So the question is, how is this property actually achieved by that potassium ion channel? And in general, how is it that these ion channels are so specific to types of ions that they move across the cell membrane? Well, to answer this question, let's take a look at the following diagram. So the purple section is basically this tetromer potassium ion channel in a simpler form. And this is the internal cavity that the potassium ions will actually pass across at the same time. That's the same cavity that will block all other ions from actually moving across.