what molecules are easy to pass through the membrane
Each method of passage through the cell membrane might be useful to different molecules. Facilitated diffusion is passive transport that uses integral membrane proteins to help larger, charged, hydrophilic, and polar molecules across a concentration gradient. Remember that integral membrane proteins span the phospholipid bilayer, connecting the inside and the outside of the cell. There are two types of integral membrane proteins that help transport molecules, like ions and polar molecules, that can't diffuse on their own through the hydrophobic layer.