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Chapter 9
Cellular Respiration:
Harvesting Chemical Energy
PowerPoint Lectures for
Biology, Seventh Edition
Neil Campbell and Jane Reece
Lectures by Chris Romero
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 many tasks
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• The giant panda
– Obtains energy for its cells by eating plants
Figure 9.1
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• Energy
– Flows into an ecosystem as sunlight and
leaves as heat
Light energy
ECOSYSTEM
Photosynthesis
in chloroplasts
Organic
CO2 + H2O
+ O2
Cellular
molecules
respiration
in mitochondria
ATP
powers most cellular work
Figure 9.2
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Heat
energy
Catabolic Pathways and Production of ATP
• The breakdown of organic molecules is
exergonic (release energy)
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• One catabolic process, fermentation
– Is a partial degradation of sugars that occurs
without 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
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• To keep working
– Cells must regenerate ATP
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Redox Reactions: Oxidation and Reduction
• Catabolic pathways yield energy
– Due to the transfer of electrons
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The Principle of Redox
• Redox reactions
– Transfer electrons from one reactant to
another by oxidation and reduction
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• In oxidation
– A substance loses electrons, or is oxidized
• In reduction
– A substance gains electrons, or is reduced
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• Examples of redox reactions
becomes oxidized
(loses electron)
Na
+
Cl
Na+
+
becomes reduced
(gains electron)
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Cl–
• Some redox reactions
– Do not completely exchange electrons
– Change the degree of electron sharing in
covalent bonds
Products
Reactants
becomes oxidized
+
CH4
CO
2O2
+
Energy
2 H2O
becomes reduced
O
O
C
O
H
O
O
H
H
H
C
+
2
H
H
Methane
(reducing
agent)
Oxygen
(oxidizing
agent)
Figure 9.3
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Carbon dioxide
Water
Oxidation of Organic Fuel Molecules During
Cellular Respiration
• During cellular respiration
– Glucose is oxidized and oxygen is reduced
becomes oxidized
C6H12O6 + 6O2
6CO2 + 6H2O + Energy
becomes reduced
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Stepwise Energy Harvest via NAD+ and the Electron
Transport Chain
• Cellular respiration
– Oxidizes glucose in a series of steps
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• Electrons from organic compounds
– Are usually first transferred to NAD+, a
coenzyme (helper of catalytic activity)
2 e– + 2 H+
NAD+
Dehydrogenase
O
NH2
H
C
CH2
O
O–
O
O P
O
H
–
O P O HO
O
N+ Nicotinamide
(oxidized form)
H
OH
HO
CH2
N
H
O
H
HO
N
H
OH
Reduction of NAD+
+ 2[H]
(from food) Oxidation of NADH
NH2
N
N
2 e– + H+
H
Figure 9.4
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H O
C
H
N
H+
NADH
NH2
Nicotinamide
(reduced form)
+
H+
• 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
Free energy, G
H2 + 1/2 O2
Figure 9.5 A
Explosive
release of
heat and light
energy
H2O
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(a) Uncontrolled reaction
• The electron transport chain
– Passes electrons in a series of steps instead of
in one explosive reaction
– Uses the energy from the electron transfer to
form ATP
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2H
1/
+
2
O2
1/
O2
(from food via NADH)
Free energy, G
2 H+ + 2 e–
Controlled
release of
energy for
synthesis of
ATP
ATP
ATP
ATP
2 e–
2
H+
H2O
Figure 9.5 B
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(b) Cellular respiration
2
The Stages of Cellular Respiration: A Preview
• Respiration is a cumulative function of three
metabolic stages
– Glycolysis
– The citric acid cycle (Kreb Cycle)
– Oxidative phosphorylation
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• Glycolysis
– Breaks down glucose into two molecules of
pyruvate
• The citric acid cycle
– Completes the breakdown of glucose
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• Oxidative phosphorylation
– Is driven by the electron transport chain
– Generates ATP
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• 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
• 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
• Concept 9.2: Glycolysis harvests energy by
oxidizing glucose to pyruvate
• Glycolysis
– Means “splitting of sugar”
– Breaks down glucose into pyruvate
– Occurs in the cytoplasm of the cell
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• Glycolysis consists of two major phases
– Energy investment phase
– Energy payoff phase
Citric
acid
cycle
Glycolysis
Oxidative
phosphorylation
ATP
ATP
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
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CH2OH
HH
H
HO H
HO
OH
H OH
Glycolysis
Glucose
ATP
1
Hexokinase
ADP
CH2OH P
HH OH
OH H
HO
H OH
Glucose-6-phosphate
2
Phosphoglucoisomerase
CH2O P
O CH2OH
H HO
HO
H
HO H
Fructose-6-phosphate
ATP
3
Phosphofructokinase
ADP
P O CH2 O CH2 O P
HO
H
OH
HO H
Fructose1, 6-bisphosphate
4
Aldolase
5
H
P O CH2 Isomerase
C O
C O
CHOH
CH2OH
CH2 O P
Figure 9.9 A
Dihydroxyacetone
phosphate
Glyceraldehyde3-phosphate
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Citric
Oxidative
acid
cycle phosphorylation
• A closer look at the energy payoff phase
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6
Triose phosphate
dehydrogenase
2 NAD+
2 Pi
2 NADH
+ 2 H+
2
P
O C O
CHOH
CH2 O P
1, 3-Bisphosphoglycerate
2 ADP
7
Phosphoglycerokinase
2 ATP
O–
2
C
CHOH
CH2 O P
3-Phosphoglycerate
8
Phosphoglyceromutase
2
O–
C
O
H C O
P
CH2OH
2-Phosphoglycerate
9
Enolase
2H O
2
2
O–
C O
C O
P
CH2
Phosphoenolpyruvate
2 ADP
10
Pyruvate kinase
2 ATP
2
O–
C O
C O
Figure 9.8 B
CH3
Pyruvate
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• 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 the cycle to glycolysis
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
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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
• Concept 9.4: 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
NADH
50
Free energy (G) relative to O2 (kcl/mol)
FADH2
40
FMN
I
Fe•S
Fe•S II
O
30
Multiprotein
complexes
FAD
III
Cyt b
Fe•S
20
Cyt c1
IV
Cyt c
Cyt a
Cyt a3
10
0
Figure 9.13
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2 H + + 12 O2
H2 O
Chemiosmosis: The Energy-Coupling Mechanism
• ATP synthase
– Is the enzyme that actually makes ATP
INTERMEMBRANE SPACE
H+
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
MITOCHONDRIAL MATRIX
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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.
• At certain steps along the electron transport
chain
– Electron transfer causes protein complexes to
pump H+ from the mitochondrial matrix to the
intermembrane space
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• The resulting H+ gradient
– Stores energy
– Drives chemiosmosis in ATP synthase
– Is referred to as a proton-motive force
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• 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
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
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 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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• About 40% of the energy in a glucose molecule
– Is transferred to ATP during cellular respiration,
making approximately 38 ATP
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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
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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 glyocolysis
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• In alcohol fermentation
– Pyruvate is converted to ethanol in two steps,
one of which releases CO2
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• 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
O–
C O
Glucose
Glycolysis
C O
CH3
2 Pyruvate
2 NADH
2 NAD+
H
2 CO2
H
H C OH
C O
CH3
CH3
2 Ethanol
2 Acetaldehyde
(a) Alcohol fermentation
2 ADP + 2
Glucose
P1
2 ATP
Glycolysis
O–
C O
C O
O
2 NAD+
2 NADH
C O
H
C OH
CH3
2 Lactate
Figure 9.17
(b) Lactic acid fermentation
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CH3
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
– 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
MITOCHONDRION
Ethanol
or
lactate
Figure 9.18
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Acetyl CoA
Citric
acid
cycle
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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• 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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• The control of cellular respiration
Glucose
Glycolysis
Fructose-6-phosphate
–
Inhibits
AMP
Stimulates
+
Phosphofructokinase
–
Fructose-1,6-bisphosphate
Inhibits
Pyruvate
Citrate
ATP
Acetyl CoA
Citric
acid
cycle
Figure 9.20
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Oxidative
phosphorylation