A student makes a model of a cell. He uses a plastic bag to represent the cell membrane. He fills it with a clear gel to represent the cytoplasm. He puts in other materials to represent parts of the cell within the cell membrane. His teacher tells him that the plastic bag is not a good representation of the cell membrane. She suggests that the student find a material that better shows how a cell membrane works. The student makes another model using a woven cloth bag to represent the cell membrane. The teacher tells him that he made a good choice.