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" 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 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 μm thick and has a surface area of 3 μm2. The concentration of oxygen in the alveolus is 1.8 x 10-16 mol μm-3 and the concentration of oxygen in the blood is 7.5 x 10-17 mol μm-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

Exam 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.

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