The plasma membrane has several functions, including separating the cell from its external environment, housing receptors, protecting the cell from external interference and allowing permeability when necessary. Other functions include adhesion, signal transduction and insulation.
One of the main roles of the cell membrane is to compartmentalize it from its surrounding environment. It keeps the nucleus and other cell compartments separated from the watery environment that exists outside the cell. By doing this, it protects the cells from substances and molecules that may be harmful to it. In some cases, it is necessary for items to enter or exit the cell, so the membrane allows for permeability when this occurs.
Recognition and response are two additional functions of the membrane. The plasma membrane is often the site where a cell's receptors are located. When a molecule from the outside recognizes this receptor, it can bind to it and initiate action within the cell.
The actions that take place within the cell after recognition occurs are called signal transduction. When cells need to stick to each other, the structure of the membrane also allows for cell adhesion. Other markers on the cell include those that help other molecules and cells to recognize and identify it.