What is an analgoy for the call membrane

The cell membrane is supposed to be permeated by a few different materials. If the chemical makeup of a material is not what the membrane wants to allow through, then the membrane holds strong and refuses entry. This is just like a drawbridge or gate. The gatekeeper will only let a person through the wall or fence if he recognizes the person who wants to enter.

A fluid mosaic, because it is always changing shape and form