The role of selective permeability in cell membranes is to regulate the passage of substances in and out of the cell, maintaining homeostasis. Cell membranes, composed of phospholipid bilayers with embedded proteins, allow certain molecules to cross while restricting others. This selective barrier enables the cell to control its internal environment, ensuring that essential nutrients enter, while waste products and harmful substances are kept out. Additionally, selective permeability plays a key role in communication and signaling processes, impacting cellular responses to external stimuli.