The Concept of Tonicity: Problems and Resolutions

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IntroductionIn this presentation, we address 6 major problems in the current understanding of the concept of tonicity that uses inconsistent descriptions/definitions of tonicity rooted in the problems with the current definitions of osmolarity (conventional osmolarity, see our abstract #2, “The Concept of Osmolarity: Problems and Resolutions” in EB2020). We then resolve these problems using two new concepts we defined in the abstract #2: 1) OC0 (the initial osmotic concentration (the concentration of the impermeant solute particles (SP)) in S‐m‐H2O (a simple osmosis system consisting of a solution (S) compartment and a water compartment separated by a selectively permeable membrane (either water‐permeable (only permeable to water) or SP‐permeable (permeable to water and some solute particle species). 2) S1‐m‐S2 (a composite osmosis system consisting of two solutions separated by a selectively permeable membrane).Methodlogical reasoning.Results1) To understand tonicity, a composite S1‐m‐S2 needs to be deconstructed into two simple osmosis systems that are mirrored (Fig 1). 2) When OC0 is used to eliminate the use of both conventional osmolarity and effective osmolarity (see the abstract #2), the meanings of the two sets of osmolarity and tonicity‐related terms (hyper/iso/hypo‐osmotic and hyper/iso/hypotonic) become consistent: If a solution is hyper/iso/hypo‐osmotic to another solution, it must also be hyper/iso/hypotonic to another solution. 3) Tonicity is a score‐like, relational concept that reflects a comparison between two OC0 in a composite osmosis system (S1‐m‐S2). It does not “belong” to a solution, but between two solutions. Its owner is a given composite osmosis system (S1‐m‐S2). 4) Tonicity can be expressed in multiple ways. 5) Tonicity has three important properties (graphical illustration will be provided in the poster): membrane‐dependency inherited from the membrane‐dependency of OC0 (see the abstract #2), reciprocity, and reference‐solution dependency or relativity. 6) Tonicity has a physical nature (Fig 2): either the ΔOC0 (the concentration gradient between two OC0 across the membrane) or the Δπ(S1‐S2) (the osmotic pressure gradient between S1 and S2). 7) The relationship among CTSP, OC0, and tonicity is illustrated in Fig 2. CTSP is the molar concentration of the total solute particles (TSP) measured in mM, not mOsm/L, defined in the abstract #2.Deconstruction of a composite S1‐m‐S2 into two simple osmosis systems: S1‐m‐H2O + H2O‐m‐S2.Figure 1A panoramic view of the concepts of CTSP, OC0, tonicity as the cause of osmosis (the ΔOC0(SX‐SY)), and tonicity as the effect of osmosis (reflected as the Δπ). Water is symbolized by the rope in the tug‐of‐war analogy. σ: reflection coefficient.Figure 2