Oligosaccharides (oligo = a few) are carbohydrates formed by condensation products of 2-9 monosaccharides.
The monosaccharides are joined by glycolsidic bonds, which are formed when hydroxyl groups attached to two adjacent monosaccharides react and eliminate a molecule of water. This is a condensation reaction forming C-O-C bond.
The oligosaccharides can be disaccharide (when two monosaccharides are involved, disaccharides (when three mono saccharine are involved), tetrasaccharides and so
Polysaccharides, commonly called as the glycans/ polyholosides/ polyosides, are polymers of many (more than 9) monosaccharides or their derivatives. When a polysaccharide contains a single type of monosaccharides it is called homopolysaccharide and when it contains different types of monosaccharides it is called heteropolysaccharide.
Polysaccharides can be branched or unbranched and may be structural polysaccharides or storage polysaccharides. Some important polysaccharides are:
It is a polymer of a-D-glucose and is a homopolysaccharide. It is a storage polysaccharide and is the most important reserve food materials of higher plants. Natural starch consists of two components: amylose and amylopectin or isoamylose. Amylose (15-20%) is a long unbranched straight chain of a-Dglucose and amylopectin (80-85%) is a branched chain polysaccharide of a-D.glucose.
It is the major reserve food in animals and is often referred to as animal starch. It is a branched chain polymer of a-D-glucose like starch and resembles the amylopection. But it has more glucose units per molecule and is about 1.5 times more branched. The chains are also shorter than amylopectin and, therefore, molecule is more compact.
It is the most abundant bio molecule in the biosphere as it is the component of cell-wall of higher plants. It is a structural homo polysaccharide composed of b- D-glucose.
It is probably the second most abundant polysaccharide in the nature. It is a structural homopolysaccharide found in fungal cell wall and exoskeleton of arthropods. It is often known as fungus cellulose. It is polymer of nitrogen containing glucose derivative known as X-acetyl glucosamine.
It is a storage polysaccharide in the members of family Composite. It is stored in the tubers and roots of the plants. It is also found in onion and garlic. It is a polymer of b-D-fructose.
6. Pectin –
It is a structural homopolysaccharide of D-galacturonic acid. It is found as intercellular substances of plant tissues and is abundant in ripe fruits like guava, apples and pears.
Xylans, xyloglucans and other related substances are collectively called hemi cellulose. They are not structurally related to cellulose. The hemi cellulose can either be homo polymers of pentoses (xylose, arabinose) or hexoses (mannose, galactose) as well as sugar acids.
The polymer of xylose (xylans) is considered as the representative of hemicellulose and is the next most abundant polysaccharide along with chitin. Hemicelluloses are structural components of plant cell walls.
The polysaccharides forming slimy substances or mucilage and composed of mixture of simple sugars and their derivatives such as amino sugars and uronic sugars are called as mucopolysaccharides. Hyaluronic acid is the most abundant member of mucopolysaccharides.