The Nature and Measures of Osmotic Pressure

Abstract

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.