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), π achieved , π applied , and π TM (TM refers to transmembrane) are the three common ways osmotic pressure ( π ) is defined, where π TM 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 ( Δ π TM ) across the m. In this presentation, we apply the concepts of a composite ECF‐m‐ICF and the Δ π TM 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‐H 2 O and H 2 O‐m‐ICF) to understand the Δ π TM . 2) Graphical (logical) analysis of the three pressures in plant and animal ECF‐m‐ICF (Fig 2). Results 1) As shown in Fig 1, Δ π TM = π TM (ECF‐m‐H 2 O) ‐ π TM (H 2 O‐m‐ICF). 2) After water enters a cell and reaches equilibrium of osmosis, Δ π TM is reduced to Δ π eq (Fig 2). 3) In a plant ECF‐m‐ICF (Fig 2a and 2b), large Δ π eq (close to Δ π TM ), high hydrostatic pressure ( ΔP H2O , thick grey arrows in Fig 2b) inside the cell, and high tension in the call wall ( ΔT wall , 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 Δ π eq , lower P H2O (the thin grey arrows, Fig 2d), and lesser tension in the cell membrane ( ΔT CM , 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 Δ π TM .