Selective permeability refers to the one of the distinctive characteristics of the plasma membrane. The plasma membrane is the membrane that contains the cytoplasm. In the animal cell, it is the outermost covering. In a plant cell, the plasma membrane is found beneath the cell wall. The plasma membrane is capable of being selectively permeable because of its structure. It is composed of a bilayer of phospholipids interspersed with proteins. The phospholipid part of the plasma membrane renders the latter hydrophobic and therefore polar molecules would not be able to easily pass through this layer. The proteins embedded on the plasma membrane act as transporters or channels for certain molecules (e.g. ions). Thus, the composition and structure of the plasma membrane makes the latter semipermeable, i.e. selective to the entry of certain molecules and substances. This means that not all substance would be able to gain entry or exit through the cell. The selective permeability of the plasma membrane helps the latter to carry out an important function of controlling or regulating the passing through of substances between the external environment and the cytoplasm.