They Form Membranes, Micelles, Liposomes, & Emulsions

In general, lipids are insoluble in water since they contain a predominance of nonpolar (hydrocarbon) groups. How ever, fatty acids, phospholipids, sphingolipids, bile salts, and, to a lesser extent, cholesterol, contain polar groups. Therefore, a part of the molecule is hydrophobic, or water insoluble, and a part is hydrophilic, or water soluble. Such molecules are described as amphipathic (Figure 1). They become oriented at oil–water interfaces with the polar group in the water phase and the nonpolar group in the oil phase. A bilayer of such amphipathic lipids is the basic structure in biologic membranes. When a critical concentration of these lipids is present in an aqueous medium, they form micelles, spherical particles in which the hydrophobic tails of the molecules are inside while the hydrophilic heads face the water. Liposomes consist of spheres of lipid bilayers that enclose part of the aqueous medium. They may be formed by sonicating an amphipathic lipid in an aqueous medium. Aggregation of bile salts into micelles and the formation of mixed micelles with the products of fat digestion are important in facilitating absorption of lipids from the intestine. Liposomes are of potential clinical use—particularly when combined with tissue-specific antibodies—as carriers of drugs in the circulation, targeted to specific organs, for example, in cancer therapy. In addition, they are used for gene trans fer into vascular cells and as carriers for topical and trans dermal delivery of drugs and cosmetics. Emulsions are much larger particles, formed usually by nonpolar lipids in an aqueous medium. These are stabilized by emulsifying agents such as amphipathic lipids (eg, phosphatidyl choline), which form a surface layer separating the main bulk of the nonpolar material from the aqueous phase (see Figure 1) .

Fig1. Formation of lipid membranes, micelles, emulsions, and liposomes from amphipathic lipids, for example, phospholipids.

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مواضيع ذات صلة

Oxidation of Fatty Acids Occurs in Mitochondria

Lipid Peroxidation is A Source of Free Radicals

Steroids Play Many Physiologically Important Roles

Glycolipids (Glycosphingolipids) are Important in Nerve Tissue & in the Cell Membrane

Phospholipids are The Main Lipid Constituents of Membranes

Fatty Acids are Aliphatic Carboxylic Acids

The Pentose Phosphate Pathway & Other Pathways of Hexose Metabolism: clinical aspects

Galactose is Needed for the Synthesis of Lactose, Glycolipids, Proteoglycans, & Glycoproteins

Ingestion of Large Quantities of Fructose has Profound Metabolic Consequences

Reactions of the Pentose Phosphate Pathway Occur in the Cytosol

Blood Glucose is Derived From the Diet, Gluconeogenesis, & Glycogenolysis

Glycolysis & gluconeogenesis share the same pathway but in opposite directions, & are reciprocally regulated