The plasma membrane, also known as the cell membrane, protects the inside of the cell and is able to select which substances are allowed into the cell. Plasma membranes are mainly made from phospholipids and cholesterol, although cholesterol is not found in plant cell membranes.
Plasma membranes contain a number of different types of protein. Structural proteins, for example, support the shape of the membrane and make sure that the cell retains its overall structure. Transport proteins have a more complex role, as they allow certain molecules to cross the membrane and enter the interior of the cell.
Another group of proteins, called receptor proteins, are also found in the plasma membrane. These proteins allow the cell to communicate with other cells. To do this, the receptor proteins release hormones and other types of molecules.
Phospholipids are an important component of all plasma membranes. These form the main structure of the membrane and align into two separate layers. The heads of the phospholipids are attracted to water and form two lines facing both inwards and outwards. In between these two lines is the tail of each phospholipid, which is repelled by water. The result is called a lipid bi-layer, which only allows specific molecules to enter the cell.