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

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