Diffusion in biological systems

Variability in the properties of agents is not the only difficulty in predicting rates of diffusion. Biological tissues present diverse resistances to molecular diffusion. Resistance to diffusion also depends on architecture tissue composition, structure, and homogeneity are important variables. This chapter explores the variation in diffusion coefficient for molecules of different size and structure in physiological environments. The first section reviews some of the most important methods used to measure diffusion coefficients, while subsequent sections describe experimental measurements in media of increasing complexity water, membranes, cells, and tissues. 

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The process of active ttanspott differs from diffusion in that molecules ate ttanspotted away from thermodynamic equilibrium hence, energy is required. This energy can come from the hydrolysis of ATP, from electron movement, ot from light. The maintenance of electtochemical gtadients in biologic systems is so important that it consumes pethaps 30—40% of the total energy expenditure in a cell. 

Figure 3.98 Comparison of a reversible conventional cyclic voltammogram (linear diffusion) and reversible steady-state voltammogram obtained at a single microelectrode disc where mass transport is solely by radial diffusion. Current axis not drawn to scale. From A.M. Bond and H.A.O. Hill, Metal Inns in Biological Systems, 27 (1991) 431. Reprinted by courtesy of Marcel...

Part A. Quantum-Mechanical Theory of Diffusion Independent Electron Transfer in Biological Systems by Ephraim Buhks (University of Delaware)... 

The design of biocatalytic electrodes for activity toward gaseous substrates, such as dioxygen or hydrogen, requires special consideration. An optimal electrode must balance transport in three different phases, namely, the gaseous phase (the source of substrate), the aqueous phase (where the product water is released and ionic transport takes place), and the solid phase (where electronic transport occurs). Whereas the selectivity of biocatalysts facilitates membraneless cells for implementation in biological systems that provide an ambient electrolyte, gas-diffusion biofuel cells require an electro-... 

As was the case for composites, there is little new in the way of fundamental concepts for mass transport in biologies that has not already been presented. However, it is possible to briefly describe extensions of some previously introduced topics that are of particular importance to biological materials—namely, diffusion of nonspherical molecnles in solution, diffusion throngh biological membranes, and convective mass transfer in biological systems. 

NO is a radical that is water soluble and can cross membranes fairly freely by diffusion. Due to its radical nature, NO has only a short lifetime in aqueous solution of ca. 4 sec. Important reaction partners of NO in biological systems are oxygen O2, the 02 radical... 

As noted earlier, peroxynitrite is formed with a diffusion-controlled rate from superoxide and nitric oxide (Reaction 10). As both these radicals are ubiquitous species, which present practically in all cells and tissues, peroxynitrite can be the most important species responsible for free radical-mediated damage in biological systems. Moreover, it is now known that NO synthases are capable of producing superoxide and nitric oxide simultaneously (see Chapter 22), greatly increasing the possible rate of peroxynitrite production. In addition, another enzyme xanthine dehydrogenase is also able to produce peroxynitrite in the presence of nitrite... 

Stanisz GJ. Diffusion MR in biological systems tissue compartments and exchange. Israel Journal of Chemistry 2003, 43, 33 14. 

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