Carbohydrates are separated into three subunits: monosaccharides, polysaccharides and oligosaccharides. Monosaccharides are the smallest of the subunits and are made up of single sugar molecules. The most common monosaccharides are the sugars fructose and glucose, and these typically take on a ring-shaped structure.
Oligosaccharides are complex carbohydrate chains made up of two to twenty simple sugars joined together with a covalent bond. The most common oligosaccharide is the disaccharide, and common examples of this include sucrose, maltose and lactose. Polysaccharides are known for their ability to store energy and are made up of long chains of sugars. The most common polysaccharides are starch, glycogen, cellulose and chitin.