It will take place spontaneously because free energy is released into the environment. Now, the second function of the selectivity filter, this sequence of five amino acids within this restricted area is to basically give that specific channel its ion specificity. So we see that the selectivity filter also determines the specific nature of that ion channel, its ability to actually move these potassium ions across that membrane. The question is why? Well, let's begin with the easy case. Let's suppose that we take an ion that has a greater radius than the potassium.