The sell membrane, other wise known as the plasma membrane, separates the inner part of the cell from the outer part of the cells domain. It's whole being of the plasma membrane is to protect the cell from everything around it. The plasma membrane must also be very flexible to allow certain cells. For example, red blood cells and white blood cells as they will pass through the capillaries. Its second roll that the plasma membrane plays is hold down the cytoskeleton, which gives the cell its shape. It then attaches to the extracellular matrix and many other cells to make tissue. All in all, if you were to think about it, the plasma membrane is like protective wall to the cell. it only lets in the cells that it knows, and protects cell form all the bad.