Biomembrane structure, phospholipid bilayer

A phospholipid bIlayer can be of almost unlimited size— from micrometers ( xm) to millimeters (mm) in length or width—and can contain tens of millions of phospholipid molecules. Because of their hydrophobic core, bilayers are virtually Impermeable to salts, sugars, and most other small hydrophilic molecules. The phospholipid bilayer is the basic structural unit of nearly all biological membranes thus, although they contain other molecules (e.g., cholesterol, gly-collplds, proteins), biomembranes have a hydrophobic core that separates two aqueous solutions and acts as a permeability barrier. The structural organization of biomembranes and the general properties of membrane proteins are described In Chapter 5. 

In a phospholipid bilayer, which constitutes the basic structure of all biomembranes, fatty acyl chains in each leaflet are oriented toward one another, forming a hydrophobic core, and the polar head groups line both surfaces and directly interact with the aqueous solution. 

The phospholipid bilayer, the basic structural unit of all biomembranes, is a two-dimensional lipid sheet with hydrophilic faces and a hydrophobic core, which is impermeable to water-soluble molecules and ions (see Figure 5-2). 

The phospholipid bilayer, the basic structural unit of biomembranes, is essentially impermeable to most water-soluble molecules, ions, and water itself. After describing the factors that influence the permeability of lipid membranes, we briefly compare the three major classes of membrane proteins that Increase the permeability of biomembranes. We then examine operation of the simplest type of transport protein to Illustrate basic features of protein-mediated transport. Finally, two common experimental systems used in studying the functional properties of transport proteins are described. 

For decades, colloid and surface scientists have known that amphiphilic molecules such as phospholipids can self-assemble or self-organize themselves into supramolecular structures of bilayer lipid membranes (planar BLMs and spherical liposomes), emulsions, and micelles [2-4]. As a matter of fact, our current understanding of the structure and function of biomembranes can be traced to the studies of these experimental systems such as soap films and Langmuir monolayers, which have evolved as a direct consequence of applications of classical principles of colloid and interfacial chemistry. As already mentioned in Section I, the seminal work on the self-assembly of planar lipid bilayers and bilayer or black lipid membranes was carried out in 1959-1963. The idea started while one of the authors was reading a paperback edition of Soap Bubbles by C. 

The central role of biomembranes in cellular function underlines the importance of membrane research, an area that is by its very nature highly interdisciplinary and ranges from molecular biology to physical chemistry. At its core, however, is an essentially supramolecular interaction, the hydrophobic effect, which causes phospholipid amphiphiles to self-assemble into bilayers and then into complex closed compartments. The elegant self-assembly process that forms these remarkably large structures is an area of keen interest in its own right, but much recent study into self-assembled membranes aims to replicate the functions of biomembranes. Indeed, for cells, phospholipid bilayers are more than just delimiting boundaries they are... 

Over the past 30 years, rapid progress has been made applying supramolecular principles to biomembrane research, and this chapter has endeavored to provide a flavor of key developments in the area. Yet much is left to discover. Many cellular processes occurring in phospholipid bilayers are currently poorly understood, but are yielding to better analytical techniques and improved structural characterization of membrane proteins. These advances will doubtlessly uncover previously unsuspected modes of biomembrane behavior, where stripped-down bionfimetic systems will be... 

Relatively few single crystal structures have been obtained of phospholipids, but those which are known reveal the same major structural features. The results are of direct relevance to biomembrane structure since phospholipids crystallize in bilayers and the characteristic features of the molecular structure are preserved in fully hydrated, fluid phospholipid bilayers. 

Abstract To understand how membrane-active peptides (MAPs) function in vivo, it is essential to obtain structural information about them in their membrane-bound state. Most biophysical approaches rely on the use of bilayers prepared from synthetic phospholipids, i.e. artificial model membranes. A particularly successful structural method is solid-state NMR, which makes use of macroscopically oriented lipid bilayers to study selectively isotope-labelled peptides. Native biomembranes, however, have a far more complex lipid composition and a significant non-lipidic content (protein and carbohydrate). Model membranes, therefore, are not really adequate to address questions concerning for example the selectivity of these membranolytic peptides against prokaryotic vs eukaryotic cells, their varying activities against different bacterial strains, or other related biological issues. 

In the fluid state, the lateral diffusion coefficient of lipids in the bilayer structure is 0( 10 1 ) m2 s-1 (the symbol O is used to indicate order of magnitude). Interestingly, it has been shown that the diffusion coefficients of phospholipids may differ greatly from the inner to the outer leaflet of the biomembrane layer [4,5]. Again, this is related to the differences in chemical... 

Phosphatidylcholines are the most prominent components of biological membranes and therefore often serve as model biomembrane systems in biophysical studies.Due to their amphiphathic character, phospholipids have a strong tendency to spontaneously form bilayer structures when... 

A typical biomembrane consists largely of amphiphilic lipids with small hydrophilic head groups and long hydrophobic fatty acid tails. These amphiphiles are insoluble in water (<10 1° mol L 1) and capable of self-organization into ultrathin bilayer lipid membranes (BLMs). Until 1977 only natural lipids, in particular phospholipids like lecithins, were believed to form spherical and related vesicular membrane structures. Intricate interactions of the head groups were supposed to be necessary for the self-organization of several ten thousands of... 

Supercritical fluid CO2 has been proposed for the encapsulation of hydrophilic and hydrophobic therapeutic agents into liposomes (144-149). Liposomes are closed bilayer structures that enclose an aqueous volume. These vesicles, comprising single or multiple bilayers, are typically formed from phospholipids. On the basis of their biological components and structure, liposomes are model cell membranes and artificial biomembranes. Their similarities to cellular membranes can be exploited for the parental administration of pharmaceuticals, an area of intense research since the 1970s (146). The presence of both hydrophilic regions of the vesicle (in the aqueous core)... 

Phospholipids Associate Noncovalently to Form the Basic Bilayer Structure of Biomembranes... 

Phospholipids of the composition present in cells spontaneously form sheetlike phospholipid hilayers, which are two molecules thick. The hydrocarbon chains of the phospholipids in each layer, or leaflet, form a hydrophobic core that is 3-4 nm thick in most biomembranes. Electron microscopy of thin membrane sections stained with osmium tetroxide, which binds strongly to the polar head groups of phospholipids, reveals the bilayer structure (Figure 5-2). A cross section of all single membranes stained with osmium tetroxide looks like a railroad track two thin dark lines (the stain-head group complexes) with a uniform light space of about 2 nm (the hydrophobic tails) between them. 

C. butyricum appears to regulate the stability of the bilayer arrangement of membranes by altering the ratio of ether versus acyl ethanolamine phospholipids in response to changes in the degree of lipid unsaturation of the membranes. Experiments with bacteria indicate that substitution of plasmenylethanolamine for phosphatidylethanolamine in biomembranes would have only small effects on lipid melting transitions, whereas the tendency to form non-lamellar lipid structures would be significantly increased. 

A biological membrane is a structure particularly suitable for study by the LB technique. The eukaryotic cell membrane is a barrier that serves as a highway and controls the transfer of important molecules in and out of the cell (Roth etal., 2000). The cell membrane consists of a bilayer or a two-layer LB film (Tien etal, 1998). Lipid bilayers are composed of a variety of amphiphilic molecules, mainly phospholipids and sterols which in turn consist of a hydrophobic tail, and a hydrophilic headgroup. The complexity of the biomembrane is such that frequently simpler systems are used as models for physical investigations. They are based on the spontaneous self-organization of the amphiphilic lipid molecules when brought in contact with an aqueous medium. The three most frequently used model systems are monolayers, black lipid membranes, and vesicles or liposomes. 

This technique is based on the large chemical shift anisotropy exhibited by lipid phosphorus. In the presence of proton decoupling, this anisotropy results in characteristically broad spectra whose shape and width depend on phospholipid motion in turn, this is related to the lipid phase. In large bilayer structures (radius greater than 200 nm), as is the case for most biomembrane preparations, the only possibility of motion at the NMR time scale (10 s) is the rotation of the phospholipid molecules about their long axis. The result is a very broad spectrum with a low-field... 

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