What is the structure and importance of the cell membrane?

The cell membrane regulates the transport of substances in and out of the cell. It is selectively permeable, and therefore allows for the passage of some substances more readily than others. As the person above me has said, it also serves as a protective barrier that protects the interior of the cell from its external surroundings, thus helping to maintain a stable internal environment. Plants have a cell wall, IN ADDITION TO a cell membrane.