The Nature and Properties of Tonicity
Authors: Kuang Serena , Yang Xiaoqi , Walter Stefan , Li Xiaonan 
Published: May 2023

We proposed that tonicity has a physical nature and three properties1. In this abstract, we prove this hypothesis to be true by logical reasoning. Two key terms will be used: 1) S1-m-S2 (a composite osmosis system that can be deconstructed into two simple osmosis systems: S1-m-H2O and H2O-m-S21; and 2) OC0, the initial osmotic concentration (concentration of the impermeant solute particles (impSP)) before osmosis occurs (time = 0)2. Results are as follows. 1) Since we defined tonicity to be a relational concept that reflects a comparison of two OC0 in S1-m-S21, if OC0 (S1) = 90 mOsm/L and OC0 (S2)= 30 mOsm/L, then Δ OC0 (S1-S2) = 90 – 30 = 60 mOsm/L, which contributes to the force that drives osmosis and an osmotic pressure gradient (Δ π) results3. Δ OC0 informs the tonicity (i.e., S1 is hypertonic to S2) directly and Δ π indirectly, so both can be used to express tonicity (the comparison)4. Either Δ OC0 or Δ π is a form of energy, thus tonicity has a physical nature. Tonicity is often inappropriately described as a solution's ability to change a cell’s volume5. As shown in this work, it is Δ OC0 (S1-S2) that has the ability to cause a change in cell volume, not any single solution in S1‐m‐S2. 2) The first property of tonicity is m-dependent: Facing different m, different fractions of the impSP result, so OC0 is m-dependent2; tonicity compares two OC0 separated by a given m, thus is m-dependent too. 3) The second property is relativity (reference-solution dependency: S2 in S1-m-S2 can be replaced by SX (X = 1, 2, 3, …n), a reference solution for S1. In contrast, in any simple osmosis system (S-m-H2O)1, the reference solution for S is always water with 0 mOsm/L, which cannot be replaced by any solution. 4) The third property is reciprocity because the same comparison can be described as either “S1 is hypertonic to S2 or S2 is hypotonic to S1” or “ Δ OC0 (S1-S2) or Δ OC0 (S2–S1)” etc. Hence, we conclude: Tonicity can be expressed using either Δ OC0 or Δ π. Either stores a form of energy representing a physical nature. OC0 (defined in a simple S-m-H2O) and tonicity (defined in a composite S1-m-S2) are both m-dependent, whereas relativity and reciprocity are properties of tonicity only. References: Kuang et al., The concept of tonicity: Problems and resolutions [Abstract], The FASEB Journal, 2020 34(S1); Kuang et al., The concept of osmolarity: Problems and resolutions [Abstract], The FASEB Journal, 2020 34(S1); Kuang et al., The pressure profiles of the equilibrium states of osmosis across plant and animal cell membranes [Abstract], The FASEB Journal, 2021 35(S1); Kuang et al., Tonicity can be expressed in multiple ways [Abstract], submitted to the APS Summit 2023; Kuang et al. A resolution for the inconsistency in the definitions of tonicity [Abstract], The FASEB Journal, 2022 36(S1). This is the full abstract presented at the American Physiology Summit 2023 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.