The type of macromolecule that can move polar substances across membranes via facilitated diffusion and/or active transport are the lipids. This group of macromolecule would belong to the family of organic substance which would include waxes, cholesterol, vegetable oils and fats. Facilitated diffusion is a process wherein there is a spontaneous transport of molecules across a membrane. It happens in almost all of the cells. Lipids would undergo this diffusion across a phospholipid bilayer by the carrier proteins. Since the bilayer contains lipids then only molecules that are soluble in lipid can pass the membrane. Most likely these are small polar uncharged molecules.
