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Biology > Biomolecules > Polysaccharides
Biomolecules

Polysaccharides

Polysaccharides are complex biomacromolecules that are made up chains of monosaccharides. The bonds that form these chains are glycosidic bonds. Commonly found monomer units in polysaccharides are glucose, fructose, mannose and galactose which are simple sugars.

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Types of Polysaccharides

Polysaccharides can be broadly classified into two classes:

  • Homo-polysaccharides – are made up of one type of monosaccharide units. ex: cellulose, starch, glycogen.
  • Hetero-polysaccharides – are made up of two or more types of monosaccharide units. ex. hyaluronic acid and they provide extracellular support for organisms.

Structure of Polysaccharides

Polysaccharides

All polysaccharides are formed by the same basic process where monosaccharides are connected via glycosidic bonds. These glycosidic bonds consist of an oxygen molecule bridging two carbon rings. The bond is formed when a hydroxyl group is lost from the carbon of one molecule, while the hydrogen is lost by the hydroxyl group of another monosaccharide. Because two molecules of hydrogen and one of oxygen are expelled, the reaction is a dehydration reaction. The structure of the molecules being combined determines the structures and properties of the resulting polysaccharide.  A polysaccharide used for energy storage will give easy access to the constituent monosaccharides whereas a polysaccharide used for support is usually a long chain of monosaccharides that form fibrous structures.

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Functions of Polysaccharides

Polysaccharides form a crucial part of cell function and structure.

  • Storage polysaccharides: Polysaccharides such as starch and glycogen are called storage polysaccharides because they are stored in the liver and muscles to be converted to energy later for body functions. Starch is found in plants whereas glycogen is found in animals.
  • Structural polysaccharides: Polysaccharides such as cellulose are structural polysaccharides which are found in the cell walls of plants. Another structural polysaccharide is chitin.

Read about Bond Linking Monomers here.

Important polysaccharides

Homopolysaccharides

Structure of Starch

(Source: Wikipedia)

  • Starch: It is the storage polysaccharide found in plant cells and exists in two forms: amylose is the helical form of starch comprised only of alpha-1,4 linkages and amylopectin that has a structure like glycogen except that the branched alpha-1,6 linkages are present on only about one in 30 monomers.

Glucose

(Source: Wikipedia)

  • Glycogen: This polysaccharide is the polysaccharide found in animals to store energy and is composed of alpha-1,4-glycosidic bonds with branched alpha-1,6 bonds present at about every tenth monomer. It is mainly produced by the liver and muscles, but it can also be made during a process called glycogenesis.
  • Cellulose: Is a structural polysaccharide that is found in the cell wall of plants and when consumed, it acts as a dietary fibre. Cellulose is said to be the most abundant organic molecule on earth. Wood, paper, and cotton are common forms of cellulose.

Heteropolysaccharides

These are found in different structural and functional roles in the human body.

  • Hyaluronic Acid: Acts as a lubricant in the synovial fluid of joints
  • Chondroitin Sulfate: It contributes to tensile strength and elasticity of cartilages, tendons, ligaments, and walls of the aorta.
  • Dermatan sulfate: It is found mainly in the skin, and also is in vessels, heart, lungs. It may be related to coagulation and vascular diseases and other conditions.
  • Keratan sulfate: Present in the cornea, cartilage bone and a variety of other structures as nails and hair.
  • Heparin: Is present as an anticoagulant in the blood.

Another type of polysaccharides that are found in the human body is glycosaminoglycans or mucopolysaccharides that are formed by the endoplasmic reticulum. These mature in the Golgi apparatus. They form important components of connective tissues and are found in collagen and elastin.

Solved Example for You

Q:  Which is the storage polysaccharide that is found in plants?

a. Starch

b. Glycogen

c. Cellulose

d. Chitin

Sol: The correct option is “a”. The storage polysaccharide that is found in plants is starch. It is made up of two parts: amylose and amylopectin. They are made up of glucose monomers.

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