Monosaccharides and Disaccharides

Human body requires a constant supply of essential nutrients for normal growth and development. The principle food components essential for the body include carbohydrates, fats and proteins. As recommended by the World Health Organization, about 55 to 75% of daily energy requirement should be fulfilled by carbohydrates. Carbohydrates, also known as saccharides, are molecular compounds that are made up of three elements, carbon, hydrogen and oxygen.

What are Monosaccharides and Disaccharides?

Carbohydrates are classified into two major types, simple and complex carbohydrates. Simple carbohydrates are made up of single, basic sugar unit. Simple carbohydrates are single sugars and are easy to digest. They can be found in food substances such as milk, honey, fruits, etc. Carbohydrates are further classified as monosaccharides and disaccharides.

Monosaccharides
Monosaccharides are often called single or simple sugars and are the simplest of all carbohydrates. They are the building blocks of all higher carbohydrates. Monosaccharides are aldehydes or ketones and generally have two or more hydroxyl groups. They are the important fuel molecules needed by our body and cannot be hydrolyzed to smaller carbohydrates. All the monosaccharides have a general molecular formula (CH₂O)_n.

Monosaccharides are further classified, based on the number of carbon atoms in a molecule, into trioses, when the carbohydrate contains 3 carbon atoms, tetroses, when the carbohydrate contains 4 carbon atoms, pentoses, if the carbohydrate contains 5 carbon atoms and so on. There is another system of classification of monosaccharides, based on the placement of its carbonyl group. The monosaccharide is called an aldose if its carbonyl group is an aldehyde and a ketose, if the carbonyl group is a ketone. These two systems are often combined and the monosaccharides are knows as aldotriose, aldotetrose, ketotriose, ketotetrose and so on.

Disaccharides
Disaccharides consist of two sugar units linked by covalent bonds. The covalent bond that holds the two sugars together is known as glycosidic linkage. It is formed by a condensation reaction that takes place between the two sugars, resulting in the loss of a hydrogen atom from one monosaccharide and a hydroxyl group from the other. Disaccharides are broken down into smaller monosaccharides, in the small intestine during the process of digestion.

Examples of Monosaccharides and Disaccharides

Aldoses

• Glyceraldehyde
• Erythrose
• Threose
• Ribose
• Arabinose
• Xylose
• Lyxose
• Allose
• Altrose
• Glucose
• Mannose
• Gulose
• Idose
• Galactose
• Talose

Ketoses

• Dihydroxyacetone
• Erythrulose
• Ribulose
• Xylulose
• Psicose
• Fructose
• Sorbose
• Tagatose

Some examaples of common disaccharides are as follows:

• Sucrose - Glucose + Fructose
• Lactulose - Galactose + Fructose
• Lactose - Galactose + Glucose
• Maltose - Glucose + Glucose
• Cellobiose - Glucose + Glucose

Let us have a look at some other rare disaccharides:

• Kojibiose
• Nigerose
• Isomaltase
• Sophorose
• Laminaribiose
• Gentiobiose
• Turanose
• Maltulose
• Palatinose
• Mannobiose
• Melibiose
• Rutinose
• Xylobiose

Monosaccharides are the simplest form of carbohydrates and are used very quickly by the cells, however, a cell may not need all the energy at one time and may have to store it. This is done by first converting the monosaccharides into disaccharides by condensation reactions and then further into polysaccharides, that can be stored in the body. These are then broken down by hydrolysis, whenever energy is required.