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Carbohydrates
Dr Seemal Jelani
Introduction to Biochemistry
1
4/12/2016
Carbohydrates
Most abundant
Their abundance in human body
is low but constitute about 75%
by mass of dry plant materials
Green (chlorophyll-containing)
plants produce carbohydrates
via photosynthesis
A major source of energy from
our diet composed of the
elements C, H and O.
They also called saccharides,
which means “sugars.”
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Biochemistry
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Carbohydrates
They are produced by
photosynthesis in plants.
such as glucose are
synthesized in plants
from CO2 from the air,
H2O from the soil, and
energy from the sun
absorbed in chlorophyll
to form carbohydrates
and O2
Oxidized in living cells to
produce CO2, H2O, and
energy.
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Biochemistry
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Uses of carbohydrates in plants
Cellulose (carbohydrates) serve as structural
elements
Starch (carbohydrates) provide energy
reserves for the plants
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Biochemistry
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Dietary intake of plant materials is the major
carbohydrate source for humans and animals
Functions:
Carbohydrate oxidation provides energy
Carbohydrate storage in the form of glycogen
provides a short-term energy reserve
Carbohydrates supply carbon atoms for the
synthesis of other biochemical substances
(proteins, lipids, nucleic acids)
Carbohydrate form part of the structural
framework of DNA and RNA
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Biochemistry
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Classification 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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Biochemistry
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General Formula
Cn H2n On
Carbohydrates are Polyhydroxy aldehydes,
Polyhydroxy ketones
The carbohydrate glucose is Polyhydroxy aldehyde
and the carbohydrate fructose is Polyhydroxy ketone
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Biochemistry
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CHIRALITY: Handedness in molecules
A chiral object is not superimposable on its
mirror image they do not possess a plane
of symmetry
Two forms of a chiral object are known as
enantiomers
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Mirror image
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Mirror image
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Non superposable
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Chiral HANDS
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Chiral
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Shells
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The plane has the same thing on
both sides for the flask
There is no mirror plane for a hand
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If an object has a plane of
symmetry it is necessarily the
same as its mirror image
The lack of a plane of symmetry
is called “handedness”, Chirality
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Hands, gloves are prime
examples of chiral object
They have a “left” and a
“right” version
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Why this chapter?
Handedness is important in
organic and biochemistry
Molecular handedness makes
possible specific interactions
between enzymes and
substrates
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Chirality in daily life
Most Biomolecules in nature are
chiral (sugars, DNA, proteins, amino
acids, steroids)
Human proteins are exclusively
built from L-amino acids; this
involves receptors which are chiral
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The Reason for Handedness:
Chirality
Molecules that are not superimposable
with their mirror images are chiral (have
handedness)
A plane of symmetry divides an entire
molecule into two pieces that are exact
mirror images
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Functional groups in
carbohydrates
Aldehydes CHO
Ketone
C=O
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Biochemistry
Introduction to
Aldose
Ketose
20
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Chirality in carbohydrates
Fischer projections
Carbohydrates may contain more than one
chiral center
FP represents a method for giving
molecularity specifications in two dimensions
FP is a two dimensional notation for showing
the spatial arrangement of groups about
chiral centers in molecule
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Biochemistry
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Stereoisomers
Enantiomers
Diastereomers
Handedness (Right and left)
D and L
Fischer Projection for 2,3,4-trihydroxybutanal
Epimers
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Properties of Enantiomers
Structural isomers have different properties
Diastereoisomers have different properties
Enantiomers have same properties expect
two:
Their interaction with plane polarized light
Their interaction with other chiral substances
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Biochemistry
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PLANE-POLARIZED LIGHT
Polarimeter: a device for measuring the extent
of rotation of plane-polarized light
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Chem-241
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Plane-Polarized Light
optical activity
Light vibrating in all planes to direction of propagation
Plane-polarized light: light vibrating only in parallel planes
Plane-polarized light the vector sum of left
and right circularly polarized light
Optically Activity
Enantiomers (chiral) interact with circularly polarized light
rotating the plane one way with R center
and opposite way with S
result: rotation of plane-polarized light clockwise (+)
or counterclockwise (-)
Plane-Polarized Light
Change in the polarized plane?
(polarimeter)
achiral
sample
no change in the plane
Plane-Polarized Light
Change in the polarized plane?
(polarimeter)
rotates the plane
Classification of Monosaccharides
Monosaccharides consist of
3-6 carbon atoms typically.
A carbonyl group (aldehyde or ketone).
Several hydroxyl groups.
2 types of monosaccharide structures:
Aldoses and ketoses
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Biochemistry
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Aldoses
O
║
C─H aldose
│
H─ C─OH
│
H─ C─OH
│
CH2OH
Aldoses are monosaccharides
with an aldehyde group
with many hydroxyl (-OH)
groups.
triose (3C atoms)
tetrose (4C atoms)
pentose (5 C atoms)
hexose (6 C atoms)
Erythose, an aldotetrose
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Biochemistry
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Ketoses
CH2OH
│
C=O
ketose
│
H─ C─OH
│
H─ C─OH
│
H─C─OH
│
CH2OH
Ketoses are monosaccharides
with a ketone group
with many hydroxyl (-OH)
groups.
Fructose, a ketohexose
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Learning Check
Identify each as aldo- or keto- and as tetrose, pentose, or
hexose:
O
CH2OH
C H
H C OH
C O
H C OH
HO C H
H C OH
H C OH
H C OH
CH2OH
CH2OH
ketopentose
aldohexose
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Biochemistry
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Structures of Monosaccharides
Copyright © 2005 by Pearson Education, Inc.
Publishing as Benjamin Cummings
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Biochemistry
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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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Biochemistry
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Chiral Carbon
Carbon linked to four different groups
CH3CHOHC6H5
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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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Biochemistry
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Examples of D and L Isomers of
Monosaccharides
O
O
C H
C H
H
O
OH
HO
H
H
OH
H
OH
CH2OH
D-glucose
Dr Seemal Jelani
Biochemistry
HO
C H
H
OH
H
H
OH
OH
CH2OH
D-ribose
Introduction to
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H
H
OH
H
OH
HO
H
CH2OH
L-galactose
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D-Glucose
found in fruits, corn
syrup, and honey.
an aldohexose with the
formula C6H12O6 known
as blood sugar in the
body.
The monosaccharide in
polymers of starch,
cellulose, and glycogen.
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Biochemistry
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D-Fructose
is a ketohexose
C6H12O6.
is the sweetest
carbohydrate.
CH2OH
C O
HO C H
is found in fruit
juices and honey.
H C OH
H C OH
converts to glucose
in the body.
CH2OH
D-Fructose
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Biochemistry
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Cyclic Structures
Cyclic structures
are the widespread form of monosaccharides with 5
or 6 carbon atoms.
O
O
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Biochemistry
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The cyclic forms of monosaccharides result
from the ability of their carbonyl group to
react intramolecularly with a –OH group
Cyclic structure is formed when the –OH
group on C-5 reacts with the aldehyde group
or ketone group
The result is a cyclic hemiacetal or cyclic
hemiketal
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Biochemistry
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Steps for cyclisation
for D-Glucose
All –OH groups to the
right in the projection
formula appear below the
ring whereas –OH gps to
the left in FP appear
above the ring
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Biochemistry
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Drawing the Cyclic Structure for Glucose
Number the carbon chain and turn clockwise to
linear open chain.
H
O
H H OH H
C
1
H
form a
2C
OH
HO
3C
H
H
4C
OH
H
5C
OH
O
HOCH2 C C C C C
H
OH OH H OH
6
5
4
3
2
1
6CH OH
2
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Biochemistry
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The –OH gp on C-5 adds across then
C=O
to give two stereoisomers
In aq. Solu of D- glucose, a dynamic equilibrium exists
among the α, β, and open chain forms and there is
continual interconversion among them
A freshly mixed solution of pure α- D- Glucose slowly
converts to a mixture of both α & β-D-glucose by an
opening and closing of the cyclic structure.
At equilibrium 63% is β-D-glucose and 37% are α-Dglucose and less than 0.01% in open chain
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Cyclic Structure for Glucose
STEP 2 Fold into a hexagon.
Bond the C5 –O– to C1.
Place the C6 group above the
ring.
Write the –OH groups on C2 and
C4 below the ring.
Write the –OH group on C3
above the ring.
Write a new –OH on C1.
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Biochemistry
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CH2OH
6
5
O
4
OH
1
OH
3
2
OH
OH
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Cyclic Structure for Glucose (cont)
STEP 3 Write the new –OH on C1
• down for the form.
• up for the form.
CH2OH
O
CH2OH
O
OH
OH
OH
OH
OH
OH
OH
OH
-D-Glucose
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Biochemistry
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-D-Glucose
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Summary of the Formation of Cyclic
Glucose
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Biochemistry
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Alcohols react with the carbonyl
groups of aldehydes and ketones to
give hemiacetal
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Biochemistry
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-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.
CH2OH
CH2OH
O
OH
OH
Dr Seemal Jelani
Biochemistry
O
O
C
OH
H
OH
OH
OH
-D-glucose
(36%)
H
O
OH
OH
CH2OH
D-glucose (open)
(trace)
Introduction to
49
OH
OH
OH
β-D-glucose
(64%)
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Intramolecular Cyclic hemiketal
Structure of Fructose
Fructose and other ketoses with a long
carbon chain also cyclizes to form hemiketal
D-fructose and D-ribose form a fivemembered ring
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Biochemistry
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Cyclic Structure of Fructose
Fructose
is a ketohexose.
forms a cyclic structure.
reacts the —OH on C-5 with the C=O on C-2.
CH2OH
C O
CH2OH
HO C H
CH2OH
OH
H C OH
H C OH
CH2OH
O
O
OH
OH
OH
CH2OH
OH
OH
α-D-fructose
-D-fructose
CH2OH
D-fructose
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Biochemistry
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Haworth Projection
This is two dimensional notation that specifies the 3dimensional structure of cyclic form of carbohydrate
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Biochemistry
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The D and L form of a monosaccharide is determined
by the position of the terminal CH2OH gp on the
highest-numbered ring carbon atom
In D-form, this group is positioned above the ring
In L-form the terminal CH2OH gp is positioned below
the ring (not encountered in biological systems)
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α and β configurations is determined by the position
of the –OH gp on carbon no 1 relative to the CH2OH
In β-configuration both of these gps point in the same
direction
In α-configuration the two gps point in opposites
direction
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Reactions of Monosaccharides
Oxidation
Reduction
Glycoside formation
Phosphate ester formation
Amino sugar formation
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Biochemistry
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Oxidation
Yield three different types of oxidation products
Oxidizing agent used to determined the product
Weak oxidizing Agent:
Tollen's Reagent
Benedict Solution
Reducing Sugars
Is a carbohydrate that gives a positive test with TR, FS
and BS
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Strong oxidizing agent oxidizes both end of
monosaccharide i.e
Terminal pri-alcohol and carbonyl group to give
dicarboxylic acid
Such polyhydroxy dicarboxylic acids are known as –
aric acids
The oxidation of glucose gives Glucaric acid
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Reduction
Carbonyl group present in a monosaccharide ( aldose,
ketose) can be reduced to a Hydroxyl group using
Hydrogen as a reducing group
Product is called Sugar Alcohol
D- Glucitol is also known as D-sorbitol
These are used as moisturizing agents in foods and
cosmetics because of their affinity for water
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Glycoside Formation
Cyclic forms of monosaccharides are hemiacetals and
hemiketals react with alcohols to form Acetals and
Ketals
The general name for monosaccharide acetals and
Ketals is Glycoside
Glycoside
It is an Acetal or a Ketal formed from a cyclic
monosaccharide
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Glycoside produced from glucose is Glycoside
Glycoside produced from galactose is Galactoside
Exist in α and β forms
Named as by listing alkyl or aryl group attached to the
oxygen followed by the name of a monosaccharide
involved with the suffix-ide
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Phosphate Ester formation
The –OH gp of a monosaccharide can react with
Oxoacids to form Esters
Phosphate esters are formed from phosphoric acid
and various monosaccharides commonly encountered
in biological system
Example
Specific enzymes in the body catalyze the
esterification of the carbonyl group (C1) and the
primary alcohol (C6) of glucose to give
Glucose-1-Phosphate
Glucose-6-Phosphate
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These phosphate esters of glucose are stable
in aqueous solution and play important roles
in the metabolism of carbohydrates
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Amino Sugar formation
Amino sugars of glucose, mannose and galactose are
common in nature
They are produced by replacing the –OH group on
carbon 2 on the monosaccharide with an amino group
Amino sugars and their N-acetyl-derivatives are
important building blocks of polysaccharides such as
cartilage
The N-acetyl derivatives of D-glucosamine and
D-galactosamine are present in the biochemical markers
on red blood cells, which distinguish the various blood
type
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Disaccharides
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Biochemistry
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Important Disaccharides
A disaccharide consists of two monosaccharides.
Monosaccharides
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Biochemistry
Disaccharide
glucose + glucose
maltose + H2O
glucose + galactose
lactose + H2O
glucose + fructose
sucrose + H2O
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Disaccharides
Monosaccharide + Monosaccharide
(Functioning as
(Functioning as
A hemiacetal or
an alcohol)
hemiketal)
Disaccharide + H2O
glycoside
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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.
The Glycosidic linkage between the two glucose units is called
(1-4) linkage
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Glycosidic Formation
Cyclic forms of monosaccharides are hemiacetals and
hemiketals, they react with alcohols to form acetals
and Ketals
The bond that links two monosaccharides of a
disaccharide together is called a Glycosidic linkage
A Glycosidic linkage is the carbon-Oxygen-carbon
bond that joins the two components of Glycoside
together
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Formation of Maltose
Free α-OH
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Cellobiose
Produced as an intermediate in the hydrolysis of the
polysaccharide cellulose
Contains two D-glucose monosaccharide units
Differ from maltose –must have a β- configuration
β- (1-4)Glycosidic linkage
Reducing sugar having three isomeric forms in Aq.
Solu and on hydrolysis produces two D-glucose
molecules
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Difference in maltose and
Cellobiose
Different in biological behaviors
Differences are related to stereochemistry of their
glycosidic linkages
Maltase enzyme which breaks glucose-glucose α (1-4)
linkage present in maltose is present in maltose is
found in human body and in yeast
That’s why maltose is easily digested by human body
and readily fermented by yeast
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Both the human body and yeast lacks enzyme
cellobiase which is needed to break the glucoseglucose β (1-4) glycosidic linkage of Cellobiose
Cellobiose cannot be digested by humans or
fermented by yeast
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Lactose
Lactose
is a disaccharide of β-Dgalactose and α- Dglucose.
contains a β -1,4glycosidic bond.
α-form
is found in milk nearly 49% and milk products.
α-form
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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-1,2-glycosidic bond.
α-D-glucose
β -D-fructose
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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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Learning Check
Identify the monosaccharides in each of the following:
A. lactose
(1) α-D-glucose
(2) β-D-fructose
(3) β-D-galactose
B. maltose
(1) α-D-glucose
(2) β-D-fructose
(3) β-D-galactose
C. sucrose
(1) α-D-glucose
(2) β-D-fructose
(3) β-D-galactose
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Polysaccharides
Copyright © 2005 by Pearson Education, Inc.
Publishing as Benjamin Cummings
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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.
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CH2OH
O
OH
OH
OH
OH
α-D-Glucose
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Structures of Amylose and Amylopectin
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Amylose
Amylose is
a polymer of α-Dglucose molecules.
linked by -1,4
glycosidic bonds.
a continuous
(unbranched) chain.
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Amylopectin
Amylopectin
is a polymer of α-Dglucose 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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Dextrins
Starches like amylose and amylopectin hydrolyze to
dextrins (smaller polysaccharides)
Contain 3-8 glucose units
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Glycogen
Glycogen
is the polysaccharide that
stores α-D-glucose in
muscle.
is similar to amylopectin,
but is more highly
branched.
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Cellulose
Cellulose
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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