The function of the lipid bilayer in cell membranes is to provide a semi-permeable barrier that regulates the movement of substances in and out of the cell. Comprised mainly of phospholipids, the lipid bilayer allows certain molecules to pass while blocking others, maintaining the internal environment of the cell. This property is critical for maintaining homeostasis and facilitating communication between cells. The lipid bilayer also plays a role in the formation of membrane proteins, which are essential for various functions, including cell signaling and transport.