Osmotic Pressure and Tonicity

Osmotic pressure and tonicity often are confusing to people. Both are scientific terms pertaining to pressure. Osmotic pressure is the pressure of a solution against a semipermeable membrane to prevent water from flowing inward across the membrane. Tonicity is the measure of this pressure. If the concentration of solutes on both sides of the membrane is equal, then there is no tendency for water to move across the membrane and no osmotic pressure. The solutions are isotonic with respect to each other. Usually, there is a higher concentration of solutes on one side of the membrane than the other. If you're unclear about osmotic pressure and tonicity it might because you're confused about how the difference between diffusion and osmosis.

Diffusion Versus Osmosis

Diffusion is the movement of particles from a region of higher concentration to one of lower concentration. For example, if you add sugar to water, the sugar will diffuse throughout the water until the concentration of sugar in the water is constant throughout the solution. Another example of diffusion is how the scent of perfume spreads throughout a room.

During osmosis, as with diffusion, there is a tendency of particles to seek the same concentration throughout the solution. However, the particles may be too large to cross a semipermeable membrane separating regions of a solution, so water moves across the membrane. If you have a sugar solution on one side of a semipermeable membrane and pure water on the other side of the membrane, there will always be pressure on the water side of the membrane to try to dilute the sugar solution. Does this mean all of the water will flow into the sugar solution? Probably not, because the fluid may be exerting pressure on the membrane, equalizing the pressure.

As an example, if you put a cell in fresh water, the water will flow into the cell, causing it to swell. Will all of the water flow into the cell? No. Either the cell will rupture or else it will swell to a point where the pressure exerted on the membrane exceeds the pressure of the water trying to enter the cell.

Of course, small ions and molecules may be able to cross a semipermeable membrane, so solutes such as small ions (Na⁺, Cl^-) behave much like they would if simple diffusion were occurring.

Hypertonicity, Isotonicity, and Hypotonicity

The tonicity of solutions with respect to each other may be expressed as hypertonic, isotonic or hypotonic. The effect of different external solute concentrations on red blood cells serves as a good example for a hypertonic, isotonic and hypotonic solution.

Hypertonic Solution or Hypertonicity

When the osmotic pressure of the solution outside the blood cells higher than the osmotic pressure inside the red blood cells, the solution is hypertonic. The water inside the blood cells exits the cells in an attempt to equalize the osmotic pressure, causing the cells to shrink or create.

Isotonic Solution or Isotonicity

When the osmotic pressure outside the red blood cells is the same as the pressure inside the cells, the solution is isotonic with respect to the cytoplasm. This is the usual condition of red blood cells in plasma.

Hypotonic Solution or Hypotonicity

When the solution outside of the red blood cells has a lower osmotic pressure than the cytoplasm of the red blood cells, the solution is hypotonic with respect to the cells. The cells take in water in an attempt to equalize the osmotic pressure, causing them to swell and potentially burst.