Oligosaccharides
Oligosaccharides are small carbohydrates which are formed by condensation of 2-9 monosaccharides. Depending upon the
number of monosaccharide molecules condensed to form oligosaccharides, the latter are known as disaccharides (e.g., sucrose,
lactose), trisaccharides (e.g., raffinose), tetrasaccharides
(e.g., stachyose) etc. The smallest and the commonest
oligosaccharides are disaccharides.
Disaccharides
Disaccharides are condensation products of two monosac-charide
units. In disaccharide, monosaccharide units (similar or dissimilar)
are held together by a glycosidic bond. It is normally formed
between carbon atoms 1 and 4 of the neighbouring units (α – 1,
4 bond). The monosaccharide units when linked up, are called
residues.
Examples of disaccharides:
(i) Sucrose is the commercial sugar, also called cane sugar
because most of it is obtained from sugarcane (Saccharum
officinarum). Sucrose is formed by condensation of one molecule
each of glucose and fructose. Glycosidic bond is established
between carbon atom 1 of glucose and carbon atom 2 of fructose
(α-1, 2 linkage) and a molecule of water is released. Sucrose
is a non-reducing sugar.
(ii) Maltose or malt sugar is a reducing sugar which is
formed by condensation of two molecules of glucose. Glycosidic
bond is established between carbon 1 of one glucose molecule
and carbon 4 of second glucose molecule (α-1, 4 linkage). It
is found in malted grains in barley, germinating seeds and tissues
where starch is broken down.
(iii) Lactose or milk sugar is a reducing sugar which is formed
inside mammary glands by β -1, 4 condensation of two hexose
molecules, glucose and galactose.
Table: Differences between Reducing and Non-reducing Sugars
Reducing Sugars Nonreducing Sugars
(i) The sugars have a free aldehyde (–CHO) or ketonic (–CO) group. A free aldehyde or ketonic group is absent.
(ii) The sugars can reduce cupric ions (Cu2+)of Benedict’s or Fehling’s
solution to cuprous ions (Cu+), during which blue colour of copper
sulphate changes to brick red.They do not reduce cupric ions of Benedict’s or Fehling’s
solution to cuprous ions.Pentose; arabinose and xylose are components of hemicellulose.Sucrose serves as reserve food for sugarcane and sugarbeet.Glucose is the main respiratory substrate.Trioses, glyceraldehyde and dihydroxy acetone are intermediates of
both respiratory and photosynthetic pathways.
Oligosaccharides are important for cell recognition, cell attachment,
receptor molecules.Functions of Small CarbohydratesPolysaccharides
Polysaccharides are condensation products of more than ten monosaccharide units; such as starch and dextrin, which may be linear
or branched polymers. Polysaccharides are also called glycans. They are long chained which may be branched or unbranched.
Depending upon the composition, polysaccharides are of two types, homopolysaccharides and heteropolysaccharides.