BIOMOLECULES
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Classification of Carbohydrates
& Glucose - Preparation and
Structure
Carbohydrates are called saccharides.
Classification
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Classification of Monosaccharides
■ Monosaccharides are classified based on the
number of carbon atoms and the functional
group present in them.
■ Different types of monosaccharides arelisted in
the given table
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Carbon atom General term aldehyde ketone
3 Triose Aldotriose Ketotriose
4 Tetrose Aldotetrose Ketotetrose
5 Pentose Aldopentose Ketopentose
6 Hexose Aldohexose Ketohexose
7 Heptose Aldoheptose Ketoheptose
3 Triose Aldotriose Ketotriose
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Glucose
method of preparation
■ Preparation of glucose
■ By boiling sucrose with dilute HCl or H2SO4 in
alcoholic solution
■ By boiling starch with dilute H2SO4, at 393 K,
under pressure
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■ Structure
■ Glucose has been assigned the above structure
based on the following evidences.
■ (i) Molecular formula − C6H12O6
(ii) Suggestion of straight chain
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■ iii) Confirmation of carbonyl (> C = O) group
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■ (iv) Confirmation of the presence of carbonyl
group as aldehydic group
■ (v) Confirmation of the presence of five −OH
groups
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■ (vi) Indication of the presence of a primary
alcohol
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■ The correct configuration of glucose is given
by
■ Glucose is correctly named as D (+) − Glucose
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Cyclic Structure of Glucose
■ The following reactions of glucose cannot be
explained by its open-chain structure.
■ Aldehydes give 2, 4-DNP test, Schiff’s test, and
react with NaHSO4 to form the hydrogen sulphite
addition product. However, glucose does not
undergo these reactions.
■ The penta-acetate of glucose does not react with
hydroxylamine. This indicates that a free −CHO
group is absent from glucose.
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■ Glucose exists in two crystalline forms, a and
β.
The a-form (m.p = 419 K) crystallises from a
concentrated solution of glucose at 303 K and
the β-form (m.p = 423 K) crystallises from a
hot and saturated aqueous solution at 371 K.
This behaviour cannot be explained by the
open-chain structure of glucose
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● Glucose exists in two cyclic forms, which
exist in equilibrium with the open- chain
structure.
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■ Representation of the cyclic structure of
glucose by Haworth structure:
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Structure of Fructose, Disaccharides &
Polysaccharides
■ Structure of Fructose
■ Open-chain structure:
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■ Cyclic structure:
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■ Representation of the structure of fructose by
Haworth structures
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Disaccharides
The monosaccharide units are
joined together by an oxide
linkage called glycosidic linkage.
It is formed by the loss of a water
molecule.
Sucrose
Sucrose is a non reducing sugar
■ Reason : A glycosidic linkage
exists between C1 of α-glucose
and C2 of β-fructose. So the
reducing groups of glucose and
fructose are involved in
glycosidic bond formation.
What is invert
sugar?
■Sucrose is dextrorotatory.
■As the product of hydrolysis of
sucrose is laevorotatory, it is called
invert sugar.
{This is because the laevorotation of
fructose (–92.4°) is more than
dextrorotation of glucose (+ 52.5)}
Maltose
■ Is a reducing sugar
■ is composed of two
α-D-glucose units in which C1
of one glucose (I) is linked to
C4 of another glucose unit
(II).
Lactose
■ known as milk sugar
■ is composed of β-D-galactose and
β-D-glucose.
■ The linkage is between C1 of
galactose and C4 of glucose.
■ is a reducing sugar.
Polysaccharides
Starch
■ main storage polysaccharide of
plants
■ is a polymer of α-glucose and
consists of two components—
Amylose and Amylopectin.
DIFFERENCE
■ Amylose is water ■ Amylopectin is
soluble component insoluble in water
■ constitutes about ■ constitutes about
15-20% of starch. 80- 85% of starch
■ is a long ■ is a branched chain
unbranched chain polymer of
with 200-1000 α-D-glucose units
α-D-(+)-glucose with C1–C4
units held by C1 - glycosidic linkage
C4 glycosidic
linkage.
Cellulose
■ occurs abundantly in plants
■ is a straight chain
polysaccharide composed of
β-D-glucose units
■ The monosaccharide units are
joined by C1 - C4 glycosidic
linkage.
Glycogen
■ carbohydrate stored in animal body
■ also known as animal starch as its
structure is similar to amylopectin
■ is present in liver, muscles and brain.
■ When the body needs glucose, enzymes
break the glycogen down to glucose.
