Cell theory is useful part of biology. It’s defined that all plants are formed of one or more cells. It is defined that structure of all living things is made by cell. Cell is the basic unit of structure and new cells are formed from already existing cells. A cell is a small mass of proplasm usually containing a nucleus or nuclear material and some other organelles, and is bounded by a cell membrane. Cell is the unit of function, reproduction, heredity and disease, besides being a unit of structure.