The primary function of a cell membrane is to protect the cell and regulate the movement of substances in and out of it. This semi-permeable membrane is composed of a lipid bilayer with embedded proteins, allowing it to maintain homeostasis by controlling what enters and exits the cell based on size, charge, and molecule type. It plays a crucial role in communication between cells through the reception of signals and can assist in transporting nutrients and waste. The cell membrane also contributes to the cell's structure and shape, reinforcing its integrity in varying environmental conditions.