DISACCHARIDES
When two monosaccharides are combined together by glycosidic linkage, a
disaccharide is formed.
The important disaccharides are
1. Sucrose
2. Maltose and isomaltose
3. Lactose.
Sucrose
It is the sweetening agent known as cane sugar. It is present in sugarcane and
various fruits.
Sucrose contains glucose and fructose.
Sucrose is not a reducing sugar; and it will not form osazone.
This is because the linkage involves first carbon of glucose and second
carbon of fructose
Free reducing groups are not available. So…
When sucrose is hydrolysed, the products have reducing action.
A sugar solution which is originally nonreducin
but becomes reducing after hydrolysis, is identified as sucrose (specific
sucrose test)
Sucrose hydrolysis is inversion
Hydrolysis of sucrose (optical rotation +66.5°) will produce one molecule of
glucose (+52.5°) and one molecule of fructose (–92°).
Therefore the products will change the dextrorotation to levorotation
the plane of rotation is inverted.
Equimolecular mixture of glucose and fructose thus formed is called invert
sugar.
The enzyme producing hydrolysis of sucrose is called sucrase or invertase.
Honey contains invert sugar. Invert sugar is sweeter than sucrose.
Lactose
It is the sugar present in milk. It is a reducing disaccharide.
On hydrolysis lactose produce glucose and galactose Beta glycosidic linkage is present in lactose.
The anomeric carbon atom of beta-galactose is attached to the 4th
hydroxyl group of glucose through beta-1,4 glycosidic linkage.
Lactose forms osazone which resemble "badminton ball" or "hedgehog" or
flower of "touch-me-not" plant.
Maltose
Maltose contains two glucose residues.
There is alpha-1,4 linkage, i.e.1st carbon atom of one glucose is combined
with 4th carbon of another glucose through alpha-glycosidic linkage.
It is a reducing disaccharide.
It forms petal shaped crystals of maltose-osazone.
Isomaltose
It contains 2 glucose units combined in alpha -1, 6 linkage.
Thus first carbon of one glucose residue is attached to the sixth carbon of
another glucose through a glycosidic linkage.
It is a reducing sugar
Partial hydrolysis of glycogen and starch produces isomaltose.
POLYSACCHARIDES
These are polymerized products of many monosaccharide units. They may be
1. Homoglycans are composed of single kind of monosaccharides, e.g. starch,
glycogen and cellulose.
2. Heteroglycans are composed of two or more different monosaccharides, e.g.
hyaluronic acid, chondroitin sulphate.
Homopolysaccharides / homoglycanes
1. Starch
A. Structure of Starch
It is the reserve carbohydrate of plant kingdom.
Sources: Potatoes, tapioca, cereals (rice, wheat) and other food grains.
Starch is composed of amylose and amylopectin When starch is treated with boiling water, 10-20% is solubilized; this part is
called amylose.
Amylose is made up of glucose units with alpha-1,4 glycosidic linkages
The insoluble part absorbs water and forms paste like gel; this is called
amylopectin.
Amylopectin is also made up of glucose units, but is highly branched
The branching points are made by alpha-1,6 linkage.
B. Hydrolysis of Starch
Starch is non reducing.
When starch is hydrolysed , It produce smaller and smaller fragments are
produced.
Thus hydrolysis produces amylodextrin .
Further hydrolysis produces erythrodextrin ,Than achrodextrins
Further it produce maltose and then glucose.
Digestion
Salivary amylase and pancreatic amylase are alpha-amylases
They act on alpha- 1,4 glycosidic bonds to split starch into smaller units
(dextrins), and finally to alpha-maltose.
2. Glycogen
It is the reserve carbohydrate in animals.
It is stored in liver and muscle.
Excess carbohydrates are deposited as glycogen.
Glycogen is composed of glucose units joined by alpha-1,4 links in the
straight chains.
It also has alpha-1,6 glycosidic linkages at the branching points
Glycogen is more branched and more compact than amylopectin.
3. Cellulose
It is the supporting tissues of plants.
Cellulose is the most abundant organic material in nature.
It is made up of glucose units combined with beta-1,4 linkages.
It has a straight line structure, with no branching points.
Beta-1,4 bridges are hydrolysed by the enzyme cellobiase.
But this enzyme is absent in animal and human digestive system, and hence
cellulose cannot be digested.
Herbivorous animals have large caecum, which harbor bacteria.
These bacteria can hydrolyse cellulose,and the glucose produced is utilized
by the animal.
4. Inulin
It is a long chain homoglycan composed of D-fructose units with repeating beta-
1,2 linkages.
It is clinically used to find renal clearance value and glomerular filtration
rate.
5. Dextrans
These are highly branched homopolymers of glucose units with 1-6, 1-4 and
1-3 linkages.
They are produced by microorganisms.
Since they will not easily go out of vascular compartment,
They are used for intravenous infusion as plasma volume expander for
treatment of hypovolemic shock
6. Chitin
It is present in exoskeletons of insects.
It is composed of units of N-acetylglucosamine with beta-1,4 glycosidic
linkages.
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