MEMBRANE POTENTIALS: Membrane Potentials

In this module we begin to consider to the way in which transport of substances across the membrane affects the membrane potential. The easiest way to visualise what is happening is to consider a couple of fairly simple experiments:

Experiment One

Let us imagine a cell with a low concentration of substance X (an uncharged molecule) in its cytoplasm and a high concentration of X in the extracellular fluid. Let us also imagine that the plasma membrane contains a protein channel which permits the movement of X when they are open but that these are closed at the beginning of the experiment. In this instance let us assume that the membrane potential at the start of the experiment is equal to 0 mV.

Clearly under these circumstances if we open the protein channel, X will move into the cell down its concentration gradient (orange arrow).

Substance X will continue to flow into the cell until the concentration inside is equal to the concentration outside at which point there will no longer be any net movement of X into the cell. Note that at this point the movement of X does not cease completely but is at equilibrium. Due to the continual random motion of the molecules as long as the channel remains open then X molecules may still cross the membrane but because this upsets the balance it is immediately followed by the movement of a molecule of X in the opposite direction to restore the balance.

The important thing about this experiment is to note that the membrane potential does not change (see voltmeter) but remains at the starting value of 0 mV. This is simply because substance X is uncharged and consequently its movement does not affect the potential difference between the inside and outside of the cell.

Experiment Two

Now let us look at a similar experiment to that above but this time considering what would happen if X were a monovalent cation (X⁺). The initial experimental setup is exactly the same with a high concentration of X⁺ outside the cell, a low concentration inside the cell, the X⁺ ion channels closed and the membrane potential equal to 0 mV.

If we now open the ion channel then X⁺ ions will move into the cell because of the concentration gradient (orange arrow). However note that in this instance as a result of the X⁺ ions entering the cell, there is a net flow of positive charge into the cell. Because of this influx of positive charge, the recorded membrane potential (the potential between the inside and outside of the cell) becomes more positive (see voltmeter).

From the previous experiment, you might expect that the X⁺ ions would continue to flow into the cell until the concentration of X⁺ inside the cell was the same as that outside the cell. However this point is rarely reached with charged substances. In this experiment this is because as more and more X⁺ ions flow into the cell the inside of the cell become more and more positive (see voltmeter) and forms an electrical gradient which begins to repel the X⁺ ions and push them out of the cell (purple arrow). In effect the concentration gradient which is driving the X⁺ ions into the cell (orange arrow) is counteracted by an electrical gradient which drives the X⁺ ions out of the cell (purple arrow). Eventually a point of equilibrium is reached where the concentration gradient is exactly balanced by the electrical gradient and no further net movement of ions occurs and the membrane potential is stable.

This idea that ion flow can change the membrane potentials of cells is very important so it is probably worth spending a little bit of time getting your head around it before moving on. For each of the subsequent experiments, try and work out what will happen when the ion channels open. Assume that the membrane potential is equal to zero at the beginning of the experiment and employ the same logical steps in the two experiments described above to complete the description. Then check your answer.

One thing that is very important to note is that when considering changes in charge we only think about what is happening inside the cell. This is because the extracellular space is so large that ionic movement has no significant effect on its net charge. In other words ions leaving a cell do affect the charge inside the cell but not the charge of the extracellular space which is always considered to be zero.

Experiment Three In this instance, when the ion channels open will flow the cell. As a consequence there will be a net flow of charge the cell. As a result the system will reach equilibrium at a membrane potential.
Experiment Four In this instance, when the ion channels open will flow the cell. As a consequence there will be a net flow of charge the cell. As a result the system will reach equilibrium at a membrane potential.
Experiment Five In this instance, when the ion channels open will flow the cell. As a consequence there will be a net flow of charge the cell. As a result the system will reach equilibrium at a membrane potential.

Hopefully you can now see that the membrane potential can be modified by movement of ions across the membrane as a result of opening of ion channels. Furthermore, if the membrane is selectively permeable to one ion then the polarity of the membrane potential will be determine by the charge of the ion and the direction that the ion is moving across the membrane (which in turn is determined by the relative concentrations of the ion across the membrane).

A logical extension of these observations is that if the membrane is selectively permeable to one ion, then the magnitude of the membrane potential is directly proportional to the concentration gradient of the ion across the membrane (i.e. if the concentration gradient is large, so is the resultant membrane potential). This is probably best explained by considering two similar experiments where the concentration gradients for the same ion are different:

Experiment Six

In the experiment shown on the right, the concentration gradient of X+ across the membrane is relatively small. If the membrane is selectively permeable to X+ ions then we know that X+ will flow into the cell making the inside of the cell more positive and thus the recorded membrane potential will be positive when equilibrium is reached. Because of the low concentration gradient (orange arrow) the inside of the cell doesn't have to become very positive before the electrical gradient (purple arrow) balances the concentration gradient and hence equilibrium is reached. In other words it only requires a small electrical gradient to balance the small concentration gradient.

Experiment Seven

If you compare this experiment to the one above you will see that the concentration gradient is much higher. As a result the inside of the cell has to become much more positive before the electrical gradient (purple arrow) balances the larger concentration gradient (orange arrow). In other words it requires a much greater electrical gradient to balance the higher concentration gradient.

In summary:

The membrane potential is simply the difference in charge between the inside and outside of a cell.
The membrane potential can vary from cell to cell and with time.
We can predict the polarity of the membrane potential if we know what ion channels are open, the valency of the ions that can pass through these channels and the relative concentration of these ions inside and outside the cell.
The magnitude of the membrane potential is dependent upon the size of the concentration gradient of the ion across the membrane.

If you don't understand any of these points then go back through this part of the lesson again. There is nothing too complex about this stuff but if you are struggling with these concepts now then some later topics are going to seem like gobbledegook. You have been warned!

Experiment One

Experiment Two

Experiment Three

Experiment Four

Experiment Five

Experiment Six

Experiment Seven

In summary: