Diffusion means that the net movement
of particles (molecules) is from an area of high concentration
to low concentration. If the particles can move through the
lipid bilayer by simple diffusion, then nothing limits the
number that can fit through the membrane. Thus, the rate of
diffusion increases linearly as we add more particles to one
side of the membrane.
If the particles can only pass through
protein channels, then the rate of diffusion is determined
by the number of channels as well as the number of particles.
Once the channels operate at their maximal rate, a further
increase in particle numbers no longer increases the apparent
rate of diffusion. At this limited rate we describe the protein
channel as being saturated.