Fick's Law of Diffusion
- Fick's Law relates the rate of diffusion to the concentration gradient, the diffusion distance and the surface area
- This relationship can be represented by the following equation, where ∝ means "proportional to"
rate of diffusion (surface area x concentration difference)thickness of membrane
- Proportionality means the rate of diffusion will double if:
- The surface area or concentration difference doubles
- The diffusion pathway halves
- Fick's Law can be written as an equation which can be used to calculate the rate of diffusion
Rate = P x A x ((C1 - C2)T)
- Where
- P = A permeability constant that is a quantitative measure of the rate at which a particular molecule can cross a particular membrane
- A = surface area
- C1 - C2 = the difference in concentration between two areas
- T = thickness of the exchange surface
Worked example
A sample of alveolar epithelium tissue from a mammal is 1.5 m thick and has a surface area of 3 m2. The concentration of oxygen in the alveolus is 1.8 x 10-16 mol m-3 and the concentration of oxygen in the blood is 7.5 x 10-17 mol m-3. The permeability constant for oxygen across alveolar epithelium is 0.012 molecule s-1.
Calculate the rate of diffusion across the section of alveolar epithelium.
Step 1: Substitute numbers into the equation
Rate = P x A x ((C1 - C2) T)
Rate = 0.012 x 3 x ((1.8 x 10-16 - 7.5 x 10-17)1.5)
Step 2: Complete the calculation
Rate = 0.012 x 3 x (1.05 x 10-16 1.5)
Rate = 0.012 x 3 x 7 x 10-17
Rate = 2.52 x 10-18 molecules m-2 s-1
Examiner Tip
You will be given the equation for Fick's Law in your exam, but it is important you understand what it means and how to interpret it.