Membranes, natural Permeability, Plasma membrane

In sum, the natural tendency will be for sodium, calcium, and chloride ions to flow into the neuron and for potassium ions to flow out, and in so doing to reduce the membrane potential to zero. In reality, this is not so easy. The plasma membrane of the neuron is not very permeable to these ions. If it were, it would be impossible to sustain concentration gradients across it. The rate of passive diffusion of these ions across this membrane is very slow, though not zero, and different for each ion. So how do ions get across the neuronal plasma membrane rapidly There are two ways gated channels and active transport by pumps. 

Perhaps the most important feature of the plasma membrane is that it is selectively permeable. Overall permeability of the membrane to a chemical depends on the nature of the membrane, its surface charge and rigidity, and the chemical in question. Therefore, only certain substances are able to pass through the membrane, depending on particular physicochemical characteristics. It will be apparent throughout this book that the physicochemical characteristics of molecules are major determinants of their disposition and often of their toxicity. 

In 1943, Davson and Danielli introduced, in their seminal book The Permeability of Natural Membranes, the idea that solute permeability was not a generalized property of the plasma membrane but rather was associated with discrete and... 

The transfer of the information described in the preceding sections of this chapter to the in vivo situation is a matter where opinions are sharply divided, even if more than 20 years have elapsed since the discovery by Vasington and Murphy [4]. One key problem, naturally, is the impossibility of reproducing the composition and the conditions of the cytosol in in vitro experiments. The above mentioned effect of Mg on the rate of Ca influx into mitochondria is but one striking example of the difficulties inherent to the extrapolation to the in situ conditions. Of interest in this respect are recent experiments [124,125] in which methods have been devised to estimate simultaneously the membrane potential across the plasma membrane and the mitochondria of nerve endings in situ. The conclusion of this work has been that the concentration of free Ca in the cytosol correlates directly to the membrane potential across the mitochondrial membrane, and is maintained at a steady-state level below 1 jaM. Simulation of the in situ conditions has also been the aim of studies [126] in which isolated liver endoplasmic reticulum has been added to media in which isolated liver mitochondria were made to take up Ca, or in which liver cells have been treated with digitonin to abolish the permeability barrier of the plasma membrane. It was found that respiring mitochondria lower the external Ca " concentration to about 0.5 /iM. The addition of endoplasmic reticulum vesicles produces a further decrease of the external Ca " to about 0.2 jaM. Thus, mitochondria... 

Membranes play an important role in natural science and for many technical applications. Depending on their purpose, their shape can be very different. For instance, membranes include porous or non-porous films, either supported or non-supported, with two interfaces surrounded by a gas or by a liquid. Important properties of non-porous membranes are their permeability for certain compounds and their stability. In biological cells their main task is to stabilize the cell and to separate the cell plasma from the environment. Furthermore, different cells and cell compartments have to communicate with each other which requires selective permeability of the membranes. For industrial applications membranes are often used for separation of gases, liquids, or ions. Foams and emulsions for instance are macroscopic composite systems consisting of many membranes. They contain the continuous liquid phase surrounded by the dispersed gas phase (foams) or by another liquid (emulsions). Beside these application possibilities membranes give the opportunity to investigate many questions related to basic research, e.g. finite size effects. 

I have already shown several syntheses of sphingolipids as microbial metabolites or marine natural products. Sphingolipids are building blocks of the plasma membrane of eukaryotic cells. Their function is to anchor lipid-bound carbohydrates to cell surfaces, and to construct the epidermal water permeability barrier. The chemistry of sphingolipids is therefore closely related to dermatology or the science of skin. This section first treats sphingolipid in human epidermis. 

The basic features of a cell membrane are given in Figure 1 to show how it consists essentially of protein molecules incorporated into a semifluid liquid bilayer structure. As a rough guide the plasma membrane of most cells is composed, on a dry weight basis, of nearly equal components of protein and lipid. Because of its nonpolar nature the lipid membrane structure is intrinsically impermeable to polar and electrically charged molecules. For example, turbidity measurements on sarcoplasmic reticulum membranes provide membrane resistance values of 2.6 x 10 and 2.5 X 10 Q cm for the permeability of calcium ions and protons, respectively, while for sodium and potassium ions the corresponding values are... 

Several studies of the preparation of gas separation membranes by low pressure plasma treatment have been reported. Kawakami et al. demonstrated that the selectivity of O2 over N2 increased somewhat by plasma treatment of natural rubber (7). Yasuda et al. have found that CO2 selectivity over CH4 of poly(dimethylsiloxane) increased by about 20-fold after Ar plasma treatment 4), Matsuyama et al. investigated in detail the OO2 permeability and selectivity over CH4 of poly(dimethylsiloxane) membranes modified by plasma treatment using Ar, N2, O2 and NH3 and demonstrated that the selectivity of CO2 over CH4 was remarlrably improved by plasma treatment. They also observed that the selectivity showed a maximum with treatment time (i). 

Furukawa K, Matsuzaki-Kobayashi M, Hasegawa T, et al. Plasma membrane ion permeability induced by mutant alpha-synuclein contributes to the degeneration of neural cells. / Neurochem. 2006 97(4) 1071-1077. Lashuel HA, Hartley D, Petre BM, Walz T, Lansbury Jr PT. Neurodegenerative disease amyloid pores from pathogenic mutations. Nature. 2002 418(6895) 291. 

Although there is a natural tendency toward equilibrium of the solute concentration on both sides of the membrane, such an equilibrium is rare in a living system, and selective permeability of the plasma membrane therefore assures the required distribution of metabolically important material inside and outside the cell. Kinetic studies of solute transport often permit characterization of the type of transmembrane movement involved (Neame and Richards, 1972). As outlined by Csaky (1965), a given substance can cross the cell membrane in several different ways free diffusion, diffusion through pores, pinocytosis, and carrier-mediated transport. 

Plasma membranes. Lamellar, and partly micellar, mosaics of lipids and proteins form a thin, semi-permeable membrane around the exterior of every cell, and also around each organelle in the cell (see Section 5.2). The permeability of natural membranes was discussed in Section 3.1. The interaction of these membranes with diuretics, cardiac glycosides, and other ionophoric effects will now be considered. 

Phospholipids, components of every cell membrane, are active determinants of membrane permeability. They are sources of energy, components of certain enzyme systems, and involved in lipid transport in plasma. Because of their polar nature, phospholipids can act as emulsifying agents. The structure of most phospholipids resembles that of triglycerides except that one fatty acid radical has been replaced by a radical derived limn phosphoric acid and a nitrogen base. c.g.. choline or serine. 

After arriving at a particular organ, the free, unbound form of the drug is able to cross first the endothelial cells of the capillaries into the interstitial space, and subsequently the cellular membrane of the tissue. Capillary permeability is largely determined by (1) capillary structure and (2) the chemical nature of the drug. The membrane-related factors influencing distribution of drugs between blood plasma and tissues are essentially the same as those described previously between the GI tract and blood plasma. 

Thin films with a broad spectrum of properties can be deposited in plasma chemical processes [30, 31]. Hard coatings such as diamond films and TiN films, soft plasma polymer films, insulating SiO films, highly conducting Si films, anti-reflection coatings, semi-permeable membranes and very effective diffusion barriers can be deposited. Important parameters for the film deposition are (i) the nature of the precursor, (ii) the gas mixture and... 

---