When a few (around three to six) monosaccharides are joined, it is called an "oligosaccharide" ("oligo-" meaning "few"). These molecules tend to be used as markers and signals, as well as having some other uses. Many monosaccharides joined together form a polysaccharide. They can be joined together in one long linear chain, or they may be branched. Two of the most common polysaccharides are cellulose and glycogen, both consisting of repeating glucose monomers. "Cellulose" is an important structural component of plant's cell walls and "glycogen" is used as a form of energy storage in animals.