The carriers for membrane transport are integral membrane proteins. In carrier-mediated transport, the carriers do not move through the thickness of the membrane, carrying its substrate with it. Rather, a ping-pong mechanism is proposed.
In this model, the carrier protein exists in two principal conformations: ping and pong. In the pong state, it is exposed to high concentrations of solute, and molecules of the solute bind to specific sites on the carrier protein.
Transport occurs when the conformation changes to the ping state, exposing the carrier to a lower concentration of solute. In facilitated diffusion, the energy for the transition between the ping and pong states comes from the binding of the substrate to the carrier. In active transport, additional energy comes from the binding of ATP to the carrier.