Chapter 24
Carbohydrate
Metabolism
Section 24.1
Digestion and Absorption of Carbohydrates
Carbohydrates are the major energy source for human beings.
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Chapter 24
Table of Contents
24.1
24.2
24.3
24.4
Digestion and Absorption of Carbohydrates
Glycolysis
Fates of Pyruvate
ATP Production for the Complete Oxidation of
Glucose
24.5 Glycogen Synthesis and Degradation
24.6 Gluconeogenesis
24.7 Terminology for Glucose Metabolic Pathways
24.8 The Pentose Phosphate Pathway
24.9 Hormonal Control of Carbohydrate Metabolism
24.10 B-Vitamins and Carbohydrate Metabolism
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Section 24.1
Digestion and Absorption of Carbohydrates
• Digestion: Breakdown of food molecules by hydrolysis
into simpler chemical units that can be used by cells in
their metabolic processes
• Carbohydrate digestion: Begins in the mouth
– Salivary enzyme “Alpha-amylase” catalyzes the hydrolysis of
alpha-glycosidic linkages of starch and glycogen to produce
smaller polysaccharides and disaccharide - maltose
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Section 24.1
Digestion and Absorption of Carbohydrates
• Only a small amount of carbohydrate digestion occurs in
the mouth because food is swallowed so quickly into the
stomach.
• In stomach very little carbohydrate is digested:
– No carbohydrate digestion enzymes present in stomach
– Salivary amylase gets inactivated because of stomach
acidity
• The primary site for the carbohydrate digestion is within
the small intestine
– Pancreatic alpha-amylase breaks down polysaccharide chains
into disaccharide – maltose
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Section 24.1
Digestion and Absorption of Carbohydrates
• The final step in carbohydrate digestion occurs on the
outer membranes of intestinal mucosal cells
• Disaccharidase enzymes present in the intestinal
mucosa convert disaccharides (maltose, sucrose and
lactose) to monosaccharides (glucose, fructose and
galactose)
– Maltase – converts maltose to glucose
– Sucrase – Converts sucrose to glucose and fructose
– Lactase – Converts lactose glucose and galactose
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Section 24.1
Digestion and Absorption of Carbohydrates
• The carbohydrate digestion products (glucose,
galactose, and fructose) are absorbed into the
bloodstream through the intestinal wall.
• The intestinal villi are rich in blood capillaries into which
the monosaccharides are actively transported.
• ATP hydrolysis and protein carriers mediate the passage
of the monosaccharides through cell membranes.
• Galactose and Fructose are converted to products of
glucose metabolism in the liver.
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Section 24.1
Digestion and Absorption of Carbohydrates
A section of the small intestine, showing its folds and the villi
that cover the inner surface of the folds. Villi greatly increase
the inner intestinal surface area.
Figure 24-1 p887
Section 24.1
Digestion and Absorption of Carbohydrates
Summary of carbohydrate digestion in the human body.
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Section 24.2
Glycolysis
Six-Carbon Stage of Glycolysis
• Glycolysis: The metabolic pathway in which glucose is
converted to two molecules of pyruvate (a C3
carboxylate), and ATP and NADH are produced.
• Occurs in two stages: 6 carbon and 3 Carbon stages
• Steps 1-3: Six carbon stage
– The six-carbon stage of glycolysis is an energyconsuming stage
– Phosphate derivatives glucose and fructose are
formed via a ATP coupling reactions.
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Section 24.2
Glycolysis
Six-Carbon Stage of Glycolysis (Steps 1-3)
• Step 1: Formation of glucose-6-phosphate:
– Phosphorylation of glucose - phosphate group from ATP is transferred
to the hydroxyl group on carbon 6 of glucose
– Reactions catalyzed by Hexokinase
– Endothermic reaction
– Energy needed is derived from ATP hydrolysis
• Step 2: Formation of Fructose-6-phosphate:
– Glucose 6 phosphate is isomerized to Fructose -6-Phosphate.
– Enzyme: Phosphoglucoisomerase
• Step 3: Formation of Fructose 1,6-bisphosphate:
–
–
–
–
Further phosphorylation of Fructose-6-bisphosphate
Endothermic reaction
Energy derived from ATP hydrolysis
Enzyme: phosphofructokinase
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Section 24.2
Glycolysis
Three-Carbon Stage of Glycolysis (Steps 4-10)
• Reaction intermediates are derivatives of glycerol and
acetone
• All reaction intermediates are phosphorylated derivatives
of dihydroxyacetone, glyceraldehyde, glycerate, or
pyruvate
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Section 24.2
Glycolysis
Steps 4-5
• Step 4: Formation of Triose Phosphates:
– C6 species is split into two C3 species
– Two C3 species formed are dihydroxyacetone phosphate and
glyceraldehyde 3-phosphate
– Enzyme : Aldolase
• Step 5: Isomerization of Triose Phosphates:
– Dihydroxyacetone phosphate is isomerized to glyceraldehyde 3phosphate
– Enzyme: Triosephosphate isomerase
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Section 24.2
Glycolysis
Steps 7-8
• Step 7: Formation of 3-Phosphoglycerate:
– Diphosphate from step 6 is converted back to monophosphate
species
– It is an ATP producing step
• C1 high energy phosphate group of 1,3-bisphosphoglycerate is
transferred to an ADP molecule to form an ATP
– Enzyme: phosphoglycerokinase
– Two ATP molecules are produced for each original glucose
molecule
• Step 8: Formation of 2-phosphoglycerate:
– Isomerization of 3-phosphoglycerate to 2-phosphoglycerate
• Phosphate group moved from C-3 to C-2
–
Enzyme: Phosphoglyceromutase
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Section 24.2
Glycolysis
Steps 9-10
• Step 9: Formation of Phosphoenolpyruvate:
– This is an alcohol dehydration reaction -- results in another high
energy phosphate group containing compound
– Enzyme: Enolase
• Step 10: Formation of Pyruvate:
– High energy phosphate is transferred from
phosphoenolpyruvate to ADP molecule to produce ATP and
pyruvate
– Enzyme: Pyruvate kinase
– Two ATP molecules are produced for each original glucose
molecule
– Note: Steps 1,3 and 10 are control points for glycolysis
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Section 24.2
Glycolysis
An overview of glycolysis.
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Section 24.2
Glycolysis
ATP Production and Consumption
• There is a net gain of two ATP molecules in glycolysis for every
glucose molecule processed
• Overall equation for glycolysis
Glucose + 2NAD+
2ADP + 2Pi
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2 Pyruvate + 2NADH + 2H+ + 2H2O
2ATP
17
Section 24.2
Glycolysis
Practice Exercise
• Indicate at what step in the glycolysis pathway each of
the following events occur:
a. Second formation of ATP occurs
b. Second “energy-rich” compound is produced
c. Second time ATP is converted to ADP
d. A hydration reaction occurs
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Section 24.2
Glycolysis
Practice Exercise
• Indicate at what step in the glycolysis pathway each of
the following events occur:
a. Second formation of ATP occurs
b. Second “energy-rich” compound is produced
c. Second time ATP is converted to ADP
d. A hydration reaction occurs
Answers:
a. Step 10
b. Step 9
c. Step 3
d. Step 9
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Section 24.2
Glycolysis
Entry of Galactose and Fructose into Glycolysis
• Both fructose and galactose are converted in liver to
intermediates that enter into the glycolysis pathway.
• Entry of fructose into the glycolytic pathway involves:
– Phosphorylation by ATP to produce fructose 1-phosphate
– Fructose 1-phosphate is converted to two trioses:
• Glyceraldehyde: phosphorylated to enter into
glycolysis
• Dihydroxyacetone phosphate - enters into
glycolysis directly
• The entry of galactose into glycolysis also needs
phosphorylation by ATP to produce glucose 1-phosphate
and is isomerized to glucose 6-phosphate
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Section 24.2
Glycolysis
Entry points for
fructose and
galactose into the
glycolysis pathway.
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Section 24.2
Glycolysis
Regulation of Glycolysis
• Control points of glycolysis: Steps 1, 3, and 10
• Step 1- Conversion of glucose to glucose 6-phosphate by
hexokinase:
– Hexokinase inhibited by glucose 6-phosphate (feedback
inhibition)
• Step 3: Fructose 6-phosphate converted to fructose 1,6bisphosphate by phosphofructokinase:
– High concentrations of ATP and citrate inhibit
phosphofructokinase
• Step 10: Conversion of phosphoenolpyruvate to pyruvate by
Pyruvate kinase:
– Enzyme is inhibited by high ATP concentrations.
– Both pyruvate kinase (Step 10) and phosphofructokinase (Step
3) are allosteric enzymes.
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Section 24.3
Fates of Pyruvate
The three common fates of pyruvate generated by glycolysis.
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Section 24.3
Fates of Pyruvate
Oxidation to Acetyl CoA
• Under aerobic (oxygen-rich) conditions, pyruvate is
oxidized to acetyl CoA by pyruvate dehydrogenase
complex
• Acetyl CoA thus formed enters the mitochondrial
matrix for further processing through the citric acid cycle
• Most pyruvate formed during glycolysis is converted to
Acetyl CoA.
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Section 24.3
Fates of Pyruvate
Lactate Fermentation
• An enzymatic anaerobic reduction of pyruvate to lactate
occurs mainly in muscles
• Purpose: Conversion of NADH to NAD+ for increased
rate of glycolysis
• Lactate is converted back to pyruvate when aerobic
conditions are reestablished in the cell
• Muscle fatigue associated with strenuous physical
activity is attributed to increased build-up of lactate
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Section 24.3
Fates of Pyruvate
Strenuous muscular activity can
result in lactate accumulation.
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Section 24.3
Fates of Pyruvate
Anaerobic lactate formation allows
for “recycling” of NAD1, providing
the NAD1 needed for Step 6 of
glycolysis.
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Section 24.3
Fates of Pyruvate
Ethanol Fermentation
•
•
•
Enzymatic anaerobic conversion of pyruvate to ethanol and
carbon dioxide
Simple organisms, e.g., yeast and bacteria, regenerate NAD+
through ethanol fermentation reactions
Involves two reactions:
– Pyruvate decarboxylation by pyruvate decarboxylase
– Acetaldehyde reduction to ethanol by alcohol
dehydrogenase
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Section 24.3
Fates of Pyruvate
• Ethanol fermentation involving yeast
causes bread and related products to rise
as a result of CO2 bubbles being released
during baking.
• Beer, wine, and other alcoholic drinks are
produced by ethanol fermentation of the
sugars in grain and fruit products.
• Overall ethanol fermentation reaction:
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Section 24.3
Fates of Pyruvate
All three of the common fates of pyruvate from glycolysis provide
for the regeneration of NAD1 from NADH.
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Section 24.3
Fates of Pyruvate
Practice Exercise
•
Which of the three common metabolic pathways for pyruvate is
compatible with each of the following characterizations concerning
the reactions that pyruvate undergoes?
a. Acetaldehyde is an intermediate in this pathway
b. An anaerobic pathway that does not function in humans
c. An anaerobic pathway that does function in humans
d. A C2 molecule is a product under aerobic reaction conditions for
this pathway
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Section 24.3
Fates of Pyruvate
Practice Exercise
•
Which of the three common metabolic pathways for pyruvate is
compatible with each of the following characterizations concerning
the reactions that pyruvate undergoes?
a. Acetaldehyde is an intermediate in this pathway
b. An anaerobic pathway that does not function in humans
c. An anaerobic pathway that does function in humans
d. A C2 molecule is a product under aerobic reaction conditions for
this pathway
Ans:
a. Ethanol fermentation
b. Ethanol fermentation
c. Lactate fermentation
d. Acetyl CoA formation
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Section 24.4
ATP Production for the Complete Oxidation of Glucose
• NADH produced during Step 6 of Glycolysis cannot
directly participate in the electron transport chain
because mitochondria are impermeable to NADH and
NAD+
• Glycerol 3-phosphate-dihydroxyacetone phosphate
transport system shuttles electrons from NADH, but not
NADH itself, across the membrane:
– Dihydroxyacetone phosphate and glycerol phosphate
freely cross the mitochondrial membrane
– The interconversion shuttles the electrons from
NADH to FADH2
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Section 24.4
ATP Production for the Complete Oxidation of Glucose
The Dihydroxyacetone Phosphate-Glycerol 3-Phosphate Shuttle
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Section 24.4
ATP Production for the Complete Oxidation of Glucose
• Total of 30 ATP molecules are produced in muscle and
nerve cells:
– 26 from oxidative phosphorylation of electron
transport chain
– 2 from oxidation of glucose to pyruvate
– 2 from conversion of GTP (guanosine triphosphate)
to ATP
• Aerobic oxidation of glucose is 15 times more efficient in
the ATP production as compared to anaerobic lactate
and ethanol processes
• In other cells such as heart and liver cells a more
complex shuttle system is used and 32 molecules are
produced instead of 30 per glucose molecule
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Section 24.4
ATP Production for the Complete Oxidation of Glucose
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Section 24.5
Glycogen Synthesis and Degradation
• Glycogen: A branched polymer form of glucose is the
storage form of carbohydrates in humans and animals
(animal starch):
– In muscle: source of glucose for glycolysis
– In liver tissue: source of glucose to maintain normal
blood glucose levels
– Produced by the process of glycogenesis
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Section 24.5
Glycogen Synthesis and Degradation
Glycogenesis
• Metabolic pathway by which glycogen is synthesized
from glucose
• Involves three steps:
– Formation of Glucose 1-phosphate
– Formation of UDP Glucose (uridine diphosphate
glucose)
– Glucose transfer to a Glycogen Chain
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Section 24.5
Glycogen Synthesis and Degradation
Steps 1-3
•
•
•
Step 1: Formation of glucose 1-phosphate:
– Starting material is glucose 6-phosphate -- from first step of glycolysis
– Enzyme phosphoglucomutase catalyzes conversion of glucose 6phosphate to glucose 1-phosphate
Step 2: Formation of UDP Glucose:
– High energy compound UTP (uridine triphosphate) activates glucose 1phosphate to uridine diphosphate glucose (UDP-glucose)
Step 3: Glucose transfer to a Glycogen Chain:
– The glucose unit of UDP-glucose is attached to the end of a glycogen
chain and UDP is produced
– UDP reacts with ATP to form UTP and ADP
– Adding one glucose unit to a glycogen chain requires the investment of
two ATP molecules
– One in the formation of glucose 6-phosphate and one in the
regeneration of UTP
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Section 24.5
Glycogen Synthesis and Degradation
Glycogenolysis
• Breakdown of glycogen to glucose-6-phosphate:
– It is not just reverse of glycogenesis because it does
not require UTP or UDP molecules
– Glycogenolysis is a two-step process
– Step 1: Phosphorylation of a glucose residue
– Step 2: Glucose 1-phosphate isomerization
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Section 24.5
Glycogen Synthesis and Degradation
Steps 1-2
• Step 1: Phosphorylation of a glucose residue:
– Glycogen phosphorylase catalyzes the removal of an
end glucose residue from a glycogen molecule as
glucose 1-phosphate.
• Step 2: Glucose 1-phosphate Isomerization:
– Phosphoglucomutase isomerizes glucose 1phosphate is to glucose 6-phosphate (reverse of the
first step of glycogenesis)
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Section 24.5
Glycogen Synthesis and Degradation
• The locally produced glucose 6-phosphate directly
enters the glycolysis pathway:
– Low glucose levels stimulates glycogenolysis in liver cells
• Glucose 6-phosphate is ionic and cannot cross the
membrane:
– Enzyme glucose 6-phosphatase found in liver, kidneys and
intestine convert glucose 6-phosphate to glucose
– This enzyme is not present in muscle and brain tissues
– The free glucose is then transported to muscle and brain via
blood
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Section 24.5
Glycogen Synthesis and Degradation
The processes of glycogenesis and glycogenolys is contrasted.
The intermediate UDP—glucose is part of glycogenesis but not of
glycogenolysis.
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Section 24.6
Gluconeogenesis
• Metabolic pathway by which glucose is synthesized
from non-carbohydrate sources:
– The process is not exact opposite of glycolysis
• Glycogen stores in muscle and liver tissue are depleted
with in 12-18 hours from fasting or in even less time from
heavy work or strenuous physical activity
• Without gluconeogenesis, the brain, which is dependent
on glucose as a fuel would have problems functioning if
food intake were restricted for even one day
• Gluconeogenesis helps to maintain normal bloodglucose levels in times of inadequate dietary
carbohydrate intake
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Section 24.6
Gluconeogenesis
• About 90% of gluconeogenesis takes place in the liver
• Non-carbohydrate starting materials for
gluconeogenesis:
– Pyruvate
– Lactate (from muscles and from red blood cells)
– Glycerol (from triacylglycerol hydrolysis)
– Certain amino acids (from dietary protein hydrolysis
or from muscle protein during starvation)
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Section 24.6
Gluconeogenesis
Overall Reaction
• 2 Pyruvate + 4ATP + 2GTP + 2NADH + 2H2O  Glucose
+ 4ADP + 2GDP + 6Pi + 2NAD+
• Pyruvate to glucose conversion requires the expenditures
of 4 ATP and 2 GTP
• Gluconeogenesis occurs at the expense of other ATPproducing metabolic processes
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Section 24.6
Gluconeogenesis
The pathway for gluconeogenesis
is similar, but not identical, to the
pathway for glycolysis.
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Section 24.6
Gluconeogenesis
Cori Cycle
• Gluconeogenesis using lactate as a source of pyruvate
is particularly important because of lactate formation
during strenuous exercise
• Lactate produced diffuses from muscle cells into the
bloodstream and transported to liver
• Enzyme lactate dehydrogenase converts lactate to
pyruvate in the liver
• Pyruvate is then converted to glucose via
gluconeogenesis
• The glucose thus produced enters the bloodstream and
transported to the muscles
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Section 24.6
Gluconeogenesis
The Cori Cycle. Lactate, formed from glucose under anaerobic conditions in muscle
cells, is transferred to the liver, where it is reconverted to glucose, which is then
transferred back to the muscle cells.
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Section 24.6
Gluconeogenesis
Cori Cycle
Nucleotide triphosphate change (gain or loss) associated with
the two parts of the Cori cycle.
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Section 24.7
Terminology For Glucose Metabolic Pathways
• Glycogenesis: 2-Step process in which glycogen is
synthesized from glucose 6-phosphate
• Gluconeogenesis: 11-step process in which pyruvate is
converted to glucose
• Glycolysis: 10 step process in which glucose is
converted to pyruvate
• Glycogenolysis: The process in which glycogen is
converted to glucose 6-phosphate
– Names ending with “lysis” - Breakdown
– Names ending with “genesis” - Synthesis
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Section 24.7
Terminology For Glucose Metabolic Pathways
Relationships Among Four Common Metabolic Pathways That
Involve Glucose
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Section 24.7
Terminology For Glucose Metabolic Pathways
Practice Exercise
• Identify each of the following as a characteristic of one or
more of the following processes: glycolysis,
glycogenesis, glycogenolysis, and gluconeogenesis.
a. Glycogen is the final product.
b. Glucose is the initial reactant.
c. Glucose 1-phosphate is produced in the first step.
d. ADP is converted to ATP in this process.
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Section 24.7
Terminology For Glucose Metabolic Pathways
Practice Exercise
• Identify each of the following as a characteristic of one or
more of the following processes: glycolysis,
glycogenesis, glycogenolysis, and gluconeogenesis.
a. Glycogen is the final product.
b. Glucose is the initial reactant.
c. Glucose 1-phosphate is produced in the first step.
d. ADP is converted to ATP in this process.
Answers:
a. Glycogenesis
b. Glycolysis
c. Glycogenesis
d. Glycolysis
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Section 24.7
Terminology For Glucose Metabolic Pathways
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Section 24.8
The Pentose Phosphate Pathway
Structure of NADPH
• The pentose phosphate pathway:
A metabolic pathway in which
glucose is used to produce
NADPH, ribose 5-phosphate (a
pentose phosphate) and
numerous other sugar phosphates
– NADPH: reduced form of NADP+
(nicotinamide adenine dinucleotide
phosphate)
– NADP+/NADPH is a phosphorylated
version of NAD+/NADH
– NADPH, like ATP, is essential for
biosynthetic reactions/pathways.
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Section 24.8
The Pentose Phosphate Pathway
Two Stages
• Oxidative stage:
– Involves three steps through which glucose 6phosphate is converted to ribulose 5-phosphate and
CO2
• Non-oxidative stage:
– In the first step of the non-oxidative stage of the
pentose phosphate pathway, ribulose 5-phosphate
(ketose) is isomerized to ribose 5-phosphate (aldose)
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Section 24.8
The Pentose Phosphate Pathway
• The pentose phosphate pathway helps meet cellular
needs in numerous ways:
– When ATP demand is high, the pathway continues to
its end products which enter glycolysis
– When NADPH demand high, intermediates are
recycled to glucose 6-phosphate (the start of the
pathway), and further NADPH is produced
– Helps generate ribose 5-phosphate for nucleic acid
and coenzyme production
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Section 24.9
Hormonal Control of Carbohydrate Metabolism
• The second major method for controlling carbohydrate
metabolism, besides enzyme inhibition by metabolism is
hormonal control
• Three major hormones control carbohydrate metabolism:
– Insulin
– Glucagon
– Epinephrine
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Section 24.9
Hormonal Control of Carbohydrate Metabolism
Insulin Hormone Produced by Beta Cells of Pancreas
•
•
•
•
•
51 amino acid polypeptide
Promotes utilization of glucose by cells
Its function is to lower blood glucose levels
Also involved in lipid metabolism
The release of insulin is triggered by high blood-glucose
levels
• The mechanism for insulin action involves insulin binding
to proteins receptors on the outer surfaces of cells which
facilitates entry of the glucose into the cells
• Insulin also produces an increase in the rate of glycogen
synthesis
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Section 24.9
Hormonal Control of Carbohydrate Metabolism
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Section 24.9
Hormonal Control of Carbohydrate Metabolism
Glucagon
•
•
•
•
29 amino acid peptide hormone
Produced in the pancreas by alpha cells
Released when blood glucose levels are low
Principal function is to increase blood-glucose
concentration by speeding up the conversion of
glycogen to glucose (glycogenolysis) in the liver
• Glucagon elicits the opposite effects of insulin
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Section 24.9
Hormonal Control of Carbohydrate Metabolism
Epinephrine
• Also called adrenaline
• Released by the adrenal glands in response to anger, fear, or
excitement
• Function is similar to glucagon, i.e., stimulates glycogenolysis
• Primary target of epinephrine is muscle cells
• Promotes energy generation for quick action
• It also functions in lipid metabolism
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Section 24.9
Hormonal Control of Carbohydrate Metabolism
The series of events by which the hormone epinephrine stimulates
glucose production.
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Section 24.10
B-Vitamins and Carbohydrate Metabolism
• Structurally modified B-vitamins function as coenzymes
in carbohydrate metabolism
• 6 B-Vitamins participate in various reactions of
carbohydrate metabolism:
–
–
–
–
–
–
Niacin – NAD+ and NADH
Riboflavin – as FAD, FADH2 and FMN
Thiamin – as TPP
Pantothenic acid - as CoA
Biotin
Vitamin B6 in the form of PLP(pyridoxal 5-phosphate)
• Without these B-vitamins body would be unable to utilize
carbohydrates as energy sources.
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Section 24.10
B-Vitamins and Carbohydrate Metabolism
B vitamin participation in
chemical reactions associated
with carbohydrate metabolism.
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Section 24.10
B-Vitamins and Carbohydrate Metabolism
p916
Section 24.10
B-Vitamins and Carbohydrate Metabolism
p916