Final answer:
The lipid bilayer of a cell membrane prevents ions from passing through due to its hydrophobic interior. Instead, ions and polar molecules can pass through the cell membrane via specific protein channels.
Step-by-step explanation:
The lipid bilayer of a cell membrane acts as a barrier to the passage of molecules and ions. The hydrophobic interior of the phospholipid bilayer does not allow ions or polar molecules to pass through because these molecules are hydrophilic. Instead, ions and some polar molecules are able to pass through the cell membrane through specific protein channels that are embedded in the lipid bilayer.