Download Cellular Respiration: Harvesting Chemical Energy

Cellular Respiration:
Harvesting Chemical Energy
Chapter 9
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
3. Explain the role of the electron transport chain
in cellular respiration
2. Name and describe the three stages of
cellular respiration; for each, state the region
of the eukaryotic cell where it occurs and the
products that result
1. Explain how redox reactions are involved in
energy exchanges
You should be able to:
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
6. Distinguish between obligate and facultative
anaerobes
5. Distinguish and explain fermentation and
anaerobic respiration
4. Explain where and how the respiratory
electron transport chain creates a proton
gradient
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
• Some animals, such as the giant panda, obtain
energy by eating plants, and some animals
feed on other organisms that eat plants
• Living cells require energy from outside
sources
Overview: Life Is Work
Fig. 9-1
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
• Cells use chemical energy stored in organic
molecules to regenerate ATP, which powers
work
• Photosynthesis generates O2 and organic
molecules, which are used in cellular
respiration
• Energy flows into an ecosystem as sunlight
and leaves as heat
Fig. 9-2
ATP
Cellular respiration
in mitochondria
Photosynthesis
in chloroplasts
Organic
+O
molecules 2
Heat
energy
ATP powers most cellular work
CO2 + H2O
ECOSYSTEM
Light
energy
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• Several processes are central to cellular
respiration and related pathways
Concept 9.1: Catabolic pathways yield energy by
oxidizing organic fuels
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
• Anaerobic respiration is similar to aerobic
respiration but consumes compounds other
than O2
• Aerobic respiration consumes organic
molecules and O2 and yields ATP
• Fermentation is a partial degradation of
sugars that occurs without O2
• The breakdown of organic molecules is
exergonic
Catabolic Pathways and Production of ATP
6 CO2 + 6 H2O + Energy
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C6H12O6 + 6 O2
(ATP + heat)
• Although carbohydrates, fats, and proteins are
all consumed as fuel, it is helpful to trace
cellular respiration with the sugar glucose:
• Cellular respiration includes both aerobic and
anaerobic respiration but is often used to refer
to aerobic respiration
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• This released energy is ultimately used to
synthesize ATP
• The transfer of electrons during chemical
reactions releases energy stored in organic
molecules
Redox Reactions: Oxidation and Reduction
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
• In reduction, a substance gains electrons, or is
reduced (the amount of positive charge is
reduced)
• In oxidation, a substance loses electrons, or is
oxidized
• Chemical reactions that transfer electrons
between reactants are called oxidation-reduction
reactions, or redox reactions
The Principle of Redox
Fig. 9-UN1
becomes reduced
(gains electron)
becomes oxidized
(loses electron)
Fig. 9-UN2
becomes reduced
becomes oxidized
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• An example is the reaction between methane
and O2
• Some redox reactions do not transfer electrons
but change the electron sharing in covalent
bonds
• The electron receptor is called the oxidizing
agent
• The electron donor is called the reducing
agent
Oxygen
(oxidizing
agent)
becomes oxidized
Reactants
Methane
(reducing
agent)
Fig. 9-3
Carbon dioxide
becomes reduced
Products
Water
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• During cellular respiration, the fuel (such as
glucose) is oxidized, and O2 is reduced:
Oxidation of Organic Fuel Molecules During
Cellular Respiration
Fig. 9-UN3
becomes reduced
becomes oxidized
Fig. 9-UN4
Dehydrogenase
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• Each NADH (the reduced form of NAD+)
represents stored energy that is tapped to
synthesize ATP
• As an electron acceptor, NAD+ functions as an
oxidizing agent during cellular respiration
• Electrons from organic compounds are usually
first transferred to NAD+, a coenzyme
• In cellular respiration, glucose and other
organic molecules are broken down in a series
of steps
Stepwise Energy Harvest via NAD+ and the Electron
Transport Chain
Fig. 9-4
NAD+
2[H]
Nicotinamide
(oxidized form)
+
Oxidation of NADH
Reduction of NAD+
Dehydrogenase
2 e– + 2 H+
2 e– + H+
Nicotinamide
(reduced form)
NADH
+ H+
H+
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• The energy yielded is used to regenerate ATP
• O2 pulls electrons down the chain in an energyyielding tumble
• Unlike an uncontrolled reaction, the electron
transport chain passes electrons in a series of
steps instead of one explosive reaction
• NADH passes the electrons to the electron
transport chain
Fig. 9-5
(a) Uncontrolled reaction
Explosive
release of
heat and light
energy
H2 + 1/2 O2
(b) Cellular respiration
2H
(from food via NADH)
Controlled
release of
+
–
2H + 2e
energy for
synthesis of
ATP
1/ O
2 2
1/ O
2 2
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– Oxidative phosphorylation (accounts for
most of the ATP synthesis)
– The citric acid cycle (completes the
breakdown of glucose)
– Glycolysis (breaks down glucose into two
molecules of pyruvate)
• Cellular respiration has three stages:
The Stages of Cellular Respiration: A Preview
ATP
Substrate-level
phosphorylation
Cytosol
Pyruvate
Glycolysis
Electrons
carried
via NADH
Glucose
Fig. 9-6-1
ATP
Substrate-level
phosphorylation
Substrate-level
phosphorylation
Mitochondrion
Citric
acid
cycle
Electrons carried
via NADH and
FADH2
ATP
Cytosol
Pyruvate
Glycolysis
Electrons
carried
via NADH
Glucose
Fig. 9-6-2
ATP
Oxidative
phosphorylation
Substrate-level
phosphorylation
Substrate-level
phosphorylation
Oxidative
phosphorylation:
electron transport
and
chemiosmosis
ATP
Mitochondrion
Citric
acid
cycle
Electrons carried
via NADH and
FADH2
ATP
Cytosol
Pyruvate
Glycolysis
Electrons
carried
via NADH
Glucose
Fig. 9-6-3
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• The process that generates most of the ATP is
called oxidative phosphorylation because it is
powered by redox reactions
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• A smaller amount of ATP is formed in
glycolysis and the citric acid cycle by
substrate-level phosphorylation
• Oxidative phosphorylation accounts for almost
90% of the ATP generated by cellular
respiration
ADP
Enzyme
P
Substrate
Fig. 9-7
Product
+
ATP
Enzyme
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– Energy payoff phase
– Energy investment phase
• Glycolysis occurs in the cytoplasm and has two
major phases:
• Glycolysis (“splitting of sugar”) breaks down
glucose into two molecules of pyruvate
Concept 9.2: Glycolysis harvests chemical energy
by oxidizing glucose to pyruvate
Fig. 9-8
Glucose
2 NAD+ + 4 e– + 4 H+
4 ATP formed – 2 ATP used
Net
2 NAD+ + 4 e– + 4 H+
4 ADP + 4 P
Energy payoff phase
2 ADP + 2 P
Glucose
Energy investment phase
formed
used
2 NADH + 2 H+
2 ATP
2 Pyruvate + 2 H2O
2 Pyruvate + 2 H2O
2 NADH + 2 H+
4 ATP
2 ATP
Fig. 9-9-1
1
Hexokinase
Glucose-6-phosphate
ADP
ATP
Glucose
1
Hexokinase
Glucose-6-phosphate
ADP
ATP
Glucose
Fig. 9-9-2
1
Hexokinase
Fructose-6-phosphate
2
Phosphoglucoisomerase
Glucose-6-phosphate
ADP
ATP
Glucose
Fructose-6-phosphate
2
Phosphoglucoisomerase
Glucose-6-phosphate
Fig. 9-9-3
1
Hexokinase
3
Phosphofructokinase
Fructose1, 6-bisphosphate
ADP
ATP
Fructose-6-phosphate
2
Phosphoglucoisomerase
Glucose-6-phosphate
ADP
ATP
Glucose
Phosphofructokinase
3
Fructose1, 6-bisphosphate
ADP
ATP
Fructose-6-phosphate
1
Hexokinase
Glucose
3
Phosphofructokinase
5
Isomerase
Glyceraldehyde3-phosphate
4
Aldolase
Fructose1, 6-bisphosphate
ADP
ATP
Fructose-6-phosphate
2
Phosphoglucoisomerase
Glucose-6-phosphate
ADP
ATP
Dihydroxyacetone
phosphate
Fig. 9-9-4
Dihydroxyacetone
phosphate
Isomerase
5
Glyceraldehyde3-phosphate
Aldolase
4
Fructose1, 6-bisphosphate
Fig. 9-9-5
6
Triose phosphate
dehydrogenase
2 Pi
2 1, 3-Bisphosphoglycerate
2 NADH
+ 2 H+
2 NAD+
6
Triose phosphate
dehydrogenase
2 Pi
2 1, 3-Bisphosphoglycerate
+ 2 H+
2 NADH
2 NAD+
Glyceraldehyde3-phosphate
Fig. 9-9-6
6
Triose phosphate
dehydrogenase
2 Pi
2
7
Phosphoglycerokinase
3-Phosphoglycerate
2 ATP
2 ADP
2 1, 3-Bisphosphoglycerate
2 NADH
+ 2 H+
2 NAD+
2
7
Phosphoglycerokinase
3-Phosphoglycerate
2 ATP
2 ADP
2 1, 3-Bisphosphoglycerate
Fig. 9-9-7
6
Triose phosphate
dehydrogenase
2 Pi
2
2
7 Phosphoglycerokinase
2-Phosphoglycerate
Phosphoglyceromutase
8
3-Phosphoglycerate
2 ATP
2 ADP
2 1, 3-Bisphosphoglycerate
2 NADH
+ 2 H+
2 NAD+
2
2
2-Phosphoglycerate
8
Phosphoglyceromutase
3-Phosphoglycerate
Fig. 9-9-8
6
Triose phosphate
dehydrogenase
2 Pi
2
2
2
7 Phosphoglycerokinase
9
Enolase
Phosphoenolpyruvate
2 H2O
2-Phosphoglycerate
Phosphoglyceromutase
8
3-Phosphoglycerate
2 ATP
2 ADP
2 1, 3-Bisphosphoglycerate
2 NADH
+ 2 H+
2 NAD+
2
2
Phosphoenolpyruvate
2 H2O
Enolase
9
2-Phosphoglycerate
Fig. 9-9-9
6
2 Pi
Triose phosphate
dehydrogenase
2 H2O
Enolase
9
2-Phosphoglycerate
Phosphoglyceromutase
8
3-Phosphoglycerate
7 Phosphoglycerokinase
2
2 ATP
Pyruvate
10
Pyruvate kinase
2 Phosphoenolpyruvate
2 ADP
2
2
2 ATP
2 ADP
2 1, 3-Bisphosphoglycerate
2 NADH
+ 2 H+
2 NAD+
2 ADP
2
Pyruvate
10
Pyruvate
kinase
Phosphoenolpyruvate
2 ATP
2
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• Before the citric acid cycle can begin, pyruvate
must be converted to acetyl CoA, which links
the cycle to glycolysis
• In the presence of O2, pyruvate enters the
mitochondrion
Concept 9.3: The citric acid cycle completes the
energy-yielding oxidation of organic molecules
CYTOSOL
Transport protein
Pyruvate
Fig. 9-10
1
CO2
NAD+
3
Coenzyme A
2
NADH + H+
Acetyl CoA
MITOCHONDRION
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• The cycle oxidizes organic fuel derived from
pyruvate, generating 1 ATP, 3 NADH, and 1
FADH2 per turn
• The citric acid cycle, also called the Krebs
cycle, takes place within the mitochondrial
matrix
Fig. 9-11
NAD+
FAD
+ H+
NADH
FADH2
Pyruvate
ATP
CoA
ADP + P i
Citric
acid
cycle
Acetyl CoA
CoA
CoA
CO2
+ 3 H+
3 NADH
3 NAD+
2 CO2
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• The NADH and FADH2 produced by the cycle
relay electrons extracted from food to the
electron transport chain
• The next seven steps decompose the citrate
back to oxaloacetate, making the process a
cycle
• The acetyl group of acetyl CoA joins the cycle
by combining with oxaloacetate, forming citrate
• The citric acid cycle has eight steps, each
catalyzed by a specific enzyme
Fig. 9-12-1
Citric
acid
cycle
Oxaloacetate
1
Citrate
CoA—SH
Acetyl CoA
Fig. 9-12-2
Citric
acid
cycle
Oxaloacetate
1
Citrate
CoA—SH
Acetyl CoA
2
Isocitrate
H2O
Fig. 9-12-3
Citric
acid
cycle
Oxaloacetate
1
Citrate
CoA—SH
Acetyl CoA
2
NAD+
3
-Ketoglutarate
CO2
NADH
+ H+
Isocitrate
H2O
Fig. 9-12-4
Citric
acid
cycle
Oxaloacetate
1
2
Succinyl
CoA
NAD+
NADH
+ H+
3
CO2
-Ketoglutarate
CO2
NADH
+ H+
Isocitrate
H2O
NAD+
4
CoA—SH
Citrate
CoA—SH
Acetyl CoA
Fig. 9-12-5
ATP
ADP
GTP GDP
Succinate
5
Pi
CoA—SH
Citric
acid
cycle
Oxaloacetate
1
2
Succinyl
CoA
NAD+
NADH
+ H+
3
CO2
-Ketoglutarate
CO2
NADH
+ H+
Isocitrate
H2O
NAD+
4
CoA—SH
Citrate
CoA—SH
Acetyl CoA
Fig. 9-12-6
FADH2
FAD
5
ATP
ADP
Pi
CoA—SH
GTP GDP
Succinate
6
Fumarate
Citric
acid
cycle
Oxaloacetate
1
2
Succinyl
CoA
NAD+
NADH
+ H+
3
CO2
-Ketoglutarate
CO2
NADH
+ H+
Isocitrate
H2O
NAD+
4
CoA—SH
Citrate
CoA—SH
Acetyl CoA
Fig. 9-12-7
H2O
FADH2
7
FAD
5
ATP
ADP
Pi
CoA—SH
Citric
acid
cycle
GTP GDP
Succinate
6
Fumarate
Malate
Oxaloacetate
1
2
Succinyl
CoA
4
NAD+
NADH
+ H+
3
CO2
-Ketoglutarate
CO2
NADH
+ H+
Isocitrate
H2O
NAD+
CoA—SH
Citrate
CoA—SH
Acetyl CoA
Fig. 9-12-8
H2O
FADH2
7
NAD+
8
FAD
Fumarate
5
ATP
ADP
Pi
CoA—SH
Citric
acid
cycle
GTP GDP
Succinate
6
1
Oxaloacetate
Malate
NADH
+H+
2
Succinyl
CoA
4
NAD+
NADH
+ H+
3
CO2
-Ketoglutarate
CO2
NADH
+ H+
Isocitrate
H2O
NAD+
CoA—SH
Citrate
CoA—SH
Acetyl CoA
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• These two electron carriers donate electrons to
the electron transport chain, which powers ATP
synthesis via oxidative phosphorylation
• Following glycolysis and the citric acid cycle,
NADH and FADH2 account for most of the
energy extracted from food
Concept 9.4: During oxidative phosphorylation,
chemiosmosis couples electron transport to ATP
synthesis
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• Electrons drop in free energy as they go down
the chain and are finally passed to O2, forming
H2O
• The carriers alternate reduced and oxidized
states as they accept and donate electrons
• Most of the chain’s components are proteins,
which exist in multiprotein complexes
• The electron transport chain is in the cristae of
the mitochondrion
The Pathway of Electron Transport
Fig. 9-13
0
10
20
30
40
50
FMN
2 e–
Fe•S
NAD+
NADH
Q
Cyt b
Fe•S
FAD
2 e–
Fe•S
FAD
FADH2
Cyt c1
Cyt a
Cyt a3
I
V
H 2O
2 H+ + 1/2 O2
2 e–
(from NADH
or FADH2)
Cyt c
Multiprotein
complexes
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• The chain’s function is to break the large freeenergy drop from food to O2 into smaller steps
that release energy in manageable amounts
• The electron transport chain generates no ATP
• Electrons are passed through a number of
proteins including cytochromes (each with an
iron atom) to O2
• Electrons are transferred from NADH or FADH2
to the electron transport chain
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• This is an example of chemiosmosis, the use of
energy in a H+ gradient to drive cellular work
• ATP synthase uses the exergonic flow of H+ to
drive phosphorylation of ATP
• H+ then moves back across the membrane,
passing through channels in ATP synthase
• Electron transfer in the electron transport chain
causes proteins to pump H+ from the
mitochondrial matrix to the intermembrane space
Chemiosmosis: The Energy-Coupling Mechanism
Fig. 9-14
ATP
Stator
MITOCHONDRIAL MATRIX
i
ADP
+
P
Catalytic
knob
Internal
rod
Rotor
H+
INTERMEMBRANE SPACE
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• The H+ gradient is referred to as a protonmotive force, emphasizing its capacity to do
work
• The energy stored in a H+ gradient across a
membrane couples the redox reactions of the
electron transport chain to ATP synthesis
H+
(carrying electrons
from food)
NADH
Protein complex
of electron
carriers
Fig. 9-16
NAD+
FAD
V
2 H+ + 1/2O2
Cyt c
H+
Oxidative phosphorylation
1 Electron transport chain
FADH2
Q
H+
H2O
H+
ATP
synthase
ATP
2 Chemiosmosis
ADP + P i
H+
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• About 40% of the energy in a glucose molecule
is transferred to ATP during cellular respiration,
making about 38 ATP
glucose
NADH
electron transport chain
proton-motive force
ATP
• During cellular respiration, most energy flows in
this sequence:
An Accounting of ATP Production by Cellular
Respiration
2 NADH
Electron shuttles
span membrane
+ 2 ATP
Glucose
2
Pyruvate
Glycolysis
CYTOSOL
Fig. 9-17
Maximum per glucose:
2
Acetyl
CoA
2 NADH
or
2 FADH2
2 NADH
About
36 or 38 ATP
+ 2 ATP
Citric
acid
cycle
6 NADH
+ about 32 or 34 ATP
Oxidative
phosphorylation:
electron transport
and
chemiosmosis
2 FADH2
MITOCHONDRION
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• In the absence of O2, glycolysis couples with
fermentation or anaerobic respiration to
produce ATP
• Glycolysis can produce ATP with or without O2
(in aerobic or anaerobic conditions)
• Most cellular respiration requires O2 to produce
ATP
Concept 9.5: Fermentation and anaerobic
respiration enable cells to produce ATP without
the use of oxygen
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• Fermentation uses phosphorylation instead of
an electron transport chain to generate ATP
• Anaerobic respiration uses an electron
transport chain with an electron acceptor other
than O2, for example sulfate
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• Two common types are alcohol fermentation
and lactic acid fermentation
• Fermentation consists of glycolysis plus
reactions that regenerate NAD+, which can be
reused by glycolysis
Types of Fermentation
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• Alcohol fermentation by yeast is used in
brewing, winemaking, and baking
• In alcohol fermentation, pyruvate is
converted to ethanol in two steps, with the first
releasing CO2
Fig. 9-18
2 NAD+
2 NAD+
(b) Lactic acid fermentation
2 Lactate
2 ATP
2 CO2
2 Pyruvate
2 NADH
+ 2 H+
2 Pyruvate
2 Acetaldehyde
2 NADH
+ 2 H+
Glycolysis
2 ADP + 2 Pi
Glucose
2 ATP
Glycolysis
(a) Alcohol fermentation
2 Ethanol
Glucose
2 ADP + 2 Pi
2 Ethanol
Glucose
2 NAD+
2 ATP
2 CO2
2 Pyruvate
2 Acetaldehyde
2 NADH
+ 2 H+
Glycolysis
2 ADP + 2 P i
(a) Alcohol fermentation
Fig. 9-18a
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
• Human muscle cells use lactic acid
fermentation to generate ATP when O2 is
scarce
• Lactic acid fermentation by some fungi and
bacteria is used to make cheese and yogurt
• In lactic acid fermentation, pyruvate is
reduced to NADH, forming lactate as an end
product, with no release of CO2
Fig. 9-18b
2 NAD+
2 ATP
2 NADH
+ 2 H+
Glycolysis
(b) Lactic acid fermentation
2 Lactate
Glucose
2 ADP + 2 P i
2 Pyruvate
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
• Cellular respiration produces 38 ATP per
glucose molecule; fermentation produces 2
ATP per glucose molecule
• The processes have different final electron
acceptors: an organic molecule (such as
pyruvate or acetaldehyde) in fermentation and
O2 in cellular respiration
• Both processes use glycolysis to oxidize
glucose and other organic fuels to pyruvate
Fermentation and Aerobic Respiration Compared
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• In a facultative anaerobe, pyruvate is a fork in
the metabolic road that leads to two alternative
catabolic routes
• Yeast and many bacteria are facultative
anaerobes, meaning that they can survive
using either fermentation or cellular respiration
• Obligate anaerobes carry out fermentation or
anaerobic respiration and cannot survive in the
presence of O2
Fig. 9-19
Ethanol
or
lactate
No O2 present:
Fermentation
CYTOSOL
Acetyl CoA
Citric
acid
cycle
MITOCHONDRION
O2 present:
Aerobic cellular
respiration
Pyruvate
Glycolysis
Glucose
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• Glycolysis probably evolved in ancient
prokaryotes before there was oxygen in the
atmosphere
• Glycolysis occurs in nearly all organisms
The Evolutionary Significance of Glycolysis
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• Gycolysis and the citric acid cycle are major
intersections to various catabolic and anabolic
pathways
Concept 9.6: Glycolysis and the citric acid cycle
connect to many other metabolic pathways
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
• Proteins must be digested to amino acids;
amino groups can feed glycolysis or the citric
acid cycle
• Glycolysis accepts a wide range of
carbohydrates
• Catabolic pathways funnel electrons from many
kinds of organic molecules into cellular
respiration
The Versatility of Catabolism
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• An oxidized gram of fat produces more than
twice as much ATP as an oxidized gram of
carbohydrate
• Fatty acids are broken down by beta oxidation
and yield acetyl CoA
• Fats are digested to glycerol (used in
glycolysis) and fatty acids (used in generating
acetyl CoA)
Fig. 9-20
NH3
Sugars
Amino
acids
Oxidative
phosphorylation
Citric
acid
cycle
Acetyl CoA
Pyruvate
Glyceraldehyde-3- P
Glucose
Glycolysis
Carbohydrates
Proteins
Glycerol
Fatty
acids
Fats
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• These small molecules may come directly from
food, from glycolysis, or from the citric acid
cycle
• The body uses small molecules to build other
substances
Biosynthesis (Anabolic Pathways)
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• Control of catabolism is based mainly on
regulating the activity of enzymes at strategic
points in the catabolic pathway
• If ATP concentration begins to drop, respiration
speeds up; when there is plenty of ATP,
respiration slows down
• Feedback inhibition is the most common
mechanism for control
Regulation of Cellular Respiration via Feedback
Mechanisms
Fig. 9-21
ATP
Inhibits
–
Oxidative
phosphorylation
cycle
Citric
acid
Acetyl CoA
Pyruvate
Fructose-1,6-bisphosphate
Phosphofructokinase
Glycolysis
Fructose-6-phosphate
Glucose
Citrate
Inhibits
–
Stimulates
+
AMP