Tonicity is a term used in biology to describe the ability of an extracellular solution to change the volume of a cell by affecting osmosis/05%3A_Structure_and_Function_of_Plasma_Membranes/5.09%3A_Passive_Transport_-Tonicity). It is a measure of the effective osmotic pressure gradient between two solutions separated by a partially-permeable cell membrane. Tonicity depends on the relative solute concentrations and the cell membranes permeability to those solutes. Three terms are used to describe the tonicity of a solution: hypertonic, hypotonic, and isotonic/05%3A_Structure_and_Function_of_Plasma_Membranes/5.09%3A_Passive_Transport-_Tonicity).

• Hypertonic solution: A solution with a greater concentration of non-permeating solutes than another solution. If a cell is placed in a hypertonic solution, there will be a net flow of water out of the cell, and the cell will lose volume/05%3A_Structure_and_Function_of_Plasma_Membranes/5.09%3A_Passive_Transport_-_Tonicity).

• Hypotonic solution: A solution with a lower concentration of non-permeating solutes than another solution. If a cell is placed in a hypotonic solution, there will be a net flow of water into the cell, and the cell will gain volume/05%3A_Structure_and_Function_of_Plasma_Membranes/5.09%3A_Passive_Transport_-_Tonicity).

• Isotonic solution: A solution with the same concentration of non-permeating solutes as another solution. If a cell is placed in an isotonic solution, there will be no net flow of water into or out of the cell, and the cells volume will remain stable/05%3A_Structure_and_Function_of_Plasma_Membranes/5.09%3A_Passive_Transport_-_Tonicity).

Tonicity is a concern for all living things, particularly those that lack rigid cell walls and live in hyper- or hypotonic environments. In healthcare settings and biology labs, it is often helpful to think about how solutions will affect water movement into and out of cells. Tonicity is tightly regulated by the equilibrium between water intake and wa...