The homosaccharides contain saym; type of moromeres in their molecules, i.e., they yield, on hydrolysis a tingle monosaccharide constituents.
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Some of the important homo- polysacchftiides are the following:
(i) Starch:
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It is a complex substance which is formed by condensation of amylose and autylopecfin which is a branched polysaccharide with shorter chains. It is found in abundance in plants, seeds, fruits and tubers.
It is an amorphous white powder which forms descent solutions in water on boiling. With iodine starch forms an adsorption compound of blue iodide of starch, the colour of which disappears on heating but appears on cooling.
Amylases or diastases hydrolyse starch but these enzymes are themselves complexes with members specific for the various linkages within the starch.
The first product of hydrolysis is maltose and the final product is glucose.
(ii) Insulin:
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It is a storage polysaccharide found in the tubers of dahlia, artichoke, etc.
It is an inert substance when injected into the body and is composed of large numbers of fructo- furanose molecules condensed together in chains.
(iii) Dextrin:
It forms hazy solution in water which when concentrated becomes thick and adhesive. It gives a port-wine colour with iodine.
It is hydrolysed with the help of dilute acids. The first product of hydrolysis is maltose and the final product is glucose.
(iv) Glycogen:
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It is main carbohydrate storage substance of animals and fungi (Haworth). It is made up of molecules rather like amylopectin but with more numerous side chains.
Physically and chemically it resembles with dextrin. It forms opalescent solutions in water as dextrin and gives brown colour on treating with iodine. It is also hydrolysed by the dilute acids into glucose.
(v) Cellulose:
It makes the cell wall of plants and the external covers of all grains also. It is made up of long chains of β-glycosides as in cellobiose and it may contain many thousands of monosaccharides to a single molecule.
These long chains may be further strengthened by hydrogen bonding from one chain to another.
It is absolutely insoluble in water and resistant to the action of dilute acids or alkalis. Strong acids would hydrolyse it into glucose.
It is also hydrolysed by cellulose enzyme into cellobiose and eventually to glucose.
(vi) Lignin:
It is a structural chemical forming the wood of plants. Unlike cellulose and other carbohydrates it is made up of aromatic units which form long chains with cross linkages. It is thought that the original units themselves synthesized from monosaccharides. Lignin is very important in the support of woody plants.