The phospholipid bilayer acts as a barrier. The inward facing tails of the phospholipids create a hydrophobic barrier that prevents water from entering and exiting the cell freely

The cell membrane is selectively permeable: it can regulate what substances go in and out of a cell. Transport proteins can act as gates and channels for only specific molecules to cross.

Glycolipids and glycoproteins on the membrane's outer face can receive signals from other cells. They help indicate a cell's role to other surrounding cells.

The phospholipid bilayer is not set in place. It constantly moves and shifts, so if a small hole is poked in the membrane, surrounding lipids can fill in the gap instantly.

The glycoproteins and glycolipids on the exterior of the cell send messages to other cells. Part of their function is to help your immune system recognize which cells are yours to be protected and which are foreign and should be eliminated.

A cell's membrane is only 8-10 nanometers thick, but still maintain a definite shape that keeps anything from going in or out. There are 10 million nanometers in a centimeter. It would take 1 million of the thicker lipid bilayers stacked on top of each other to measure up to a centimeter.