All the living organisms, whether plants or animals, from smaller to the largest, from microscopic to huge whales and trees are made up of small, tiny structures or compartments called cells. All the life processes, i.e., metabolism, responsiveness, reproduction, etc. are carried out by the cells. The cell is the seat of all Metabolic (anabolic as well as catabolic) processes. The cell has mitochondria for the oxidation of food and to produce energy. It utilizes nutrient molecules to synthesize complex molecules. It has the power of division to increase its number for growth and healing up of the wounds. Thus, respiration, nutrition, and release of energy for the body are carried out within the cells only, and the growth of the body occurs because cells grow and multiply. Even the organisms reproduce because the cells reproduce individually. The cell can regulate their activities to maintain their physicochemical environment.
Microscopic and one-celled organisms like Amoeba, Paramecium, Euglena, Chlamydomonas, etc. are called unicellular organisms. All such organisms perform their life processes within the framework of their one-celled body. There are organisms which are bigger in size and are made up of several million cells and are called multicellular organisms. Unlike unicellular organisms, the cells in the body of multicellular organisms share their biological activities to maintain their life. In fact, there is a division of labor in the cells forming the body of multicellular organisms. Due to this division of labor, the cells in the body of multicellular organisms undergo a wide range of differentiation and specialization and co-ordinate their activities which maintain their life. Therefore, in the body of multicellular organisms, tissues, organs, and organ systems have developed. But the basic structure of all tissues, organs, and organ systems are the cells only, and these work because the cells work. It is, thus, clear that the cell is the structural and functional unit of a living body. In other words, the cell is the basic unit of life.