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Chapter 22 Metabolic Pathways
for Carbohydrates
22.1 Metabolism and Cell Structure
22.2 ATP and Energy
Copyright © 2007 by Pearson Education, Inc.
Publishing as Benjamin Cummings
1
Metabolism
Metabolism involves
 Catabolic
reactions that
break down large,
complex
molecules to
provide energy
and smaller
molecules.
 Anabolic reactions
that use ATP
energy to build
larger molecules.
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2
Stages of Metabolism
Catabolic reactions are organized as
Stage 1:
Digestion and hydrolysis break down
large molecules to smaller ones that
enter the bloodstream.
Stage 2:
Degradation breaks down molecules to
two- and three-carbon compounds.
Stage 3:
Oxidation of small molecules in the citric
acid cycle and electron transport
provide ATP energy.
3
Stages of Metabolism
4
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Cell Structure
Metabolic reactions occur in specific sites within cells.
5
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Cell Components and Function
TABLE 22.1
6
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ATP and Energy
Adenosine triphosphate (ATP)
 Is the energy form stored in cells.
 Is obtained from the oxidation of food.
 Consists of adenine (nitrogen base), a ribose sugar,
and three phosphate groups.
 Requires 7.3 (31 kJ) per mole to convert ADP + Pi
to ATP.
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ATP and Energy
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Hydrolysis of ATP
 The hydrolysis of ATP to ADP releases 7.3 kcal (31
kJ/mole).
ATP
ADP + Pi + 7.3 kcal (31 kJ/mole)
 The hydrolysis of ADP to AMP releases 7.3 kcal (31
kJ/mole).
ADP
AMP + Pi + 7.3 kcal (31 kJ/mole)
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Hydrolysis of ATP to ADP and
ADP to AMP
10
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ATP and Muscle Contraction
Muscle fibers
 Contain the protein fibers actin and myosin.
 Contract (slide closer together) when a nerve
impulse increases Ca2+.
 Obtain the energy for contraction from the
hydrolysis of ATP.
 Return to the relaxed position as Ca2+ and ATP
decrease.
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ATP and Muscle Contraction
12
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Learning Check
Match the following:
1) ATP
2) ADP + Pi
A.
B.
C.
D.
Used in anabolic reactions.
The energy-storage molecule.
Coupled with energy-requiring reactions.
Hydrolysis products.
13
Solution
Match the following:
1) ATP
2) ADP + Pi
A.
B.
C.
D.
1
1
1
2
Used in anabolic reactions
The energy-storage molecule.
Coupled with energy-requiring reactions.
Hydrolysis products.
14
Chapter 22 Metabolic Pathways
for Carbohydrates
22.3 Important Coenzymes in
Metabolic Pathways
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15
Coenzyme NAD+
NAD+ (nicotinamide adenine dinucleotide)
 Participates in reactions that produce a carbonoxygen double bond (C=O).
 Is reduced when an oxidation provides 2H+ and 2e-.
Oxidation
CH3—CH2—OH
O
||
CH3—C—H + 2H+ + 2e-
Reduction
NAD+ + 2H+ + 2e-
NADH + H+
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Structure of Coenzyme NAD+
NAD+
 Is nicotinamide
adenine
dinucleotide.
 Contains ADP,
ribose, and
nicotinamide.
 Reduces to NADH
when the
nicotinamide group
accepts H+ and 2e-.
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17
Coenzyme FAD
FAD (flavin adenine dinucleotide)
 Participates in reactions that produce a carbon-carbon
double bond (C=C).
 Is reduced to FADH2.
Oxidation
—CH2—CH2—
Reduction
FAD + 2H+ + 2e-
—CH=CH— + 2H+ + 2e-
FADH2
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Structure of Coenzyme FAD
FAD
 Is flavin adenine
dinucleotide.
 Contains ADP
and riboflavin
(vitamin B2).
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Coenzyme A
Coenzyme A.
 Consists of vitamins B3, pantothenic acid, and ADP.
 Activates acyl groups such as the two-carbon acetyl
group for transfer.
O
||
CH3—C— + HS—CoA
acetyl group
O
||
CH3—C—S—CoA
acetyl CoA
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Structure of Coenzyme A
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21
Learning Check
Match the following:
1) NAD+
2) FAD
4) FADH2
5) Coenzyme A
3) NADH + H+
A. Coenzyme used in oxidation of carbon-oxygen
bonds.
B. Reduced form of flavin adenine dinucleotide.
C. Used to transfer acetyl groups.
D. Contains riboflavin.
E. The coenzyme after C=O bond formation.
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Solution
Match the following:
1) NAD+
2) FAD
4) FADH2
5) Coenzyme A
3) NADH + H+
A. 1 Coenzyme used in oxidation of carbon-oxygen
bonds.
B. 4 Reduced form of flavin adenine dinucleotide.
C. 5 Used to transfer acetyl groups.
D. 2 Contains riboflavin.
E. 3 The coenzyme after C=O bond formation.
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Chapter 22 Metabolic Pathways for
Carbohydrates
22.4
Digestion of Carbohydrates
22.5
Glycolysis: Oxidation of Glucose
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Stage 1: Digestion of Carbohydrates
In Stage 1, the digestion of carbohydrates
 Begins in the mouth where salivary amylase breaks
down polysaccharides to smaller polysaccharides
(dextrins), maltose, and some glucose.
 Continues in the small intestine where pancreatic
amylase hydrolyzes dextrins to maltose and glucose.
 Hydrolyzes maltose, lactose, and sucrose to
monosaccharides, mostly glucose, which enter the
bloodstream for transport to the cells.
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Digestion of Carbohydrates
26
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Stage 2: Glycolysis
Stage 2: Glycolysis
 Is a metabolic
pathway that uses
glucose, a digestion
product.
 Degrades six-carbon
glucose molecules to
three-carbon.
pyruvate molecules.
 Is an anaerobic (no
oxygen) process.
27
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Glycolysis: Energy-Investment
In reactions 1-5 of glycolysis,
 Energy is required to add phosphate groups to
glucose.
 Glucose is converted to two three-carbon molecules.
28
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Glycolysis: Energy Investment
4
1
3
5
5
2
29
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Glycolysis: Energy-Production
In reactions 6-10 of glycolysis, energy is generated as
 Sugar phosphates are cleaved to triose phosphates.
 Four ATP molecules are produced.
30
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Glycolysis: Reactions 6-10
9
6
8
10
7
31
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Glycolysis: Overall Reaction
In glycolysis,
 Two ATP add phosphate to glucose and fructose-6phosphate.
 Four ATP are formed in energy-generation by direct
transfers of phosphate groups to four ADP.
 There is a net gain of 2 ATP and 2 NADH.
C6H12O6 + 2ADP + 2Pi + 2NAD+
Glucose
2C3H3O3- + 2ATP + 2NADH + 4H+
Pyruvate
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Regulation of Glycolysis
Glycolysis is regulated by three enzymes,
 Reaction 1 Hexokinase is inhibited by high levels of
glucose-6-phosphate, which prevents the
phosphorylation of glucose.
 Reaction 3 Phosphofructokinase, an allosteric
enzyme, is inhibited by high levels of ATP and
activated by high levels of ADP and AMP.
 Reaction 10 Pyruvate kinase, another allosteric
enzyme is inhibited by high levels of ATP or acetyl
CoA.
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Learning Check
In glycolysis, what compounds provide phosphate
groups for the production of ATP?
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Solution
In glycolysis, what compounds provide phosphate
groups for the production of ATP?
In reaction 7, phosphate groups from two
1,3-bisphosphoglycerate molecules are transferred to
ADP to form two ATP.
In reaction 10, phosphate groups from two
phosphoenolpyruvate molecules are used to form two
more ATP.
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Chapter 22 Metabolic Pathways for
Carbohydrates
22.6
Pathways for Pyruvate
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Pyruvate: Aerobic Conditions
Under aerobic conditions (oxygen present),
 Three-carbon pyruvate is decarboxylated.
 Two-carbon acetyl CoA and CO2 are produced.
O O
pyruvate
|| ||
dehydrogenase
CH3—C—C—O- + HS—CoA + NAD+
pyruvate
O
||
CH3—C—S—CoA + CO2 + NADH
acetyl CoA
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Pyruvate: Anaerobic Conditions
Under anaerobic conditions (without oxygen),
 Pyruvate is reduced to lactate.
 NADH oxidizes to NAD+ allowing glycolysis to continue.
O O
|| ||
CH3—C—C—O- + NADH + H+
lactate
dehydrogenase
pyruvate
OH O
|
||
CH3—CH—C—O- + NAD+
lactate
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Lactate in Muscles
During strenuous exercise,
 Oxygen in the muscles is depleted.
 Anaerobic conditions are produced.
 Lactate accumulates.
OH
│
C6H12O6 + 2ADP + 2Pi
2CH3–CH–COO- + 2ATP
glucose
lactate
 Muscles tire and become painful.
After exercise, a person breathes heavily to repay the
oxygen debt and reform pyruvate in the liver.
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Fermentation
Fermentation
 Occurs in anaerobic microorganisms such as yeast.
 Decarboxylates pyruvate to acetaldehyde, which is
reduced to ethanol.
 Regenerates NAD+ to continue glycolysis.
O
OH
||
|
CH3—C—COO- + NADH + H+
CH3—CH2 + NAD+ + CO2
pyruvate
ethanol
40
Pathways for Pyruvate
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41
Learning Check
Match the following terms with the descriptions
1) Catabolic reactions
2) Coenzymes
3) Glycolysis
4) Lactate
A. Produced during anaerobic conditions.
B. Reaction series that converts glucose to pyruvate.
C. Metabolic reactions that break down large
molecules to smaller molecules + energy.
D. Substances that remove or add H atoms in
oxidation and reduction reactions.
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Solution
Match the following terms with the descriptions:
1) Catabolic reactions
2) Coenzymes
3) Glycolysis
4) Lactate
A. 4 Produced during anaerobic conditions.
B. 3 Reaction series that converts glucose to
pyruvate.
C. 1 Metabolic reactions that break down large
molecules to smaller molecules + energy.
D. 2 Substances that remove or add H atoms in
oxidation and reduction reactions.
43
Chapter 22 Metabolic Pathways for
Carbohydrates
22.7
Glycogen Metabolism
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Glycogenesis
Glycogenesis
 Stores glucose by converting glucose to glycogen.
 Operates when high levels of glucose-6-phosphate
are formed in the first reaction of glycolysis.
 Does not operate when energy stores (glycogen)
are full, which means that additional glucose is
converted to body fat.
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Diagram of Glycogenesis
46
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Formation of Glucose-6-Phosphate
In glycogenesis
 Glucose is initially converted to
glucose-6-phosphate using ATP.
O
-
O P O CH2
O-
O
OH
OH
OH
OH
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glucose-6-phosphate
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Formation of Glucose-1-Phosphate
Glucose-6-phosphate is converted
to glucose-1-phosphate.
O
H O CH2
-
O P O CH2
O-
O
O
O
OH
OH
OH
OH
OH
glucose-6-phosphate
O P O-
OH
OH
O-
glucose-1-phosphate
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UDP-Glucose
UTP activates glucose-1-phosphate
to form UDP-glucose and
pyrophosphate (PPi).
O
CH2OH
H
O
OH
O
OH
O
O
O P O P O CH2
OH
O-
OOH
N
N
O
OH
UDP-glucose
49
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Glycogenesis: Glycogen
 The glucose in UDP-glucose adds to glycogen.
UDP-Glucose + glycogen
glycogen-glucose + UDP
 The UDP reacts with ATP to regenerate UTP.
UDP + ATP
UTP + ADP
50
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Glycogenolysis
In glycogenolysis
 Glycogen is broken
down to glucose.
 Glucose molecules
are removed one by
one from the end of
the glycogen chain
to yield glucose-1phosphate.
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Glycogenolysis
Glycogenolysis
 Is activated by glucagon (low blood glucose).
 Bonds glucose to phosphate to form glucose-1phosphate.
glycogen-glucose + Pi
glycogen + glucose-1-phosphate
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Isomerization of Glucose-1phosphate
 The glucose-1-phosphate isomerizes to glucose6-phosphate, which enters glycolysis for energy
production.
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Glucose-6-phosphate
Glucose-6-phosphate
 Is not utilized by brain and skeletal muscle because
they lack glucose-6-phosphatase.
 Hydrolyzes to glucose in the liver and kidney, where
glucose-6-phosphatase is available providing free
glucose for the brain and skeletal muscle.
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54
Learning Check
Match each description with
1) Glycogenesis
2) Glycogenolysis
A. Activated by low levels of blood glucose.
B. Converts glucose-1-phosphate to glucose-6phosphate.
C. Activated by high levels of glucose-6-phosphate.
D. Glucose + UTP
UDP-glucose + PPi
55
Solution
Match each description with:
1) Glycogenesis
2) Glycogenolysis
A. 2 Activated by low levels of blood glucose.
B. 2 Converts glucose-1-phosphate to glucose-6phosphate.
C. 1 Activated by high levels of glucose-6-phosphate.
D. 1 Glucose + UTP
UDP-glucose + PPi
56
Chapter 22 Metabolic Pathways for
Carbohydrates
22.8
Gluconeogenesis: Glucose Synthesis
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Utilization of Glucose
Glucose
 Is the primary
energy source for
the brain, skeletal
muscle, and red
blood cells.
 Deficiency can
impair the brain
and nervous
system.
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Gluconeogenesis: Glucose Synthesis
Gluconeogenesis is
 The synthesis of glucose from carbon atoms of
noncarbohydrate compounds.
 Required when glycogen stores are depleted.
59
Gluconeogenesis: Glucose Synthesis
60
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Gluconeogenesis: Glucose Synthesis
In gluconeogenesis,
 Glucose is synthesized from noncarbohydrates such
as lactate, some amino acids, and glycerol after they
are converted to pyruvate or other intermediates.
 Seven reactions are the reverse of glycolysis and use
the same enzymes.
 Three reactions are not reversible.
Reaction 1
Hexokinase
Reaction 3
Phosphofructokinase
Reaction 10
Pyruvate kinase
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Gluconeogenesis: Pyruvate to
Phosphoenolpyruvate
 Pyruvate adds a carbon to form oxaloacetate by two
reactions that replace the reverse of reaction 10 of
glycolysis.
 Then a carbon is removed and a phosphate added to
form phosphoenolpyruvate.
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Phosphoenolpyruvate to Fructose1,6-bisphosphate
 Phosphoenolpyruvate is converted to fructose-1,6bisphosphate using the same enzymes in glycolysis.
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Glucose Formation
Glucose forms when
 A loss of a phosphate from fructose-1,6-bisphosphate
forms fructose-6-phosphate and Pi.
 A reversible reaction converts fructose-6-phosphate to
glucose-6-phosphate.
 A phosphate is removed from glucose-6-phosphate.
64
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Cori Cycle
The Cori cycle
 Is the flow of lactate and glucose between the muscles
and the liver.
 Occurs when anaerobic conditions occur in active
muscle and glycolysis produces lactate.
 Operates when lactate moves through the blood stream
to the liver, where it is oxidized back to pyruvate.
 Converts pyruvate to glucose, which is carried back to
the muscles.
65
Pathways for Glucose
are derived from
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66
Regulation of Glycolysis and
Gluconeogenesis
Regulation occurs as
 High glucose levels and insulin promote glycolysis.
 Low glucose levels and glucagon promote
gluconeogenesis.
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Regulation of Glycolysis and
Gluconeogenesis
TABLE 22.2
68
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Learning Check
Identify each process as:
1) glycolysis
2) glycogenesis
3) glycogenolysis
4) gluconeogenesis
A.
B.
C.
D.
The synthesis of glucose from noncarbohydrates.
The breakdown of glycogen into glucose.
The oxidation of glucose to two pyruvate.
The synthesis of glycogen from glucose.
69
Solution
Identify each process as:
1) glycolysis
2) glycogenesis
3) glycogenolysis
4) gluconeogenesis
A. 4 The synthesis of glucose from
noncarbohydrates.
B. 3 The breakdown of glycogen into glucose.
C. 1 The oxidation of glucose to two pyruvate.
D. 2 The synthesis of glycogen from glucose.
70