Plasma membranes asymmetric composition

In the plasma membrane of animals (1), the amount of cholesterol is usually around 20-30 mol%. The rest of the lipids are mainly PC, PE, and sphingomyelin (SM) lipids, with smaller amounts of PS, PI, and glycolipids. These lipids are distributed asymmetrically across the membrane, because most cholesterol, PC, and glycolipids are located in the extracellular (outer) leaflet, whereas PS and PE lipids are located mainly in the intracellular (inner) monolayer. The lipid composition can be highly different in other organelles, however, as is the case in mitochondria (1), in which the mitochondrial membrane is composed of two (inner and outer) membranes. There, the amounts of cholesterol, SM, and PS are negligible most lipids are PC and PE. The major difference compared with plasma membrane is the concentration of cardiolipins. They are actually found only in bacterial and in mitochondrial membranes, where their numbers are significant even in mitochondria they are located mainly on the iimer membrane. 

Although a great deal is known about the chemical composition of the mitochondrial membrane and it is established that the membrane contains a number of catalytic proteins e.g., the ATPase synthetase system, an ion transport molecular machinery and electron transport chain), the topological distribution of these proteins in the membrane is not known. All topological models proposed are at present hypothetical [177]. However, it is accepted that the mitochondrial membrane, like most if not all biological membranes, is of the fluid mosaic model and is composed of a lipid bilayer traversed by proteins (see plasma membrane in Chapter 16). Electron microscopic studies of the freeze-edge fractured faces of the outer and the inner membrane [178] indicate that the proteins are asymmetrically distributed not only when the inner is compared to the outer membrane, but also when the inner and outer faces of each of the fractured membranes are compared (Table 1-3). 

WETZEL, 1976 GOTTLIEB et, 1974). Because such periodic changes must have a basis on the molecular level, we can look forward to a bewildering array of membrane chemistries. This means that the biochemical definition of membrane organization will require a substantial increase in the resolution of our separation and observation techniques. My review of the asymmetrical composition of the plasma membrane categorizes membrane elements essentially as if they were part of the simple envelope which we expected from our early views of the red cell. 

Several other techniques have been exploited to answer queries about asymmetrical compositions of plasma membranes. To make an observation of the transverse asymmetry in the distribution of certain membrane molecules requires that one side of the membrane be excluded from a reaction medium while a constituent is chemically altered, labeled or identified by its properties. Many such experiments have been conducted with intact erythrocytes which were reacted and then lysed and reacted again. The reactant saw initially only the outer surface of the membrane and eventually both sides. Qualitative or quantitative differences observed under these conditions allowed conclusions about the... 

We should evaluate our observations on the asymmetrical composition of the plasma membrane by assuming that the "statistical mean surface" of the cell represents a datum. Microelectrode work indicates that we should retain our concept of a contiguous lipid bi-layer over the entire cell. However, a part of the cell surface, or even the entire cell, may for brief periods expose the naked cytoplasm to the environment (CHAMBERS 6e CHAMBERS, 1961). Regeneration or biosynthesis of the membrane from pre-formed sub-assemblies in the cytoplasm can be achieved in a very brief period, as if in quantum increases (BARASSI 6e BAZAN, 1974 TB S ad., 1974). Nature has invented a very flexible apparatus to maintain or build whatever membrane is required at specific times. The liquid crystalline state of the bi-layer and the presumed fluidity of its constituent molecules are seemingly well adapted to such requirements. 

I have also reviewed briefly the asymmetric composition of the lipid bi-layer portion of the membrane. It seems that most of the plasma membrane enzymes either contain lipid moieties or require the presence of lipids for activity. Conversely, the organization of the lipid bi-layer in the membrane may depend on these proteins. Organization requires information and I suggest that we adopt the term collocation for the information transfer which directs the organization of the lipid phase in the membrane. In this review... 

A membrane designated "Solrox" made by Sumitomo Chemical Company is closely related to the above plasma polymerized composite membranes. A 1980 report by T. Sano described the Sumitomo process (31). A support film was cast from a polyacrylonitrile copolymer containing at least 40 mole percent acrylonitrile. The support film was dried and exposed to a helium or hydrogen plasma to form a tight cross-linked surface skin on the porous polyacrylonitrile support film. Data in a U.S. Patent issued in 1979 to Sano et al showed that the unmodified support film had a water flux of 87 gfd (145 L/ sq m/hr) at 142 psi (10 kg/sq cm). After the plasma treatment a reverse osmosis test using 0.55 percent NaCl at 710 psi (4895 kPa) showed 10.5 gfd (17.5 L/sq m/hr) flux at 98.3 percent salt rejection (32). This membrane appears to fall between a conventional asymmetric membrane and a composite membrane. If the surface skin is only cross-linked, one might call it a modified asymmetric membrane. However, if the surface skin is substantially modified chemically to make it distinct from the bulk of the membrane it could be considered as a composite type. 

Wang et al. (2002) demonstrated a composite membrane (subjected to PV) of an asymmetric poly(4-methyl-l-pentene) (TPX) membrane dip-coated with poly(acrylic acid) (PAA). To improve the interface peeling of the PA A/TPX composite membranes, the surfaces of TPX membranes were modified by residual air plasma in a tubular-type reactor. The plasma treatments were effective in rendering the asymmetric TPX membrane hydrophilic. Optimal results were obtained with PAA/TPX composite membrane prepared from the PAA/ethylene glycol (EG)/aluminum nitrate = 1/2/0.05 coating solution at 5 W/30 s plasma treatment condition. Concentration of the water in the permeate was nearly 100%, and a permeate flux of 960 g/m h was obtained with a 3 wt% feed acetic acid concentration. 

It is also possible to employ composite membranes which are skiimed asymmetric membranes. However, in conqx>site membranes, the toplayer and sublayer originate from different polymeric materials each layer can be optimised independently. Generally die supportlayer is already an asymmetric membrane on which a thin dense layer is deposited. Several methods have been developed to achieve this such as dip-coating, interfadal polymerisation, in-situ polymerisation and plasma polymerisation. 

Plasma etching is a new technique which allows the measurement of the thickness of the top layer in asymmetric and composite membranes. The uniformity of the structure in the... 

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