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Diffusion Across a Selectively Permeable Membrane

In the simulations we have seen so far, the particles have been free to move within a (electronic) container but prevented absolutely from moving outside of it. In terms of the Fick equations discussed on the previous page, the container has a relatively high permeability coefficient, but the walls are impermeable (their permeability is zero).

Sometimes diffusing particles encounter regions in which the permeability is reduced, but not zero. One example of such a region is called a membrane, a thin barrier that might be made of cellulose, plastic, glass, or phospholipid or the barrier might be a biological membrane consisting of a complex mixture of phospholipids and proteins.

Membranes alter the rate at which particles can diffuse, and they do so selectively. That is, they might cause little hindrance to one type of particle but completely prevent the passage of a different. Indeed, cells use a whole array of membrane proteins, called carriers and channels, to regulate (and sometimes rapidly change) the permeability of what would otherwise be an impermeable phospholipid bilayer.

Membranes, through their selective alteration of the otherwise inexorable process of diffusion, cause a number of interesting cellular phenomena. When water can cross the membrane and dissolved particles cannot, osmosis occurs. When particles of one charge can cross and not those with the opposite charge, diffusion potentials are generated. These subjects will be examined more thoroughly in subsequent pages.