State the major function of the cell membrane

A major function of the cell membrane is to maintain the characteristic integrity of the cell by forming a selective barrier between the cell and its surroundings. It regulates the movement of various molecules into and out of the cell. For this purpose, the cell membrane is endowed with various transport processes. In general, nonpolar molecules are transported more readily across the membrane than polar or charged molecules. While some substances can diffuse by passive diffusion across the membrane (e.g. water), substances like glucose are transported by facilitated diffusion, involving specific proteins in the membrane. Most biomolecules are, however, transported into and out of the cell in an active process involving expenditure of energy.