MCAT Content / Carbohydrates / 1d Polysaccharides

Polysaccharides

Topic: Carbohydrates

Polysaccharides are long chains of monosaccharides linked together by glycosidic bonds.

The chain of a polysaccharide may be branched or unbranched, and it may contain different types of monosaccharides. Common polysaccharides include starch, glycogen, and cellulose.

Plants are able to synthesize glucose, and the excess glucose is stored as starch in different parts of the plant, including the roots and seeds. Starch is the stored form of sugars in plants. In starch, the glucose monomers are in the α form (with the hydroxyl group of carbon 1 sticking down below the ring), and they are connected primarily by α-1-4 glycosidic linkages (i.e., linkages in which carbon atoms 1 and 4 of the two monomers form a glycosidic bond). Amylose consists entirely of unbranched chains of glucose monomers connected by 1-4 linkages. Amylopectin is a branched polysaccharide. Although most of its monomers are connected by 1-4 linkages, additional 1-6 linkages occur periodically and result in branch points. You can see this in the figure below:

Glycogen is the storage form of glucose in humans and other vertebrates. It is similar to starch, except that it has more α-1-6 glycosidic bonds (approximately 1 for every 10 glucose molecules), which makes it a highly branched compound. 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.


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.



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