Polysaccharide - definition, examples, function and structure (2023)

Polysaccharide Definition

A polysaccharide is a large molecule made up of manymonosaccharides. Monosaccharides are simple sugars, like glucose. Special enzymes join these tiny monomers together to create large sugar polymers, or polysaccharides. A polysaccharide is also calledglycan. A polysaccharide can be ahomopolysaccharide, where all monosaccharides are the same, or oneheteropolysaccharidein which the monosaccharides vary. Depending on which monosaccharides are connected and which carbons of the monosaccharides are connected, polysaccharides take a variety of forms. A molecule with a linear chain of monosaccharides is called a linear polysaccharide, while a chain that has arms and loops 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 to 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 are normally folded and can contain many monosaccharides in a dense area. Also, because the side chains of monosaccharides form as many hydrogen bonds as possible with themselves, water cannot penetrate the molecules, making themhydrophobic. This property allows the molecules to stick together and not dissolve in the cytosol. This reduces the concentration of sugar in a cell and more sugar can be absorbed. Polysaccharides not only store energy, but also allow for changes in the concentration gradient, which can influence cellular uptake of nutrients and water.

cellular communication

Many polysaccharides are convertedglycoconjugateswhen covalently bound to proteins or lipids. Glycolipids and glycoproteins can be used to send signals between and within cells. Proteins destined for a specific organelle can be "tagged" by certain polysaccharides that help the cell move towards a specific organelle. Polysaccharides can be identified by special proteins, which then help attach the protein, vesicle, or other substance to a microtubule. The system of microtubules and associated proteins within cells can carry any substance to its destination location once tagged by specific polysaccharides. Additionally, multicellular organisms have immune systems that are 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 polysaccharides and different glycoproteins, it goes into action and destroys the invading cells.

Mobile support

By far one of the main functions of polysaccharides is that of support. All plants on Earth are sustained, in part, by the polysaccharidecellulose. Other organisms, such as insects and fungi, usechitinto support the extracellular matrix around your cells. A polysaccharide can be mixed with any number of other components to create stiffer fabrics, less stiff fabrics, or even materials with special properties. Between chitin and cellulose, both polysaccharides made from glucose monosaccharides, living organisms create hundreds of billions of tons each year. Everything from the wood of trees to the shells of sea creatures is produced by some form of polysaccharide. By simply rearranging the structure, polysaccharides can change from storage molecules to much stronger fibrous molecules. The ring structure of most monosaccharides aids in this process, as seen below.

Structure of a polysaccharide

All polysaccharides are formed by the same basic process: monosaccharides are connected viaglycosidic bonds. When in a polysaccharide, the individual monosaccharides are known aswaste. Below are just a few of the many monosaccharides created in nature. 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 its 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 give easy access to monosaccharides while maintaining a compact structure. A polysaccharide used as a scaffold is usually assembled as a long chain of monosaccharides, which acts as 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.

Glycosidic bonds between monosaccharides consist of an oxygen molecule linking two carbon rings. The bond is formed when a hydroxyl group is lost from the carbon of one molecule, while the hydroxyl group of another monosaccharide loses hydrogen. The carbon of the first molecule will replace the oxygen of the second molecule as its own and the glycosidic bond will be formed. As two hydrogen molecules and one oxygen molecule are expelled, the reaction also produces a water molecule. This type of reaction is calleddehydration reactionas water is removed from the reactants.

Examples of a polysaccharide

Cellulose and Chitin

Cellulose and chitin are structural polysaccharides consisting of many thousands of glucose monomers combined into long fibers. The only difference between the two polysaccharides is the side chains attached to the carbon rings of the monosaccharides. In chitin, the glucose monosaccharides were modified with a group containing more carbon, nitrogen, and oxygen. The side chain creates a dipole, which increases hydrogen bonding. While cellulose can produce hard structures like wood, chitin can produce even harder structures like bark, limestone, and even marble when compressed.

Both polysaccharides form long linear chains. These chains form long fibers, which lie outside the cell membrane. Certain proteins and other factors help the fibers intertwine into a complex shape, which is held in place by hydrogen bonding between the side chains. Therefore, simple glucose molecules that were previously used for energy storage can be converted into molecules with structural rigidity. The only difference between structural polysaccharides and storage polysaccharides is the monosaccharides used. By changing the configuration of glucose molecules, instead of a structural polysaccharide, the molecule will branch and store many more links in a smaller space. The only difference between cellulose and starch is the glucose configuration used.

glycogen and starch

Probably the most important storage polysaccharides on the planet, glycogen and starch, are produced by animals and plants, respectively. These polysaccharides form from a central starting point and spiral outward due to their complex branching patterns. With the help of various proteins that bind to individual polysaccharides, the large branched molecules are formed.granulesor groupings. This can be seen in the image below of the glycogen molecules and associated proteins seen in the middle.

When a glycogen or starch molecule is broken down, the responsible enzymes start at the ends furthest from the center. This is important as you will notice that due to the extensive branching, there are only 2 starting points but many endings. This means that the monosaccharides can be quickly extracted from the polysaccharide and used for energy. The only difference between starch and glycogen is the number of branches that are produced per molecule. This is caused by different parts of the monosaccharides forming bonds and different enzymes acting on the molecules. In glycogen, a branch occurs every 12 or more residues, while in starch a branch occurs only after every 30 residues.

• monosaccharide– The smallest unit of sugar molecules, or a sugar monomer.
• monomer– A single entity that can be combined to form a larger entity, or polymer.
• Polymer– Includes proteins, polysaccharides, and many other existing molecules of smaller combined units.
• Polypeptide– A polymer of amino acid monomers, also called a protein.

Proof

1. If you haven't brushed your teeth for a while, you may notice yellow plaque starting to form. Part of the plaque consists of dextrans, or polysaccharides, which are used by bacteria to store energy. Where do bacteria get the monosaccharides to create these polysaccharides?
A.They synthesize them from sunlight.
B.They create them from your genetic code.
C.They collect them from the food scraps you eat.

2. Plants produce both the amylose from starch and the structural polymer of cellulose from glucose units. Most animals cannot digest cellulose. Even ruminants, such as cattle, cannot digest cellulose and rely on internal symbiotic organisms to break down the cellulose bonds. However, all mammals produce amylase, an enzyme that can break down amylose. Why can't amylase break cellulose bonds?
A.Cellulose and amylose are structurally different, and amylase does not recognize cellulose.
B.Cellulose's glycosidic bonds are stronger.
C.The extracellular matrix created by cellulose cannot be broken down.

3. Hyaluronan is a molecule found in vertebrate joints that provides support by creating a gelatinous matrix to cushion the bones. Hyaluronan is created from several different monosaccharides linked together in long chains. Which of the following describes hyaluronan?
1. Homopolisacarideo
2. Saccharide heteropolis
3. Polymer
4. Monomer
A.All of them
B.1, 3
C.2, 3