Biochemistry is a branch of Biology, which deals with the study of chemical components and the chemical processes in living organisms. A basic knowledge of Biochemistry is essential for understanding anatomy and physiology, because all of the structures of an organism have biochemical organization. For example, photosynthesis, respiration, digestion, muscle contraction can all be described in biochemical terms.

All living things are made of certain chemical compounds, which are generally classified as organic and inorganic. Most important organic compounds in living organisms are carbohydrates, proteins, lipids and nucleic acids. Among inorganic substances are water, carbon dioxide, acids, bases, salts.

Typically an animal and a bacterial cell consists of chemicals as shown in the following table.

Table 2.1 Chemical composition of a Bacterial and a Mammalian cell.

Chemical components

% total cell weight

Bacterial cell       mammalian cell


70                               70


15                              18


3                                4


2                                3


1                             .25


6                              1.1

Other organic molecules (enzymes, hormones, metabolites)

       2                               2

Inorganic ions


    1                                      1


The survival of an organism depends upon its ability to take some chemicals from its environment and sue them to make chemicals of its living matter. For this reason, cells of every organism are constantly taking are constantly taking in new substances and changing them chemically in various ways i.e. building new cellular materials and obtaining energy for their needs. Life of an organism depends upon the ceaseless chemical activities in its cells.

This chemical activity is maintained with a high degree of organization. All the chemical reactions taking place within a cell are collectively called metabolism. Metabolic processes are characterized as anabolism and catabolism. Those reactions in which simpler substances are combined to form complex substances are called anabolic reactions.

Anabolic reactions need energy. Energy is released by the break down of complex molecules into simpler ones, such reactions are called catabolic reactions. Anabolic and catabolic reactions go hand in hand in the living cells. Complex molecules are broken down and the resulting smaller molecules are reused to form new complex molecules. Interconversions of carbohydrates, proteins, and lipids that occur continuously in living cells are examples of co-ordinate catabolic and anabolic activities.