# Final Review

- Flux density is the movement of particles per time (through a given area).
- In diffusion, individual particles are moving at random, and net movement
is a result of more particles moving from high to low concentrations than
are moving from low to high, thus, (Fick’s First Law)

- Fick’s First Law is phenomenological, i.e., it cannot be derived from first principles, but it shares the same form as several other transport equations.
- D (the diffusion coefficient) has units of cm
^{2}/sec, and is NOT equal to the speed at which an individual particle moves. - In diffusion, the time to diffuse across a distance is given by

,

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 math that goes along with it) can be used as an analogy to understand the movements of cells and more complex organisms.

*If you want a printer-friendly version of this module, you can find it here in a Microsoft Word 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.*

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