Polysaccharides are large, complex carbohydrates formed by connecting many smaller sugar units. These intricate molecules play diverse roles within living organisms, from storing energy to providing structural support.
The Fundamental Building Blocks
The individual sugar units that make up polysaccharides are called monosaccharides, with glucose being a very common example. These monosaccharide units connect to each other through a specific type of covalent bond known as a glycosidic bond. This bond forms between the hydroxyl group of one sugar and the anomeric carbon of another sugar molecule, releasing a water molecule in the process.
The orientation of this glycosidic bond is important, as it can be either alpha (α) or beta (β). An alpha glycosidic bond forms when the hydroxyl group on the anomeric carbon is on the opposite side of the ring from the CH2OH group. Conversely, a beta glycosidic bond forms when the hydroxyl group is on the same side as the CH2OH group. This subtle difference in bond orientation influences the overall shape and properties of the resulting polysaccharide.
Arrangement of Monosaccharide Chains
Polysaccharide chains can exhibit two primary arrangements: linear or branched. A linear arrangement describes an unbranched sequence of monosaccharide units joined end-to-end, forming a straight or helical chain.
A branched arrangement, on the other hand, involves a main backbone chain with additional side chains extending outwards. Some monosaccharide units within the main chain form glycosidic bonds with three or more other units, creating points of branching.
How Structure Determines Function
The specific arrangement and bonding within a polysaccharide directly dictate its biological role, evident in molecules like starch and glycogen. Starch, a storage polysaccharide in plants, consists of two types of glucose polymers: amylose and amylopectin. Amylose is a linear chain of glucose units linked by alpha-1,4 glycosidic bonds, forming a helical structure. Amylopectin, however, is highly branched, featuring both alpha-1,4 linkages in its main chain and alpha-1,6 linkages at its branch points, occurring approximately every 24 to 30 glucose residues.
This branched structure of amylopectin and glycogen allows for multiple points where enzymes can rapidly break off glucose units, making them efficient forms of stored energy.
In contrast, structural polysaccharides like cellulose in plants and chitin in fungi and arthropods demonstrate how different linkages lead to rigidity. Cellulose is a linear polymer of glucose units joined by beta-1,4 glycosidic bonds. These beta linkages promote the formation of long, straight chains that can align parallel to each other. Numerous hydrogen bonds form between adjacent cellulose chains, creating strong microfibrils that provide tensile strength to plant cell walls.
Chitin, a structural component of fungal cell walls and the exoskeletons of insects and crustaceans, is a linear polymer of N-acetylglucosamine units. Like cellulose, chitin also utilizes beta-1,4 glycosidic bonds, which enable its chains to form strong, parallel arrangements. The extensive hydrogen bonding between these parallel chains creates a rigid material that offers protection and structural support.
Classifying by Monomer Composition
Polysaccharides can be classified based on the types of monosaccharide units they contain. Homopolysaccharides are composed of one type of monosaccharide monomer. For instance, starch, glycogen, and cellulose are all homopolysaccharides made up of glucose units. Chitin is also a homopolysaccharide, consisting of N-acetylglucosamine monomers.
Conversely, heteropolysaccharides are made up of two or more different types of monosaccharide units, which can include various combinations of sugars and their derivatives. An example of a heteropolysaccharide is hyaluronic acid, found in connective tissues, skin, and joint fluid. It is composed of repeating disaccharide units of D-glucuronic acid and N-acetylglucosamine, contributing to its viscous and lubricating properties.