Penetration mechanism

P Ashton, WC Shen, VHL Lee. (1989). Penetration mechanism of a protein across the conjunctiva of the pigmented rabbit. Pharm Res 6 S 116. 

Penetration The means by which the vims penetrates into the cell depends on the nature of the host cell, especially on its surface stmctures. Cells with cell walls, such as bacteria, are infected in a different manner from animal cells, which lack a cell wall. The most complicated penetration mechanisms have been found in viruses that infect bacteria. The bacteriophage T4, which infects E. coli, can be used as an example. 

Non-enveloped viruses generally use the first two penetration mechanisms, while the enveloped viruses enter a cell by endocytosis followed by binding with the membrane of an endosome. In addition to this mechanism, the enveloped viruses fuse directly with the cell membrane. The fusion of the viral envelope with the cell membrane requires the interaction of the glycoproteins of the virus with a cell receptor. After the internalization of the viral particle, the genome is freed for later expression. This process is known as unwrapping and it involves both cellular and viral enzymes. 

The actual calculation of the repulsive forces needs of course a very exact knowledge of the charge distribution on the surface of the molecules, and therefore presents considerable difficulties hitherto, a detailed calculation could only be carried out for the very simplest case of He. The most successful attempts in this direction so far have applied the ingenious Thomas-Fermi method which takes the Pauli Principle directly as a basis and is accordingly able, neglecting many unessential details, to account for just that effect which is characteristic of this penetration mechanism. 

In contrast to SCC of carbon and low-alloy steels in chloride, sulfide, and sulfuric acid environments by hydrogen-embrittlement mechanisms, cracking in several environments is attributed to passive-film cracking and/or active-corrosion-path anodic-dissolution penetration mechanisms (Ref 124). These environments include nitrates, hydroxides, ammonia, carbon-dioxide/carbonate solutions, and aqueous car-bon-monoxide/carbon-dioxide. Nitrate-bearing solutions are encountered in coal distillation and fertilizer plants hydroxide solutions in the production of NaOH and in crevices of steam boilers and ammonia cracking has occurred in tanks and distribution systems for agricultural ammonia applications. 

These models predict different forms of the equations of state and the adsorption isotherms of the soluble surfactant depend on the mechanism. Based on the shape of experimental n -A isotherms respective conclusions can be drawn on the actual penetration mechanism. 

The whole virus or its genetic material alone (DNA or RNA) enters the cell s cytoplasm (penetration and uncoating). A virus may have different penetration mechanisms in the host cell. For enveloped virus, fusion of membrane sometimes occurs. Most viruses are introduced into the cell by a kind of phagocytosis named viropexis. Virus particles are transported along the network of cytoplasmic microtubules to a specific cell site where subsequent replication takes place. Uncoating results in the liberation of viral nucleic acids into the cell which makes them sensitive to nucleases. 

Under production conditions, the transcutaneous penetration mechanism into the organism, i.e. through undamaged skin, follows inhalation in degree of danger. The toxicological experience of the transcutaneous effects of Cl has proved to have a local character, characterized by inflammation (skin reddening) that may be complicated by a suppurative process with deep sores. This local affect is common to the majority of Cl. 

Three main mechanisms are being discussed for the processes leading to the breakdown of passivity and the nucleation of corrosion pits [19] (Fig. 15). The penetration mechanism involves the migration of aggressive anions from the electrolyte through the passive layer to the... 

The penetration mechanism requires the transfer of the aggressive anions from the electrolyte to the metal-oxide interface. The application of surface analytical methods such as X-Ray Photoelectron Spectroscopy (XPS), Auger Electron Spectroscopy (AES), and Secondary Ion Mass Spectroscopy (SIMS) do not clearly support this mechanism. Careful measurements begin with a specimen prepassivated in a solution without any aggressive anions and with their being added later to avoid incorporation during film growth to study their penetration. Eor these conditions. 

Other serious objections to the penetration mechanism refer to the kinetics of pit formation. A poorly prepassivated specimen is submitted to pitting within... 

Fig. IS Schematic diagram demonstrating the three mechanisms leading to the breakdown of passivity and pit nucleation [19], (a) penetration mechanism with migration of aggressive anions in the high electrical field of the passive layer ...

The point defect model of passivity and its breakdown is a variant of the penetration mechanism [51]. The transport of cations from the metal surface to the oxide-electrolyte interface corresponds to an inward movement of cation vacancies Vm+- This inward transport of Vm+ is supported by their high concentration at... 

As mentioned already for the discussion of the penetration mechanism, pit nucleation is an extremely fast process of a few ms only for nonstationary conditions of the passive layer. Stepping of the potential in either direction, positive or negative, causes excessive formation of corrosion pits, especially for potentials well above the critical value and in the presence of a high concentration of aggressive anions. Even for stationary conditions, a... 

Yang M.J. et al. 1997. Exploration of Cement Grouting Penetration Mechanism. Mechanics in Engineering 19(5) 45-47 (in Chinese). 

Laible, R. C. In Ballistic Materials and Penetration Mechanics, edited by R. C. Laible. Elsevier, Amsterdam, 1980, pp. 73-115. 

The penetration mechanism of a slash attack has been discussed in detail in section 7.2.4.3. Eig. 7.2 shows the different forces applied by a knife attack. It can be observed from Eig. 7.2 that the load apphed by a knife attack increases rapidly upon contact, whereas the loads applied by different cut resistant test methods are constant load. There is a rapid increase in load when using BS EN 1082-3, but all the load is applied on the pointed tip of the blade, while in a slash, all the load is applied on the edge of the blade (Fig. 7.3). 

The fast penetration of water into the highly sized paper which is observed at higher temperatures seems at first counterintuitive, since the water surface tension is only weakly dependent on the temperature. However, the important penetration mechanism is, in this case, most likely vapour transport and capillary condensation, where the latter occurs in accord with the Kelvin equation, as follows ... 

---