By Anup Pokhrel


All the living things i.e animals, plants, and microorganisms are composed of basic functional units called cells. The cells combine to form tissue which then form organs and the organs make the system in the body of living organisms. An organism performs its vital activities due to the assembly of various systems. Their vital activities are called as life processes, which mainly constitute growth, movement, ageing, reproduction, etc. All the processes are functional of chemical reactions between various organic and inorganic molecules in the cell of living beings. Most of the molecules found in living cells are the complex organic substances like carbohydrates, amino acids, protein, fats, vitamins, hormones etc. Water and other inorganic substances are also the components of living beings. Such molecules of the organic compound which build up living systems and are required for their growth and maintenance are called biomolecules or the molecules of life. Water constituents up to 90% by mass in living beings. There are about 50 elements which occur in the body of living organisms. Some 22 elements out of these are known to have considerable functions. Carbon, hydrogen, oxygen, nitrogen, calcium, chlorine, magnesium, phosphorous, potassium, sodium, sulphur etc. are major elements of our body. Whereas boron, aluminium, silicon, vanadium, manganese, iron, cobalt, copper, zinc, molybdenum, iodine etc. are the trace elements in living beings.

The study of chemical processes and the material in living systems is known as biochemistry. The chemical processes that take place in the synthesis of an organic compound is called as metabolism. Metabolism may be of two types i.e., catabolism and anabolism. The breaking down of complex organic molecules to release energy is called as catabolism and the building up of more complex compounds from simple molecules is called as anabolism. For example, breaking of sugar into carbon dioxide, water and energy is catabolism and synthesis of proteins from amino acids are anabolism.


Carbohydrates are organic compounds of vital importance in living beings. These are made of carbon, hydrogen, and oxygen in various structural forms. The general formula of carbohydrates is Cn(H2O)m, i.e. carbon hydrate. Hence, the name of this class of organic compounds is carbohydrate. In carbohydrates, the ratio of oxygen and hydrogen is same in water but the two are not present as in water molecules. Hydrogen may be present in the form of –CHO, -OH or directly linked to the carbon atoms. Similarly, oxygen may be present in the form of –CHO, -OH, -O etc. groups. In a more common way, carbohydrates are the polyhydroxy, aldehydes, polyhydroxy ketones or large polymeric molecules which on hydrolysis produce polyhydroxy aldehydes or ketones. Carbohydrates include familiar substances like glucose, sucrose, starch, cellulose, glycogen, chitin etc.

Glucose, C6H12O6 or C6(H2O)6

Sucrose, C12H22O11 or C12(H2O)11

Classification of carbohydrates

Carbohydrates can be classified on the basis of following types:

  1. On the basis of hydrolysis

Hydrolysis is the process of breaking of complex molecules into simpler molecules. This process can be a basis for the classification of carbohydrates. Carbohydrates can be classified into following three classes according to their behavior on hydrolysis.

  • Monosaccharides: The carbohydrates which do not break down into any simple molecules upon hydrolysis are called as monosaccharides. So, monosaccharides are simplest carbohydrates and represent a complete carbohydrate unit. These may contain 3 to 6 carbons atom in their molecules and can be absorbed directly into the bloodstream of living beings. About 20 monosaccharides occur normally. For example glucose (C6H12O6), galactose, fructose rodpse, glucose etc are some common monosaccharides. The composition of monosaccharides, in general, can b represented by the formula CnH2nOn.


Monosaccharides are water soluble substances and undergo carrying on heating. These may act as reducing agent by themselves undergoing oxidation into carboxylic acids. They may undergo polymerization with the elimination of water molecules.

  • Oligosaccharides: The carbohydrates whose molecules give two or more molecules of same or different monosaccharides upon hydrolysis are called as oligosaccharides. These are soluble in water and sweet in the test. These are further classified into different classes depending upon the no of molecules of monosaccharides produced by hydrolysis.
  1. Disaccharides
  2. Trisaccharide
  • Tetrasaccharides
  • Polysaccharides: The carbohydrates which give a large number of monosaccharide molecules upon hydrolysis are called as polysaccharides. These are formed by the polymerization of monosaccharides of same or different types. The individual sugar units may be connected to one another to form linear, branched or circular polysaccharides. General formula of polysaccharide is (C6H10O5)
  1. On the basis of functional groups

Carbohydrates can be classified into following two classes on the basis of the functional group present in their molecules.

  • Aldoses: The carbohydrates whose molecules possess an aldehyde group in addition to hydroxyl group are called as aldolases. The presence of an aldehydic group in the molecules differentiates aldoses from other classes of the carbohydrates. The aldehyde group is present in at the end of the carbon chain. Examples: glyceraldehyde, erythrose, ribose, glucose etc.
  • Ketoses: The carbohydrates whose molecules possess a keto group are called as ketoses. These compounds are characterized by the properties of a ketonic group. The ketonic group may be present anywhere in the carbon chain except the terminal carbon atoms. In most of the naturally occurring monosaccharides, the keto group is present at the second carbon atom.Examples: dihydroxyacetone, ribulose, fructose, etc.
  1. On the basis of number of carbon atoms

Carbohydrates have got simple to complex structures ranging from carbon atom 3 to 3,000. The simple carbohydrates or the monosaccharides possess 3 to 6 carbon atoms. The other carbohydrates are polymerization products of these monosaccharides. So the monosaccharides can be classified on the basis of number of carbon atoms present in their molecules in following four types:

  • Triose
  • Tetrose
  • Pentose
  • Hexose
  1. On the basis of taste

Carbohydrates can be classified on the basis of their test into following two classes:

  • Sugars: The carbohydrates which are soluble in water and are sweet in the test are called as sugars. These are crystalline organic compounds.
  • Non-sugars: The carbohydrates which are tasteless and insoluble in water are called as non-sugars. They do not undergo hydrolysis to give simpler carbohydrates. Nonsugar is generally amorphous In nature.

Important Questions
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