What are the main functions of the plasma membrane?

The plasma membrane acts as a physical barrier that encloses cells. Therefore, the plasma membrane physically separates the cytoplasm and the extracellular fluid. Additionally, separating the cell cytoplasm from the extracellular fluid is critical in shielding the cell constituents from the external settings. Similarly, the plasma membrane enables various cellular processes to occur both within and outside the cells. Subsequently, by surrounding cells, the plasma membrane offers structural reinforcement to the cells. The selective permeability of the plasma membrane selects the molecules that pass through. Further, selective permeability of the plasma membrane causes materials or molecules to enter the cell through specific pores.  

Cell endocytosis and exocytosis are critical processes that are controlled by the plasma membrane. Therefore, endocytosis involves the ingestion of whole substances that are usually bigger than the cells.  Subsequently, exocytosis is the process through which cells release these substances.