From the simplest bacteria to the most complex human cell, the plasma membrane has retained such an important function in cells that its structure has been conserved over a wide range of species. The plasma membrane is selectively permeable; it regulates what passes from one side to the other.
A selectively permeable membrane will allow water to flow freely while limiting the passage of molecules, especially large molecules or those with a charge. A plasma membrane provides some protection to the insides of cells, but its main life-giving function is to control the passage of substances into and out of cells