Biological membrane thickness

As a point of interest, it is possible to form very thin films or membranes in water, that is, to have the water-film-water system. Thus a solution of lipid can be stretched on an underwater wire frame and, on thinning, the film goes through a succession of interference colors and may end up as a black film of 60-90 A thickness [109]. The situation is reminiscent of soap films in air (see Section XIV-9) it also represents a potentially important modeling of biological membranes. A theoretical model has been discussed by Good [110]. 

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. 

Ion-selective bulk membranes are the electro-active component of ion-selective electrodes, which sense the activity of certain ions by developing an ion-selective potential difference according to the Nernst equation at their phase boundary with the solution to be measured. The main differences to biological membranes are their thickness and their symmetrical structure. Nevertheless they are used as models for biomembranes. 

Ion-selective bulk membranes are the electro-active component of ion-selective electrodes. They differ from biological membranes in many aspects, the most marked being their thickness which is normally more then 105 times greater, therefore electroneutrality exists in the interior. A further difference is given by the fact that ion-selective membranes are homogeneous and symmetric with respect to their functioning. However, because of certain similarities with biomembranes (e.g., ion-selectivity order, etc.) the more easily to handle ion-selective membranes were studied extensively also by many physiologists and biochemists as model membranes. For this reason research in the field of bio-membranes, and developments in the field of ion-selective electrodes have been of mutual benefit. 

The use of Upid bilayers as a relevant model of biological membranes has provided important information on the structure and function of cell membranes. To utilize the function of cell membrane components for practical applications, a stabilization of Upid bilayers is imperative, because free-standing bilayer lipid membranes (BLMs) typically survive for minutes to hours and are very sensitive to vibration and mechanical shocks [156,157]. The following concept introduces S-layer proteins as supporting structures for BLMs (Fig. 15c) with largely retained physical features (e.g., thickness of the bilayer, fluidity). Electrophysical and spectroscopical studies have been performed to assess the appUcation potential of S-layer-supported lipid membranes. The S-layer protein used in aU studies on planar BLMs was isolated fromB. coagulans E38/vl. 

The thickness of the membrane phase can be either macroscopic ( thick )—membranes with a thickness greater than micrometres—or microscopic ( thin ), i.e. with thicknesses comparable to molecular dimensions (biological membranes and their models, bilayer lipid films). Thick membranes are crystalline, glassy or liquid, while thin membranes possess the properties of liquid crystals (fluid) or gels (crystalline). 

Carotenoids are hydrophobic molecules and thus are located in lipophilic sites of cells, such as bilayer membranes. Their hydrophobic character is decreased with an increased number of polar substitutents (mainly hydroxyl groups free or esterified with glycosides), thus affecting the positioning of the carotenoid molecule in biological membranes. For example, the dihydroxycarotenoids such as LUT and zeaxanthin (ZEA) may orient themselves perpendicular to the membrane surface as molecular rivet in order to expose their hydroxyl groups to a more polar environment. In contrast, the carotenes such as (3-C and LYC could position themselves parallel to the membrane surface to remain in a more lipophilic environment in the inner core of the bilayer membranes (Parker, 1989 Britton, 1995). Thus, carotenoid molecules can have substantial effects on the thickness, strength, and fluidity of membranes and thus affect many of their functions. 

Biological membranes, such as those surrounding cells, are made up of lipids and proteins exhibiting a more or less ordered arrangement. These membranes are about 70 A thick and allow ions and nutrients to... 

Let us first consider the lipid molecular structures required. First is the hydrophobic matching. The length of the hydrophobic chain determines the thickness of the hydrophobic part of the lipid bilayer, this should correspond closely to the dimension of the native membrane. As most biological membranes contain diacylglycerol lipids with hydrophobic chain lengths of 16 18 carbon atoms. Thus, synthetic lipids should possess relatively long hydrocarbon chain length, e.g., 16-18 carbon atoms. 

All biological membranes are constructed according to a standard pattern. They consist of a continuous bilayer of amphipathic lipids approximately 5 nm thick, into which proteins are embedded, in addition, some membranes also carry carbohydrates (mono- and oligosaccharides) on their exterior, which are bound to lipids and proteins. The proportions of lipids, proteins, and carbohydrates differ markedly depending on the type of cell and membrane (see p. 216). 

Closed bilayer aggregates, formed from phospholipids (liposomes) or from surfactants (vesicles), represent one of the most sophisticated models of the biological membrane [55-58, 69, 72, 293]. Swelling of thin lipid (or surfactant) films in water results in the formation of onion-like, 1000- to 8000-A-diameter multilamellar vesicles (MLVs). Sonication of MLVs above the temperature at which they are transformed from a gel into a liquid (phase-transition temperature) leads to the formation of fairly uniform, small (300- to 600-A-diameter) unilamellar vesicles (SUVs Fig. 34). Surfactant vesicles can be considered to be spherical bags with diameters of a few hundred A and thickness of about 50 A. Typically, each vesicle contains 80,000-100,000 surfactant molecules. 

I wish to respond to Professor Ubbelohde s question regarding what thinness, per se, of biological membranes could be important. For ion movements across membranes as mediated catalytically by carriers or channels, the thinness permits local deviations from the electroneutrality constraint that, for example, enables neutral molecules to carry cations across the membrane as charged species, leaving their counterions behind in the aqueous solutions. This is not possible when the thickness of the system becomes large. 

When the concentration in the donor compartment remains constant during the experiment, the concentration in the receptor compartment increases regularly at first, then attains a plateau. For an ideal system with the thickness of a biological membrane a maximum increase in concentration of 130 times can be predicted by computer simulation. This system gives an experimental physicochemical example of a transformation of scalar chemical energy into a vectorial catalysis effect. ... 

Dr. Thomas drew our attention to the fact that, although biological membranes are thin in comparison to his preparations, their diffusion constants may be long. I have measured permeability coefficients (transmembrane "diffusion ) for a number of solutes for artificial phospholipid bilayers (liposomes). The values follow and are to be compared for calculated permeabilities for a solute diffusion across a comparable thickness of water. 

I can easily understand a thickness of the order of 50 to 100 ptm for the unstirred diffusion layers of the flat and thin macromembranes you discussed. In microsystems such as mitochondria with diameters around a few p.m these unstirred layers must, however, be considerably smaller. Would you please comment on this substantial difference between the model membranes you studied and actual biological membrane systems ... 

FIGURE 11-1 Biological membranes. Viewed in cross section, all cell membranes share a characteristic trilaminar appearance. When an erythrocyte is stained with osmium tetroxide and viewed with an electron microscope, the plasma membrane appears as a three-layer structure, 5 to 8 nm (50 to 80 A) thick. The trilaminar image consists of two electron-dense layers (the osmium, bound to the inner and outer surfaces of the membrane) separated by a less dense central region. 

Biological membranes are constructed of lipid bilayers 3 nm (30 A) thick, with proteins protruding on each side. The hydrocarbon core of the membrane, made up of the —CH2— and —CH3 of the fatty acyl groups, is about as nonpolar as decane, and liposomes formed in the laboratory from pure lipids are essentially impermeable to polar solutes, as are biological membranes (although the latter, as we shall see, are permeable to solutes for which they have specific transporters). 

Length of a Fatty Acid Molecule The carbon carbon bond distance for single-bonded carbons such as those in a saturated fatty acyl chain is about 1.5 A. Estimate the length of a single molecule of palmitate in its fully extended form. If two molecules of palmitate were placed end to end, how would their total length compare with the thickness of the lipid bilayer in a biological membrane ... 

The model system for biological membranes is the so-called artificial lipid membrane made of two monolayers of lipid between two (aqueous) electrolyte solutions. The thickness of such a membrane can vary over a wide range and this introduces some ambiguity in deciding what kind of model should be the basis for the mathematical description of ion transport and other properties. Two extreme concepts can be distinguished. 

The peptide group can be fixed at the ice lattice with p = 0127. Ice-like distances of the H-bond acceptors are in different molecules for example Biotin. Thymine, Triglyceride, 1,4-Quinone etc.141. Hechter gave a model for biologic membranes too142. In the membrane surface Hechter assumes a water double hydrate lamella of 4.9 A thickness in which K+ could be placed, but Na+ would be too big. 

Carlson EC, Audette JL, Veitenheimer NJ, Risan JA, Latumus DI, Epstein PN. Ultrastructural morphometry of capillary basement membrane thickness in normal and transgenic diabetic mice. Anatomical Record Part A Discoveries in Molecular, Cellular, and Evolutionary Biology 2003, 271, 332-341. 

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