(glycan) A high-molecular-weight polymer of monosaccharides or monosaccharide derivatives. The major functions of polysaccharides are as energy storage molecules (reserve polysaccharides) or as structural elements in cell walls and intercellular spaces (structural polysaccharides). *Starch
are the most abundant plant polysaccharides.
Glucose is the most commonly occurring monosaccharide residue in polysaccharides; both starch and cellulose are made up exclusively of glucose subunits. Other sugars important in polysaccharides include galactose, man-nose, fructose, xylose, and glucuronic and galacturonic acids. Polysaccharides differ in the nature of their monosaccharide units, in the types of bonding between units, in chain length, and in degree of chain branching. Polysaccharides containing only one type of sugar residue are known as homopolysaccharides; these include glucans (glucose polymers), *mannans
(mannose polymers), *galactans
, and fructans. Heteropolysaccharides contain two or more different monosaccharides; examples are the *hemicelluloses and some *pectic substances