What is the function of a cell's selectly permeable membrane

Definition of Selectively Permeable Membranes
All cells are enclosed with a cell membrane. A selectively permeable cell membrane is one that allows certain molecules or ions to pass through it by means of active or passive transport. Active transport processes require the cell to expend energy to move the materials, while passive transport can be done without using cellular energy. It is important to note that the membrane can also block the passage of materials depending on the conditions inside or outside of the cell.