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Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Biology, Seventh EditionNeil Campbell and Jane Reece
Lectures by Chris Romero
Chapter 9Chapter 9
Cellular Respiration: Harvesting Chemical Energy
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• Overview: Life Is Work
• Living cells
– Require transfusions of energy from outside sources to perform their different tasks
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• The giant panda
– Obtains energy for its cells by eating plants
Figure 9.1
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• Energy
– Flows into ecosystems as sunlight and leaves as heat
Light energy
ECOSYSTEM
CO2 + H2O
Photosynthesisin chloroplasts
Cellular respirationin mitochondria
Organicmolecules
+ O2
ATP
powers most cellular work
HeatenergyFigure 9.2
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• Photosynthetic organisms trap a portion of the sunlight energy and transform it into chemical energy (organic molecules) with O2 is released.
• Cells use some of the chemical energy in organic molecules to make ATP; the energy source for cellular work.
• Energy leaves organisms as it dissipates as heat
• The products of respiration (CO2 and H2O) are the raw materials for photosynthesis.
• Photosynthesis produces glucose and oxygen, the raw materials for respiration
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• Concept 9.1: Catabolic pathways yield energy by oxidizing organic fuels
Catabolic Pathways and Production of ATP
• Organic compounds store energy in their arrangement of atoms
• With the help of enzymes, a cell systematically degrades complex organic molecules that are rich in potential energy to simpler waste products that have less energy
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• Fermentation a catabolic process
– Is a partial degradation of sugars that occurs without oxygen (anaerobic)
• Cellular respiration
– Is the most prevalent and efficient catabolic pathway
– Consumes oxygen and organic molecules such as glucose
– Yields ATP
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Respiration can be summarized:
organic + oxygen → carbon + water + Energy compounds dioxide
cellular respiration is most often described as the oxidation of glucose:
C6H12O6 + 6 O2 → 6 CO2 + 6H2O + Energy (ATP + heat)
The breakdown of glucose is exergonic (free energy change) (ΔG= – 686 kcal per mol)
– ΔG → the products of the chemical process store less energy than reactants and the reaction can happen spontaneously (without an input of energy)
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• To keep working
– Cells must regenerate ATP from ADP + Pi
– To understand how cellular respiration accomplishes this, let’s examine the fundamental process known as oxidation and reduction
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Redox Reactions: Oxidation and Reduction
• Why 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 is used to synthesize ATP
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
The Principle of Redox
• Redox reactions
– Transfer electrons from one reactant to another by oxidation and reduction
• In oxidation
– A substance loses electrons, or is oxidized
• In reduction
– A substance gains electrons, or is reduced
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• Examples of redox reactions
Na + Cl Na+ + Cl–
becomes oxidized(loses electron)
becomes reduced(gains electron)
Xē + Y X + Yē
becomes oxidized
becomes reduced
We could generalize a redox reaction this way:
X is the electron donor, is called the reducing agent, it reduces Y which accepts the donated electronY is the electron acceptor, it oxidizes X by removing its electron
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• Some redox reactions
– Do not completely exchange electrons
– Change the degree of electron sharing in covalent bonds
– An electron loses potential energy when it shifts from a less electronegative atom toward more electronegative one → this energy can be put to work
CH4
H
H
HH
C O O O O OC
H H
Methane(reducingagent)
Oxygen(oxidizingagent)
Carbon dioxide Water
+ 2O2 CO2 + Energy + 2 H2O
becomes oxidized
becomes reduced
Reactants Products
Figure 9.3
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Oxidation of Organic Fuel Molecules During Cellular Respiration
• Oxidation of methane is the main combustion reaction that occurs at the burner of a gas stove
• During cellular respiration
– Glucose is oxidized and oxygen is reduced
C6H12O6 + 6O2 6CO2 + 6H2O + Energy
becomes oxidized
becomes reduced
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• 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 when they are transferred to oxygen
• By oxidizing glucose, respiration liberates stored energy from glucose and makes it available for ATP synthesis
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Stepwise Energy Harvest via NAD+ and the Electron Transport Chain
• Cellular respiration
– Oxidizes glucose in a series of steps each is catalyzed by an enzyme
– The hydrogen atoms are not transferred to oxygen, but instead are usually passed first to a coenzyme called NAD+ (nicotinamide adenine dinucleotide).
– NAD functions as coenzyme in the redox reactions thus is an oxidizing agent. It is found in all cells and helps in e transfer.
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• Electrons from organic compounds
– Are usually first transferred to NAD+, a coenzyme
– NAD+ accept electrons and act as an oxidizing agent during respiration
NAD+
H
O
O
O O–
O
O O–
O
O
O
P
P
CH2
CH2
HO OHH
HHO OH
HO
H
H
N+
C NH2
HN
H
NH2
N
N
Nicotinamide(oxidized form)
NH2+ 2[H]
(from food)
Dehydrogenase
Reduction of NAD+
Oxidation of NADH
2 e– + 2 H+
2 e– + H+
NADH
OH H
N
C +
Nicotinamide(reduced form)
N
Figure 9.4
How does NAD+ trap electrons from
glucose and other organic molecules? Enzymes called dehydrogenases removes a pair of hydrogen atoms (2 electrons and 2 protons) from the substrate (a sugar for example) thereby oxidizing it
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• NADH, the reduced form of NAD+
– Passes the electrons to the electron transport chain
– 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 oxygen
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
How do electrons that are extracted from food and stored by NADH finally reach oxygen?
• If electron transfer is not stepwise
– A large release of energy occurs
– As in the reaction of hydrogen and oxygen to form water
(a) Uncontrolled reaction
Fre
e en
ergy
, G
H2O
Explosiverelease of
heat and lightenergy
Figure 9.5 A
H2 + 1/2 O2
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• The electron transport chain
– Passes electrons in a series of steps instead of one explosive reaction
– Uses the energy from the electron transfer to form ATP
– The transport chain consists of a number of molecules, mostly proteins, built into the inner membrane of a mitochondrion
– ET from NADH to O2 is an exergonic reaction with a free energy change of – 53 kcal/mol
– Food → NADH → ETC → Oxygen
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
2 H 1/2 O2
(from food via NADH)
2 H+ + 2 e–
2 H+
2 e–
H2O
1/2 O2
Controlled release of energy for synthesis of
ATP ATP
ATP
ATP
Electro
n tran
spo
rt chain
Fre
e en
ergy
, G
(b) Cellular respiration
+
Figure 9.5 B
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
The Stages of Cellular Respiration: A Preview
• Respiration is a cumulative function of three metabolic stages
– Glycolysis
– The citric acid cycle
– Oxidative phosphorylation
does not require O2
require O2
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• Glycolysis
– Breaks down glucose into two molecules of pyruvate
• The citric acid cycle
– Completes the breakdown of glucose
• Oxidative phosphorylation
– Is driven by the electron transport chain
– Generates ATP
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• An overview of cellular respiration
Figure 9.6
Electronscarried
via NADH
GlycolsisGlucos
ePyruvate
ATP
Substrate-levelphosphorylation
Electrons carried via NADH and
FADH2
Citric acid cycle
Oxidativephosphorylation:
electron transport and
chemiosmosis
ATPATP
Substrate-levelphosphorylation
Oxidativephosphorylation
MitochondrionCytosol
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
An overview of cellular respiration
• During glycolysis each glucose molecule is broken down into 2 pyruvate molecules
• Pyruvate inters into mitochondria where it will be oxidized by the citric acid cycle to CO2.
• NADH and FADH2 transfer electrons from glucose to ETCs in the inner mitochondria membrane
• During oxidative phosphorylation, ETCs convert chemical energy to a form of energy used for ATP synthesis in a process called chemiosmosis.
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• Both glycolysis and the citric acid cycle
– Can generate ATP by substrate-level phosphorylation
– 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
Figure 9.7
Enzyme Enzyme
ATP
ADP
Product
SubstrateP
+
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• Concept 9.2: Glycolysis harvests energy by oxidizing glucose to pyruvate
• Glycolysis (can occur in the presence or absence of O2)
– Means “splitting of sugar”
a 6-C sugar (glucose) to 3-C sugar (pyruvate)
– Breaks down glucose into pyruvate
– Occurs in the cytoplasm of the cell
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• Glycolysis consists of two major phases
– Energy investment phase
– Energy payoff phaseGlycolysis Citric
acidcycle
Oxidativephosphorylation
ATP ATP ATP
2 ATP
4 ATP
used
formed
Glucose
2 ATP + 2 P
4 ADP + 4P
2 NAD+ + 4 e- + 4 H +
2 NADH + 2 H+
2 Pyruvate + 2 H2O
Energy investment phase
Energy payoff phase
Glucose 2 Pyruvate + 2 H2O
4 ATP formed – 2 ATP used 2 ATP
2 NAD+ + 4 e– + 4 H +
2 NADH
+ 2 H+
Figure 9.8
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Dihydroxyacetonephosphate
Glyceraldehyde-3-phosphate
HH
H
HH
OHOH
HO HO
CH2OHH H
H
HO H
OHHO
OH
P
CH2O P
H
OH
HO
HO
HHO
CH2OH
P O CH2O CH2 O P
HOH HO
HOH
OP CH2
C OCH2OH
HCCHOHCH2
O
O P
ATP
ADPHexokinase
Glucose
Glucose-6-phosphate
Fructose-6-phosphate
ATP
ADP
Phosphoglucoisomerase
Phosphofructokinase
Fructose-1, 6-bisphosphate
Aldolase
Isomerase
Glycolysis
1
2
3
4
5
CH2OHOxidative
phosphorylation
Citricacidcycle
Figure 9.9 A
• A closer look at the energy investment phase
This is the reaction from which glycolysis Gets its name
This reaction never reaches equilibrium in the cell
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
2 NAD+
NADH2+ 2 H+
Triose phosphatedehydrogenase2 P i
2P C
CHOH
O
P
O
CH2 O
2 O–
1, 3-Bisphosphoglycerate2 ADP
2 ATP
Phosphoglycerokinase
CH2 O P
2
C
CHOH
3-Phosphoglycerate
Phosphoglyceromutase
O–
C
C
CH2OH
H O P
2-Phosphoglycerate
2 H2O
2 O–
Enolase
C
C
O
PO
CH2
Phosphoenolpyruvate2 ADP
2 ATP
Pyruvate kinase
O–
C
C
O
O
CH3
2
6
8
7
9
10
Pyruvate
O
Figure 9.8 B
• A closer look at the energy payoff phase
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• 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
– completes glucose oxidation by breaking down pyruvate derivitatives into carbon dioxide.
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• Before the citric acid cycle can begin
– Pyruvate must first be converted to acetyl CoA, which links the cycle to glycolysis
CYTOSOL MITOCHONDRION
NADH + H+NAD+
2
31
CO2 Coenzyme APyruvate
Acetyle CoA
S CoA
C
CH3
O
Transport protein
O–
O
O
C
C
CH3
Figure 9.10
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• An overview of the citric acid cycle
ATP
2 CO2
3 NAD+
3 NADH
+ 3 H+
ADP + P i
FAD
FADH2
Citricacidcycle
CoA
CoA
Acetyle CoA
NADH
+ 3 H+
CoA
CO2
Pyruvate(from glycolysis,2 molecules per glucose)
ATP ATP ATP
Glycolysis Citricacidcycle
Oxidativephosphorylation
Figure 9.11
For each turn of Krebs cycle, two carbons exit completely as CO2, three NADH and one FADH2 are formed.One ATP is made by substrate-level phosphorylation
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Figure 9.12
Acetyl CoA
NADH
Oxaloacetate
CitrateMalate
Fumarate
SuccinateSuccinyl
CoA
-Ketoglutarate
Isocitrate
Citricacidcycle
S CoA
CoA SH
NADH
NADH
FADH2
FAD
GTP GDP
NAD+
ADP
P i
NAD+
CO2
CO2
CoA SH
CoA SH
CoAS
H2O
+ H+
+ H+ H2O
C
CH3
O
O C COO–
CH2
COO–
COO–
CH2
HO C COO–
CH2
COO–
COO–
COO–
CH2
HC COO–
HO CHCOO–
CH
CH2
COO–
HO
COO–
CH
HC
COO–
COO–
CH2
CH2
COO–
COO–
CH2
CH2
C O
COO–
CH2
CH2
C O
COO–
1
2
3
4
5
6
7
8
Glycolysis Oxidativephosphorylation
NAD+
+ H+
ATP
Citricacidcycle
Figure 9.12
• A closer look at the citric acid cycle
NAD is reduced to NADH+
CoA is displaced by a phosphate groupWhich is transferred to GDP forming GTP
And then to ATP.
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• During oxidative phosphorylation, chemiosmosis couples electron transport to ATP synthesis
• NADH and FADH2
– Donate electrons to the electron transport chain, which powers ATP synthesis via oxidative phosphorylation
Concept 9.4:
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The Pathway of Electron Transport
– Electrons from NADH and FADH2
lose energy in several steps
– Couples this exergonic slide of electrons to ATP synthesis or oxidative phosphorylation
– The electron transport chain is made of electron carrier molecules embedded in the inner membrane of mitochondria.
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
– Each carrier in the chain has a higher electronegativity than the carrier before it, so electrons are pulled downhill towards oxygen
– Most carriers are protein molecules except for ubiquinone (Q)
– At the end of the chain, electrons are passed to oxygen, forming water
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
H2O
O2
NADH
FADH2
FMN
Fe•S Fe•S
Fe•S
O
FAD
Cyt b
Cyt c1
Cyt c
Cyt a
Cyt a3
2 H + + 12
I
II
III
IV
Multiproteincomplexes
0
10
20
30
40
50
Fre
e e
ner
gy
(G)
rela
tive
to O
2 (k
cl/m
ol)
Figure 9.13
NADH is oxidized and flavoprotein is reduced as highenergy electrons from NADH are transferred to FMN
↓Flavoprotein is oxidized as it passes electrons to an iron sulfur protein, FeS.
↓FeS is oxidized as it pass electrons to ubiquinone Q
↓Q passes electrons on to a succession of electron carriers, most of which are cytochromes.
↓cyt a3 , the last cytochrome passes electrons to oxygen.
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Chemiosmosis: The Energy-Coupling Mechanism
• ATP synthase
– Is the enzyme that actually makes ATPINTERMEMBRANE SPACE
H+
H+
H+
H+
H+
H+ H+
H+
P i
+ADP
ATP
A rotor within the membrane spins clockwise whenH+ flows past it down the H+
gradient.
A stator anchoredin the membraneholds the knobstationary.
A rod (for “stalk”)extending into the knob alsospins, activatingcatalytic sites inthe knob.
Three catalytic sites in the stationary knobjoin inorganic Phosphate to ADPto make ATP. MITOCHONDRIAL MATRIXFigure 9.14
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• At certain steps along the electron transport chain
– Electron transfer causes protein complexes to pump H+ from the mitochondrial matrix to the intermembrane space
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• Chemiosmosis
– Is an energy-coupling mechanism that uses energy in the form of a H+ gradient across a membrane to drive cellular work
• The resulting H+ gradient
– Stores energy
– Drives chemiosmosis in ATP synthase
– Is referred to as a proton-motive force
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• Chemiosmosis and the electron transport chain
Oxidativephosphorylation.electron transportand chemiosmosis
Glycolysis
ATP ATP ATP
InnerMitochondrialmembrane
H+
H+H+
H+
H+
ATPP i
Protein complexof electron carriers
Cyt c
I
II
III
IV
(Carrying electronsfrom, food)
NADH+
FADH2
NAD+
FAD+ 2 H+ + 1/2 O2
H2O
ADP +
Electron transport chainElectron transport and pumping of protons (H+),
which create an H+ gradient across the membrane
ChemiosmosisATP synthesis powered by the flowOf H+ back across the membrane
ATPsynthase
Q
Oxidative phosphorylation
Intermembranespace
Innermitochondrialmembrane
Mitochondrialmatrix
Figure 9.15
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An Accounting of ATP Production by Cellular Respiration
• During respiration, most energy flows in this sequence
– Glucose to NADH to electron transport chain to proton-motive force to ATP
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• There are three main processes in this metabolic enterprise
Electron shuttlesspan membrane
CYTOSOL 2 NADH
2 FADH2
2 NADH 6 NADH 2 FADH22 NADH
Glycolysis
Glucose2
Pyruvate
2AcetylCoA
Citricacidcycle
Oxidativephosphorylation:electron transport
andchemiosmosis
MITOCHONDRION
by substrate-levelphosphorylation
by substrate-levelphosphorylation
by oxidative phosphorylation, dependingon which shuttle transports electronsfrom NADH in cytosol
Maximum per glucose:About
36 or 38 ATP
+ 2 ATP + 2 ATP + about 32 or 34 ATP
or
Figure 9.16
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• About 40% of the energy in a glucose molecule
– Is transferred to ATP during cellular respiration, making approximately 38 ATP
1 ATP → - 7.3 kcal/mol
38 ATP X 7.3 / 686 = 40%
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• 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
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• Glycolysis
– Can produce ATP with or without oxygen, in aerobic or anaerobic conditions
– Couples with fermentation 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
• In alcohol fermentation
– Pyruvate is converted to ethanol in two steps, one of which releases CO2
• During lactic acid fermentation
– Pyruvate is reduced directly to NADH to form lactate as a waste product
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2 ADP + 2 P1 2 ATP
GlycolysisGlucose
2 NAD+ 2 NADH
2 Pyruvate
2 Acetaldehyde 2 Ethanol
(a) Alcohol fermentation
2 ADP + 2 P1 2 ATP
GlycolysisGlucose
2 NAD+ 2 NADH
2 Lactate
(b) Lactic acid fermentation
H
H OH
CH3
C
O –
OC
C O
CH3
H
C O
CH3
O–
C O
C O
CH3O
C O
C OHH
CH3
CO22
Figure 9.17
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Fermentation and Cellular Respiration Compared
• Both fermentation and cellular respiration
– Use glycolysis to oxidize glucose and other organic fuels to pyruvate
• 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
No O2 presentFermentation
O2 present Cellular respiration
Ethanolor
lactate
Acetyl CoA
MITOCHONDRION
Citricacidcycle
Figure 9.18
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The Evolutionary Significance of Glycolysis
• Glycolysis
– Occurs in nearly all organisms
– Probably evolved in ancient prokaryotes before there was oxygen in the atmosphere
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The Versatility of Catabolism
• Catabolic pathways
– Funnel electrons from many kinds of organic molecules into cellular respiration
• Concept 9.6: Glycolysis and the citric acid cycle connect to many other metabolic pathways
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• The catabolism of various molecules from food
Amino acids
Sugars Glycerol Fattyacids
Glycolysis
Glucose
Glyceraldehyde-3- P
Pyruvate
Acetyl CoA
NH3
Citricacidcycle
Oxidativephosphorylation
FatsProteins Carbohydrates
Figure 9.19
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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
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• The control of cellular respirationGlucose
Glycolysis
Fructose-6-phosphate
Phosphofructokinase
Fructose-1,6-bisphosphateInhibits Inhibits
Pyruvate
ATPAcetyl CoA
Citricacidcycle
Citrate
Oxidativephosphorylation
Stimulates
AMP
+
– –
Figure 9.20
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