How Nerve Cells Work

2. Movement of Ions: Diffusion and Membranes

Silvia Helena Cardoso, PhD  and Renato M. E. Sabbatini, PhD

Animations: André Malavazzi

In a nervous cell, different ions (sodium, potassium, chloride, etc.) have different concentrations on each side of the cell membrane.

But, as we saw in the previous section, ions tend to diffuse rapidly around the solutions which makes the inner and outer environments of cells, and gradually become balanced, that is, their concentration becomes the same in all parts of the solution.

How this phenomenon happens?

Free Diffusion


Molecules in solution move randomly about. In this beaker full of water, an impermeable membrane divides the beaker in half. On the left side there is a collection of molecules. The membrane prevents movement of the water and the molecules from crossing from one side of the beaker to the other 

When the membrane is removed, it 
allows the water and molecules to move freely (diffusion)  to the other side. The speed of diffusion is proportional to the temperature of the solution.

Now, biological membranes, such as the cell membrane. are permeable to many ions. This is so because of the existence of pores or passive transport channels bridging one side to the other, through which ions can pass freely.

For instance, if in the compartment on one side of the membrane there is a higher concentration of common salt (made of sodium and chloride ions) than in the other side, these ions will pass gradually to the other side, following the difference of concentration. Eventually, both compartments will have the same concentration of ions. The rate of passage is regulated by the number and size of pores (it is not an instant phenomenon, because it is a kind of slower diffusion).

 Diffusion Across a Permeable Membrane

Small pores in the surface of the permeable membrane allow the selective passage of ions. There are specific channels for each ion (sodium, chloride, potassium, etc.). The rate of passage is regulated by the number and size of pores. After a while, the concentration of both ions (green and yellow bars) will be the same on both sides of the membrane.
Next: Electrical Potential

Copyright 1999 Universidade Estadual de Campinas
An initiative: Center for Biomedical Informatics
Published on 25.July.1999