![]() ![]() On both sides the ions are moving randomly in all directions. Now let's suppose that there are passive ion channels in the membrane that only permit positive (blue) ions to pass through. Any positive ion entering them from either side can passes through, but negative ions cannot. By Fick's law of diffusion, more positive ions will move from the right to the left than from the left to the right. There is a higher concentrationon the right and a lower concentration on the left. Positive ions are shown as blue and negative ions are shown as red. There are lots of ions (Na + and Cl -) on both sides of the membrane, and each side has a roughly equal number of positive and negative ions.īut if the concentrations are not the same, there will be more positive ions and more negative ions on one side of a membrane than the other. ![]() We'll make sense of this by telling the story of how a charge separation arises from diffusion and entropic effects, starting with a membrane that is impenetrable to ions but dissolved salt (NaCl) present on both sides, but with different concentrations on each side. The potential arises from a balance between an unbalanced diffusion of ions across the membrane arising from a concentration difference - a kind of entropic force, and the electric force that arises from the resulting separation of charge. There is a deep and interesting physical mechanism that is responsible for creating this potential difference. Somehow, it seems as if membranes can act like "batteries", maintaining an electric potential differences (voltage) across themselves. This is important to many biochemical processes, including the electrical signals in neurons. An essential characteristic of the functioning of membranes is the fact that they maintain an electric potential difference across them. ![]()
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