Because of the way these little guys work, they can only carry so many molecules, only work so fast. An interesting perspective can be seen in figure 2, flux and solute. Since the protein can only work so fast, there is a maximum flux, or velocity,at which flow occurs. The important point here is that no matter how large the concentration, the maximum flow does not change. What is next in importance is the direction of flow.
Free diffusion flows along the gradient.
Assisted or facilitated diffusion also flows along the gradient. It takes energy to do work. Our cells use a molecule called Adenosine TriPhosphate, ATP, to work. A classic pump is the Sodium - Potassium ATPase pump. The pump pumps potassium into the cell, sodium out of the cell and utilizes the energy contained in a molecule of ATP to accomplish this.