9 Chapter Lecture
Respiration
CHAPTER 9
CELLULAR RESPIRATION:
HARVESTING CHEMICAL ENERGY
Section A: The Principles of Energy Harvest
1. Cellular respiration and fermentation are catabolic, energy-yielding
pathways
2. Cells recycle the ATP they use for work
3. Redox reactions release energy when electrons move closer to
electronegative atoms
4. Electrons fall from organic molecules to oxygen during cellular
respiration
5. The fall of electrons during respiration is stepwise, via NAD + and an
electron transport chain
Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Introduction
Living is work.
To perform their many
tasks, cells require
transfusions of energy
from outside sources.
In most ecosystems,
energy enters as sunlight.
Light energy trapped in
organic molecules is
available to both
photosynthetic organisms
and others that eat them.
Fig. 9.1
Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings
1. Cellular respiration and fermentation are
catabolic, energy-yielding pathways
Organic molecules store energy in their
arrangement of atoms.
Enzymes catalyze the systematic degradation of
organic molecules that are rich in energy to
simpler waste products with less energy.
Some of the released energy is used to do work
and the rest is dissipated as heat.
Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Metabolic pathways that release the energy
stored in complex organic molecules are
catabolic.
One type of catabolic process, fermentation,
leads to the partial degradation of sugars in the
absence of oxygen.
A more efficient and widespread catabolic
process, cellular respiration, uses oxygen as a
reactant to complete the breakdown of a variety
of organic molecules.
Most of the processes in cellular respiration occur
in mitochondria.
Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Cellular respiration is similar to the combustion
of gasoline in an automobile engine.
The overall process is:
Organic compounds + O2 -> CO2 + H2O + Energy
Carbohydrates, fats, and proteins can all be used
as the fuel, but it is traditional to start learning
with glucose.
C6H12O6 + 6O2 -> 6CO2 + 6H2O + Energy (ATP +
heat)
The catabolism of glucose is exergonic with a
delta G of - 686 kcal per mole of glucose.
Some of this energy is used to produce ATP that will
perform cellular work.
Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings
2. Cells recycle the ATP they use for work
ATP, adenosine triphosphate, is the pivotal
molecule in cellular energetics.
It is the chemical equivalent of a loaded spring.
The close packing of three negatively-charged
phosphate groups is an unstable, energy-storing
arrangement.
Loss of the end phosphate group relaxes the
spring.
The price of most cellular work is the conversion
of ATP to ADP and inorganic phosphate (Pi).
An animal cell regenerates ATP from ADP and Pi
by the catabolism of organic molecules.
Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings
The transfer of the terminal phosphate group
from ATP to another molecule is
phosphorylation.
This changes the shape of the receiving molecule,
performing work (transport, mechanical, or
chemical).
When the
phosphate
groups leaves
the molecule,
the molecule
returns to its
alternate
Fig. 9.2
shape.
Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings
3. Redox reactions release energy when
electrons move closer to electronegative
atoms
Catabolic pathways relocate the electrons stored
in food molecules, releasing energy that is used to
synthesize ATP.
Reactions that result in the transfer of one or
more electrons from one reactant to another are
oxidation-reduction reactions, or redox
reactions.
The loss of electrons is called oxidation.
The addition of electrons is called reduction.
Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings
The formation of table salt from sodium and
chloride is a redox reaction.
Na + Cl -> Na+ + Cl Here sodium is oxidized and chlorine is reduced (its
charge drops from 0 to -1).
More generally: Xe- + Y -> X + Ye X, the electron donor, is the reducing agent and
reduces Y.
Y, the electron recipient, is the oxidizing agent and
oxidizes X.
Redox reactions require both a donor and
acceptor.
Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Redox reactions also occur when the movement
of electrons is not complete but involve a change
in the degree of electron sharing in covalent
bonds.
In the combustion of methane to form water and
carbon dioxide, the nonpolar covalent bonds of
methane (C-H) and oxygen (O=O) are converted
to polar covalent bonds (C=O and O-H).
Fig. 9.3
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When these bonds shift from nonpolar to polar,
the electrons move from positions equidistant
between the two atoms for a closer position to
oxygen, the more electronegative atom.
Oxygen is one of the most potent oxidizing agents.
An electron looses energy as it shifts from a less
electronegative atom to a more electronegative
one.
A redox reaction that relocates electrons closer
to oxygen releases chemical energy that can do
work.
To reverse the process, energy must be added to
pull an electron away from an atom.
Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings
4. Electrons fall from organic molecules
to oxygen during cellular respiration
In cellular respiration, glucose and other fuel
molecules are oxidized, releasing energy.
In the summary equation of cellular respiration:
C6H12O6 + 6O2 -> 6CO2 + 6H2O
Glucose is oxidized, oxygen is reduced, and
electrons loose potential energy.
Molecules that have an abundance of hydrogen
are excellent fuels because their bonds are a
source of hilltop electrons that fall closer to
oxygen.
Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings
The cell has a rich reservoir of electrons
associated with hydrogen, especially in
carbohydrates and fats.
However, these fuels do not spontaneously
combine with O2 because they lack the
activation energy.
Enzymes lower the barrier of activation energy,
allowing these fuels to be oxidized slowly.
Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings
5. The fall of electrons during
respiration is stepwise, via NAD+
and an electron transport chain
Cellular respiration does not oxidize glucose in a
single step that transfers all the hydrogen in the
fuel to oxygen at one time.
Rather, glucose and other fuels are broken down
gradually in a series of steps, each catalyzed by a
specific enzyme.
At key steps, hydrogen atoms are stripped from
glucose and passed first to a coenzyme, like
NAD+ (nicotinamide adenine dinucleotide).
Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Dehydrogenase enzymes strip two hydrogen
atoms from the fuel (e.g., glucose), pass two
electrons and one proton to NAD+ and release
H+.
H-C-OH + NAD+ -> C=O + NADH + H+
Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings
This changes the oxidized form, NAD+, to the
reduced form NADH.
NAD + functions as the oxidizing agent in many of the redox
steps during the catabolism of glucose.
Fig. 9.4
Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings
The electrons carried by NADH loose very little
of their potential energy in this process.
This energy is tapped to synthesize ATP as
electrons fall from NADH to oxygen.
Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Unlike the explosive release of heat energy that
would occur when H2 and O2 combine, cellular
respiration uses an electron transport chain to
break the fall of electrons to O2 into several
steps.
Fig. 9.5
Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings
The electron transport chain, consisting of
several molecules (primarily proteins), is built
into the inner membrane of a mitochondrion.
NADH shuttles electrons from food to the top
of the chain.
At the bottom, oxygen captures the electrons
and H+ to form water.
The free energy change from top to bottom
is -53 kcal/mole of NADH.
Electrons are passed by increasingly
electronegative molecules in the chain until they
are caught by oxygen, the most electronegative.
Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Introduction
Living is work.
To perform their many
tasks, cells require
transfusions of energy
from outside sources.
In most ecosystems,
energy enters as sunlight.
Light energy trapped in
organic molecules is
available to both
photosynthetic organisms
and others that eat them.
Fig. 9.1
Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings
1. Cellular respiration and fermentation are
catabolic, energy-yielding pathways
Organic molecules store energy in their
arrangement of atoms.
Enzymes catalyze the systematic degradation of
organic molecules that are rich in energy to
simpler waste products with less energy.
Some of the released energy is used to do work
and the rest is dissipated as heat.
Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Metabolic pathways that release the energy
stored in complex organic molecules are
catabolic.
One type of catabolic process, fermentation,
leads to the partial degradation of sugars in the
absence of oxygen.
A more efficient and widespread catabolic
process, cellular respiration, uses oxygen as a
reactant to complete the breakdown of a variety
of organic molecules.
Most of the processes in cellular respiration occur
in mitochondria.
Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Cellular respiration is similar to the combustion
of gasoline in an automobile engine.
The overall process is:
Organic compounds + O2 -> CO2 + H2O + Energy
Carbohydrates, fats, and proteins can all be used
as the fuel, but it is traditional to start learning
with glucose.
C6H12O6 + 6O2 -> 6CO2 + 6H2O + Energy (ATP +
heat)
The catabolism of glucose is exergonic with a
delta G of - 686 kcal per mole of glucose.
Some of this energy is used to produce ATP that will
perform cellular work.
Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings
2. Cells recycle the ATP they use for work
ATP, adenosine triphosphate, is the pivotal
molecule in cellular energetics.
It is the chemical equivalent of a loaded spring.
The close packing of three negatively-charged
phosphate groups is an unstable, energy-storing
arrangement.
Loss of the end phosphate group relaxes the
spring.
The price of most cellular work is the conversion
of ATP to ADP and inorganic phosphate (Pi).
An animal cell regenerates ATP from ADP and Pi
by the catabolism of organic molecules.
Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings
The transfer of the terminal phosphate group
from ATP to another molecule is
phosphorylation.
This changes the shape of the receiving molecule,
performing work (transport, mechanical, or
chemical).
When the
phosphate
groups leaves
the molecule,
the molecule
returns to its
alternate
Fig. 9.2
shape.
Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings
3. Redox reactions release energy when
electrons move closer to electronegative
atoms
Catabolic pathways relocate the electrons stored
in food molecules, releasing energy that is used to
synthesize ATP.
Reactions that result in the transfer of one or
more electrons from one reactant to another are
oxidation-reduction reactions, or redox
reactions.
The loss of electrons is called oxidation.
The addition of electrons is called reduction.
Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings
CHAPTER 9
CELLULAR RESPIRATION:
HARVESTING CHEMICAL ENERGY
Section B: The Process of Cellular Respiration
1. Respiration involves glycolysis, the Krebs cycle, and electron transport: an
overview
2. Glycolysis harvests chemical energy by oxidizing glucose to pyruvate: a
closer look
3. The Krebs cycle completes the energy-yielding oxidation of organic
molecules: a closer look
4. The inner mitochondrial membrane couples electron transport to ATP
synthesis: a closer look
5. Cellular respiration generates many ATP molecules for each sugar molecule
it oxidizes: a review
Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings
1. Respiration involves glycolysis, the Krebs
cycle, and electron transport:
an overview
Respiration occurs in three metabolic stages:
glycolysis, the Krebs cycle, and the electron
transport chain
and oxidative
phosphorylation.
Fig. 9.6
Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Glycolysis occurs in the cytoplasm.
It begins catabolism by breaking glucose into two
molecules of pyruvate.
The Krebs cycle occurs in the mitochondrial
matrix.
It degrades pyruvate to carbon dioxide.
Several steps in glycolysis and the Krebs cycle
transfer electrons from substrates to NAD+,
forming NADH.
NADH passes these electrons to the electron
transport chain.
Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings
In the electron transport chain, the electrons
move from molecule to molecule until they
combine with oxygen and hydrogen ions to
form water.
As they are passed along the chain, the energy
carried by these electrons is stored in the
mitochondrion in a form that can be used to
synthesize ATP via oxidative
phosphorylation.
Oxidative phosphorylation produces almost 90% of
the ATP generated by respiration.
Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Some ATP is also generated in glycolysis and
the Krebs cycle by substrate-level
phosphorylation.
Here an enzyme
transfers a phosphate
group from an
organic molecule
(the substrate)
to ADP, forming
ATP.
Fig. 9.7
Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Respiration uses the small steps in the
respiratory pathway to break the large
denomination of energy contained in glucose
into the small change of ATP.
The quantity of energy in ATP is more appropriate
for the level of work required in the cell.
Ultimately 38 ATP are produced per mole of
glucose that is degraded to carbon dioxide and
water by respiration.
Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings
2. Glycolysis harvests chemical energy by
oxidizing glucose to pyruvate:
a closer look
During glycolysis, glucose, a six carbon-sugar, is
split into two, three-carbon sugars.
These smaller sugars are oxidized and rearranged
to form two molecules of pyruvate.
Each of the ten steps in glycolysis is catalyzed by
a specific enzyme.
These steps can be divided into two phases: an
energy investment phase and an energy payoff
phase.
Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings
In the energy investment phase, ATP provides
activation energy by phosphorylating glucose.
This requires 2 ATP per glucose.
In the energy payoff
phase, ATP is
produced by
substrate-level
phosphorylation
and NAD+ is
reduced to NADH.
4 ATP (net) and
2 NADH are produced
per glucose.
Fig. 9.8
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Fig. 9.9a
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Fig. 9.9b
Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings
The net yield from glycolysis is 2 ATP and 2
NADH per glucose.
No CO2 is produced during glycolysis.
Glycolysis occurs whether O2 is present or not.
If O2 is present, pyruvate moves to the Krebs cycle
and the energy stored in NADH can be converted to
ATP by the electron transport system and oxidative
phosphorylation.
Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings
energy-yielding oxidation of
organic molecules:
a closer look
More than three quarters of the original energy in
glucose is still present in two molecules of
pyruvate.
If oxygen is present, pyruvate enters the
mitochondrion where enzymes of the Krebs cycle
complete the oxidation the of organic fuel to
carbon dioxide.
Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings
As pyruvate enters the mitochondrion, a
multienzyme complex modifies pyruvate to
acetyl CoA which enters the Krebs cycle in the
matrix.
A carboxyl group is removed as CO2.
A pair of electrons is transferred from the remaining
two-carbon fragment to NAD+ to form NADH.
The oxidized
fragment, acetate,
combines with
coenzyme A to
form acetyl CoA.
Fig. 9.10
Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings
The Krebs cycle is named after Hans Krebs
who was largely responsible for elucidating its
pathways in the 1930s.
This cycle begins when acetate from acetyl CoA
combines with oxaloacetate to form citrate.
Ultimately, the oxaloacetate is recycled and the
acetate is broken down to CO2.
Each cycle produces one ATP by substrate-level
phosphorylation, three NADH, and one FADH2
(another electron carrier) per acetyl CoA.
Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings
The Krebs
cycle
consists of
eight steps.
Fig. 9.11
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The conversion of
pyruvate and the
Krebs cycle
produces large
quantities of
electron carriers.
Fig. 9.12
Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings
4. The inner mitochondrial membrane
couples electron transport to ATP
synthesis: a closer look
Only 4 of 38 ATP ultimately produced by
respiration of glucose are derived from substratelevel phosphorylation.
The vast majority of the ATP comes from the
energy in the electrons carried by NADH (and
FADH2).
The energy in these electrons is used in the
electron transport system to power ATP
synthesis.
Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Thousands of copies of the electron transport
chain are found in the extensive surface of the
cristae, the inner membrane of the
mitochondrion.
Most components of the chain are proteins that are
bound with prosthetic groups that can alternate
between reduced and oxidized states as they accept
and donate electrons.
Electrons drop in free energy as they pass down
the electron transport chain.
Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Electrons carried by
NADH are transferred
to the first molecule in
the electron transport
chain, flavoprotein.
The electrons continue
along the chain which
includes several
cytochrome proteins
and one lipid carrier.
The electrons carried
by FADH2 have lower
free energy and are
added to a later point in
the chain.
Fig. 9.13
Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Electrons from NADH or FADH2 ultimately
pass to oxygen.
For every two electron carriers (four electrons), one
O2 molecule is reduced to two molecules of water.
The electron transport chain generates no ATP
directly.
Its function is to break the large free energy
drop from food to oxygen into a series of
smaller steps that release energy in manageable
amounts.
The movement of electrons along the electron
transport chain does contribute to chemiosmosis
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A protein complex,
ATP synthase, in the
cristae actually makes
ATP from ADP and Pi.
ATP used the energy of
an existing proton
gradient to power ATP
synthesis.
This proton gradient
develops between the
intermembrane space
and the matrix.
Fig. 9.14
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The proton gradient is produced by the
movement of electrons along the electron
transport chain.
Several chain molecules can use the exergonic
flow of electrons to pump H+ from the matrix to
the intermembrane space.
This concentration of H+ is the proton-motive
force.
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Fig. 9.15
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The ATP synthase molecules are the only place
that will allow H+ to diffuse back to the matrix.
This exergonic flow of H+ is used by the
enzyme to generate ATP.
This coupling of the redox reactions of the
electron transport chain to ATP synthesis is
called chemiosmosis.
Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings
The mechanism of ATP
generation by ATP
synthase is still an area
of active investigation.
As hydrogen ions flow
down their gradient, they
cause the cylinder portion
and attached rod of ATP
synthase to rotate.
The spinning rod causes a
conformational change in
the knob region,
activating catalytic sites
where ADP and inorganic
phosphate combine to
make ATP.
Fig. 9.14
Chemiosmosis is an energy-coupling mechanism
that uses energy stored in the form of an H+
gradient across a membrane to drive cellular
work.
In the mitochondrion, chemiosmosis generates ATP.
Chemiosmosis in chloroplasts also generates ATP,
but light drives the electron flow down an electron
transport chain and H+ gradient formation.
Prokaryotes generate H+ gradients across their
plasma membrane.
They can use this proton-motive force not only to generate
ATP but also to pump nutrients and waste products across
the membrane and to rotate their flagella.
Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings
5. Cellular respiration generates many ATP
molecules for each sugar molecule it
oxidizes: a review
During respiration, most energy flows from
glucose -> NADH -> electron transport chain ->
proton-motive force -> ATP.
Considering the fate of carbon, one six-carbon
glucose molecule is oxidized to six CO2
molecules.
Some ATP is produced by substrate-level
phosphorylation during glycolysis and the Krebs
cycle, but most comes from oxidative
phosphorylation.
Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Each NADH from the Krebs cycle and the
conversion of pyruvate contributes enough
energy to generate a maximum of 3 ATP
(rounding up).
The NADH from glycolysis may also yield 3 ATP.
Each FADH2 from the Krebs cycle can be used
to generate about 2ATP.
In some eukaryotic cells, NADH produced in
the cytosol by glycolysis may be worth only 2
ATP.
The electrons must be shuttled to the
mitochondrion.
Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings
In some shuttle systems, the electrons are passed to
Assuming the most energy-efficient shuttle of
NADH from glycolysis, a maximum yield of 34
ATP is produced by oxidative phosphorylation.
This plus the 4 ATP from substrate-level
phosphorylation gives a bottom line of 38 ATP.
This maximum figure does not consider other uses
of the proton-motive force.
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Fig. 9.16
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How efficient is respiration in generating ATP?
Complete oxidation of glucose releases 686 kcal per
mole.
Formation of each ATP requires at least 7.3
kcal/mole.
Efficiency of respiration is 7.3 kcal/mole x 38
ATP/glucose/686 kcal/mole glucose = 40%.
The other approximately 60% is lost as heat.
Cellular respiration is remarkably efficient in
energy conversion.
Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings
CHAPTER 9
CELLULAR RESPIRATION:
HARVESTING CHEMICAL ENERGY
Section C: Related Metabolic Processes
1. Fermentation allows some cells to produce ATP without the help of oxygen
2. Glycolysis and the Krebs cycle connect to many other metabolic pathways
3. Feedback mechanisms control cellular respiration
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1. Fermentation enables some cells to
produce ATP without the help of oxygen
Oxidation refers to the loss of electrons to any
electron acceptor, not just to oxygen.
In glycolysis, glucose is oxidized to two pyruvate
molecules with NAD+ as the oxidizing agent, not O2.
Some energy from this oxidation produces 2 ATP
(net).
If oxygen is present, additional ATP can be generated
when NADH delivers its electrons to the electron
transport chain.
Glycolysis generates 2 ATP whether oxygen is
present (aerobic) or not (anaerobic).
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Anaerobic catabolism of sugars can occur by
fermentation.
Fermentation can generate ATP from glucose by
substrate-level phosphorylation as long as there is a
supply of NAD+ to accept electrons.
If the NAD+ pool is exhausted, glycolysis shuts down.
Under aerobic conditions, NADH transfers its electrons to
the electron transfer chain, recycling NAD +.
Under anaerobic conditions, various fermentation
pathways generate ATP by glycolysis and recycle
NAD+ by transferring electrons from NADH to
pyruvate or derivatives of pyruvate.
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In alcohol fermentation, pyruvate is converted
to ethanol in two steps.
First, pyruvate is converted to a two-carbon
compound, acetaldehyde by the removal of CO2.
Second, acetaldehyde is reduced by NADH to
ethanol.
Alcohol fermentation
by yeast is used in
brewing and
winemaking.
Fig. 9.17a
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During lactic acid fermentation, pyruvate is
reduced directly by NADH to form lactate
(ionized form of lactic acid).
Lactic acid fermentation by some fungi and bacteria
is used to make cheese and yogurt.
Muscle cells switch from aerobic respiration to
lactic acid fermentation to generate ATP when O2 is
scarce.
The waste product,
lactate, may cause
muscle fatigue, but
ultimately it is
converted back to
pyruvate in the liver.
Fig. 9.17b
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Fermentation and cellular respiration are
anaerobic and aerobic alternatives, respectively,
for producing ATP from sugars.
Both use glycolysis to oxidize sugars to pyruvate
with a net production of 2 ATP by substrate-level
phosphorylation.
Both use NAD+ as an electron acceptor.
In fermentation, the electrons of NADH are
passed to an organic molecule, regenerating
NAD+.
Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings
In respiration, the electrons of NADH are
ultimately passed to O2, generating ATP by
oxidative phosphorylation.
In addition, even more ATP is generated from
the oxidation of pyruvate in the Krebs cycle.
Without oxygen, the energy still stored in
pyruvate is unavailable to the cell.
Under aerobic respiration, a molecule of
glucose yields 38 ATP, but the same molecule
of glucose yields only 2 ATP under anaerobic
respiration.
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Some organisms (facultative anaerobes),
including yeast and many bacteria, can survive
using either fermentation or respiration.
At a cellular level, human
muscle cells can behave
as facultative anaerobes,
but nerve cells cannot.
For facultative anaerobes,
pyruvate is a fork in the
metabolic road that leads
to two alternative routes.
Fig. 9.18
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The oldest bacterial fossils are over 3.5 billion
years old, appearing long before appreciable
quantities of O2 accumulated in the atmosphere.
Therefore, the first prokaryotes may have
generated ATP exclusively from glycolysis.
The fact that glycolysis is also the most
widespread metabolic pathway and occurs in
the cytosol without membrane-enclosed
organelles, suggests that glycolysis evolved
early in the history of life.
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2. Glycolysis and the Krebs cycle connect to
many other metabolic pathways
Glycolysis can accept a wide range of
carbohydrates.
Polysaccharides, like starch or glycogen, can be
hydrolyzed to glucose monomers that enter
glycolysis.
Other hexose sugars, like galactose and fructose, can
also be modified to undergo glycolysis.
The other two major fuels, proteins and fats, can
also enter the respiratory pathways, including
glycolysis and the Krebs cycle, used by
carbohydrates.
Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Proteins must first be digested to individual
amino acids.
Amino acids that will be catabolized must have
their amino groups removed via deamination.
The nitrogenous waste is excreted as ammonia,
urea, or another waste product.
The carbon skeletons are modified by enzymes
and enter as intermediaries into glycolysis or
the Krebs cycle depending on their structure.
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The energy of fats can also be accessed via
catabolic pathways.
Fats must be digested to glycerol and fatty
acids.
Glycerol can be converted to glyceraldehyde
phosphate, an intermediate of glycolysis.
The rich energy of fatty acids is accessed as fatty
acids are split into two-carbon fragments via beta
oxidation.
These molecules enter the Krebs cycle as acetyl
CoA.
In fact, a gram of fat will generate twice as
much ATP as a gram of carbohydrate via
aerobic respiration.
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Carbohydrates, fats,
and proteins can all
be catabolized
through the same
pathways.
Fig. 9.19
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The metabolic pathways of respiration also play
a role in anabolic pathways of the cell.
Not all the organic molecules of food are
completely oxidized to make ATP.
Intermediaries in glycolysis and the Krebs cycle
can be diverted to anabolic pathways.
For example, a human cell can synthesize about half
the 20 different amino acids by modifying
compounds from the Krebs cycle.
Glucose can be synthesized from pyruvate and fatty
acids from acetyl CoA.
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Glycolysis and the Krebs cycle function as
metabolic interchanges that enable cells to
convert one kind of molecule to another as
needed.
For example, excess carbohydrates and proteins can
be converted to fats through intermediaries of
glycolysis and the Krebs cycle.
Metabolism is remarkably versatile and
adaptable.
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3. Feedback mechanisms control
cellular respiration
Basic principles of supply and demand regulate
the metabolic economy.
If a cell has an excess of a certain amino acid, it
typically uses feedback inhibition to prevent the
diversion of more intermediary molecules from the
Krebs cycle to the synthesis pathway of that amino
acid.
The rate of catabolism is also regulated, typically
by the level of ATP in the cell.
If ATP levels drop, catabolism speeds up to produce
more ATP.
Control of catabolism is
based mainly on
regulating the activity of
enzymes at strategic
points in the catabolic
pathway.
One strategic point
occurs in the third step
of glycolysis, catalyzed
by phosphofructokinase.
Fig. 9.20
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Allosteric regulation of phosphofructokinase
sets the pace of respiration.
This enzyme is inhibited by ATP and stimulated by
AMP (derived from ADP).
It responds to shifts in balance between production and
degradation of ATP: ATP <-> ADP + Pi <-> AMP + Pi.
Thus, when ATP levels are high, inhibition of this
enzyme slows glycolysis.
When ATP levels drop and ADP and AMP levels
rise, the enzyme is active again and glycolysis
speeds up.
Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Citrate, the first product of the Krebs cycle, is
also an inhibitor of phosphofructokinase.
This synchronizes the rate of glycolysis and the
Krebs cycle.
Also, if intermediaries from the Krebs cycle are
diverted to other uses (e.g., amino acid synthesis),
glycolysis speeds up to replace these molecules.
Metabolic balance is augmented by the control
of other enzymes at other key locations in
glycolysis and the Krebs cycle.
Cells are thrifty, expedient, and responsive in
their metabolism.
Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings