PLANT BIOPHYSICS - Y. I. Posudin - 2004

I. PHYSICAL PROPERTIES OF PLANT CELLS AND TISSUES

2. MOLECULAR PROCESSES IN CELLS

2.3. OSMOSIS

Osmosis is the diffusion of a substance (usually a solvent) through a semipermeable membrane separating a solution from a pure solvent. The movement of solvent molecules is driven by osmotic pressure. Equalization of concentrations on both sides of a membrane—which allows small solvent molecules to pass while blocking larger solute molecules—is possible only through unidirectional solvent diffusion. For this reason, osmosis always proceeds in the direction from the pure solvent to the solution (or from a dilute solution to a concentrated one).

If a solution separated from the external environment by a membrane is immersed in a pure solvent (Fig. 2.3) under equal hydrostatic pressure, a net diffusion of the solvent into the solution will occur. Diffusion stops once a specific osmotic pressure is reached within the solution. Osmotic pressure can be measured; it is equal to the excess pressure that must be applied to the solution side to halt osmosis. Biological fluids are dilute aqueous solutions in which Water accounts for up to 95% of the volume; consequently, osmotic fluxes across Introduction/36.html">Biological Membranes induce water flows from regions of high water concentration (dilute solutions) to regions of low water concentration (more concentrated solutions). Plant Cell vacuoles contain salt solutions, sugars, organic acids, and Amino Acids. As a result, Cells constantly absorb water osmotically and generate an internal hydrostatic pressure known as turgor pressure. This pressure exerts force against The Cell wall, making it rigid. Simultaneously, The Cell wall resists the osmotic pressure; this action of the cell wall on the Cytoplasm is characterized by the osmotic potential. When dynamic equilibrium is established, the influx of water into the cell ceases. The osmotic concentration of the vacuolar sap in ROOT cells ranges from 0.3 to 1.2 MPa, whereas in the Cells of the aerial PARTS OF THE plant it reaches 1.0 to 2.6 MPa [Musienko, 2001]. It is precisely thanks to this vertical gradient of osmotic concentration that sap reaches the top of the plant.

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Fig. 2.3. Net diffusion of a solvent into a solution through a semipermeable membrane: a - initial state; b - final state; 1 - solution; 2 - membrane; 3 - water; Ар - excess pressure.

Thus, osmotic pressure is the pressure exerted by a solution on a semipermeable membrane that separates it from a solvent or from a solution of lower concentration. Osmosis and diffusion share both Similarities and differences. Both processes occur spontaneously, moving from higher to lower concentrations due to the random thermal motion of molecules, and are Temperature-dependent. At the same time, osmosis differs in that it involves a displacement of substance volume, whereas diffusion takes place within the same volume.

Osmotic pressure is determined by an expression known as the van 't Hoff equation:

where R is the universal gas constant (R = 8.31 J·mol^-1K^-1), T is the absolute temperature, and С_А is the molar concentration of the solute.

For example, if the molar concentration of the solute is 450 mol·m³ at a temperature of 300 K, the osmotic pressure According to the van 't Hoff equation is p_osm — 1.12 MPa, which exceeds atmospheric pressure by nearly an order of magnitude.

Thus, osmotic pressure in plants is one of the most critical Factors influencing the distribution of water and solutes within plant Tissues.

Control Questions

1. Name the Functions of biological membranes.

2. What is meant by The Lipid Bilayer?

3. Explain The Essence of the fluid-mosaic model of the membrane.

4. What is diffusion?

5. What factors determine the diffusion coefficient?

6. What driving force causes the diffusion of molecules into or out of a cell?

7. What determines the membrane permeability coefficient?

8. What is osmosis?

9. Write down the van 't Hoff equation.