This Demonstration shows the diffusion of oxygen molecules across a membrane (represented by a box) according to Fick’s law. The graphic is not to scale and the molecular motion is idealized.
The rate at which molecules diffuse through the membrane is a function of the membrane thickness and surface area. The diffusion rate is calculated using Fick’s law:
is the diffusion constant, equal to 0.009048 (
is the surface area (
is the oxygen pressure difference (mmHg), which is set to 60 mmHg; and
represents the thickness (
). Flux is also defined as
; it is displayed in one of the plots as
). Known diffusion constant and oxygen pressure values actually determine the thickness and the surface area.
The thickness and surface area of the membrane can be selected using the sliders. The play button for molecular motion shows the molecules moving across the membrane. Plots are shown for thickness versus flux, thickness versus rate and surface area versus rate.