MEMBRANE TRANSPORT 

A. INTRODUCTION

In the previous module we examined the manner by which various physiologically important solutes traverse the plasma membrane of cells. One substance which we have not discussed to any great extent in our consideration of membrane transport is water. This is perhaps surprising because water makes up around 70% of the volume of most cells and moves across the plasma membrane far more frequently than any other substance. In red blood cells for example a volume of water equivalent to approximately 100 times the volume of the whole cell moves into and out off the cell each second.

Osmosis is the term given to the movement of any solvent across a selectively permeable barrier. As was indicated previously, both the extracellular fluid (ECF) and intracellular fluid (ICF) are aqueous solutions and so in physiological systems the only important solvent is water. The plasma membrane is of course the selectively permeable barrier. So for our purposes osmosis refers simply to the movement of water across the plasma membrane.

Somewhat surprisingly, the plasma membrane of most cells is highly water permeable. Water molecules are small enough (and have sufficient kinetic energy) to pass straight through both the lipid bilayer (despite its hydrophobic core) and protein channels. Consequently water can move across the plasma membrane down its concentration gradient in the same way that we previously saw lipid soluble molecules or small ions move through the membrane (i.e. by simple diffusion). 

What then is the difference between the diffusion of solutes and the diffusion of water (which we refer to as osmosis)? The answer to this is that diffusion of solutes across the membrane does not result in volume changes whilst osmosis does produce changes in volume.

Let us illustrate this answer with a simple example using a model system consisting of two compartments filled with and an aqueous solution containing only one solute separated by an artificial membrane (see opposite) . If we design the membrane so that it is completely permeable to water (small blue particles) and completely impermeable to the solute (larger red particles) and put a solution with a slightly higher solute concentration in the compartment on the right side of the membrane, what will happen? Well the solute concentration in the left compartment is lower than the solute concentration on the right so there is a concentration gradient which favours the movement of the solute from the right to the left. However because the membrane is impermeable to the solute, this cannot occur. However, because the solute concentration in the right compartment is higher than the solute concentration in the left compartment, it follows that the water concentration in the left compartment must be higher than the right compartment. In this instance there is a concentration gradient which favours the movement of water from left to right. Because the membrane is water permeable, water moves from the left compartment to the right by osmosis . Note that because of the net flow of water from left to right, the volume of the right compartment increases and the left compartment decreases. This example illustrates the fundamental difference between the diffusion of solutes and osmosis: there is no change in volume following diffusion of solutes but there is following osmosis.

B. OSMOTIC PRESSURE

Note that although the driving force for water movement by osmosis is ultimately the concentration gradient of water across the membrane, this concentration gradient is a direct consequence of the different concentrations of the solute on the two sides of the membrane. To put this another way, water will always move across the membrane in the direction of the higher solute concentration because this is down the concentration gradient for water. If you are having trouble visualising this then just think of the higher solute concentration "attracting" the water.

In the model system illustrated above, if we increase the solute concentration in the right compartment even more then it follows that the water concentration must be lower than in the previous example. Therefore the concentration gradient for water from the left compartment to the right compartment is increased and more water will flow from left to right increasing the volume of the right compartment to a greater extent that was illustrated above.

From these simple examples you should now be able to see that it is the concentration of the solutes which are unable to cross the membrane (referred to as the non-penetrating solutes) that determines the amount of osmosis that takes place. So if the difference in the concentration of non-penetrating solutes is small, then there is only a small amount of osmosis. If the concentration difference of non-penetrating solutes across the membrane is high, then there will be a greater movement of water by osmosis.

Because of this we refer to the total concentration of non-penetrating solutes as the osmotic pressure as it is in reality the driving force for osmosis.

C. OSMOLARITY

In order to get some idea of the osmotic pressure of a solution we need to know its osmolarity. Osmolarity is a measure of the number of solute particles in a solution and reflects more accurately its osmotic pressure. The reason for this is that the osmotic pressure of a solution is much more dependent upon the total number of solute particles in solution than the total weight of solute in solution. This is because some solutes (for example salts) dissociate into their constituent ions when in solution and therefore contribute a larger number of solute particles than substances which don't dissociate.

For example, NaCl dissociates into Na⁺ and Cl^- when in solution and therefore contributes two particles. Glucose on the other hand doesn't dissociate in solution and therefore only gives rise to one particle. For this reason NaCl contributes to a greater extent to the osmotic pressure of a solution than the same weight of glucose.

Osmolarity therefore provides us with a measure of the osmotic effectiveness of a solution.

Osmolarity is measured in Osmoles/Litre where 1 Osmole (Osm) is simply the molarity of the solute multiplied by the number of particles into which the solute dissociates when in solution. Here are a couple of simple examples it help clarify this:

Example One: If we have a 1 M solution of glucose then the osmolarity of this solution = 1 Osm/L because glucose doesn't dissociate in solution (i.e. for substances which don't dissociate osmolarity and molarity are the same).

Example Two: If we have a 0.5 M solution of NaCl then the osmolarity of this solution = 1 Osm/L because NaCl dissociates into Na⁺ and Cl^- when is solution (i.e. the osmolarity is 2 x molarity).

Example Three: If we have a 0.1 M solution of MgCl₂ then the osmolarity of this solution = 0.3 Osm/L because MgCl₂ dissociates into a Mg⁺and two Cl^- ions when is solution (i.e. the osmolarity is 3 x molarity).

D. TONICITY

As was alluded to at the beginning of this lesson, a volume of water equivalent to approximately 100 times the volume of the whole cell moves across the membrane of a single red blood each second. However, as the volume of water flowing into the cell is at equilibrium with the water flowing out of the cell there is no net movement of water and so the cell remains happy (well as happy as anything without a brain and a typical life span of 110 days can be). This is because the ECF (in this instance plasma) has almost exactly the same osmotic pressure as the ICF. Thus there is no concentration gradient for water and consequently there is no net movement of water. (Note that due to the random kinetic energy a water molecule may end up moving across the membrane but immediately another water molecule will move in the opposite direction to restore the balance).

When a cell is placed in a solution which doesn't produce any net movement of water, the solution is known as an isotonic solution.

When a cell is placed in a solution which has a higher concentration of solutes than the ICF then water will flow out of the cell (down its concentration gradient) by osmosis and the cell will shrink. This type of solution is said to be a hypertonic solution.

Finally, when a cell is placed in a solution which has a lower concentration of solutes than the ICF then water will flow into the cell (down its concentration gradient) by osmosis and the cell will swell. This type of solution is said to be a hypotonic solution.

From this you can see that the nature of the solutions which surround cells can have fairly dramatic effects on the movement of water into and out off cells.

Osmosis is an extremely important mechanism which is responsible for the movement of water in a variety of very important physiological processes. If you grasp the concept now then your understanding of key elements in the nervous, cardiovascular, digestive and renal systems will be made all the easier.