The function of the cell membrane is to regulate the entry and exit of substances in and out of the cell. It acts as a selective barrier that maintains homeostasis by controlling the movement of ions, nutrients, and waste products. The membrane is composed of a phospholipid bilayer with embedded proteins, allowing for various functions such as communication, signaling, and transport. Additionally, the cell membrane plays a crucial role in cell recognition and protection against harmful substances. This dynamic structure is essential for maintaining the overall functionality of the cell.