Answer:
The correct answer would be plasma membrane or cell membrane.
Plasma membrane is phospholipid bilayer structure which separates the intra-cellular components from extra-cellular environment.
One of the most important characteristics of the plasma membrane is that it is selectively permeable in nature.
It allows some substances to move freely in and out of the cell while restricts the movement of other substances. It is similar to function of security guard who decides whom may enter and whom may not.
Small molecules and uncharged particles can travel across the plasma membrane easily by simple diffusion. For example, exchange or diffusion of gases (carbon dioxide and oxygen) across the cell. The movement of such substance depends upon its concentration gradient across the membrane.
Charged particles and large sized substances require protein channels to travel across the cell. Such substances are transported either with the help of facilitated diffusion or active transport.
