Osmotic pressure biological membranes

The osmotic pressure is a property that has proven to be especially valuable in the study of solutions of macromolecules, including those of biologic and polymeric interest. The apparatus for measuring this quantity is shown schematically in Figure 7.10. Two compartments are separated by a membrane that will allow the flow of liquid solvent between the two chambers. If solvent is added, flow will occur until the liquid level on the two sides of the membrane is the same. 

Osmotic pressure plays an important role in biological chemistry because the cells of the human body are encased in semipermeable membranes and bathed in body fluids. Under normal physiological conditions, the body fluid outside the cells has the same total solute molarity as the fluid inside the cells, and there is no net osmosis across cell membranes. Solutions with the same solute molarity are called isotonic solutions. 

A semi-permeable membrane, which is unequally permeable to different components and thus may show a potential difference across the membrane. In case (1), a diffusion potential occurs only if there is a difference in mobility between cation and anion. In case (2), we have to deal with the biologically important Donnan equilibrium e.g., a cell membrane may be permeable to small inorganic ions but impermeable to ions derived from high-molecular-weight proteins, so that across the membrane an osmotic pressure occurs in addition to a Donnan potential. The values concerned can be approximately calculated from the equations derived by Donnan35. In case (3), an intermediate situation, there is a combined effect of diffusion and the Donnan potential, so that its calculation becomes uncertain. 

It is now universally accepted that a-LTX can insert into, andpermeabilize, artificial and biological membranes. Cation currents can explain some, but not all, of the toxin s effects. For example, it is not clear how the a-LTX channel could mediate Ca2+ -independent exocytosis in neurons. Although it would be tempting to assign some of a-LTX actions in the absence of Ca2+ to Na+ currents, lack of Na+ does not prevent Ca2+-independent secretion (Tsang et al. 2000). Cation flux-associated incursion of terminals by water could be involved, but a-LTX effect on intracellular osmotic pressure has not been characterized yet. 

For biological particles, for example, cells which have a semi-permeable membrane and semi-permeable microcapsules, their mechanical integrity can be characterised by exposuring them to media with different osmotic pressures (Van Raamsdonk and Chang, 2001). 

Osmotic pressure is another property due to dissolved substances. The presence of solute particles lowers the ability of solvent molecules to pass through a semipermeable membrane. Osmotic pressure is very important in biological systems, and an application of the theory behind osmotic pressure allows for the purification of seawater. The osmotic pressure of a solution, IT, is proportional to the molarity (the number of moles per liter) ... 

Osmosis, as you should have learned in high school biology, is the ability of a solvent (usually water) to pass from a dilute solution (or pure solvent) on one side of a membrane to a more concentrated solution on the other side (Figure 12-3). The osmotic pressure is the pressure that must be applied to the solution to prevent this occurring. 

Osmotic pressure becomes important from a physiological standpoint because a majority of biological membranes are semipermeable, and body fluids such as blood and tears exhibit significant osmotic pressure owing to a number of solutes dissolved in them. As noted above in the Introduction, if a small quantity of blood is mixed with a solution containing 0.9% w/v NaCl, the red blood cells remain intact and retain their normal size and shape. The NaCl solution is considered to be isotonic because it maintained the tone of the membrane of the red blood cell. In contrast, if the blood is mixed with the hypertonic 1.8% w/v NaCl solution, cells shrink and become wrinkled or crenated owing to its content being sucked out. It is because the red blood cell content exerts a lower osmotic pressure... 

Osmometry is a technique for measuring the concentration of solute particles that contribute to the osmotic pressure of a solution. Osmotic pressure governs the movement of solvent (water in biological systems) across membranes that separate two solutions. Different membranes vary in pore size and thus in their ability to select molecules of different size and shape. Examples of biologically important selective membranes are those enclosing the glomerular and capillary vessels that are permeable to water and to essentially aU small molecules and ions, but not to large protein molecules. Differences in the concentrations of osmoticaUy active molecules that carmot cross a membrane cause those molecules that can cross the membrane to move to establish an osmotic equilibrium. This movement of solute and permeable ions exerts what is known as osmotic pressure. 

Water is the ideal biological solvent. It easily dissolves a wide variety of the constituents of living organisms. Examples include ions (e.g., Na+, K+, and CF), sugars, and many of the amino acids. Its inability to dissolve other substances, such as lipids and certain other amino acids, makes supramolecular structures (e.g., membranes) and numerous biochemical processes (e.g., protein folding) possible. In this section the behavior of hydrophilic and hydrophobic substances in water is described. This discussion is followed by a brief review of osmotic pressure, one of the colligative properties of water. Colligative properties are physical properties that are affected not by the specific structure of dissolved solutes, but rather by their numbers. 

The osmosis phenomenon, stemming from biological systems with biological semipermeable membrane, initially represents a nature net transport of solvent molecules from a region of higher water chemical potential (e.g., dilute solution) to a region of lower water chemical potential (e.g., concentrate solution). The driving force is the pure chemical potential difference, i.e., osmotic pressure difference, across the membrane. 

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