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3.2
Moving to the Mitochondrion
E X P E C TAT I O N S
Describe the energy transformations that occur in aerobic cellular respiration.
Explain the role of enzymes in metabolic reactions in the mitochondrion.
Interpret qualitative data in a laboratory investigation of enzyme activity in
the Krebs cycle.
Explain the process of anerobic cellular metabolism.
Research and describe how cellular processes are used in the food industry.
Whereas glycolysis occurs in the cell cytosol, the
remaining reactions of aerobic cellular respiration
take place inside the mitochondria. As you have
learned in previous studies, mitochondria are
small organelles found in eukaryotic cells. A
mitochondrion is composed of an outer and inner
membrane, separated by an intermembrane space,
as shown in Figure 3.7. The inner membrane folds
as shelf-like cristae and contains the matrix, an
enzyme-rich fluid. The cristae and the matrix
are the sites where ATP synthesis occurs. More
mitochondria are found in cells that require
more energy, such as muscle and liver cells.
The two mitochondrial membranes have
important differences in their biochemical
composition. The outer membrane contains a
transport protein called porin that makes it
permeable to all molecules of 10 000 u (atomic
mass units) or less. It also contains enzymes that
help convert fatty acids to molecules that can pass
into the interior of the mitochondrion to be broken
down further. The inner membrane contains a high
proportion of cardiolipin, a phospholipid that
makes this membrane especially impermeable to
ions. This relative impermeability, as you will
discover when you learn about chemiosmosis, is
very important in the production of ATP.
The intermembrane space is a fluid-filled area
containing enzymes that use ATP. During the
synthesis of ATP, the intermembrane space serves
as a hydrogen ion (H+ ) reservoir, storing the
hydrogen ions that will be used for ATP synthesis.
Finally, the inner mitochondrial compartment,
comprised of the matrix and the cristae, remains
relatively isolated from the outer mitochondrial
membranes. The inner membrane contains the
multienzyme complexes and the electron carriers
of the electron transport chain. During aerobic
cellular respiration, the transfer of electrons
through this chain creates a high hydrogen ion (H+ )
cristae
matrix
outer
membrane
intermembrane space
200 nm
inner membrane
Figure 3.7 Structure of the mitochondrion
Chapter 3 Cellular Energy • MHR
69
concentration in the intermembrane space. This
process creates a positively charged gradient in
which there is a greater concentration of hydrogen
ions on one side of the membrane than the other. The
passage of these protons to the inner mitochondrial
compartment drives the synthesis of ATP.
to escape. This compromises the electron transport
chain, which is indirectly involved in ATP production. Aerobic respiration stops. Cytochrome c, a
key component of the electron transport chain, is
released into the cytosol. Here, cytochrome c may
combine with ATP and a protein, forming an apoptosome, or apoptosis activator. Endonuclease and
protease enzymes in the cytosol are activated and
break down the nucleus and the rest of the cell.
BIO FACT
Eukaryotic cells are believed to have arisen from a
relationship, called endosymbiosis, between two prokaryotic
cells, or bacteria. In this relationship, one cell, the
endosymbiont, lived inside the other cell, called the host
cell. The endosymbiont provided the host cell with a surplus
of ATP molecules, while the host cell provided some
metabolic functions for the endosymbiont. The endosymbiont
eventually took the role of the mitochondrion found in
eukaryotic cells of today.
ELECTRONIC LEARNING PARTNER
To learn more about how oxygen and nutrient levels can
affect ATP production in mitochondria, go to your Electronic
Learning Partner now.
The Transition Reaction
Mitochondria are the main source of ATP
molecules in cells. When the mitochondria are not
functioning properly, a depletion of ATP molecules
can occur in tissues, such as muscle, brain, and
heart. These tissues require large amounts of energy
to function properly. Insufficient amounts of ATP can
result in cell damage and even cell death. Eventually,
entire body systems can fail, and the health of the
organism may be severely compromised.
The intact membranes of the mitochondria
are important to the overall health of the cell.
Mitochondria are believed to be key activators for
apoptosis, or programmed cell death. When the
mitochondrion produces ATP, the membranes are
polarized by the high concentration of H+ ions in
the intermembrane space. One of the early steps
towards cell death is depolarization, the loss of
this concentration gradient. Pores in both the
outer membrane and inner membrane cause ions
cytosol
The transition reaction, which occurs in the
matrix of the mitochondrion, is the first step in the
process of aerobic cellular respiration. This process
continues as long as sufficient levels of oxygen are
available in the mitochondrion. If little or no oxygen
is available, pyruvate in the cytosol can be oxidized
through one of two fermentation processes. These
processes will be described later in this section.
Pyruvate crosses the mitochondrion’s outer
membrane and then enters the matrix by way of a
transport protein in the inner membrane (see
Figure 3.8). Once in the matrix, the pyruvate
dehydrogenase complex (a complex of enzymes)
aids the process of oxidative decarboxylation.
NAD+ removes two electrons, oxidizing pyruvate.
Carbon dioxide is removed, leaving a two-carbon
acetyl group that combines with coenzyme A to
form acetyl-CoA. Coenzyme A is a compound that
A One carbon atom and two oxygen atoms are
removed from pyruvate as a CO2 molecule.
mitochondrion
transport protein
NAD+
O
NADH + H+
−
B
C
O
C
O
CH3
pyruvate
outer membrane
70
S
CoA
C
O
CH3
A
C
CO2
Coenzyme A
C The acetyl group of the acetate ion is transferred
to coenzyme A, forming acetyl CoA.
acetyl CoA
matrix
inner membrane
inner membrane space
MHR • Unit 1 Metabolic Processes
B The remaining two-carbon fragment is oxidized
to form an acetate ion. Electrons from this
reaction are picked up by NAD+ , which is
reduced to form NADH.
Figure 3.8 In the transition reaction, pyruvate is
converted to acetyl-CoA. This reaction marks the
junction between glycolysis and the Krebs cycle.
contains a sulfur-based functional group. This
electronegative sulfur-based functional group binds
to a carbon in the acetyl group to make the reactive
acetyl-CoA.
This product, acetyl-CoA, is a key reactant in the
next step in aerobic respiration, the Krebs cycle. At
this point, you have seen that carbohydrates provide
the energy for ATP production. Lipids and proteins
can be broken down to acetyl-CoA in the cell, and
produce ATP in the mitochondrion. If ATP levels
are high, acetyl-CoA can be directed into other
metabolic pathways, such as the production of fatty
pyruvate
NAD
CO2
+
A The cycle starts when twocarbon acetate and four-carbon
oxaloacetate react to form a
six-carbon molecule, citric acid.
NADH
C
C
S
CoA
acetyl-CoA
C
CoA
C
E Dehydrogenation of
malate forms a third
NADH, and the
cycle begins again.
C
C
C
9
C
C
oxaloacetate (C4)
H+ + NADH
C
C
1
aconitate
(intermediate)
NAD+
8
C
C
C
citrate (C6)
C
2
C
isocitrate (C6 )
C
malate (C4)
NAD+
3
7
C
C
C
C
C
C
C
H2O
NADH + H+
CO2
B Oxidative
decarboxylation
forms NADH
and CO2 .
fumarate (C4)
C
α-ketoglutarate (C5)
C
C
FADH2
D Oxidation
of succinate
forms FADH2. FAD
6
C
CO2
NAD+
4
C
succinate (C4)
NADH + H+
5
C
C
succinyl-CoA (C4)
C
C
C A second
oxidative
decarboxylation
forms another
NADH and CO2 .
C
C
C
C
GTP
GDP
C
C
ADP
S
CoA
ATP
Figure 3.9 The Krebs cycle, which includes nine (numbered) reactions, oxidizes
acetyl-CoA. Can you identify where substrate-level phosphorylation takes place?
Chapter 3 Cellular Energy • MHR
71
acids that are required to produce lipids. The
energy stored in these molecules can be released
later, as needed. You can find a more detailed
description of these other metabolic pathways at
the end of this section.
As the product of the transition reaction from
glycolysis, acetyl-CoA enters the Krebs cycle to
produce ATP.
The Krebs Cycle
The Krebs cycle, named after scientist Hans Krebs,
is a cyclical metabolic pathway that oxidizes
acetyl-CoA to carbon dioxide and water, forming a
molecule of ATP (see Figure 3.9 on page 71). In
Investigation
addition to producing one ATP molecule, the series
of redox reactions that form the Krebs cycle involve
the transfer of electrons. This electron transfer
leads to the formation of three NADH molecules
and one FADH2 molecule.
The Krebs cycle involves a total of nine reactions.
First, an enzyme removes the acetyl group from
acetyl-CoA and combines it with a four-carbon
oxaloacetate molecule to produce a six-carbon
citrate molecule. With the acetyl group removed,
coenzyme A is released to participate in another
reaction in the matrix.
At this point, a series of oxidation-reduction
reactions begins that will result in the formation of
SKILL FOCUS
3 • A
Predicting
Enzyme Activity in the Krebs Cycle
The enzyme succinic dehydrogenase catalyzes the oxidation of the four-carbon
molecule succinic acid to fumaric acid. This is an essential step in the Krebs cycle.
In this investigation, you will use a solution of methylene blue to observe the action
of succinic dehydrogenase. Fresh beef heart tissue will serve as a source of
succinic dehydrogenase.
Pre-lab Questions
What is the purpose of oxidizing succinic acid in the
Krebs cycle?
How does methylene blue indicate succinic
dehydrogenase activity?
Problem
Where is succinic acid oxidized in a cell? In other words,
where does succinic dehydrogenase activity occur?
Prediction
Predict how you could observe succinic dehydrogenase
activity.
Performing and recording
Analyzing and interpreting
Communicating results
Materials
500 mL beaker
3 test tubes
scalpel or sharp knife
3 medicine droppers
hot plate
thermometer
grease pencil
colour chart
blue coloured pencils
(several shades)
0.5 mol/L succinic acid
methylene blue (0.01%)
distilled water
mineral oil
two pieces of beef heart
(each about 2–3 cm3 )
Procedure
1. Set up a hot water bath in the 500 mL beaker, and
maintain it at 37°C.
2. Using the grease pencil, number each test tube.
CAUTION: The indicator methylene blue is a dye.
Avoid any contact with skin, eyes, or clothes.
Flush spills on your skin immediately with copious
amounts of water and inform your teacher.
Exercise care when handling hot objects. Handle
thermometers with care. Follow your teacher’s
instructions on the safe use of scalpels. Dispose
of all chemicals according to your teacher’s
instructions and wash your hands before leaving
the laboratory.
72
MHR • Unit 1 Metabolic Processes
3. Using the scalpel, cut away any fat tissue from the
beef muscle. Cut away two small pieces of tissue
and set aside.
4. In test tube 1, add a piece of beef heart, 4 drops
of succinic acid, 8 drops of methylene blue, and
8 drops of distilled water.
5. In test tube 2, add 4 drops of succinic acid, 8 drops
of methylene blue, and sufficient distilled water to
equal the volume of the mixture in test tube 1.
two carbon dioxide molecules and one ATP molecule
(produced by substrate-level phosphorylation).
During the cycle, energetic electrons reduce NAD+
and FAD, which combine with H+ ions to form
NADH and FADH2 . Remember that an additional
NADH was produced in the transition reaction.
As you learned in Chapter 2, FAD is a coenzyme
involved in redox reactions. Figure 3.9 shows the
various reactions involved in the Krebs cycle.
Four NADH molecules and one FADH2 molecule
are produced for each molecule of pyruvate that
enters the mitochondrion for aerobic respiration.
Because two molecules of pyruvate enter the
matrix for each molecule of glucose oxidized,
6. In test tube 3, add a piece of beef heart, 8 drops of
methylene blue, and sufficient distilled water to
equal the volume of the mixture in test tube 1.
7. Gently swirl the contents of each test tube. Then,
quickly but carefully, use a medicine dropper to add
about a 2 cm layer of mineral oil to each test tube.
To do this, tip the test tube slightly to one side and
allow the mineral oil to flow down the inside of the
test tube.
eight NADH and two FADH2 are produced for each
molecule of glucose. Of these, six NADH and two
FADH2 , along with two ATP molecules, result
from the reactions of the Krebs cycle. At this
point, oxygen has not been used in the reactions
described. Oxygen plays a crucial role in the
electron transport chain, during oxidative
phosphorylation. NADH and FADH2 molecules that
have been formed by redox reactions in the Krebs
cycle will donate electrons to the electron transport
chain. Energy from these electrons fuels ATP
synthesis by aerobic respiration.
At the end of the cycle, the glucose molecule
that entered glycolysis has been completely
2. Methylene blue is a dye (indicator) that changes
from blue to colourless when it is reduced by other
substances in a chemical reaction. Describe the
chemical reaction that was responsible for changing
the colour of the methylene blue in the test samples.
3. What other controls could be incorporated into this
procedure?
4. Why was it necessary to add a layer of oil to the
surface of each mixture?
Conclude and Apply
5. What cell structures (organelles) contain succinic
dehydrogenase? Why would you expect to find high
concentrations of these cell organelles in
mammalian heart tissue?
6. Would this procedure work with other types of
animal tissue? Explain briefly.
8. Place the test tubes in the water bath. Record the
initial colour of the mixture in each test tube. Use a
colour chart or make a series of categories, such as
dark blue, medium blue, light blue, colourless. You
can also make a series of sketches using coloured
pencils. If possible, take a series of photographs of
the samples to augment your written observations.
9. Observe the colour of the mixture in each test tube
every 5 min. for about 30 min. Use a data chart to
record your observations.
Post-lab Questions
7. How could you modify this procedure to obtain
more accurate evidence of succinic dehydrogenase
activity?
Exploring Further
8. Repeat the investigation using the additional
controls you outlined in answer to question 3.
How would these controls help you to interpret
the results?
9. Does plant tissue produce similar succinic
dehydrogenase activity? Repeat the procedure
using germinating white beans in place of beef heart
tissue. What other plant parts might be suitable for
this investigation? Explain briefly.
1. Which sample(s) showed the most pronounced
change in colour? Why?
Chapter 3 Cellular Energy • MHR
73
catabolized. As Table 3.1 shows, by the end of
the Kreb’s cycle, energy contained in the original
molecule of glucose has been used to form four
ATP molecules and 12 electron carriers.
2e−
reduction
NADH
Figure 3.9 shows that succinate, the ionic form
of succinic acid, is oxidized to produce one
molecule of FADH2 . The enzyme that catalyzes this
reaction is succinic dehydrogenase. You will study
the action of this enzyme in Investigation 3-A.
Table 3.1
NAD+ + H+
The output of energy molecules up to the end
of the Krebs Cycle
A NADH and FADH2 bring electrons
to the electron transport chain.
ATP is produced at the ATP
synthase complex, but the
diagram shows the power of the
ADP + Pi
proton pumps for chemiosmosis.
NADH
reductase
oxidation
H+
+
H
2e−
ATP
2e−
reduction
FADH2
B Each pair of electrons from
NADH pulls 3 pairs of H+
ions into the intermembrane
space to make 3 ATP with
ATP synthase.
FAD + 2H
coenzyme
Q
2e−
D Each of the electron carriers
becomes reduced and then
oxidized as the electrons
move down the chain.
reduction
cytochrome
b, c1
ADP + Pi
oxidation
2e−
ATP
reduction
cytochrome
c
oxidation
E As a pair of electrons is
passed from carrier to
carrier, proton pumps
carry H+ ions into the
intermembrane space.
2e−
reduction
cytochrome c
oxidase
ADP + Pi
oxidation
ATP
2e−
F Oxygen is the final
acceptor of the electrons,
and together with hydrogen
becomes water and joins
the general water content
of the cell.
2H+
1
O
2 2
Figure 3.10 Overview of the electron transport chain
74
MHR • Unit 1 Metabolic Processes
glycolysis
ATP
produced
Energy molecules
2 ATP
2 NADH (in cytosol)
oxidation
(decarboxylation)
of pyruvate (× 2)
2 NADH
Krebs cycle (× 2)
2 ATP
6 NADH 2 FADH2
Total
4 ATP
10 NADH 2 FADH2
+
C Each pair of electrons from
FADH2 pulls 2 pairs of H+
ions into the intermembrane
space to make 2 ATP with
ATP synthase.
oxidation
Metabolic
process
H2O
Chemiosmosis and
ATP Production
The final stage of energy transformation in
aerobic cellular respiration includes the
electron transport chain and oxidative
phosphorylation of ADP by chemiosmosis.
The reduced coenzymes NADH and FADH2
shuttle electrons and H+ ions from the
Krebs cycle in the matrix to the electron
transport chain embedded on the cristae,
the folds of the mitochondrion’s inner
membrane. The electron transport chain
involves a series of electron carriers and
multienzyme complexes. These carriers
and complexes oxidize NADH and FADH2
molecules. With their extra electrons
removed, the additional H+ ions also leave.
As a result, the oxidized molecules NAD+
and FAD can participate in a redox reaction
in the matrix, such as the Krebs cycle.
The oxidation and reduction of the
electron carriers in the electron transport
chain releases small amounts of energy.
This energy is then used to power proton
pumps that pull H+ ions across the inner
membrane into the intermembrane space.
These H+ ions are now trapped between
two membranes, building a concentration
gradient between the intermembrane space
and the matrix. The movement of these H+
ions through special channels drives the
production of ATP. (As you learned in the
previous section, this movement of H+ ions
through a special protein complex is called
chemiosmosis.) Figure 3.10 shows how electron
transfer moves H+ ions.
Recall that during glycolysis and the Krebs cycle,
ATP molecules are produced through substratelevel phosphorylation. In this process, the ADP
molecule is phosphorylated. A phosphate group
is moved from another substrate (like PEP) to ADP
to make ATP. In the electron transport chain, the
carriers are reduced (accept electrons) and then
oxidized (lose electrons to the next carrier) in a
sequence. At each step, some energy is liberated and
used to pump H+ (against a concentration gradient)
across the inner membrane to the intermembrane
space. In addition, some of the liberated energy is
lost to the environment as thermal energy. At the
end of this process, the spent (low energy) electrons
must be removed. Oxygen must be present in the
matrix to oxidize the last component of the electron
transport chain. When oxygen is combined with
available H+ ions in the matrix, water is formed. This
allows additional electrons to enter the electron
transport chain and release the energy needed to
pump more H+ ions into the intermembrane space.
ATP is produced when the high concentration of
H+ ions diffuses through the channel of the ATP
synthase complex that is embedded in the inner
membrane, as shown in Figure 3.11. Because of
the crucial role played by oxygen, this process of
A As electrons (e− ) move through the
electron transport chain, hydrogen
ions (H+ ) are pumped from the matrix
into the intermembrane space.
H+
+
H
B A hydrogen ion gradient is formed,
with a higher concentration of ions
in the intermembrane space than
in the matrix.
H+
+
+
H
H
proton
pump
chemiosmosis is also called oxidative
phosphorylation. In the next section, you will
see that a similar process is used to make ATP
in photosynthesis.
When NADH is oxidized, electrons enter the
electron transport chain. The electrons first transfer
to NADH dehydrogenase complex, a multienzyme
system that oxidizes NADH. The liberated
hydrogen ions are released into the matrix, leaving
NAD+ . The electrons are passed along from one
carrier to another, as shown in Figure 3.10. Some
of these carriers are cytochromes, proteins with a
heme group containing an iron atom that can be
oxidized or reduced reversibly.
The two electrons from the FADH2 molecules,
which have less energy than the electrons carried
by NADH, enter the electron transport chain at a
different point. As a result, the less energetic
electrons from FADH2 are responsible for the
production of fewer ATP molecules.
Energy from the electrons powers the
multienzyme complexes, called proton pumps, to
move hydrogen ions (H+ ) into the intermembrane
space, as shown in Figure 3.11. As a result of this
high concentration of H+ ions, the inner membrane
of the mitochondrion becomes positively charged.
At the same time, negative ions are attracted to
the exterior of the outer membrane, helping the
movement of protons through the proton pumps.
C When hydrogen ions flow back into
the matrix down their concentration
gradient, ATP is synthesized from
ADP + Pi by an ATP synthase complex.
H
ATP
synthase
+
electron
carriers
intermembrane
space
H+
+
H
H+
H+
inner
membrane
electron
pathway
NADH
NAD+
FADH2
H+
FAD
4 H+ + O2
H2O
H2O
ADP + Pi
ATP
H+
matrix
Figure 3.11 Mitochondria synthesize ATP by chemiosmosis. ATP production is
based on a gradient of hydrogen ion (H+ ) concentration. The gradient is established
by pumping hydrogen ions into the intermembrane space of the mitochondrion.
Chapter 3 Cellular Energy • MHR
75
The charge difference creates an electrical gradient,
while the concentration difference creates a
chemical gradient. The H+ ions can leave the
intermembrane space through a special channel in
the ATP synthase complex, as shown in Figure 3.11.
Chemiosmosis, the movement of these H+ ions,
creates an electric current of positively charged
particles. This current provides the energy needed
to phosphorylate ADP with inorganic phosphate
ions in the matrix, forming ATP.
The multienzyme ATP synthase complex is not
part of the electron transport chain. The electrons
moving along the electron transport chain are not
directly involved in the production of ATP. These
electrons activate the proton pumps that move
the H+ ions into the intermembrane space. ATP
synthase complex is both a collection of enzymes
that phosphorylate ADP, and a tunnel that allows
the H+ ions to move from the intermembrane space
back into the matrix. This flow of charged particles
provides energy that is used by the enzymes to
phosphorylate ADP, and release H+ ions back into
the matrix for other reactions. These reactions
include the formation of water at the end of the
electron transport chain. This process produces
about 90 percent of the ATP molecules in a cell.
Most ATP production takes place during the
reactions of the electron transport chain and
chemiosmosis, because of the input of NADH and
FADH2 molecules (see Figure 3.12). NADH that is
produced by glycolysis in the cytosol can donate
electrons to the electron transport chain by way of
the outer membrane of the mitochondrion. This
additional step has a cost, however, and the
electrons from the cytosol NADH can only pump
enough protons for two ATP. In contrast, the NADH
molecules that are produced in the matrix can
produce three ATP.
WEB LINK
www.mcgrawhill.ca/links/biology12
Since Boyer and Walker’s work on ATP synthase, biochemists
worldwide have been studying the workings of ATP synthase
complex. To find out more about the intricacies of ATP
synthase, go to the web site above, and click on Web Links.
Prepare an abstract about research into one aspect of the
ATP synthase complex.
cytoplasm
glucose
2
glycolysis
ATP
2 NADH
electron transport chain
2 pyruvate
2 NADH
mitochondrion
2 acetyl-CoA
2 CO2
6 NADH
2
Krebs
cycle
ATP
Substrate-level phosphorylation during glycolysis
and the Krebs cycle produces four ATP for
each molecule of glucose oxidized. Oxidative
phosphorylation from the electron transport chain
and chemiosmosis accounts for the remaining
32 ATP molecules. The total number of ATP
molecules produced from one molecule of glucose
is 36, as shown in
Figure 3.12. The concept
organizer, shown in
Figure 3.13 on the next
page, summarizes the
4
ATP
important concepts
you have learned that
are involved in ATP
6
ATP
synthesis. Refer to the
chapters and sections
listed to review these
concepts.
2 FADH2
18
ATP
4
ATP
4 CO2
O2
ATP Yield
4
+
ATP
H2O
32
ATP
Figure 3.12 Number of ATP
36
76
MHR • Unit 1 Metabolic Processes
ATP
molecules produced for each
molecule of glucose used in
aerobic cellular respiration
CONCEPT ORGANIZER
ATP Synthesis
Enzymes
(Chapter 2, section 2.2)
Energy
(Chapter 2, section 2.1)
Coupled reactions
(Chapter 2, section 2.3)
ATP synthesis
Electrons
(Chapter 1, section 1.1;
Chapter 3, section 3.2)
Chemiosmosis
(Chapter 3, section 3.2)
Enzymes catalyze exothermic chemical reactions, which release energy. Energy
from these reactions is coupled to ATP formation. In the electron transport
chain, energy from electrons is used to move H+ ions into the intermembrane
space. During chemiosmosis, the movement of hydrogen ions (H+ ) across the
inner membrane releases energy, which is used to form ATP molecules.
Figure 3.13 ATP formation during cellular respiration
THINKING
LAB
Metabolic Rate and Exercise
Background
Researchers working on the impact of exercise and diet on
human metabolic processes have come up with six actions
to improve resting or basal metabolic rate (BMR). These six
actions increase the efficiency with which cells metabolize
food molecules into ATP and anabolize macromolecules for
cellular work.
1. Exercise frequently, for a long period of time (between
30 and 60 minutes), and with sufficient intensity.
2. Increase metabolically active tissue (or muscles) through
total-body exercise.
3. Eat well between five and six times per day.
4. Eat within 30 to 60 minutes after exercising.
5. Split your exercise routine into two sessions per day,
and drink a carbohydrate drink 30 to 60 minutes after
each session.
2. How would any of these actions increase the rate at
which you metabolize food? Record your findings in
a notebook.
3. How could you calculate your basal metabolic rate?
4. If you are able, experiment with any two or more of the
above actions to improve your basal metabolic rate.
First, determine your basal metabolic rate before you
begin your study. Then practise the actions for at least
three weeks, recording your exercise and eating
regimen daily.
Calculate your BMR
again and compare
the result to your
original BMR. Did
a three-week
period of prescribed
exercise and diet
increase or decrease
your BMR?
6. Concentrate on exercising large muscle groups.
You Try It
1. Using the library and/or Internet resources, conduct
further research on human basal metabolic rate
focussing on the actions listed above.
Chapter 3 Cellular Energy • MHR
77
Anaerobic Cellular Respiration
When oxygen is not present, there is no electron
acceptor to remove electrons at the end of the
electron transport chain. As a result, no further
oxidation of NADH and FADH2 is possible. No H+
ions can be pumped into the intermembrane space.
Without the oxidative coenzymes NAD+ and FAD,
the Krebs cycle stops. The transition reaction, which
requires NAD+ , also stops. Pyruvate can enter the
mitochondrion, but it cannot be processed. The
Biology Magazine
concentration of pyruvate inside the mitochondrion
increases until it matches the concentration in the
cytosol. As a result, there is no net movement of
pyruvate into the mitochondrion. This leads to a
high level of pyruvate in the cytosol, where one
of two possible fermentation pathways may be
followed. These pathways are examples of anaerobic
cellular respiration, the production of ATP in
the cell without the use of oxygen. Figure 3.14
compares aerobic and anaerobic cellular respiration.
TECHNOLOGY • SOCIETY • ENVIRONMENT
Nutrition and Cellular Energy
then also enter the Krebs cycle in the mitochondria to
produce ATP.
Most people know that a healthy, balanced diet is
important for good health. Dieticians recommend that
the average person should consume a balanced diet in
which 60 to 65 percent of daily energy requirements
come from carbohydrates, 30 to 35 percent from fat, and
the remainder from protein. What role does each type of
food play in the production of cellular energy?
Sources of dietary fats include butter, cream, margarine,
and oil, and any food item prepared with these
ingredients. Fats are also found in meats and nuts. Health
professionals usually recommend choosing unsaturated
fats such as olive or canola oil over saturated fats such
as butter or margarine.
Carbohydrates
Protein
Carbohydrates are essential to cellular respiration. They
break down to form simple sugars such as glucose, which
is the primary reactant for glycolysis. Glucose undergoes
glycolysis to produce pyruvic acid, which combines with
coenzyme A (CoA) to form acetyl-CoA. Acetyl-CoA enters
the Krebs cycle to produce ATP molecules.
Once in the body, protein is broken down into amino
acids. The amino acids contribute to the production of
cellular energy via several routes.
Excess carbohydrates are converted into glycogen, a
large molecule that is stored in muscle and liver cells.
When the body requires energy, the glycogen can be
broken down into molecules of glucose to begin glycolysis.
Amino acids can also be converted into lipids and
stored in the body’s fatty tissue.
Good sources of dietary carbohydrates include pasta,
rice, whole grains, and bread. Dieticians usually advise
choosing sources of complex carbohydrates (such as
brown rice or whole-wheat bread) over sources of refined
carbohydrates (such as sugar, white rice, or white bread)
whenever possible. Complex carbohydrates contain
vitamins and dietary fibre, which are missing from refined
carbohydrates.
Amino acids can be converted into glucose or glycogen
to enter glycolysis, eventually entering the Kreb’s cycle
as pyruvic acid.
Amino acids are used for numerous other functions within
the body, including the synthesis of new tissue proteins,
nucleic acids, hormones, and antibodies.
Dietary protein is found in meat, including red meat,
poultry, and seafoods. Protein is also found in eggs and
milk products. You can get protein from plants in the form
of beans, peas, and nuts. Cereals and pasta also contain
certain kinds of protein.
Fats
Fats belong to the group of substances known as
lipids. In the body, lipids are hydrolyzed into glycerol
and fatty acids.
Glycerol is converted to PGAL and then enters
glycolysis. The pyruvic acid produced can then enter
the Krebs cycle in the mitochondria to produce ATP.
Fatty acids may be stored as tissue fat or be
converted to a two-carbon fragment that can enter the
mitochondrion to form acetyl CoA. The acetyl CoA can
78
MHR • Unit 1 Metabolic Processes
Dietary carbohydrates, fats, and protein come from a wide
variety of foods.
Figure 3.15, on page 80, shows the two possible
fermentation pathways. In each case, NADH
molecules in the cytosol (formed by glycolysis II)
are oxidized to form NAD+ , allowing glycolysis
to continue.
Figure 3.14 Following the formation of pyruvate from
glucose, pyruvate may enter either aerobic cellular
respiration or anerobic cellular respiration.
Manipulating Nutrient Intake
People who want to lose weight or reduce body fat
sometimes turn to restrictive low-fat or low-carbohydrate
diets. These types of diets may lead to weight loss in the
short term, but often have detrimental effects on the
body’s metabolic function.
The Effects of Extreme Low-Fat Diets
For example, some people try to control their weight by
severely restricting their fat intake, instead ingesting large
amounts of pasta, bread, and other carbohydrates
Someone who severely restricts their fat intake will not
necessarily prevent further accumulation of fat on their
body. This is particularly true when someone who is
not eating enough fat compensates by overeating
carbohydrates. Glucose from carbohydrates can be
converted to fatty acids via acetyl Co-A during the
transition reaction. Glucose can also be converted to
glycerol through glyceraldegyde-3-phosphate, as shown
below. In short, if you ingest food that contains more
energy than you can use, your body will store it as
fat, regardless of whether the food is carbohydrate,
protein, or fat.
glucose
glyceraldehyde-3-phosphate
glycerol
fats
pyruvate
fatty acids
acetyl-CoA
Excess glucose is used to synthesize fats.
The Effects of Extreme
Low-Carbohydrate Diets
On the opposite end of the scale, extreme lowcarbohydrate diets, which tend also to be low in nutrients
glucose
pyruvate
oxygen present
oxygen absent
aerobic cellular respiration
(reactions inside
mitochondria)
anerobic cellular respiration
(reactions outside
mitochondria)
overall, can also cause metabolic problems. People on
these diets are forced to derive their energy primarily from
stored fats. Breaking down lipids to form acetyl-CoA for
the Krebs cycle, however, produces byproducts that can
be harmful in large amounts.
These byproducts are compounds with ketone groups.
Some of these compounds are acids. The condition
caused by the build-up of ketones is called ketosis, and
is characterized by the smell of acetone on the breath. If
the condition progresses, the acidic ketones can lower
the blood pH to dangerous levels. Brain damage or even
death can result. This extreme condition is called
ketoacidosis.
Most experts agree that the best way to stay healthy and
maintain a healthy weight is to eat a broad variety of
foods that provide the right balance and quantity of
carbohydrates, protein, and fats.
Follow-Up
1. What percentage of your daily food intake comes
from carbohydrates, lipids, and proteins? Keep a
food log for several days to monitor your intake of
carbohydrates, proteins, and fats. Should you make
any adjustments to your diet to provide a healthier
mix of dietary energy sources?
2. To help people to follow a healthy, balanced diet,
organizations and governments issue dietary
guidelines. Do some research on the Internet to
find out about several different dietary guidelines
produced for the general public. For example, you
might choose to compare the Canada Food Guide
and the Federal Dietary Guidelines (U.S.).
(a) How do the guidelines compare with respect
to recommended nutrient sources and the
percentage of each type of nutrient?
(b) Which set of guidelines is easier to understand
and follow?
(c) Make recommendations on how you would
improve or expand upon each set of guidelines.
Chapter 3 Cellular Energy • MHR
79
Alcohol Fermentation
In alcohol fermentation, two reactions of
fermentation convert pyruvate to ethanol. The first
reaction reduces pyruvate to CO2 and acetaldehyde.
In the second reaction, NADH reduces the
acetaldehyde to ethanol (see Figure 3.15). Two NAD+
are formed and participate in glycolysis II. This
process ensures that glycolysis can continue. Yeast
cells, shown in Figure 3.16, are eukaryotes that
carry out alcohol fermentation. This process is
used in the manufacture of wine, for example. The
glucose
C6
2
ATP
2 ADP
2
PGAL
C3
P
2 Pi
2 NAD +
2 NADH
2
PGAP
P
P
C3
4 ADP
4
ATP
2
2 NADH
pyruvate
Lactic Acid Fermentation
Lactic acid fermentation occurs in certain fungi,
bacteria, and muscle cells. For example, when
human muscle cells undergo strenuous activity
they may experience a depletion of oxygen. When
there is insufficient oxygen for muscle cells to
undergo aerobic cellular respiration, the cells will
continue to produce ATP through fermentation.
During this process, no CO2 is produced. Instead,
lactic acid is produced. During strenuous activity,
lactic acid levels increase in muscle cells. In the
short term, lactic acid buildup can cause muscle
fatigue and pain resulting from muscle cramps. The
excess lactic acid is eventually removed from the
cells by the circulatory system and muscle pain
begins to ease. The lactic acid is cleansed from the
bloodstream by the liver. There is a physiological
advantage to producing lactic acid when energy
output exceeds oxygen intake. Through lactic acid
fermentation, muscles can continue to function in
the absence of oxygen.
Table 3.2 compares the number of ATP
molecules produced through anerobic and aerobic
cellular respiration.
Table 3.2
C3
2 NAD
+
CO2
or
formation of ethanol from pyruvate occurs naturally
if grapes are allowed to ferment on the vine or
ferment in winemaking vats. When the ethanol
concentration reaches a certain level, 12–16 percent
depending on the variety of yeast being used, the
toxic effects of the ethanol kill the yeast cells.
Lactic acid and ethanol fermentation produce the same
number of ATP molecules. These processes are far less
efficient in ATP synthesis than is aerobic cellular respiration.
Cellular respiration
acetaldehyde
Anerobic
2 lactic acid (C3 )
2 ethanol (C2 )
Figure 3.15 Possible pathways of pyruvate metabolism
Lactic acid
glucose
glycolysis
pyruvate
fermentation
Ethanol
glucose
glycolysis
Aerobic
glucose
glycolysis
pyruvate
fermentation
2 lactic acid
+
CO2
+
2 ATP
2 ethanol
+
2 ATP
pyruvate
Krebs cycle
and electron
transport chain
CO2
+
water
+
36 ATP
Figure 3.16 These yeast cells can survive in an oxygen-free
environment. How do yeast cells produce ATP molecules?
80
MHR • Unit 1 Metabolic Processes
Probeware
Try the investigation: Aerobic or Anaerobic: Can You Tell
When the Switch Occurs?
BIO FACT
Athletes train to improve the ability of their muscle cells
to accommodate lactic acid buildup. They do this by using
breathing techniques and “sprinting” exercises that serve
to saturate muscle cells with lactic acid. Such exercises
involve working muscles at a maximum heart rate for about
20 minutes, then resting until the body has recovered and
removed the lactic acid from the blood.
Other Metabolic Pathways
Glycolysis, the transition reaction, the Krebs cycle,
and oxidative phosphorylation are the key steps in
aerobic cellular respiration. However, it is important
to consider that intermediate compounds produced
in aerobic cellular respiration may be shunted
sideways into other reactions that are not part
of aerobic cellular respiration. For instance,
carbohydrate metabolism in the mitochondrion,
including the oxidation of glucose by aerobic
cellular respiration, produces many molecules
important in cellular processes (see Figure 3.17).
Anabolic reactions take the carbon-based molecules
that result from cellular metabolism and build
them into such complex macromolecules as steroid
hormones, lipids, porphyrins (e.g. hemoglobin),
and proteins.
Although glucose molecules are the main
substrate in cellular respiration, ATP can be
manufactured through the breakdown of proteins
and fats as well. The breakdown of proteins in this
type of reaction is called deamination which
involves the initial removal of an −NH2 group from
the protein. In fats, β-oxidation removes twocarbon acetate units from the carboxyl end of longchain fatty acids. This reaction occurs in the outer
mitochondrial membrane where enzymes catalyze
the Coenzyme A reaction to produce acetyl-CoA.
This is the same acetyl-CoA that is produced by the
transition reaction of pyruvate. The acetyl-CoA
enters the Krebs cycle in exactly the same manner.
Some organisms, such as anaerobic bacteria,
make enough ATP using fermentation to survive.
Eukaryotes, however, tend to rely on mitochondria
to produce energy. The mitochondria are more
efficient, using the product of glycolysis —
carbohydrate
glucose-6- P
glycerol
GAP
alanine
protein
lipid
pyruvate
fatty acids
acetyl-CoA
cholesterol
aspartate
oxaloacetate
steroid
hormones
citrate
Krebs
cycle
succinate
α-ketoglutarate
porphyrins
(e.g. hemoglobin)
protein
glutamate
pyrimidine
nucleotides
Figure 3.17 Molecules formed in
nucleic acids
conjunction with aerobic cellular
respiration
Chapter 3 Cellular Energy • MHR
81
pyruvate — to make ATP. The organisms that use
aerobic cellular respiration also rely, to a certain
extent, on the anaerobic process of lactic acid
fermentation. The fermentation of lactic acid can
continue to provide muscles with energy in ATP
molecules when oxygen is not available.
In the next section, you will learn that green
plants also use aerobic cellular respiration to
SECTION
REVIEW
9.
I A strain of cells undergoes a mutation that
increases the permeability of the inner mitochondrial
membrane to hydrogen ions.
1.
K/U Explain how enzymes within the mitochondrion
catalyze metabolic reactions that involve the products
of glycolysis.
2.
How many turns of the citric acid cycle are
required to catabolize one molecule of glucose?
(a) What effect would you expect this mutation to
have on the process of cellular respiration?
3.
Draw a diagram that shows three possible
reaction pathways for pyruvate following glycolysis.
Identify the main products of each reaction.
(b) Assuming the mutant cells can survive, how might
the metabolic requirements of these cells differ
from those of a non-mutant strain of the same
variety?
4.
K/U Under what conditions does fermentation take
place in an animal muscle cell? Explain.
5.
K/U The conversion of pyruvate to lactic acid does
not produce any ATP. How, then, does this reaction
contribute to the production of energy by a cell?
6.
K/U
C
C Working with a partner or in a small group, use
an analogy to explain the role of the mitochondrion in
cellular respiration. Outline how various details in the
analogy relate to various components of respiration in
the mitochondrion.
7.
MC In your local grocery store, compare the prices
of foods that are rich in fat with those rich in
carbohydrates. What general pattern can you see?
How could you explain this pattern in terms of cellular
metabolism?
8.
A baker wishes to make a loaf of bread.
According to the recipe, she should first prepare a
yeast culture by mixing some dried yeast with warm
water and a little sugar. The other ingredients are
added to this mixture later.
MC
(a) Draw a diagram that illustrates the process of
respiration taking place in the yeast cells. Why is
the yeast necessary for the bread to rise?
(b) If a strain of yeast existed that employed lactic
acid fermentation, could this yeast be used in
place of ordinary baker’s yeast? Explain.
82
convert pyruvate to ATP molecules. However,
unlike animals, plants must first manufacture
glucose through photosynthesis. Although often
considered the reverse of aerobic cellular
respiration, photosynthesis involves many different
enzymes and metabolic pathways to produce
glucose. You will study photosynthesis in some
detail in the next section.
MHR • Unit 1 Metabolic Processes
10.
I The “four-minute mile” is often cited as an
example of the limit of physical performance. That is,
no matter how much athletes train, there will always
be a limit to their endurance.
(a) Why does this limit exist?
(b) Design an experiment that you could conduct to
test your hypothesis.
11.
MC Crash diets that focus on highly regimented
eating routines often produce yo-yo syndrome, in
which weight lost is quickly regained. In many cases,
dieters gain back more weight than they lost. How
does an understanding of basal metabolic rate
explain this? What kind of advice would help
someone in this situation?
UNIT INVESTIGATION PREP
As you have learned, different types of cells can use
different types of cellular respiration. What experiment
could you perform to distinguish between yeast and
muscle cells in an oxygen-free environment?