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The Structure of Carbohydrates – Monosaccharides, Disaccharides, and Polysaccharides

The structure of carbohydrates can be simple or extremely complex. Learn about the different types of carbohydrate structures including monosaccharides, disaccharides, and polysaccharides.

Carbohydrates are molecules that are synthesized primarily from carbon, oxygen, and hydrogen atoms. Some types of carbohydrates consist of a single unit consisting of a few atoms, while other carbohydrates consists of thousands of units linked together through chemical bonds. Glucose, maltose, and glycogen are three carbohydrates that are similar, but structurally different.

Monosaccharides

The simplest form of carbohydrates is the monosaccharide. ‘Mono’ means ‘one’ and ‘saccharide’ means ‘sugar’. Monosaccharides are either aldoses or ketoses. Aldoses such as glucose consists of a carbon backbone and a carbonyl group (C=O) located at the end of the chain. Ketoses such as fructose consists of a carbon backbone with a carbonyl group located at any other carbon in the chain. The remaining carbon atoms are bound to hydroxyl groups (-OH).

Disaccharides

When two monosaccarides are joined together through a bond, a disaccharide is formed. Maltose is a sugar composed of two glucose molecules. Other common disaccharides include lactose and sucrose. The structure of carbohydrates featuring two or more monosaccharides is held together covalently with a glycosidic bond.

Polysaccharides

When dozens, hundreds, or even thousands of sugars are linked together, the molecule is considered a polysaccharide. Carbohydrate structures with a single repeating monosaccharide are considered homopolysaccharides, while carbohydrates that consist of more than one monosaccharide is considered a heteropolysaccharide. Polysaccharides can also be identified by whether or not the chain is linear or branched. Glycogen is an example of a homopolysaccharide that is branched. It consists of repeating glucose molecules. Glycosaminoglycan is an example of a heteropolysaccharide. It consists of repeating units of acetylglucosamine and glucuronic acid.


Reference:

Nelson, David L. Lehninger Principles of Biochemistry 4th ed. W.H. Freeman and Company. 2005

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