The internal composition of the cell is maintained because the plasma membrane is selectively permeable to small molecules. A lot of biological molecules are unable to distribute by the phospholipid bilayer so the plasma membrane develops a barrier which blocks the free exchange of molecules between the cytoplasm and the external environment of the cell. Exact transport proteins carrier proteins and channel proteins then mediate the careful passage of small molecules across the membrane, permitting the cell to control the composition of its cytoplasm.