Cells control osmosis through selective permeability. Selective permeability is the process whereby only certain molecules can enter or leave the cell membrane.
Osmosis is a type of diffusion where water passes from an area of high concentration to one of lower concentration within the cell through a semi-permeable membrane. Osmosis may be described as the diffusion of water from an area of low solute concentration to one of high solute concentration.
Water will move through the membrane if an osmotic gradient is present. The osmotic gradient is the difference between the solute concentrations on either side of the semi-permeable membrane. Solutes exist in two states:
- Hypertonic: higher concentration
- Hypotonic: lower concentration
In hypertonic states, there is more solute and less water. Hypotonic states contain more water and less solute. If there is equilibrium, meaning that the concentration of solutes is the same both on the inside and outside of the cell, the cell is isotonic. In such a case, there is no osmotic gradient and water does not move in any particular direction. In osmosis, the movement of water occurs from the hypotonic to the hypertonic.
If an overabundance of water flows into a cell, the cell may experience lysis, or breakage. The pressure exerted on the cell walls at the point of lysis is called osmotic pressure.