The significance of cell membranes in cellular function lies in their role as selectively permeable barriers. They regulate the entry and exit of substances, maintaining homeostasis within a cell. Composed of a phospholipid bilayer with embedded proteins, cell membranes facilitate communication, signaling, and transport processes critical for cellular metabolism. They also play a key role in protecting the cell from the external environment and are involved in cell recognition through specific molecular interactions, which are vital for immune responses and tissue formation.