The cell membrane is made up of phospholipid bilayer and proteins. It is a living semi-permeable membrane and regulates the solutes which can pass through it. The membrane is in general impermeable to ions because they are charged and, hence, insoluble in lipids. The cell membrane has special transport proteins which allow passage of specifically selected ions only. This is important for maintenance of proper electrochemical balance between the cell and environment. Thus, the correct answer is option D.