Biological membranes semipermeable

The discovery of galvanic electricity (i.e. electrical phenomena connected with the passage of electric current) by L. Galvani in 1786 occurred simultaneously with his study of a bioelectrochemical phenomenon which was the response of excitable tissue to an electric impulse. E. du Bois-Reymond found in 1849 that such electrical phenomena occur at the surface of the tissue, but it was not until almost half a century later that W. Ostwald demonstrated that the site of these processes are electrochemical semipermeable membranes. In the next decade, research on semipermeable membranes progressed in two directions—in the search for models of biological membranes and in the study of actual biological membranes. 

Osmotic effects are very important from a physiological standpoint. This is because biological membranes including the membrane of red blood cells behave like semipermeable membranes. Consequently, when red blood cells are immersed in a hypertonic solution (e.g., D5 A NS or D5NS), they shrink as water leaves the blood cells in an attempt to dilute and establish a concentration equilibrium across the blood cell membrane. Thus, when hypertonic solutions are administered into the blood stream, the fluid moves from interstitial and cellular space into the intravascular space. Conversely, when cells are placed in hypotonic environment (e.g., V2 NS), they swell because of the entry of fluid from the intravascular compartment, and may eventually undergo lysis. 

Such materials are known as semipermeable membranes. They are essential components of nearly all living things, and the development of new materials of this type is an important component of biomedical research. The control of diffusion of molecules through a membrane can be accomplished by variations in the hydrophilicity of the polymer molecules that constitute the membrane. As in biological membranes, hydrophobic molecules are more likely to pass through the hydrophobic domains of a synthetic membrane than through the hydrophilic regions, and vice versa. 

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... 

Most biological membranes are semipermeable, more permeable to water than to ions or most other solutes. Water moves by osmosis across membranes from a solution of lower solute concentration to one of higher solute concentration. 

Biological membranes are bilayers and contain several types of lipids some more often associated with the outside face of the cell, and others face the inside. Biological membranes also contain a large number of protein components. Membranes are semipermeable, naturally excluding hydrophilic compounds (carbohydrates, proteins, and ions, for example) while allowing oxygen, proteins, and water to pass freely. 

The search for models of biological membranes started at the end of the 19th century. This general interest in membranes was expressed by the famous German physico-chemist Wilhelm Ostwald who wrote Not only mysterious phenomena of electric fish but also processes occurring in muscles and nerves will be, in the future, explained by semipermeable membranes [1]. 

Semipermeable Membrane. A semipermeable membrane is a boundary that restricts the flow of some kinds of particle, while allowing others to cross. Dialysis is performed with semipermeable membranes. Biological membranes are semipermeable. 

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. 

In contrast to mechanics, where the term membrane (Lat. membrana = parchment) designates an elastic, two-dimensional plate, this term is used in chemistry, biophysics and biology to designate a solid or liquid phase (usually, but not always, with a thickness substantially smaller than its other dimensions) separating two, usually liquid, phases. The transport (permeation) of the various components of both phases through the membrane occurs at different rates relative to those in the homogeneous phases with which the membrane is in contact. The membrane is consequently called semipermeable. 

A semipermeable membrane is placed between two liquids, and the analytes transfer from one liquid to the other. This technique is used for investigating extracellular chemical events as well as for removing large proteins from biological samples prior to HPLC analysis. 

Absorption is necessary for the chemical to exert a systemic biological/toxic effect and involves crossing membranes. Membranes are semipermeable phospholipid/protein bilayers. The phospholipids and proteins are of variable structure, and the membrane is selectively permeable. The physicochemical characteristics of foreign molecules that are important include size/shape, lipid solubility, structure, and charge/polarity. 

You can demonstrate the size of colloidal particles with a dialysis experiment in which two solutions are separated by a semipermeable membrane that has pores with diameters of 1—5 nm.3 Small molecules diffuse through these pores, but large molecules (such as proteins or colloids) cannot. (Collecting biological samples by microdialysis was discussed at the opening of Chapter 25.)... 

Phase equilibrium across semipermeable membranes is of special interest in biological applications. First, we will consider two-phase aqueous systems without chemical reactions, then introduce reactions, and finally electric potential differences between phases. The numbers of intensive degrees of freedom F and... 

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