State one reason why molecule may not be able to pass into or out of a cell

An ion is a molecule that is charged because it has lost or gained an electron. The cell membrane is made of a bilayer of phospholipids, with an inner and outer layer of charged,hydrophilic "heads" and a middle layer of fatty acid chains, which are hydrophobic, or uncharged. Charged ions cannot permeate the cell membrane for the same reason that oil and water don't mix: uncharged molecules repel charged molecules. Even the smallest of ions -- hydrogen ions -- are unable to permeate through the fatty acids that make up the membrane. If ions "want" to enter the cell due to a high concentration of that type of ion on one side of the cell, they can do so by entering through the protein channels that are embedded between the lipids.

Might be a bit long but I hope that this helps!

They repel eachother or the shell is to thick so they cannot enter the molecule

Probably a behavioral adaptation