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

• 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

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