Disaccharides

What are Disaccharides?

Disaccharides are those carbohydrates which on hydrolysis with acids or enzymes give two molecules of monosaccharides which can either be same or different. The oxide linkage is formed after the loss of the water molecule and then the two monosaccharides are formed by that linkage. When two monosaccharide units are joined via the oxygen atom then that linkage is called a glycosidic linkage.

Disaccharides

Structure of Disaccharides (Sucrose)

The most common disaccharide is sucrose which gives D -(+)- glucose and D-(-)- fructose on hydrolysis.

Both the monosaccharides i.e. glucose and fructose are connected through glycosidic linkage between alpha glucose and second carbon beta fructose. Sucrose is a non-reducing sugar as both the reducing groups of glucose and fructose are involved in the glycosidic bond formation.

Examples Of Disaccharides:

  1. Sucrose:

Sucrose being dextrorotatory in nature gives dextrorotatory glucose as well as laevorotatory fructose on hydrolysis. The overall mixture is laevorotatory and this is because the laevorotation of fructose (-92.4) is more than the dextrorotation of glucose (+52.5).

Disaccharides - Sucrose

  1. Maltose:

Maltose is also one of the disaccharides which have two α -D-glucose units which are connected by the first carbon of the glucose and also linked to the fourth carbon of another glucose unit. In the solution, a free aldehyde can be produced at the first carbon of the second glucose of the solution and it is a reducing sugar as it shows reducing properties.

Disaccharides - Maltose

  1. Lactose:

Commonly it is called as milk sugar as this disaccharide is found in milk. It is made up of Beta-D-galactose and β-D-glucose. The bond is between the first carbon of galactose and the fourth carbon of glucose. This is also a reducing sugar.

Disaccharides - Lactose

Some More Types Of Disaccharides

There few more types which are not that popular, such as:

  • Trehalose

It is made up of 2 molecules of glucose which are linked in a different manner. This can be found in fungi, plants, and insects.

  • Lactulose

It is formed from galactose and fructose. It is helpful for the treatment of constipation and liver diseases.

  • Cellobiose

It is also made up of two glucose molecules which are also arranged in a different way. These can be seen bacteriology which is a form of chemical analyses.

  • Chitobiose

It comprises two glucosamine molecules which are linked. It is seen in some bacteria, exoskeletons of insects and is also found in fish, octopus, and squid.

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Practise This Question

 Assertion  : Sucrose is a non-reducing sugar.

Reason :  It has a glycosidic linkage.