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

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Cara Head

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Cara Head

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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 proportional to (surface area x concentration difference)divided bythickness 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)divided byT)

  • 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 straight mum thick and has a surface area of 3 straight mum2. The concentration of oxygen in the alveolus is 1.8 x 10-16 mol straight mum-3 and the concentration of oxygen in the blood is 7.5 x 10-17 mol straight mum-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)divided by T)

Rate = 0.012 x 3 x ((1.8 x 10-16 - 7.5 x 10-17)divided by1.5)

Step 2: Complete the calculation

Rate = 0.012 x 3 x (1.05 x 10-16 divided by1.5)

Rate = 0.012 x 3 x 7 x 10-17

Rate = 2.52 x 10-18 molecules straight mum-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.

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Cara Head

Author: Cara Head

Expertise: Biology

Cara graduated from the University of Exeter in 2005 with a degree in Biological Sciences. She has fifteen years of experience teaching the Sciences at KS3 to KS5, and Psychology at A-Level. Cara has taught in a range of secondary schools across the South West of England before joining the team at SME. Cara is passionate about Biology and creating resources that bring the subject alive and deepen students' understanding