Answer: Police officer who allows selected people to enter and leave.
The cell membrane acts as the protective covering of the cell. This membrane is selectively permeable.
The membrane which is selectively permeable that allows only certain molecules to pass through it. The smaller molecules and the non polar molecules are passed through this membrane.
The molecules such as sugar and protein are not allowed to pass through the selectively permeable membrane.
In this way the cell membrane acts as policeman that allows the movement of selected people only.
