Glucose, Galactose, Fructose, pentoses, hexoses
Monosaccharides have following properties they are small with low molecular mass, they are sweet tasting, crystalline, and are readily soluble in water.
The simplest monosaccharides have three carbon atoms (n=3) and are called trioses. An important triose, glyceraldehydes, is formed as an intermediate in the metabolic pathways of respiration and photosynthesis. Its structural formula is shown below.
Pentoses are monosaccharides with five carbon atoms in the molecule and the general formula C5H10O5 .Like trioses, the pentoses have a carbonyl group and at least two hydroxyl groups. Ribose, an important constituent of RNA (ribonucleic acid), is an aldose and can exist as a chain or in a ring form. Below is an example photo of pentose.
Hexoses are the six carbon sugars, all having the general formula C6H12O6 .
They can exist as straight chains or rings, below the straight and ring forms of two common hexoses, galactose and fructose, are shown.
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 (jams). 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 1, 4 glycosidic bonds.
Disaccharides
Disaccharides are small molecules with low molecular mass, they are sweet tasting but they are not sweet as monosaccharides, they are also crystalline and also they are soluble in water, but less readily than monosaccharides. They have 2[(CH2O)n] – H2O as their general formula>
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 is made up of fructose and glucose. In plants, sugars formed as a result of photosynthesis are transported throughout the plant by the phloem in the form of sucrose. This sucrose is transported via the phloem tissue. Sucrose is very soluble. Despite sucrose’s solubility it is used as a energy storage in onions. Sucrose is found in sugar cane and the juices of sugar beet plant.
Lactose consists of galactose and glucose. It is found in mammalian milk, where it provides energy to the newborn infant.
Maltose 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 making of the 1, 4 glycosidic bonds with expulsion of water.
Polysaccharides
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 (cellulose fibers). Cellulose fibers join together by the action of hydrogen bonds on adjacent sides of the fiber
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 30% (1, 4 unbranched chains) and amylopectin 70% (1, 4 and 1, 6 – tightly packed and branched chain).Below are the properties that suite starch and glucose molecules to be a perfect energy storage molecules,
- They are compact and do not take up much space
- They are insoluble so they cannot move out of the cells in which they are stored
- They do not create any osmotic effect
- They do not become involved in chemical reactions in the cells
- They are easily hydrolysed to soluble products by enzyme action when required
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 branched more often.
Below are the images of the polysaccharides
