Particle movements from high to lower concentrations are called diffusion . Only after an even distribution is achieved can no further net movements be observed, the system has reached an equilibrium. 1 and c 2 are the concentrations of the chosen planes, x 1 - x 0 is the distance between them. Since the concentration decreases with growing distance has the concentration gradient a negative value. The precept expressed in this formula is also called FICK's first law of diffusion .
Diffusion is very quick over short distances but extremely slow at long ones.
It is the square of the distance that has an influence on the formula and it is proportional to the available area. Diffusion is important when regarding the molecular movements within cells or between neighbouring cells. But it looses its significance when regarding whole tissues and ceases to have any effect at all when, for example, thinking of the assimilate transport from leaves to roots. Permeability Permeability is the term that describes the diffusion of particles through membranes. We will in a rough approximation assume that the membrane equals a solvent layer of the thickness d in order to obtain a quantitative statement.