Membrane lipids, physical properties

This idea of a membrane as a bubble is of course highly simplified, and the real cell membrane is not only a simple surfactant bilayer but also a complex structure composed of many different lipid molecules, cholesterol, and proteins. However, despite the bilayer s complicated composition, the surfactant-like lipids largely define the cell membrane s physical properties. 

The lipids found in biological systems are either hydrophobic (containing only nonpolar groups) or amphipathic, which means they possess both polar and nonpolar groups. The hydrophobic nature of lipid molecules allows membranes to act as effective barriers to more polar molecules. In this chapter, we discuss the chemical and physical properties of the various classes of lipid molecules. The following chapter considers membranes, whose properties depend intimately on their lipid constituents. 

Cullis PR, Fenske DB, Fiope MJ Physical properties and functional roles of lipids in membranes. In Biochemistry of Lipids, Lipoproteins and Membranes. Vance DE, Vance JE (editors). Elsevier, 1996. 

The lipid molecule is the main constituent of biological cell membranes. In aqueous solutions amphiphilic lipid molecules form self-assembled structures such as bilayer vesicles, inverse hexagonal and multi-lamellar patterns, and so on. Among these lipid assemblies, construction of the lipid bilayer on a solid substrate has long attracted much attention due to the many possibilities it presents for scientific and practical applications [4]. Use of an artificial lipid bilayer often gives insight into important aspects ofbiological cell membranes [5-7]. The wealth of functionality of this artificial structure is the result of its own chemical and physical properties, for example, two-dimensional fluidity, bio-compatibility, elasticity, and rich chemical composition. 

Further progress may derive from a more accurate definition of the chemical and physical properties of the humic substances present at the rhizosphere and how they interact with the root-cell apoplast and the plasma membrane. An interaction with the plasma membrane H -ATPase has already been observed however this master enzyme may not be the sole molecular target of humic compounds. Both lipids and proteins (e.g., carriers) could be involved in the regulation of ion uptake. It therefore seems necessary to investigate the action of humic compounds with molecular approaches in order to understand the regulatory aspects of the process and therefore estimate the importance of these molecules as modulators of the root-soil interaction. 

Gruszecki, W.I. and K. Strzalka. 2005. Carotenoids as modulators of lipid membrane physical properties. Biochim. Biophys. Acta 1740 108-115. 

Subczynski, W. K., J. Widomska, and J. B. Feix. 2009. Physical properties of lipid bilayers from EPR spin labeling and their influence on chemical reactions in a membrane environment. Free Radic. Biol. Med., 46, 707-718. 

Subczynski, W. K. and A. Wisniewska. 1998. Effects of P-carotene on physical properties of lipid membranes-comparison with effects of polar carotenoids. Curr. Top. Biophys. 22 44—51. 

Widomska, J., M. Raguz, J. Dillon, E. R. Gaillard, and W. K. Subczynski. 2007. Physical properties of the lipid bilayer membrane made of calf lens lipids EPR spin labeling studies. Biochim. Biophys. Acta 1768 1454-1465. 

Subczynskil, W.K. and Wisniewska, A. (2000) Physical properties of lipid bilayer membranes relevance to membrane biological functions. Acta Biochimica Polonica, 47 (3), 613-625. 

We thank Dr. M. Hirano at Toray Research Center, Kamakura for providing us with the purified PS II. We also thank Dr. H. Minamikawa for the synthesis of Maln(Phyt)2 and Dr. K. Kawasaki for the FFEM experiments. This work was performed as a part of a R D project of AIST (subject Physical properties of membrane protein/lipid assemblies) and of International Joint Research Program FY 2001-2002 supported by NEDO. 

The first decision to be made in designing an experiment to measure the motional properties of membrane lipids concerns the type of probe molecule. Too often, this choice is made from the point of view of convenience or tradition rather than suitability, although there is now a considerable range of suitable fluorophores from which to choose. The second consideration is the type of measurement to be made. The most detailed and complete motional information is obtained from a time-resolved fluorescence anisotropy measurement which is able to separate the structural or orientational aspects from the dynamic aspects of fluorophore motion. Steady-state anisotropy measurements, which are much easier to perform, provide a more limited physical parameter relating to both of these aspects. 

Natural biological membranes consist of lipid bilayers, which typically comprise a complex mixture of phospholipids and sterol, along with embedded or surface associated proteins. The sterol cholesterol is an important component of animal cell membranes, which may consist of up to 50 mol% cholesterol. As cholesterol can significantly modify the bilayer physical properties, such as acyl-chain orientational order, model membranes containing cholesterol have been studied extensively. Spectroscopic and diffraction experiments reveal that cholesterol in a lipid-crystalline bilayer increases the orientational order of the lipid acyl-chains without substantially restricting the mobility of the lipid molecules. Cholesterol thickens a liquid-crystalline bilayer and increases the packing density of lipid acyl-chains in the plane of the bilayer in a way that has been referred to as a condensing effect. 

Some workers have suggested that the lauryl chain is of intrinsic biological importance in relation to its ability to disrupt lipid bilayers, having the optimal physical properties of lipophilicity and size, but as C12 compounds are also maximally irritant to the skin (28) where simple lipoidal barrier membranes are probably not involved, other factors are no doubt implicated. Dominguez al. (29) have considered Schott s (26) approach to the biological uniqueness of the dodecyl chain, but have postulated that its properties of skin penetration are related to the conformation of the chain, especially when adsorbed to or interacting with protein. Dominguez e postulate that... 

The phospholipid molecules are such that in aqueous media they spontaneously form extended bilayers with a hydrophobic core. Although membrane proteins vary enormously they all form compact structures. This minimises the surface of interaction with the lipid, so that, although protein may account for 30-80% of the weight of the membrane, it does not affect the basic physical properties of the lipid bilayer. 

Prades, J, Vogler, O, Alemany, R, Gomez-florit, M. Funari, S.S Ruiz-Gutierrez, V. Barcelo, F. ((2011). Plant pentacyclic triterpenic acids as modulators of lipid membrane physical properties Biochimica et Biophysica Acta (BBA)- Biomenbranes., 1808... 

An important approach to the study of biological membranes has been the preparation and study of model membranes. According to current usage, model membranes include lipid bilayers and lipid bilayers into which have been incorporated additional components such as one or more membrane proteins. It is through the study of such model membranes that one has the best opportunity to isolate and study fundamental physical chemical and biophysical processes, and it is for this reason that the present report emphasizes these systems. A discussion of model membranes necessarily starts with a description of the chemical compositions and physical properties of lipid molecules. 

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