What makes a cell membrane selectively permeable?

A cell membrane is selectively permeable because it only allows particles to move across them due to different concentrations. Molecules move from areas of low concentration to high concentration.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.

The Lipid bi-layer of the cellular membrane allows simple molecules to pass through. However, for certain proteins, a special transportation mode is needed.