The smallest unit of living material is the cell. The cell theory is a unifying concept of biology and recognizes that all life is composed of one or more cells. The theory also states all cells are derived from pre-existing cells.
All cells interact with their environment.
In order to function cells must obtain chemicals required for life and eliminate waste materials must be removed. All substances moving into or out of a cell must pass through the plasma membrane. The plasma membrane is the boundary layer of a cell and consists of a phospholipid bilayer. Other chemical reactions occur within a cell. Various compartments within the cell are also membrane-bound.