Membrane contact point

The effect differs according to time and place. So far as the time effect is concerned, there is a need to distinguish acute effect, which appears a short while after the substance penetration, from the long term or chronic effect, for which effects can be identified after several years of exposure. The action can be local, ie contact point with the substance, or systemic , reaching organs that are distant from the penetration point.The local effect affects skin and eyes and/or mucous membranes, especially the inhalation ones. The local effects are irritant and sensitive. 

Synapses are the contact points of two nerve cells or of a nerve cell with an effector cell (such as a muscle, glandular or sensory cell). It is at the synapse, exactly at the synaptic cleft, where the transfer of information from one cell to the next takes place. It is estimated that the diameter of a synaptic cleft, Le. the distance between the presynaptic membrane (part of the first cell) and the postsvnaptic membrane (part of the second cell), is about 100 300 pm. Depending on the carrier of information, electrical and chemical synapses can be distinguished ... 

FIGURE 7.27 Micromachined ISE chip, (a) Schematic drawing of a sensor chip design with channels and reservoirs point A, inlet for U-channel point B, outlet for U-channel point C, inlet for sample channel point D, junction structure, where membrane contacts sample solution and point E, outlet for sample channel. The diagram illustrates the complete filling of the silanized U-channel with an organic membrane cocktail, (b) Scaled diagram of the 12- x 6-mm chip [766]. Reprinted with permission from the American Chemical Society. 

Concave double layers, l.e. double layers at the hollow side of curved interfaces, are of more general interest, "nius double layers inside spherical cavities are relevant for micro-emulsions and vesicles, and those inside cylinders for electrokinetics and membrane transport. Certain contact points exist with double layers in overlap, as occurring in colloid stability. 

Remarkably enough, statements of high chemical stability under harsh corrosive environments such as pH < 3 or pH > 9 are not substantiated in the literature by reliable measurements on membrane systems and much more work is needed here. Problems that occur with the bursting pressure of support tubes after long-term usage might indicate local corrosion at the contact points between the ceramic particles making up the microstructure. 

Most of the contact points of peptides could be on the membrane surface rather than in the transmembrane region, while high-affinity small molecule ligands typically have contact points in the hydrophobic transmembrane region of the receptor. 

This load has to balance the tractions and (see Figure 5.3). Therefore, computing requires determining the vectors n, a, and m. The normal is computed w.r.t to the known substrate surface at the contact point, by considering the closest point projection as mentioned in the surface contact, while the tangent is determined at the membrane point x as... 

The principle of electrofiision is the simultaneous electric breakdown of plasma membranes at the contact points with adjacent cells, when a short pulse of supercritical voltage for electric breakdown is applied to the cell pair. A lumen is formed between the coaxial pores at the breakdown site, which eventually enlarges to connect the cytoplasm of the fusion partner cells. The force involved in making cell-cell contact before, during and after pulse application, and the force causing the simultaneous membrane breakdown, can be calculated from the conductivities and dielectric constants of the cytoplasm, the membrane, and the external medium. When all parameters are known precisely, the outcome of electrofiision can be predicted with certainty. 

In this regard, Hackenbrock has described an ultrastructural feature of isolated rat liver mitochondria which he has termed contact points, representing regions where the inner and outer mitochondrial membranes have come together and are possibly fused. Our studies with yeast mitochondria indicate that bound 80 S ribosomes are attached preferentially to contact points. This finding not only implies that ribosome... 

The problem of how to transmit, unaltered, the potential of the ion-sensitive phase still remains. It is not always possible to simply imbed one end of the instrument lead in the material which carries out the specific ion exchange with the test solution. In this case an additional phase boundary, with its own potential, must be introduced into the measuring circuit. In order for a constant equilibrium Galvani potential to arise at this interface, a reversible reaction is required. This contact point must also behave like an unpolarizable electrode, a requirement which is not as easily fulfilled. One already has, however, a reversible and unpolarizable electrode reaction on the measuring side of the membrane (otherwise the electrode response would not follow the Nemst equation). The obvious solution, then, is to allow the same electrode reaction to occur on the inner side of the membrane, only this time with a constant activity of the corresponding measured ion. 

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