Origin of the Term “Isotonic”

Introduction In our 1 st and 2 nd abstracts to EB2022, we addressed the inconsistency in the definitions of tonicity 1 and the problems of introducing both osmolarity and effective osmolarity 2 and resolved these issues. In this abstract, we introduce the origin of the term “isotonic”, which further clarifies the confusion about tonicity and leads to an understanding of the membrane (m)‐dependency of tonicity, one of the 3 properties of tonicity 3 . Method Logical reasoning. Results 1) The term “isotonic” was coined by the botanist Hugo de Vries in the 19 th century 4 . In his experimental setup, if S 1 caused a certain degree of shrinkage of a cell and S 2 caused the same degree of shrinkage of the same cell, then S 1 and S 2 were considered “isotonic solutions” because they have an equal “water attracting force”. We now know that it is the impermeant solute particles (imp‐SP) in S 1 and S 2 that pull water to their compartments in the osmosis systems S 1 ‐m‐S 3 and S 2 ‐m‐S 3 , respectively, where S 3 refers to the intracellular fluid (ICF) of the cell. That time, De Vries seemed not to be aware that when his S 1 is isotonic to S 2 , S 1 and S 2 are both hypertonic to S 3 , or vice versa, S 3 is hypotonic to S 1 and S 2 . 2) The term “x‐tonic (i.e., hypertonic or isotonic or hypotonic)” compares the 2 water attracting forces in an osmosis system, such as S 1 ‐m‐S 3 or S 2 ‐m‐S 3 . Obviously, it is the imp‐SP that exert this “water attracting force” (more appropriately this “pressure”). If we call this pressure “the osmotic tone” (i.e., the osmotic energy or strength), then the meaning of the term “x‐ tonic ” is clear: It compares the 2 osmotic tones in an osmosis system such as S 1 ‐m‐S 2 . Lack of an understanding of this point leads to the following illogical descriptions about tonicity: a common saying “S 1 is isotonic to the cell” compares an apple with an orange and another common saying “S 1 is isotonic” is not a complete sentence. These illogical statements result from the inconsistency in the definitions of tonicitywe addressed previously 1 and increase the level of confusion about what tonicity is, which we have addressed previously 1, 3 . 3) The logic that if a cell swells/shrinks in a solution, the solution is hypotonic/hypertonic to ICF is commonly reflected in the literature, but this reverses the cause and effect : It is because the ICF is hypertonic/hypotonic to the solution, so that the ICF wins/loses the water competing game (i.e., osmosis) across the cell membrane, or in other words, the winner (ICF) gains water and volume (cell swelling)/the loser (ICF) loses water and volume (cell shrinkage). Conclusions This abstract and our previous works listed in the References section together eliminate all problems related to the concept of tonicity. References 1. Kuang et al. “A Resolution for the Inconsistency in the Definitions of Tonicity” [Abstract], submitted to EB2022 2. Kuang et al. “Resolutions to the Problems Caused by Introducing both Osmolarity and Effective Osmolarity” [Abstract], submitted to EB2022 3. Kuang et al., The Concept of Tonicity: Problems and Resolutions. The FASEB Journal , Vol 34(S1) 4. Hamburger HJ. Twenty‐five years of osmotic pressure in the medical sciences. Science 34: 583‐589, 1911.