Natural membranes like the cell membrane is selectively permeable, which means that it allows only certain molecules to get in or go out of the cell. This prevents random molecules from getting in like toxins which would otherwise be harmful for the cell.

Example of a cell membrane is where it allows only non-polar and small molecules through the lipid bilayer along the concentration gradient. To move other important molecules, it needs either energy (active transport) against the concentration gradient or needs aid from proteins like carrier and channel proteins that facilitate the movement of other specific molecules along the concentration gradient.