Polysaccharides (sometimes called glycans) are relatively complex carbohydrates.
They are polymers made up of many monosaccharides joined together by glycosidic linkages. They are therefore very large, often branched, molecules. They tend to be amorphous, insolubile in water, and have no sweet taste.
When all the constituent monosaccharides are of the same type they are termed homopolysaccharides; when more than one type of monosaccharide is present they are termed heteropolysaccharides.
Examples include storage polysaccharides such as starch and glycogen and structural polysaccharides such as cellulose.
Starches are polymers of glucose in which glucopyranose units are bonded by alpha-linkages. Amylose consists of a linear chain of several hundred glucose molecules. Amylopectine is a branched molecule made of several thousand of glucose units.
Starches are insoluble in water. They can be digested by hydrolysis catalyzed by enzymes called amylases, which can break the alpha-linkages. Humans and other animals have amylases, so they can digest starches. Potato, rice, wheat, and maize are major sources of starch in the human diet.
Glycogen is the storage form of glucose in animals. It is a branched polymer of glucose. Glycogen can be broken down to form substrates for respiration, through the process of glycogenolysis. This involves the breaking of most of the C-O-C bonds between the glucose molecules by the addition of a phosphate, rather than a water as in hydrolysis. This process yields phosphorylated glucose molecules, which can be metabolized with a savings of one ATP molecule.
The structural components of plants are formed primarily from cellulose. Wood is largely cellulose and lignin, while paper and cotton are nearly pure cellulose. Cellulose is a polymer made with repeated glucose units bonded together by beta-linkages. Humans and many other animals lack an enzyme to break the beta-linkages, so they do not digest cellulose. Certain animals can digest cellulose, because bacteria possesing the enzyme are present in their gut.
Acidic polysaccharides are a group of polysaccharides that contain carboxyl groups and/or sulfuric ester groups .