Pentoses and Hexoses are monosaccharides.
Pentoses are monosaccharides with 5 carbon atoms. The general formula of pentoses is C5H10O5. Ribose is an example of a pentose, since it is made up of 5 carbon atoms in its molecule. Ribose is an important component of RNA (ribonucleic acid). Deoxyribose is another pentose monosaccharide. Deoxyribose is an important constituent of DNA. Its chemical formula is C5H10O4.
Hexoses are monosaccharides made up of 6 carbon atoms. Their general formula is C6H12O6. Glucose, fructose and galactose make up the principal monosaccharides. They are all hexoses. Glucose is mildly sweet and is the important carbohydrate in the blood. There are two types of glucose, α-glucose and β-glucose. These are isomers i.e. they have the same chemical formula but different structural formula. Below are the structures of the glucose.
It is used in the process of respiration to produce energy in animals. Fructose on the other hand is extremely sweet and found in fruits (strawberry jam) and vegetables. Galactose is found in mammalian milk as a constituent of lactose. It is used to provide energy to the infant.
Monosaccharides are formed by the hydrolysis of disaccharides, which involves the addition of water to the disaccharides and the breaking of the glycosidic bond.
Disaccharides consist of two monosaccharides joined by the glycosidic bond. They are sweet but not as sweet as monosaccharides. The most important disaccharides are sucrose, lactose and maltose.
Sucrose (table sugar) is made up of fructose and glucose. In plants, sugars formed as a result of photosynthesis are transported throughout the plant in the form of sucrose. This sucrose is transported via the phloem tissue. Sucrose is very soluble. Despite sucrose’s solubility they are used as a energy storage. Sucrose is found in sugar cane and the juices of sugar beet plant.
Lactose (milk sugar) consists of galactose and glucose. It is found in mammalian milk, where it helps in giving energy to the newborn infant.
Maltose (malt sugar) is made up of two glucose molecules. The break down of starch leads to the formation of maltose. This catabolic reaction takes place in the presence of maltase.
Disaccharides are made by the condensation process. This involves the breaking of the glycosidic bonds with expulsion of water.
Polysaccharides are non-sugars. The joining of many monosaccharides forms polysaccharides. The three important polysaccharides include cellulose, starch and glycogen.
Cellulose is found throughout the plant in the cell walls of the plant cell walls. It is assists in support, strength and protection of the plant. This is a polysaccharide of β-glucose joined by 1,4 glycosidic bonds, thereby forming straight unbranched chains.
Starch is found in the leaves of plants. It is used as energy storage, as it is insoluble. Starch is a polysaccharide of α-glucose monomers and is a mixture of amylose (1,4 unbranched chains) and amylopectin (1,4 and 1,6 – tightly packed and branched chain).
Glycogen is found in animals. It is known as animal fat, as it is used as energy storage. It is made up of α-glucose monomers with 1,4 and more 1,6 glycosidic bonds. They are very branched.
