Passive transport, biological membranes

Diffusion is one means of passive transport across membranes. Let s now suppose that a biological membrane is present and that a substance is found at one concentration outside the membrane and at half that concentration inside the membrane. If the solute can pass through the membrane, diffusion will occur with net transport of material from the region of initial high concentration to the region of initial low concentration, and the substance will equilibrate across the cell membrane (Figure 18.16). After a while, the concentration of the substance will be the same on both sides of the membrane the system will be at equilibrium, and no more net change will occur. 

Eischer, H. Passive diffusion and active transport through biological membranes - binding of drugs to... 

Substances pass through membranes primarily by passive diffusion. In addition, in biological membranes, substances may penetrate through specific transport mechanisms. 

In addition to the passive diffusional processes over lipid membranes or between cells, substances can be transferred through the lipid phase of biological membranes through specialized systems, i.e., active transport and facilitated diffusion. Until recently, the active transport component has been discussed only for nutrients or endogenous substances (e.g., amino acids, sugars, bile acids, small peptides), and seemed not to play any major role in the absorption of pharmaceuticals. However, sufficient evidence has now been gathered to recognize the involvement of transporters in the disposition of pharmaceuticals in the body [50, 127]. 

Transport of an organic pollutant by passive diffusion across a biological membrane (kmt = Is-1) 1... 

A parameter (usually symbolized by P, and often containing a subscript to indicate the specific ion) that is a measure of the ease with which an ion can cross a unit area of membrane by simple (or passive) diffusion through a membrane experiencing a 1.0 M concentration gradient. For a particular biological membrane, the permeabilities are dependent on the concentration and activity of various channel or transporter proteins. In an electrically active cell (e.g., a neuron), increasing the permeability of K+ or CF will usually result in hyperpolarization of the membrane. Increasing will cause depolarization. 

Membranes play essential roies in the functions of both prokaryotic and eukaryotic cells. There is no unicellular or multicellular form of life that does not depend on one or more functional membranes. A number of viruses, the enveloped viruses, also have membranes. Cellular membranes are either known or suspected to be involved in numerous cellular functions, including the maintenance of permeability barriers, transmembrane potentials, active as well as specific passive transport across the membranes, hornione-receptor and transmitter-receptor responses, mitogenesis, and cell-cell recognition. The amount of descriptive material that might be included under the title of biological membranes is encyclopedic. The amount of material that relates or seeks to relate structure and function is less, but still large. For introductory references see Refs. 53, 38, 12, 47, 34, 13. Any survey of this field in the space and time available here is clearly out of the question. For the purposes of the present paper we have selected a rather narrow, specific topic, namely, the lateral diffusion of molecules in the plane of biological mem-branes.38,12,43,34 We consider this topic from the points of view of physical chemistry and immunochemistry. 

It should be mentioned here that, in living systems the transport of mass sometimes takes place apparently against the concentration gradient. Such uphill mass transport, which usually occurs in biological membranes with the consumption of biochemical energy, is called active transport, and should be distinguished from passive transport, which is the ordinary downhill mass transport as discussed in this chapter. Active transport in biological systems is beyond the scope of this book. 

Movement of polar compounds and ions across biological membranes requires protein transporters. Some transporters simply facilitate passive diffusion across the membrane from the side with higher concentration to the side with lower. Others bring about active movement of solutes against an electrochemical gradient such transport must be coupled to a source of metabolic energy. 

In order to be able to distinguish between active and passive transport through biological membranes, P. Meares and H. H. Ussing (95) likewise made a study of the fluxes through a membrane under the influence of diffusion together with an electric current. They studied the influxes and the outfluxes of sodium- and chloride ions at a cation exchange resin membrane. They started from the Nemst-Planck flux equations of the type ... 

Meares and his collaborators are especially interested in transport processes across biological membranes. They wish to distinguish experimentally between the active and the passive transport of a solute. For that purpose they determined the fluxes of the sodium ions in each direction through the membrane, using the technique of radio-tracers. The ratio of these experimental fluxes was compared with the theoretical ratios. The same is done with regard to the chlorine ions. 

Drugs and other substances that pass through biologic membranes usually do so via passive diffusion, active transport, facilitated diffusion, or some special process such as endocytosis (Fig. 2-2). Each of these mechanisms is discussed here. 

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