Why is a cell membrane selectively permeable?
it´s selectively permeable because some substances can pass across the membrane and others cannot, and it´s always trying to keep the proper amount of water and nutrients inside and outside the cell.
The cell needs to control what enters and leaves. It needs to keep somethings in and other things out. However, only the Selectively cells can enter and go out of the cell! This is why the selectively permeable membrane is permeable! A cell membrane (the outer casing or inner casing in a plant cell) is selectively permeable because some materials can pass freely through the cell membrane while others cannot.
Selectively permeable means that only certain substances are able pass through the cell membrane. A selectively permeable membrane is a membrane that only can be permeated by selective things, in other words a surface that lets some molecules in but not others. Like a cell membrane will let water diffuse into it :)