The cell membrane is semipermeable or selectively allow the passage of ions of molecules through it. Different routes of transport are present to transport molecules with different properties.
For example, the ions are transported by selective ion channels such as sodium ion channel, potassium ion channel; means they are specific to certain ions.
Thus, the selectivity of cell membrane may facilitate the movement of ions, impede the movement of ions by closure of ion channels or pump ions across the membrane.