Xia has been looking forward to the Elementary Science Fair for weeks. Her fourth-grade class had just finished learning about the biological cell, and she could not wait for you to walk through the greatly over-sized cell model that is at the fair so she could explain all of the parts and what they do. When the both of you finally got to the cell model your first challenge was figuring out how to get inside. The "membrane" of the cell had no holes in it. You both walked around the cell until you finally found a small doorway. A sign above the doorway identified this type of structure, through which you could both crawl to enter the cell.

Which membrane structure could these hydrophilic molecules that you (and Xia) pass through be? What other functions do similar membrane structures perform? Is there anyway that anything else could pass through the membrane without the use of these membrane structures? If so how and if not, why not?