MONOSACCHARIDES

DEFINITION

Monosaccharides are the most basic units of biologically important carbohydrates. They are the simplest form of sugar and are usually colourless,water-soluble,crystalline solids and sweet tasting.Examples of monosaccharidesare glucose , fructose, galactose,xylose and ribose. Monosaccharides are the building blocks of disaccharides (such as sucrose) and polysaccharides (such as cellulose and starch). Further, each carbon atom that supports hydroxyl group (except for the first and last) is chiral, giving rise to a number of isomeric forms all with the same chemical formula. For instance, galactose and glucose are both aldohexoses, but have different chemical and physical properties.Monosaccharides can be classified by the number x of carbon atoms they contain: diose (2) triose (3) tetrose (4), pentose (5), hexose (6), heptose (7), and so on.

Linear-chain monosaccharides

Simple monosaccharides have a linear and unbranched carbon skeleton with one carbonyl (C=O) functional group, and one hydroxyl (OH) group on each of the remaining carbon atoms. Therefore, the molecular structure of a simple monosaccharide can be written as H(CHOH)_n(C=O)(CHOH)_mH, where n+1+m = x; so that its elemental formula is C_xH_2xO_x.
By convention, the carbon atoms are numbered from 1 to x along the backbone, starting from the end that is closest to the C=O group.
If the carbonyl is at position 1 (that is, n or m is zero), the molecule begins with an formyl group H(C=O)-, and is technically an aldehyde. In that case, the compound is termed an aldose. Otherwise, the molecule has a   group, a carbonyl -(C=O)- between two carbons; then it is formally a ketone, and is termed a ketose.

GLUCOSE
Gucose is a simple monosaccharides found in plants. It is one of the three dietary monosaccharides, along with fructose and galactose, that are absorbed directly into the bloodstream during digestion. An important carbohydrate in biology cells use it as the primary source of energy and a metabolic intermediate. Glucose is one of the main products of photosynthesis and fuels for cellular respiration.

GALACTOSE
Galactose is a monosaccharides.When combined with glucose (monosaccharide), through a condensation reaction, the result is the disaccharide lactose. The hydrolysis of lactose to glucose and galactose is catalyzed by the enzyme lacatase and β-galactose.Galactose is found in dairy products sugar beets, other gums and mucilages. It is also synthesized by the body, where it forms part of glycolipids and glycoproteins in several tissues.

FRUCTOSE

Fructose, or fruit sugar, is a simple monosaccharides found in many plants, where it is often linked to glucose forming sucrose.It is one of the three dietary monosaccharides, along with glucose and galactose that are absorbed directly into the bloodstream during digestion. Fructose was discovered by French chemist Augustin-Pierre Dubrunfaut in 1847.Pure, dry fructose is a very sweet, white, odorless, crystalline solid and is the most water-soluble of all the sugars.From plant sources, fructose is found in honey, tree and vine fruits, flowers, berries, and most root vegetables. In plants, fructose may be present as the monosaccharide or as a molecular component of sucrose, which is a disaccharides.