The functional significance of cell membranes in cellular biology lies in their role as a selective barrier. Composed of a phospholipid bilayer, they regulate the passage of substances in and out of the cell, maintaining homeostasis. Cell membranes also facilitate communication and signaling through embedded proteins, allowing cells to respond to their environment. They contribute to cellular structure and protect the genetic material within. Overall, cell membranes are crucial for the integrity and functionality of cells.