If there's no selective permeability, the cell would be able to allow all substances into the cell, in other words, the cell will be "permeable".
Normally, the biological membranes have a selective permeability, that is to say, they let themselves through some substances more easily than others, quickly pass through the membrane, because they dissolve in the lipid bilayer.
*Hydrophobic substances rapidly cross the membrane because they dissolve in the lipid bilayer.
*The small polar molecules (like H2O and CO2) thanks to their small size, these polar molecules also cross the double lipid layer.
*Large molecules and ions pass through the membrane through specific transport proteins, such as uniport, symport or antiport.
