* Small, nonpolar molecules can easily slip through the phospholipid bilayer of the membrane, as they are attracted to the hydrophobic tails of the phospholipids. Examples include oxygen, carbon dioxide, and some small lipid-soluble molecules.
* Large, polar molecules have difficulty crossing the membrane because they are repelled by the hydrophobic tails and cannot easily form interactions with the phospholipids. They often require the assistance of transport proteins to move across the membrane. Examples include sugars, amino acids, and ions.
