Cells have numerous membrane bound organelles which divide the cell into different compartments. Each organelle has separate identity within the cell and can perform their functions independently and effciently. For example - the membrane of mitochondria divides it into two compartments one between the inner and outer membrane and the other inside the inner membrane.
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.