The plasma membrane is more than just a sack to hold the contents of the cell. It plays an important role in cellular function and the maintenance of homeostasis. One obvious function is to regulate what enters and leaves the cell. This process is highly coordinated and very specific. In addition, the cell membrane responds to countless chemical messengers in ways that alter the activity of the cell. As we discuss the structure of the plasma membrane, keep in mind that this description also applies to other membranes that are components of intracellular organelles.
Our modern model of the cell membrane is called the Fluid Mosaic Model of the Cell Membrane. The word fluid implies that the membrane is constantly changing and moving. Indeed, it is not a static structure but one that changes depending on cellular need and environment. The term mosaic conjures up an image of numerous small and different pieces. Indeed, the membrane contains many different components including lipids, proteins, and carbohydrates.