Chapter I Introduction

Carbohydrates are naturally occurring and well distributed, and are the most important building blocks of the biosphere. These evolutionary and biologically important organic compounds are present on Earth in different forms. Traditionally, on the basis of the number of sugar units, carbohydrates are classified into three groups: monosaccharides, oligosaccharides, and polysaccharides. The natural macromolecules composed of several monosaccharide units (more than ten) are known as polysaccharides and are synthesized at different stages of life cycle of every living organisms for different purposes. The monosaccharide units of polysaccharides are joined to each other by an acetal linkage. These acetal linkages are formed by the reaction of a hemiacetal hydroxyl group of one unit with an alcohol group of another unit which liberates water to give a glycosidic bond. Polysaccharides not only have different sequences of monomeric units but also have different sequences of glycosidic linkages and different types of branching. They may be amorphous or even insoluble in water. All these factors together give polysaccharides a great diversity of structure, property, and functions.