The rate of simple diffusion is directly proportional to the concentration gradient across the membrane and the permeability of the membrane to the solute.
Examples of simple diffusion include diffusion of gases across the respiratory membrane and diffusion of solutes from blood to the cells through the interstitial spaces.
The rate of facilitated diffusion is governed by the kinetics of carrier- mediated transport and cannot exceed a certain maximum, called the Vmax (Fig. 4.1). However, at low concentration gradients, its rate is higher than in simple diffusion.
Examples of facilitated diffusion are: (i) the cotransport of Na+ with sugars (glucose, galactose, fructose) and amino acids in renal tubular cells and intestinal mucosal cells, and (ii) the counter-transport of CI-HC03" in renal tubular cells and gastric parietal cells.