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Biology
A Guide to the Natural World
Chapter 7 • Lecture Outline
Vital Harvest: Deriving Energy from Food
Fifth Edition
David Krogh
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7.1 Energizing ATP
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Energizing ATP
• The molecule adenosine triphosphate (ATP)
supplies the energy for most of the activities
of living things.
• For ATP to be produced, a third phosphate
group must be added to adenosine
diphosphate (ADP) through a redox
reaction.
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Storing and Releasing Energy
• The substance that loses electrons in a redox
reaction is said to have been oxidized,
while the substance that gains electrons is
said to have been reduced.
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NAD
• Electrons are carried between one part of
the energy-harvesting process and another
by electron carriers, the most important of
which is nicotinamide adenine
dinucleotide, or NAD.
• In its “empty” state, this molecule exists as
NAD+.
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NAD
• Through a redox reaction, NAD+ picks up
one hydrogen atom and another single
electron from food, thus becoming NADH.
• It will retain this form until it drops off its
energetic electrons (and a proton) in a later
stage of the energy-harvesting process.
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The Electron Carrier NAD+
empty
loaded
proton
(oxidized)
empty
goes to pick
up more electrons
used in later
stage of respiration
(reduced)
used in later
stage of respiration
1. NAD+ within a cell,
along with two hydrogen
atoms that are part of the
food that is supplying
energy for the body.
3. NADH carries the electrons
2. NAD+ is reduced to NAD
to a later stage of respiration
by accepting an electron from
then drops them off,
a hydrogen atom. It also picks
becoming oxidized to
up another hydrogen atom to
its original form, NAD+.
become NADH.
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Figure 7.3
Storing and Releasing Energy
2. Energy from food is then
stored as a phosphate bond
in ATP.
3. Energy is then released when
the phosphate bond is
broken, and can be used to
fuel our everyday activities.
1. Energy from food is
required to push a third
phosphate group onto ADP.
energy
in
ATP
energy
out
energy hill
P +
P +
ADP
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ADP
Figure 7.1
7.2 The Three Stages of Cellular
Respiration
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Cellular Respiration
• Cellular respiration is the harvesting of
energy from food.
• It has three stages:
1. Glycolysis
2. Krebs Cycle (Citric Acid Cycle)
3. Electron Transport Chain (ETC)
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Cellular Respiration
• Some organisms rely solely on glycolysis for
energy harvesting.
• For most organisms, however, glycolysis is a
primary process of energy extraction only in
certain situations, when quick bursts of energy
are required.
• But, it is a necessary first stage to the Krebs
cycle and the ETC.
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Cellular Respiration
• Glycolysis takes place in the cell’s cytosol,
while the Krebs cycle and the ETC take
place in cellular organelles, called
mitochondria, that lie within the cytosol.
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Cellular Respiration
• Glycolysis yields two net molecules of ATP
per molecule of glucose, as does the Krebs
cycle.
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Cellular Respiration
Suggested Media Enhancement:
Cellular Respiration
To access this animation go to folder C_Animations_and_Video_Files
and open the BioFlix folder.
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Cellular Respiration
• The net yield in the ETC is a maximum of
about 32 ATP molecules per molecule of
glucose.
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Cellular Respiration
• Glycolysis and the Krebs cycle are critical
in that they yield electrons that are carried
to the ETC for the final high-yield stage of
energy harvesting.
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(b) In schematic terms
reactants
products
glycolysis
(a) In metaphorical terms
2 ATP
insert 1 glucose
glucose
cytosol
glycolysis
glucose derivatives
2 energy
tokens
2 energy
tokens
Krebs
cycle
Krebs
cycle
2 ATP
32 energy
tokens
electron
transport
chain
electron
transport
chain
32 ATP
mitochondrion
36 ATP maximum per glucose molecule
Just as the video games in some arcades can use only tokens
(rather than money) to make them function, so our bodies can use
only ATP (rather than food) as a direct source of energy. The energy
contained in food—glucose in the example—is transferred to ATP in
three major steps: glycolysis, the Krebs cycle, and the electron
transport chain. Though glycolysis and the Krebs cycle contribute
only small amounts of ATP directly, they also contribute electrons
(on the left of the token machine) that help bring about the large
yield of ATP in the electron transport chain. Our energy-transfer
mechanisms are not quite as efficient as the arcade machine
makes them appear. At each stage of the conversion process, some
of the original energy contained in the glucose is lost to heat.
As with the arcade machine, the starting point in this example is a
single molecule of glucose, which again yields ATP in three major sets
of steps: glycolysis, the Krebs cycle, and the electron transport chain
(ETC). These steps can yield a maximum of about 36 molecules of ATP:
2 in glycolysis, 2 in the Krebs cycle, and 32 in the ETC. As noted,
however, glycolysis and the Krebs cycle also yield electrons that move
to the ETC, aiding in its ATP production. These electrons get to the ETC
via the electron carriers NADH and FADH2, shown on the left. Oxygen is
consumed in energy harvesting, while water and carbon dioxide are
produced in it. Glycolysis takes place in the cytosol of the cell, but the
Krebs cycle and the ETC take place in cellular organelles, called
mitochondria, that lie within the cytosol.
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Figure 7.4
7.3 First Stage of Respiration:
Glycolysis
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First Stage of Respiration:
Glycolysis
• Glycolysis begins with a single molecule of
glucose. The ultimate products are two
molecules of NADH (which move to the
ETC, bearing their energetic electrons) and
two molecules of ATP (which are ready to
be used).
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Glycolysis
• Glycolysis also produces two molecules of
pyruvic acid—the derivatives of the original
glucose molecule—which move on to the
Krebs cycle.
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glycolysis
2 ATP
glucose
molecules out
molecules in
2 NADH
Krebs
cycle
glucose
ATP
Steps in glycolysis
ADP
electron
transport
chain
1. Delivered by the bloodstream, glucose enters a cell and
immediately has a phosphate group from ATP attached
to it. Because this process, called phosphorylation,
attaches the phosphate to the sixth carbon of glucose,
it now goes under the name glucose-6-phosphate. Note
that one molecule of ATP has been used in this step.
1.
glucose-6-phosphate
ATP ledger now reads: -1 ATP
Black balls are carbons
and gold ovals are
phosphate groups
2.
2. Glucose 6-phosphate is rearranged to become
a molecule called fructose-6-phosphate.
3. Another molecule of ATP is used to add a second
phosphate to fructose-6-phosphate, which now
becomes fructose-1,6-diphosphate.
fructose-6-phosphate
ATP
ADP
3.
ATP ledger now reads: -2 ATP
4. The single, six-carbon sugar fructose-1,6-diphosphate
now becomes two molecules of a 3-carbon sugar,
glyceraldehyde-3-phosphate, each with a phosphate
group attached. From here on out, glycolysis happens
in duplicate: What happens to one of the glyceraldehyde
molecules happens to the other.
5. An enzyme brings together glyceraldehyde-3phosphate, the electron carrier NAD+, and a
phosphate group. The glyceraldehyde-3-phosphate
molecule is oxidized by NAD+, which in its new form,
NADH, moves to the electron transport chain bearing
its electron cargo. The oxidation of NAD+ is energetic
enough that it allows the phosphate group to become
attached to the main molecule, now called
1,3-diphosphoglyceric acid. Because everything is
happening in duplicate, two NADH molecules are produced.
fructose-1,6-diphosphate
4.
glyceraldehyde-3-phosphate
5.
1,3-diphosphoglyceric acid
ATP ledger now reads: 0 ATP
2 ATP
2 ADP
6. 1,3-diphosphoglyceric acid loses one of its phosphate
groups, thus becoming 3-phosphoglyceric acid. The
reaction is energetic enough to push this phosphate
group onto an ADP molecule, yielding ATP. Because
of duplication, two ATP are produced.
6.
3-phosphoglyceric acid
2 ATP
2 ADP
7. In two reactions, 3-phosphoglyceric acid becomes
phosphoenolpyruvic acid, which generates more ATP
as it transfers its phosphate group to ADP. Two more
ATP molecules are produced. The phosphate transfer
turns phosphoenolpyruvic acid into pyruvic acid—the
derivative of the original glucose that now will enter
the Krebs cycle.
7.
pyruvic acid
ATP ledger now reads: +2 ATP
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Figure 7.5
7.4 Second Stage of Respiration:
The Krebs Cycle
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Intermediate Step
• There is a transition step in respiration
between glycolysis and the Krebs cycle.
• In it, each pyruvic acid molecule that was
produced in glycolysis combines with
coenzyme A, thus forming acetyl coenzyme
A (acetyl CoA), which enters the Krebs
cycle.
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Intermediate Step
• There are also two other products of this
reaction:
1. One molecule of carbon dioxide, which
diffuses to the bloodstream.
2. One more molecule of NADH, which moves to
the ETC.
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Transition Between Glycolysis
and the Krebs Cycle
mitochondrion
glycolysis
pyruvic
acid
to electron
transport chain
acetyl coenzyme A
coenzyme A
cytosol
inner compartment
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Krebs
cycle
Figure 7.7
The Krebs Cycle
• For each starting molecule of glucose, two
molecules of pyruvic acid go through this
step.
• Thus, the step’s product per molecule of
glucose is two molecules of carbon dioxide,
two NADH, and two acetyl CoA.
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The Krebs Cycle
• In the Krebs (or citric acid) cycle, the
derivatives of the original glucose molecule
are oxidized.
• The result is that more energetic electrons
are transported by the electron carriers
NADH and to the ETC.
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The Krebs Cycle
• The net energy yield of the Krebs cycle per
molecule of glucose is:
• six molecules of NADH
• two molecules of FADH2
• two molecules of ATP
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glycolysis
Krebs
cycle
2 ATP
electron
transport
chain
acetyl coenzyme A
Steps in the Krebs cycle
1. Acetyl CoA combines with the four-carbon
oxaloacetic acid and the CoA fragment
separates from this compound. The result is the
energetic six-carbon molecule citric acid, which
will now be oxidized.
1.
oxaloacetic acid
2.
2. A citric acid derivative is oxidized by NAD+; the
resulting NADH carries electrons to the ETC. An
intermediate molecule then loses a CO2
molecule. Citric acid is now alpha-ketoglutaric
acid.
3. Alpha-ketoglutaric acid loses a CO2 molecule
and the resulting four-carbon molecule is
oxidized by NAD+.
4. An alpha-ketoglutaric acid derivative is split,
releasing enough energy to attach a phosphate
to ADP, making it ATP. Alpha-ketoglutaric acid
has become succinic acid.
5. Succinic acid is oxidized by FAD, losing two
complete hydrogen atoms to it. The resulting
FADH2 then moves to the ETC. In a series of
steps, succinic acid is transformed into malic
acid.
citric acid
6.
-Ketoglutaric acid
malic acid
3.
FAD
5.
ADP
succinic acid
6. Malic acid is oxidized by NAD+. This step
transforms malic acid to oxaloacetic acid—the
molecule that enters the first step of the Krebs
cycle.
4.
-ketoglutaric
acid derivative
ATP
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Figure 7.8
7.5 Third Stage of Respiration:
The Electron Transport Chain
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Third Stage of Respiration: The Electron
Transport Chain
• The ETC is a series of molecules located
within the mitochondrial inner membrane.
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The Electron Transport Chain
• On reaching the ETC, the electron carriers
NADH and FADH2 are oxidized by
molecules in the chain.
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The Electron Transport Chain
• Each carrier in the chain is then reduced by
accepting electrons from the carrier that
came before it.
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The Electron Transport Chain
• The last electron acceptor in the ETC is
oxygen.
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The Electron Transport Chain
• The movement of electrons through the
ETC releases enough energy to power the
movement of hydrogen ions (H+ ions)
through the three ETC protein complexes.
• They move from the mitochondrion’s inner
compartment to its outer compartment.
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The Electron Transport Chain
• The movement of these ions down their
concentration and charge gradients, back
into the inner compartment through an
enzyme called ATP synthase, drives the
synthesis of ATP from ADP and phosphate.
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The Electron Transport Chain
• In the inner compartment, oxygen accepts
the electrons from the ETC and hydrogen
ions, thus forming water.
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The Electron
Transport Chain
Mitochondrion
inner membrane
outer compartment
inner compartment
Electron transport chain
ATP synthesis
outer compartment
inner
membrane
ATP
synthase
inner compartment
ATP
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Figure 7.9
The Electron Transport Chain
• If oxygen is not present to accept the ETC
electrons, the entire energy-harvesting
process downstream from glycolysis comes
to a halt.
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7.6 Other Foods, Other Respiratory
Pathways
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Other Foods, Other Respiratory Pathways
• Different nutrients and their derivatives can
be channeled through different pathways in
cellular respiration in accordance with the
needs of an organism.
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Many Respiratory Pathways
• Proteins and lipids enter the metabolic
pathway for ATP production at different
points than does glucose.
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food
proteins
carbohydrates
amino acids
sugars
fats
glycerol
fatty acids
glucose
glycolysis
pyruvic acid
acetyl CoA
Krebs
cycle
NH3
(ammonia)
electron
transport
chain
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Figure 7.10