Introduction to Carbohydrates

Do you know table sugar, washing starch and cellulose in plants all belong to same type of molecules? All of them are carbohydrates. Carbohydrates are the organic molecules that are present in the highest amount in nature. They have a variety of functions, including providing a significant portion of the dietary calories for most organisms, behaving as a storage form of energy in the body, and serving as a part of cell membranes that help some forms of intercellular communication. The empiric formula for many of the simpler carbohydrates is (CH2O)n, hence the name “hydrate of carbon.”

Classification and structure of carbohydrates

Simple sugars, also known as monosaccharides, can be divided ino classes according to the number of carbon atoms present in them. Examples of some monosaccharides commonly found in humans are Glyceraldehyde, Erythrose, Ribose, Glucose and fructose. Carbohydrates ha possess an aldehyde as their functional group are called aldoses, whereas those having a keto group as the functional group are called ketoses. For example Glucose is a aldose while fructose is a ketose. Monosaccharide connect together to form larger structures by glycosidic bonds.

 The molecules having two monosaccharide units are termed as disaccharides. Oligosaccharides are the one having three to about ten monosaccharide units. Polysaccharides contain more than ten monosaccharide units, and can be hundreds of sugar units in length.

Isomers, epimers and Enantiomers

The compounds that have same chemical formula but differ in their structure are known as isomers. Carbohydrates like glucose, fructose, mannose and galactose are all isomers of eachother having same chemical formula(C6H12O6) but different structurally and functionally. Carbohydrate isomers that differ in configuration around only one specific carbon atom are called epimers. Glucose and Galactose are the example of C-4 epimers. The example of C-2 epimers is glucose and mannose that differ in only in the position of the –OH group at carbon 2. The carbohydrates that are mirror images of each other are called enantiomers. The two pairs are known as either L-sugar or D-sugar. If  the –OH group on the carbon linked to four different groups is on the right then it is D-sugar. If on left, it is termed as L-sugar.

Formation of  disaccharides

Monosaccharides can be joined to form disaccharides. The bonds that link carbohydrates are called glycosidic bonds. Glycosidic bonds between sugars are named according to the numbers of the connected carbons, and with regard to the position of the –OH  group of the sugar involved in the bond. Sucrose, for example, is synthesized by forming a glycosidic bond between carbon 1 of α-D-glucose and carbon 4 of β-D-fructose. The linkage is, therefore, α-1,β-2- glycosidic bond. Other important disaccharides include maltose and lactose formed by linkage between two glucose and one glucose other galactose espectively


Glucose subunits polymerize to give rise to more storable polymers called Polysaccharides including starch, glycogen, cellulose etc. Amylopectin is a glucose polymer with mainly α (1→4) linkages, but it also has branches formed by α (1→6) linkages. The branches produce a compact structure & provide multiple chain ends at which enzymatic cleavage can occur. The glucose storage polymer in animals is called Glycogen. The glycogen has more α (1→6) branches. Cellulose, a major part of plant cell walls, consists of long linear chains of glucose with β (1→4) linkages.

Complex carbohydrates

If carbohydrates are linked to non-carbohydrate structures by glycosidic bond, they are termed as complex carbohydrates. This structure can be purines and pyrimidines as in DNA and RNA, aromatic rings as in bilirubin, lipids as in glycolipids and proteins as in glycoproteins. If the non-carbohydrate group is attached to carbohydrate by –NH2 group, then they are called N-glycosides. When a non-carbohydrate molecule to which the sugar is attached is an –OH group, the structure is an O-glycoside. All sugar to sugar glycosidic linkages are actually O type bonds.

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