Monosaccharides and Oligosaccharides

Carbohydrates:

               These are present in all organisms and in all parts of cells.  These play both structural and functional roles in the body of living organisms. As these are the sources of energy and are also found in the cell wall and cell membrane of the cells. Those compounds which upon hydrolysis yields aldehydes and ketones. The primary source of carbohydrates is green plants which are formed it by photosynthesis. 

Monosaccharides:

     These are non-reducing sugar. These cannot be further hydrolyzed into simplest sugars. These are soluble in water. All the carbons in monosaccharides except  1  have OH a group. The remaining carbon is either a part of the aldehyde or of the ketone. These are sweet in taste. The sugar with an aldehyde is called aldo-sugar while the sugar with ketone is called keto-sugar. In monosaccharides, 3 to 7 carbons are found. The saccharide with 3 carbon is called triose, with four carbons is called tetrose, with five carbon is called pentose, and so on.

Pentose and hexsoses are most common. Glucose is the most important hexose. Most of the monosaccharides form ring-like structures in solution. for example, ribose forms five cornered ring structures called ribofuranose while glucose forms six cornered ring structures called glucopyranose. 

     In a free state, glucose is present in all fruits, abundant in grapes, figs, and dates. Our blood normally contains 0.08% glucose. In combined form, it is present in disaccharides and in polysaccharides. Glucose is naturally found in green plants by photosynthesis. For the synthesis of 10g glucose, 717.6 kcal of solar energy is required. This energy is stored as chemical energy in glucose molecules and is available when required.   

• If the OH group at carbon number one is in a downward position, then it is called alpha sugar, and if If OH group at carbon number one is in an upward position, then it is called beta sugar. 
•  If two glucose molecules are mirror images of each other and are non-superimposable, then these are called enantiomers. For example, L glucose which is left-handed glucose, and D-glucose which is right-handed. All the naturally occurring glucose is D-glucose. While L-Glucose is used as an artificial sweetener. D glucose is digested by the stomach while L glucose is not digested by the human stomach.                                                                                                                                           

Oligosaccharides:

          Those carbohydrates which on hydrolysis yield 2 to 10 saccharides are called oligosaccharides. Based on several units, the oligosaccharides are classified as disaccharides( the most important disaccharide), trisaccharides, tetrasaccharides, and so on. These are comparatively less sweet in taste, and less soluble in water than monosaccharides. These can be hydrolyzed into simple monosaccharides.

The most important disaccharides are:

• . sucrose
• .Lactose
• .Maltose

Sucrose:

               The most familiar disaccharide is sucrose, formed by the condensation reaction between glucose and fructose. Sucrose or sugar cane is only non-reducing sugar in disaccharides. Its molecule formula is C12H22O11. It is commonly used as a sweetener at home. The prepared food in plants is transported as sucrose, so it is called transport disaccharides. It is very soluble in water.  It is moved efficiently in high concentrations in plants. It is chemically unreactive. The OH group of C1 of glucose and the OH group of C2 of fructose combine through alpha 1-2 glycosidic linkage.

Maltose:

    It is an intermediate disaccharide produced during the breakdown of starch and glycogen. It is also known as malt sugar. It is found in germinating seeds. It is formed by the condensation of two alpha glucose by forming alpha 1-4 glycosidic linkage.

Lactose:

   It is formed by a condensation reaction between beta glucose and beta galactose by forming a beta 1-4 glycosidic linkage. It is commonly known as milt sugar.