We know that cell membranes act as a barrier to most but not all molecules. That is why cell membranes are called differentially (or semi-) permeable membranes separating their inner cellular environment from the outer environment.

Functions of cell membranes:

While exchanging matter with the cell environment cell membrane maintains equilibrium inside the cell walls as outside. This control of the passage of molecules into and out of the cells is made possible through the phenomena of diffusion, osmosis, filtration, active transport, endocytosis, and exocytosis.