The structure that encloses and protects a cell is known as the cell membrane, or plasma membrane. This selective barrier is composed of a phospholipid bilayer with embedded proteins that regulate the movement of substances in and out of the cell. It plays a critical role in maintaining the cell's internal environment, facilitating communication with other cells, and anchoring structures within the cell. The cell membrane is essential for cellular function and integrity, influencing processes such as signal transduction and metabolism.