3. Tendency of water to flow from a hypotonic solution (low concentration of dissolved substances) to hypertonic solution (higher concentration of dissolved substances) across a semipermeable membrane
In biological systems, osmosis is essential since many biological membranes are semipermeable, and it leads to different physiological effects. For example, when an animal cell is exposed to a hypertonic surrounding (or lower water concentration) the water will leave the cell causing the cell to shrink.
When an animal cell is placed in a hypotonic surrounding (or higher water concentration), the water molecules will move into the cell causing the cell to swell. If osmosis continues and becomes excessive the cell will eventually burst. In a plant cell, excessive osmosis is prevented due to the osmotic pressure exerted by the cell wall thereby stabilizing the cell. In fact, osmotic pressure is the main cause of support in plants. However, if a plant cell is placed in a hypertonic surrounding, the cell wall cannot prevent the cell from losing water. It results in cell shrinking (or cell becoming flaccid).