How does polarity of molecules affect diffusion?
The polarity of molecules has little effect on their diffusion through inorganic materials.
The situation is different when molecules in solution have to pass through a cell membrane.
Cell membranes consist of a phospholipid bilayer. The phospholipids have a polar head and a nonpolar tail.
They arrange themselves to form a bilayer in which a row of heads faces the outside of the cell. Another row of heads faces the inside of the cell. The nonpolar tails are between the rows of heads and thus not in contact with water.
Nonpolar molecules such as CO₂ and O₂ can diffuse through the lipid bilayer in each direction. Polar molecules cannot diffuse through the bilayer.
Polar molecules need polar protein channels to diffuse through the membrane. This is "facilitated diffusion".