Movie 6. P_low is 0.01; fast. For this simulation, the probability of being in the low-affinity state (P_low) is reduced to 1%. The simulation is run for 200,000 steps, displaying every 200th frame. Because the K-binding site is almost always in the high-affinity state, a K ion is occupying the filter almost continuously. This very efficiently prevents Na ions from permeating, resulting in high ion selectivity (K/Na flux ratio is » 100). Whenever the binding site switches to the low-affinity state it releases the bound ion, which is almost always a K ion that was trapped.