BIOL 2011L Lecture Notes - Lecture 2: Chemical Potential, Transport Phenomena, Advection

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23 Nov 2017
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DIFFUSION
Diffusion is the net movement of molecules or atoms from a region of high concentration
with high chemical potential to a region of low concentration with low chemical potential.
This is also referred to as the movement of a substance down a concentration gradient.
A gradient is the change in the value of a quantity e.g. concentration, pressure, or
temperature with the change in another variable, usually distance. A change in
concentration over a distance is called a concentration gradient, a change in pressure over
a distance is called a pressure gradient, and a change in temperature over a distance is a
called a temperature gradient.
The word diffusion derives from the Latin word, diffundere, which means "to spread out". A
substance that “spreads out” is moving from an area of high concentration to an area of low
concentration.
A distinguishing feature of diffusion is that it dependends on particle random walk, and
results in mixing or mass transport without requiring directed bulk motion. Bulk motion, or
bulk flow, is the characteristic of advection.[1] The term convection is used to describe the
combination of both transport phenomena.
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