Disaccharides
They are formed when 2 monosaccharides, usually hexoses, combine by means of a chemical reaction known as a condensation. This means removal of water.
The bond formed between 2 monosaccharides as a result
of condensation is called a glycosidic bond and it normally forms
between carbon atoms 1 and 4 of neighboring units (a 1,4 bond or
1,4 linkage). The process can be repeated many times to build up
the giant molecules of polysaccharides. The monosaccharide units
are called residues once they have been linked. Thus a maltose
molecule contains 2 glucose residues.
The most common disaccharides are maltose, lactose and sucrose:
maltose = a-D-glucose + a-D-glucose
lactose = b-D-glucose + b-D-galactose
sucrose = a-D-glucose + b-D-fructose
Disaccharides are often used by plants or animals to transport monosaccharides from 1 cell to another. The monosaccharides and disaccharides generally have names ending in –ose. These water-soluble carbohydrates, which have a characteristically sweet taste, are also called sugars.
1. Sucrose
Sucrose, or cane sugar, is the most
abundant disaccharide in nature. It is most commonly found in
plants, where it is transported in large quantities through
phloem tissue. It makes a good transport sugar because it is very
soluble, and can therefore be moved efficiently in high
concentrations. It is also relatively unreactive chemically. This
means it tends not to enter into general metabolism on its way
from 1 place to another. It is sometimes stored for the same
reasons. It is obtained commercially form sugar cane and sugar
beet and is the ‘sugar’ in shops.
Structure
Non-reducing sugar, because of no free carboxyl group, sucrose consists of an a-D-glucopyranose unit and a b-D-fructofuranose unit. An oxygen bridge from carbon 1 on glucose to carbon 2 on fructose joins these monosaccharides--- that is, by an a-1,2-glycosidic linkage.
2.Lactose
Lactose, or milk sugar, is found exclusively in milk and is an important energy source for young mammals. It can only be digested slowly, so gives a slow steady release of energy.
Structure
Common lactose consists of a b-D-galactopyranse unit linked to an a-D-glucopyranose unit. These are joined by a b-1,4-glycosidic linkage from carbon 1 on galactose to carbon 4 on glucose. The more systematic name for lactose is b-D-galactopyranosyl-(1-4)-a-D-glucopyranose. Although glycosidic bonds are straight carbon-oxygen linkages the structural formula represents them in a bent fashion to provide stereochemical information. Carbon 1 of the galactose is in a b configuration, so its bent bond initially up. The oxygen on carbon 4 of the glucose is below the carbon in the Haworth formula, and thus the bent bond initially points down.
3. Maltose
Maltose occurs mainly as a breakdown product during digestion of starch by enzymes called amylase. This commonly occurs in animals and in germinating seeds. The latter is made use of in brewing beer when barley grain is used as the source of starch. Germination of the barley is stimulated and this results in the conversion of the starch to maltose, a process known as malting. The maltose is then fermented by yeast to alcohol. This involves conversion of maltose to glucose by the action of the enzyme maltase, a process that also occurs in animals during digestion.
Structure
The structure in maltose may be considered as being derived from 2 glucose molecules by the elimination of a molecule of water between the –OH group on carbon 1 of 1 glucose unit and the –OH group of carbon 4 on the other glucose unit. This is an a-1,4-glycosidic linkage, since the glucose units have the a configuration and are joined at carbons 1 and 4. In a more systematic nomenclature, this form of maltose is known as a-D-glucopyranosyl-(1,4)-a-D-glucopyranose. If the structure of glucose is known, this name provides a complete description for drawing the formula.
4. Property
5. Function