All living systems need matter as well as energy in one form or the other for their growth and maintenance. The bodies of living system can be compared to factories. Just as factories require fuels for liberation of energy and machines for the proper utilization of energy, the living system also require fuel and machines. Food constitutes fuel for living beings and enzymes act as machines. The major constituents of food are carbohydrates, fats and proteins. The enzymes help in the conversion of food into body parts through the series of chemical processes which are also called as biological reactions.

The branch of science that deals with the study of the chemical composition and the structure of living organisms and also various chemical changes taking place within them is called as biochemistry. The complex organic molecules which form the basis of life; i.e., which built up living organisms and are also required for their growth and maintenance are called biomolecules.

The cell is the fundamental unit of life. A cell contains 50 elements. The eleven most abundant elements in living organisms are O, C, H, N, Ca, P, K, S, Cl, Na and Mg. The first four elements oxygen, carbon, hydrogen and nitrogen together account for 96% of man's total mass. The most abundant substance in a living cell is water which amounts to about 70% of the weight. In addition to water, the cell contains a large number of carbon compounds. These compounds can be divided into two classes. The compounds of first class are small molecules having molecular masses in the range of 100 to 1000 and containing upto 30 carbon atoms. These are found in free solution in the cytoplast of the cell. They form a number of intermediates from which the molecules belonging to the second class are synthesized.

The compounds of the second class are macromolecules. Among these the most important are carbohydrates, proteins, lipids and nucleic acids. Carbohydrates constitute an important class of compounds like glucose, fructose, sucrose etc., which play an important role in everyday life and having a general formula Cx(H2O)y, and they are considered to be hydrates of carbon. Proteins are the complex organic substances which are the basis of protoplasm and are found in all living organisms. They are polymers of a-amino acids. Like peptides the amino acid units in proteins are held up by peptide linkage. In fact, polymeric products of a-amino acids with molecular mass upto 10000 are called polypeptides while those having molecular mass more than 10000 are considered as proteins.