Oligosaccharides and polysaccharides

1. Lecture 4
Disaccharides
Polysaccharides

2. Diasaccharides
 Disaccharides consists of two monosaccharide
units (similar or dissimilar) held together by a
glycosidic bond
 They are crystalline, water-soluble and sweet to
taste
 The diasaccharides are of two types
 Reducing diasacchrides with free aldehyde or
keto group e.g. Maltose, lactose
 Non reducing disaccharides with no free
aldehyde or keto group e.g. sucrose, trehalose

3. Maltose
 Composed of two α-D-glucose units held together
by α (1-4) glycosidic bond.
 Maltose can be hydrolysed by enzyme maltase to
liberate two molecules of α-D-glucose

4. Isomaltose
The glucose units are held together by α(1-6)
glycosidic linkage

5. Cellubiose
Is a disaccharide identical in structure with maltose except
that the former has β (1-4) glycosidic linkage.
It is formed during the hydrolysis of cellulose

6. Sucrose
 (Cane sugar) mostly produced by sugar cane and
sugar beets.
 It is made up of α-D-glucose and β-D-fructose.
 The two monosaccharides are held together by
glycosidic bond (α1-β2)
 It is an important source of dietary carbohydrate.
 It is sweeter than most other common sugars like
fructose glucose and maltose.

7.  It is employed as a sweetening agent in food
industry
 The intestinal enzyme- sucrase- hydolyses
sucrose to glucose and fructose which are
absorbed.

8. Lactose
 It is more commonly known as milk sugar
 It is composed of β-D-galactose and β-D-
glucose(β (1-4) glycosidic bond)

9. Polysaccharides
 Polysaccharides or simply glycans consists of
repeat units of monosaccharides or their
derivatives held together by glycosidic bonds
 They are primarily concerned with two important
functions- structural and storage of energy
 They are linear as well as branched polymers

10.  The occurrence of branches in polysaccharide is
due to the fact that the glycosidic linkages can be
formed at any one of the hydroxyl groups of
monosacchraides.

11. Homopolysaccharides
On hydrolysis yield only a single type of monosacchraide
e.g.Glucans are polymers of glucose
Fructosans are polymers of fructose

12. Heteropolysaccharides
 On hydrolysis yield a mixture of few
monosaccharides or their derivatives

13. Homopolysaccharides
Starch :
 It is the carbohydrate reserve of the plants which
is the most important dietary source for higher
animals
 High content is found in cereals, roots, tubers,
vegetables
 It is a homopolymer composed of D-glucose units
held together by α-glycosidic bonds (glucans or
glucosans)

14.  It consists of two poysaccharides components
 Water soluble amylose (15-20%)
 Water insoluble amylopectin (80-85%)
 Chemically amylose is a long unbranched chain
with 200-1000 glucose units held by α (1-4)
glycosidic linkages
 Amylopectin is a branched chain with α (1-6)
glycosidic bonds at the branching points and α (1-
4) linkages everywhere else.

16.  Starches are hydrolyzed by amylase to liberate
dextrins and finally maltose and glucose units
 Amylase acts specifically on α (1-4) glycosidic
bonds
Dextrins
 The breakdown products of starch by the enzyme
amylase or dilute acids
 Starch is sequentially hydrolyzed through different
dextrins and finally to maltose and glucose

17. Dextrans
 They are polymers of glucose produced by micro-
organisms.
 They are used as plasma volume expanders in
transfusion
Inulin
 Is a polymer of fructose i.e. fructosan
 Inulin is not utilized by the body
 Used for accessing kidney functions through
measurement of glomerular filtration rate (GFR)

18.  Occurs in a garlic, union
 Easily soluble in water
Glycogen
 The carbohydrate reserve in animals (animals
starch)
 Present in high concentration in liver, followed by
muscle, brain etc.
 Also found in plants that do not possess
chlorophyll (yeast, fungi)

19.  Structurally, glycogen is similar to that of
amylopectin with more number of branches
 Glucose is the repeating unit held together by α
(1-4) glycosidic bonds and α (1-6) glycosidic
bonds at branching points
 Number of glucose units (upto 25000) vary in
glycogen depending on the source from which
glycogin is obtained

21. Cellulose
 Occurs exclusively in plants
 Predominant constituent of plant cell-wall
 Totally absent in animal body
 Composed of β-D-Glucose units linked by β (1-4)
glycosidic bonds
 Cannot be digested by mammals due to lack of
enzymes
 Certain ruminants and herbivores animals contain
micro-organisms in the gut which enzymes which
can cleave the bond

22.  Hydrolysis of cellulose yields a disaccharide
cellobiose, followed by β-D-Glucose
 Cellulose is the major constituent of fiber, the
non-digestable carbohydrate
 The functions of dietary fibres include:
Decreasing the absorption of glucose and
cholesterol from the intestines besides increasing
the bulk of feces.

23. Chitin
 Composed of N-acetyl D-glucosamine units held
together by β (1-4) glycosidic bond
 It is a structural polysaccharide found in the
exoskeleton of some invertebrates, for example,
insects, crustaceans