Cell membrane Charge

To put it even more simply, drugs can get into the brain if they dissolve in the fats in cell membranes. Charged molecules are usually water-soluble (i.e., are hydrophilic ) and tend not to dissolve in fat (i.e., are lipophobic). Adding charge—making the molecule an ion changes how the molecule is transported in the body. 

The reason for the variation in sensitivity to cadmium between individual endothelial cells in sensory ganglia is not completely understood. Schlaepfer " has suggested a number of possible explanations, such as differences in metabolic activity, in cell membrane charge or porosity, and in intrinsic chelation mechanisms between endothelial cells. 

M.p. 103°C. Noradrenaline is released in the adrenal medulla with adrenaline, and also at the sympathetic nerve endings. Its release from a nerve fibre is followed by binding to a receptor molecule on the next nerve or muscle fibre, probably causing a change in the electrical charge of the receptor-cell membrane. Biosynthetically it normally serves as a precursor for adrenaline. 

The ability of living organisms to differentiate between the chemically similar sodium and potassium ions must depend upon some difference between these two ions in aqueous solution. Essentially, this difference is one of size of the hydrated ions, which in turn means a difference in the force of electrostatic (coulombic) attraction between the hydrated cation and a negatively-charged site in the cell membrane thus a site may be able to accept the smaller ion Na (aq) and reject the larger K (aq). This same mechanism of selectivity operates in other ion-selection processes, notably in ion-exchange resins. 

The net electrochemical driving force is determined by two factors, the electrical potential difference across the cell membrane and the concentration gradient of the permeant ion across the membrane. Changing either one can change the net driving force. The membrane potential of a cell is defined as the inside potential minus the outside, i.e. the potential difference across the cell membrane. It results from the separation of charge across the cell membrane. 

The electrical signals are carried by the movement of charged ions across the cell membrane. This makes use of the potential energy stored across the cell membrane in the form of ionic gradients. Concentration gradients for the principal ions across a typical nerve cell membrane are indicated in Fig. 2.1(a). The cell interior has a high concentration of K+ ions and a low concentration of Na+, Cl and Ca + ions relative to the exterior. 

The important attributes of liposomes as a drug carrier are (a) they are biologically inert and completely biodegradable (b) they pose no concerns of toxicity, antigenicity, or pyrogenicity, because phospholipids are natural components of all cell membranes (c) they can be prepared in various sizes, compositions, surface charges, and so forth, depending on the requirements of... 

Trimethylaminodiphenylhexatriene chloride (TMADPH Fig. 7.45) is a fluorescent quaternary ammonium molecule that appears to permeate cell membranes [595]. TMADPH fluoresces only when it is in the bilayer, and not when it is dissolved in water. Therefore, its location in cells can be readily followed with an imaging fluorescence microscope. One second after TMADPH is added to the extracellular solution bathing HeLa cell types, the charged molecule fully equilibrates between the external buffer and the extracellular (outer) leaflet bilayer. Washing the cells for one minute removes >95% of the TMADPH from the outer leaflet. If the cells are equilibrated with TMADPH for 10 min at 37°C, followed by a one-minute wash that removed the TMADPH from the outer leaflet, the fluorescent molecule is... 

Figure 1 General pathways through which molecules can actively or passively cross a monolayer of cells. (A) Endocytosis of solutes and fusion of the membrane vesicle with the opposite plasma membrane in an active process called transcytosis. (B) Similar to A, but the solute associates with the membrane via specific (e.g., receptor) or nonspecific (e.g., charge) interactions. (C) Passive diffusion between the cells through the paracellular space. (C, C") Passive diffusion (C ) through the cell membranes and cytoplasm or (C") via partitioning into and lateral diffusion within the cell membrane. (D) Active or carrier-mediated transport of an otherwise poorly membrane permeable solute into and/or out of a cellular barrier.

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