Inorganic Chemistry For Dummies
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Osmotic pressure is a basic chemical principle, but it also plays a role in biology. In fact, osmosis is going on in your body right now as your biological systems work to maintain balance in your blood chemistry.

Suppose that you take a container and divide it into two compartments using a thin membrane that contains microscopic pores; the pores are large enough to allow water molecules to pass through but not solute particles. This membrane type is called a semipermeable membrane; it lets only some small particles pass through.

You then add a concentrated salt solution to one compartment and a more dilute salt solution to the other. Initially, the volumes of the two solutions start out the same. But after a while, you notice that the level on the more concentrated side has risen and that the level on the more dilute side has dropped. This change in levels is due to the passage of water molecules from the more dilute side to the more concentrated side through the semipermeable membrane. This process is called osmosis, the passage of a solvent through a semipermeable membrane into a solution of higher solute concentration. The pressure that you’d have to exert on the more concentrated side in order to stop this process is called osmotic pressure.

Osmotic pressure.
Osmotic pressure.

The solvent always flows through the semipermeable membrane from the more dilute side to the more concentrated side. In fact, you can have pure water on one side and any salt solution on the other, and water always goes from the pure-water side to the salt-solution side. The more concentrated the salt solution, the more pressure it takes to stop the osmosis (the higher the osmotic pressure).

But what if you apply more pressure than is necessary to stop the osmotic process, exceeding the osmotic pressure? Water is forced through the semipermeable membrane from the more concentrated side to the more dilute side in a process called reverse osmosis. Reverse osmosis is a good, relatively inexpensive way of purifying water. The world has many reverse osmosis plants that extract drinking water from seawater. Navy pilots even carry small reverse osmosis units with them in case they have to eject at sea.

Cell walls often act as semipermeable membranes. Do you ever eat pickles? Cucumbers are soaked in a brine solution in order to make pickles. The concentration of the solution inside the cucumber is less than the concentration of the brine solution, so water migrates through the cell walls into the brine, causing the cucumber to shrink.

One of the most biologically important consequences of osmotic pressure involves the cells within your own body. For example, inside a red blood cell is an aqueous solution, and outside the cell is another aqueous solution (intercellular fluid). When the solution outside the cell has the same osmotic pressure as the solution inside the cell, that outside solution is said to be isotonic.

Water can be exchanged in both directions, helping to keep the cell healthy. However, if the intercellular fluid becomes more concentrated and has a higher osmotic pressure (the solution is hypertonic), water flows primarily out of the blood cell, causing it to shrink and become irregular in shape. This process, called crenation, may occur if the person becomes seriously dehydrated.

The crenated cells are not as efficient in carrying oxygen. If, on the other hand, the intercellular fluid is more dilute than the solution inside the cells and has a lower osmotic pressure (the solution is hypotonic), the water flows mostly into the cell. This process, called hemolysis, causes the cell to swell and eventually rupture.

Crenation and hemolysis of red blood cells.
Crenation and hemolysis of red blood cells.

The processes of crenation and hemolysis explain why the concentration of IV solutions is so critical. If they’re too dilute, then hemolysis can take place, and if they’re too concentrated, crenation is a possibility.

You can calculate the osmotic pressure (π) by using the following equation:


In this equation, π is the osmotic pressure in atmospheres, n is the number of moles of solute, R is the ideal gas constant, T is the Kelvin temperature, V is the volume of the solution in liters, and i is the van’t Hoff factor (the number of moles of particles that will be formed from 1 mole of solute); n/V may be replaced by M, the molarity of the solution.


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