All organisms begin their life as a single cell. Certain organisms complete their life cycle as a single cell. They are called unicellular or acellular organisms, e.g., Amoeba, Chlamydomonas, bacteria and yeast. In other organisms, single cell undergoes divisions to form multicellular body.
Body of human being, is made up of trillion of cells.
All the cells of an organism carry the same genetic material, developed from same pre-existing cells and possess several organelles to perform various life activities. The cells are therefore, basic unit of life and structural unit of an organism.