Like simple diffusion, facilitated diffusion is a passive process.It relies on diffusion through a protein channel or carrier protein.Small molecules which cannot diffuse through the plasma membrane because of the hydrophobic fatty acid tails of the phospholipid forming an impenetrable barrier to polar or charged particles such as charged ions and polar molecules, must pass through transmembrane proteins that span the membrane.The molecules either pass through protein channels or use carrier proteins to go across the membrane.