Transport in biological membranes

The biological membrane is a barrier for the motion of molecules. Hydrophilic molecules are obstructed by the core of the Upid bilayer but their mobility at the polar peripheral regions of the manhrane may be enhanced. Conversely, crossing the polar peripheries by hydrophobic substances is strongly impeded so that these substances may be trapped in the hydrophobic core, where they have considerable freedom to move laterally. 

the bilayer forms an obstacle for both hydrophilic and hydrophobic molecules. However, to allow for life processes to occur various compounds have to be transported in and ont the cells and snbceUnlar organelles and therefore they have to traverse the membrane. This transport is rarely, if ever, governed by the lipid bilayer properties. Instead, special regnlatory mechanisms control manbrane permeation. Fignre 19.4 iUnstrates different types of transport throngh a membrane. 

FIGURE 19.4 Types of ion transport across a biomembrane. (a) Diffusion without transport mediator carrier-mediated transport using (b) the traveling or (c) the hopping mode (d) transport through a transmembrane channel. 

Whatever the transport mechanism is, it should be realized that biological membranes are dynamic structures that respond to ambient changes. Thus, a membrane often responds to solute-manbrane interactions through a feedback mechanism. 

With respect to the driving force for m brane permeation, we can distinguish betweeu active aud passive trausport. Transport of a component i is called passive if, at constant temperature and pressure, its flux is driven by a gradient of the (elec-tro)chanical potential d(p, -Hz,/ )/dx 0. The flux of i is toward the region where its (electro)chanical potential has a less positive (or more negative) value. Active 

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The coupling between chemical reactions and transport in biological membranes, such as the sodium and potassium pumps, is known as active transport, in which the metabolic reactions cause the transport of substances against the direction imposed by their thermodynamic force of mainly electrochemical potential gradients. 

Impedance spectroscopy (IS) is a measurement of the conductive and dielectric properties of electroactive systems over a wide range of frequencies. Its popularity and applicability has increased dramatically over the past 25 years with the advent of fast-response potentiostats and frequency response analyzers. Impedance spectroscopy has been applied extensively in electrochemistry, especially in battery and sensor research, and it has been used to study active transport in biological membranes. Skin impedance has also been investigated with IS, but many of these studies attempted to correlate impedance with hydration and provided no insight into the mechanism of charge transport. More recent studies have used IS to elucidate the pathways of ion transport through skin, with special emphasis on understanding the mechanism... 

Zimniak, P., S. Pikula, J. Bandorowicz-Pikula and Y.C. Awasthi. Mechanisms for xenobiotic transport in biological membranes. Toxicol. Lett. 106 107—118, 1999. 

Water Transport in Biological Membranes Benga, G., Ed. CRC Press Boca Raton, FL, 1989 p 41. 

Electron transfer during photosynthesis, according to the tunnel mechanism, was examined in detail in Refs. 116-121. The most interesting of all for the modeling of electron transport in biological membranes is the case when the redox reaction at the membrane/electrolyte interface leads to ion permeability, and not only to electron permeability. Let us assume that ion B is insoluble in the membrane, and, therefore the membrane is impermeable to it. However, if at the interface the ion undergoes redox transformations ... 

Hypothesis on the Mechanism of Proton Transport in Biological Membranes... 

Solute transfer kinetics across the interface between two immiscible liquids has relevance in several areas of physical chemistry, chemical engineering and biology. Examples include ion extraction at free [1] or polarized interfaces [2, 3] and ion transport in biological membranes [4, 5, 6],... 

The distribution of proton concentration Ch+ and potential in solution is governed by the Poisson-Nernst-Planck (PNP) model, widely used in the theory of ion transport in biological membranes (Coalson and Kurnikova, 2007 Keener and Sneyd, 1998). Oxygen diffusion is determined by Pick s law. Inside the pore, the continuity and transport equations for protons and oxygen are... 

Miller, I. R. Structural and energetic aspects of charge transport in lipid layers and in biological membranes, in Topics in Bioelectrochemistry and Bioenergetics, Vol. 4 (ed.) Milazzo, G., New York, Wiley 1981... 

Urry, D. W. Chemical Basis of Ion Transport Specificity in Biological Membranes. 128, 175-218... 

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