The plasma membrane is the membrane that surrounds cells and controls what comes in and what goes out of that cell. It is made up of layers of phospholipids whos tails are nonpolar (no charge) and the heads of the Phospholipids are Polar (have a charge). Because of the charges of the phospholipids, some substances can not come into the cell. The reason is their charges are different then the layer and therefore, it repels the substance. However, if the substance has the right charge or is small enough to slip through membrane holes, the substance can get past the Phospholipid bilayer and into the cell. Transport proteins can allow some substances in that would normally not be let in.
