Membrane inner side composition

Asymmetry. Biological membranes are asymmetric that is, the lipid composition of each half of a bilayer is different. For example, the human red blood cell membrane possesses substantially more phosphatidylcholine and sphingomyelin on its outside surface. Most of the membrane s phosphatidylserine and phos-phatidylethanolamine are on the inner side. Membrane asymmetry is not unexpected, because each side of a membrane is exposed to a different environment. Asymmetry originates during membrane synthesis, because phospholipid biosynthesis occurs on only one side of a membrane (Special Interest Box 12.3). The protein components of membranes (discussed below) also exhibit considerable asymmetry with distinctly different functional domains within membrane and on the cytoplasmic and extracellular faces of membrane. 

Glycerides consist of glycerin, an alcohol from the C< pool, which is esterified with three fatty acids (Fig. 8) to form flits as an energy store. In phospholipids one fatty acid is replaced by phosphoric acid. Phospholipids form membranes that isolate the inner part of cells from the surrounding environment because of their arrangement as a bilayer. The hydrophobic alkyl chains of the fatty acids are directed toward the inner side of the bilayer and the hydrophilic phosphate ends form the surface of the membrane. Membranes are most important for cellular function and therefore are part of all organisms. The composition of fatty acids in membranes is specific to source organisms and hence is used to describe microbial community structures (Olsson, 1999). 

After the ion-exchange of MFI zeolite membranes from Na-type to H-type, the alkylation of toluene with methanol was carried out at temperatures of 450 and 500°C. The reactants, in a molar composition of 2 toluene 1 methanol, were fed with a syringe pump. The carrier gas used from the outer to the inner side of the cylindrical membranes was He. The composition was analyzed by gas chromatography using a xylene-isomer separation column (Ben-tone 34, Supelco). The pressure difference across the membrane was controlled at approximately 10 kPa. 

The strong influence of the inner solution composition on the resulting calibration curves is shown in Fig. 9.21. A very high detection limit can be obtained in practice with concentrated and lipophilic electrolytes that result in substantial electrolyte extraction into the back side of the membrane (here, c (5 ) = 0.02M). The predominant mode of transport is then co-diffusion with its cotmterion. 

The membrane establishes in and out. The membrane is asymmetric because the inner and outer leaflets can have a different lipid composition and contain different proteins (Fig. 3-3). Proteins can be associated with either side of the membrane, or they can pass through the membrane using membrane-spanning segments. The functional part of the protein can be on the cytosolic side, the external side, or even in the membrane itself. A common structure for spanning a membrane is an a-helix (but there are examples of sheets spanning a membrane). It takes about 20 amino acid residues arranged in a helix to span to a 30 A hydrophobic interior of the bilayer. 

Biological membranes consist of lipids, proteins, and carbohydrates (see p. 214). These components occur in varying proportions (left). Proteins usually account for the largest proportion, at around half. By contrast, carbohydrates, which are only found on the side facing away from the cytoplasm, make up only a few percent. An extreme composition is seen in myelin, the insulating material in nerve cells, three-quarters of which consists of lipids. By contrast, the inner mitochondrial membrane is characterized by a very low proportion of lipids and a particularly high proportion of proteins. 

This structure (Fig. 7.14) has received much attention due to the role it plays in the hatching process (see below). It is not a typical unit membrane but resembles a membranous lamina and consists of a layer of regularly arranged granules bounded on both sides by a number of lamina (442). Its chemical composition has not been determined but, in taeniid cestodes, there is some histochemical evidence that it may be a lipoprotein (442). It is apparently formed by the delamination of the inner part of the inner envelope, detaching from it as a thin, separate layer. 

Seawater or brackish water can be purified by reverse osmosis. To maximise the flow of water through a polymer membrane, the polymer must have a high water permeability, yet a low permeability for the salts. To maximise efficiency, the membrane area must be large and its thickness as small as possible consistent with a lack of pinholes. A high pressure is applied to the salt water side of the membrane. Because it is thin, a cellulose triacetate membrane is supported on a porous cellulose nitrate-cellulose acetate support structure to resist the pressure. To make the unit compact the composite membrane is spirally wound on to an inner cylinder, and the edges glued together. When a pressure of 70 bar is applied to the seawater side, NaCl rejection levels in excess of 99.7% can be achieved. 

The membrane potential, that is, the potential difference between the two phases separated by the membrane, is the sum of the two phase-boundary potentials and the transmembrane potential difference. The latter is negligible in all practical cases since only one dominating exchangeable ion prevails in the manhrane. The phase-boundary potential on the inner membrane side does not depend on the composition of the sample and is thus constant. The observed electromotive force is the sum of all contributions in a measuring cell. Besides the ISM, the only sample-dependent contribution comes from the reference electrode. In well-designed systems, its contribution is nearly constant and can be included together with the other contributions, such as the transmembrane potential and the phase-boundary potential on the inner membrane side, into one potential term of the cell, K . Thus, the observed electromotive force, anf, is... 

The membrane potential is directly related to the ion-activities in the solutions either side of the membrane and the cell potential can be described by the Nemst equation if the liquid junction potential can be considered to be constant and the composition of the inner filling solution is kept unchanged. 

Analysis of the amino acid composition of the mitochondrially synthesized subunits of cytochrome oxidase of the rutamycin-sensitive ATP-ase (another membrane component which requires mitochondrial protein synthesis for some of its component polypeptides) and cytochrome b displayed a high proportion of nonpolar amino acid residues. The resulting unusually hydrophobic composition explains their insolubility in aqueous solutions. This hydrophobic character of the mitochondrially translated polypeptides may have relevance for speculations on the existence of mitochondrial protein synthesis. It has been suggested that their hydro-phobic properties make it necessary that these polypeptides be delivered to the inner mitochondrial membrane from the matrix side, since they cannot be transported through the cytoplasm and the intercristae space. 

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