Final answer:
The hydrophobic interior of protein-free lipid bilayers acts as a barrier to the passage of molecules and ions into and out of the cell.
Step-by-step explanation:
Phospholipid bilayers are critical components of cell membranes. The lipid bilayer acts as a barrier to the passage of molecules and ions into and out of the cell.
However, an important function of the cell membrane is to allow selective passage of certain substances into and out of cells. This is accomplished by the embedding of various protein molecules in and through the lipid bilayer. These proteins form channels through which certain specific ions and molecules are able to move.
Many membrane proteins also contain attached carbohydrates on the outside of the lipid bilayer, allowing it to form hydrogen bonds with water.