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 $\propto$ (surface area x concentration difference) $\div$ 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) $\div$ 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 $μ$ m thick and has a surface area of 3 $μ$ m². The concentration of oxygen in the alveolus is 1.8 x 10^-16 mol $μ$ m^-3 and the concentration of oxygen in the blood is 7.5 x 10^-17 mol $μ$ 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) $\div$ T)

Rate = 0.012 x 3 x ((1.8 x 10^-16- 7.5 x 10^-17) $\div$ 1.5)

Step 2: Complete the calculation

Rate = 0.012 x 3 x (1.05 x 10^-16 $\div$ 1.5)

Rate = 0.012 x 3 x 7 x 10^-17

Rate = 2.52 x 10^-18 molecules $μ$ 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.

Fick's Law of Diffusion (Edexcel A (SNAB) AS Biology)

Revision Note