The energy is very high for that reaction and it doesn't actually take place. And so we see that proteins do not flip flop. They cannot rotate or move from one leaflet to the opposing leaflet. And so we see that this is precisely what prevents the asymmetry from being lost. So the membrane asymmetry is preserved over long periods of time, because membrane proteins do not actually rotate from one side of the membrane to the other side of the membrane, because this is too energetically unfavorable. Now, in addition, what also retains or preserves the asymmetry of the membrane is the fact that membranes are always constructed from preexisting membranes, and those preexisting membranes are always asymmetric.