11: Summary

  • In diffusion, the time to diffuse across a distance is given by
    T = 1/2D * x2
    so that time to diffuse increases with the square of distance (consequence of Fick’s Second Law).
  • Diffusion is efficient only at very short distances. If organisms need to transport materials over longer distances, they need mechanisms such as lungs and circulatory systems.
  • Diffusion (and all the maths that goes along with it) can be used as an analogy to understand the movements of cells and more complex organisms.

Learning Outcomes

You should now be able to:

  • Explain how the rate of movement of a substance is proportional to the distance squared (Fick’s Second Law).
  • Measure the time it takes for a substance to move from one area to another as a function of distance and the permeability of the medium.

If you want a printer-friendly version of this module, you can find it here in a PDF document. This printer-friendly version should be used only to review, as it does not contain any of the interactive material, and only a skeletal version of problems solved in the module.