of particles (molecules) is from an area of high concentration to low concentration.
Graph of the simple and facilitated diffusion taking into account the rate of uptake and the concentration
If the particles can move through the lipid bilayer by simple diffusion, then there is
no limit to the number that can fit
through the membrane. The rate of diffusion increases linearly as we add
more particles to one side of the membrane.
If the particles can only pass through protein channels, then the rate of
diffusion is determined by the number of channels as well as the number of particles.
Once the channels operate at their maximal rate, a further increase in
particle numbers no longer increases the apparent rate of diffusion. At this limited rate we describe the protein channel as being saturated.
The cartoon illustrates several points about facilitated diffusion. The particles are more concentrated on one side of the membrane, and yet they can move in both directions. However, the net movement is from high particle concentration to low. If the number of particles gets so high on one side of the membrane that they
interfere with diffusion through the
protein channel, then we observe a limit to the rate of diffusion at the point of saturation.
Illustration of facilitated diffusion
animation used with permission of the Virtual Cell Web Page
Problem 7 | Answer | Problem 8
The Biology Project > Cell Biology > Cell Membranes > Problem Set
The Biology Project
Department of Biochemistry and Molecular Biophysics
University of Arizona
May 2002
Revised: August 2004
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Simple diffusion does not require energy: facilitated diffusion requires a source of ATP. Simple diffusion can only move material in the direction of a
concentration gradient; facilitated
diffusion moves materials with and against a concertion gradient.
