Energy and Diffusion
I know that diffusion is a passive transport, i.e. does not require energy
expenditure. But certainly there must be energy that drives the movement
of molecules from high concentration to low concentration. So I am
wondering what is the energy that causes diffusion to occur? and how does
concentration gradient provides a driving force for movement?
At all temperatures above absolute zero (-273C), atoms and molecules are is a state of
incessant vibrational, rotational, and translational motion. Those motions (the extent
to which are proportional to the absolute temperature) are what enables diffusion to
occur. The second law of thermodynamics describes the direction of diffusion processes --
that is, from high concentration (a state of order) to low concentration (a state of
increased disorder). Overall, uninfluenced natural processes tend to proceed from order
There need not be any energy driving diffusion. It is the change in entropy that is the
driving force. In simple terms here is what happens. Consider only the translation of the
particles. Each particle is free to occupy any translational state consistent with the
energy of the system (which depends only upon the temperature). So every particle
'bounces' from one allowed translational state to another with equal random probability.
The NUMBER of such states (one can show) is proportional to the volume occupied by the
particles, so we can say the "density" of such translational states is proportional to
the volume. The dependence of the
density of such states is so vastly increases with volume that a particle "likes" the
added space so much that it will diffuse to occupy those newly accessible states. No
energy is required, Now it turns out that the increase in entropy S2 - S1 = R* ln(V2/V1).
It is the random shuffling of the particles between all translational states (consistent
with the temperature) that "drives" the particles to diffuse from regions of higher
concentration to lower concentration. A mathematically rigorous derivation is somewhat
involved (see e.g. "The Principles of Statistical Mechanics" by Richard Tolman) but it
is a driving force nonetheless. We are so used to thinking that going from higher energy
to lower energy is the only driving force that the more subtle, but equally strong driving
force, increasing the density of allowed states, is frequently overlooked.
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Update: June 2012