Basic Cell Physiology

Why do cells have a membrane potential?

Designing a simple cell (Continued)

Solutions to the Osmotic Problem

To recapitulate, a cell with very simple basic characteristics faces a problem that inward osmotic movement of water will tend to make it swell indefinitely. What solutions are there to this fundamental problem?

Solution 1: extrude water

Since the fundamental problem is that water is continually entering the cell, we could provide a mechanism to counteract that entry with an extrusion mechanism (we might call this a water pump). If the water could be moved out as quickly as it enters, this would prevent the swelling.

Note that the water enters down a concentration gradient, so the water must be extruded against this gradient and hence will require energy. Providing the energy will immediately seriously complicate the design of the cell which started out so simple!

Real examples? The contractile vacuole of some single celled organisms is probably a means of extruding water. Mammalian cells do not have such a mechanism.

Solution 2: Resist the swelling

Despite the comment in the original exposition of the problem that the forces generated by osmosis are very large, we can still consider reinforcing the cell membrane so that swelling is prevented.

Real examples? Plant cells and bacteria have external cell walls that assist in solving the osmotic problem. This has the advantage of being an energy independent solution once it is in place. It has the disadvantage of restricting mobility in a serious way.

Solution 3: Extrude salt

Although water entry is the fundamental problem, it is a consequence of the inward movement of the salt making up the salty solution we started with that distinguishes Case 2 and Case 3 of the imaginary cells. So another solution would be to extrude the salt as rapidly as it enters. Once again this extrusion (which we could also refer to as pumping) would be against a concentration gradient and would be energy dependent. Otherwise it is a satisfactory solution.

A catch is that the extrusion of salt equates to the extrusion of whatever ions make up the saline solution. If it is anything like seawater, there are many different ionic species. A simpler solution would be to pick one ionic species and provide a pump for it. Surprisingly, this will work, though it introduces a new complexity, the separation of charge across the membrane - a transmembrane voltage that physiologists like to call the Membrane Potential. Let's see how this happens and why the single ion pump will work.

Solution 3 when the pump is for a single ion

Let us now specify that the saline is actually a solution of NaCl, i.e. our cell picture looks like:

Now suppose the membrane has some mechanism for extruding Na⁺ ions, a sodium pump of sorts. We indicate this pump with a circle and arrow thus:

Since Na⁺ ions will be moved against their concentration gradient, this mechanism will require energy, so adds a complexity to the cell, but a more important issue is that the mechanism moves a positive ion out of the cell without a corresponding negative ion. This separates a tiny amount of positive and negative ions, and an electrical potential difference results. Because this is manifest across the membrane, it is called a membrane potential. We can indicate this potential with a plus and minus across the membrane thus:

This potential is very important, for once it is present, tends to also affect the movement of ions. For example, the potential difference will tend to move Na⁺ ions into the cell (as does the concentration gradient) making the energy required to pump the Na⁺ ions greater. But it will also influence the movement of other ions.

Importantly, the membrane potential will favour the outward movement of chloride ions, and some chloride ions will move in response to it:

Thus what started out being a sodium pump, effectively moves NaCl out of the cell. In the context of our simple cell, this mechanism can solve the osmotic problem. Thus, the membrane potential is a side-effect of solving the osmotic problem. Once again the solution requires energy, because the movement of Na⁺ is against a concentration and electrical gradient. Na⁺ ions will continually leak back into the cell. The pump must work forever, If it stops, the osmosis takes over, and the cell will die.

Although real cells are more complex than the simple one with which we started this cell design exercise, the implications of our very basic assumptions include the outcome of membrane potential, and a non-equilibrium, energy-dependent mechanism. Since real cells must have a solution to the osmotic problem, this may explain why cells must have membrane potentials! (But as we said at the outset, the evolutionary history is lost, so this is only speculation.)

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