The main steps of the sodium-potassium pump system in terms of rate trends. Steps of the sodium-potassium pump determined by the transition rate trends. There are arrows only for the rates that exhibit positive trends. Pump states in gray are those whose the rate in the forward direction has positive trend, whereas those in white have the backward rate exhibiting a positive trend. The thickness of the central arrow in the forward direction indicates the strength to the general trend of the pump in this direction. Thus, the thicker the arrow, the greater the tendency of the pump to run in the forward direction. (A) Initial step. The double circle represents the initial state of the system. (B) Intermediate step. (C) Final step.