Polysaccharides, also called glycanos, differ each other in the identity of their recurring monosaccharides units, in the length of their chains, in the types of bonds linking the units, and in the degree of branching. About the identity of their recurring monosaccharides units, polysaccharides can be homopolysaccharides, or heteropolysaccharides.

Os homopolissacarídeos são aqueles constituídos por apenas uma unidade monomérica e são formas de armazenamento de monossacarídeos que servirão de reserva energética, como o amido e o glicogênio, ou ainda como elementos estruturais, tal qual é o caso da celulose na parede bacteriana ou o da quitina, componente do exoesqueleto de artrópodes.

Homopolysaccharides contain only a single type of monomeric unit. Some of them serve as a storage forms of monosaccharides used as fuels; starch and glycogen are homopolysaccharides of this type. Other homopolysaccharides such as cellulose and chitin serve as structural elements in plant cell walls and animal exoskeletons.

Starch contains two types of glucose polymers, amylose and amilopectina. The forms consists of long, unbranched chains of D-glucose units connected by a1?4 linkages. Amylopectin, however, is highly branched. The glycosidic linkages joining successive glucose residues in Amylopectin chains are a1?4, but the branch points, occurring every 24 to 30 residues, are a1?6 linkages.

Glycogen is the main storage polysaccharide of animal cells and is specially abundant in the liver, where it may constitute as much as 7% of the wet weight. Like amylopectin, glycogen is a polymer of a1?4 linked subunits of glucose, with a1?6 linked branches, but glycogen is more extensively branched (branches occur every 8 to 12 residues) and more compact than starch.

Cellulose constitutes much of the mass of wood, and cotton is almost pure cellulose. Because cellulose is a linear, unbranched homopolysaccharides of D-glucose units, it resembles amylose and the main chains of glycogen, but in cellulose, the glucose residues are linked by ß1?4 glycosidic bonds, This difference gives cellulose and amylose very different three-dimensional structures and physical properties.

Chitin is a linear homopolysaccharides composed of N-acetyl-D-glucosamine residues in ß linkage. The only chemical difference form cellulose is the replacement of a hydroxyl group at C-2 with an acetylated amino group.

Heteropolysaccharides contain two or more different kinds of anomeric units and provide extracellular support to cells, tissues or organs.

The rigid component of bacterial cell walls is a heteropolymer of alternating ß1?4 linked N-acetylglucosamine and N-acetylmuramic acid units. This cross linked peptidoglycan structure is degraded by the enzyme lysozyme, which hydrolyses the glycosidic bond between N-acetylglucosamine and N-acetylmuramic acid units, killing bacteria cells.

The heteropolysaccharides called glycosaminoglycans are a family of linear polymers composed of repeating disaccharide units. One of the two monosaccharides is always either N-acetylglucosamine or N-acetylgalactosamine; the other is in most case a uronic acid, usually glucuronic acid. In some glycosaminoglycans, one or more of the hydroxyls of the amino sugar is esterified with sulphate which gives a very high density of negative charge.

Glycosaminoglycans joined to proteins are called proteoglycans.