Transport mechanisms activated diffusion

Transport Mechanism Facilitated Diffusion Active Transport... 

K). The changes in slope indicate changes in the mechanism of surface diffusion. While, at low temperatures, adatom diffusion or adatom-surface atom exchange appears to be the dominant atom-transport mechanism, the diffusion of surface vacancies created by thermal roughening is likely to be dominant at high temperatures to account for the increased activation energies. For example, copper adatom and vacancy diffusion rates in the Cu(l 10) crystal face are given by... 

Structure and water sorption characteristics of fuel cell media determine their transport properties. The dynamic properties of water determine microscopic transport mechanisms and diffusion rates of protons in PEM and CLs. Protons must be transported at sufficiently high rates, away from or toward the active Pt catalyst in anode and cathode catalyst layers, respectively. Effective rates of proton transport in nanoporous PEM and CLs result from a convolution of microscopic transport rates of protons with random network properties of aqueous pathways. Accounting for the geometry of these materials, namely, their external surface area and thickness, gives their resistances. 

The cellular mechanism of action of hydrocortisone, a glucocorticoid, is also related to proteins but not by the enhancement of cAMP production. Hydrocortisone is transported by simple diffusion across the membrane of the cell into the cytoplasm and binds to a specific receptor The steroid-receptor complex is activated and enters the nucleus, where it regulates transcription of specific gene sequences into ribonucleic acid (RNA). Eventually, messenger RNA (mRNA) is translated to form specific proteins in the cytoplasm that are involved in the steroid-induced cellular response. 

The nucleus is surrounded by the nuclear envelope, which takes on a lumenal structure connected to the endoplasmic reticulum. The transport of proteins into (and out of) the nucleus occurs through the nuclear pore complex (NPC), a large complex composed of more than 100 different proteins (Talcott and Moore, 1999). Because NPC forms an aqueous pore across the two membranes, small proteins less than 9 nm in diameter can pass through it simply by diffusion. However, most of the transports of both proteins and RNAs are mediated by an active transport mechanism. It is now clear that there is heavy traffic through the NPC in both directions. Proteins are not only imported into the nucleus but also actively exported from it as well. There are many reasons for nuclear export. One reason is to send some shuttle proteins back after their import another is for some viral proteins to export their replicated genomes outside the nucleus. 

Transport mechanism Diffusion or active transport through tunneling nanotubes Diffusion or endocytotic uptake of carbon nano tubes... 

The transport mechanisms that operate in distribution and elimination processes of drugs, drug-carrier conjugates and pro-drugs include convective transport (for example, by blood flow), passive diffusion, facilitated diffusion and active transport by carrier proteins, and, in the case of macromolecules, endocytosis. The kinetics of the particular transport processes depend on the mechanism involved. For example, convective transport is governed by fluid flow and passive diffusion is governed by the concentration gradient, whereas facilitated diffusion, active transport and endocytosis obey saturable MichaeUs-Menten kinetics. 

In the central nervous system (brain and spinal cord), capillary endo-theUa lack pores and there is little transcytotic activity. In order to cross the blood-brain barrier, drugs must diffuse transcellularly, i.e., penetrate the luminal and basal membrane of endothelial cells. Drug movement along this path requires specific physicochemical properties (p. 26) or the presence of a transport mechanism (e.g., L-dopa, p. 188). Thus, the blood-brain barrier is permeable only to certain types of drugs. 

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