The function of a cell membrane is to act as a barrier that regulates the movement of substances in and out of the cell. Composed of a lipid bilayer, the cell membrane maintains the cell's internal environment, ensuring essential molecules such as nutrients and ions can enter while waste products are expelled. It also plays critical roles in cell communication and signaling, enabling cells to respond to changes in their surroundings. The cell membrane’s selective permeability is vital for maintaining homeostasis.