Lipid bimolecular leaflet

The phenomena of association colloids in which the limiting structure of a lamellar micelle may be pictured as composed of a bimolecular leaflet are well known. The isolated existence of such a limiting structure as black lipid membranes (BLM) of about two molecules in thickness has been established. The bifacial tension (yh) on several BLM has been measured. Typical values lie slightly above zero to about 6 dynes per cm. The growth of the concept of the bimolecular leaflet membrane model with adsorbed protein monolayers is traceable to the initial experiments at the cell-solution interface. The results of interfacial tension measurements which were essential to the development of the paucimolecular membrane model are discussed in the light of the present bifacial tension data on BLM. 

A solution of brain lipids was brushed across a small hole in a 5-ml. polyethylene pH cup immersed in an electrolyte solution. As observed under low power magnification, the thick lipid film initially formed exhibited intense interference colors. Finally, after thinning, black spots of poor reflectivity suddenly appeared in the film. The black spots grew rapidly and evenutally extended to the limit of the opening (5, 10). The black membranes have a thickness ranging from 60-90 A. under the electron microscope. Optical and electrical capacitance measurements have also demonstrated that the membrane, when in the final black state, corresponds closely to a bimolecular leaflet structure. Hence, these membranous structures are known as bimolecular, black, or bilayer lipid membranes (abbreviated as BLM). The transverse electrical and transport properties of BLM have been studied usually by forming such a structure interposed between two aqueous phases (10, 17). 

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 widespread interest in transport across membranes of living cells has led to studies of diffusion in lyotropic liquid crystals. Biological membranes are generally thought to contain single bimolecular leaflets of phospholipid material, leaflets which are like the large, flat micelles of lamellar liquid crystals. No effort is made here to review the literature on transport either across actual cell membranes or across single bimolecular leaflets (black lipid films) which have often been used recently as model systems for membrane studies. Instead, experiments where lamellar liquid crystals have been used as model systems are discussed. 

Fig. 3 Structural model of the cell membrane. The membrane is composed of a bimolecular leaflet of phospholipid with the polar head groups facing the extracellular and cytosolic compartments and the acyl groups in the middle of the bilayer. Integral membrane proteins are embedded in the lipid bilayer. Integral proteins are glycosylated on the exterior surface and may be phosphorylated on the cytoplasmic surface. Extrinsic membrane proteins, peripheral proteins, are linked to the cytosolic surface of the intrinsic proteins by electrostatic interactions. (From Ref. l)...

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. 

One of the regulatory structures in living organisms is the cell membrane. The simple bimolecular leaflet structure for the lipid portion of the membrane probably still is considered to form the main bulk of the mammalian membrane, but the concept of a more flexible and mobile structure has been suggested by Lucy and others [340]. While the current views on membrane morphology are constantly shifting and recent texts should be consulted, it is salutory to consider work which has discussed the micellar nature of membrane lipids. Much of this is speculative but illustrates one approach to the problem. 

On the basis of X-ray diffraction studies, the ultrastructure of chloroplast membrane has been analyzed by Kreutz - s, and the subject reviewed by him. Using the freeze-etching technique, Muehlethaler has suggested a picture for the thylakoid membrane based upon the bimolecular leaflet model. The salient feature in all these proposed models lies in their oriented bilayer lipid core, onto which other important cellular constituents, such as proteins and pigments, may interact through either ionic or van der Waals attraction, or both. Muehlethaler s interpretation is of special interest, in view of the experiments using the pigmented... 

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... 

Biological membranes arc composed of lipids and proteins, the lipids building up the bimolecular leaflet, which is the major permeability barrier, and the proteins providing the essential biological functions, The proteins can be either intrinsic proteins embedded in the lipid biiayers and having accessible polar surfaces on both sides of the bilayer, or they are mainly bound to the bilayer surface by electrostatic interactions. When proteins are incorporated into lipid bilayers or bound to the surface of the lamellae, the lipid packing in the gel phase is perturbed and the transition profiles obtained by DSC are changed in a characteristic way. Numerous studies of lipid-protein interactions have been performed in the past and the effects on the DSC transition curves have been modeled [99-104]. 

Implicit in the bimolecular leaflet model is the idea that the protein is spread as an extended sheet ( 8-conformation) over the ionic heads of the phospholipids and that the binding between phospholipid and protein is essentially electrostatic. A number of observations about membrane lipids and proteins are not consistent with this model as it stands. 

Alternatively, the strongly opposed preferences of the hydrophilic and hydrophobic moieties of membrane lipids can be satisfied by forming a lipid bilayer, composed of two lipid sheets (Figure 12,10). A lipid bilayer is also called a bimolecular sheet. The hydrophobic tails of each individual sheet interact with one another, forming a hydrophobic interior that acts as a permeability barrier. The hydrophilic head groups interact with the aqueous medium on each side of the bilayer. The two opposing sheets are called leaflets. 

Haydon, D.A. Taylor, J.M. (1963) The Stability and Properties of Bimolecular Lipid Leaflets in Aqueous Solutions , Journal of Theoretical Biology, 4, 281-96 Heckels, J.E., Archibald, A.R. Baddiley, J. (1975) Studies on the Linkage Between... 

Although the structural hypothesis of the molecular conformation of biological membranes was introduced over five decades ago (Gorter and Grendel, 1925), we still do not know the real structure of the cell membrane. Early research into membrane structure dates back to 1895, when Overton proposed the existence of lipid components in biological membranes. On the basis of experiments with human red cells, Gorter and Grendel (1925) concluded that the lipid was spread over the red cell surface in a bimolecular layer, with hydrophobic tails directed toward the center of the lipid leaflet and polar, hydrophilic heads on the surface. 

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