Membrane concepts, uses

The concepts of conformation and dynamics developed in Part I—especially those dealing with the specificity and catalytic power of enzymes, the regulation of their catalytic activity, and the transport of molecules and ions across membranes—enable us to now ask questions fundamental to biochemistry ... 

Common types of membrane materials used are listed in Table 3. This gets us into the concept of geometry. There are three types of modules generally used, namely Tubular, Spiral wound, and Hollow fiber. A comparison of the various geometries is given in Table 4. 

The concept presented in Fig. 6 could use also other type of ordered mesoporous membranes, based on silica for example. As discussed before, oxides such as Ti02 provide better multi-functionalities for the design of such a type of nanofactory catalysts. Worth to note is that in the cover picture of the recent US DoE report Catalysis for Energy a very similar concept was reported. This cover picture illustrates the concept, in part speculative, that to selectively convert biomass-derived molecules to fuels and chemicals, it is necessary to insert a tailored sequence of enzyme, metal complexes on metal nanoparticles in a channel of a mesoporous oxide. 

We can say, following these experiments, that the structural permeability loses its physical meaning. In fact, such nonlinearities between fluxes and forces and permeability variations establish the very existence of a chemical effect, leading us to the concept of the dynamical membrane permeability. 

In the very interesting report by Dr. Thomas, the diffusion aspect of chemicals through artificial membranes has been beautifully analyzed. However, I have some hesitation if one attempts to extend a concept proved for macroscopic (bulk) phenomena to microscopic (surface) ones. Let us start with the simple consideration that a biological membrane to a... 

Imagine a container with a solution that is separated from its pure solvent by a membrane that allows the solvent to pass through, but not the solute. The membrane that allows specific things through, but not others, is a semipermeable membrane. Since substances diffuse (tend to move from higher concentration to lower concentration), the solvent will diffuse through the semipermeable membrane into the solution (osmosis). Let us carry the concept one step further. 

Owing to the complementary roles of NEP and APN in enkephalin inactivation, selective inhibitor of only one of the two peptidases gives weak antinoceptive effects even after ICV administration. This led us to propose the concept of mixed inhibitors, that is, compounds able to simultaneously block NEP and APN activities [reviews in 9,20]. This was possible owing to the fact that these two membrane-bound enzymes belong to the superfamily of zinc metallopeptidases. 

The relationship between the structure of the disordered heterogeneous material (e.g., composite and porous media) and the effective physical properties (e.g., elastic moduli, thermal expansion coefficient, and failure characteristics) can also be addressed by the concept of the reconstructed porous/multiphase media (Torquato, 2000). For example, it is of great practical interest to understand how spatial variability in the microstructure of composites affects the failure characteristics of heterogeneous materials. The determination of the deformation under the stress of the porous material is important in porous packing of beds, mechanical properties of membranes (where the pressure applied in membrane separations is often large), mechanical properties of foams and gels, etc. Let us restrict our discussion to equilibrium mechanical properties in static deformations, e.g., effective Young s modulus and Poisson s ratio. The calculation of the impact resistance and other dynamic mechanical properties can be addressed by discrete element models (Thornton et al., 1999, 2004). 

The main problem with this concept is one of solubility. The site of attack is the double-bonded part of the lipids, embedded in the centre of the membrane. Not only are Fe(II) complexes unlikely to be present, but also H202 is unlikely to be present in the inner region of the phospholipid double layer. Thus, the probability of there being sufficient of each to initiate autoxidation seems to us to be very low indeed. Nevertheless, catalysis by metal ions does occur. Possibly, membrane proteins are involved in some way. 

In order to discuss the concept of solvent drag in quantitative terms let us consider a membrane barrier separating two dilute solutions of uncharged solute S. 

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