Water is found in all living cells. An organism's metabolism refers to its ability to use energy for daily functions. The cell theory states that all organisms are made of one or more cells, cells are the basic unit of structure and function, and all cells come from pre-existing cells. The three scientists most responsible for developing the cell theory are Theodore Schwann, Matthias Schleiden, and Rudolf Virchow. For an entity to be considered living, it must exhibit all six characteristics shared by living things.