Jardetzky’s allosteric model for membrane pumps was a thought experiment from the 1960s.
The model demonstrates a method membrane pumps may work by. The substrate binds to a binding site, the protein changes conformation allowing the substrate access to the other side of the membrane, and the substrate is released. This method ensures the pump only has access to one side of the membrane at a time, preventing the concentration gradients of other compounds from being diminished.
On the side of the membrane with a high concentration of the substrate (the P side), the binding site has a low affinity to the substrate, and so it is easier to release it. On the side of the membrane with a lower concentration of substrate, the binding site has a higher affinity. This encourages binding, and allows the uptake even in low concentrations.