3.1
Cellular Respiration
E X P E C TAT I O N S
Identify and describe the four stages of aerobic cellular respiration.
Outline the key steps of glycolysis.
Identify the products of glycolysis.
You are running late for school. The bus stop is
five minutes’ walking distance away, and the bus
is due to arrive in only three minutes. Taking a
shortcut, you sprint the full distance, arriving just
as the bus comes to the stop. You board the bus
and collapse into a seat, breathing heavily. During
your sprint, molecules of glucose were broken
down in your cells. This process provided your
muscles with the energy they needed to carry you
to the bus. As you recall from Chapter 2, the
breakdown of glucose is an exothermic reaction
that releases energy. In this reaction, hydrogen
atoms and electrons are removed from glucose and
added to oxygen. Glucose is oxidized, and oxygen
is reduced. Energy from this reaction is used for
the production of ATP molecules, which are the
energy source for cells. Metabolic pathways that
contribute to the production of ATP molecules in
cells are collectively referred to as aerobic cellular
respiration. The term aerobic means that the
process requires oxygen. The overall equation of
aerobic cellular respiration is shown in Figure 3.1.
ADP + Pi
C6H12O6 + 6O2
glucose
oxygen
ATP
6CO2 + 6H2O + energy
carbon
dioxide
water
Figure 3.1 Aerobic cellular respiration most often involves
the breakdown of glucose, coupled with the manufacture
of ATP.
When a molecule of glucose undergoes aerobic
cellular respiration, 36 molecules of ATP are
produced. Glucose is an energy-rich molecule. The
breakdown of glucose results in the formation of
low-energy molecules and energy. ATP synthesis
requires energy; it involves a series of endothermic
reactions. The exothermic breakdown of glucose
is coupled (linked) to the endothermic reactions
involved in the synthesis of ATP. This coupling
of reactions results in about 40 percent of the
chemical energy in the glucose molecule being
64
transformed into energy in ATP molecules. The rest
of the energy is waste thermal energy.
Releasing energy from a stable molecule, such as
glucose, within the cell requires controlled oxidation.
Controlled oxidation is made possible by a series of
reactions involving various enzymes and metabolic
pathways, as well as coupled reactions. In this way,
each reaction releases only a small portion of
energy, while other reactions conserve this energy
in molecules of ATP.
MHR • Unit 1 Metabolic Processes
Figure 3.2 All organisms, including these E. coli bacteria,
manufacture ATP to fuel cellular functions.
Stages of Aerobic
Cellular Respiration
The process of aerobic cellular repiration can be
divided into four distinct stages. These stages are
summarized in Figure 3.3. Refer to the figure as you
read the following overview. Section 3.1 describes
glycolysis in detail. Section 3.2 examines the three
remaining steps in aerobic cellular respiration.
Overview of Cellular Respiration
The first step in this process is a chain of reactions
called glycolysis. “Glycolysis” means breaking
sugar. This process is anaerobic (without oxygen)
and occurs in the cytosol of cells, outside the
organelles. The process of glycolysis produces
ATP molecules. During this stage, the six-carbon
glucose is broken down into molecules of threecarbon pyruvate. Two pyruvate are produced from
each molecule of glucose. The pyruvate can be used
without oxygen in the process of fermentation, but
no further ATP is produced during this process.
You will learn more about fermentation in section
3.2. If oxygen is present, the pyruvate molecules
enter the mitochondria and the process of aerobic
cellular respiration can occur. Aerobic cellular
respiration is a series of redox reactions that
produce water, carbon dioxide, and additional
ATP molecules.
glucose
C6
Glycolysis
ATP
pyruvate
C3
CO2
Transition
reaction
Fermentation
Glycolysis:
Reactions in the Cytoplasm
acetyl group
C2
cytoplasm
ATP
mitochondrion
Krebs
cycle
e−
ATP
e−
combines with coenzyme A to form acetyl-CoA,
thus connecting glycolysis to the next stage, the
Krebs cycle.
The Krebs cycle, also known as the citric acid
cycle, is a cyclical metabolic pathway located in
the matrix of a mitochondrion. The Krebs cycle
occurs twice (once for each acetyl-CoA molecule)
to oxidize the products of the transition reaction to
carbon dioxide. Only one ATP molecule results
from one cycle of this metabolic pathway.
The final stage, oxidative phosphorylation,
requires oxygen to produce ATP by chemiosmosis,
the movement of concentrated H+ ions through a
special protein complex. Oxidative phosphorylation
relies on the electron transport chain. This is a
series of molecules that are embedded on the inner
membrane of the mitochondrion. The molecules
in the electron transport chain are sequentially
reduced and oxidized to move electrons to a final
step where water is produced.
You will now learn about each stage of cellular
respiration. Each stage is complex. The diagrams in
sections 3.1 and 3.2 offer detailed descriptions of
each stage. Study the diagrams as you read through
the descriptions.
CO2
Krebs
cycle
H2O Electron
transport
chain
O2
Figure 3.3 Steps of cellular respiration
The next stage in the process is the transition
reaction, also called oxidative decarboxylation. In
this reaction each pyruvate loses a carbon atom, or
is decarboxylated, by the oxidative activity of NAD+ .
This reaction changes a three-carbon pyruvate to a
two-carbon acetyl group. This smaller molecule
Glucose is the primary reactant for glycolysis. The
source of glucose may be from either carbohydrates
or from glycogen (a molecule made of many
glucose molecules) stored in muscle and liver cells.
Glycolysis occurs in the cytoplasm of all cells, and
it produces two pyruvate molecules and two ATP
molecules. To accomplish this process, 11 different
enzymes are used. Both prokaryotes (cells without
nuclei) and eukaryotes (cells with nuclei) use
glycolysis in some stage of ATP production. A few
eukaryotes (yeast and mature human red blood
cells) and many prokaryotes (some bacteria) can
survive on the energy produced by glycolysis
alone. However, this amount of energy is not
sufficient for most eukaryotes, which use aerobic
respiration in the mitochondria to increase ATP
production. (Aerobic respiration will be covered in
the next section.) Cellular respiration starts with
glycolysis, which has two main phases:
Glycolysis I: the endothermic activation phase,
which uses ATP
Glycolysis II: the exothermic phase, which
produces ATP molecules and pyruvate
Chapter 3 Cellular Energy • MHR
65
Glycolysis I
Glycolysis I involves a series of endothermic
reactions. In order for glycolysis to begin, activation
energy, from an ATP molecule, must be provided.
This is accomplished in the first reaction of
glycolysis by substrate-level phosphorylation, the
transfer of an inorganic phosphate group (Pi ) from
one substrate to another by way of an enzyme, as
shown in Figure 3.4. This process can remove
(dephosphorylate) a phosphate from ATP or it can
add (phosphorylate) a phosphate to ADP. As shown
in Figure 3.5, one ATP is used to phosphorylate
glucose to form glucose-6-phosphate. This molecule
is then rearranged to form fructose-6-phosphate.
At this point, another ATP molecule must
phosphorylate the fructose-6-phosphate, producing
fructose-1,6-diphosphate. In turn, this molecule is
split into two PGAL (glyceraldehyde-3-phosphate).
These PGALs act as the reactants for glycolysis II.
Glycolysis II
Glycolysis II is a sequence of exothermic reactions
that provides energy for the cell. Following
glycolysis I, each PGAL is oxidized. When PGAL is
oxidized, energetic electrons move to NAD+ , which
is reduced. A hydrogen ion attaches to the reduced
NAD− to form NADH. The oxidized form of PGAL
is now able to attract a free phosphate ion in the
cytosol, forming PGAP (1,3-biphosphoglycerate), as
shown in Figure 3.6. Two PGAP are produced for
each glucose molecule that enters glycolysis.
Following the formation of PGAP, two ADP
molecules (with the help of enzymes) each remove
one phosphate group from each PGAP to form PGA
(3-phosphoglycerate), as shown in Figure 3.6. Here,
substrate-level phosphorylation produces two ATP,
one for each PGAP. At this point, because two
molecules of ATP have been made, and two
molecules of ATP were used to start glycolysis, the
net change in the number of ATP molecules is zero.
glucose
ATP
A Phosphorylation
of glucose by ATP
ADP
glucose-6phosphate
fructose-6phosphate
B Rearrangement,
followed by a second
phosphorylation by ATP
ATP
ADP
fructose-1,
6-diphosphate
glyceraldehyde-3phosphate (PGAL)
C The six-carbon
molecule is split
into two threecarbon molecules.
glyceraldehyde-3phosphate (PGAL)
Figure 3.5 Glycolysis involves the formation of two PGAL
molecules.
Next, the two PGA molecules are each oxidized,
forming two water molecules and two PEP
(phosphoenolpyruvate) molecules. Finally, another
substrate-level phosphorylation occurs — two ADP
molecules each remove the remaining phosphate
group from each PEP molecule. The result is the
production of two ATP molecules and two
pyruvate molecules.
ct
du
substrate
o
pr
P
P
enzyme
ATP
P
P
P
P
ADP
n
ne
osi
ade
adenosine
A
66
B
MHR • Unit 1 Metabolic Processes
Figure 3.4 Substrate-level
phosphorylation. Here, substrate
1 donates one phosphate group to
substrate 2 (ADP), making ATP. The
reverse reaction dephosphorylates
substrate 1, producing ADP.
The entire glycolysis process, including
glycolysis I and glycolysis II, produces a net gain
of two ATP molecules. The energy stored in these
ATP can now be used for cellular respiration in
mitochondria.
glucose
C6
ATP
ATP
ADP
ADP
A Two ATP molecules are used in the first
phase of glycolysis to activate glucose.
−2 ATP
C6
P
P
B The six-carbon molecule is split to form
two molecules of PGAL (glyceraldehyde3-phosphate).
PGAL
PGAL
C3
C3
P
Pi
Pi
NAD
+2 NADH
+
NAD +
NADH
NADH
C3
P
P
C3
ADP
P
ATP
ATP
C3
PGA
P
C3
P
H2O
H2O
PEP
C3
E Oxidation of each PGA molecule removes
water and forms two molecules of PEP
(phosphoenolpyruvate).
PEP
P
C3
ADP
2 ATP
(net gain)
D Phosphorylation removes a phosphate
group from each PGAL and produces two
molecules of ATP and two molecules of
PGA (3-phosphoglycerate).
ADP
PGA
+2 ATP
C Oxidation and phosphorylation of PGAL
results in the formation of two NADH
and two molecules of PGAP
(1,3-bisphosphoglycerate).
PGAP
PGAP
P
+2 ATP
P
P
ADP
ATP
ATP
pyruvate
pyruvate
C3
C3
F Phosphorylation removes a phosphate
group from each PEP molecule to form
two molecules of ATP and two molecules
of pyruvate.
Figure 3.6 Glycolysis including glycolysis I and glycolysis II, starts with one
molecule of glucose and produces two pyruvate molecules. There is a net
gain of two NADH and two ATP molecules from glycolysis.
Chapter 3 Cellular Energy • MHR
67
1.
K/U
What is the purpose of glycolysis?
3.
K/U
List the products of glycolysis.
4.
K/U
How is NAD+ important to glycolysis?
5.
K/U
How is ATP important to start glycolysis?
C Use Figures 3.5 and 3.6 as a guide to make a
flowchart for glycolysis. Include the names of the
intermediate molecules.
7.
K/U Give an example of substrate-level
phosphorylation.
8.
C Using diagrams, explain the process of
substrate-level phosphorylation.
9.
K/U List the coupled reactions that transfer energy
to or from glycolysis.
10.
11.
(a) After the initial investment of two ATP, 38 ATP
are produced. How much energy was used to
phosphorylate one ADP?
K/U List in order the four stages of aerobic cellular
respiration. For each molecule of glucose that enters
the respiration process, identify the number of ATP
molecules produced in each stage.
2.
6.
68
REVIEW
C The process of glycolysis is thought to have
emerged very early in the origins of life. What
evidence to support this theory can you find in the
material presented in this section? Discuss your ideas
with a partner, and then write a brief summary of
your findings.
I The total amount of energy released from the
chemical bonds of glucose is 2870 kJ/mol, after the
initial investment of two ATP. The total energy that
can be harvested to form ATP molecules is about
1200 kJ/mol glucose.
MHR • Unit 1 Metabolic Processes
(b) What percentage of the total energy contained
in glucose is captured during glycolysis?
12.
13.
C Glycolysis is often described as being an
inefficient process. List points in support of this
statement and points against it. Devise a new
statement using a term other than “inefficient”
to describe the process of glycolysis.
I
Increasing energy level
SECTION
glucose
fructose
phosphate
fructose
diphosphate
PGAL
(a) Copy and complete the graph by plotting the
relative energy level of each molecule.
(b) Which molecule has the highest amount of
stored energy?
(c) Which molecule has the least amount of
stored energy?