The Concept of Osmotic Pressure: Two Common Misunderstandings and Resolutions

Abstract

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.