Amphipathic lipid

Amphipathic lipids spontaneously form a variety of structures when added to aqueous solution. All these structures form in ways that minimize contact between the hydrophobic lipid chains and the aqueous milieu. For example, when small amounts of a fatty acid are added to an aqueous solution, a mono-layer is formed at the air-water interface, with the polar head groups in contact with the water surface and the hydrophobic tails in contact with the air (Figure 9.2). Few lipid molecules are found as monomers in solution. 

Further addition of fatty acid eventually results in the formation of micelles. Micelles formed from an amphipathic lipid in water position the hydrophobic tails in the center of the lipid aggregation with the polar head groups facing outward. Amphipathic molecules that form micelles are characterized by a unique critical micelle concentration, or CMC. Below the CMC, individual lipid molecules predominate. Nearly all the lipid added above the CMC, however, spontaneously forms micelles. Micelles are the preferred form of aggregation in water for detergents and soaps. Some typical CMC values are listed in Figure 9.3. 

AMPHIPATHIC LIPIDS SELF-ORIENT AT OILrWATER INTERFACES... 

Lipids have the common property of being relatively insoluble in water (hydrophobic) but soluble in nonpolar solvents. Amphipathic lipids also contain one or more polar groups, making them suitable as constituents of membranes at lipidiwater interfaces. 

Cholesterol, an amphipathic lipid, is an important component of membranes. It is the parent molecule from which all other steroids in the body, including major hormones such as the adrenocortical and sex hormones, D vitamins, and bile acids, are synthesized. 

Lipoproteins Consist of a Nonpolar Core a Single Surface Layer of Amphipathic Lipids... 

Since nonpolar lipids are insoluble in water, for transport between the tissues in the aqueous blood plasma they are combined with amphipathic lipids and proteins to make water-miscible lipoproteins. 

Nutrients. Amphipathic lipids are used by cells to build membranes (see p. 214). Typical membrane lipids include phospholipids, glycolipids, and cholesterol. Fats are only weakly amphiphilic and are therefore not suitable as membrane components. 

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). 

Most lipids are barely soluble in water, and many have amphipathic properties. In the blood, free triacylglycerols would coalesce into drops that could cause fat embolisms. By contrast, amphipathic lipids would be deposited in the blood cells membranes and would dissolve them. Special precautions are therefore needed for lipid transport in the blood. While long-chain fatty acids are bound to albumin and short-chain ones are dissolved in the plasma (see p. 276), other lipids are transported in lipoprotein complexes, of which there several types in the blood plasma, with different sizes and composition. 

Lung surfactant is a mixture of proteins and amphipathic lipids that acts like a detergent or soap to greatly decrease the surface tension forces at the alveolar fluid-air interface. 

The bilayer is composed of amphipathic lipid molecules oriented according to their preferences for interaction with water. 

A. The three major types of amphipathic lipids found in membranes are the gly-cerophospholipids (also called phosphoglycerides), the sphingolipids, and cholesterol. 

DaTorre SD, Creer MH, Pogwizd SM, Corr PB Amphipathic lipid metabolites and their relation to arrhythmogenesis in the ischemic heart. J Mol Cell Cardiol 1991 23(suppl 1) 11-22. 

Depending on the precise conditions and the nature of the lipids, three types of lipid aggregates can form when amphipathic lipids are mixed with water (Fig. 11-4). Micelles are spherical structures that contain anywhere from a few dozen to a few thousand amphipathic molecules. These molecules are arranged with... 

FIGURE 11-4 Amphipathic lipid aggregates that form in water, (a) In... 

Membranes Membranes contain amphipathic lipid molecules that... 

The plasma membrane of epithelial cells, in common with other cell types, is selectively permeable, allowing the penetration of some substances but not others. The construction of the membrane from amphipathic lipid molecules forms a highly impermeable barrier to most polar and charged molecules, thereby preventing the loss of most water-soluble contents of the cell. This selective permeability presents a physical barrier to drag absorption, limiting absorption to specific routes and mechanisms, as described below (see Section 1.3.3). 

Figure 9.23 Two possible means of aggregation by amphipathic lipids. The micelle is formed by lysolecithin, for example, whereas the bilayer may be formed by lecithins, sphingomyelins, or cephalins.

Dolichol phosphate is an amphipathic lipid of the endoplasmic reticulum consisting of isoprenoid units and initiating the biosynthesis of the oligosaccharide portion of glycoproteins. 

Figure 111-43 Formation of micelles and monolayers by amphipathic lipid molecules in aqueous solution.

The phospholipid bilayer is the basic structure of all biological membranes. In addition to the phospholipids noted above a variety of others exist. Thus cardiolipin (in which a glycerol diester links two phosphatidates) is present in mitochondrial and bacterial membranes. Sphingosine is an amphipathic lipid having the structure ... 

Liposomes are vesicles in which an aqueous volume is entirely surrounded by a phospholipid membrane and their size can range between 30 and 50 nm up to several micrometers. They can consist of one (unilamellar) or more (multilamellar) homocentric bilayers of amphipathic lipids (mainly phospholipids). Based on their lamellarity (number of lamellae)—and size—they are characterized as SUVs/LUVs... 

An obvious hypothesis is that this unusual membrane lipid composition is related directly to membrane function in some way. Within the restricted area of lipid bilayers, lipid composition is known to be an important determinant of physical properties. There are several prominent examples. First, the temperature at which the hydrocarbon chains melt when assembled in bilayers (the gel-to-liquid-crystalline transition temperature, marks an abrupt change in many of the physical properties of such bilayer systems for example, water permeability through such bilayers increases by several orders of magnitude above the transition. Second, the presence of cholesterol within bilayers composed of amphipathic lipids has a profound effect on lipid motion, mechanical properties (such as resistance to shear), and permeability to water. 

A/ An is an amphipathic lipid containing a polar head group and... 

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