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Chapter 9 – Cellular Respiration
• Overview: Life Is Work
• Living cells
– Require transfusions of energy from outside
sources to perform their many tasks
• Assemble polymers
• Pump substances against the membrane
• Move
• Reproduce
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Cellular Respiration
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Basics of Respiration
Biologists define respiration as the aerobic
harvesting of energy from food molecules by
cells
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• Concept 9.1: Catabolic pathways yield energy
by oxidizing organic fuels
– Cells break down complex organic molecules
(rich in potential energy) to simpler waste
products (low in potential energy)
• To get energy to do work
• Rest is given off as heat
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•
Cells do this in two ways:
1) Fermentation
2) Cellular respiration
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1) One catabolic process, fermentation
– Is a partial degradation of sugars that occurs
without oxygen
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2) Cellular respiration
– Most prevalent and efficient catabolic pathway
– Consumes oxygen and organic molecules
such as glucose
– Yields more ATP
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• The equation for the reaction of cellular
respiration:
C6H12O6 + 6O2
6CO2 + 6H2O + Energy
(ATP & Heat)
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Catabolic Pathways and Production of ATP
• The breakdown of organic molecules is
exergonic
– But catabolic pathways don’t do work directly
• Just release energy so work can be done
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Redox Reactions: Oxidation and Reduction
• Catabolic pathways yield energy
– Due to the transfer of electrons
– When 1 or more electrons transfers from one
reactant to another the process is called
oxidation reduction reaction or redox for
short
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The Principle of Redox
• Oxidation – loss of an electron
– The substance that gives up its electron is the
reducing agent
• Reduction the gain of an electron
– The substance that gains the electron is the
oxidizing agent
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• Examples of redox reactions
becomes oxidized
(loses electron)
Reducing agent
Na
+
Cl
Na+
becomes reduced
(gains electron)
Oxidizing agent
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+
Cl–
• Specifically in cellular respiration:
becomes oxidized
C6H12O6 + 6O2
6CO2 + 6H2O + Energy
becomes reduced
• Glucose is oxidized and oxygen is reduced
• Electrons lose potential energy and energy is
released
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To Sum Up
• Remember an electron loses potential energy
when it shifts from a less electronegative atom
to a more electronegative atom (oxygen is very
electronegative)
• So by oxidizing glucose, respiration frees
stored energy (in glucose) and makes it
available for work (as ATP)
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Stepwise Energy Harvest via NAD+ and the Electron
Transport Chain
• Cellular respiration
– Oxidizes glucose in a series of steps
• Because energy can’t be released all at
once and still be useful
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Stepwise Energy Harvest
• Glucose is broken down in a series of steps
with the help of enzymes
– First an electron is taken away from a glucose
molecule. But a proton goes with the electron,
so in effect a Hydrogen atom was removed
– Second it is picked up by a coenzyme NAD+
which will act as an oxidizing agent
• NAD+ is reduced to NADH
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Stepwise Energy Harvest
– Second it is picked up by a coenzyme NAD+
which will act as an oxidizing agent
• NAD+ is reduced to NADH
• Electrons lose little potential energy going
from food to NAD+
• NADH molecule now represents stored
energy
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Stepwise Energy Harvest
– Third NADH will release their electrons down
a series of reactions with oxygen being the
final electron acceptor (or receptor)
• This is known as the Electron Transport
Chain and uses the energy from the
electron transfer to form ATP
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To Sum Up:
• The electrons removed from food by NAD+ fall
down an energy gradient in the ETC to a far
more stable location in the electronegative
oxygen atom
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The Stages of Cellular Respiration: A Preview
– Glycolysis
– The citric acid cycle
– Oxidative phosphorylation
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• The first two stages are catabolic pathways
that decompose glucose
1) Glycolysis
– Occurs in the cytosol
– Breaks down glucose into two molecules of
pyruvate
2) The citric acid cycle
– Occurs in the mitochondrial matrix
– Completes the breakdown of glucose
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Remember, why do we do this?
•
Goal of cellular respiration is ATP production
1) Oxidative phosphorylation - redox rxns of the
ETC
2) Substrate level phosphorylation
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• Substrate level phosphorylation
– ATP is formed directly
• Enzymes transfer a phosphate group from a
substrate molecule to an ADP (adenosine
diphosphate)
– Generates ATP directly
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• Both glycolysis and the citric acid cycle
– Can generate ATP by substrate-level
phosphorylation
Enzyme
Enzyme
ADP
P
Substrate
+
Figure 9.7
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Product
ATP
• An overview of cellular respiration
Electrons carried
via NADH and
FADH2
Electrons
carried
via NADH
Citric
acid
cycle
Glycolsis
Pyruvate
Glucose
Cytosol
Mitochondrion
ATP
Figure 9.6
Oxidative
phosphorylation:
electron
transport and
chemiosmosis
Substrate-level
phosphorylation
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ATP
Substrate-level
phosphorylation
ATP
Oxidative
phosphorylation
Glycolysis
• Concept 9.2: Glycolysis harvests energy by
oxidizing glucose to pyruvate
• Glycolysis
– Breaks down 1 glucose into 2 pyruvate
– Occurs in the cytoplasm of the cell
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• Glycolysis consists of two major phases
– Energy investment phase
Citric
acid
cycle
Glycolysis
ATP
ATP
– Energy payoff phase
Oxidative
phosphorylation
ATP
Energy investment phase
Glucose
2 ATP + 2 P
2 ATP
used
Energy payoff phase
4 ADP + 4 P
2 NAD+ + 4 e- + 4 H
+
4 ATP formed
2 NADH + 2 H+
2 Pyruvate + 2 H2O
Glucose
4 ATP formed – 2 ATP used
Figure 9.8
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2 NAD+ + 4 e– + 4 H
+
2 Pyruvate + 2 H2O
2 ATP + 2 H+
2 NADH
• A closer look at the energy investment phase
– Be thinking:
• What do I start with?
• What do I add in to this?
• What do I get out of this?
• Where does this occur?
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Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• A closer look at the energy payoff phase
– Be thinking:
• What do I start with?
• What do I add in to this?
• What do I get out of this?
• Where does this occur?
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Citric Acid Cycle
• Concept 9.3: The citric acid cycle completes
the energy-yielding oxidation of organic
molecules
• The citric acid cycle
– Takes place in the matrix of the mitochondrion
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• Before the citric acid cycle can begin:
– Pyruvate must first be converted to acetyl CoA,
which links glycolysis to the citric acid
cycle
CYTOSOL
MITOCHONDRION
NAD+
NADH
+ H+
O–
S
CoA
C
O
2
This only
happens if
oxygen is
present!
C
C
O
O
1
3
CH3
Pyruvate
Transport protein
Figure 9.10
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CH3
Acetyle CoA
CO2
Coenzyme A
1) The carboxyl group is removed (it is fully
oxidized so it has little chemical energy left)
–
First step where CO2 is released
CYTOSOL
MITOCHONDRION
NAD+
NADH
+ H+
O–
S
CoA
C
O
2
C
C
O
O
1
3
CH3
Pyruvate
Transport protein
Figure 9.10
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CH3
Acetyle CoA
CO2
Coenzyme A
2) Electrons are removed and transferred to
NAD+ which stores them as NADH
CYTOSOL
MITOCHONDRION
NAD+
NADH
+ H+
O–
S
CoA
C
O
2
C
C
O
O
1
3
CH3
Pyruvate
Transport protein
Figure 9.10
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CH3
Acetyle CoA
CO2
Coenzyme A
3) Coenzyme A attaches via an unstable bond, so
compound is now highly reactive
CYTOSOL
MITOCHONDRION
NAD+
NADH
+ H+
O–
S
CoA
C
O
2
C
C
O
O
1
3
CH3
Pyruvate
Transport protein
Figure 9.10
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CH3
Acetyle CoA
CO2
Coenzyme A
• An overview of the citric acid cycle
Pyruvate
(from glycolysis,
2 molecules per glucose)
Glycolysis
Citric
acid
cycle
ATP
ATP
Oxidative
phosphorylatio
n
ATP
CO2
CoA
NADH
+ 3 H+ Acetyle CoA
CoA
CoA
Citric
acid
cycle
2 CO2
3 NAD+
FADH2
FAD
3 NADH
+ 3 H+
ADP + P i
ATP
Figure 9.11
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• A closer look at the citric acid cycle
– Be thinking:
• What do I start with?
• What do I add in to this?
• What do I get out of this?
• Where does this occur?
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Glycolysis
Citric
Oxidative
acid phosphorylation
cycle
S
CoA
C
O
CH3
Acetyl CoA
CoA SH
O
NADH
+ H+
C COO–
COO–
1
CH2
COO–
NAD+
8 Oxaloacetate
HO C
COO–
COO–
CH2
COO–
HO CH
H2O
CH2
CH2
2
HC COO–
COO–
Malate
Figure
CH2
HO
Citrate
9.12
COO–
Isocitrate
COO–
H2O
COO–
CH
CO2
Citric
acid
cycle
7
3
NAD+
COO–
Fumarate
HC
CH
CH2
CoA SH
6
CoA SH
COO–
FAD
CH2
CH2
COO–
C O
Succinate
Pi
S
CoA
GTP GDP Succinyl
CoA
ADP
ATP
Figure 9.12
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4
C O
COO–
CH2
5
CH2
FADH2
COO–
NAD+
NADH
+ H+
+ H+
a-Ketoglutarate
CH2
COO–
NADH
CO2
Oxidative Phosphorylation . . . ETC
• Concept 9.4: During oxidative phosphorylation,
chemiosmosis couples electron transport to
ATP synthesis
• So far, only 4 ATP have been made
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The Pathway of Electron Transport
• Electron Transport Chain – is a collection of
molecules embedded in the inner membrane of
the mitochondria
– Most components are proteins
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• An NADH molecule begins the process by
“dropping off” its electron at the first electron
carrier molecule
NADH
50
Free energy (G) relative to O2 (kcl/mol)
FADH2
40
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I
Fe•S
30
20
Multiprotein
complexes
FAD
Fe•S II
O
III
Cyt b
Fe•S
Cyt c1
IV
Cyt c
Cyt a
Cyt a3
10
0
Figure 9.13
FMN
2 H + + 1  2 O2
H2 O
NADH
50
FADH2
Free energy (G) relative to O2 (kcl/mol)
• Remember: each component
will be reduced when it
accepts the electron and
oxidized when it passes the
electron down to the more
electronegative carrier
molecule in the chain
40
FMN
I
Fe•S
Multiprotein
complexes
FAD
Fe•S
II
O
III
Cyt b
30
Fe•S
Cyt c1
IV
Cyt c
Cyt a
Cyt a3
20
10
0
Figure 9.13
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2 H + + 12
O2
H2 O
• Finally the electron is passed to oxygen, which
is very electronegative.
NADH
50
The oxygen also picks up 2
H+ ions from the aqueous
solution and forms water
Free energy (G) relative to O2 (kcl/mol)
FADH2
40
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I
FAD
Fe•S
Fe•S II
Multiprotein
complexes
O
III
Cyt b
30
Fe•S
Cyt c1
IV
Cyt c
Cyt a
Cyt a3
20
10
0
Figure 9.13
FMN
2 H + + 12
O2
H2 O
• FADH goes through mostly the
same processes, except it
drops off its electron at a lower
point on the ETC
NADH
50
Free energy (G) relative to O2 (kcl/mol)
FADH2
40
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I
FAD
Fe•S
Fe•S II
O
30
Multiprotein
complexes
III
Cyt b
Fe•S
Cyt c1
IV
Cyt c
Cyt a
Cyt a3
20
10
0
Figure 9.13
FMN
2 H + + 12
O2
H2 O
WARNING: The ETC makes no ATP directly!
The ETC releases energy in a step-wise series
of reactions. It powers ATP synthesis via
oxidative phosphorylation.
But it needs to be coupled with chemiosmosis
to actually make ATP.
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Chemiosmosis: The Energy-Coupling Mechanism
• Inner membrane of mitochondria has many
protein complexes called ATP synthase
– ATP synthase – enzyme that makes ATP from
ADP and inorganic phosphate
• It uses the energy of an existing gradient to
do this.
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• The existing gradient is the difference in H+ ion
concentration on opposite sides of the inner
membrane of the mitochondria
Oxidative
phosphorylation.
electron transport
and chemiosmosis
Glycolysis
ATP
Inner
Mitochondrial
membrane
ATP
ATP
H+
H+
H+
Intermembrane
space
Q
I
Inner
mitochondrial
membrane
Mitochondrial
matrix
Figure 9.15
H+
Cyt c
Protein complex
of electron
carners
IV
III
II
FADH2
NADH+
FAD+
NAD+
(Carrying electrons
from, food)
2 H+ + 1/2 O2
ATP
synthase
H2O
ADP +
ATP
Pi
H+
Chemiosmosis
Electron transport chain
Electron transport and pumping of protons (H+), ATP synthesis powered by the flow
which create an H+ gradient across the membrane Of H+ back across the membrane
Oxidative phosphorylation
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So what is this process called?
• Chemiosmosis – the process in which energy
stored in the form of a hydrogen ion gradient
across a membrane is used to drive cellular
work (like the synthesis of ATP)
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• It is the job of the ETC to create this H+ ion
gradient
Oxidative
phosphorylation.
electron transport
and chemiosmosis
Glycolysis
ATP
ATP
ATP
H+
H+
H+
Q
I
Mitochondrial
matrix
H+
Cyt c
Protein complex
Intermembrane of electron
space
carners
Inner
mitochondrial
membrane
Inner
Mitochondria
membrane
IV
III
II
FADH2
NADH+
NAD+
FAD+
2 H+ + 1/2 O2
ATP
synthase
H2O
ADP +
(Carrying electrons
from, food)
ATP
Pi
H+
Chemiosmosis
Electron transport chain
ATP synthesis powered by the flow
Electron transport and pumping of protons (H+),
Of H+ back across the membrane
which create an H+ gradient across the membrane
Figure 9.15
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Oxidative phosphorylation
• H+ ions are pumped into the intermembrane
space by the ETC
– The H+ ions want to drift back into the matrix
• But they can only come into the matrix
easily through ATP synthase channels
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• The rushing of H+ ions through ATP synthase
acts like water turning a water wheel
INTERMEMBRANE SPACE
H+
Result is that
inorganic
phosphate
can be
joined to
ADP to make
ATP
H+
H+
H+
H+
H+
H+
A rotor within the
membrane spins
clockwise when
H+ flows past
it down the H+
gradient.
A stator anchored
in the membrane
holds the knob
stationary.
H+
ADP
+
Pi
Figure 9.14
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MITOCHONDRIAL MATRIX
ATP
A rod (for “stalk”)
extending into
the knob also
spins, activating
catalytic sites in
the knob.
Three catalytic
sites in the
stationary knob
join inorganic
Phosphate to ADP
to make ATP.
To Sum Up
Chemiosmosis is an energy coupling mechanism
that uses energy stored in the form of a H+
gradient across a membrane to drive cellular
work
In mitochondria, the energy for gradient formation
comes from the exergonic redox reactions of
the ETC and ATP synthesis is the work
performed by ATP synthase
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• Chemiosmosis and the electron transport chain
Oxidative
phosphorylation.
electron transport
and chemiosmosis
Glycolysis
ATP
Inner
Mitochondrial
membrane
ATP
ATP
H+
H+
H+
Intermembrane
space
Protein complex
of electron
carners
Q
I
Inner
mitochondrial
membrane
IV
III
ATP
synthase
II
FADH2
NADH+
Mitochondrial
matrix
H+
Cyt c
FAD+
NAD+
2 H+ + 1/2 O2
H2O
ADP +
(Carrying electrons
from, food)
ATP
Pi
H+
Chemiosmosis
Electron transport chain
+
ATP
synthesis
powered by the flow
Electron transport and pumping of protons (H ),
+
+
which create an H gradient across the membrane Of H back across the membrane
Figure 9.15
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Oxidative phosphorylation
• There is a problem though, when we try to
account how much ATP is made from glucose
(or any organic molecule)
We can not state exactly how many ATP come
from each glucose because of 3 reasons . . .
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1) The ETC and ATP synthase work together,
but not 100% in synchronicity
So there isn’t an exact correlation between
NADH (or FADH) and ATP production
1 NADH synthesizes 2.5 – 3.3 ATP
1 FADH synthesizes 1.5 – 2 ATP
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(
2) The electron shuttle into the mitochondria can
vary (either NAD+ or FAD) at the transition
stage
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3) The H+ ion gradient does other things in the
mitochondria, it is not just used for the
synthesis of ATP
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Bottom Line
• We’ll say there are 38 ATP:
Electron shuttles
span membrane
CYTOSOL
MITOCHONDRION
2 NADH
or
2 FADH2
2 NADH
2 NADH
Glycolysis
Glucose
2
Pyruvate
6 NADH
Citric
acid
cycle
2
Acetyl
CoA
+ 2 ATP
by substrate-level
phosphorylation
Maximum per glucose:
+ 2 ATP
2 FADH2
Oxidative
phosphorylation:
electron transport
and
chemiosmosis
+ about 32 or 34 ATP
by substrate-level by oxidative phosphorylation, depending
on which shuttle transports electrons
phosphorylation
from NADH in cytosol
About
36 or 38 ATP
Figure 9.16
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An interesting note:
• Cellular respiration harvests about 40% of the
energy in a glucose molecule
– This is perhaps the most efficient energy
conversion known
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In comparison:
• The most efficient cars only convert about 25%
of the energy stored in gas into energy that
moves the car
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• Concept 9.5: Fermentation enables some cells
to produce ATP without the use of oxygen
• Cellular respiration
– Relies on oxygen to produce ATP
• In the absence of oxygen
– Cells can still produce ATP through
fermentation
– Not as much ATP is made
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• Glycolysis
– Can produce ATP with or without oxygen, in
aerobic or anaerobic conditions
– Couples with fermentation – an extension of
glycolysis, to produce ATP
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Types of Fermentation
• Fermentation consists of
– Glycolysis plus reactions that regenerate
NAD+, which can be reused by glycolysis
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Type 1
•In alcohol fermentation
– Pyruvate is converted
to ethanol in two
steps, one of which
releases CO2
– The second
regenerates NAD+ so
glycolysis can
continue
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Type 2
•During lactic acid
fermentation
– Pyruvate is reduced
directly to NADH to
form lactate as a
waste product
– No CO2 is produced
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Fermentation and Cellular Respiration Compared
• Both fermentation and cellular respiration
– Use glycolysis to oxidize glucose and other
organic fuels to pyruvate
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Fermentation and Cellular Respiration Contrasted
• Fermentation and cellular respiration
– Differ in their final electron acceptor
• Cellular respiration
– Produces more ATP
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• Pyruvate is a key juncture in catabolism
Glucose
CYTOSOL
Pyruvate
Some organisms can
produce ATP using either
cellular respiration or
fermentation. They are
called facultative
anaerobes.
Ex: our muscle cells
Figure 9.18
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No O2 present
Fermentation
O2 present
Cellular respiration
MITOCHONDRION
Ethanol
or
lactate
Acetyl CoA
Citric
acid
cycle
• Concept 9.6: Glycolysis and the citric acid
cycle connect to many other metabolic
pathways
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The Versatility of Catabolism
• Catabolic pathways
– Funnel electrons from many kinds of organic
molecules into cellular respiration
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• The catabolism of various molecules from food
Proteins
Carbohydrates
Amino
acids
Sugars
Fats
Glycerol
Glycolysis
Glucose
Glyceraldehyde-3- P
NH3
Pyruvate
Acetyl CoA
Citric
acid
cycle
Figure 9.19
Oxidative
phosphorylation
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Fatty
acids
Biosynthesis (Anabolic Pathways)
• The body
– Uses small molecules to build other
substances
• These small molecules
– May come directly from food or through
glycolysis or the citric acid cycle
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Regulation of Cellular Respiration via Feedback
Mechanisms
• Cellular respiration
– Is controlled by allosteric enzymes at key
points in glycolysis and the citric acid cycle
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