The cell membrane functions as a selective barrier that regulates the movement of substances into and out of the cell, playing a crucial role in homeostasis. It maintains the internal environment by controlling the exchange of ions, nutrients, and waste products. The membrane's structure includes a phospholipid bilayer embedded with proteins that facilitate transport and communication. This selective permeability ensures that essential molecules enter the cell while harmful substances are kept out, contributing to the overall balance and health of the cell.