Unit membrane model

Figure 2. The unit membrane model (after Robinson). Extended protein layers (CD, m) cover phospholipid bilayers.

If ffcf is substituted for a, it depends on the membrane model how c( is expressed. For a pore model, cs is the concentration of ion i in gequiv. per liter imbibed solution. For a homogeneous gel, ct is the number of gequiv. of the ion i per unit of volume of the swollen resin. In the activity coefficient must then be incorporated the interaction energy with the charged resin skeleton. 

In the classical model, as well as its more contemporary variant, the Robertson unit membrane hypothesis, membranes are seen to consist of a blmolecular leaflet of phospholipid sandwiched between two layers of globular protein. The non-polar fatty acid chains of the phospholipid are oriented Inward, perpendicular to the plane of the membrane system. The polar groups of the phospholipid at the external surface of the blmolecular leaflet are presumed to bond electrostatically to the protein, affording... 

Hence, the fluid mosaic model proposes a pattern of organisation which is consistent with the demonstrable physical and biological properties of plasma membranes and with their known chemistry. A major advantage of this model is that it can describe many different membranes and so reinforces the concept of the unit membrane. 

The figure demonstrates that the assumed simple temperature dependence correctly describes that A at unit activity decreases in smaller intervals as the temperature increases and that this effect is more pronounced at higher activities. Because a simple temperature dependence is used, a minimum in the value of A with respect to temperature is not observed. However, since the upswing is relatively small, neglecting the minimum should not be a significant source of error. The above way of treating temperature effects is consistent with the overall approach of our membrane model and is more... 

The goal of the model for membrane unit for gas separation is to predict the flow rate and composition of retentate and permeate streams, for a given feed stream containing n components, membrane type and area, and permeate pressure. Here, the process boundary and variables are limited to one of the membrane modules shown in Figure 4.5. In this section, the solution-diffusion mechanism is used to predict the separation behavior of the membrane. In the development of a membrane model, it is assumed that the process is at steady state, pressure is constant on feed side, and permeability of a component through the... 

Davis, R.A. (2002) Simple gas permeation and pervaporation membrane unit operation models for process simulators. Chemical Engineering Technology, 25 (7), 717-722. 

Of most note, spherules made from proteinoid and lecithin have been recently shown to possess electrical membrane phenomena. Membrane, action, and oscillatory potentials were next recorded in the microspheres made from proteinoid only. While many of the functions of the model protocells are much weaker than the corresponding functions in modern cells, e.g., the catalytic functions, the electrical behavior is quantitatively comparable. Consequently the units are models of modern excitable cells, as well as of excitable protocells, and are treated mainly as such in this chapter. 

The permeation of wettable liquids through SPG membrane can be analyzed using the nonuniform cylindrical capillary model. Consider a piece of SPG membrane (tube or disc) with a cross-sectional area of Am, a thickness of Sm, and a dry weight of nim containing nonuniform cylindrical pores. The number of pores per unit membrane weight in the fth range of pore sizes, the mean diameter of... 

The membrane model is able to describe the mass transfer through membranes and takes into account the specific effects of different membrane materials. Simulation studies with the non-equilibrium model for distillation and the semi-empirical membrane model illustrate the influence of the mass flow of the side stream and the heating energy on the required membrane area. Both parameters have a major effect on the membrane area. Rigorous models for both unit operations are necessary to perform detailed process studies of the integrated process, because all physical effects have to be taken into account especially for membrane separation. 

L. Maeler and A. Graeslund, Artificial Membrane Models for the Study of Macromolecular Delivery , in Methods in Molecular Biology (Totowa, NJ, United States), ed. M. Belting, 2009, vol. 480, Macromolecular Drug Delivery, p. 129. 

Fig. 5. Diagrammatic portrayal of the unit membrane in the lysosome-vacuolar system, freely derived from membrane models of Lucy, Glauert, and Bangham. The membrane is envisaged as being in dynamic resonance between globular and bimolecular phases. Mosaicism is interposed by way of protein molecules with enzymic or antigenic determinant roles. No representation of the attactor or repulsive field forces is made.

The unit membrane, the structure of the cell and its organelles is based, to a large extent, on the existence of membrane systems. Consider the plasmalemma, the tonoplasts, the nuclear membrane, the membrane system of plastids and mitochondria, the dictyosomes and the endoplasmic reticulum. It is assumed that these membrane structures might all be based on a common structural principle, namely, the so-called unit membrane. There are several models of the molecular structure of the unit membrane, one of which is shown in Fig. 81 two protein layers enclose two lipoid layers. The lipoids are so oriented that their polar ends are directed towards the protein and their lipophilic ends towards the other lipoid layer. 

Fig. 81. Construction of a model of the unit membrane (from Lehninger 1969).

Robertson s unit membrane (Robertson, 1960, 1964, 1967) differs from the Danielli model from which it is derived by specifying a narrow range, 70-80 A, for the membrane thickness. Legitimate objections have been raised against this unduly restrictive specification since the width of membranes, as measured by X-ray diffraction, can reach 120 A (Finean et ah, 1966a). 

There are two main issues in an ultrafiltration process productivity and selectivity. Productivity is quantified in terms of the permeate flux, this being defined as the permeation rate per unit membrane surface area. Factors that affect permeate flux are solute type, solute concentration, membrane type, solution pH, solution ionic strength, apphed pressure (also called the transmembrane pressure), and the hydrodynamic conditions on the feed side. The volumetric permeate flux, which is the volume of permeate collected per unit time per unit membrane area is given by the following generalized equation, based on a resistance model ... 

On the basis of electron microscopic studies, Robertson (1967) has modified the Danielli-Davson model, presenting a new model—the unit membrane hypothesis—for all cell membranes. This model was similar to that of Danielli-Davson but with a few significant changes. The number of bimolecular phospholipid layers in Robertson s model was restricted to one, and the membrane was considered to be asymmetrical, with mucopolysaccharide or mucoprotein on the outside and unconjugated protein on the inner part of the membrane (Fig. 2). The... 

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