Movement of molecules

Generally, electrophoresis is carried out not in free solution but in a porous support matrix such as polyacrylamide or agarose, which retards the movement of molecules according to their dimensions relative to the size of the pores in the matrix. 

Passive diffusion is the simplest transport process. In passive diffusion, the transported species moves across the membrane in the thermodynamically favored direction without the help of any specific transport system/molecule. For an uncharged molecule, passive diffusion is an entropic process, in which movement of molecules across the membrane proceeds until the concentration of the substance on both sides of the membrane is the same. For an uncharged molecule, the free energy difference between side 1 and side 2 of a membrane (Figure 10.1) is given by... 

All of us are familiar with the process of vaporization, in which a liquid is converted to a gas, commonly referred to as a vapor. In an open container, evaporation continues until all the liquid is gone. If the container is closed, the situation is quite different. At first, the movement of molecules is primarily in one direction, from liquid to vapor. Here, however, the vapor molecules cannot escape from the container. Some of them collide with the surface and reenter the liquid. As time passes and the concentration of molecules in the vapor increases, so does the rate of condensation. When the rate of condensation becomes equal to the rate of vaporization, the liquid and vapor are in a state of dynamic equilibrium ... 

Equation 10.4, which describes the mass transfer rate arising solely from the random movement of molecules, is applicable to a stationary medium or a fluid in streamline flow. If circulating currents or eddies are present, then the molecular mechanism will be reinforced and the total mass transfer rate may be written as ... 

The observation of the system NO2/N2O4 provided essential empirical evidence to support the idea that the reactant and product could coexist. According to the questions posed in the activity, this evidence could not only be made explicit in the representation of their models but also be explained by the models. The students who were able to establish relationships between the movement of molecules and the occurrence of a chemical reaction (according to the kinetic particle model that had been studied earlier), were also able to include dynamic components in their models. Those who were not able to do so had the opportunity to think about this from the general discussion of the models - when all groups presented and justified their ideas - or from other empirical evidence that was obtained next. 

Modeling of water may be extended to properties involving the movement of molecules into space, a process of evaporation. For this the grid must be structured at the initial setting to have two different areas, one with occupied cells and the other with unoccupied cells (Figure 3.7). The rate of evaporation can be measured from a model allowing for water movement into an empty part of the grid. This is illustrated in Example 3.5. 

H. Trauble, The movement of molecules across hpid membranes amolecular theory. J. Membr. Biol. 1971, 4, 193-208. 

Equation can be applied directly to the movement of molecules escaping from a container into a vacuum. This process is effusion. Effusion is exemplified by the escape of molecules from the oven of Figure 5-7. 

Fig. 9 Schematic representation depicting the movement of molecules from the absorbing (mucosal or apical) surface of the GIT to the basolateral membrane and from there to blood. (A) transcellular movement through the epithelial cell. (B) Paracellular transport via movement between epithelial cells. (Q Specialized carrier-mediated transport into the epithelial cell. (D) Carrier-mediated efflux transport of drug out of the epithelial cell. (Copyright 2000 Saguaro Technical Press, Inc., used with permission.)...

Mass transfer phenomena exist everywhere in nature and are important in the pharmaceutical sciences. We may think of drug synthesis preformulation studies dosage form design and manufacture and drug absorption, distribution, metabolism, and excretion. Mass transfer plays a significant role in each. Mass transfer is referred to as the movement of molecules caused not only by diffusion but also by convection [1],... 

Stein WD. (1967). The Movement of Molecules Across Cell Membranes. New York Academic Press, pp 65-125. 

Molecules and ions are in constant motion and the velocity of their motion is proportional to their temperature. This passive movement of molecules and ions from one place to another is referred to as diffusion. When a molecule is unevenly distributed across a permeable membrane with a higher concentration on one side and a lower concentration on the opposite side, there is said to be a concentration gradient or a concentration difference. Although all of the molecules are in motion, the tendency is for a greater number of molecules to move from the area of high concentration toward the area of low concentration. This uneven movement of molecules is referred to as net... 

Defects of mitochondrial transport interfere with the movement of molecules across the inner mitochondrial membrane, which is tightly regulated by specific translocation systems. The carnitine cycle is shown in Figure 42-2 and is responsible for the translocation of acyl-CoA thioesters from the cytosol into the mitochondrial matrix. The carnitine cycle involves four elements the plasma membrane carnitine transporter system, CPT I, the carnitine-acyl carnitine translocase system in the inner mitochondrial membrane and CPT II. Genetic defects have been described for each of these four steps, as discussed previously [4,8,9]. 

Dislocations occur profusely in nonmetallic materials. As mentioned above, ceramics are brittle at ordinary temperatures, not because of a lack of dislocations but because these cannot easily glide due to strong bonding between the component atoms. Organic crystals, which are usually composed of molecules consisting of strongly bound atoms, linked by weak external bonds, usually glide by movement of molecules rather than atoms, and dislocations can be referred to the molecular array rather than the atom array. 

Structural characteristics. Both natural and synthetic mordenite have an orthorhombic structure that consists of parallel, 12-membered ring channels in the c-direction, having an eliptical cross-section with dimensions of 6,7 x 7.0 A (Figure 9). Smaller 8-membered ring channels with dimensions of 2.9 x 5.7 A, which are perpendicular to the main channels, are too small to allow the movement of molecules from one main channel to another. Mordenite has been synthesized in a "large -port" and "small-port" form that have different sorption properties. A typical unit cell content is Na0[(A10o)o(Si0o)/rJ. 24 HO. 8 28 2 40... 

To determine the movement of molecules, the following algorithm (15) is often used. The force acting on the ith atom in a molecule (Fj) is determined from the spatial derivative of the total interaction potential energy of that particle ... 

Passive Diffusion Diffusion is the random movement of molecules in fluid. If a fluid is separated by a semipermeable membrane, more dissolved molecules will diffuse across the membrane from the higher concentration side to the lower concentration side than in the reverse direction. This process will continue until equihbrium is achieved, whereby both sides have the same concentration. When equilibrium is reached, there are equal numbers of molecules crossing the membrane in both directions. 

Daltom A unit of measurement equal to the mass of a hydrogen atom, electrophoresis The differential movement of molecules through a gel under the influence of an electric field. 

A membrane can be described as a semipermeable barrier between two phases which prevents intimate contact. This barrier must be permselective which means that it restricts the movement of molecules in it in a very specific way. The barrier can be solid, liquid or gas. Permselectivity can be obtained by many mechanisms ... 

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