Plasma membrane is the outer membrane of the cell. It is composed of lipids bilayer which is quasi fluid in nature. The plasma membrane is semi permable in nature. Besides, the lipid content it also has protein and carbohydrates. It provides protection and structural support to the cell. The plasma membrane helps in the transport of material in and out of the cell. The fluid nature of the membrane helps in the formation of cell wall, secretion, cell growth etc.
Receptor molecules are specific proteins in the cell's plasma membrane that receive chemical signals from outside the cell. When such chemical signals bind to a receptor, they cause some form of cellular response.