BIO 101 Catabolic Pathways and Production of ATP Discussion

9
Cellular Respiration
and Fermentation
KEY CONCEPTS
9.1 Catabolic pathways yield energy by
oxidizing organic fuels p. 165
9.2 Glycolysis harvests chemical energy by
oxidizing glucose to pyruvate p. 170
9.3 After pyruvate is oxidized, the citric acid
cycle completes the energy-yielding
oxidation of organic molecules p. 171
9.4 During oxidative phosphorylation,
chemiosmosis couples electron
transport to ATP synthesis p. 174
9.5 Fermentation and anaerobic
respiration enable cells to produce ATP
without the use of oxygen p. 179
9.6 Glycolysis and the citric acid cycle
connect to many other metabolic
pathways p. 182
Study Tip
Make a visual study guide: Draw a cell
with a large mitochondrion, labeling
the parts of the mitochondrion. As you
go through the
Mitochondrion
chapter, add key
reactions for each
stage of cellular
respiration,
linking the stages
together. Label the
carbon molecule(s)
with the most
energy and the carbon molecule(s) with
the least energy. Your cell can be a simple
sketch, as shown here.
Figure 9.1 This hoary marmot (Marmota caligata) obtains energy for its cells by
feeding on plants. In the process of cellular respiration, mitochondria in the cells
of animals, plants, and other organisms break down organic molecules, generating
ATP and waste products: carbon dioxide, water, and heat. Note that energy flows
one way, but chemicals are recycled.
How is the chemical energy stored in food used to
generate ATP, the molecule that drives most cellular work?
Light
energy
Photosynthesis
Organic
+ O2
molecules
CO2 + H2O
Go to Mastering Biology
generates
For Students (in eText and Study Area)
• Get Ready for Chapter 9
• BioFlix® Animation: Cellular Respiration
• Figure 9.12 Walkthrough: Free-Energy
Change During Electron Transport
used in
Cellular respiration in mitochondria
For Instructors to Assign (in Item Library)
• BioFlix® Tutorial: Glycolysis
• BioFlix® Tutorial: Cellular Respiration:
Inputs and Outputs
Ready-to-Go Teaching Module
(in Instructor Resources)
• Oxidative Phosphorylation (Concept 9.4)
generates
used in
breaks down organic
molecules,generating
Plant cell
ATP
Heat
164
powers most
cellular work
Animal cell
CONCEPT 9.1
Catabolic pathways yield energy
by oxidizing organic fuels
Living cells require transfusions of energy from outside sources
to perform their many tasks—for example, assembling polymers, pumping substances across membranes, moving, and
reproducing. The outside source of energy is food, and the
energy stored in the organic molecules of food ultimately comes
from the sun. As shown in Figure 9.1, energy flows into an ecosystem as sunlight and exits as heat; in contrast, the chemical
elements essential to life are recycled. Photosynthesis generates
oxygen, as well as organic molecules used by the mitochondria
of eukaryotes as fuel for cellular respiration. Respiration breaks
this fuel down, using oxygen (O2) and generating ATP. The
waste products of this type of respiration, carbon dioxide (CO2)
and water (H2O), are the raw materials for photosynthesis.
Mastering Biology Animation: Energy Flow
and Chemical Recycling
Let’s consider how cells harvest the chemical energy stored
in organic molecules and use it to generate ATP, the molecule
that drives most cellular work. Metabolic pathways that release
stored energy by breaking down complex molecules are called
catabolic pathways (see Concept 8.1). Transfer of electrons
from food molecules (like glucose) to other molecules plays a
major role in these pathways. In this section, we consider these
processes, which are central to cellular respiration.
Catabolic Pathways and Production of ATP
Organic compounds possess potential energy as a result
of the arrangement of electrons in the bonds between
their atoms. Compounds that can participate in exergonic
reactions can act as fuels. Through the activity of enzymes
(see Concept 8.4), a cell systematically degrades complex
organic molecules that are rich in potential energy to
simpler waste products that have less energy. Some of the
energy taken out of chemical storage can be used to do work;
the rest is dissipated as heat.
One catabolic process, fermentation, is a partial degradation of sugars or other organic fuel that occurs without the
use of oxygen. However, the most efficient catabolic pathway
is aerobic respiration, in which oxygen is consumed as
a reactant along with the organic fuel (aerobic is from the
Greek aer, air, and bios, life). The cells of most eukaryotic and
many prokaryotic organisms can carry out aerobic respiration. Some prokaryotes use substances other than oxygen as
reactants in a similar process that harvests chemical energy
without oxygen; this process is called anaerobic respiration (the
prefix an- means “without”). Technically, the term cellular
respiration includes both aerobic and anaerobic processes.
However, it originated as a synonym for aerobic respiration
because of the relationship of that process to organismal
respiration, in which an animal breathes in oxygen. Thus,
cellular respiration is often used to refer to the aerobic process,
a practice we follow in most of this chapter.
Although very different in mechanism, aerobic respiration is in principle similar to the combustion of gasoline in
an automobile engine after oxygen is mixed with the fuel
(hydrocarbons). Food provides the fuel for respiration, and
the exhaust is carbon dioxide and water. The overall process
can be summarized as follows:
Organic
Carbon
+ Oxygen S
+ Water + Energy
compounds
dioxide
Carbohydrates, fats, and proteins from food can all be processed and consumed as fuel. In animal diets, a major source
of carbohydrates is starch, a storage polysaccharide that can
be broken down into glucose (C6H12O6) subunits. Here, we
will learn the steps of cellular respiration by tracking the
degradation of the sugar glucose:
C6H12O6 + 6 O2 S 6 CO2 + 6 H2O + Energy (ATP + heat)
Mastering Biology BioFlix® Animation: Introduction to
Cellular Respiration
This breakdown of glucose is exergonic, having a freeenergy change of -686 kcal (-2,870 kJ) per mole of glucose
decomposed (?G = – 686 kcal/mol). Recall that a negative
?G1 ?G 6 02 indicates that the products of the chemical
process store less energy than the reactants and that the reaction can happen spontaneously—in other words, without an
input of energy (see Concept 8.2).
Catabolic pathways do not directly move flagella, pump
solutes across membranes, polymerize monomers, or perform other cellular work. Catabolism is linked to work by a
chemical drive shaft—ATP (see Concept 8.3). To keep working, the cell must regenerate its supply of ATP from ADP and
P i (see Figure 8.12). To understand how cellular respiration
~
accomplishes this, let’s examine the fundamental chemical
processes known as oxidation and reduction.
Redox Reactions: Oxidation and Reduction
How do the catabolic pathways that decompose glucose
and other organic fuels yield energy? The answer is based on
the transfer of electrons during the chemical reactions. The
relocation of electrons releases energy stored in organic molecules, and this energy ultimately is used to synthesize ATP.
The Principle of Redox
In many chemical reactions, there is a transfer of one or more
electrons (e – ) from one reactant to another. These electron
transfers are called oxidation-reduction reactions, or redox
reactions for short. In a redox reaction, the loss of electrons
CHAPTER 9
Cellular Respiration and Fermentation
165
from one substance is called oxidation, and the addition of
electrons to another substance is known as reduction. (Note
that adding electrons is called reduction; adding negatively
charged electrons to an atom reduces the amount of positive
charge of that atom.)
To take a simple, nonbiological example, consider the
reaction between the elements sodium (Na) and chlorine (Cl)
that forms table salt:
. Figure 9.2 Methane combustion as an energy-yielding redox
reaction. The reaction releases energy to the surroundings because the
electrons lose potential energy when they end up being shared unequally,
spending more time near electronegative atoms such as oxygen.
Reactants
1
Cl–
becomes reduced
(gains electron)
We could generalize a redox reaction this way:
becomes oxidized
Xe
–
1
Y
X
1 Ye –
becomes reduced
In the generalized reaction, substance Xe -, the electron donor,
is called the reducing agent; it reduces Y, which accepts the
donated electron. Substance Y, the electron acceptor, is the
oxidizing agent; it oxidizes Xe – by removing its electron.
Because an electron transfer requires both an electron donor and
an acceptor, oxidation and reduction always go hand in hand.
Not all redox reactions involve the complete transfer of electrons from one substance to another; some change the degree
of electron sharing in covalent bonds. Methane combustion,
shown in Figure 9.2, is an example. The covalent electrons in
methane are shared nearly equally between the bonded atoms
because carbon and hydrogen have about the same affinity for
valence electrons; they are about equally electronegative (see
Concept 2.3). But when methane reacts with O2, forming CO2,
electrons end up shared less equally between the carbon atom
and its new covalent partners, the oxygen atoms, which are
very electronegative. In effect, the carbon atom has partially
“lost” its shared electrons; thus, methane has been oxidized.
Now let’s examine the fate of the reactant O2. The two
atoms of O2 share their electrons equally. But after the reaction with methane, when each O atom is bonded to two H
atoms in H2O, the electrons of those covalent bonds spend
more time near the oxygen (see Figure 9.2). In effect, each O
atom has partially “gained” electrons, so the oxygen molecule
(O2) has been reduced. Because the O atom is so electronegative, O2 is one of the most powerful of all oxidizing agents.
Energy must be added to pull an electron away from an
atom, just as energy is required to push a ball uphill. The more
electronegative the atom (the stronger its pull on electrons),
the more energy is required to take an electron away from it.
An electron loses potential energy when it shifts from a less
electronegative atom toward a more electronegative one, just
as a ball loses potential energy when it rolls downhill. A redox
reaction that moves electrons closer to an O atom, such as the
166
UNIT TWO
The Cell
2 O2
CO2 +
H
C
Energy + 2 H2O
becomes reduced
H
Na+
Cl
1
+
CH4
becomes oxidized
(loses electron)
Na
Products
becomes oxidized
H
O
O
O
C
O H
O
H
H
Methane
(reducing
agent)
Oxygen
(oxidizing
agent)
Carbon dioxide
Water
VISUAL SKILLS Is the carbon atom oxidized or reduced during this
reaction? Explain.
Mastering Biology Animation: Redox Reactions
burning (oxidation) of methane, therefore releases chemical
energy that can be put to work.
Oxidation of Organic Fuel Molecules
During Cellular Respiration
The oxidation of methane by O2 is the main combustion
reaction that occurs at the burner of a gas stove. The combustion of gasoline in an automobile engine is also a redox
reaction; the energy released pushes the pistons. But the
energy-yielding redox process of greatest interest to biologists
is respiration: the oxidation of glucose and other molecules in
food. Examine again the summary equation for cellular respiration, but this time think of it as a redox process:
becomes oxidized
C6H12O6
1 6 O2
6 CO2 1
6 H2O
1 Energy
becomes reduced
As in the combustion of methane or gasoline, the fuel (glucose) is oxidized and O2 is reduced. The electrons lose potential energy along the way, and energy is released.
In general, organic molecules that have an abundance of
hydrogen are excellent fuels because their bonds are a source
of “hilltop” electrons, whose energy may be released as these
electrons “fall” down an energy gradient during their transfer
to oxygen. The summary equation for respiration indicates
that hydrogen is transferred from glucose to the O atoms in
O2. But the important point, not visible in the summary equation, is that the energy state of the electron changes as hydrogen (with its electron) is transferred to oxygen. In respiration,
the oxidation of glucose transfers electrons to a lower energy
state, liberating energy that becomes available for ATP synthesis. So, in general, we see fuels with multiple C ¬ H bonds
oxidized into products with multiple C ¬ O bonds.
The main energy-yielding foods—carbohydrates and fats—
are reservoirs of electrons associated with hydrogen, often in
the form of C ¬ H bonds. Only the barrier of activation energy
holds back the flood of electrons to a lower energy state
(see Figure 8.13). Without this barrier, a food substance like
glucose would combine almost instantaneously with O2.
If we supply the activation energy by igniting glucose, it burns
in air, releasing 686 kcal (2,870 kJ) of heat per mole of glucose
(about 180 g). Body temperature is not high enough to initiate burning, of course. Instead, if you swallow some glucose,
enzymes in your cells will lower the barrier of activation
energy, allowing the sugar to be oxidized in a series of steps.
Stepwise Energy Harvest via NAD·
and the Electron Transport Chain
If energy is released from a fuel all at once, it cannot be
harnessed efficiently for constructive work. For example,
if a gasoline tank explodes, it cannot drive a car very far.
Cellular respiration does not oxidize glucose (or any other
organic fuel) in a single explosive step either. Rather, glucose
is broken down in a series of steps, each one catalyzed by an
enzyme. At key steps, electrons are stripped from the glucose.
As is often the case in oxidation reactions, each electron
travels with a proton—thus, as a hydrogen atom. The hydrogen atoms are not transferred directly to O2, but instead are
usually passed first to an electron carrier, a coenzyme called
nicotinamide adenine dinucleotide, a derivative of the vitamin niacin. This coenzyme is well suited as an electron carrier
because it can cycle easily between its oxidized form, NAD1,
and its reduced form, NADH. As an electron acceptor, NAD +
functions as an oxidizing agent during respiration.
How does NAD + trap electrons from glucose and the other
organic molecules in food? Enzymes called dehydrogenases
remove a pair of hydrogen atoms (2 electrons and 2 protons)
from the substrate (glucose, in the preceding example),
thereby oxidizing it. The enzyme delivers the 2 electrons
along with 1 proton to its coenzyme, NAD + , forming NADH
(Figure 9.3). The other proton is released as a hydrogen ion
(H + ) into the surrounding solution:
H C OH 1 NAD+
2 e– + H+
NAD+
C
CH2
O
O
P
O–
O
O
P
O
O
N+ Nicotinamide
(oxidized form)
H
O
+ 2H
(from food)
Reduction of NAD+
Oxidation of NADH
H
H
OH
CH2
N
N
H
H
HO
OH
NH2
N Nicotinamide
(reduced form)
+
H+
VISUAL SKILLS
Describe the structural
differences between
the oxidized form and
the reduced form of
nicotinamide.
NH2
N
H
O
O
C
H
HO
–
NH2
H+
NADH
Dehydrogenase
O
C O 1 NADH 1 H+
By receiving 2 negatively charged electrons but only 1 positively charged proton, the nicotinamide portion of NAD + has
its charge neutralized when NAD + is reduced to NADH. The
name NADH shows the hydrogen that has been received in
the reaction. NAD + is the most versatile electron acceptor in
cellular respiration and functions in several of the redox steps
during the breakdown of glucose.
Electrons lose very little of their potential energy when
they are transferred from glucose to NAD + . Each NADH molecule formed during respiration represents stored energy that
can be tapped to make ATP when the electrons complete their
“fall” down an energy gradient from NADH to O2.
How do electrons that are extracted from glucose and
stored as potential energy in NADH finally reach oxygen?
It will help to compare the redox chemistry of cellular respiration to a much simpler reaction: the reaction between
hydrogen and oxygen to form water (Figure 9.4a). Mix H2
and O2, provide a spark for activation energy, and the gases
combine explosively. In fact, combustion of liquid H2 and
O2 is harnessed to help power the rocket engines that boost
satellites into orbit and launch spacecraft. The explosion
represents a release of energy as the electrons of hydrogen
“fall” closer to the electronegative oxygen atoms. Cellular
respiration also brings hydrogen and oxygen together to
form water, but there are two important differences. First, in
cellular respiration, the hydrogen that reacts with oxygen
2 e– + 2 H+
H
Dehydrogenase
N
H
m Figure 9.3 NAD· as an electron shuttle. The full name for NAD + ,
nicotinamide adenine dinucleotide, describes its structure—the molecule consists
of two nucleotides joined together at their phosphate groups (shown in yellow).
(Nicotinamide is a nitrogenous base, although not one that is present in DNA or
RNA; see Figure 5.23.) The enzymatic transfer of 2 electrons and 1 proton (H+ ) from
an organic molecule in food to NAD + reduces the NAD + to NADH: Most of the
electrons removed from food are transferred initially to NAD + , forming NADH.
CHAPTER 9
Cellular Respiration and Fermentation
167
is derived from organic molecules rather than H2. Second,
instead of occurring in one explosive reaction, respiration
uses an electron transport chain to break the fall of electrons
to oxygen into several energy-releasing steps (Figure 9.4b).
An electron transport chain consists of a number of
molecules, mostly proteins, built into the inner membrane
of the mitochondria of eukaryotic cells (and the plasma
membrane of respiring prokaryotes). Electrons removed
from glucose are shuttled by NADH to the “top,” higherenergy end of the chain. At the “bottom,” lower-energy end,
O2 captures these electrons along with hydrogen nuclei (H + ),
forming water. (Anaerobically respiring prokaryotes have
an electron acceptor at the end of the chain that is different
from O2.)
Electron transfer from NADH to oxygen is an exergonic reaction with a free-energy change of -53 kcal/mol (-222 kJ/mol).
Instead of this energy being released and wasted in a single
explosive step, electrons cascade down the chain from one carrier molecule to the next in a series of redox reactions, losing a
small amount of energy with each step until they finally reach
oxygen, the terminal electron acceptor, which has a very great
affinity for electrons. Each “downhill” carrier has a greater affinity for electrons than, and is thus capable of accepting electrons
from (oxidizing), its “uphill” neighbor, with O2 at the bottom
of the chain. Therefore, the electrons transferred from glucose
to NAD +, reducing it to NADH, fall down an energy gradient
in the electron transport chain to a far more stable location in
an electronegative oxygen atom from O2. Put another way, O2
pulls electrons down the chain in an energy-yielding tumble
analogous to gravity pulling objects downhill.
In summary, during cellular respiration, most electrons
travel the following “downhill” route: glucose S NADH S
electron transport chain S oxygen. Later in this chapter, you
will learn more about how the cell uses the energy released
from this exergonic electron fall to regenerate its supply of
ATP. For now, having covered the basic redox mechanisms of
cellular respiration, let’s look at the entire process by which
energy is harvested from organic fuels.
The Stages of Cellular Respiration: A Preview
The harvesting of energy from glucose by cellular respiration is
a cumulative function of three metabolic stages. We list them
here along with a color-coding scheme we will use throughout
the chapter to help you keep track of the big picture:
1. GLYCOLYSIS (color-coded blue throughout the chapter)
2. PYRUVATE OXIDATION (light orange) and the
CITRIC ACID CYCLE (dark orange)
3. OXIDATIVE PHOSPHORYLATION: Electron transport and
chemiosmosis (purple)
Free energy, G
Free energy, G
t
spor
tran
tron ain
ch
Elec
Biochemists usually reserve the term cellular respiration for
stages 2 and 3 together. In this text, however, we include glycolysis as a part of cellular respiration because most respiring
cells deriving energy from glucose use glycolysis to produce
the starting material for the citric acid cycle.
As diagrammed in Figure 9.5, glycolysis
and then pyruvate oxidation
. Figure 9.4 An introduction to electron transport chains.
and the citric acid cycle are the catabolic
(a) Uncontrolled reaction.
(b) Cellular respiration. In cellular respiration, the
pathways that break down glucose and
The one-step exergonic reaction
same reaction occurs in stages: An electron
other organic fuels. Glycolysis, which
of hydrogen with oxygen to
transport chain breaks the “fall” of electrons in this
form water releases a large
reaction into a series of smaller steps and stores
occurs in the cytosol, begins the degradaamount of energy in the form
some of the released energy in a form that can be
tion process by breaking glucose into two
of heat and light: an explosion.
used to make ATP. (The rest of the energy is
released as heat.)
molecules of a compound called pyruvate.
In eukaryotes, pyruvate enters the mito1
+
/2 O2
H2 + 1/2 O2
chondrion and is oxidized to a compound
2H
(from food via NADH)
called acetyl CoA, which enters the citric
Controlled
acid cycle. There, the breakdown of
release of
glucose to carbon dioxide is completed.
+
–
2H + 2e
energy for
(In prokaryotes, these processes take place
synthesis of
ATP
in the cytosol.) Thus, the carbon dioxide
ATP
produced by respiration represents fragExplosive
ATP
ments of oxidized organic molecules.
release of
Some of the steps of glycolysis and
heat and light
ATP
energy
the citric acid cycle are redox reactions in
which dehydrogenases transfer electrons
2 e–
from substrates to NAD + or the related
12 O
2
+
2H
electron carrier FAD, forming NADH or
FADH2. (You’ll learn more about FAD and
H 2O
H2O
FADH2 later.) In the third stage of respiration, the electron transport chain accepts
168
UNIT TWO
The Cell
c Figure 9.5 An overview of cellular
respiration. During glycolysis, each glucose
molecule is broken down into two molecules
of pyruvate. In eukaryotic cells, as shown
here, the pyruvate enters the mitochondrion.
There it is oxidized to acetyl CoA, which will
be further oxidized to CO2 in the citric acid
cycle. The electron carriers NADH and FADH2
transfer electrons derived from glucose to
electron transport chains. During oxidative
phosphorylation, electron transport chains
convert the chemical energy to a form
used for ATP synthesis in the process called
chemiosmosis. (During earlier steps of cellular
respiration, a few molecules of ATP are
synthesized in a process called substrate-level
phosphorylation.) To visualize these processes
in their cellular context, see Figure 6.32b.
Mastering Biology Animation: Overview
of Cellular Respiration
GLYCOLYSIS
Glucose
Pyruvate
CYTOSOL
PYRUVATE
OXIDATION
CITRIC
ACID
CYCLE
Acetyl CoA
OXIDATIVE
PHOSPHORYLATION
(Electron transport
and chemiosmosis)
MITOCHONDRION
ATP
ATP
ATP
Substrate-level
phosphorylation
Substrate-level
phosphorylation
Oxidative
phosphorylation
electrons from NADH or FADH2 generated during the first two
stages and passes these electrons down the chain. At the end of
the chain, the electrons are combined with molecular oxygen
(O2) and hydrogen ions (H +), forming water (see Figure 9.4b).
The energy released at each step of the chain is stored in a form
the mitochondrion (or prokaryotic cell) can use to make ATP
from ADP. This mode of ATP synthesis is called oxidative
phosphorylation because it is powered by the redox
reactions of the electron transport chain.
In eukaryotic cells, the inner membrane of the mitochondrion is the site of electron transport and another process
called chemiosmosis, together making up oxidative phosphorylation. (In prokaryotes, these processes take place in the plasma
membrane.) Oxidative phosphorylation accounts for almost
90% of the ATP generated by respiration. A smaller amount
of ATP is formed directly in a few reactions of glycolysis and
the citric acid cycle by a mechanism called substrate-level
phosphorylation (Figure 9.6). This mode of ATP synthesis
. Figure 9.6 Substrate-level phosphorylation. Some ATP is
made by direct transfer of a phosphate group from an organic
substrate to ADP by an enzyme. (For examples in glycolysis, see
Figure 9.8, steps 7 and 10.)
Enzyme
Enzyme
Electrons carried
via NADH
and FADH2
Electrons carried
via NADH
ADP
occurs when an enzyme transfers a phosphate group from a
substrate molecule to ADP, rather than adding an inorganic
phosphate to ADP as in oxidative phosphorylation. “Substrate
molecule” here refers to an organic molecule generated as
an intermediate during the catabolism of glucose. You’ll see
examples of substrate-level phosphorylation later in the chapter, in both glycolysis and the citric acid cycle.
You can think of the whole process this way: When you
withdraw a relatively large sum of money from an ATM, it is not
delivered to you in a single bill of a large denomination. Instead,
the machine dispenses a number of smaller-denomination
bills that you can spend more easily. This is analogous to ATP
production during cellular respiration. For each molecule of
glucose degraded to CO2 and H2O by respiration, the cell makes
up to about 32 molecules of ATP, each with 7.3 kcal/mol of free
energy. Respiration cashes in the large denomination of energy
banked in a single molecule of glucose (686 kcal/mol under
standard conditions) for the small change of many molecules of
ATP, which is more practical for the cell to spend on its work.
This preview has introduced you to how glycolysis, the
citric acid cycle, and oxidative phosphorylation fit into the
process of cellular respiration so you can keep the big picture in mind as you take a closer look at each of these three
stages of respiration. As you read about the chemical reactions, remember that each reaction is catalyzed by a specific
enzyme, some of which are shown in Figure 6.32b.
CONCEPT CHECK 9.1
P
ATP
Substrate
Product
MAKE CONNECTIONS Review Figure 8.9. In the reaction shown
above, is the potential energy higher for the reactants or for the
products? Explain.
1. Compare and contrast aerobic and anaerobic respiration,
including the processes involved.
2. WHAT IF? If the following redox reaction occurred, which
compounds would be oxidized? Reduced?
C 4H6O5 + NAD + S C 4H4O5 + NADH + H+
For suggested answers, see Appendix A.
CHAPTER 9
Cellular Respiration and Fermentation
169
. Figure 9.7 The inputs and outputs of glycolysis.
CONCEPT 9.2
Glycolysis harvests chemical energy
by oxidizing glucose to pyruvate
Mastering Biology
Animation: Glycolysis
GLYCOLYSIS
The word glycolysis means “sugar splitting,” and that is
exactly what happens during this pathway. Glucose, a sixcarbon sugar, is split into two three-carbon sugars. These
smaller sugars are then oxidized and their remaining atoms
rearranged to form two molecules of pyruvate. (Pyruvate is
the ionized form of pyruvic acid.)
As summarized in Figure 9.7, glycolysis can be divided
into two phases: the energy investment phase and the energy
payoff phase. During the energy investment phase, the cell
actually spends ATP. This investment is repaid with interest during the energy payoff phase, when ATP is produced
by substrate-level phosphorylation and NAD + is reduced to
NADH by electrons released from the oxidation of glucose.
The net energy yield from glycolysis, per glucose molecule, is
2 ATP plus 2 NADH. The ten steps of the glycolytic pathway
are shown in Figure 9.8.
All of the carbon originally present in glucose is
accounted for in the two molecules of pyruvate; no carbon is
released as CO2 during glycolysis. Glycolysis occurs whether
or not O2 is present. However, if O2 is present, the chemical energy stored in pyruvate and NADH can be extracted
by pyruvate oxidation, the citric acid cycle, and oxidative
phosphorylation.
GLYCOLYSIS
OXIDATIVE
PHOSPHORYLATION
CITRIC
ACID
CYCLE
PYRUVATE
OXIDATION
GLYCOLYSIS:
OXIDATIVE
PHOSPHORYLATION
CITRIC
ACID
CYCLE
PYRUVATE
OXIDATION
ATP
Energy Investment Phase
Glucose
2
ATP
2 ADP + 2 P
used
Energy Payoff Phase
4 ADP + 4 P
4
2 NAD+ + 4 e– + 4 H+
formed
ATP
2 NADH + 2 H+
2 Pyruvate + 2 H2O
Net Inputs and Outputs
Glucose
2 Pyruvate + 2 H2O
4 ATP formed – 2 ATP used
2 NAD+ +
2 ATP
4 e– + 4 H+
2 NADH + 2 H+
. Figure 9.8 The steps of glycolysis. Glycolysis, a source of ATP and NADH, takes place in
the cytosol. Two of the enzymes (in steps 1 and 3 ) are shown in Figure 6.32b.
GLYCOLYSIS: Energy Investment Phase
WHAT IF? What would happen if you removed the dihydroxyacetone
ATP
phosphate generated in step 4 as fast as it was produced?
Glyceraldehyde
3-phosphate (G3P)
Glucose
CH2OH
O H
H H
OH H
OH
HO
H
ADP
CH2O
H H
Hexokinase HO
OH
OH
1
H
Hexokinase transfers
a phosphate group
from ATP to glucose,
making it more
chemically reactive.
The charged
phosphate also traps
the sugar in the cell.
170
UNIT TWO
Fructose
ATP
6-phosphate
Glu

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