Diffusivity in biological solutions

Diffusion in Biological Fluids. Recall from Section 4.3.3.1 that polymer molecnles in dilute solutions can be characterized by a friction coefficient, f, which describes the resistance to motion throngh the solvent, and that the diffnsivity of these molecnles can be related to the friction coefficient by... 

Ushida, K. (2008) Anomalous diffusion in polymer solution as probed by fluorescence correlation spectroscopy and its universal importance in biological systems. A1P Conference Proceedings, 982, pp. 464-469. 

Experimental methods to determine diffusivity. Methods to determine the diffusivity of biological solutes are similar to those discussed previously in Section 6.3 with some modifications. In the diaphragm diffusion cell shown in Fig. 6.3-1, the chamber is made of Lucite or Teflon instead of glass, since protein molecules bind to glass. Also, the porous membrane through which the molecular diffusion occurs is composed of cellulose acetate or other polymers (G5, G6, Kl). 

Prediction of diffusivities for biological solutes. For predicting the diffusivity of small solutes alone in aqueous solution with molecular weights less than about 1000 or solute molar volumes less than about 0.500 m /kg mol, Eq. (6.3-9) should be used. For larger solutes the equations to be used are not as accurate. As an approximation the Stokes-Einstein equation (6.3-8) can be used. 

Diffusion in biological stem is, however, mostly associated with chemical reactions. In such situations the simplest assumption that can be made regairding the diffusing stem is that within the diffusing space the rate of utilization of the diffusing solute is constant. l.e. independent of time and place. Equation (5) in such conditions will take the form... 

This measurement of tracer diffusion in dilute solution is a good strategy. Such a use of radioactive tracers provides a near-unique opportunity for a specific chemical analysis in highly dilute solution. Such analysis is especially important in biological systems, where complex chemistry may compromise analysis. Moreover, in dilute solution, the diffusion coefficients found with radioactive tracers are almost always indistinguishable from those measured in other ways. Exceptions occur in those systems in which the solute moves by a jump mechanism like that for protons (see Fig. 6.1.-1) or in which the solute s molecular weight is significantly altered by the isotopic mass. 

Fig. 5. Tentative mixed potential model for the sodium-potassium pump in biological membranes the vertical lines symbolyze the surface of the ATP-ase and at the same time the ordinate of the virtual current-voltage curves on either side resulting in different Evans-diagrams. The scale of the absolute potential difference between the ATP-ase and the solution phase is indicated in the upper left comer of the figure. On each side of the enzyme a mixed potential (= circle) between Na+, K+ and also other ions (i.e. Ca2+ ) is established, resulting in a transmembrane potential of around — 60 mV. This number is not essential it is also possible that this value is established by a passive diffusion of mainly K+-ions out of the cell at a different location. This would mean that the electric field across the cell-membranes is not uniformly distributed.

The field of modified electrodes spans a wide area of novel and promising research. The work dted in this article covers fundamental experimental aspects of electrochemistry such as the rate of electron transfer reactions and charge propagation within threedimensional arrays of redox centers and the distances over which electrons can be transferred in outer sphere redox reactions. Questions of polymer chemistry such as the study of permeability of membranes and the diffusion of ions and neutrals in solvent swollen polymers are accessible by new experimental techniques. There is hope of new solutions of macroscopic as well as microscopic electrochemical phenomena the selective and kinetically facile production of substances at square meters of modified electrodes and the detection of trace levels of substances in wastes or in biological material. Technical applications of electronic devices based on molecular chemistry, even those that mimic biological systems of impulse transmission appear feasible and the construction of organic polymer batteries and color displays is close to industrial use. 

The insertion of proteins into intracellular membranes has incising effects upon the kinetic and thermodynamic properties of the corresponding biological interactions. Although diffusion in membranes is approx. 100-fold slower than in aqueous solution the probability for two molecules to meet... 

Against simplistic views of the FIAM, it is necessary to stress that the model does not imply that the free metal ion is the only species available to the microorganism [2,14], Indeed, the internalisation flux (i.e. the rate of acquisition) depends on the free metal ion concentration at the biological interphase (which in the FIAM is practically cj ), but metal bound to a ligand in the solution can dissociate, can diffuse (under a negligible gradient according to the FIAM), and can eventually be taken up. 

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... 

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