Polysaccharides are long-chain polymers of monosaccharide units, joined together by glycosidic linkages. They are also known as glycans. They are the most abundant carbohydrates found in food.
Carbohydrates are vital macromolecules required for the essential functions of organisms. They are classified into the following three classes depending on two criteria, whether they undergo hydrolysis, and if they do, the number of products they form.
They are composed of a single unit of polyhydroxy aldehyde and ketone. These are simple sugars with general chemical formula (CH2O)n.
Examples of monosaccharides include glucose, fructose, galactose, ribose, deoxyribose, etc. Glucose is the most abundant monosaccharide in nature. They serve as an energy source in respiration and act as building blocks for large molecules that are essential to life.
They are composed of two monosaccharide units joined together by an O-glycosidic linkage. Some examples of disaccharides include maltose, lactose, sucrose, trehalose, melibiose, etc. After a hydrolysis reaction, they split into two sugar molecules.
Different disaccharides have different functions to perform. For example, lactose is a major animal energy source, trehalose is a major circulatory sugar in insects, and sucrose is a primary sugar that transports carbon within a plant.
They are ubiquitous. When many monomer units are joined together by a condensation reaction, they form a polymer called polysaccharides. They are involved in the mechanical stability of cells, organs, organisms, and act as carbohydrate stores in some other organisms.
This article presents you with the fundamentals of polysaccharides, their definition, classification, and functions in different organisms. It also introduces you to the physical and chemical properties.
Polysaccharides are an essential class of biological polymers. They are mainly involved in the structural or storage functions of the organism. One key information about polysaccharides is if a sugar molecule contains 3-10 monomeric units, it’s considered oligosaccharides, and if it’s composed of 11 or more monomeric units, they are considered true polysaccharides.
The physical and biological properties of polysaccharides depend on the components and the architecture of the molecule, and its interaction with the available enzymatic machinery. They are classified either based on the functions they perform, the type of monosaccharide units they contain, or their origin.
Polysaccharides are categorized into two classes based on the type of monosaccharide units they contain: homopolysaccharides and heteropolysaccharides.
They are composed of repeating units of only one type of monomer. The major homopolysaccharides include cellulose, chitin, starches (amylose and amylopectin), glycogen, and xylans.
It is a linear, unbranched polymer of glucose units joined by beta 1-4 linkages. It was discovered by a French scientist named Anselme Payen. It’s one of the most abundant organic compounds in the biosphere and it’s a basic structural component of the plant cell wall.
Though cellulose forms a part of the human diet, it’s indigestible in them because of the absence of the enzymes required to hydrolyze cellulose in the human body.
Celluloses are easily digestible by animals. These herbivore animals retain food for a longer time, where digestion is slowly performed by a group of microorganisms present in their alimentary tract.
It is a linear, long-chain polymer of N-acetyl-D-glucosamine (a derivative of glucose) residues/units. These units are joined together by glycosidic beta 1-4 glycosidic linkages. Chitin is the second most abundant natural biopolymer after cellulose.
It’s biodegradable in the natural environment, in which the reaction is catalyzed by the enzyme, chitinases, secreted by some bacteria and fungi. Chitin is closely related to cellulose, which is a linear unbranched chain of glucose units, rather than N-acetyl-glucosamine present in chitin.
It’s made of repeating units of D-glucose that are joined together by alpha-linkages. Starch is one of the most abundant polysaccharides found in plants. It’s composed of a mixture of amylose (15-20%) and amylopectin (80-85%).
The breakdown of starch is done by the enzyme, amylase. It’s present in both humans and animals, and that’s why both of them can easily digest starch.
It’s a polysaccharide composed of repeating units of xylose (a pentose sugar) joined together by beta-1, 4-linkages. Xylan is the third most abundant polymer on Earth and it is found in the cell walls of plants. Xylans account for 25-30% dry biomass of woody tissues of dicots and lignified tissues of monocots. In some cereals and grasses, it accounts for up to 50% of their weight.
They are composed of two or more repeating units of different types of monomers. Some of the heteropolysaccharides include glycosaminoglycans, agarose, and peptidoglycans. Naturally, most of the heteropolysaccharides are connected with peptides, proteins, and lipids.
They are negatively charged unbranched heteropolysaccharides. They are composed of repeating units of disaccharides that include acidic and amino sugars. The formula for GAG structure is n.
GAGs are found in animals and bacteria but are absent in plants. The amino sugar in GAGs is either N-acetylglucosamine or N-acetylgalactosamine. And the acidic sugar is usually a uronic acid (like glucuronic acid). The chart given below shows the building units of some major glycosaminoglycans.
| GAGs | Acidic sugar | Amino sugar |
|---|---|---|
|
Hyaluronic acid |
D-Glucuronic acid |
N-acetylglucosamine |
|
Chondroitin sulfate |
D-Glucuronic acid |
N-acetylgalactosamine |
|
Heparan sulfate |
D-Glucuronic acid or L-iduronic acid |
N-acetylglucosamine |
|
Heparin |
D-Glucuronic acid or L-iduronic acid |
N-acetylglucosamine |
|
Dermatan sulfate |
D-Glucuronic acid or L-iduronic acid |
N-acetylgalactosamine |
|
Keratan sulfate |
D-Galactose |
N-acetylglucosamine |
It is a heteropolymer of alternating units of N-acetylglucosamine (NAG) and N-acetylmuramic acids (NAM), linked together by beta-1,4-glycosidic linkage.
The 40-90% dry weight of the cell wall of gram-positive bacteria is composed of peptidoglycans whereas, in gram-negative bacteria, it only accounts for 10% of the dry weight.
It’s a polysaccharide composed of repeating units of agarobiose. Agarobiose is a disaccharide consisting of D-galactose and 3,6-anhydro-L-galactopyranose. It is generally extracted from red seaweed.
Polysaccharides are long-chain polymers of repeating units of sugars. They perform different life-supporting functions in different organisms. Some polysaccharides, because of their strong long polymer chain structure, elasticity, and rigidity, are used in several biomedical applications. Some of them, like agars and gellan gums, provide nutritional value and are used in the food industry.
The study of polysaccharides is still an ongoing research area. Many natural polysaccharides are being explored for their use as nanoformulation, microparticles, tablets, aerogels, hydrogels, and transdermal drug-delivery systems.
The natural physico-chemical properties of polysaccharides pose several challenges for their use in pharmaceuticals. However, scientists are working towards chemically modifying the physico-chemical properties of natural polysaccharides to develop novel semisynthetic natural polysaccharides that support their application in biomedical and pharmaceutical areas.
Monday – Friday
9 AM – 5 PM EST
DISCLAIMER: ConductScience and affiliate products are NOT designed for human consumption, testing, or clinical utilization. They are designed for pre-clinical utilization only. Customers purchasing apparatus for the purposes of scientific research or veterinary care affirm adherence to applicable regulatory bodies for the country in which their research or care is conducted.
