Final answer:
The cell membrane's selective permeability is due to transport proteins that help polar molecules, ions, and larger molecules to pass through the phospholipid bilayer.
Step-by-step explanation:
The cell membrane is selectively permeable due to the molecular structure of the phospholipid bilayer and the presence of transport proteins. Small nonpolar molecules like carbon dioxide (CO2) and oxygen (O2) can easily diffuse through the bilayer. However, polar molecules, ions, and large molecules cannot pass through the hydrophobic core of the membrane on their own. They require the help of specialized membrane proteins such as channel proteins, gated channel proteins, and carrier proteins to cross the membrane.
There are two major classes of proteins in the cell membrane that assist in transport: integral proteins, which span the hydrophobic interior of the bilayer, and peripheral proteins, which are associated with the surface of the bilayer. These proteins facilitate the movement of ions, polar molecules, and other substances that cannot freely diffuse through the lipid bilayer, making membranes selectively permeable to specific molecules.