During osmosis, water molecules move from an area where there are fewer molecules of a solute to the area where there are more of them. The former solution is called hypotonic and the latter is called hypertonic.
Osmosis is movement of water across a semi-permeable membrane. In the case of biology, it is a cell membrane. Osmosis takes place when solutions have different concentrations of a solute. Solution is a solvent, in this case water, with an addition of a solute or any substance that can be dissolved in water. Sugar and salt are examples of a solute. If two solutions have equal concentrations of a solute, they are called isotonic and osmosis does not occur.
The movement of water towards a hypertonic solution can be demonstrated if plant cells are placed in a concentrated sugar or salt solution. The plant cells shrink as the water leaves them to travel to the solution with more solute, such as sugar or salt. An important example of osmosis for human life is movement of water across the cell wall of a red blood cell. If distilled water is introduced into the blood flow, it creates a hypotonic solution. The water flows into the blood cells, which swells and potentially bursts. By contrast, plant cells placed into distilled water do not burst, thanks to their rigid cell walls.