Download Section 3 Cellular Respiration

Cellular Respiration
Section 3
Cellular Energy
Objectives
● Summarize how glucose is
broken down in the first
stage of cellular respiration.
● Describe how ATP is made
in the second stage of cellular respiration.
4B
● Identify the role of fermentation in the second stage of
cellular respiration.
4B
● Evaluate the importance
of oxygen in aerobic
respiration.
4B
Most of the foods we eat contain usable energy. Much of the energy
in a hamburger, for example, is stored in proteins, carbohydrates,
and fats. But before you can use that energy, it is transferred to ATP.
Like in most organisms, your cells transfer the energy in organic
compounds, especially glucose, to ATP through a process called
cellular respiration. Oxygen in the air you breathe makes the production of ATP more efficient, although some ATP is made without
oxygen. Metabolic processes that require oxygen are called aerobic
(ehr OH bihk). Metabolic processes that do not require oxygen are
called anaerobic (AN ehr oh bihk), meaning “without air.”
The Stages of Cellular Respiration
Cellular respiration is the process cells use to harvest the energy in
organic compounds, particularly glucose. The breakdown of glucose
during cellular respiration can be summarized by the following
equation:
Key Terms
aerobic
anaerobic
glycolysis
NADH
Krebs cycle
FADH2
fermentation
enzymes
C6H12O6 ! 6O2 ⎯⎯⎯⎯→ 6CO2 ! 6H2O ! energy
glucose
oxygen
gas
carbon
dioxide
water
ATP
As Figure 10 shows, cellular respiration occurs in two stages:
Stage 1 Glucose is converted to pyruvate (PIE roo vayt), producing
a small amount of ATP and NADH.
Figure 10
Cellular respiration
Cellular respiration occurs
in two stages.
NAD+
NADH
1. First, glucose is
broken down to
pyruvate.
Anaerobic
(without O2)
Ethanol
and CO2, or
lactate
2. Then, either aerobic
respiration or anaerobic
processes occur.
104
Stage 2 When oxygen is present, pyruvate and NADH are used
to make a large amount of ATP. This process is called aerobic respiration. Aerobic respiration
Glucose
occurs in the mitochondria of eukaryotic cells and in the cell membrane of
prokaryotic cells. When oxygen is not
present, pyruvate is converted to either
ADP
lactate (LAK tayt) or ethanol (ethyl alcoATP
hol) and carbon dioxide.
The equation above does not show
Stage 1
how cellular respiration occurs. It simply
Aerobic
Pyruvate
shows that the complete enzyme-assisted
(with O2)
breakdown of a glucose molecule uses
six oxygen molecules and forms six
Stage 2
carbon dioxide molecules, six water
molecules, and ATP. Aerobic respiration
produces most of the ATP made by cells.
Mitochondrion
Intermediate products of aerobic respiration form the organic compounds that
ATP
help build and maintain cells.
CHAPTER 5 Photosynthesis and Cellular Respiration
Copyright © by Holt, Rinehart and Winston. All rights reserved.
Stage One: Breakdown of Glucose
The primary fuel for cellular respiration is glucose, which is formed
when carbohydrates such as starch and sucrose are broken down. If
too few carbohydrates are available to meet an organism’s glucose
needs, other molecules, such as fats, can be broken down to make
ATP. In fact, one gram of fat contains more energy than two grams of
carbohydrates. Proteins and nucleic acids can also be used to make
ATP, but they are usually used for building important cell parts.
Glycolysis
In the first stage of cellular respiration, glucose is broken down in the cytoplasm during a process called
glycolysis (glie KAHL uh sihs). Glycolysis is an
enzyme-assisted anaerobic process that breaks down
one six-carbon molecule of glucose to two threecarbon pyruvate ions. Recall that a molecule that has
lost or gained one or more electrons is called an ion.
Pyruvate is the ion of a three-carbon organic acid
called pyruvic acid. The pyruvate produced during glycolysis still contains some of the energy that was
stored in the glucose molecule.
As glucose is broken down, some of its hydrogen
atoms are transferred to an electron acceptor called
NAD!. This forms an electron carrier called NADH .
For cellular respiration to continue, the electrons carried by NADH are eventually donated to other organic
compounds. This recycles NAD!, making it available
to accept more electrons. Glycolysis is summarized in
Figure 11.
Step
Step
IO
B
graphic
Step
Two NADH molecules are produced, and one
more phosphate group is transferred to each
three-carbon compound.
Step
In a series of four reactions, each three-carbon
compound is converted to a three-carbon
pyruvate, producing four ATP molecules in
the process.
Glycolysis uses two ATP molecules but produces four
ATP molecules, yielding a net gain of two ATP molecules. Glycolysis is followed by another set of reactions
that use the energy temporarily stored in NADH to
make more ATP.
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Glycolysis
Glucose
C C C C C C
1
2 ADP
6-carbon
compound
C C C C C C
2
In a series of three reactions, phosphate
groups from two ATP molecules are transferred to a glucose molecule.
In two reactions, the resulting six-carbon compound is broken down to two three-carbon
compounds, each with a phosphate group.
Figure 11
Two 3-carbon
compounds
C C C
C C C
2 NAD+
3
2 NADH + 2H+
2
Two 3-carbon
compounds
C C C
C C C
4 ADP
4
Two 3-carbon
pyruvates
C C C
C C C
SECTION 3 Cellular Respiration
105
Stage Two: Production of ATP
When oxygen is present, pyruvate produced during glycolysis enters
a mitochondrion and is converted to a two-carbon compound. This
reaction produces one carbon dioxide molecule, one NADH molecule, and one two-carbon acetyl (uh SEET uhl) group. The acetyl
group is attached to a molecule called coenzyme A (CoA), forming a
compound called acetyl-CoA (uh SEET uhl-koh ay).
Krebs Cycle
Acetyl-CoA enters a series of enzyme-assisted reactions called the
Krebs cycle , summarized in Figure 12. The cycle is named for the
biochemist Hans Krebs, who first described the cycle in 1937.
O
I
B
p
a
r
g hic
Step
Acetyl-CoA combines with a four-carbon compound, forming a six-carbon compound and releasing coenzyme A.
Step
Carbon dioxide, CO2, is released from the six-carbon compound, forming a five-carbon compound. Electrons are
transferred to NAD+, making a molecule of NADH.
Figure 12
Krebs Cycle
The Krebs cycle produces electron carriers that temporarily store chemical energy.
22. CO2 is released
11. Acetyl-CoA combines with a
four-carbon compound,
forming a six-carbon
compound.
Acetyl-CoA
from the six-carbon
compound, leaving a
five-carbon compound.
CoA
C C
6-carbon
compound
C CO2
NAD+
C C C C C C
NADH + H+
4-carbon
compound
5-carbon
compound
C C C C
C C C C C
33. CO2 is released from
the five-carbon
compound, leaving
a four-carbon
compound.
C CO2
NAD+
NADH + H+
NADH + H+
NAD+
55. The new four-carbon
compound is converted
to the four-carbon
compound that began
the cycle.
4-carbon
compound
4-carbon
compound
C C C C
C C C C
ADP + P
ATP
44. The four-carbon compound
FAD
is converted to a new
four-carbon compound.
FADH2
106
CHAPTER 5 Photosynthesis and Cellular Respiration
Copyright © by Holt, Rinehart and Winston. All rights reserved.
Step
Carbon dioxide is released from the five-carbon compound,
resulting in a four-carbon compound. A molecule of ATP is
made, and a molecule of NADH is also produced.
Step
The existing four-carbon compound is converted to a new
four-carbon compound. Electrons are transferred to an
electron acceptor called FAD, making a molecule of FADH2.
FADH2 is another type of electron carrier.
Step
The new four-carbon compound is then converted to the
four-carbon compound that began the cycle. Another molecule of NADH is produced.
www.scilinks.org
Topic: Aerobic Respiration
Keyword: HX4004
After the Krebs cycle, NADH and FADH2 now contain much of the
energy that was previously stored in glucose and pyruvate. When the
Krebs cycle is completed, the four-carbon compound that began the
cycle has been recycled, and acetyl-CoA can enter the cycle again.
Electron Transport Chain
In aerobic respiration, electrons donated by NADH and FADH2 pass
through an electron transport chain, as shown in Figure 13. In
eukaryotic cells, the electron transport chain is located in the inner
membranes of mitochondria. The energy of these electrons is used to
pump hydrogen ions out of the inner mitochondrial compartment.
Hydrogen ions accumulate in the outer compartment, producing a
concentration gradient across the inner membrane. Hydrogen ions
diffuse back into the inner compartment through a carrier protein
that adds a phosphate group to ADP, making ATP. At the end of the
electron transport chain, hydrogen ions and spent electrons combine
with oxygen molecules, O2, forming water molecules, H2O.
Figure 13 Electron transport chain of aerobic respiration
In the inner membranes of mitochondria, electron transport chains (represented by the red lines) make ATP.
Outer compartment
+
H
H+
H
H+
+
H+
H+
3. ATP is produced as hydrogen ions
diffuse into the inner compartment
through a channel protein.
eATP-producing
carrier protein
e-
H+
H+
NAD+
H+
NADH + H+
Inner compartment
1. The electron transport
chain pumps hydrogen
ions, H+, out of the
inner compartment.
Copyright © by Holt, Rinehart and Winston. All rights reserved.
4H+ + O2
H+
Inner
mitochondrial
membrane
2H2O
H+
2. At the end of the chain,
electrons and hydrogen
ions combine with
oxygen, forming water.
ADP + P
ATP
SECTION 3 Cellular Respiration
107
Fermentation in the Absence
of Oxygen
Figure 14 Fermentation.
In cheese making, fungi or
prokaryotes added to milk
carry out lactic acid fermentation on some of the sugar in
the milk.
What happens when there is not enough oxygen for aerobic respiration to occur? The electron transport chain does not function
because oxygen is not available to serve as the final electron acceptor. Electrons are not transferred from NADH, and NAD! therefore
cannot be recycled. When oxygen is not present, NAD! is recycled in
another way. Under anaerobic conditions, electrons carried by
NADH are transferred to pyruvate produced during glycolysis. This
process recycles NAD! needed to continue making ATP through glycolysis. The recycling of NAD! using an organic hydrogen acceptor
is called fermentation . Prokaryotes carry out more than a dozen
kinds of fermentation, all using some form of organic hydrogen
acceptor to recycle NAD!. Two important types of fermentation are
lactic acid fermentation and alcoholic fermentation. Lactic acid fermentation by some prokaryotes and fungi is used in the production
of foods such as yogurt and some cheeses, as shown in Figure 14.
Lactic Acid Fermentation
In some organisms, a three-carbon pyruvate is converted to a threecarbon lactate through lactic acid fermentation, as shown in
Figure 15. Lactate is the ion of an organic acid called lactic acid. For
example, during vigorous exercise pyruvate in muscles is converted
to lactate when muscle cells must operate without enough oxygen.
Fermentation enables glycolysis to continue producing ATP in muscles as long as the glucose supply lasts. Blood removes excess lactate
from muscles. Lactate can build up in muscle cells if it is not
removed quickly enough, sometimes causing muscle soreness.
Figure 15 Two types of fermentation
When oxygen is not present, cells recycle NAD+ through fermentation.
In lactic acid fermentation, pyruvate is converted to lactate.
Glucose
Glycolysis
C C C C C C
In alcoholic fermentation, pyruvate is broken down
to ethanol, releasing carbon dioxide, CO2.
Pyruvate
Glucose
C C C
C C C C C C
Glycolysis
Pyruvate
C C C
C
NAD+
NADH + H+
Lactate
NAD+
C C
Lactic acid fermentation
CHAPTER 5 Photosynthesis and Cellular Respiration
CO2
2-carbon
compound
Ethanol
C C C
108
NADH + H+
C C
Alcoholic fermentation
Copyright © by Holt, Rinehart and Winston. All rights reserved.
Alcoholic Fermentation
In other organisms, the three-carbon pyruvate is broken down to
ethanol (ethyl alcohol), a two-carbon compound, through alcoholic fermentation. Carbon dioxide is released during the process. As shown in
Figure 15, alcoholic fermentation is a two-step process. First, pyruvate
is converted to a two-carbon compound, releasing carbon dioxide.
Second, electrons are transferred from a molecule of NADH to the twocarbon compound, producing ethanol. As in lactic acid fermentation,
NAD! is recycled, and glycolysis can continue to produce ATP.
Alcoholic fermentation by yeast, a fungus, has been used in the
preparation of many foods and beverages. Wine and beer contain
ethanol made during alcoholic fermentation by yeast. Carbon
dioxide released by the yeast causes the rising of bread dough and
the carbonation of some alcoholic beverages, such as beer. Ethanol
is actually toxic to yeast. At a concentration of about 12 percent
ethanol kills yeast. Thus, naturally fermented wine contains about
12 percent ethanol.
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Topic: Fermentation
Keyword: HX4080
Muscle Fatigue and Endurance Training
A
nyone who runs or exercises
for a long period of time
soon learns about muscle fatigue.
As you continue vigorous exercise, the muscles you are using
become fatigued—that is, tired
and less able to generate force.
The reasons for muscle fatigue are
not fully understood, but in most
cases the fatigue increases when
the production of lactic acid by
the exercising muscle increases.
Anaerobic Threshold
Why does an exercising muscle
produce lactic acid? A resting
muscle obtains most of its energy
from aerobic respiration. A continuously exercising muscle, however, soon depletes its available
oxygen. At this point, called the
anaerobic threshold, the exercising muscle begins to obtain the
ATP needed anaerobically. In the
absence of oxygen, glycolysis
extracts the required ATP from
glycogen in the muscle. Glycogen
Copyright © by Holt, Rinehart and Winston. All rights reserved.
is a storable form of glucose
that acts as an energy reserve.
Glycolysis converts the muscle
glycogen to pyruvate, which is
then fermented to lactic acid.
The ability to perform continuous exercise is limited by
the body’s stored glycogen. So,
physical endurance can increase
if glycogen stored in muscles is
spared during exercise. Trained
athletes such as cyclist Lance
Armstrong, shown at right, get a
relatively large portion of their
energy from aerobic respiration.
Thus, their muscle glycogen
reserve is depleted more slowly
than that in untrained individuals.
In fact, the greater the level of
physical training, the higher the
proportion of energy the body
derives from aerobic respiration.
Athletic Endurance
Endurance-trained athletes generally have more muscle mass
than untrained people. But it is
Lance Armstrong
endurance-trained athletes’ high
aerobic capacity—rather than
their greater muscle mass—that
allows these athletes to exercise
more before lactic acid production and glycogen depletion
cause muscle fatigue.
www.scilinks.org
Topic: Anaerobic Threshold
Keyword: HX4192
SECTION 3 Cellular Respiration
109
Figure 16 Effect of oxygen on ATP production
Most ATP is produced during aerobic respiration.
Glucose
Glycolysis
Fermentation
Lactate
Without O2
Pyruvate
(Net)
With O2
2 ATP
Krebs cycle
Ethanol
and CO2
Electron
transport
chain
Anaerobic processes
2 ATP
(Up to)
34 ATP
Aerobic respiration
Production of ATP
The total amount of ATP that a cell is able to harvest from each glucose
molecule that enters glycolysis depends on the presence or absence of
oxygen. As shown in Figure 16, cells use energy most efficiently when
oxygen is present. In the first stage of cellular respiration, glucose is
broken down to pyruvate during glycolysis. Glycolysis is an anaerobic
process, and it results in a net gain of two ATP molecules. In the second stage of cellular respiration, the pyruvate passes through either
aerobic respiration or (anaerobic) fermentation. When oxygen is
present, aerobic respiration occurs. When oxygen is not present,
fermentation occurs instead. The NAD! that gets recycled during fermentation allows glycolysis to continue producing ATP. Thus, a small
amount of ATP is produced even during fermentation. Most of a cell’s
ATP is made, however, during aerobic respiration. For each molecule
of glucose that is broken down, as many as two ATP molecules are
made directly during the Krebs cycle, and up to 34 ATP molecules
are produced later by the electron transport chain.
Section 3 Review
List the products of glycolysis. What is the
role of each of these products in cellular
4B
respiration?
Summarize the roles of the Krebs cycle and
the electron transport chain during aerobic
4B
respiration.
Describe the role of fermentation in the second
4B
stage of cellular respiration.
Critical Thinking Comparing Functions
Critical Thinking Inferring Conclusions
Excess glucose in your blood is stored in your
liver as glycogen. How might your body senses
when to convert glucose to glycogen and glyco4B
gen back to glucose?
TAKS Test Prep When oxygen is present,
most of the ATP made in cellular respiration is
9B
produced by
A aerobic respiration. C alcoholic fermentation.
B glycolysis.
D lactic acid fermentation.
Explain why cellular respiration is more efficient
4B
when oxygen is present in cells.
110
CHAPTER 5 Photosynthesis and Cellular Respiration
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