The plasma membrane is the thin (about 10 nanometers thick) layer of tightly packed fat and protein molecules that separates the interior of the cell from the surrounding fluid chamber. Plasma membranes - cell membranes - should not be confused with the cell wall, which often consists of cellulose or cellulose-like substances and is 100 times thicker than cell membranes.
Among the protein molecules, one should note the functionally very important ones: the receptor proteins, the channel proteins, the pump and transport proteins, the G proteins, etc.