Mass transport in biological systems

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. 

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. 

There exist a large number of phenomenological laws for example, Fick s law relates to the flow of a substance and its concentration gradient, and the mass action law explores the reaction rate and chemical concentrations or affinities. When two or more of these phenomena occur simultaneously in a system, they may couple and induce new effects, such as facilitated and active transport in biological systems. In active transport, a substrate can flow against the direction imposed by its thermodynamic force. Without the coupling, such uphill transport would be in violation of the second law of thermodynamics. Therefore, dissipation due to either diffusion or chemical reaction can be negative only if these two processes couple and produce a positive total entropy production. 

Mass transfer Irreversible and spontaneous transport of mass of a chemical component in a space with a non-homogeneous field of the chemical potential of the component. The driving force causing the transport can be the difference in concentration (in liquids) or partial pressures ( in gases) of the component. In biological systems. 

The specific methods discussed in this chapter are representative of those which have found use in the study of mass transport in systems of pharmaceutical interest. Many of these methods represent updated versions of methods taken from the biological, chemical, and engineering sciences and adapted to the study... 

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...

Our main concern here is to present the mass transfer enhancement in several rate-controlled separation processes and how they are affected by the flow instabilities. These processes include membrane processes of reverse osmosis, ultra/microfiltration, gas permeation, and chromatography. In the following section, the different types of flow instabilities are classified and discussed. The axial dispersion in curved tubes is also discussed to understand the dispersion in the biological systems and radial mass transport in the chromatographic columns. Several experimental and theoretical studies have been reported on dispersion of solute in curved and coiled tubes under various laminar Newtonian and non-Newtonian flow conditions. The prior literature on dispersion in the laminar flow of Newtonian and non-Newtonian fluids through... 

Mass transport phenomena in biological systems can be investigated with SECM if the species of interest can be detected either by an potenti-ometric or amperometric microelectrode. Theory and selected studies of localized mass transport are covered in Chapter 9 of this volume. SECM is particularly appropriate in these studies in view of the intimate connection of the imaging mechanism to mass transport effects. The investigation of oxygen and ion transport in various tissues and under a variety of driving forces (concentration gradient, electric field, convection) has been demon-... 

Accurate model prediction, no matter how practical it is, can be obtained only by understanding the underlying science. In this case, the prediction of mass transport in the subsurface can only be accurate if we understand the geochemical reactions that occur in the aquifer. This requires that we understand the geochemical properties of the aquifer, have the thermodynamic and kinetic properties of the chemical system in hand, and understand the interplay among chemical, physical, and biological processes. 

Facilitated transport involves the coupling of a reversible chemical reaction to the mass transfer through the polymer. This type of transport is very common in biological systems, and is capable of substantially increasing the flux and selectivity of synthetic polymers. Facilitated transport still proceeds by the solution-diffusion... 

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