Passive membrane processes

A membrane is defined as an intervening phase separating two phases forming an active or passive barrier to the transport of matter. Membrane processes can be operated as (1) Dead-end filtration and (2) Cross-flow filtration. Dead-end filtration refers to filtration at one end. A problem with these systems is frequent membrane clogging. Cross-flow filtration overcomes the problem of membrane clogging and is widely used in water and wastewater treatment. 

Kansy, M., Senner, F., Gubemator, K. Physicochemical high throughput screening parallel artificial membrane permeation assay in the description of passive absorption processes. /. Med. Chem. 1998, 43, 1007-1010. 

In addition to the passive diffusional processes over lipid membranes or between cells, substances can be transferred through the lipid phase of biological membranes through specialized systems, i.e., active transport and facilitated diffusion. Until recently, the active transport component has been discussed only for nutrients or endogenous substances (e.g., amino acids, sugars, bile acids, small peptides), and seemed not to play any major role in the absorption of pharmaceuticals. However, sufficient evidence has now been gathered to recognize the involvement of transporters in the disposition of pharmaceuticals in the body [50, 127]. 

The use of artificial membranes to investigate passive permeation processes has a long history, going back more than 40 years [68], The parallel artificial membrane permeation assay (PAMPA) is an application of the filter-supported lipid membrane system [149] and was first introduced by Kansy and... 

The membrane is a unit of the process separated from the reactor. It maintains only the separation function (passive membrane) and there is almost no interaction between reaction and separation. In fact, we have here two different processes connected in series. Stream purification from catalyst poisoning substances or feed enrichment of a recycle stream belong to the possibilities of this configuration. 

A membrane reactor offers the possibility of combining two individual processes in the same unit operation. (1) Selective permeation (thus separation) can be coupled directly with the reaction by means of either a catalytically active membrane or of a passive membrane placed next to the... 

Kansy M., Sermer, F. and Gubernator, K. (1998) Physicochemical High Throughput Screening Parallel Artificial Membrane Permeation Assay in the Description of Passive Absorption Processes. Journal of Medicinal Chemistry, 41, 1007-1010. 

Because there are many different ways to combine a catalyst with a membrane, there are numerous possible classifications of the CMRs. However, one of the most useful classifications is based on the role of the membrane in the catalytic process we have a catalytically active membrane if the membrane has itself catalytic properties (the membrane is functionalized with a catalyst inside or on the surface, or the material used to prepare the membrane is intrinsically catalytic) otherwise if the only function of the membrane is a separation process (retention of the catalyst in reactor and/or removal of products and/or dosing of reagents) we have a catalytically passive membrane. The process carried out with the second type of membrane is also known as membrane-assisted catalysis (a complete description of the different CMRs configurations will be presented in a specific chapter). 

Many substances can be transported into the cell (and vice versa) against a concentration gradient. This is an active transport process, and it requires energy in the form of ATR It is to be distinguished from a passive transport process, which is simple diffusion across membranes. One of the better understood systems of this type is the sodium-potassium ATPase (or Na/K) pump, which maintains high potassium and low sodium levels in the cell. These are up to 160 meq/L for K+ and about 10 meq/L of Na+ inside the cell. Extracellular fluid contains about 145 meq/L of Na+ and 4 meq/L of K+. The simultaneous movement of one substance out of the cell and another into the cell is an antiport. A substantial percentage of the basal metabolic rate (see Chapter 21) is accounted for by the activity of the Na/K pump. ATPase (Na/K pump) is lo-... 

For a passive transport process, the concentration gradient across the membrane can be considered constant and therefore the gradient can be approximated by differences as in (2.13) to obtain... 

Sometimes it is useful to apply a combination of the descriptors based on the global properties of the molecule and those based on a pharmacophoric representation. Conceptually, the global properties would better describe the imtial passive membrane permeation required to reach the site of action. Then, the specific protein interactions could be explained by the pharmacophoric descriptors. This has been demonstrated successfully in the P-glycoprotein case [26], where two processes are important for the transport passive transport to the cell and active... 

Generally speaking, two principal mechanisms operate in the biology of membrane processes, such as membrane transport and permeation. One involves a network of active sites and operates by metabolic energy, and it is referred to as active another is directed by passive diffusion, and it is called passive. This passive mechanism is determined by various aspects of lipid dynamics and lipid-protein interactions, and it can be described in quantitative terms of chemical and phase equilibrium and molecular physics. However, in the highly anisotropic... 

Fig. 3.9 Passive Transportation Processes Across the Cell Membrane...

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