Osmosis is the process of diffusion of water molecules across a membrane (The Cell). The contents of cells are basically solutions of many different solutes, and the more concentrated this solution is, the more solute molecules there are in a given volume, and the less water molecules. Water molecules can diffuse freely across a membrane, but only down a concentration gradient, so water always diffuses from a dilute solution to a more concentrated solution.
Osmosis can be quantified using water potential, so it is possible to calculate which way water molecules will move and how fast (The Cell). The water potential, ?, is just the effective concentration of water. It is measured in units of pressure (Pa or more commonly kPa), and the rule is that water always "falls" from a high to a low potential. One hundred percent pure water has a ? = 0, which is the highest possible water potential, so all solutions have a ? < 0, and it is impossible to have ? > 0.
The more concentrated a solution is, the higher the osmotic
The concentration of solutes in the solution surrounding a cell will affect the state of the cell due to osmosis because it affects the osmotic pressure (The Cell). An isotonic solution is one in which the concentration of solutes is equal to that of the contents of the cell, i.e. the two have the same osmotic pressure. In this case, there will be no change. If the solution is hypertonic, it will have a higher concentration of solutes than the cell, and will therefore draw water out of the cell, and the cell will shrink. If the solution is hypotonic, the concentration of solutes will be less than those in the cell, and so water will flow from this solution into the cell, and may cause the cell to burst. These three states are illustrated in Figure 1 (take from
http://www.biologymad.com/cells/cellmembrane.htm )
The effects of different solutions can cause problems for living cells (The Cell). Fo...