Polysaccharides

Polysaccharides are large carbohydrates composed of long chains of monosaccharide units linked by glycosidic bonds. They vary in monomer composition, linkage type (alpha or beta), degree of polymerization, and branching. Their properties reflect these features and influence their biological roles.

They are broadly categorized into storage polysaccharides, which provide energy, and structural polysaccharides, which contribute to cell shape and integrity. In plants, starch stores glucose as amylose and amylopectin; in animals and fungi, glycogen serves a similar energy reservoir. Structural examples include cellulose in plant cell walls; chitin in arthropod exoskeletons and fungal cell walls; and peptidoglycan in bacterial cell walls. Other polysaccharides, such as pectin, hemicellulose, agar, and carrageenan, are important components of plant cell walls or are used as gelling agents in industry.

Polysaccharides can be linear or highly branched. Their solubility and physical behavior depend on their molecular weight, branching, and the nature of the glycosidic linkage. In humans, only some polysaccharides are digestible: starch and glycogen yield glucose, whereas cellulose and many fibers resist mammalian digestion but may be fermented by gut microbes.

Biosynthesis occurs via enzyme-catalyzed polymerization of activated sugar nucleotides, often in different cellular compartments. Industrially, polysaccharides are valued as thickeners, stabilizers, gelling agents, and biomaterials, with applications ranging from food additives to pharmaceuticals.