So far, you know of two forces that are involved in the movement of molecules in and around cells: diffusion (remember that osmosis is simply a special case of diffusion) and active transport. There is a third force that acts on ions - the electrical force. This force is the basis of electricity and is relatively easy to comprehend: opposite charges attract each other and like charges repel each other. You also need to understand that, the number of positive charges in an area should equal the number of negative charges. If they do not, an electric field is formed which will exert a force on all charged particles. Let=s see how the consideration of this electric force will affect the movement of ions across a cell (do not forget, that we cannot ignore diffusion).
Suppose that we have a cell with a certain concentration proteins inside the cell. Proteins, on the average, have a negative charge and are NOT permeable across the cell membrane. Let us assume that the cell is in electric balance so the number of positive charges inside the cell is equal to the number of negative charges inside the cell. Assuming that the only negative ions inside the cell at this time are the impermeable proteins, we need to include a positive counter ion inside the cell. Let us pick potassium (just for fun, any positive ion would do!). Since the number of positive ions must equal the number of negative ones, the concentration of potassium much equal the concentration of protein charges. If this cell was in distilled water, water would move into the cell by osmosis until the cell bursts. Since we do want this to happen, we must add some substance outside the cell to make it osmotically balanced. Let us pick KCl at a concentration where the concentration of potassium inside the cell exactly equals the concentration of potassium outside the cell. In this manner, there is no diffusion gradient for potassium as the concentrations are equal and we assume that there is also no diffusion gradient for water (and we will actually ignore the movement of water for the rest of this thought experiment). Everyone is happy!! Well, not everyone! Chloride Awants@ to diffuse inward as the concentration of chloride outside the cell is greater than that inside (remember, there is no chloride inside the cell at the start of this experiment). If we assume that chloride is permeable through the cell membrane, chloride will now begin to diffuse inward (high to low, the blue arrow on the following fugure).
If we let chloride diffuse inward until the intracellular and extracellular concentrations are equal (thus no diffusion gradient will exist) and we ignore the movement of potassium (we will come back to it), the movement of the chloride inside will produce an excess of negative charges inside the cell (and a simultaneous increase in positive charges outside). This will cause the formation of an electical field across the cell membrane, with the inside being negative. This negative charge will force the clhloride back out (see the red arrow in the following figure).
If we now let all of the chloride leave until there is no electrical field, all of it will now be extracellular and there will now be a diffusion gradient again (see above). Something must give!
We are now dealing with a situation in which there are two opposing forces acting on the chloride, diffusion and the electrical force. The initial diffusion of chloride into the cell immediately sets up an imbalance of charges forming a electric field, with the inside of the cell being negative. This pushes chloride back out. Note, whenever we reach a state in which the concentration gradient is removed, there is an electical force still present. Whenever we reach a state in which the electical force is removed, there is a concentration gradient. The chloride ion must feel like a rope in a tug-of-war, being push in two different directions! The electric force pushing chloride out of the cell and diffusion pushing it into the cell. So who wins! Actually, no one. If we think of a tug-of-war, the rope will not move if the two sides (forces) are pulling with the same force. The same is true with chloride. If the force of diffusion pushed chloride out with the same force as the electrical force pushes it our, there will be no NET movement of chloride - this would mean that chloride is in a state of equilibrium. This would only be true in our example if (at equilibrium) diffusion pushes chloride in while the electical force pushes it out at the same time with the same amount of force. This means that at equilibrium,
There must be a greater concentration of chloride in the extracellular fluid compared to the intracellular concentration (thus making a diffusion gradient pushing chloride in).
The inside of the cell must have a net negative charge (thus forming the electrical field pushing chloride out). See the following figure.
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