Diffusion mechanisms schematic representations

Molecules can passively traverse the bilayer down electrochemical gradients by simple diffusion ot by facilitated diffusion. This spontaneous movement toward equilibrium contrasts with active transport, which requires energy because it constitutes movement against an electrochemical gradient. Figure 41-8 provides a schematic representation of these mechanisms. 

Figure 1. Schematic representation of remodelling mechanisms. (Adapted form Langst and Becker, 2004.) The schemes show nucleosomes from the top. (a) The twist diffusion model - Twisting of DNA moves it over the histone surface in one base pair increments. This changes the position of the DNA with respect to the histone, as shown by the open and closed circles, (b) The Loop recapture model - Extranucleosomal DNA is pulled into the nucleosomes to replace a DNA segment which consequently loops out. This loop is then propragated over the histone surface like ripples of a wave. The star,, indicates how this leads to a change in the position of DNA relative to the nucleosome. (See Colour Plate 4.)...

FI G U RE 10.2 Schematic representation of alveolar cells and possible mechanism of transport of molecules from the alveolar space into the circulation. Particles will release molecules of interest (gray circles) into the mucus in which the particle is embedded. The molecule can either be lost in the mucus, taken up by alveolar macrophages by phagocytosis or diffusion, taken up by alveolar epithelial cells by passive or active transport, or bypass the alveolar cells via paracellular transport depending upon the properties of the drug. Once a molecule has reached the extracellular space, the same mechanisms are possible for transport from the extracellular space into the blood. Molecules in the extracellular space may also reach to circulation via the lymph. 

The model is based on the schematic representation of the commercial reactor shown in Figure le. The wafers are supported concentrically and perpendicular to the flow direction within the tube. The heats of reaction associated with the deposition reactions are small because of the low growth rates obtained with LPCVD ( 2 A/s). Furthermore, at high temperatures (1000 K) and low pressures (100 Pa), radiation is the dominant heat-transfer mechanism. Therefore, temperature differences between wafers and the furnace wall will be small. This small temperature difference eliminates the need for an energy balance. Moreover, buoyancy-driven secondary flows are unlikely. In fact, because of the rapid diffusion, the details of the flow field... 

Fig. 4. Schematic representation of suggested mechanisms of PET across membrane for some systems of Table 1 (a) — System 3 (b) — System 12 (c) — System 27 (d) — System 21 (e) — System 45. White and black arrows indicate, respectively, reaction steps that do and do not requi re light quanta to occur. Dotted arrows indicate transmembrane diffusion of substances... 

Fig. 5.18 Schematic representation of the mechanism of haloperoxidases. In the presence of Cl", HOCI is formed that (a) diffuses from the active site and oxidizes substrates in the medium, although in some cases, (b) oxidation may occur within the active site. In the absence of Cl", thiol-ligated haloperoxidases can (c) catalyze oxygen transfer to their substrates in a cytochrome P450-like reaction...

Figure 5.2 Schematic representation of the dissolution mechanisms according to (A) the diffusion layer model, and (B) the interfacial barrier model.

Figure 2. Schematic representation of surface transport processes (A) unrolling-carpet mechanism (B) transport by defect diffusion.

Figure 4. Schematic representation of transport mechanisms in porous media (a) Poiseuille flow (b) Knudscn diffusion (c) surface diffusion (d) capillary condensation (e) molecular sieving...

Figure 3. Schematic representation of the oxygen isotope zonation developed in a mineral by exchange with low 5 0 fluids through different mechanisms (A) volume diffusion inward from the grain boundary (B) dissolution and reprecipitation (C) exchange along a set of microfractures and (D) exchange along multiple sets of microfractures. Distinguishing among these processes requires microanalysis (Elsenheimer and Valley 1992).

Figure 1.6 Schematic representation of (a) the diffusion-precipitation mechanism of carbon filament growth from the gas phase [32], and (b) the carbon-fiber growth mechanism proposed by OberUn et al. [33]. Important details regarding the effects of metal particle size and shape on the chemical reactions occurring at the metal-carbon interface, and thus on the nature and size of the filaments or nanotubes produced, have yet to be sorted out.

Finally, the migration of point defects from the damage layer can extend very much deeper into the solid, e.g., several microns below the surface. These defects can cause enhanced diffusion or segregation over this depth and result in changes in composition well beyond the ion range. Fig. 5 shows a schematic representation of the characteristic depths for the various mechanisms which are listed and characterized in Table 2. 

Fig. 8.11 Left) parallel and radial wrinkles morphologies observed when solvent diffuses in the PS layer from an edge or a point-like defect, respectively. Right) schematic representation of the wrinkle mechanism induced by solvent diffusion. First, the thermal deposition process generates compression in the upper membrane. Subsequently, solvent diffusion triggers the transition from unbuckled to buckled state...

FIGURE 9.3 Schematic representation of the pervaporation transport mechanism (a) solution-diffusion model and (b) pore flow model. 

In membrane-based gas separation, the movement of penetrant gases is driven by the pressure gradient imposed between upstream and downstream. A membrane will separate gases oidy if some components pass through the membrane more rapidly than others, as shown in Fig. 3.3. There are three general transport mechanisms for membrane-based gas separation Knudsen diffusion, solution-diffusion, and molecular sieving [156,163]. A schematic representation of the mechanisms of membrane-based gas separations is shown in Fig. 3.4. 

On the other hand, the vehicular mechanism involves the movement of the hydrated proton aggregate. Here, in response to the electrochemical difference, hydrated proton (H30 ) diffuses through the aqueous medium [244,245]. A schematic representation of the vehicular mechanism is presented in Fig. 3.17. In the vehicular mechanism, hydrated protons carry one or more molecules of water (H+[H20] ) through the membrane and are transferred with them as a result of electro-osmohc drag. The major condition for proton transport through the vehicular mechanism is the existence of free volumes within the polymer matrix of a PEM, which allow the passage of hydrated protons through the membrane. 

Figure 15. Schematic representation of the mechanism of formation of porous nanoparticles [15]. (a) A fine droplet of NMP, PAA, and PA formed immediately after injection (b, c) possible intermediate states, in which the yellow regions represent zones in which NMP and cyclohexane exchange through mutual diffusion processes (d) the resulting porous PAA NP.

FIG. 21 Schematic representation of a diffusion mechanism of penetration of a surfactant solution into a hydrophobic capillary with advancing contact angles 0a more than 90°. 

FIGURE 23.9 Schematic representation of (a) chemical bonding, (b) molecular entanglement, (c) mechanical interlocking, (d) diffusion adhesion, and (e) electrostatic adhesion between the matrix and the CNTs. 

Figure 1 A schematic representation of copper ion extraction with an emulsion liquid membrane. Copper(II) is transported to the emulsion/feed phase interface and reacts with the complexing agent (RH) to form a soluble copper complex (CuRj). This complex diffuses to the interior of the emulsion droplet until it encounters a droplet of the internal phase where the metal ion is exchanged for a hydrogen ion. The net effect is a unidirectional mass transport of the cation from the original fe to the receiving phase with counter-transport of hydrogen ions. Mercury exhibits a comparable mechanism for transport in these systems.

FIGURE 11.9 Schematic representation of membrane-based gas separations, (a) Knudsen-flow separation, (b) surface-diffusion, (c) capillary condensation, (d) molecular-sieving separation, and (e) solution-diffusion mechanism. 

Fig. 3.3 Specific volume of polymers as a function of temperature for totally amorphous (a), semicrystalline (b), and crystalline (c) polymers and schematic representation of diffusion mechanisms of small molecules amorphous and crystalline domains...

Figure 5.2. A schematic representation of the mechanism for the transport of monomer between a small monomer droplet and a large droplet. Monomer molecules tend to diffuse from the small monomer droplet to the large droplet due to the Ostwald ripening effect. This will cause a concentration gradient for costabilizer between these two monomer droplets. However, the very hydrophobic costabilizer in the small monomer droplet cannot be dissolved in water, diffuse across the continuous aqueous phase, and then enter the large droplet. Thus, monomer molecules in the large monomer droplet are forced to migrate back to the small droplet in order to relax the concentration gradient for costabilizer (temned the osmotic pressure effect), and a relatively stable miniemulsion product is obtained.

Figure 24.2 Schematic representation of gas permeation steps across polymer membranes according to the solution-diffusion mechanism.

Fig. 74. Schematic representation of two possible models of the mechanism of solid-state ion exchange in microporous materials. Mechanism A, (top) NaCI molecule diffuses. Mechanism B, (bottom) Na" and counter-diffuse (see text)...

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