The primary function of the cell membrane is to act as a barrier that separates and protects the cell from its external environment. It is often described as a selectively permeable membrane, allowing certain substances to pass while blocking others. This enables the cell to maintain homeostasis by regulating the movement of ions, nutrients, and waste products in and out of the cell. The cell membrane also plays an essential role in communication between cells through receptor proteins, facilitating responses to various signals. Additionally, it helps maintain cell shape and integrity.