Lipid bilayers are completely impermeable to charged ion and large polar molecules. Some very small water soluble molecules can slowly difuse through the lipid bilayer.
Regulation is accomplished using membrane proteins.
Channel proteins - forms a hydrophilic pore across a membrane Pore is selective but does not bind molecule being transported. Channels typically flicker open in response to a stimulus and shut automatically. Must be down conc. Transporter proteins - binds to ion or molecule being transported may be up or down conc. Inside of cell is negative with respect to the outside of the cell.