Although plant and animal cells make a large number of different polysaccharides, for all kinds of roles, the dominant ones are those made from glucose.
Cellulose is a polymer of glucose monosaccharides that plants use as their primary building material. Threads of cellulose are bound by hydrogen bonds into bundles of great strength and flexibility. These are used by plants to surround each cell in a way that protects them from the effects of osmosis and also gives them shape and form.
Each plant cell wall, however, is more than just an inert box. About 0.5 micometers thick, it is a complex of pure cellulose (40% to 60%), a similar polysaccharide made of pentose sugars, and a special bonding agent called lignin. As the cells grow, expand, shrink or change their shape, the wall is adapted and modified accordingly, and when the cell divides, a new wall is formed between the daughter cells.
A cellulose-like material, called chitin, is used by insects and arthropods to stiffen and give form to their outer exoskeleton, and other complex polysaccharides are used in animals in places where tensile strength is needed.
Starch is a polymer of the alternate anomer of glucose and is used by plants as a way of storing glucose. It is a major reserve of energy that can be quickly mobilized as necessary.
Most plants cells have stored starch reserves in the form of tiny granules. Within these granules are two kinds of starch; amylose and amylopectin, which differ from one another in the amount of branching taking place in the molecule.
Many plants also have specialized regions of starch storage in which parenchymatous cells process and package starch molecules for long-term use. Tubers, such as potatoes, and seeds with their valuable embryos, are both plant structures with high concentrations of stored starch.
Mobile animals, such as humans, need energy reserves in much the same way. A small amount of these reserves is in the form of an amylopectin-like molecule called glycogen, which is found in the liver and some muscles. However, carbohydrates like starch or glycogen only produce about 4 kilocalories of energy per gram of weight, about the same as that for protein.
While this kind of efficiency is fine for plants (which don't have to move), it is not enough for animals with their higher metabolic needs. Lipids store about 9 kilocalories of energy per gram, almost twice that of carbohydrates, so they are the preferred fuel in the animal body.