Polysaccharides are long chains composed of monosaccharide units bound together by glycosidic linkages.

A long chain of monosaccharides linked by glycosidic bonds is known as a polysaccharide (poly- = “many”). The chain may be branched or unbranched, and it may contain different types of monosaccharides. Starch, glycogen, cellulose, and chitin are primary examples of polysaccharides.

Starch is the stored form of sugars in plants and is made up of a mixture of amylose and amylopectin (both polymers of glucose). Starch is made up of glucose monomers that are joined by α 1-4 or α 1-6 glycosidic bonds. The numbers 1-4 and 1-6 refer to the carbon number of the two residues that have joined to form the bond. Amylose is starch formed by unbranched chains of glucose monomers (only α 1-4 linkages), whereas amylopectin is a branched polysaccharide (α 1-6 linkages at the branch points).

Glycogen is the storage form of glucose in humans and other vertebrates and is made up of monomers of glucose. Glycogen is the animal equivalent of starch and is a highly branched molecule usually stored in liver and muscle cells. Whenever blood glucose levels decrease, glycogen is broken down to release glucose in a process known as glycogenolysis.

Cellulose is the most abundant natural biopolymer. The cell wall of plants is mostly made of cellulose; this provides structural support to the cell. Wood and paper are mostly cellulosic in nature. Cellulose is made up of glucose monomers that are linked by β-1-4 glycosidic bonds. These chains cluster together to form parallel bundles that are held together by hydrogen bonds between hydroxyl groups. This gives cellulose its rigidity and high tensile strength.

Cellulose is specific to plants, but polysaccharides also play an important structural role in non-plant species. For instance, arthropods (such as insects and crustaceans) have a hard external skeleton, called the exoskeleton, which protects their softer internal body parts. This exoskeleton is made of the macromolecule chitin, which resembles cellulose but is made out of modified glucose units that bear a nitrogen-containing functional group.

Practice Questions

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Key Points

• Polysaccharides are formed by glycosidic bonding of carbohydrates, and the polymers cellulose, starch, and glycogen are most commonly found in nature.

• Starch is a glucose molecule joined by α-1-4 linkage; it is the stored form of sugars in plants.

• Glycogen is the same as starch but with additional α-1-6 linkages for branching; it is the stored form of sugars in animals (stored in the liver).

• Cellulose is made up of glucose molecules that are linked by β-1-4 glycosidic bonds; it is the main structural component for plant cell walls and the main source of fiber in the human diet. 

Key Terms

Polysaccharides: Long chains of monosaccharides linked together by glycosidic bonds.

Glycosidic bonds (linkages): A type of covalent bond that joins two carbohydrate molecules.

Starch: The stored form of sugars in plants; amylose is unbranched and made through α-1-4 bonds while amylopectin is branched and also includes α-1-6 bonds.

Glycogen: The storage form of glucose in humans and other vertebrates; has many α-1-6 glycosidic bonds and is highly branched.

Cellulose: The main component of plant cell walls; made through β-1-4 glycosidic bonds, providing strength to the biopolymer.

α-1-4 glycosidic linkages: Linkages in which carbon atoms 1 and 4 of the two monomers form a glycosidic bond.

Amylose: An unbranched plant starch made from monosaccharides connected through α-1-4 bonds.

Amylopectin: A branched plant starch made from monosaccharides connected through α-1-4 and α-1-6 bonds.

α-1-6 glycosidic bonds: Linkages in which carbon atoms 1 and 6 of the two monomers form a glycosidic bond.

Biopolymer: Any macromolecule of a living organism that is formed from the polymerization of smaller entities.

β-1-4 glycosidic bonds: Linkages in which carbon atoms 1 and 4 of two glucose monomers in their β form connect through a glycosidic bond.

Chitin: Similar to cellulose, except the glucose monomers feature a nitrogen-containing functional group; makes up the exoskeleton of insects.