Difference Between Disaccharide and Polysaccharide

Disaccharide vs Polysaccharide
 

Carbohydrates are a group of compounds, which are defined as “polyhydroxy aldehydes and ketones or substances that hydrolyze to yield polyhydroxy aldehydes and ketones.” Carbohydrates are the most abundant type of organic molecules on earth. They are the source of chemical energy for living organisms. Not only this, they serve as important constituents of tissues. Carbohydrates are synthesized in plants and some microorganisms by photosynthesis. Carbohydrates got its’ name because it has the formula C_x(H₂O)_x, and this looked like hydrates of carbon. Carbohydrate can be again categorized into three as monosaccharide, disaccharides and polysaccharides. Monosaccharides are the simplest carbohydrate type.

Disaccharide

Disaccharide is the combination of two monosaccharides. When two monosaccharides are joined together, an ester bond is formed between any two –OH groups. Commonly this happens between the 1st and 4th –OH groups in two monosaccharides. The bond formed between the two monomers is known as a glycosidic bond. During this reaction, a water molecule is removed. Hence, this is a condensation reaction. Sometimes, both the monomers in a disaccharide are the same and sometimes they are different. For example, to produce maltose, two glucose molecules are participating. Fructose is made by the condensation reaction between a glucose and fructose, whereas lactose is made from glucose and galactose. Disaccharides are also common in nature. For example, sucrose is found in fruits and vegetables. And lactose is found in milk. Disaccharides can be hydrolyzed and produce the relevant monomers back. They are sweet in taste and can be crystallized. Most of the disaccharides can be hydrolyzed except sucrose.

Polysaccharide

When ten or higher number of monosaccharides is joined by glycosidic bonds, they are known as polysaccharides. They are also known as glycans. There chemical formula is C_x(H₂O)_y. Polysaccharides are polymers and, therefore, have a larger molecular weight, typically more than 10000. Monosaccharide is the monomer of this polymer. There can be polysaccharides made out of a single monosaccharide and these are known as homopolysaccharides. These can be also classified based on the type of monosaccharide. For example, if the monosaccharide is glucose, then the monomeric unit is called a glucan. Polysaccharides made out of more than one type of monosaccharide are known as heteropolysaccharides. Polysaccharides can be liners molecules with 1,4-glycosidc bonds. They also can form branched molecules. At the branching points, 1,6- glycosdic bonds are forming. There is a wide variety of polysaccharides. Starch, cellulose, and glycogen are some of the polysaccharides we are familiar with. Starch is abundant in our food sources. Glycogen is the storage polysaccharide in our bodies. Polysaccharides do not have a sweet taste. Some are partially soluble in water, whereas some are insoluble. Like disaccharides, polysaccharides can be hydrolyzed.