The phospholipid bilayer's function is to maintain a barrier between the cell and its external environment and to store and transport a variety of proteins that are essential to the cell's function. It controls what enters and exits the cell.
The phospholipid bilayer serves as the main barrier between the cell's internal components and its extracellular environment, which consists mainly of the cytoplasm. By doing this, it controls and maintains a balance of molecules that are present in the cell, such as proteins and ions. Several proteins exist in the bilayer that help to control what enters and exits the cell. These are often referred to as transmembrane or transporter proteins. The cell alters the gene expression of these proteins to create more or less in response to its need to transport items across the membrane.
While the uptake of molecules into the cell is one major function, the bilayer also serves to prevent certain molecules from entering that may be harmful to the cell. Molecules housed on the surface of the bilayer have several other functions in addition to transport. For example, they may serve as communicating molecules, signaling messages between the internal part of the cell and external molecules such as proteins.