(1) A property of a solution that depends on the osmotic force exerted across the membrane as influenced by the differing concentrations of solutes in and out of the cell; the osmotic pressure or tension of a solution, as in the cells would swell or shrink depending on the tonicity of the environment
In biology, tonicity pertains to two definitions. The first one is associated with the osmotic pressure exerted upon a membrane and the other is about tone or tension (in a muscle or an organ). Relevant terms include isotonicity, hypertonicity, and hypotonicity.
At the cellular level, isotonicity refers to a condition or property of a solution in which its solute concentration is the same as the solute concentration of another solution with which it is compared. Hypertonicity is a property of a solution wherein the amount of solutes is higher than that of another solution. In contrast, hypotonicity pertains to a property of a solution with a comparatively lower concentration of solutes relative to the amount of solutes in another solution.
At the tissue level, isotonicity is a condition wherein the muscle remains to be in a relatively constant tension while its length changes. Hypotonicity of a muscle means a lesser degree of tone or tension as its length changes. This is in contrast to a hypertonic muscle that has a greater degree of tone or tension.
Word origin: Greek tonos (“tone")