate results in not only NADH, but also CO2.
•
The citric acid cycle also takes place in the matrix of
mitochondria. As oxidation occurs, NADH and FADH2
result and more CO2 is released. The citric acid cycle
is able to produce 2 ATP per glucose molecule.
1
1. Why is breathing necessary to cellular respiration?
2. Explain why glucose is broken down slowly, rather than
quickly, during cellular respiration.
3. List the four phases of complete glucose breakdown.
The electron transport chain requires a series of
electron carriers in the cristae of mitochondria. NADH
and FADH2 give up electrons to the chain. Energy
is released and captured as the electrons move from
a higher energy to a lower energy state. Later, this
energy will be used for the production of ATP. Oxygen
(O2 ) finally shows up here as the last acceptor of
electrons from the chain. Combination with hydrogen
ions (Hⴙ) produces water (H2O).
Answers: 1. Breathing takes in oxygen needed for cellular respiration and rids
the body of carbon dioxide, a waste product of cellular respiration. 2. Slow
breakdown allows much of the released energy to be captured and utilized by
the cell. 3. Glycolysis, the preparatory reaction, the citric acid cycle, and the
electron transport chain.
•
Check Your Progress
NAD = Nicotinamide adenine dinucleotide; FAD = Flavin adenine dinucleotide
Cytoplasm
NADH and
FADH2
Figure 7.2
Glycolysis
glucose
e–
pyruvate
Citric acid
cycle
Preparatory reaction
b.
a.
c.
2
ATP
a. The enzymatic
reactions of glycolysis
take place in the
cytoplasm. b. The
preparatory reaction,
(c) the citric acid cycle,
and (d) the electron
transport chain occur in
mitochondria.
Electron
transport
chain
d.
2
ATP
34
The four phases of
complete glucose
breakdown.
ATP
100
Part I The Cell
Cytoplasm
NADH and
FADH2
e–
Glycolysis
Preparatory reaction
Citric acid
cycle
Electron
transport
chain
Energy-investment steps
glucose
2
ATP
2
ATP
34
ATP
–2 ATP
ATP
ATP
ADP
ADP
P
P
P
P
G3P
G3P
Energy-harvesting steps
P
P
NAD+
NADH
P
P
NAD+
P
P
BPG
BPG
+2 ATP
ATP
NADH
ADP
ATP
ADP
P
P
3PG
RuBP
ribulose 1,5-bisphosphate
3PG
3-phosphoglycerate
BPG
1,3-bisphosphoglycerate
G3P
glyceraldehyde-3-phosphate
3PG
H 2O
H 2O
P
P
+2 ATP
Figure 7.3
ATP
ADP
ADP
Glycolysis.
This metabolic pathway begins with glucose and ends
with pyruvate. A net gain of 2 ATP molecules can be
calculated by subtracting those expended during the
energy-investment steps from those produced during the
energy-harvesting steps.
Net gain:
2 ATP
pyruvate
pyruvate
ATP
Chapter 7 Energy for Cells
7.2
101
Outside the Mitochondria:
Glycolysis
In eukaryotes, such as plants and animals, glycolysis takes place within the
cytoplasm outside the mitochondria. During glycolysis, glucose, a C6 molecule, is broken down to 2 molecules of pyruvate, a C3 molecule. Glycolysis
is divided into (1) the energy-investment steps when ATP is used, and (2) the
energy-harvesting steps, when NADH and ATP are produced (Fig. 7.3).
Energy-Investment Steps
During the energy-investment steps, 2 ATP transfer phosphate groups to substrates, and 2 ADP 䊊
P result. In other words, ATP has been broken down,
not built up. However, the phosphate groups activate the substrates so that
they can undergo reactions.
Energy-Harvesting Steps
During the energy-harvesting steps, substrates are oxidized by the removal of
hydrogen atoms, and 2 NADH result.
Oxidation produces substrates with high-energy phosphate groups, which
are used to synthesize 4 ATP. As a phosphate group is transferred to ADP, ATP
results. The process is called substrate-level ATP synthesis (Fig. 7.4).
What is the net gain of ATP from glycolysis? Confirm that 2 ATP are
used to get started, and 4 ATP are produced by substrate-level ATP synthesis.
Therefore, there is a net gain of 2 ATP from glycolysis.
If oxygen is available, pyruvate, the end product of glycolysis, enters
mitochondria, where it undergoes further breakdown. If oxygen is not available,
pyruvate undergoes reduction. In humans, if oxygen is not available, pyruvate is
reduced to lactate, as discussed on page 107.
enzyme
P
P
ADP
P
ATP
Figure 7.4
Substrate-level ATP synthesis.
The net gain of 2 ATP from glycolysis is the result of substrate-level
ATP synthesis. At an enzyme’s active sites, ADP acquires a phosphate
group from a substrate, and ATP results.
Glycolysis
inputs
outputs
glucose
2 pyruvate
2 NAD+
2 NADH
2 ATP
4 ATP
Check Your Progress
2 ATP
net
1. Contrast the energy-investment steps of glycolysis with
the energy-harvesting steps.
2. What happens to pyruvate when oxygen is available in
a cell?
Answers: 1. During the energy-investment steps, ATP breakdown provides the
phosphate groups to activate substrates. During the energy-harvesting steps,
NADH and ATP are produced. 2. Pyruvate enters the mitochondria for further
breakdown.
ADP + P
102
Part I The Cell
7.3
Inside the Mitochondria
The other three phases of cellular respiration occur inside the mitochondria
(Fig. 7.5).
Preparatory Reaction
cytoplasm
location of glycolysis
Occurring in the matrix, the preparatory (prep) reaction is so called because
it produces a substrate that can enter the citric acid cycle. The preparatory reaction occurs twice per glucose molecule because glycolysis results in 2 pyruvate
molecules. During the prep reaction:
•
•
•
Pyruvate is oxidized, and a CO2 molecule is given off. This is part of the
CO2 we breathe out!
NAD accepts a hydrogen atom, and NADH results.
A C2 acetyl group is attached to coenzyme A (CoA), forming acetylCoA.
The Citric Acid Cycle
outer membrane
matrix
cristae
70,000
The citric acid cycle is a cyclical metabolic pathway located in the matrix of
mitochondria (Fig. 7.6). It was originally called the Krebs cycle to honor the
man who first studied it. At the start of the citric acid cycle, the C2 acetyl group
carried by CoA joins with a C4 molecule, and a C6 citrate molecule results. The
CoA returns to the preparatory reaction to be used again.
During the citric acid cycle:
•
matrix
location of the prep
reaction and the citric
acid cycle
•
•
The acetyl group is oxidized, and the rest of the CO2 we
breathe out per glucose molecule is released.
Both NAD and FAD accept hydrogen atoms, resulting in
NADH and FADH2.
Substrate-level ATP synthesis occurs (see Fig. 7.4), and an
ATP results.
Because the citric acid cycle turns twice for each original glucose
molecule, the inputs and outputs of the citric acid cycle per glucose
molecule are as follows:
inner membrane
forms cristae
Citric acid cycle
intermembrane
space
cristae
location of the electron
transport chain
inputs
outputs
2 acetyl-CoA
4 CO2
6
NAD+
2 FAD
Figure 7.5
Mitochondrion structure and function.
A mitochondrion is bounded by a double membrane. The inner membrane
invaginates to form the shelflike cristae. Glycolysis takes place in the cytoplasm
outside the mitochondria. The preparatory reaction and the citric acid cycle
occur within the mitochondrial matrix. The electron transport chain is located
on the cristae of a mitochondrion.
2 ADP + 2 P
6 NADH
2 FADH2
2 ATP
Chapter 7 Energy for Cells
NADH and
FADH2
1 Pyruvate from glycolysis is
oxidized to a C2 acetyl group
that is carried by CoA to the
citric acid cycle.
2
Preparatory
reaction
e–
Glycolysis
Preparatory reaction
Citric acid
cycle
Electron
transport
chain
pyruvate
2
2 NADH
2
matrix
ATP
2
ATP
34
ATP
NAD+
2 CoA
CO2
6 Additional oxidation reactions
produce another NADH and an
FADH2 and regenerate the
original C4 molecule.
2 CoA
NAD+
NAD+
NADH
CO2
Citric acid
cycle
NAD+
FADH2
NADH
FAD
ADP + P
5 ATP is produced by
substrate-level ATP
synthesis.
3 Twice over, oxidation
reactions produce
NADH, and CO2
is released.
ATP
CO2
4 The loss of two CO2
results in a new C4
molecule.
Figure 7.6
The citric acid cycle.
The acetyl-CoA from the preparatory reaction enters the citric
acid cycle. The net result of one turn of this cycle of reactions is
the oxidation of the acetyl group to 2 molecules of CO2 and the
formation of 3 molecules of NADH and 1 molecule of FADH2.
Substrate-level ATP synthesis occurs, and the result is 1 ATP
molecule. The citric acid cycle turns twice per glucose molecule.
Check Your Progress
1. A C2 acetyl group enters the citric acid cycle. Where does it
come from?
2. What are the products of the citric acid cycle as a result of
further breakdown of glucose?
Answers: 1. The C2 acetyl group comes from the prep reaction. 2. The citric
acid cycle turns twice per glucose molecule, producing 2 CO2, 3 NADH,
1 FADH2, 1 ATP per turn.
NADH
2 The C2 acetyl group
combines with a C4
molecule to produce
citric acid, a C6
molecule.
103
104
Part I The Cell
The Electron Transport Chain
The electron transport chain located in the cristae of mitochondria is a series
of carriers that pass electrons from one to the other. NADH and FADH2 deliver
electrons to the chain. Consider that the hydrogen atoms attached to NADH and
FADH2 consist of an e and an H. The members of the electron transport chain
accept only electrons (e) and not hydrogen ions (H).
In Figure 7.7, high-energy electrons enter the chain, and low-energy
electrons leave the chain. When NADH gives up its electrons, the next carrier
gains the electrons and is reduced. This oxidation-reduction reaction starts the
process, and each of the carriers in turn becomes reduced and then oxidized as
the electrons move down the system. As the pair of electrons is passed from
carrier to carrier, energy is released and captured for ATP production. The final
acceptor of electrons is oxygen (O2), the very O2 we breathe in. It’s remarkable
to think that the role of oxygen in cellular respiration is to keep the electrons
moving from the first to the last carrier. Why? Because oxygen attracts electrons
to a greater degree than the carriers of the chain. Once oxygen accepts electrons
it combines with H, and the other end product of cellular respiration (i.e., water)
results (see the equation on page 98).
When NADH delivers electrons to the first carrier of the electron transport chain, enough energy is captured by the time the
electrons are received by O2 to permit the production of three ATP molecules. When FADH2
delivers electrons to the electron transport
chain, only 2 ATP are produced.
Once NADH has delivered
5
electrons to the electron transport
chain, NAD is regenerated and
can be used again. In the same
O2
manner, FAD is regenerated and
can be used again. The recycling
of coenzymes, and for that matter ADP, increases cellular efficiency since it does away with
the need to synthesize NAD,
FAD, and ADP anew.
NADH and
FADH2
e–
Glycolysis
Preparatory reaction
2
ATP
Citric acid
cycle
2
ATP
Electron
transport
chain
34
ATP
1
2
3
4
NADH
and
FADH2
NAD+
2e–
NADH
1 oxidized
2 H+
2
3 oxidized
oxidized
The Cristae of a
Mitochondrion
4
5
oxidized
3 ATP
1
–
2
O2
H 2O
2 H+
Figure 7.7
The electron transport chain.
An electron transport chain operates like a bucket brigade. Each electron carrier (#1–5)
is alternatively reduced (orange) and oxidized as if the electrons were a bucket being
passed from person to person. As oxidation-reduction occurs, energy is released that
will be used to make ATP.
The carriers of the electron transport
chain are located in molecular complexes
within the inner mitochondrial membrane. ATP
synthesis is carried out by ATP synthase complexes
also located in this membrane (Fig. 7.8).
The carriers of the electron transport chain
accept electrons from NADH or FADH2 and then pass
them from one to the other by way of two additional
Chapter 7 Energy for Cells
Intermembrane
space
H+
H+
H+
electron transport
H+
carriers in a molecular
complex
H+
H+
H+
H+
H+
H+
H+
H+
H+
mobile carrier
H+
+
H
H+
H+
NAD+
NADH
H+
H+
+
1
–
2
O2
H
matrix
cristae
H 2O
ADP + P
ATP
2 H+
ATP
synthase
complex
H+
Matrix
a. Electron transport chain
Figure 7.8
H+
H+
e–
intermembrane
space
105
b. ATP synthesis
The organization of cristae.
Molecular complexes that contain the electron transport carriers are located in the cristae as are ATP synthase complexes. a. As electrons move from one carrier to
the other, hydrogen ions (H) are pumped from the matrix into the intermembrane space. b. As hydrogen ions flow back down a concentration gradient through
an ATP synthase complex, ATP is synthesized by the enzyme ATP synthase.
mobile carriers (orange arrow). What happens to the hydrogen ions (H) carried
by NADH and FADH2? The complexes use the energy released by oxidationreduction to pump H from the mitochondrial matrix into the intermembrane
space located between the outer and inner membrane of a mitochondrion. The
pumping of H into the intermembrane space establishes an unequal distribution of H ions; in other words, there are many H in the intermembrane space
and few in the matrix of a mitochondrion.
The energy stored in the H gradient is now used to drive forward ATP
synthesis. The cristae of mitochondria (like the thylakoid membrane of chloroplasts) contain an ATP synthase complex that allows H to return to the matrix.
The flow of H through the ATP synthase complex brings about a conformational change, which causes the enzyme ATP synthase to synthesize ATP from
P . ATP leaves the matrix by way of a channel protein. This ATP
ADP ⫹ 䊊
remains in the cell and is used for cellular work.
Check Your Progress
Explain how the electron transport chain results in the synthesis
of ATP.
Answer: As electrons move from one carrier to another in the cristae, energy
is released, and this energy is used to pump hydrogen ions from the matrix
to the intermembrane space. The flow of hydrogen ions back down the
concentration gradient into the matrix drives the synthesis of ATP by ATP
synthase.
106
Part I The Cell
Phase
NADH
FADH2
ATP Yield
Glycolysis
2
–
2
Prep reaction
2
–
–
Citric acid
cycle
6
2
2
2
30
4
Electron
transport chain
10
Total ATP
Figure 7.9
38
Calculating ATP energy yield per glucose
molecule.
Substrate-level ATP synthesis during glycolysis and the citric acid
cycle accounts for 4 ATP. The electron transport chain produces a
maximum of 34 ATP, and the maximum total is 38 ATP. Some cells,
however, produce only 36 ATP per glucose molecule or even less.
Energy Yield from Glucose Metabolism
Figure 7.9 calculates the ATP yield for the complete breakdown of glucose to
CO2 and H2O. Per glucose molecule, there is a net gain of 2 ATP from glycolysis, which takes place in the cytoplasm. The citric acid cycle, which occurs in
the matrix of mitochondria, accounts for 2 ATP per glucose molecule. This
means that a total of 4 ATP form due to substrate-level ATP synthesis outside
the electron transport chain.
Most of the ATP produced comes from the electron transport chain and
the ATP synthase complex. Per glucose molecule, 10 NADH and 2 FADH2
take electrons to the electron transport chain. The maximum number of ATP
produced by the chain is therefore 34 ATP, and the maximum number produced
by both the chain and substrate-level ATP synthesis is 38. However, for reasons
beyond the scope of this book, the maximum number of ATP produced per
glucose molecule in some cells is only 36 ATP or lower. A yield of 36–38 ATP
represents about 40% of the available energy in a glucose molecule. The rest of
the energy is lost in the form of heat.
Alternative Metabolic Pathways
Food
Proteins
Carbohydrates
amino acids
glucose
NH3
Fats and oils
glycerol
fatty acids
Glycolysis
ATP
pyruvate
Acetyl-CoA
Citric
acid
cycle
ATP
ATP
Electron
transport
chain
H 2O
O2
Figure 7.10
Alternative metabolic pathways.
All the types of food in a pizza can be used to generate ATP.
Let’s say you are on a low-carbohydrate diet. Will you then run out of ATP?
No, because your cells can also utilize other energy sources—the components
of fats and oils, namely glycerol and fatty acids, and amino acids, which are
derived from proteins (Fig. 7.10).
Because glycerol is a carbohydrate, it enters the process of cellular respiration during glycolysis. Fatty acids can be metabolized to acetyl groups, which
enter the citric acid cycle. A fatty acid with a chain of 18 carbons can make
three times the number of acetyl groups as does glucose. For this reason, fats
are an efficient form of stored energy—there are three long fatty acid chains
per fat molecule. The complete breakdown of glycerol and fatty acids to carbon
dioxide and water results in many more ATP molecules per molecule than does
the breakdown of glucose.
Only the hydrocarbon backbone of amino acids, not the amino group,
can be used by the cellular respiration pathways. The amino group becomes
ammonia (NH3), which becomes part of urea, the primary excretory product
of humans. Just where the hydrocarbon backbone from an amino acid begins
degradation to produce ATP molecules depends on its length. Figure 7.10 shows
that the hydrocarbon backbone from an amino acid can enter cellular respiration
pathways at pyruvate, at acetyl-CoA, or during the citric acid cycle.
The smaller molecules in Figure 7.10 can also be used to synthesize larger
molecules. In such instances ATP is used instead of generated. You already
know that amino acids can be employed to synthesize proteins. Also, some substrates of the citric acid cycle can become amino acids through the addition of
an amino group. Of the 20 most common amino acids, humans have the ability
to synthesize 11 amino acids in this way, but we cannot synthesize the other
9. These nine are called the essential amino acids, meaning that they must be
present in the diet or else we suffer a protein deficiency.
Similarly, substrates from glycolysis can become glycerol, and acetyl
groups can be used to produce fatty acids. When glycerol and three fatty
acids join, a fat results. This explains why you can gain weight from eating
carbohydrate-rich foods.
glucose
7.4
Fermentation
–2 ATP
Fermentation is the anaerobic breakdown of glucose resulting in the buildup
of 2 ATP and lactate, a toxic by-product (Fig. 7.11). During fermentation in
animal cells, the pyruvate formed by glycolysis accepts 2 hydrogen atoms and
is reduced to lactate. Notice in Figure 7.11 that 2 NADH pass hydrogen atoms
to pyruvate, reducing it. Why is it beneficial for pyruvate to be reduced to
lactate when oxygen is not available? The answer is that this reaction regenerates NAD, which can then pick up more electrons during the earlier reactions
of glycolysis. This keeps glycolysis going, during which ATP is produced by
substrate-level ATP synthesis.
The 2 ATP produced by fermentation represent only a small fraction of
the potential energy stored in a glucose molecule. Following fermentation,
most of this potential energy is still waiting to be released.
The inputs and outputs of fermentation are as follows:
2 P
2 NADH
2 P
+4 ATP
glucose
2 lactate
or
2 alcohol and 2 CO2
2 ATP
4 ATP
4 ADP
pyruvate
outputs
P
P
2
inputs
ADP +
2 NAD+
2 P
Fermentation
2 ATP
2 ATP
2 ADP
2 NADH
net
2 NAD+
Net gain:
2 ATP
Despite its low yield of only 2 ATP, fermentation is essential. It can provide
a rapid burst of ATP, and muscle cells are more apt than other cells to
carry on fermentation. When our muscles are working vigorously over
a short period of time, as when we run, fermentation is a way to produce ATP even though oxygen is temporarily in limited supply.
However, one of its by-products, lactate, is toxic to cells. At first,
blood carries away all the lactate formed in muscles. But eventually, lactate begins to build up, changing the pH and causing the muscles to “burn”
and then to fatigue so that they no longer contract. When we stop running,
our bodies are in oxygen deficit, as signified by the fact that we continue to
breathe very heavily for a time. Recovery is complete when all the lactate
is transported to the liver, where it is reconverted to pyruvate. Some of the
pyruvate is oxidized completely, and the rest is converted back to glucose.
2 ethyl
alcohol
2 lactate
Bread
Athlete
Wine
Figure 7.11
Microorganisms and Fermentation
2 CO2
or
Fermentation.
Fermentation consists of glycolysis followed by a reduction of pyruvate by
NADH. This regenerates NAD, which returns to the glycolytic pathway to
pick up more hydrogen atoms.
Bacteria utilize fermentation to produce an organic acid, such as lactate,
or an alcohol and CO2, depending on the type of bacterium.
Yeasts are good examples of microorganisms that generate
ethyl alcohol and CO2 when they carry out fermentation. When
Check Your Progress
yeast is used to leaven bread, the CO2 makes the bread rise.
When yeast is used to ferment grapes for wine production or to
What are the drawbacks and benefits of fermentation?
ferment wort—derived from barley—for beer production, ethyl
alcohol is the desired product.
Answer: Drawbacks: Most of the energy in a glucose molecule is unused and
it results in a toxic end product. Benefits: The 2 ATP gained can be used as a
burst of energy when oxygen is not available for complete glucose breakdown.
108
Part I The Cell
THE CHAPTER IN REVIEW
Citric Acid Cycle
Summary
Cellular Respiration
7.1
During cellular respiration, glucose from food is oxidized to CO2,
which we exhale. Oxygen (O2), which we breathe in, is reduced to
H2O. When glucose is oxidized, energy is released. Cellular respiration
captures the energy of oxidation and uses it to produce ATP
molecules. The following equation gives an overview of these events:
C6H12O6
glucose
7.2
+
6 O2
6 CO2
+
6 H2O
+
ATP
Outside the Mitochondria: Glycolysis
Glycolysis, the breakdown of glucose to 2 molecules of pyruvate, is
a series of enzymatic reactions that occur in the cytoplasm. During
glycolysis:
•
•
Glucose is oxidized by removal of hydrogen atoms.
When NAD accepts these electrons, NADH results.
Acetyl groups enter the citric acid cycle, a series of reactions occurring
in the mitochondrial matrix. During one turn of the cycle, oxidation
results in 2 CO2 molecules, 3 NADH molecules, and 1 FADH. One
turn also produces 1 ATP molecule. The cycle must turn twice per
glucose molecule.
Electron Transport Chain
The final stage of cellular respiration involves the electron transport
chain located in the cristae of the mitochondria. The chain is a series
of electron carriers that accept electrons (e) from NADH and FADH2
and pass them along until they are finally received by oxygen, which
combines with H to produce water.
The carriers of the electron transport chain are located in
molecular complexes on the cristae of mitochondria. These carriers
capture energy from the passage of electrons and use it to pump H
into the intermembrane space of the mitochondrion. When H flows
down its gradient into the matrix through an ATP synthase complex,
energy is released and used to form ATP molecules from ADP and 䊊
P.
Energy Yield
Of the maximum 38 ATP formed by complete glucose breakdown,
4 are the result of substrate-level ATP synthesis, and the rest are
produced as a result of the electron transport chain and ATP synthase:
Cytoplasm
Breakdown releases enough energy to immediately give a net
gain of 2 ATP by substrate-level ATP synthesis. The inputs and outputs
of glycolysis are summarized here:
10 NADH and
2 FADH2
2 NADH
2 NADH
6 NADH and
2 FADH2
2 acetyl-CoA
Citric acid
cycle
Glycolysis
e–
Glycolysis
inputs
outputs
glucose
2 pyruvate
2 NAD+
2 NADH
glucose
pyruvate
2 ATP
2 ATP
Electron
transport
chain
34 ATP
2 ATP
ADP + P
4 ATP
Alternative Metabolic Pathways
2 ATP
net
When oxygen is available, pyruvate from glycolysis enters a
mitochondrion.
7.3
Inside the Mitochondria
Preparatory Reaction
During the preparatory reaction in the matrix:
•
•
•
Oxidation occurs as CO2 is removed from pyruvate.
NAD accepts hydrogen atoms, and NADH results.
An acetyl group, the end product, combines with CoA.
This reaction takes place twice per glucose molecule.
Besides carbohydrates, glycerol and fatty acids from fats, and amino
acids from proteins can undergo cellular respiration by entering
glycolysis and/or the citric acid cycle. These metabolic pathways also
provide substrates for the synthesis of fats and proteins.
7.4
Fermentation
Fermentation involves glycolysis followed by the reduction of pyruvate
by NADH, either to lactate or to alcohol and CO2. The reduction of
pyruvate regenerates NAD, which can accept more hydrogen atoms
from glycolysis.
•
•
Although fermentation results in only 2 ATP molecules, it still
provides a quick burst of ATP energy for short-term, strenuous
muscular activity.
The accumulation of lactate puts the individual in oxygen
deficit, which is the amount of oxygen needed when lactate is
completely metabolized to CO2 and H2O.
Chapter 7 Energy for Cells
Thinking Scientifically
8. Match the descriptions below to the lettered events in the
preparatory reaction and citric acid cycle.
1. Occasionally, you’ll hear a news story about a bin of grain
that has undergone spontaneous combustion, resulting in a
spectacular fire. It may seem odd that wet grain is more likely
to burn than dry grain. However, the grain contains living plant
seeds that are physiologically more active when moist than
when dry. In addition, the surfaces of the kernels of grain are
covered with microorganisms that increase their growth rates
when moist. Explain how the consumption of oxygen by these
organisms can contribute to a grain-bin fire.
Pyruvate is broken down to an acetyl group.
Acetyl group is taken up and a C6 molecule results.
Oxidation results in NADH and CO2.
ATP is produced by substrate-level ATP synthesis.
Oxidation produces more NADH and FADH2.
a.
2
pyruvate
Preparatory
2 NADH
reaction
2. One of the major risk factors for diabetes in the elderly is insulin
resistance, which is decreased tissue sensitivity to the action of
insulin. Tissues then compensate by increasing insulin secretion.
Insulin resistance can result from the accumulation of fatty acids
in muscle and liver tissue. Researchers have recently found a
connection between fatty acid accumulation and mitochondrial
function in elderly people. Logically, what might be this
connection? Using this knowledge, how might elderly people
reduce their risk of diabetes?
2 NAD+
2 CoA
CO2
2 CoA
b.
e.
NADH
NAD+
NAD+
NADH
CO2
NAD+
Choose the best answer for each question.
FADH2
a. glucose, oxygen
b. glucose, water
c. oxygen, water
d. water, oxygen
e. oxygen, carbon dioxide
a. water.
b. oxygen.
c. water and carbon dioxide.
d. oxygen and carbon dioxide.
e. oxygen and water.
c. ATP and NAD
d. ADP and NAD
4. The end product of glycolysis is
c. phosphoglyceraldehyde.
d. pyruvate.
5. Acetyl-CoA is the end product of
c. the citric acid cycle.
d. the electron transport chain.
6. The citric acid cycle results in the release of
a. carbon dioxide.
b. pyruvate.
c. oxygen.
d. water.
7. The following reactions occur in the matrix of the mitochondria:
a. glycolysis and the preparatory reaction
b. the preparatory reaction and the citric acid cycle
c. the citric acid cycle and the electron transport chain
d. the electron transport chain and glycolysis
CO2
ATP
9. The strongest and final electron acceptor in the electron
transport chain is
a. NADH.
b. FADH2.
3. During the energy-harvesting steps of glycolysis, which are
produced?
a. glycolysis.
b. the preparatory reaction.
ADP + P
d.
2. The products of cellular respiration are energy and
NADH
FAD
1. During cellular respiration, _________ is oxidized and _________
is reduced.
a. phosphoenol pyruvate.
b. glucose.
c.
Citric acid
cycle
Testing Yourself
a. ATP and NADH
b. ADP and NADH
109
c. oxygen.
d. water.
For questions 10–16, match the items to those in the key. Answers can
be used more than once, and each question can have more than one
answer.
Key:
a. glycolysis
b. preparatory reaction
c. citric acid cycle
d. electron transport chain
10. Produces ATP.
11. Uses ATP.
12. Produces NADH.
13. Uses NADH.
14. Produces carbon dioxide.
15. Occurs in cytoplasm.
16. Occurs in mitochondria.
17. The carriers in the electron transport chain undergo
a. oxidation only.
b. reduction only.
c. oxidation and reduction.
d. the loss of hydrogen ions.
e. the gain of hydrogen ions.
110
Part I The Cell
18. The final acceptor for hydrogen atoms during fermentation is
a. O2.
b. acetyl CoA.
c. FAD.
d. pyruvic acid.
19. Which of the following do not enter the cellular respiration
pathways?
a. fats
b. amino acids
c. nucleic acids
d. carbohydrates
20. When animals carry out fermentation, they produce _________,
while yeasts produce _________.
a. lactate, malate
b. lactate, ethyl alcohol
c. malate, ethyl alcohol
d. malate, lactate
e. ethyl alcohol, lactate
21. Fermentation does not yield as much ATP as cellular respiration
does because fermentation
a. generates mostly heat.
b. makes use of only a small amount of the potential energy in
glucose.
c. creates by-products that require large amounts of ATP to
break down.
d. creates ATP molecules that leak into the cytoplasm and are
broken down.
22. Which type of human cells carries on the most fermentation?
a. fat
b. muscle
c. nerve
d. bone
23. Cellular respiration cannot occur without
a. sodium.
b. oxygen.
c. lactate.
d. All of these are correct.
24. The metabolic process that produces the most ATP molecules is
a.
b.
c.
d.
glycolysis.
the citric acid cycle.
the electron transport chain.
fermentation.
25. The greatest contributor of electrons to the electron transport
chain is
a. oxygen.
b. glycolysis.
c. the citric acid cycle.
d. the preparatory reaction.
e. fermentation.
26. Substrate-level ATP synthesis takes place in
a. glycolysis and the citric acid cycle.
b. the electron transport chain and the preparatory reaction.
c. glycolysis and the electron transport chain.
d. the citric acid cycle and the preparatory reaction.
27. Which of the following is not true of fermentation? Fermentation
a. has a net gain of only 2 ATP.
b. occurs in the cytoplasm.
c. donates electrons to the electron transport chain.
d. begins with glucose.
e. is carried on by yeast.
28. Match the terms to their definitions. Only four of these terms
are needed:
anaerobic
citric acid cycle
fermentation
oxygen deficit
pyruvate
preparatory reaction
a. Occurs in mitochondria and produces CO2, ATP, NADH, and
FADH2.
b. Growing or metabolizing in the absence of oxygen.
c. End product of glycolysis.
d. Anaerobic breakdown of glucose that results in a gain of
2 ATP and end products such as alcohol and lactate.
Go to www.mhhe.com/maderessentials for more quiz questions.
Bioethical Issue
For millennia, humans have taken advantage of a product of
fermentation in microbes—ethyl alcohol. However, alcohol is toxic
to human cells, and alcohol abuse is a serious problem from many
perspectives. For example, a woman who consumes large amounts of
alcohol during pregnancy may cause her child to have fetal alcohol
syndrome. Children with this syndrome suffer from mental retardation
and physical problems. Some people believe the unborn child has
a right to be protected from harm and argue that intervention is
justified if a pregnant woman drinks heavily despite her doctor’s
orders. In extreme cases, it may be necessary to incarcerate the
woman throughout the pregnancy. The alternative point of view is
that every person has a right to freedom of choice, and society has no
authority to intervene in the life of a pregnant woman. In addition,
forcing a pregnant woman to follow medical treatment against her
will, for the sake of her fetus, is imposing an obligation that we do
not impose on others—that is, other members of our society are not
forced to change their lifestyles purely for the sake of others.
Do you think society has an obligation to do whatever is
necessary to prevent women from drinking excessively during
pregnancy? Will this lead to attempts to control the lives of pregnant
women in other ways as well, such as requiring them to exercise more
or to abstain from caffeine?
Understanding the Terms
acetyl-CoA 102
citric acid cycle 99, 102
coenzyme A (CoA) 99
electron transport chain 99
fermentation 107
glycolysis 99
intermembrane space 105
oxygen deficit 107
preparatory (prep) reaction 99, 102
Match the terms to these definitions:
a. _______________ First step in cellular respiration.
b. _______________ This metabolic pathway breaks down pyruvate in
the mitochondrial matrix.
c. _______________ Reduction of pyruvate when oxygen is not
available.
d. _______________ Hydrogen ions are pumped into this region during
the electron transport chain.
e. _______________ Acetyl-CoA is needed during this phase of cellular
respiration.