Polysaccharides are biological molecules which belong to a more general class of compounds called carbohydrates (organic materials that contain carbon, hydrogen, and oxygen).
They are the most abundant component of plants and are typically produced by photosynthesis. Polysaccharides are also produced by some types of bacteria, where they can have a prominent role in disease. For example, the polysaccharide elaborated by Pseudomonas aeruginosa protects the bacteria from host immune factors and antibiotics when the bacteria establish a lung infection in people with cystic fibrosis.
Polysaccharides have the largest molecular structure of any other type of carbohydrate. They are macromolecules, or polymers, made up of thousands of monosaccharide units typically linked together by an oxygen atoms. The term polysaccharide refers to polymers that are composed of ten or more monosaccharide units. Polymers that consist of three to nine monosaccharides are called oligosaccharides.
The chemical reactions that produce polysaccharides from monosaccharides are reversible. When a polysaccharide is reacted with water and an acid, monosaccharides result. These are sugar molecules that are classified by their functional group. Depending on the location of their carbonyl group, the sugar can either be an aldose or a ketose. Monosaccharide sugars are also grouped according to the number of carbon atoms they contain. The smallest is triose, which is a three carbon sugar. Pentose sugars have five carbon atoms. They include ribose and deoxyribose, which are key components of nucleic acids. One of the most important types of monosaccharides are hexoses, which contain six carbon atoms. These include sugars such as galactose, fructose and glucose.
Glucose is probably the most abundant organic compound found in nature. It is a monosaccharide that is present in honey and many fruit juices. In the body it is found in greatest concentration in the blood. In both plants and animals, it provides the major source of energy. Several important polysaccharides are built from repeating glucose units. These include storage polysaccharides like starch and glycogen, and structural polysaccharides such as glycogen and cellulose.
Starch is a mixture of two types of polysaccharides. One is a straight chain polymer made up of glucose monomers. It is called amylose and comprises 20% of the weight of the starch. The other is a branched polymer known as amylopectin. It is also made up of glucose monomers, and it makes up the other 80%. Starch is water-soluble, which makes it an important food source. In fact, starch is the most abundant polysaccharide that humans eat. A large quantity is found in cereals, potatoes, and vegetables. It is produced by plants and is used to store the energy collected during photosynthesis. When starch is digested, it is ultimately reduced to glucose.
Glycogen is also known as animal starch because it is the molecule in which animals store excess glucose. It is a branched polymer made up of glucose units and has a similar structure to the amylopectin in starch. However, it is more highly branched and exists in much greater molecular weights. Glycogen is found mainly in the liver and skeletal muscle. A small amount is found in almost all tissues. In the body, excess glucose is transformed into glycogen by a process called glycogenesis. When the body needs it, the glycogen is rapidly reduced to glucose and used for energy.
Cellulose is another type of glucose-based polysaccharide. It is by far the most abundant polysaccharide in nature. It is found in plant cell walls and is also the main structural component of stems, leaves and bark. Unlike starch and glycogen, the glucose molecules in cellulose are connected through a beta linkage. This molecular configuration makes this polysaccharide rigid, fibrous, tough and insoluble in water. For this reason, it can not be digested by most organisms. Two notable exceptions are cows and termites who have microorganisms in their guts that can breakdown cellulose. This allows them to use cellulose as part of their diet. Cellulose is used for a variety of industrial applications. A modified version of it is used to make permanent press fabric. Rayon, cellophane, explosives, and celluloid (film) are also made with cellulose. In many consumer products, cellulose is used as a thickener.
A variety of other polysaccharides have important biological roles. Chitin is a structural polysaccharide that makes up the exoskeleton of insects and other arthropods. It is composed of an amino sugar that is a nitrogen containing glucose. Pectin is a cementing substance found in plants. Finally, hyaluronic acid is a polysaccharide that helps cement connective tissue in the body.