Difference Between Disaccharide and Monosaccharide

Disaccharide vs Monosaccharide
 

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. Disaccharides and monosaccharides are readily soluble in water, and they are sweet in taste. They can be crystallized. As like there are some similarities between these two, there are a number of differences too.

Monosaccharide

Monosaccharides are the simplest carbohydrate type. Monosaccharide has the formula of C_x(H₂O)_x. These cannot be hydrolyzed into simpler carbohydrates. They are sweet in taste. All monosaccharides are reducing sugars. Therefore, they give positive results with benedicts’ or Fehling’s reagents.  Monosaccharides are classified according to,

• The number of carbon atoms present in the molecule
• Whether they contain an aldehyde or keto group

Therefore, a monosaccharide with six carbon atoms is called a hexose. If there are five carbon atoms, then it is a pentose. Further, if the monosaccharide has an aldehyde group, it is called as aldose. A monosaccharide with a keto group is called a ketose. Among these, the simplest monosaccharides are glyceraldehyde (an aldotriose) and dihydroxyacetone (a ketotriose).  Glucose is another common example for a monosaccharide. For monosaccharides, we can draw a linear or a cyclic structure. In solution, majority of the molecules are in the cyclic structure. For example, when a cyclic structure is forming in glucose, the -OH on carbon 5 is converted into the ether linkage to close the ring with carbon 1. This forms a six member ring structure. The ring is also called a hemiacetal ring, due to the presence of carbon that has both an ether oxygen and an alcohol group.

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 1^st and 4^th –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. 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.