Simple Osmosis Research Articles

How to Study Osmosis If the Membrane Water Moves Across Is Nonideal

In our past works, we introduced two fundamental osmosis systems (a simple S-m-H 2 O and a composite S 1 -m-S 2 ) to study osmosis 1,2 , where m is a selectively permeable membrane that separates two fluid compartments. A composite S 1 -m-S 2 can be deconstructed into two simple osmosis systems (S 1 -m-H 2 O and H 2 O-m-S 2 ) 1,2 . When applying these two fundamental osmosis systems, a given m can either be permeable to water only (an ideal m) or its permeability to one species of non-charged solute particles (SP, e.g., urea) can be ignored. If m' represents a nonideal membrane whose permeability to urea cannot be ignored, then a new research question emerges: how can osmosis be studied in S-m'-H 2 O and S 1 -m'-S 2 , respectively? This abstract provides the answers by logical reasoning. First, S-m'-H 2 O can be viewed as the overlap of a pure osmosis system with an ideal m (S-m-H 2 O) and a permeant SP (pSP), urea diffusion system: S-m'-H 2 O = S-m-H 2 O + S urea -m'-H 2 O, so that osmosis and urea diffusion can be studied separately. The former is governed by van ’t Hoff’s law and the latter by Fick’s law. Second, similarly, S 1 -m'-S 2 = S 1 -m-S 2 + S 1urea -m'-S 2urea . Third, the effect of urea diffusion on osmosis depends on several conditions: 1) In S-m'-H 2 O, urea diffusion and osmosis are always in opposite directions, so urea diffusion reduces the resulting osmotic pressure (π). 2) In S 1 -m'-S 2 , if S 1 is hypertonic to S 2 and [urea] S1 > [urea] S2 , then the diffusion and osmosis are in opposite directions, so the diffusion reduces the resulting osmotic pressure gradient (Δ π). 3) If S 1 is hypotonic to S 2 and [urea] S1 > [urea] S2 , then osmosis and diffusion are in the same direction, so the diffusion augments the resulting Δ π. In conclusion: 1) Deconstruction of an osmosis system that has a nonideal m' allows separate studies of osmosis and pSP diffusion, respectively. 2) The effect of pSP diffusion on osmosis needs to be differentiated according to the type of osmosis systems and the direction of osmosis and pSP diffusion. 3) While the simple S-m-H 2 O and composite S 1 -m-S 2 are fundamental osmosis systems, S-m'-H 2 O and S 1 -m'-S 2 are complex passive membrane transport systems that transport both water and pSP. 4) If a given nonideal m' is permeable to two types of pSP (e.g., urea and glucose), then the diffusion system needs to be further deconstructed into a urea diffusion system and a glucose diffusion system.

The Nature and Properties of Tonicity

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.

A Resolution for the Inconsistency in the Definitions of Tonicity

IntroductionThere is great inconsistency in the current definitions of tonicity, which hinders research, teaching, and learning about osmosis across the cell membrane. In this abstract, we 1) show the inconsistency in the literature; 2) reveal what caused it and resolve it.MethodLogical reasoning.Results1) The inconsistencyTable 1 shows a collection of 14 descriptions/definitions of tonicity classified into 5 categories (Cat.), which indicates that the level of the inconsistency is high and the truth of tonicity is elusive.2) Causes and resolution of the inconsistencyAs we illustrated previously1, tonicity should be defined in a composite osmosis system, which can be deconstructed into 2 simple osmosis systems that are mirrored: S1‐m‐S2 = S1‐m‐H2O + H2O‐m‐S2, where S1 and S2 are two solutions separated by a membrane (m) that is only permeable to H2O in this abstract. Lack of this understanding is the first cause.Second, the owner of tonicity is wrongly reflected in the definitions of Cat. 1, 2, and 3 in the Table, phrased as “a solution’s tonicity” or “the tonicity of a solution”. Neither S1 or S2 in S1‐m‐S2owns tonicity because it reflects a comparison of the 2 initial concentrations of the impermeant solute particles in S1‐m‐S2(i.e., OC0(S1) and OC0(S2), where OC0 refers to osmotic concentration before osmosis occurs, i.e., time = 01, 2. As in a game that has two players, the result of the game (a qualitative statement of “player 1 won the game” or “player 2 lost the game” or a quantitative score of 12 to 9) reflects a comparison of the points (strengths) of the 2 players, so the game system owns the result. Osmosis in S1‐m‐S2 is like a water‐competing game between OC0(S1) and OC0(S2). Tonicity expressed qualitatively as “S1 is hypertonic to S2” or quantitatively as “300 : 200” if OC0(S1) = 300 mOsm/L and OC0(S2) = 200 mOsm/L, reflects a comparison of the osmotic strengths of OC0(S1) and OC0(S2), thus predicts the result of osmosis, i.e., a change in cell volume1. Hence, it is the composite S1‐m‐S2 system that owns tonicity (the comparison or prediction or the result of osmosis). “A solution’s tonicity” makes it impossible to define tonicity. Knowing the truth of tonicity, the definitions in Cat. 3 fail to define what tonicity is and reflect the result of osmosis inappropriately. The definitions in Cat. 5 obviously make no sense.The 3rd and 4th causes are addressed and resolved in our 2nd and 3rd abstracts titled Resolutions to the Problems of Introducing both Osmolarity and Effective Osmolarity and Origin of the Term “Isotonic”, respectively.ConclusionsTonicity can be expressed in multiple ways. The definitions in Cat. 4 express tonicity appropriately as something relational between S1 and S2 if OC0is the focus, not osmolarity or effective osmolarity. The last definition in Cat. 4 is accurate among all 14 definitions, with which, the result of osmosis (i.e., the osmotic pressure gradient after osmosis)3 is used to express tonicity.

Unification of the Multiple Forms of van 't Hoff’s Law into One General Form

IntroductionThe first Nobel Prize in Chemistry was awarded to Jacobus Henricus van 't Hoff in 1901 for his discoveries of the laws of chemical dynamics and osmotic pressure (π) in solutions. The original form of van 't Hoff’s law illustrates the relationship between π and the molar concentration (C) of a solution (S) facing a membrane (m) that separates the S and water compartments and is only permeable to water: π = C·RT, where R is the gas constant and T is the absolute temperature. However, in reality, both biological and artificial membranes can be extremely complex, so that facing different m, the resulting fractions of the impermeant solute particles (imp‐SP) contributing to πis different. The composition of the total solute particles (TSP that includes imp‐SP and permeant SP (p‐SP)) in S and their interactions may also be complex. For these reasons, multiple forms of van 't Hoff’s law have been developed over time to adapt to the different levels of complexity of solutions and/or membranes. In our previous works, we have done the following: 1) Defined the osmosis setting above as a simple osmosis system (S‐m‐H2O) and the setting of S1‐m‐S2 as a composite osmosis system that can be deconstructed into two mirrored simple osmosis systems: S1‐m‐H2O and H2O‐m‐S21, 2. 2) Addressed the many problems in the current definitions of osmolarity (osmotic concentration, OC) and tonicity1‐6. 3) Reasoned out what real osmolarity is: the osmotic concentration (OC, the concentration of the imp‐SP resulting from the interaction between the composition of S and m) in S‐m‐H2O1. OC in boldface is differentiated from OC in regular face. OC is a m‐dependent variable during osmosis whose initial value before osmosis occurs (i.e,, time = 0) is OC0, a constant of practical use. 4) Proved the correctness and effectiveness of OC0 in eliminating all the problems we addressed with the current definitions of osmolarity and tonicity1‐6. Moreover, by applying OC0 to van 't Hoff’s law, the multiple forms of the law can be unified into one general form. This presentation demonstrates 1) how this unification occurs; 2) how the unified form can be applied to a composite S1‐m‐S2.MethodLogical reasoningResults1. Multiple forms ofvan 't law and their unificationTable 1 shows the unification of the multiple forms of the law into one general form: π = OC0·RT.2. Applying the unified form ofvan 'tHoff’s law to a composite osmosis system (S1‐m‐S2)Figure. 1 illustrates the application of the unified form of the law in S1‐m‐S2: ∆π = ∆OC0·RT.ConclusionsThe unification of the multiple forms of van 't Hoff’s law using OC0 into a general form (π = OC0·RT) is a significant theoretical development of the law. The original form of the law is a special case of the general form.

Resolutions to the Problems Caused by Introducing both Osmolarity and Effective Osmolarity

IntroductionIn the literature, 2 types of osmolarity are introduced: osmolarity (OS, the concentration of the total solute particles (SP) of a solution (S)) and effective osmolarity (EOS, the concentration of the impermeant SP (imp‐SP) in S). Introducing 2 types of osmolarity causes 3 problems in dealing with osmosis. In this presentation, we address these problems and resolve them.MethodLogical reasoning.Results1) Addressing the 3 problemsi. The fact that both OS and EOS are introduced in the literature is one of the four causes of inconsistency in the definitions of tonicity addressed in our first abstract1 to EB2022.ii. The fact causes a great confusion between the meanings of the following terms: hyper‐osmotic or iso‐osmotic or hypo‐osmotic (referred to as x‐osmotic) vs. hypertonic or isotonic or hypotonic (x‐tonic). Some literature specifies that x‐osmotic is used to compare 2 OS and x‐tonic to compare 2 EOS. Consequently, x‐osmotic and x‐tonic compare two different properties (apples and oranges) and knowing one relationship does not mean we know the other, e.g., “iso‐osmotic is not always isotonic”2. Moreover, if S1 (with an OS of 600 mOsm/L) is hyper‐osmotic to S2 (its OS = EOS = 300 mOsm/L), S1 can be hypertonic or isotonic or hypotonic to S2 if it contains 200 mM permeant SP (p‐SP) + 400 mM impermeant SP (imp‐SP), or 300 mM p‐SP + 300 mM imp‐SP, or 400 mM p‐SP + 200 mM imp‐SP, respectively.iii. It is unclear what real osmolarity is.2) The resolutionsi. As illustrated previously3, real osmolarity is the osmotic concentration (OC, the concentration of the imp‐SP, a variable during osmosis in a simple osmosis system (S‐m‐H2O) where m refers to a semipermeable membrane separating S and H2O compartments); OC0is the initial OC before osmosis occurs (time = 0) and of practical use. OC0 is membrane (m)‐dependent: Facing different m, the resulting fraction of the imp‐SP in S‐m‐H2O is different. Hence, real osmolarity is neither OS nor EOS: OS contains permeant SP that do not exert an osmotic pressure; OS is a misconception of the term CTSP (the molar concentration of the total SP in S that is inherent to S and m‐independent)3. It is unclear whether EOS is a variable or a constant. OC0 is a system parameter owned by a simple osmosis system (S‐m‐H2O), whereas OS and EOS are owned by a solution.ii. If OS is replaced by CTSP and EOS is replaced by OC0, then all problems addressed in this and our first abstracts to EB2022 are eliminated and x‐osmotic has the same meaning as x‐tonic, so that if S1 is x‐osmotic to S2, it is also x‐tonic to S2. See our third EB2022 abstract titled Origin of the Term “Isotonic”.ConclusionsBy applying CTSP to replace OS and OC0 to replace EOS, we have proved the effectiveness of CTSP and OC0 in eliminating the problems presented in this and our first abstracts1.

The Nature and Measures of Osmotic Pressure

Introduction In our first abstract titled “The Concept of Osmotic Pressure: Two Common Misunderstandings and Resolutions” in EB2021, we addressed that osmotic pressure should be defined in the context of a simple osmosis system (i.e., S-m-H2O, with a solution compartment (S) and a water compartment separated by a selectively permeable membrane (m)), not in the context of a composite osmosis system (i.e., S1-m-S2), which can instead be deconstructed into two mirrored simple osmosis systems: S1-m-H2O and H2O-m-S2. In this presentation, we 1) use a new set of terminology to differentiate the osmotic pressure (π) defined in three common ways in physiology and biology as well as in chemistry and physics; 2) address which of these ways represents the nature of π; and 3) illustrate the osmotic pressure difference/gradient in S1-m-S2. Method Logical reasoning as shown in Fig 1 and 2. Results 1) In S-m-H2O (Fig 1a to 1b), osmosis occurs and reaches equilibrium (1). We define the gravitational hydrostatic pressure exerted by the resulting liquid column as πachieved. 2) We define the π applied to S to prevent osmosis as πapplied (Fig 1c). 3) According to van't Hoff's Law, πachieved or πapplied = RTOC0 (OC0 is the initial osmotic concentration in S). 4) In S-m-H2O, a transmembrane pressure drives osmosis (illustrated using the blue arrow in Fig 1c), which can be viewed as exerted by either the water potential difference (Δμ0, not shown) or the OC0 difference (ΔOC0, the red gradient in Fig 1a and 1c) across the m: during osmosis, Δμ0 pushes or ΔOC0 pulls water toward S. We define this pressure to be πTM (TM refers to transmembrane). In physical chemistry, π (our πTM) = const×Δμ01. Hence, πTM = -RTOC0 = const×Δμ0. Conclusion 1) Among the three common ways to define π (πachieved, πapplied, πTM), πTM is thenature ofosmotic pressure; 2) πachieved and πapplied are two different ways to measure πTM. 3) In S1-m-S2, it is the osmotic pressure difference (ΔπTM) that drives osmosis, which can be measured using either Δπachieved or Δπapplied (Fig 2). In our third abstract, titled “The Pressure Profiles of the Equilibrium States of Osmosis Across Plant and Animal Cell Membranes”in EB2021, we will apply the concepts of ΔπTM to analyze the pressure profiles that characterize the equilibrium states of osmosis (after water enters a cell) across plant and animal cell membranes. 1Chang R. Physical Chemistry for the Chemical and Biological Sciences. Sausalito, CA: University Science Books, 2000, p. 235.

The Pressure Profiles of the Equilibrium States of Osmosis across Plant and Animal Cell Membranes

Introduction In our previous abstract titled “The Nature and Measures of Osmotic Pressure” in EB2021, we illustrated that 1) in a simple osmosis system where a solution compartment and a water compartment are separated by a selectively permeable membrane (m), πachieved, πapplied, and πTM (TM refers to transmembrane) are the three common ways osmotic pressure (π) is defined, where πTM is the nature of osmotic pressure that drives osmosis; and 2) in a composite osmosis system where a cell membrane separates two solutions (extracellular and intracellular fluids, i.e., ECF and ICF), the driving pressure of osmosis is the osmotic pressure difference/gradient (ΔπTM) across the m. In this presentation, we apply the concepts of a composite ECF-m-ICF and the ΔπTM to illustrate how three different types of pressure characterize the equilibrium states of osmosis (after water enters the cells) across plant and animal cell membranes. Method 1) Deconstructing a composite ECF-m-ICF (Fig 1) into two simple osmosis systems (ECF-m-H2O and H2O-m-ICF) to understand the ΔπTM. 2) Graphical (logical) analysis of the three pressures in plant and animal ECF-m-ICF (Fig 2). Results 1) As shown in Fig 1, ΔπTM = πTM(ECF-m-H2O) - πTM(H2O-m-ICF). 2) After water enters a cell and reaches equilibrium of osmosis, ΔπTM is reduced to Δπeq (Fig 2). 3) In a plant ECF-m-ICF (Fig 2a and 2b), large Δπeq (close to ΔπTM), high hydrostatic pressure (ΔPH2O, thick grey arrows in Fig 2b) inside the cell, and high tension in the call wall (ΔTwall, thick white bidirectional arrows, Fig 2b) characterize this equilibrium state. To focus on the pressure profiles and simplify the Fig 2, the vacuoles that store water in the plant cell are not shown. In contrast, in an animal ECF-m-ICF, smaller Δπeq, lower PH2O (the thin grey arrows, Fig 2d), and lesser tension in the cell membrane (ΔTCM, the white bidirectional arrows, Fig 2d) characterize the equilibrium state. Conclusion This is the first work to illustrate in detail the differences in the three pressure profiles across plant and animal cell membranes after water enters the cells due to osmosis using the method of deconstruction of ICF-m-ECF and the concept of ΔπTM.

The Concept of Osmotic Pressure: Two Common Misunderstandings and Resolutions

Introduction In a previous publication titled “The Concept of Osmolarity: Problems and Resolutions” (Vol 34, Issue S1, The FASEB Journal), we defined two osmosis systems: 1) a simple osmosis system (S-m-H2O, in which a solution compartment (S) and a pure water (H2O) compartment are separated by a selectively permeable membrane (m)) and 2) a composite osmosis system (S1-m-S2, in which two solution compartments (S1 and S2) are separated by m). In this presentation, we will apply the concepts of S-m-H2O and S1-m-S2 to address two common misunderstandings in the measurement/definition of the concept of osmotic pressure (π) in physiology and biology, as well as in chemistry and physics. Method Deconstructing a composite osmosis S1-m-S2 into two mirrored simple osmosis systems: S1-m-H2O and H2O-m-S2 (Fig 1). Results From Fig 1, the following points are clear and thus can be addressed: 1) π should be measured/defined in the context of a simple S-m-H2O not a composite S1-m-S2. 2) The first common misunderstanding that plagues the areas mentioned above is the measurement/definition of π in the context of S1-m-S2. What is measured is not π but the difference between two osmotic pressures (π(S1-m-H2O) and π(H2O-m-S2)) in S1-m-S2. 3) π is a system parameter because it is m-dependent or S-m-H2O-dependent: Facing various m, a solution can form various simple osmosis systems with pure water, resulting in various magnitudes of π. Hence π is specific to each given, simple osmosis system and thus a system parameter rather than belonging to a solution. Therefore, 4) the second common misunderstanding is that π is the solution's π. 5) Osmosis across a cell membrane occurs in a composite osmosis system, ECF-m-ICF = ECF-m-H2O + H2O-m-ICF, where ECF and ICF refer to intracellular and extracellular fluids. This step is a prerequisite for us to develop our other abstract titled “The Pressure Profiles of the Equilibrium States of Osmosis Across Plant and Animal Cell Membranes” in EB2021. Conclusion 1)The missingstep of defining simple and composite osmosis systems is essential to define osmotic pressure and study osmotic pressure difference/gradient (Δπ) across cell membrane in the human body. 2) The view of osmotic pressure as a system parameter of a simple osmosis system derives from this essential step, which advances our understanding of the concept of osmotic pressure.

The Concept of Osmolarity: Problems and Resolutions

IntroductionIn this presentation, we address and resolve the problems with the current definitions of osmolarity (conventional osmolarity) that result in great confusion about the definition of tonicity. These problems are partially rooted in issues described in our first abstract in the EB2020, “The Problems with the Concepts of Osmole and Osmotically Active Particles”.Methodlogical reasoning.Results1) CTSP (the molar concentration of the total solute particles (TSP) in a solution measured using mM,not mOsm/L) is defined as the first prerequisite to understanding what true osmolarity should be. CTSP is a missing concept in current teaching and research on solutions and osmosis. 2) A thorough differentiation of the vague term “solute” (Fig 1) is the second prerequisite. 3) Our amended definition that reflects true osmolarity (bolded osmolarity) is the impermeant fraction of CTSP (or the concentration of the impermeant SP) in the solution (S) compartment of a simple osmosis system (S‐m‐H2O consisting of a S compartment and a water compartment separated by a selectively permeable membrane (m) (irrespective whether the m is water‐permeable (only permeable to water) or SP‐permeable (permeable to water and some species of solute particles (SP)). The unit of osmolarity is mOsm/L. This single definition of osmolarity eliminates the use of two types of osmolarity (conventional and effective) that are problematic. 4) Osmolarity is a variable during osmosis. In keeping with the recommendation by the International Union of Pure and Applied Chemistry (IUPAC) Gold Book 1996 to use “osmotic concentration” to replace “osmolarity”, we promote the use of bolded osmotic concentration (OC) to present bolded osmolarity. The initial OC or OC0 is of practical use. 5) A solution has one CTSP but may interact with many different membranes and result in many values of OC0 (Fig 2). 6) A composite osmosis system (S1‐m‐S2) has two solution compartments separated by an m, which can be deconstructed into two simple osmosis systems that are mirrored: S1‐m‐H2O and H2O‐m‐S2. 7) The application of OC0 produces two by‐products: First, the multiple forms of van ‘t Hoff’s Law can be unified into one single general form because OC0 is not only membrane‐dependent, but also embraces multiple factors that influence its value and thus is of complexity; and second, by using OC0 and S1‐m‐S2 to illustrate tonicity, all confusion about tonicity is eliminated in our third abstract titled “The Problems with Tonicity and Resolutions” in EB2020.Conclusion1) OC0 is a membrane‐dependent system parameter affiliated with S‐m‐H2O (not S). 2) By replacing conventional osmolarity with CTSP, effective osmolarity with OC0, and osmotically active particles with the membrane‐dependent osmotically effective SP (irrespective whether the membrane is water‐permeable or SP‐permeable), all problems with conventional osmolarity are resolved.Solute differentiationFigure 1The one‐to‐many relationship between CTSP and OC0: facing different 3 membranes, 3 different values of OC0 emerge in the SA compartment of the 3 simple osmosis systems. σ: reflection coefficient.Figure 2

The Concept of Tonicity: Problems and Resolutions

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

The Problems with the Concepts of Osmole and Osmotically Active Particles

IntroductionIn this presentation, we address and resolve the problems with the current definitions of osmole and the widely used term “osmotically active particles”. This presentation precedes and connects to our second abstract titled “The Concept of Osmolarity: Problems and Resolutions” in EB2020.MethodLogical reasoning.Results1) A simple osmosis system is first defined as S‐m‐H2O: a solution (S) compartment and a water compartment separated by a selectively permeable membrane (m) that can be either water‐permeable (only permeable to water) or SP‐permeable (permeable to water and some species of solute particles (SP)). This is a missing step to define osmole and true osmolarity. 2) The problems mentioned above can be identified in three sample definitions of osmole: “One osmole (Osm) is equal to 1 mole (mol) (6.02 × 1023) of solute particles”; “One mole of osmotically active particles is called one osmole”; and “the osmole (Osm or osmol) is a non‐SI unit of measurement that defines the number of moles of a chemical compound that contribute to a solution’s osmotic pressure”. These definitions are consistent if the m in S‐m‐H2O is water‐permeable but have different meanings when the m is SP‐permeable: the first includes osmotically ineffective, permeant SP, while the third excludes them. The second definition has a problem with the ambiguous term “osmotically active particles”: are these particles osmotically active when facing a water‐permeable membrane or facing a SP‐permeable membrane or both? Only the third definition still works when the membrane is SP‐permeable, since the moles of particles “that contribute to a solution’s osmotic pressure” are the moles of the impermeant SP regardless of whether m is water‐permeable or SP‐permeable. 3) To fix the problem with the term “osmotically active particles”, we replace it with a new term “osmotically effective SP,” defined as the physical quantity representing the amount of the impermeant (i.e., osmotic pressure (π)‐contributable) SP in the S compartment of S‐m‐H2O. 4) Osmosis does not take place in an isolated solution but in S‐m‐H2O. When the m is given, the total solute particles (TSP) of S are differentiated to be permeant SP measured using millimole (mmol) and impermeant SP measured using milliosmole (mOsm). Since the unit mOsm defined above excludes any permeant SP to a given m, the unit of osmolarity (i.e., mOsm/L) defines true osmolarity: the molar concentration of the impermeant SP in the S compartment of S‐m‐H2O.ConclusionOur definitions of osmole, osmotically effective SP, and true osmolarity are all given membrane‐dependent, system parameters of S‐m‐H2O, not owned by an isolated S. These definitions are essential but not enough to thoroughly resolve the many unaddressed problems with the current definitions of osmolarity. Our second abstract addresses these problems and offers resolutions.

Pancreatic secretion and pressure biomechanics in pancreatic acinus

The article highlights the mechanism of the mathematical model of acinus, the components of the formation of pressure in its cavity and the formation of pancreatic juice. It has been established that the mechanism for creating pressure in the acinus cavity is similar to the intraductal one. In this case, the question remains open about the causes of such high pressure, which is measured in several hundred millimeters of a mercury column, especially since, as histologically established, the pancreas and its ducts do not have muscle structures, and those rudiments of myofibrils, which are noted in some places of the flow system, of course, cannot ensure the development of such pressure. The increase in pressure in the cavity of the acinus is associated with the phenomenon of osmosis in its cells. Since cell membranes have the property of conductivity, as a result of osmosis, water through the membrane first passes from the blood to the cell, then from the cell through the membrane into the acinus cavity. In addition to the mechanism of osmosis through the membrane, in the cells of the acinus epithelium, there is a filtering mechanism through the pores of the layer of connective tissue to the lymph channel. It has now been established that, together with simple osmosis, the phenomenon of electroosmosis takes place in secreting cells and organs of excretion, not only accelerates the transfer of substances, but also increases the pressure on the other side of the membrane against the gradient by almost several first-order units. Thus, the outflow of fluid from the acinus cavity proceeds continuously, but only with a change in the speed of movement, it is determined by the pressure drop in the acinus – tubule – excretory duct system, the opening of the Oddi sphincter and the pulse of the cardiovascular wave, which creates dynamic pressure in the capillary. This whole mechanism, as a result, leads to the filling of the cavity of the acinus and the creation of a certain pressure in it.

Water-Transporting Proteins

Transport through lipids and aquaporins is osmotic and entirely driven by the difference in osmotic pressure. Water transport in cotransporters and uniporters is different: Water can be cotransported, energized by coupling to the substrate flux by a mechanism closely associated with protein. In the K(+)/Cl(-) and the Na(+)/K(+)/2Cl(-) cotransporters, water is entirely cotransported, while water transport in glucose uniporters and Na(+)-coupled transporters of nutrients and neurotransmitters takes place by both osmosis and cotransport. The molecular mechanism behind cotransport of water is not clear. It is associated with the substrate movements in aqueous pathways within the protein; a conventional unstirred layer mechanism can be ruled out, due to high rates of diffusion in the cytoplasm. The physiological roles of the various modes of water transport are reviewed in relation to epithelial transport. Epithelial water transport is energized by the movements of ions, but how the coupling takes place is uncertain. All epithelia can transport water uphill against an osmotic gradient, which is hard to explain by simple osmosis. Furthermore, genetic removal of aquaporins has not given support to osmosis as the exclusive mode of transport. Water cotransport can explain the coupling between ion and water transport, a major fraction of transepithelial water transport and uphill water transport. Aquaporins enhance water transport by utilizing osmotic gradients and cause the osmolarity of the transportate to approach isotonicity.

Water transport controversies--an overview.

The last 20 years have seen major advances in the understanding of the mechanisms and molecular bases of water transport across cell membranes. Prominent among these advances was the discovery of a family of transmembrane proteins that permeate exclusively or dominantly water (aquaporins), followed by structural studies identifying the intramolecular permeation pathway, as well as by studies directed to a detailed understanding of these molecules in health and disease. Agre and colleagues (Agre et al. 2002) discuss the molecular structure of aquaporins and the physiology and pathophysiology of aquaporins in diverse organs and tissues. Another important advance was the development of methods for rapid measurements of changes in cell volume. This resulted in accurate estimates of cell-membrane water permeability, and in the prediction that very small osmotic gradients would suffice to drive transepithelial transport, without the need for the existence of compartments of much higher osmolality than that of the bathing solution. Around 1990, faced with the question of what pathways and driving forces underlie net water fluxes across cell membranes, most physiologists working on animal cells would have had direct and clear answers. Water moves across the phospholipid moiety of the plasma membrane (although this membrane can be extremely water-tight in certain cases), and through water pores, in cells that express these proteins. The driving force is the difference in effective osmolality across the cell membrane. Enlightened physiologists working on epithelia would add that transepithelial water absorption or secretion is always a passive phenomenon, secondary to net solute transport in the same direction. Further, complicated compartment models, such as the standing osmotic gradient hypothesis, are unnecessary to explain water transport, because very small osmotic gradients, perhaps too small to measure directly, are a sufficient driving force. The question of how water fluxes are partitioned between transcellular and paracellular pathways remained difficult to answer because of the lack of direct measurements. The two central questions in the problem of water transport across simple or complex membranes are the pathway and the mechanism. Considering the cell membrane, one asks whether water permeates the lipid moiety and/or transmembrane proteins. These pathways are expected to have different properties, including the possibility that transmembrane water pores could also be permeable to other molecules. Concerning the driving force for water transport, in principle it could be primary active, secondary active or passive. At the time prior to the discoveries that provide the experimental bases for the Topical Reviews in this Special Issue, the virtually unanimous opinion of the experts would have been that water transport is passive. With respect to transepithelial water transport, the relative contributions of the transcellular and paracellular routes remain to be determined. There is no agreement on the molecular mechanism, i.e. whether there is secondary-active water transport at the cell membrane in addition to simple osmosis, or on the precise nature of osmotic water flow, i.e. is there truly isosmotic transport? This Special Issue highlights the current excitement in this area of physiology. Three current controversies in the field of water transport are discussed. In each case, the ‘controversial’ novel theory is presented (in the form of a Topical Review) and discussed critically in the form of a Perspective or a Research Paper. The three controversies presented in this Special Issue were debated at the XXXIV International Congress of Physiological Science in Christchurch, New Zealand.

Concentration with Selective Acid Removal from Americium Solutions Using Ion Exchange Membranes

Donnan dialysis and diffusion dialysis techniques are investigated for deacidification and possible concentration of actinide bearing acidic streams. Cation and anion exchange membranes are used in a two compartment dialysis cell to selectively remove acid from americium solutions. Acid concentrations, as high as 8.0M HNO3 are used and the stability of membranes are ascertained. Possibility of simultaneous deacidification and concentration of acidic americium solutions are indicated by combining simple osmosis with diffusion dialysis. All the experiments are carried out under non stirring conditions at room temperature.

Distribution of osmotic flow in stomach and gallbladder

Stomach and gallbladder actively transport fluids which are nearly isotonic to plasma. Consideration of the measured areas of the appropriate transporting surfaces gives a more realistic view of the osmotic gradient required to account for the observed net flow of water. Simple osmosis may be adequate if the transporting membrane has an osmotic permeability in the range observed for synthetic lecithin-cholesterol bilayer membranes.

Fluid transport in the rabbit blastocyst.

Osmolaritätsmessungen zeigen, daß während früher Schwangerschaftsstadien die Blastocoel-Flüssigkeit isoosmotisch ist mit der uterinen Flüssigkeit, daß sie aber in vitro hypotonisch wird mit Bezug auf das Inkubationsmedium. Blastozysten können Flüssigkeit aufnehmen gegen einen erheblichen osmotischen Gradienten von Saccharose oder Mannitol. Diese Beobachtungen eliminieren die Möglichkeit, daß einfache Osmose für die Flüssigkeitsakkumulation verantwortlich ist, die während des Wachstums von Blastozysten erfolgt. Aktiver Transport wird nahegelegt durch die Tatsache, daß definierte Konzentrationen von Zyanidionen oder 2,4-Dinitrophenol die Akkumulation von Flüssigkeit verhindern.Ergebnisse von Experimenten von kultivierten Embryonen zeigen, daß optimale Natrium- und Chloridionenkonzentrationen nötig sind für die Ausdehnung des Blastozysten, während Natrium-, Calcium- und Magnesiumionen den Flüssigkeitstransport nicht direkt zu beeinflussen scheinen.Das elektrochemische Verhalten von Trophoblasten wurde durch Mikroelektroden untersucht, die in das Blastocoel eingeführt wurden. Kleine, aber dauernde elektrische Potentiale existieren; die Seite, gegen welche Flüssigkeittransport gerichtet war, wurde negativ bei etwa zwei mV. Hohe Osmolarität des Mediums bewirkte, daß das Potential um einen Faktor 3 anstieg, während Zyanidionen oder Dinitrophenol das Potential elimierten. Ein Natriummengelmedium erhielt ein Transtrophoblast-Potential vergleichbar mit einem in normalem F 10, während in einem Chloridionenmangelmedium die Potentialdifferenz verschwand. Wenn F 10 Natriumsulfat statt Natriumchlorid enthielt, zeigte der Trophoblast kein Potential; wenn aber Sodiumchlorid durch Sodiumbromid ersetzt wurde, kam es zu einer relativ hohen negativen Spannung.Diese Ergebnisse stehen im Einklang mit Beobachtungen an der Gallenblase des Kaninchens, für welche das Curran Doppelmembranmodell in Anwendung kommt. Wenn man physiologische, elektrische und strukturelle Eigenschaften betrachtet, ist dieses Modell relevant für die Flüssigkeitsaufnahme der Kaninchenblastozysten.

The experimental production of watery vacuolation in the acinar cells of the submandibular gland.

1. Watery vacuolation in the acinar cells of the rat submandibular gland is described. The vacuoles are cytoplasmic, membrane-walled, and 2-20 mu in diameter. They are visible in living cells and appear to contain a watery fluid.2. Vacuolation occurred regularly in the following experimental situations: (1) in vitro-under anoxic conditions (2) post mortem-in animals killed by anoxia, and (3) in vivo-during secretion.3. By in vitro experiments it was shown that vacuolation occurs only when the cells are both anoxic and exposed to an excess of extracellular fluid containing calcium and bicarbonate. It was further shown that vacuolation is reversible in oxygen and that both its development and recovery are temperature dependent.4. Evidence is presented that the vacuolation is not a degenerative or necrotic change, that it is accompanied by the entry of fluid into the cells, and that it is not caused by simple osmosis.5. The mechanism of vacuolation and its possible relation to secretion are discussed. It is suggested that vacuolation represents an imbalance between the ingestion and secretion of water and salts.6. Similar vacuoles, apparently produced by the same mechanism, were observed in the acinar cells of the parotid gland and the pancreas of the rat.7. The close similarity of this vacuolation to that previously described in rat liver cells was noted.

Hypo-osmotic Re-absorption due to Active Salt Transport in Perfused Collecting Ducts of the Rat Renal Medulla

URINE reaches its maximal osmolality in the collecting ducts of the renal medulla1, but the mechanisms causing the transformation from isotonic tubular fluid are in dispute. Most modern attempts to explain the concentrating mechanism are based on the hypothesis of Wirz, Hargitay and Kuhn2, that the loops of Henle function as countercurrent multipliers to raise the medullary interstitial osmolality so that water can be extracted from the collecting duct urine by simple osmosis. Experimental evidence3,4 supports the validity of this hypothesis only in the outer medulla; the major increase of osmolality occurs in the inner medulla where the loops of Henle are not capable of active salt transport5 and so cannot be countercurrent multipliers. In this communication I shall propose an alternative hypothesis for the inner medulla and present the results of experimental tests of its more unusual features.

Personnel Selection in the Medical Sciences

<h3>To the Editor:—</h3> The process of selection and appointment of individuals to important professional positions in the United States is difficult to describe and even more difficult to defend. The usual mechanism is the designation by a dean and advisors of a "search" committee, which arrive at a slate of names on the basis of discussions among themselves and contacts with their friends or persons that presumably know the personnel market in the field. Into this pot are often thrown names of individuals, backed by influential administrative or faculty members, who are known to be interested through informal conversations or, perhaps, simple osmosis. It is seldom that the availability of the post becomes generally known. This occurs often as an open secret after the offer has been turned down by some candidates, contacted after the ceremony of obtaining permission from the employer in order to avoid the criticism of personnel