Bimolecular lipid layers

Phospholipids e.g. form spontaneously multilamellar concentric bilayer vesicles73 > if they are suspended e.g. by a mixer in an excess of aqueous solution. In the multilamellar vesicles lipid bilayers are separated by layers of the aqueous medium 74-78) which are involved in stabilizing the liposomes. By sonification they are dispersed to unilamellar liposomes with an outer diameter of 250-300 A and an internal one of 150-200 A. Therefore the aqueous phase within the liposome is separated by a bimolecular lipid layer with a thickness of 50 A. Liposomes are used as models for biological membranes and as drug carriers. 

Figure 8-4 Bimolecular lipid layers and membranes. (Top) A molecule of phosphatidylcholine. (Center) Lipid bilayer structure. (Bottom) Bilayer structure as seen by the electron microscope with osmium tetroxide staining.

The three fundamental lyotropic liquid crystal structures are depicted in Figure 1. The lamellar structure with bimolecular lipid layers separated by water layers (Figure 1, center) is a relevant model for many biological interfaces. Despite the disorder in the polar region and in the hydrocarbon chain layers, which spectroscopy reveals are close to the liquid states, there is a perfect repetition in the direction perpendicular to the layers. Because of this one-dimensional periodicity, the thicknesses of the lipid and water layers and the cross-section area per lipid molecule can be derived directly from x-ray diffraction data. 

Most of the properties attributed to living organisms (e.g., movement, growth, reproduction, and metabolism) depend, either directly or indirectly, on membranes. All biological membranes have the same general structure. As previously mentioned (Chapter 2), membranes contain lipid and protein molecules. In the currently accepted concept of membranes, referred to as the fluid mosaic model, membrane is a bimolecular lipid layer (lipid bilayer). The proteins, most of which float within the lipid bilayer, largely determine a membrane s biological functions. Because of the importance of membranes in biochemical processes, the remainder of Chapter 11 is devoted to a discussion of their structure and functions. 

As already mentioned, the generally accepted bimolecular leaflet model of the plasma membrane is that first proposed by Gorter and Grendel in 1925, which has dominated our thinking ever since [2]. Until then, our knowledge or the properties of bimolecular lipid layers was derived entirely from indirect experimental evidence. In 1961, the reconstitution of membranous structures from lipids of bovine brain was finally achieved [1-4]. These reconstituted membranes not only had a thickness ranging from 6 to 9 nm. 

The expected channel shown in Figure 12 is of a bimolecular structure. The rigid channel mouth may prohibit the consecutive long alkyl chains from assembling themselves and to prevent lipid molecules from invading the area. The space thus provided may accommodate water molecules to make the domain sufficiently hydrophilic to pass ions. Such a domain would recognize its counterpart located in another lipid layer to make a tail-to-tail dimer of 8, i.e. a symmetric transmembrane channel, as in the case of Gramicidin A dimer. ... 

It has long been established that all cell membranes in the body are composed of a fundamental structure called plasma membrane. This boundary surrounds single cells such as epithelial cells. More complex membranes such as intestinal epithelium and skin, are composed of multiples of this fundamental structure, which has been visualized as a bimolecular layer of lipid molecules with a monolayer of protein adsorbed into each surface. Cell membranes are further interspersed with small pores that can be protein line channels through the lipid layer or, simply, spaces between the lipid molecules. In membranes composed of many cells, the spaces between the cells constimte another kind of membrane pores (2). 

Membran systems are known to play an important role in functioning biological objects (in mass transfer processes, passive and active transport of substance, regulation of an endocellular metabolism, in bio-energetics, etc.). Unique properties of biomembranes are caused by their structure, in particular, presence of bimolecular focused layers of lipids. At the same time, one of the main disadvantages of modelling lipid membran systems (monolayers, flat bilayers, liposomes), is their low stability in time and to action of external factors. 

The prevalent view is that the gastrointestinal membrane consists of a bimolecular lipoid layer that is covered on each side by protein with the lipid molecule oriented perpendicular to the cell surface (Fig. 9.4). The lipid layer is interrupted by small, water-filled pores with a radius of approximately 4 A, and a molecule with a radius of 4 A or less may pass through these water-filled pores. Thus, membranes have a specialized transport system to assist the passage of water-soluble material and ions through the lipid interior, a process sometimes termed to as convective absorption. The rate of permeation of such small molecules through the pore is affected not only by the relative sizes of the holes and the molecules but also by the interaction between permeating molecules... 

Figure 8.1 Characteristic structure of lipids in the solid state. A fragment of the crystal is indicated showing the parallel arrangement of the chains and the packing of the polar heads in surface layers so that bimolecular unit layers are formed.

In aqueous solutions most phospholipids and glycolipids will assemble themselves into double layers in which the polar ends of the molecules are directed outwards and the long fatty acid chains are directed inwards as befits their amphiphilic nature (Figure 8.2b). Bimolecular lipid structures of this type are an essential feature of membrane structure. 

By far the most intensively studied phase transitions induced by temperature are those between different lamellar phases. This is so, because the bimolecular leaflets of the lamellar phases serve as biomembrane models (see above). Lamellar phases with various degrees of order of the hydrocarbon chains can occur. The high temperature lamellar phase (L -phase) is a so-called liquid-crystalline phase. This consists of bimolecular lipid leaflets separated by water layers of defined... 

Bimolecularly thick films formed at a pinhole (1 -2 mm) separating two aqueous solutions. The BLM is formed by painting the surfactant (or lipid), dissolved in a hydrocarbon solvent, across the pinhole and allowing it to thin to two layers of closely packed molecules which are apposed tail-to-tail. 

Although the present finding that BLM formed from simple lipids alone can possess intrinsic low yb without the presence of protein layers, it in no sense invalidates the bimolecular leaflet model. Our study does suggest, however, that natural lipids such as lecithin when in a bilayer configuration could exist in natural membranes to give the results of low yt observed by earlier workers, which has been thought essential in the development of the concept of the bimolecular lipo-protein model based upon interfacial tension measurements. 

The lipids exhibit a unique property of forming mono- or bimolecular layers, depending on the nature of the surrounding material or interfaces ... 

Biological membranes are by far the most important electrified interfaces in living systems. They consist of a bimolecular layer of lipids (the lipid bilayer) incorporating proteins. Lipid molecules are amphiphilic, that is, they consist of a hydro-phobic section (the hydrocarbon tail) and a hydrophilic section (the polar head). In biological membranes the two lipid monolayers are oriented with the hydrocarbon tails directed toward each other and the polar heads turned toward the aqueous solutions that are in contact with the two sides of the membrane. The resulting lipid bilayer is a matrix that incorporates different proteins performing a variety of functions. 

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