How do cell membranes protect the cell?
The cell membrane regulates the entry of molecules/ions into the cell and also controls the quantity of the substance entering the cell.
Cell membranes are also called selectively permeable membranes, because they are selective in allowing entry of particles into the cell.
Think about the mesh net of the windows of your house, they prevent unwanted material(eg: insects) from entering your houses but at the same time it does allow air which contains oxygen (vital for survival) to enter.
Similarly the cell membrane regulates the entry of molecules/ions into the cell and also controls the quantity of any substance entering the cell.
Only molecules essential for the survival of the cell and for vital biological processes , are allowed to pass into the cell.