Cellular Respiration
1.18
On a warm summer day in 1974, 8-year-old Sarah suddenly felt pins and
needles in the muscles of her legs as she walked. Within a year’s time, she could
no longer walk without experiencing muscle pain, shortness of breath, and a
racing heart. By the age of 16, Sarah was attending school in a wheelchair. Tests
showed that she had high levels of acidity in her blood. Her doctors were
puzzled. After extensive study, a team of researchers traced Sarah’s problem to a
defect in her mitochondria. Her muscles could not make sufficient amounts of
ATP and instead produced large amounts of a compound called lactic acid. In
time, doctors discovered that a simple mixture of vitamin C and vitamin K
reversed the condition. After taking the medication, Sarah began feeling better
almost instantly. By the age of 20, she no longer used a wheelchair and was
completely free of symptoms. The vitamin mixture did not cure Sarah’s
mitochondrial defect; it simply filled a gap in the chemical reactions that
produce ATP. Without a team of investigators and laboratory technicians who
understood the reactions that produce ATP (cellular respiration), Sarah would
have been denied the opportunity to lead a normal life.
Cellular Respiration
Virtually all organisms (autotrophs and heterotrophs) transfer chemical energy
from glucose to ATP by a process called cellular respiration. As you know,
autotrophs, such as green plants, produce glucose by photosynthesis, and
heterotrophs, such as humans, obtain glucose by eating other organisms.
There are two forms of cellular respiration, aerobic cellular respiration and
anaerobic cellular respiration. Aerobic cellular respiration uses oxygen, and
produces large amounts of ATP. Anaerobic cellular respiration does not use
oxygen, and produces small amounts of ATP. Large organisms like humans
rely on aerobic cellular respiration to provide the large amounts of ATP they
need to maintain their daily activities.
Aerobic Cellular Respiration
Aerobic cellular respiration occurs in two stages, glycolysis and oxidative
respiration. Glycolysis is a series of 10 enzyme-catalyzed reactions, occurring in
the cytoplasm, that essentially breaks one molecule of glucose (C6H12O6) into
two molecules of pyruvate (C3H6O3). The reactions of glycolysis are exergonic
reactions, meaning that they release energy. Some of the released energy is
captured by producing two molecules of ATP. This is accomplished by attaching
a phosphate group to each of two molecules of ADP. The formation of ATP
absorbs the chemical energy released by glucose. A reaction that absorbs energy,
like the formation of ATP, is called an endergonic reaction. The equation on the
next page summarizes the process of glycolysis.
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aerobic cellular respiration a
form of cellular respiration that uses
oxygen and produces large amounts
of ATP
anaerobic cellular respiration a
form of cellular respiration that does
not use oxygen and produces small
amounts of ATP
glycolysis a series of reactions
occurring in the cytoplasm that
breaks a glucose molecule into two
pyruvate molecules and forms two
molecules of ATP
Cellular Biology 77
glycolysis
C6H12O6
glucose
2 C3H6O3
pyruvate
2 ADP 2 Pi
2 ATP
(Note: Pi symbolizes a phosphate group.)
oxidative respiration a series of
reactions occurring in the
mitochondrion that uses oxygen to
convert pyruvate into carbon
dioxide, water, and ATP
The two molecules of pyruvate formed in glycolysis are transported into the
matrix of a mitochondrion where, through a series of exergonic reactions
involving oxygen, they are broken down into six molecules of carbon dioxide
(CO2) and six molecules of water. Because oxygen is used, this series of
reactions is known as oxidative respiration. This process is also called the
Krebs’ cycle in honour of Hans Krebs, the biochemist who discovered these
reactions. Much of the large amount of energy released in this process is
captured by the production of 34 molecules of ATP. The following equation
summarizes the process of oxidative respiration.
oxidative respiration
2C3H6O3 6O2
pyruvate oxygen
DID YOU
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Cyanide
Cyanide blocks the reactions of
oxidative respiration. This disruption
severely reduces the amount of ATP
that is produced in an animal’s
body, resulting in coma and death.
This is why cyanide is a poison.
6CO2 6H2O
carbon dioxide water
34 ADP 34 Pi
34 ATP
The oxygen used in oxidative respiration is obtained from air or water;
that is why land animals have lungs and fish have gills. Carbon dioxide is
released to the environment as a waste product.
The overall process of aerobic respiration (glycolysis oxidative
respiration) uses oxygen to convert glucose into carbon dioxide, water, and
ATP. The following reaction summarizes the overall process of aerobic
respiration:
aerobic respiration
C6H12O6 6O2
glucose oxygen
6CO2 6H2O
carbon dioxide water
36 ADP 36 Pi
36 ATP
The Mitochondrion and Aerobic Cellular
Respiration
mitochondrial DNA the DNA in
mitochondria
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Unit 1
The mitochondrion (Figure 1) is the site of oxidative respiration, and is
therefore the location in a cell where most ATP is produced. That is why the
mitochondrion is sometimes called the powerhouse of the cell. Some of the
enzymes that catalyze the reactions of aerobic respiration are free floating in
the matrix of the mitochondrion, while others are embedded in the inner
mitochondrial membrane. Mitochondria are so important to the cells of
larger organisms that they possess their own DNA molecules, called
mitochondrial DNA, or mDNA. Mitochondrial DNA allows mitochondria
to reproduce.
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Section 1.18
mitochondrial matrix
cristae
outer mitochondrial
membrane
inner mitochondrial
membrane
Figure 1
Oxidative ATP synthesis occurs in
the matrix and the cristae of a
mitochondrion.
Anaerobic Respiration
When oxygen is not available, organisms may still produce ATP from glucose
through the process of anaerobic cellular respiration. Some cells accomplish
this through a series of reactions called fermentation. There are two forms of
fermentation, ethanol fermentation and lactate (lactic acid) fermentation.
Ethanol fermentation occurs in yeast cells (single-celled fungi), and lactate
fermentation occurs in human muscle cells during periods of strenuous
exercise.
Ethanol Fermentation
In ethanol fermentation, glucose is first broken down by the reactions of
glycolysis (Figure 2). This produces two molecules of pyruvate and two
molecules of ATP. An enzyme in the cytoplasm called pyruvate decarboxylase
removes a carbon dioxide molecule from pyruvate, converting it into ethanol,
the alcohol found in alcoholic beverages. The reactions of ethanol fermentation
all occur in the cytoplasm of the cells. The two ATP molecules produced satisfy
the organism’s energy needs, and the ethanol and carbon dioxide are released
as waste products. Humans have learned ways of making use of these cellular
wastes. Ethanol fermentation carried out by yeast cells is of great historical,
economic, and cultural importance. Breads and pastries, wine, beer, liquor, and
soy sauce are all products of fermentation (Figure 3, on the next page).
2 ADP
glucose
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2 ATP
GLYCOLYSIS
CO2
2 pyruvate
2 ethanol
ethanol fermentation a form of
anaerobic respiration in which
glucose is converted into ethanol,
ATP, and carbon dioxide gas
Figure 2
Ethanol fermentation creates
ethanol, carbon dioxide, and ATP
from glucose.
Cellular Biology 79
Figure 3
Ethanol fermentation is used in the
production of baked goods, wine,
and beer.
lactate (lactic acid) fermentation
a form of anaerobic respiration in
which glucose is converted into
lactate and ATP
oxygen debt the extra oxygen
required to clear the body of lactate
that accumulates in muscle cells
during vigorous exercise
Bread may be leavened (assisted to rise) by mixing live yeast cells with
starches (in flour) and water. The yeast cells ferment the glucose (from the
starch in flour) and release carbon dioxide and ethanol. Small bubbles of
carbon dioxide gas cause the bread to rise, and the ethanol evaporates away
when the bread is baked. In beer making and winemaking, yeast cells ferment
the sugars found in sugar-rich fruit juices such as grape juice. The mixture
bubbles as the yeast cells release carbon dioxide gas and ethanol during
fermentation. In winemaking, fermentation ends when the concentration of
ethanol reaches approximately 12%. At this point, the yeast cells die as a result
of ethanol accumulation and the product is used as a beverage. Plants with
flooded roots also undergo ethanol fermentation (in the roots) and may die
if the roots are not exposed to oxygen.
Lactate (Lactic Acid) Fermentation
When oxygen is plentiful, animals such as humans respire glucose by aerobic
respiration. However, during strenuous exercise, muscle cells break down
glucose faster than oxygen can be supplied. Under such low-oxygen conditions,
oxidative respiration slows down and lactate fermentation begins. Like ethanol
fermentation in yeast cells, lactate fermentation begins with glucose being
broken down in the reactions of glycolysis (Figure 4). As usual, this produces
two molecules of pyruvate and two molecules of ATP. An enzyme in the
cytoplasm called lactate dehydrogenase then converts pyruvate into lactate.
glucose
GLYCOLYSIS
Figure 4
Lactate fermentation
Figure 5
Marathon runners are fatigued after
a race because of the accumulation
of lactate in their muscles. Panting
provides the oxygen needed to
respire the excess lactate.
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Unit 1
2 ADP
2 pyruvate
2 lactate
2 ATP
The accumulation of lactate molecules in muscle tissue causes stiffness,
soreness, and fatigue. When vigorous exercise ends, lactate is converted back to
pyruvate, which then goes into mitochondria and proceeds through the
reactions of oxidative respiration. As usual, this produces lots of ATP, but
requires oxygen. The extra oxygen needed to clear the body of lactate that
accumulates during vigorous exercise is referred to as the oxygen debt.
Panting after bouts of strenuous exercise is the body’s way of “paying” the
oxygen debt (Figure 5).
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Section 1.18
Clothespins and Muscle Fatigue
TRY THIS activity
In order to contract, your muscles require energy in the
form of ATP. Muscles can produce ATP by using oxygen
(aerobic respiration) or not (anaerobic respiration).
Anaerobic respiration in muscle cells produces lactic
acid. When muscles do a lot of work quickly, the buildup
of lactic acid reduces their ability to contract, until
eventually exhaustion sets in and contraction stops
altogether. This is called muscle fatigue.
Materials: clothespin, timer
1. Hold a clothespin between the thumb and index finger
of your dominant hand (the one you use the most).
2. Count the number of times you can open and close
the clothespin in a 20-s period while holding the
other fingers of the hand straight out. Make sure to
squeeze quickly and completely to get the maximum
number of squeezes for each trial.
3. Repeat the process for nine more 20-s periods,
4.
(a)
(b)
(c)
(d)
recording the result for each trial in a suitable table.
Do not rest between trials.
Repeat steps 1, 2, and 3 for the nondominant hand.
What happened to your strength as you progressed
through each trial?
Describe how your hand and fingers felt near the end
of your trials.
Were your results different for the dominant and the
nondominant hand? If yes, explain why.
Your muscles will likely recover after about 10 min of
rest. Explain why.
Photosynthesis and Cellular Respiration
Photosynthesis and cellular respiration are closely related. Plants carry out
photosynthesis and cellular respiration because they contain chloroplasts and
mitochondria. Animals can carry out cellular respiration but not photosynthesis
because they possess mitochondria but not chloroplasts. Nevertheless, both
types of organisms rely on each other for the raw materials of both processes.
Plants use carbon dioxide for photosynthesis, and animals, plants, fungi, and
bacteria give off carbon dioxide as a waste product of cellular respiration. Plants
give off oxygen as a byproduct of photosynthesis, and all aerobic organisms use
oxygen in cellular respiration. Thus, the products of one process are the raw
materials of the other (Figure 6).
rb
ca
mitochondria
ATP
usable energy
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r a te and ox
yge
oh y d
n
high-energy
compound
plants
and
animals
light
energy
plants
low-energy
compounds
C O a n d H 2O
2
chloroplast
Figure 6
Photosynthesis uses the products of
cellular respiration, and cellular
respiration uses the products of
photosynthesis.
Cellular Biology 81
DID YOU
KNOW
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Rigor Mortis
When an animal dies, all cells of the
body do not die at the same time.
Immediately after death, the
temperature of the body begins to
drop and muscles become stiff. This
is called rigor mortis. However, rigor
mortis is not caused by the drop in
body temperature but by the
fermentation of glucose in muscle
cells, leading to high levels of lactic
acid. The lactic acid causes muscle
tissue to become rigid. Rigor mortis
sets in much sooner if death occurs
immediately following strenuous
activity such as running.
The relationship that exists between photosynthesis and cellular
respiration reveals a dependency between autotrophs and heterotrophs.
Autotrophs produce almost all of the oxygen in the environment, and
heterotrophs produce almost all of the carbon dioxide in the environment.
Section 1.18 Questions
Understanding Concepts
1. Why is it possible for organisms to carry out photosynthesis and cellular
respiration but not photosynthesis only?
2. (a) What is the key difference between aerobic respiration and anaerobic
respiration?
(b) Which form of respiration is more suited to large organisms such as
humans, cows, and fish? Which form is adequate for single-celled
organisms such as yeast cells and bacteria? Explain.
3. Describe three cell processes that require the use of ATP.
4. (a) What is meant by the term exergonic reaction?
(b) Is the formation of ATP during the reactions of glycolysis an endergonic
reaction or an exergonic reaction? Explain.
5. (a) Write an overall chemical equation for glycolysis.
(b) Where in a cell do the reactions of glycolysis occur?
(c) In which organelle do the reactions of oxidative respiration occur?
6. (a) How many ATP molecules are created in glycolysis from one glucose
molecule?
(b) How many ATP molecules are created in the overall process of aerobic
respiration from one glucose molecule?
7. (a) What is the cause of the pain and stiffness felt after strenuous exercise?
(b) Why does the pain eventually subside?
8. How many ATP molecules does a yeast cell obtain from every glucose
molecule that undergoes ethanol fermentation?
Applying Inquiry Skills
9. A sealed, flexible container of apple juice is left on a counter at room
temperature. After one week, the container looks inflated.
(a) State a hypothesis that may explain this observation.
(b) Describe an experiment you could perform to test your hypothesis.
Making Connections
10. Describe two ways in which humans make use of ethanol fermentation.
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