Download Chapter 13 Carbohydrates

Chapter 13 Carbohydrates
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Carbohydrates
Carbohydrates are
• a major source of energy from
our diet
• made from elements carbon,
hydrogen, and oxygen
• also called saccharides, which means
“sugars”
Carbohydrates
• are produced by photosynthesis in
plants
• such as glucose are synthesized in
plants from CO2, H2O, and energy
from the sun
• are oxidized in living cells to
produce CO2, H2O, and energy
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Types of Carbohydrates
The types of carbohydrates are
• monosaccharides, the simplest carbohydrates
• disaccharides, which consist of two monosaccharides
• polysaccharides, which contain many monosaccharides
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Monosaccharides
Monosaccharides
• consist of three to eight carbon
chains with one carbon in a
carbonyl group
• are further classified as an
aldose, which contains an
aldehyde group, or a ketose,
which contains a ketone
carbonyl group
• have several hydroxyl groups
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Types of Monosaccharides
Monosaccharides are also classified by the number of
carbon atoms present.
triose (3 C atoms)
tetrose (4 C atoms)
pentose (5 C atoms)
hexose (6 C atoms)
An aldopentose indicates a five-carbon saccharide with an
aldehyde group.
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Aldoses
Aldoses are monosaccharides with
• an aldehyde group
• many hydroxyl (OH) groups
O
║
C─H
aldose
HCOH
HCOH
CH2OH
Erythose, an aldotetrose
(4 carbons, aldehyde group)
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Ketoses
Ketoses are monosaccharides with
• a ketone group
• many hydroxyl (OH) groups
CH2OH
C=O
HCOH
HCOH
HCOH
Fructose, a ketohexose
(6 carbons, ketone group)
CH2OH
ketose
Fischer Projections
A Fischer projection
• is used to represent carbohydrates
• places the most oxidized group at the top
• shows chiral carbons as the intersection of vertical and
horizontal lines
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D and L Notations
In a Fischer projection, the OH group on the
• chiral carbon farthest from the carbonyl group determines
an L or D isomer.
• left is assigned the letter L for the L form
• right is assigned the letter D for the D form
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Examples of D and L Isomers
of Monosaccharides
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Learning Check
Identify each as the D or L isomer.
A.
B.
C.
CH2OH
O
C H
HO
H
HO
H
H
HO
H
HO
CH2OH
__-Ribose
O
O
C H
HO
H
OH
H
OH
H
H
OH
CH2OH
__-Threose
CH2OH
__-Fructose
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Solution
Identify each as the D or L isomer.
A.
B.
C.
CH2OH
O
C H
HO
H
HO
H
H
HO
H
HO
CH2OH
L-Ribose
O
O
C H
HO
H
OH
H
OH
H
H
OH
CH2OH
L-Threose
CH2OH
D-Fructose
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Glucose and Fructose
D-glucose is
• found in fruits, corn syrup, and honey
• an aldohexose with the formula C6H12O6
• known as dextrose and blood sugar in the body
D-fructose is
• obtained from lactose
• an aldohexose with the formula C6H12O6
• important in cell membranes of the brain and
nerve cells
The sweet taste of honey comes from the
monosaccharides
D-glucose and D-fructose.
Blood Glucose Level
In the body,
• glucose has a normal blood level of 70–90 mg/dL
• a glucose tolerance test measures blood glucose for several
hours after ingesting glucose
D-Galactose
D-Galactose
•
•
•
•
is an aldohexose, C6H12O6
is not found free in nature
is obtained from lactose, a disaccharide
has a similar structure to glucose except
for the OH on carbon 4
In a condition called galactosemia, an
enzyme needed to convert galactose to
glucose is missing.
O
C H
H
OH
HO
H
HO
H
H
OH
CH2OH
D-Galactose
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Cyclic Structures
Cyclic structures
• are the prevalent form of monosaccharides with five or
six carbon atoms
O
O
• form when the hydroxyl group on carbon 5 reacts with the
aldehyde group or ketone group
Guide for Drawing Hayworth Structures
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Drawing the Cyclic Structure for Glucose
Step 1 Number the carbon chain and turn clockwise to form a
linear open chain.
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Drawing the Cyclic Structure
for Glucose
Step 2 Fold clockwise to make a hexagon.
• Bond the carbon 5 –O– to carbon 1.
• Place the carbon 6 group above the ring.
• Write the –OH groups on carbon 2 and 4 below the ring.
• Write the –OH group on carbon 3 above the ring.
• Write a new –OH on carbon 1.
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Drawing the Cyclic Structure
for Glucose
Step 3 Write the new –OH on carbon 1 down for the  form
and up for the  form.
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Mutarotation of -D-Glucose and
β-D-Glucose in Solution
When placed in solution,
• cyclic structures open and close
• -D-glucose converts to β-D-glucose and vice versa
• at any time, only a small amount of open chain forms
C H 2O H
C H 2O H
O
OH
OH
OH
OH
-D-Glucose
(36%)
H
O
OH
O
O
C
OH
OH
D-Glucose (open)
(trace)
C H2O H
H
OH
OH
β-D-Glucose OH
(64%)
OH
Cyclic Structure of Fructose
Fructose
• is a ketohexose
• forms a cyclic structure
• reacts the —OH on carbon 5 with the C=O on carbon 2
C H 2O H
C
O
HO
C
H
H
C
OH
H
C
OH
C H 2O H
C H 2O H
O
C H 2O H
OH
OH
O
OH
OH
OH
C H 2O H
OH
C H 2O H
D-Fructose
α-D-Fructose
-D-Fructose
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Learning Check
Write the cyclic forms of D-galactose.
O
C H
H
OH
HO
H
HO
H
H
OH
CH2OH
D-Galactose
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Solution
Step 1 Number the carbon chain and turn clockwise to form a
O linear open chain.
1
1C
H
HO
HO
H
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3
4
5
H
OH
H
H
H
OH OH H
C
HOH2C
6
O
5
4
OH H
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H
2
OH
1 H
OH
6
CH2OH
D-Galactose
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Solution
Step 2 Fold clockwise to make a hexagon.
• Bond the carbon 5 O to carbon 1 and change
the =O to OH on carbon 1.
• Place the carbon 6 group above the ring.
CH2OH
• Write the OH groups on carbon 2 and
6
carbon 4 below the ring and
the OH group on
carbon 3 above the ring.
OH
4
H
5
OH
H
OH H
3
O
C
1
H
2
H HO
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Solution
Step 3 Write the new –OH on carbon 1 down for the  form
or up for the  form.
H2C OH
O
OH
OH
H2C OH
O
OH
H
OH
OH
H

OH
-D-Galactose
OH
OH
-D-Galactose

Reducing Sugars
Reducing sugars
• are monosaccharides that can be oxidized to give a
carboxylic acid
• contain an aldehyde group in the open chain form
• undergo reaction in the Benedict’s test
• include the monosaccharides glucose, galactose,
and fructose
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Oxidation of D-Glucose
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Conversion of Fructose to Glucose
• Fructose, a ketohexose, is also a reducing sugar.
• In Benedict’s solution, which is basic, a rearrangement
occurs between the ketone group on carbon 2 and the
hydroxyl group on carbon 1.
• This produces an aldehyde group with an adjacent
hydroxyl, thereby converting fructose to glucose.
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Reduction of Monosaccharides
The reduction of monosaccharides
• produces sugar alcohols,
or alditols
• involves the reduction of the
carbonyl group to an alcohol
• such as D-glucose gives
D-glucitol, also called sorbitol
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Learning Check
Write the products of the oxidation and reduction
of D-mannose.
O
C H
HO
H
HO
H
H
OH
H
OH
CH2OH
D-Mannose
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Solution
Write the products of the oxidation and reduction of
D-mannose.
CH2OH
HO
H
HO
H
Reduction
O
O
C H
C OH
HO
H
HO
H
Oxidation
HO
H
HO
H
H
OH
H
OH
H
OH
H
OH
H
OH
H
OH
CH2OH
D-Mannitol
CH2OH
D-Mannose
CH2OH
D-Mannonic
acid
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Important Disaccharides
A disaccharide consists of two monosaccharides.
Monosaccharides
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Disaccharide
glucose + glucose
maltose + H2O
glucose + galactose
lactose + H2O
glucose + fructose
sucrose + H2O
Maltose
Maltose is
• a disaccharide also known as
malt sugar
• composed of two D-glucose
molecules
• obtained from the hydrolysis
of starch
• used in cereals, candies, and
brewing
• found in both the  and β
forms
Maltose is linked by an
-1,4-glycosidic bond formed
from the  OH on carbon 1 of
the first glucose and OH on
carbon 4 of the second glucose.
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Lactose
Lactose
• is found in milk and
milk products
• is a disaccharide of
β-D-galactose and
α- or β-D-glucose
• contains a
β-1,4-glycosidic bond
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Sucrose
Sucrose, or table sugar,
• is obtained from sugar
cane
and sugar beets
• consists of α-D-glucose
and β-D-fructose
• has an α,β-1,2glycosidic bond
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Learning Check
For the following disaccharide, list the two monosaccharide
units and the type of glycosidic bond present.
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Solution
For the following disaccharide, provide the reaction for two
monosaccharide units and the type of glycosidic bond present.
galactose + glucose
melibiose + H2O
The sugar melibiose is formed from two monosaccharides,
galactose and glucose, which are joined by a -1,6-glycosidic bond.
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Sweetness of Sweeteners
Sugars and artificial
sweeteners
• differ in sweetness
• are compared to
sucrose (table sugar),
which is assigned a
value of 100
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Artificial Sweeteners - Sucralose
Sucrolose is made from
sucrose by replacing some
of the hydroxyl groups
with chlorine atoms.
Sucralose is the sweetening
agent in Splenda.
Artificial Sweeteners - Aspartame
Aspartame is
• marketed as Nutra-Sweet
• a noncarbohydrate sweetener made from aspartic acid
and a methyl ester of the amino acid phenylalanine
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Polysaccharides
Polysaccharides
• are polymers of D-glucose
• include amylose and amylopectin,
starches made of α-D-glucose
• include glycogen (animal starch in
muscle), which is made of α-D-glucose
• include cellulose (plants and wood),
which is made of β-D-glucose
CH2OH
O
OH
OH
OH
OH
α-D-Glucose
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Polysaccharide: Amylose
Amylose is
• a straight-chain polysaccharide made of glucose molecules
• linked by -1,4 glycosidic bonds
Polysaccharide: Amylopectin
Amylopectin, found in plants,
• is a polymer of α-D-glucose
molecules
• is a branched-chain
polysaccharide
• has α-1,4-glycosidic bonds
between the glucose units
• has α-1,6 bonds to branches
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Glycogen
Glycogen, or animal starch,
• is the polysaccharide that stores α-D-glucose in muscle
• is similar to amylopectin, but is more highly branched
• the glucose units are joined by α-1,4-glycosidic bonds,
and branches occurring about every 10–15 glucose units
are attached by α-1,6-glycosidic bonds
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Cellulose
Cellulose, the major structural unit of wood,
• is a polysaccharide of glucose units in unbranched chains
• has β-1,4-glycosidic bonds
• cannot be digested by
humans because humans
cannot break down
β-1,4-glycosidic bonds
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Concept Map for Carbohydrates
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