A living system grows, sustains and reproduces itself. The most amazing thing about a living system is that it is composed of non-living atoms and molecules. The pursuit of knowledge of what goes on chemically within a living system falls in the domain of Biochemistry. Even though there are thousands of different types of molecules in a cell, there are only a few basic classes of bimolecular like carbohydrates, proteins, nucleic acids, lipids, etc. Proteins and carbohydrates are essential constituents of our food. In addition, some simple molecules like vitamins and mineral salts also play an important role in the functions of organisms. The complexity of even the simplest of life forms, the single cell, cannot be overstated. Nevertheless, from a chemical perspective, cellular components can be segregated into macromolecules (DNA, RNA, proteins, etc.), relatively simple molecules (amino acids, monosaccharideâ€™s, and lipids), and their precursors: CO2,H2O, and NH3. In general, the macromolecules tend to be polymers of small bimolecular; however, each of these molecules, whether simple or complex, is involved in a myriad of intricate metabolic reactions. A case in point is the monosaccharide glucose which is synthesized from H2O and CO2. When degraded to its precursors, it provides the cell with its energy requirements for such diverse processes as macroscopic movement as well as the synthesis of complex macromolecules. In addition, glucose is the fundamental building block of macromolecules such as starch and cellulose. This basic theme, in which the cell uses a simple small molecule in a multitude of processes, is typical of how relatively small bimolecules are used in living systems. In this chapter we will elaborate the chemistry, properties and metabolism of four bimolecules: amino acids, carbohydrates, lipids, and nucleotides and their roles in metabolism. You are aware of that our body, plants and other animals are made up of many chemical substances. There are certain complex organic molecules which form the basis of life. These build up living organisms and are also required for their growth and maintenance. Such molecules are called bimolecules. The main classes of bimolecules are carbohydrates, proteins, lipids, nucleic acids, enzymes, hormones etc. In this lesson, you will study about the structures and functions of some important bimolecules.