All organisms are composed of one or more cells.
The cell ( from Latin cella , meaning `` small room '' ) is the basic structural , functional , and biological unit of all known living organisms . Organisms can be classified as unicellular ( consisting of a single cell ; including bacteria ) or multicellular ( including plants and animals ) . Cell theory , first developed in 1839 by Matthias Jakob Schleiden and Theodor Schwann , states that all organisms are composed of one or more cells , that cells are the fundamental unit of structure and function in all living organisms , that all cells come from preexisting cells , and that all cells contain the hereditary information necessary for regulating cell functions and for transmitting information to the next generation of cells .