These types of proteins are tunnels that selectively control the transport of ions and other molecules into and out of a cell. This is a VERY rough analogy, but they are sort of like a pet door, for say a cat. These doors are very small and allow only small cat to pass through. Large dogs are block by their size. It's much more complicated than that, but this may work for teaching elementary students.
METAMIA is a free database of analogy and metaphor. Anyone can contribute or search. The subject matter can be anything. Science is popular, but poetry is encouraged. The goal is to integrate our fluid muses with the stark literalism of a relational database. Metamia is like a girdle for your muses, a cognitive girdle.