Department of Physiology

Basic Cell Physiology

Why do cells have a membrane potential?

Designing a simple cell

Since cells arise by division and are at their birth very complex, we can learn little about why cells are like they are by asking the question "how do they get that way?". Similarly, since most of the evolutionary history of cells is unavailable, we gain little from asking "how did they evolve?". Is there any more fruitful question?

We can ask "If we give a cell the minimum set of sensible properties, do we see why some less obvious characteristics must be present?"

So let's play a game of designing the simplest cell we can conceive and see what the consequences are. Consider the properties we need:

The interior and exterior of the cell should be an aqueous salty solution, since all real cells are so.
The cell membrane must be leaky to water, since all real cell membranes are.
The cell must contain (and retain), a range of biological molecules.

So let us consider as a starting point, the creation of a cell by enclosing a volume of the salty environment with a membrane bag (indicated by the green line):

Then we will add some biological molecules:

As soon as we do, we have to specify the characteristics of the cell membrane (over and above saying it will be leaky to water).

Lets consider some possibilities:

Case 1: A membrane permeable to all solutes

This is a bit like casting a net into the sea rather than a bag. Any solute we put inside will leak out. This includes the biological molecules. We can reject this model!

Case 2: A membrane impermeable to all solutes

Remembering that we said the membrane would be leaky to water, this defines the membrane as being semipermeable. The biological molecules we add will not leak out, so this is a better design. But nothing else can leak out, so if we want a simple method to eliminate waste products by letting them leak out, or to acquire useful molecules from outside by letting them leak in (say e.g. O₂), we have a problem.

There is an interesting side effect of this design. By adding the biological molecules we effectively reduce the concentration of the water inside the cell, This produces an inward directed concentration gradient for water (i.e. an osmotic gradient) and because the cell is permeable to water, water will enter. This will dilute the intracellular solutes until the water concentration rises to that outside. From studies of osmosis by physical chemists and biologists, we can predict that this equilibrium will occur when:

	[Biological Molecules]_i + [Saline]_i = [Saline]_o

because this implies

	[H₂O]_i = [H₂O]_o

But note also that it implies

	[Saline]_i < [Saline]_o

which can be sustained because the membrane is not permeable to the saline, so it will not enter.

Case 3: Membrane impermeable to biological molecules, but permeable to saline

This is a compromise between the extremes of Case 1 and Case 2. Now the biological molecules will not leak out as in Case 1, but simple diffusional exchange of important metabolites or waste products will be possible (unlike case 2).

But there is a catch.

The osmotic consequence of adding the biological molecules is exactly as before, and the tendency will be for the saline concentration inside to be reduced by water entry. But in this case the membrane is permeable to the salt ions so the salt will enter the cell, and will tend toward the same concentration as outside. This process will mean that:

	[Biological Molecules]_i + [Saline]_i > [Saline]_o

and

	[H₂O]_i < [H₂O]_o

so the cell will continue to swell indefinitely through associated salt and water entry. There is no equilibrium and the swelling will destroy the cell, because the pressures that arise from osmosis are very large - too large to be resisted by a cell membrane. (For a real example, see haemolysis - serious swelling of a red blood cells causing them to lose their contents).

This is The Osmotic Problem

Before you follow this suggested solutions link , try to pose one or more solutions of your own.

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