Final answer:
The lipid bilayer acts as a selective permeability barrier, controlling the passage of ions and molecules through the cell membrane, with some requiring protein channels or pumps and others passing by diffusion.
Step-by-step explanation:
The role of the lipid bilayer is to act as a selective permeability barrier. This structure is essential in controlling the movement of ions and molecules into and out of the cell. The phospholipid bilayer is tightly packed, with a hydrophobic interior that causes the membrane to allow only certain substances to pass through it. Only relatively small, nonpolar materials can move through the lipid bilayer without assistance. Substances that do not meet these criteria require transport via passive or active processes, utilizing protein channels and pumps or diffusion mechanisms such as osmosis.