A Polysaccharide is a carbohydrate polymer that is formed from monosaccharides joined together by glycosidic bonds. They can contain several units of the same sugar or several different sugars and can have a branched or unbranched molecular structure. Polysaccharides can be found as storage molecules in both plants and animals and are also located in cell walls. Much of the energy that humans receive from food comes from polysaccharides. Significant types of polysaccharides include starch, glycogen, and cellulose.
Structure and Properties
Polysaccharides consist of long chains of monosaccharides, or sugars.  They are a kind of complex carbohydrate. The different sugar units are held together by glycosidic bonds. Two types of polysaccharides include homopolysaccharides and heteropolysaccharides. Heteropolysaccharides are chains of several different sugars, while homopolysaccharides contain only one kind of sugar.
A polysaccharide's chemical formula typically contains carbon atoms and water molecules. Its structure can either be branched, such as in gum Arabic, or linear, like in cellulose. Most polysaccharides do not dissolve in water and cannot enter or leave cells through osmosis. Although Polysaccharides are a type of sugar, they often do not taste sweet. A white powdery substance can be made from extracting polysaccharides.
Polysaccharides occur frequently in the natural world. Most carbohydrates can be found as polysaccharides.They form starch, the storage carbohydrate for plants, and glycogen, the storage carbohydrate for animals. For example, the starch found in many foods high in carbohydrates, such as bread and potatoes, are polysaccharides.In plants, a polysaccharide called cellulose provides structure for cells.
Polysaccharides also occur in animals and other organisms. Chitin, another polysaccharide, composes about one fifth of the shells of several invertebrates. These compounds are also located in the cell walls of organisms such as fungi and bacteria. In mammals, certain heteropolysaccharides called mucopolysaccharides form joint fluid and some connective tissue. Connective tissues containing polysaccharides include cartilage and the tissue found in umbilical cords.
History and Uses
Polysaccharides have several practical uses. People frequently consume polysaccharides as part of their diets in the form of carbohydrates. Since ancient times, humans have grown crops such as wheat, rice, and other grains. They understood that they needed to consume these foods as a source of energy. Foods such as breads and starches contain a large amount of carbohydrates, or polysaccharides. Carbohydrates provide much of the energy used by the human body.
Polysaccharides also could have the potential to help cure bacterial infections. Certain types of polysaccharides, known as bacterial capsule polysaccharides, occur as membranes that defend human cells against harmful bacteria. Scientists are studying how they can use these polysaccharides in the medical field. One polysaccharide, protein glycosylation, could prove helpful for fighting bacterial infections in the human body. Many scientists believe that consuming carbohydrates can reduce the risk of certain diseases, like colon cancer, and contribute to having a long and healthy life.
Types of Polysaccharides
Starch Starch is a type of polysaccharide also known as amylose or amylopectin. It consists of units of glucose held together by glycosidic bonds. Starch occurs as a white powder that does not dissolve in water or many other solvents. Amylose, the unbranched form of starch, provides storage for plants, including plants regularly consumed by humans such as potatoes. Humans can obtain energy from both amylose and amylopectin, the branched form of starch. When the starch breaks apart during a chemical reaction with water called hydrolysis, energy is produced. 
Glycogen Glycogen is the polysaccharide used for energy storage in animals. It is classified as a branched polysaccharide. Like in starch, energy can be obtained from glycogen when it undergoes hydrolysis with the help of certain enzymes, which can speed up chemical reactions. Glycogen forms glucose after being broken down. In the human body, this molecule is normally found in the muscles and the liver.
Cellulose Cellulose, another polysaccharide, forms the cell walls in plants. It consists of several units of β-D glucose, a type of sugar molecule. Hydrogen bonds involving OH groups on the different glucose units make cellulose extremely strong. Some common examples of where this polysaccharide can be found are wood and cotton, but most plants contain cellulose. Humans cannot obtain energy from cellulose because they do not have the enzymes needed to digest it.
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