Membrane components, diffusion

For multi-component systems it seems intuitive that single-component diffusion and adsorption data would enable one to predict which component would be selectively passed through a membrane. This is only the case where molecular sieving is observed for all other separations where the molecules interact with one another and with the zeolite framework their behavior is determined by these interactions. Differences in membrane properties such as quahty, microstructure, composition and modification can also play a large role in the observed separation characteristics. In many cases, these properties can be manipulated in order to tailor a membrane for a specific apphcation or separation. 

Solid membranes without an internal diffusion potential [3,8, 9, 54,56,62, 63] contain a nonporous layer of a poorly soluble salt in contact with a solution that contains either a cation or anion that is also a membrane component. 

Rates of lateral diffusion of membrane components have also been determined using optical methods. The early experiments of Frye and Ediden16 demonstrated lateral motion of fluorescent-labeled surface antigens in heterokaryons of mouse and human cells. They observed intermixing of fluorescent-labeled antibodies against mouse cell and human cell antigens. Optical methods may also be characterized as either transient or steady state. The use of fluorescence correlation spectroscopy as... 

Absorption of cholesterol in the small intestine contributes to maintaining whole-body cholesterol homeostasis, yet the mechanisms of absorption have not been completely defined. For many years it was believed that cholesterol, a normal component of cell membranes, simply diffused through the brush border membrane of enterocytes (Grundy, 1983 Westergaard and Dietschy, 1974). However, the discovery of specific transporters, receptors,... 

Various analytical techniques make use of both porous and nonporous (semipermeable) membranes. For porous membranes, components are separated as a result of a sieving effect (size exclusion), that is, the membrane is permeable to molecules with diameters smaller than the membrane pore diameter. The selectivity of such a membrane is thus dependent on its pore diameter. The operation of nonporous membranes is based on differences in solubility and the diffusion coefficients of individual analytes in the membrane material. A porous membrane impregnated with a liquid or a membrane made of a monolithic material, such as silicone rubber, can be used as nonporous membranes. 

S02) easily reacts with the same membrane components, and need therefore to be removed before the separation unit. The material designer must also consider possible evaporation of membrane components. The high temperature in combination with steam can lead to increased evaporation by metal-hydroxy components. Kinetic demixing seems to be an unavoidable phenomena originating from difference in diffusivity of the metal components in thermodynamic potential gradients... 

Flip-Flop Diffusion The inner leaflet (monolayer) of the human erythrocyte membrane consists predominantly of phosphatidylethanolamine and phosphatidylserine. The outer leaflet consists predominantly of phosphatidylcholine and sphingomyelin. Although the phospholipid components of the membrane can diffuse in the fluid bilayer, this sidedness is preserved at all times. How ... 

Other processes that lead to nonlinear compartmental models are processes dealing with transport of materials across cell membranes that represent the transfers between compartments. The amounts of various metabolites in the extracellular and intracellular spaces separated by membranes may be sufficiently distinct kinetically to act like compartments. It should be mentioned here that Michaelis-Menten kinetics also apply to the transfer of many solutes across cell membranes. This transfer is called facilitated diffusion or in some cases active transport (cf. Chapter 2). In facilitated diffusion, the substrate combines with a membrane component called a carrier to form a carrier-substrate complex. The carrier-substrate complex undergoes a change in conformation that allows dissociation and release of the unchanged substrate on the opposite side of the membrane. In active transport processes not only is there a carrier to facilitate crossing of the membrane, but the carrier mechanism is somehow coupled to energy dissipation so as to move the transported material up its concentration gradient. 

Most membranes do not possess nonspeeific pores allowing rapid diffusion of solutes, and for them, simple diffusion of solutes is a very slow process. Solutes can cross such membranes much more rapidly by carrier-mediated transport. The membrane components that mediate transport are proteins. They are known as carriers or permeases or transport proteins. The term translocase has sometimes been used but should be avoided since this term is also used to describe enzymes involved in protein biosynthesis. 

For convenience, we have been discussing facilitated diffusion into a cell, but the same principles apply for exit and for fluxes at the organelle level. Let us assume that a transporter for K+ exists in the membrane of a certain cell and that it is used as a shuttle for facilitated diffusion. Not only does the carrier lead to an enhanced net flux density toward the side with the lower chemical potential, but also both the unidirectional fluxes and i ut can be increased over the values predicted for ordinary diffusion. This increase in the unidirectional fluxes by a carrier is often called exchange diffusion. In such a case, the molecules are interacting with a membrane component, namely, the carrier hence the Ussing-Teorell equation [Eq. 3.25 = c /(ctjeljFEM/RT)] is not obeyed because it does not consider... 

ESR is very useful in the study of dynamic properties of membrane components because of its high sensitivity and favorable time scale. Early ESR studies of short-range lateral diffusion in membranes were based on Heisenberg exchange (HE) effects of nitroxide spin-label line width. The HE contribution to the ESR linewidth is given for nitroxides by = )dDNACf, where d is the encounter dis-... 

Diffusion The sorbed components diffuse across the swollen membrane under a chemical potential gradient. 

The nanoporous carbon membrane consists of a thin layer (<10pm) of a nanoporous (3-7 A) carbon film supported on a meso-macroporous solid such as alumina or a carbonized polymeric structure. They are produced by judicious pyrolysis of polymeric films. Two types of membranes can be produced. A molecular sieve carbon (MSC) membrane contains pores (3-5 A diameters), which permits the smaller molecules of a gas mixture to enter the pores at the high-pressure side. These molecules adsorb on the pore walls and then they diffuse to the low-pressure side of the membrane where they desorb to the gas phase. Thus, separation is primarily based on differences in the size of the feed gas molecules. Table 7 gives a few examples of separation performance of MSC membranes. ° Component 1 is the smaller component of the feed gas mixture. 

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