The Nature and Properties of Tonicity

Abstract

We proposed that tonicity has a physical nature and three properties 1 . In this abstract, we prove this hypothesis to be true by logical reasoning. Two key terms will be used: 1) S 1 -m-S 2 (a composite osmosis system that can be deconstructed into two simple osmosis systems: S 1 -m-H 2 O and H 2 O-m-S 2 1 ; and 2) OC 0 , 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 OC 0 in S 1 -m-S 2 1 , if OC 0 (S 1 ) = 90 mOsm/L and OC 0 (S 2 )= 30 mOsm/L, then Δ OC0 (S 1 -S 2 ) = 90 – 30 = 60 mOsm/L, which contributes to the force that drives osmosis and an osmotic pressure gradient (Δ π) results 3 . Δ OC 0 informs the tonicity (i.e., S 1 is hypertonic to S 2 ) directly and Δ π indirectly, so both can be used to express tonicity (the comparison) 4 . Either Δ OC 0 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 volume 5 . As shown in this work, it is Δ OC 0 (S 1 -S 2 ) that has the ability to cause a change in cell volume, not any single solution in S 1 ‐m‐S 2 . 2) The first property of tonicity is m-dependent: Facing different m, different fractions of the impSP result, so OC 0 is m-dependent 2 ; tonicity compares two OC 0 separated by a given m, thus is m-dependent too. 3) The second property is relativity (reference-solution dependency: S 2 in S 1 -m-S 2 can be replaced by S X (X = 1, 2, 3, …n), a reference solution for S 1 . In contrast, in any simple osmosis system (S-m-H 2 O) 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 “S 1 is hypertonic to S 2 or S 2 is hypotonic to S 1 ” or “ Δ OC 0 (S 1 -S 2 ) or Δ OC 0 (S 2 –S 1 )” etc. Hence, we conclude: Tonicity can be expressed using either Δ OC 0 or Δ π. Either stores a form of energy representing a physical nature. OC 0 (defined in a simple S-m-H 2 O) and tonicity (defined in a composite S 1 -m-S 2 ) are both m-dependent, whereas relativity and reciprocity are properties of tonicity only.