2.2.2—Biological molecule terminology

A monosaccharide is one sugar unit. A disaccharide contains two monosaccharide units joined together. A polysaccharide contains many monosaccharide units joined into a long carbohydrate chain. These are carbohydrate categories defined by how many sugar units are present, not by a memorised example name.

• Monomer: a small subunit that can join to other subunits. A monosaccharide can act as the monomer of a carbohydrate polymer.
• Polymer: a large molecule made from many repeating monomer units. A polysaccharide is a carbohydrate polymer; a disaccharide is not a long polymer.
• Macromolecule: a very large biological molecule. Some macromolecules are polymers, but the terms are not interchangeable.
• Examples: glucose, fructose and galactose are monosaccharides; sucrose and maltose are disaccharides; starch, glycogen and cellulose are polysaccharides.

When sugar units join, covalent glycosidic bonds form and condensation removes water. Hydrolysis uses water to break those bonds, so a disaccharide can yield two monosaccharides and a polysaccharide can be broken into smaller sugar units. The category therefore describes the current level of joining, not an irreversible label.

Do not define a monosaccharide, disaccharide or polysaccharide only by listing examples. Do not call every macromolecule a polymer: a polymer must be built from repeating subunits, whereas macromolecule only describes very large size.