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CHAPTER 9:
CELLULAR RESPIRATION
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Living cells
Require transfusions of energy from outside sources
to perform their many tasks
Overview: Life Is Work
The giant panda Obtains energy for its cells by eating plants
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Energy
Flows into an ecosystem as sunlight and leaves as heat; thechemical elements essential to life are recycled
Light energy
ECOSYSTEM
CO2 + H2O
Photosynthesis
in chloroplasts
Cellular
respiration
in mitochondria
Organic
molecules+ O2
ATP
powers most cellular work
Heat
energ
y
How cells harvest the
chemical energy stored in
organic molecules & use it
to generate ATP, the
molecule that drives most
cellular work?
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Catabolic Pathways and Production of ATP
Metabolic pathways that release stored energy bybreaking down complex molecules catabolic pathways
The breakdown of organic molecules is exergonic
Catabolic pathways yield energy by oxidizing
organic fuels
Fermentation (catabolic process)
Is a partial degradation of sugars that occurs without
the use of oxygen
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Cellular respiration
Is the most prevalent and efficient catabolic pathway Consumes oxygen and organic molecules such as
glucose
Yields ATP
Catabolism is linked to work by a chemical drive shaft -
ATP
To keep working
Cells must regenerate its supply of ATP from ADPand Pi (Inorganic phosphate)
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Redox Reactions: Oxidation and Reduction
Catabolic pathways yield energy
Due to the transfer ofelectrons during the chemical
reactions
The relocation of electrons releases energy stored in
organic molecules
And this energy ultimately is used to synthesize ATP
by using enzyme.
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The Principle of Redox
Redox reactions
Transfer electrons from one reactant to another byoxidation and reduction
In oxidation
A substance loses electrons, or is oxidized
In reduction
A substance gains electrons, or is reduced
Examples of redox reactions
Na + Cl Na+ + Cl
becomes oxidized
(loses electron)
becomes reduced
(gains electron)
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Some redox reactions
Do not completely exchange electrons
Change the degree of electron sharing in covalent bonds
CH4
H
H
HH
C O O O O OCH H
Methane
(reducing
agent)
Oxygen
(oxidizing
agent)
Carbon dioxide Water
+ 2O2CO
2 +Energ
y + 2 H2O
becomes oxidized
becomes
reduced
Reactants Products
Methane combustion as an energy-yielding
redox reaction
Energy must be added to pull an e-
away from an atom.
The more electronegative the atom(the stronger its pull on e-), the more
energy is required to take away an e-.
An e- loses potential energy when it
shifts from a less electronegative
atom to a more electronegative one.
A redox reaction that relocates e-
closer to O2 (eg. burning of methane),
releases chemical energy that can be
used for work.
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Stepwise Energy Harvest via NAD+ and the Electron
Transport Chain
Cellular respiration
Oxidizes glucose in a series of steps, each step
catalyzed by an enzyme.
At key steps, e- are stripped from the glucose, each
electron travels with a proton (H atom)
H atom from organic compounds are not transferred
directly to oxygen But are usually first transferred to NAD+, a coenzyme
As an electron acceptor, NAD+ functions as an oxidizing
agent during respiration
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NAD+H
O
O
O O
O
O O
O
O
O
P
P
CH2
CH2
HO OH
H
H
HO 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
H+
H+
NAD+ as an electron shuttle.
The enzymatic transfer of two electrons and
one proton (H+) from an organic molecule in
food to NAD+reduces the NAD+ to NADH;
the second proton (H+
) is released.
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Most of the electrons removed from food are transferred
initially to NAD+
NADH, the reduced form of NAD+
Passes the electrons to the electron transport chain
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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
Freeenergy,
G
H2O
Explosive
release of
heat and light
energy
H2 +1/2 O2
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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
ATPATP
ATP
ATP
Electrontr
ansp
ortc
hain
Freeenergy,G
(b) Cellular respiration
+
The electron transport chain
Passes electrons in a
series of steps instead ofin one explosive reaction
Uses the energy from the
electron transfer to form
ATP
e- removed from food are shuttled by
NADH to the top, higher-energy end
of the chain
At the bottom, lower-energy end,
oxygen captures these e- along with
H+, forming water.
In cellular respiration, e- travel this
downhill route
Food NADH Electron Transport
Chain oxygen
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The Stages of Cellular Respiration:A Preview
Respiration is a cumulative function of three metabolic
stages
Glycolysis: Breaks down glucose into two molecules of
pyruvate The citric acid cycle: Completes the breakdown of glucose
Oxidative phosphorylation (electron transport &
chemiosmosis): Is driven by the electron transport chain ,
Generates ATP
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An overview of cellular respiration
Electr
ons
carrie
d
via
NAD
H
GlycolysisGlucos
e
Pyruvate
ATP
Substrate-level
phosphorylation
Electrons carried
via NADH and
FADH2
Ci
tri
cac
id
cy
cl
e
Oxidative
phosphorylation:
electron
transport
and
chemios
mosis
ATPATP
Substrate-level
phosphorylationOxidative
phosphorylation
MitochondrionCytosol
Glycolysis occurs in
the cytosol.
Begins degradation
by breaking glucose
into 2 molecules of
pyruvate
The citric acid cycletakes place within
the mitochondrial
matrix.
Completes the
breakdown ofglucose by oxidizing
a derivative of
pyruvate to CO2
In the third stage of
respiration, the ETC
accepts e- from the 1st
two stages breakdown
products via NADH &
passes these e- from 1
molecule to another.
At the end of the chain,
e- are combined with
molecular O2 and H+ to
form water.
The energy released at
each step of the chain is
stored in a form the
mitochondrion can use
to make ATP.
This mode of ATP synthesis is called oxidative
phosphorylation because it is powered by the redox
reactions of the ETC.
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Glycolysis
Means splitting of sugar
Breaks down glucose into pyruvate Occurs in the cytoplasm of the cell
Glycolysis harvests energy by oxidizing
glucose to pyruvate
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Glycolysis consists oftwo major phases
Energy investment
phase
Energy payoff
phase
Glycolysis Citricacidcycle
Oxidativephosphorylation
ATP ATP ATP
2 ATP
4 ATP
used
formed
Glucose
2 ATP + 2 P
4 ADP + 4 P
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+
Net
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Dihydroxyacetonephosphate
Glyceraldehyde-3-phosphate
HH
H
HH
OH
OH
HOHO
CH2OH
HH
H
HO H
OHHO
OH
P
CH2O P
H
O
H
HO
HO
HHO
CH2OH
P O CH2 O CH2 O P
HO
H HO
HOH
OP CH2
C O
CH2OH
H
C
CHOH
CH2
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
CH2OH
Oxidative
phosphorylation
Citric
acid
cycle
A closer look at the energy investment phase
1 Glucose is phosphorylated by hexokinase.
Hexokinase transfers a P group from ATP to
the sugar.
2 Glucose-6-phosphate is rearranged to itsisomer, fructose-6-phosphate
3 Enzyme transfers a P group from ATP tothe sugar.
P groups on opposite sides, sugar is ready
to be split in half.
4 Enzyme cleaves the sugar molecule intotwo different 3-carbon sugars (isomers)
5 Net result of steps 4 and 5 is cleavage ofa 6-carbon sugar into two molecules of
glyceraldehyde-3-phosphate.
A closer look at the energy payoff phase
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2 NAD+
NADH2
+ 2 H+
Triose phosphatedehydrogenase
2 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
CH2Phosphoenolpyruvate
2 ADP
2 ATP
Pyruvate kinase
O
C
C
O
O
CH3
2
6
8
7
9
10
Pyruvate
O
A closer look at the energy payoff phase
6 Sugar is oxidized by transfer of electronsand H+ to NAD+, forming NADH (exergonic).
Enzyme uses this energy to attach a P group to
the oxidized substrate.
7 Phosphate group is transferred to ADP toproduce ATP.
8 Enzyme relocates the P group.
9 Enolase causes a double bond to form in thesubstrate, extracting a water molecule, yielding
PEP.
10 Pyruvate kinase transfers P group from PEPto ADP.
Net gain of 2 ATP (2 ATP used, 4 ATP
produced)
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Less than a of chemical energy in glucose is released in
glycolysis; most energy is stockpiled in 2 molecules of
pyruvate
The citric acid cycle
Takes place in the matrix of the mitochondrion
The citric acid cycle completes the energy-yielding
oxidation of organic molecules
If molecular O2 is present, pyruvate enters mitochondrion;
enzymes of the citric acid cycle complete the oxidation ofthe organic fuel.
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Before the citric acid cycle can begin
Pyruvate must first be converted to acetyl CoA, which links the cycle
to glycolysis
CYTOSOL MITOCHONDRION
NAD
H
+
H+NAD+
2
31
CO2Coenzyme
APyruvate
Acetyle
CoA
SCo
A
C
CH3
O
Transport protein
O
O
O
C
C
CH3
1 Pyruvatescarboxyl group
(-COO-) which is fully
oxidized and has little
chemical energy, is
released as CO2.
2 The remaining 2C fragment isoxidized to acetate.
An enzymetransfers the extracted
electrons to NAD+, storing energy in
the form of NADH.
3 Coenzyme A (sulfur-containing compound derived
from a B vitamin) is attached to
the acetate by an unstable bond.
This acetyl
group, very
reactive.
Acetyl CoA is
now ready to
feed its acetyl
group into thecitric acid
cycle for
further
oxidation.
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An overview of the citric acid cycle
ATP
2 CO2
3 NAD+
3 NADH
+ 3 H+
ADP + Pi
FAD
FADH2
Citric
acid
cycle
CoA
CoA
Acetyle CoA
NADH
+ 3 H+
CoA
CO2
Pyruvate
(from glycolysis,
2 molecules per glucose)ATP ATP ATP
Glycolysis Citricacidcycle
Oxidative
phosphorylation
The cycle generates 1 ATP per turn by
substrate-level phosphorylation, but
most of the chemical energy is
transferred to NAD+ and the related
coenzyme FAD during redox
reactions.
The reduced coenzymes, NADH and
FADH2, shuttle their high-energy
electrons to the ETC.
Summary of the inputs and outputs as pyruvate is broken
down to 3 CO2 molecules, including the molecule of CO2
released
A l l k t th it i id l
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Figure 9.12
Acetyl CoA
NADH
Oxaloacetate
CitrateMalate
Fumarate
Succinate
Succinyl
CoA
-Ketoglutarate
Isocitrate
Citric
acid
cycle
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 CH
COO
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 Oxidative
phosphorylation
NAD+
+ H+
ATP
Citric
acid
cycle
A closer look at the citric acid cycle
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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
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The Pathway of Electron Transport
In the electron transport chain
Electrons from NADH and FADH2 lose energy in
several steps
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At the end of the chain
Electrons are passed to
oxygen, forming water
H2O
O2
NADH
FADH2
FMN
FeS FeS
FeS
O
FAD
Cyt b
Cyt c1
Cyt c
Cyt a
Cyt a3
2 H + + 12
I
II
III
IV
Multiprotein
complexes
0
10
20
30
40
50
Free
energy(G)relativetoO
2(kcl/mo
l)
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Chemiosmosis: The Energy-Coupling Mechanism
ATP synthase
Is the enzyme thatactually makes ATP
INTERMEMBRANE SPACE
H+
H+
H+
H+
H+
H+H+
H+
P i
+
ADP
ATP
A rotorwithin themembrane spins
clockwise when
H+ flows past
it down the H+
gradient.
A statoranchored
in the membrane
holds the knobstationary.
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.
MITOCHONDRIAL MATRIX
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At certain steps along the electron transport chain
Electron transfer causes protein complexes to pumpH+ from the mitochondrial matrix to the intermembrane
space
The resulting H+ gradient
Stores energy Drives chemiosmosis in ATP synthase
Is referred to as a proton-motive force
Chemiosmosis Is an energy-coupling mechanism that uses energy in
the form of a H+ gradient across a membrane to drive
cellular work
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Chemiosmosis and the electron transport chain
Oxidative
phosphorylation.
electron transport
and chemiosmosis
Glycolysis
ATP ATP ATP
InnerMitochondrial
membrane
H+
H+H+
H+
H+
ATPP i
Protein complex
of electron
carners
Cyt c
I
II
III
IV
(Carrying electrons
from, food)
NADH+
FADH2
NAD+FAD+ 2 H+ +1/2 O2 H
2O
ADP +
Electron transport chain
Electron transport and pumping of protons (H+),
which create an H+ gradient across the membrane
Chemiosmosis
ATP synthesis powered by the flow
Of H+ back across the membrane
ATP
synthase
Q
Oxidative phosphorylation
Intermembrane
space
Inner
mitochondrialmembrane
Mitochondrial
matrix
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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
There are three main processes in this metabolic
enterprise
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Electron shuttles
span membraneCYTOSOL 2 NADH
2 FADH2
2 NADH 6 NADH 2 FADH22 NADH
Glycolysis
Glucose
2
Pyruvate
2
Acetyl
CoA
Citric
acid
cycle
Oxidative
phosphorylation:
electron transport
andchemiosmosis
MITOCHONDRION
by substrate-level
phosphorylation
by substrate-level
phosphorylation
by oxidative phosphorylation, depending
on which shuttle transports electrons
from NADH in cytosol
Maximum per glucose:About
36 or 38 ATP
+ 2 ATP + 2 ATP + about 32 or 34 ATP
or
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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
Glycolysis
Can produce ATP with or without oxygen, in aerobic
or anaerobic conditions
Couples with fermentation to produce ATP
f
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Types of Fermentation
Fermentation consists of
Glycolysis plus reactions that regenerate NAD+, which
can be reused by glyocolysis
In alcohol fermentation
Pyruvate is converted to ethanol in two steps, one ofwhich 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 Acetaldehyde2 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
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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
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 present
Fermentation
O2 present
Cellular respiration
Ethanol
or
lactate
Acetyl CoA
MITOCHONDRION
Citric
acid
cycle
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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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Glycolysis and the citric acid cycle connect to many
other metabolic pathways
The Versatility of Catabolism
Catabolic pathways
Funnel electrons from many kinds of organicmolecules into cellular respiration
The catabolism of various molecules from food
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Amino
acids
Sugars Glycerol Fatty
acids
Glycolysis
Glucose
Glyceraldehyde-3- P
Pyruvate
Acetyl CoA
NH3
Citric
acid
cycle
Oxidative
phosphorylation
FatsProteins Carbohydrates
Bi th i (A b li P th )
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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
The control of cellular respiration
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The control of cellular respiration
Glucose
Glycolysis
Fructose-6-phosphate
Phosphofructokinase
Fructose-1,6-bisphosphateInhibits Inhibits
Pyruvate
ATPAcetyl CoA
Citric
acid
cycle
Citrate
Stimulates
AMP
+