The plasma membrane
The plasma membrane is a thin layer about 75 Å thick that surrounds the cell and separates it from the external environment. It is a very flexible layer that allows movements and deformations of the cell.
The plasma membrane has the function of regulating the exchange of substances between the cytoplasm and the exterior of the cell.
This type of membrane constitutes, with very few differences, the membrane of the cell's membranous organelles, which is why it is called a unit membrane.
Chemically, it is a double layer of lipids (with amphipathic behavior) with proteins embedded or located on both sides of the membrane. Under the electron microscope, this membrane is seen to be made up of three bands: two dark protein bands and, in the middle, a light band of the lipid double layer.