Amphipathic molecules phospholipids

The separation of oil and water (B) can be prevented by adding a strongly amphipathic substance. During shaking, a more or less stable emulsion then forms, in which the surface of the oil drops is occupied by amphipathic molecules that provide it with polar properties externally. The emulsification of fats in food by bile acids and phospholipids is a vital precondition for the digestion of fats (see p.314). 

The plasma lipoproteins include chylomicrons, very-low-density lipoproteins (VLDL), low-density lipoproteins (LDL), and high-density lipoproteins (HDL). They function to keep lipids (primarily triacylglyc-erol and cholesteryl esters) soluble as they transport them between tissues. Lipoproteins are composed of a neutral lipid core (containing triacylglycerol, cholesteryl esters, or both) surrounded by a shell of amphipathic apolipoproteins, phospholipid, and nonesterified cholesterol. Chylomicrons are assembled in intestinal mucosal cells from dietary lipids (primarily, triacylglycerol) plus additional lipids synthesized in these cells. Each nascent chylomicron particle has one molecule of apolipoprotein B-48 (apo B-48). They are released from the cells into the lymphatic system and travel to the blood, where they receive apo C-ll and apo E from HDLs, thus making the chylomicrons functional. Apo C-ll activates lipoprotein lipase, which degrades the... 

Lipid synthesis is unique in that it is almost exclusively localized to the surface of membrane structures. The reason for this restriction is the amphipathic nature of the lipid molecules. Phospholipids are biosynthesized by acylation of either glycerol-3-phosphate or dihydroxyacetone phosphate to form phosphatidic acid. This central intermediate can be converted into phospholipids by two different pathways. In one of these, phosphatidic acid reacts with CTP to yield CDP-diacylglycerol, which in bacteria is converted to phosphatidylserine, phosphatidylglycerol, or diphos-... 

Changes in cholesterol content. A third type of intrinsic change involves alteration in the amount of cholesterol in a membrane (Robertson and Hazel, 1997). Cholesterol can be incorporated into a membrane up to an approximately one-to-one ratio with phospholipids. Most membrane-localized cholesterol is found in the plasma membrane. Cholesterol is an amphipathic molecule, that is, different regions of the molecule have affinities for either polar or nonpolar environments (figure 7.19). In a membrane, the flexible alkyl tip of the molecule penetrates into the bilayer the 3-/1-hydroxyl group remains near the surface of the membrane, near the ester linkages between the acyl chains and the glycerol moiety. 

During the surge in interest in the phosphatidylinositol derivatives, Takai et al. (1979) noted that an unsaturated diacylglycerol diminished the Ca2+ and phospholipid concentration required for complete activation of a Ca2+-activated, phospholipid-dependent protein kinase (now known as protein kinase C). In this latter system, phosphatidylserine was most active (as the required phospholipid), with phosphatidylethanolamine and phosphatidyl inositol much less effective and phosphatidylcholine without any activity. Probably the enzyme was activated by an amphipathic molecule bearing a net negative charge, and any available cellular phospholipid mixture with the requisite surface change served this purpose. 

Figure 1. A schematic representation of the cross section of the lipid-globular protein mosaic model of membrane structure. The globular proteins (with dark lines denoting the polypeptide chain) are amphipathic molecules with their ionic and highly polar groups exposed at the exterior surfaces of the membranes the degree to which these molecules are embedded in the membrane is under thermodynamic control. The bulk of the phospholipids (with filled circles representing their polar head groups and thin wavy lines their fatty acid chains) is organized as a discontinuous bilayer.

Phospholipids are amphipathic molecules, that is, they have parts of different polarity. The fatty acyl chains are nonpolar and hydrophobic whereas the phosphoryl alcohol head group is polar and can be solvated by H20. Phospholipids form bimolecular membranes in which the hydrophobic fatty acyl chains are located in the interior of the membrane (away from H20) and the head groups are on the surface (on either side of the membrane) and exposed to H20. Representing phospholipids as =0 (where = represents the fatty acyl chain and O the head group), we can represent such a phospholipid bilayer thus ... 

Phospholipids and sphingolipids are major components of cell membranes. They are amphipathic molecules that is, one portion of the molecule is hydrophilic and associates with H20, and another portion contains the hydrocarbon chains derived from fatty acids, which are hydrophobic and associate with lipids (see Figure 6-4). 

Introduction - Liposomes are vesicles composed of one or more lipid bilayers completely surrounding an internal aqueous space. They are usually composed of phospholipids either in pure form or In combination with other amphipathic molecules such as sterols, long chain bases or acids, or membrane proteins. The structure of liposomes varies from large (0.5->5y) multllamellar vesicles to small ( 300 A) unilamellar vesicles.2,3 More recently, new methods have been reported describing the formation of unilamellar vesicles of intermediate size. >5.6 xhe general properties of liposomes and their interaction with various macromolecules have been described in several reviews. ... 

Detergents, phospholipids and sphingolipids are alike in that they are amphipathic molecules. In a polar solvent like water they will aggregate so that their nonpolar portions are away from the solvent. 

Phospholipids are amphipathic molecules that play important roles in living organisms as membrane components, emulsifying agents, and surface active agents. There are two types of phospholipids phosphoglyc-erides and sphingomyelins. 

MEMBRANE LIPIDS When amphipathic molecules are suspended in water, they spontaneously rearrange into ordered structures (Figure 11.6). As these structures form, hydrophobic groups become buried in the water-depleted interior. Simultaneously, hydrophilic groups become oriented so that they are exposed to water. Phospholipids form into bimolecular layers when sufficiently concentrated. This property of phospholipids (and other amphipathic lipid molecules) is the basis of membrane structure. 

Phospholipids are amphipathic molecules with a hydro-phobic tail (often two fatty acyl chains) and a hydrophilic head (see Figure 2-19). 

Some cell-surface proteins are anchored to the phospholipid bllayer not by a sequence of hydrophobic amino acids but by a covalently attached amphipathic molecule, gfyco sylphosphatidylinositol (GPI) (Figure 16-14a). These proteins are synthesized and initially anchored to the ER membrane exactly like type I transmembrane proteins, with a cleaved N-terminal signal sequence and Internal stop-transfer anchor sequence directing the process (see Figure... 

A FIGURE 18-4 Phospholipid synthesis. Because phospholipids are amphipathic molecules, the last stages of their multistep synthesis take place at the interface between a membrane and the cytosol and are catalyzed by membrane-associated enzymes. Step D Fatty acids from fatty acyl CoA are esterified to the phosphorylated glycerol backbone, forming phosphatidic acid, whose two long hydrocarbon chains anchor... 

Cell membranes are constructed principally of phospholipids, sphingolipids, and proteins, together with small amounts of cholesterol. The common feature of the membrane lipids is their similarity to soap and detergent molecules—the presence of both hydrophilic and hydrophobic portions in the same molecule. Such molecules are referred to as amphipathic molecules. 

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