Polysaccharides: Definition, Functions, Structure, Energy Storage and Cellular Communication

It is a large molecule made of many smaller monosaccharides. Monosaccharides are simple sugars, like glucose.

Special enzymes bind these small monomers together creating large sugar or polysaccharide polymers.

A polysaccharide is also called a glucan . A polysaccharide can be a homopolysaccharide, in which all monosaccharides are the same, or a heteropolysaccharide in which the monosaccharides vary.

Depending on which monosaccharides are connected, and which carbons in monosaccharides are connected, polysaccharides take a variety of forms.

A molecule with a straight chain of monosaccharides is called a linear polysaccharide, while a chain that has arms and turns is known as a branched polysaccharide.

Functions of a polysaccharide

Depending on their structure, polysaccharides can have a wide variety of functions in nature. Some polysaccharides are used to store energy, some to send cellular messages, and others to provide support for cells and tissues.

Energy storage

Many polysaccharides are used to store energy in organisms.

While energy-producing enzymes only work on monosaccharides stored in a polysaccharide, polysaccharides generally fold and can contain many monosaccharides in a dense area.

Furthermore, since the side chains of monosaccharides form as many hydrogen bonds as possible with themselves, water cannot intrude on the molecules, making them hydrophobic.

This property allows the molecules to stay together and not dissolve in the cytosol. This reduces the concentration of sugar in a cell and it can absorb more sugar.

Polysaccharides not only store energy, but allow changes in the concentration gradient, which can influence cellular absorption of nutrients and water.

Cellular communication

Many polysaccharides become glycoconjugates when they are covalently attached to proteins or lipids.

Glycolipids and glycoproteins can be used to send signals between and within cells.

Proteins that target a specific organelle can be “tagged” by certain polysaccharides that help the cell move to a specific organelle.

Polysaccharides can be identified by special proteins, which then help bind the protein, vesicle, or other substance to a microtubule.

The system of microtubules and associated proteins within cells can bring any substance to its intended location once tagged by specific polysaccharides.

Furthermore, multicellular organisms have immune systems driven by the recognition of glycoproteins on the surface of cells.

The cells of a single organism will produce specific polysaccharides to adorn its cells. When the immune system recognizes other different polysaccharides and glycoproteins, it goes into action and destroys the invading cells.

Cell support

By far one of the most important roles of polysaccharides is that of support. All plants on earth are compatible, in part, with polysaccharide cellulose.

Other organisms, such as insects and fungi, use chitin to support the extracellular matrix around their cells.

A polysaccharide can be mixed with any number of other components to create stiffer, less rigid fabrics or even materials with special properties.

Between chitin and cellulose, both polysaccharides made up of glucose monosaccharides, living organisms create hundreds of billions of tons each year.

Everything from the wood in trees to the shells of sea creatures is produced by some kind of polysaccharide.

By simply rearranging the structure, polysaccharides can go from storage molecules to much stronger fibrous molecules. The ring structure of most monosaccharides helps this process, as seen below.

Structure of a polysaccharide

All polysaccharides are made up of the same basic process: monosaccharides are connected through glycosidic bonds.

When they are in a polysaccharide, the individual monosaccharides are known as residues. Depending on the polysaccharide, any combination of them can be combined in series.

The structure of the molecules that are combined determines the structures and properties of the resulting polysaccharide.

The complex interaction between their hydroxyl (OH) groups, other side groups, the configurations of the molecules, and the enzymes involved affect the resulting polysaccharide produced.

A polysaccharide used for energy storage will provide easy access to monosaccharides while maintaining a compact structure.

A polysaccharide used as a support is generally assembled as a long chain of monosaccharides, which acts like a fiber.

Many fibers together produce hydrogen bonds between the fibers that strengthen the overall structure of the material, as seen in the image below.

The glycosidic bonds between monosaccharides consist of an oxygen molecule that bridges two carbon rings.

The bond is formed when a hydroxyl group is lost from the carbon of one molecule, while hydrogen is lost to the hydroxyl group of another monosaccharide.

The carbon in the first molecule will replace the oxygen in the second molecule as its own and a glycosidic bond is formed.

Because two hydrogen molecules and one oxygen are expelled, the reaction also produced a water molecule. This type of reaction is called a dehydration reaction as the water is removed from the reagents.