Each organism has its own unique marker proteins on its membranes. One of the functions of marker proteins is to enable a person's immune system to distinguish its cells from those of invading cells. Active transport requires the use of energy. They move materials in and out of the cell. Some of them function as pores that allow substances to diffuse through the membrane.
Receptor proteins extend through the cell membrane.
As the communication office of the cell, they allow the cell to interact with other cells. This causes the portion of the protein on the inside of the cell to change shape, triggering a reaction within the cell. The specificity of receptor proteins allow the cell to respond to the outside environment in many different ways. These three classes of proteins are the real workers of the plasma membrane. They allow the membrane to be a dynamic structure that allows materials to be transported and messages to be communicated to the cell.