The term ‘monosaccharides’ is employed for such sugars that on hydrolysis yield no further, lower sugars. The general formula of monosaccharides is Cn H2n On. The monosaccharides are subdivided as bioses, trioses, tetroses, pentoses, hexoses, heptoses, depending upon the number of carbon atoms they possess.
CLASSIFICATION
Monosaccharides
The term ‘monosaccharides’ is employed for such sugars that
on hydrolysis yield no further, lower sugars. The general formula of
monosaccharides is Cn H2n On. The monosaccharides are subdivided as bioses,
trioses, tetroses, pentoses, hexoses, heptoses, depending upon the number of
carbon atoms they possess.
Bioses
They contain two carbon atoms. They do not occur free in
nature.
Trioses
They contain three carbon atoms, but in the form of
phosphoric esters, for example, glyceraldehydes.
Tetroses
They contain four carbon atoms, for example, erythrose,
threose, etc.
Pentoses
They are very common in plants and are the products of hydrolysis
of polysaccharides like hamicelluloses, mucilages and gums, for example,
ribose, arabinose and xylose.
Hexoses
They are monosaccharides containing six carbon atoms and are
abundantly available carbohydrates of plant kingdom. They are further divided
into two types: aldoses and ketoses. They may be obtained by hydrolysis of
polysaccharides like starch, insulin, etc.
Depending upon the type of product of hydrolysis these are
further classified as Pentosans and Hexosans. Xylan is pentosan, whereas
starch, insulin and cellulose are the examples of hexosans.
Cellulose is composed of glucose units joined by β-1, 4
linkages, whereas starch contains glucose units connected with α- 1, 4 and α-
1, 6 units. Polyuronides, gums and
Aldoses : Glucose, mannose, galactose
Ketoses : Fructose and sorbose
Heptoses
They contain seven carbon atoms, vitally important in the
photosynthesis of plant and glucose metabolism of animals and are rarely found
accumulated in plants, for example, glucoheptose and manoheptose.
Disaccharides
Carbohydrates, which upon hydrolysis yield two molecules of
monosaccharides, are called as disaccharides.
Trisaccharides
As the name indicates, these liberate three molecules of
monosaccharides on hydrolysis.
Tetrasaccharides
Stachyose, a tetrasaccharide, yields on hydrolysis, four
molecules of monosaccharide, found in manna.
Polysaccharides
On hydrolysis they give an indefinite number of
monosaccharides. By condensation, with the elimination of water,
polysaccharides are produced from monosaccharides.
Depending upon the type of product of hydrolysis these are
further classified as Pentosans and Hexosans. Xylan is pentosan, whereas
starch, insulin and cellulose are the examples of hexosans.
Cellulose is composed of glucose units joined by β-1, 4 linkages, whereas starch contains glucose units
connected with α- 1, 4 and α- 1, 6 units. Polyuronides, gums and mucilages are the other
pharmaceutically important poly-saccharide derivatives.
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