Which substances cannot diffuse through the lipid bilayer?

What are proteins that serve as tunnels for specific substances through the lipid bilayer?

• Answer:

  The lipid bilayer is a tightly packed membrane of hydrophobic (dont like water) molecules, so only hydrophobic or very very small molecules can go directly through it. Large molecules such as hormones need to have a special hole in the membrane that they can fit through, but some are so large that if the membrane created a plain old hole, everything would be able to fit through, even things the cell doesnt want. The proteins create special openings that allow only a specific variety of molcule to go through, like only a special type of hormone. They work a lot like a lock and key, so when the molecule encounters its protein, it acts like a key and opens the portal. once the molecule passes, the protein closes the opening and nothing else can go through.