The tonicity of a solution is related to its effect on the volume of a cell. A hypotonic solution causes a cell to swell, whereas a hypertonic solution causes a cell to shrink. Although it is related to osmolality, tonicity also takes into consideration the ability of the solute to cross the cell membrane.

Tonicity in living systems If a cell is placed in a hypertonic solution, water will leave the cell, and the cell will shrink. In the case of a red blood cell, isotonic conditions are ideal, and your body has homeostatic (stability-maintaining) systems to ensure these conditions stay constant.

Cells will always aim to be in an isotonic environment, where the concentration of solute outside the cell is the same as the concentration of solute inside the cell. This is possible because cells have a semipermeable membrane.

Tonicity is a measure of the effective osmotic pressure gradient; the water potential of two solutions separated by a semipermeable cell membrane. In other words, tonicity is the relative concentration of solutes dissolved in solution which determine the direction and extent of diffusion.

Having a good understanding of tonicity gives a good insight into how dehydration can affect the cells of the body. An isotonic solution is best because the osmotic pressure within and outside the cell is equal, so there is no net movement of water and the cells will retain their shape and function.

• Relocation Diffusion.
• Hierarchical Diffusion.
• Stimulus Diffusion.
• Contagious Diffusion (diseases not allowed)

For example, cultural diffusion brought many ideas and technologies from China to Europe and the rest of the world. Inventions like paper, the compass, gunpowder and printing over thousands of years spread around the world.