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You are watching: Why do phospholipids form a bilayer in water?

Berg JM, Tymoczko JL, Stryer L. Biochemistry. 5th edition. New York: W H Freeman; 2002.


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What properties enable phospholipids to kind membranes? Membrane development is a repercussion of the amphipathic nature of the molecules. Their polar head teams favor contact with water, whereas their hydrocarbon tails connect with one another, in preference to water. How have the right to molecules v these preferences arrange us in aqueous solutions? One means is to kind a micelle, a globular framework in which polar head groups are surrounded by water and hydrocarbon tails room sequestered inside, connecting with one one more (Figure 12.9).


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Figure 12.9

Diagram the a ar of a Micelle. Ionized fat acids readily kind such structures, however most phospholipids do not.


Alternatively, the strong opposed choices of the hydrophilic and hydrophobic moieties the membrane lipids deserve to be to solve by forming a lipid bilayer, created of 2 lipid sheets (Figure 12.10). A lipid double layer is also called a bimolecular sheet. The hydrophobic tails of each individual sheet communicate with one another, creating a hydrophobic interior that acts together a permeability barrier. The hydrophilic head groups connect with the aqueous medium on every side that the bilayer. The 2 opposing sheets are called leaflets.


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The favored framework for many phospholipids and glycolipids in aqueous media is a bimolecular sheet quite than a micelle. The factor is that the two fatty acyl chain of a phospholipid or a glycolipid room too bulky to fit right into the internal of a micelle. In contrast, salt of fat acids (such as sodium palmitate, a constituent of soap), i beg your pardon contain just one chain, readily kind micelles. The formation of bilayers instead of micelles by phospholipids is of an important biological importance. A micelle is a restricted structure, usually much less than 20 nm (200 Å) in diameter. In contrast, a bimolecular sheet can have macroscopic dimensions, such as a millimeter (106 nm, or 107 Å). Phospholipids and also related molecule are important membrane constituents due to the fact that they readily kind extensive bimolecular sheets (Figure 12.11).


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The development of lipid bilayers is a self-assembly process. In various other words, the framework of a bimolecular sheet is inherent in the framework of the constituent lipid molecules. The development of lipid bilayers from phospholipids is a rapid and spontaneous procedure in water. Hydrophobic interactions room the major driving pressure for the development of lipid bilayers. Recall that hydrophobic interactions likewise play a dominant role in the urgent of proteins (Sections 1.3.4 and also 3.4) and in the stacking that bases in nucleic acids (Section 5.2.1). Water molecules are released indigenous the hydrocarbon tails of membrane lipids as these tails become sequestered in the nonpolar internal of the bilayer. Furthermore, valve der Waals attractive forces between the hydrocarbon tails donate close pack of the tails. Finally, there are electrostatic and also hydrogen-bonding attractions in between the polar head groups and water molecules. Thus, lipid bilayers are stabilized by the full range of forces that mediate molecular interactions in biological systems.

Because lipid bilayers are held together by numerous reinforcing, noncovalent interactions (predominantly hydrophobic), they are participating structures. This hydrophobic interactions have three far-reaching biological consequences: (1) lipid bilayers have actually an inherent propensity to it is in extensive; (2) lipid bilayers will have tendency to close on us so that there are no edges with exposed hydrocarbon chains, and so they type compartments; and also (3) lipid bilayers space self-sealing because a feet in a bilayer is energetically unfavorable.


12.4.1. Lipid Vesicles have the right to Be formed from Phospholipids

The propensity that phospholipids to kind membranes has been offered to create crucial experimental and clinical tool. Lipid vesicles, or liposomes, aqueous compartments enclosed by a lipid bilayer (Figure 12.12), can be offered to study membrane permeability or to supply chemicals come cells. Liposomes are formed by suspending a perfect lipid, such together phosphatidyl choline, in an aqueous medium, and then sonicating (i.e., agitating by high-frequency sound waves) to provide a dispersion of close up door vesicles that are fairly uniform in size. Vesicles formed by these approaches are practically spherical in shape and have a diameter of around 50 nm (500 Å). Larger vesicles (of the order of 1 μm, or 104 Å, in diameter) have the right to be ready by gradually evaporating the organic solvent from a suspension the phospholipid in a blended solvent system.


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Figure 12.12

Liposome. A liposome, or lipid vesicle, is a little aqueous compartment surrounded by a lipid bilayer.


Ions or molecules can be trapped in the aqueous compartments the lipid engine by developing the engine in the visibility of these substances (Figure 12.13). Because that example, 50-nm-diameter vesicles developed in a 0.1 M glycine systems will trap about 2000 molecules of glycine in each inner aqueous compartment. These glycine-containing vesicles deserve to be separated native the surrounding solution the glycine by dialysis or through gel-filtration color layer analyzer (Section 4.1.3). The permeability of the bilayer membrane come glycine deserve to then be identified by measure the price of efflux that glycine indigenous the inner compartment that the vesicle to the ambient solution. Details membrane proteins have the right to be solubilized in the presence of detergents and then added to the phospholipids indigenous which liposomes will be formed. Protein-liposome complexes provide an useful experimental devices for analyzing a range of membrane-protein functions.


Figure 12.13

Preparation that Glycine-Containing Liposome. Liposomes include glycine are created by sonication the phospholipids in the existence of glycine. Cost-free glycine is eliminated by gel filtration.


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Experiments are underway to construct clinical offers for liposomes. Because that example, liposomes containing drugs or DNA for gene-therapy experiments have the right to be injected right into patients. These liposomes fuse v the plasma membrane of numerous kinds the cells, introducing right into the cell the molecules that they contain. Drug delivery with liposomes additionally alters the circulation of a medicine within the body and often lessens its toxicity. For instance, less of the drug is spread to regular tissues since longcirculating liposomes have been displayed to concentration in regions of boosted blood circulation, such together solid tumors and also sites the inflammation. Moreover, the selective fusion of lipid motor with specific kinds of cell is a promising method of managing the delivery of medicine to target cells.

Another well-defined man-made membrane is a planar bilayer membrane. This structure have the right to be formed throughout a 1-mm hole in a partition between two aqueous compartments through dipping a well paintbrush into a membrane-forming solution, such as phosphatidyl choline in decane. The guideline of the brush is climate stroked throughout a feet (1 mm in diameter) in a partition between two aqueous media. The lipid film across the feet thins spontaneously right into a lipid bilayer. The electric conduction properties of this macroscopic bilayer membrane are conveniently studied by inserting electrodes right into each aqueous compartment (Figure 12.14). Because that example, that is permeability to ions is identified by measure the current across the membrane as a role of the applied voltage.


Figure 12.14

Experimental setup for the study of Planar double layer Membrane. A bilayer membrane is formed across a 1-mm hole in a septum that separates 2 aqueous compartments. This plan permits dimensions of the permeability and electrical conductance (more...)


12.4.2. Lipid Bilayers Are extremely Impermeable to Ions and also Most Polar Molecules

The results of permeability research studies of lipid vesicles and electrical-conductance dimensions of planar bilayers have shown that lipid bilayer membranes have actually a very low permeability for ions and also most polar molecules. Water is a conspicuous exception to this generalization; it readily traverses together membranes since of its tiny size, high concentration, and also lack of a complete charge. The range of measured permeability coefficients is very wide (Figure 12.15). For example, Na+ and K+ traverse these membranes 109 time as gradually as walk H2O. Tryptophan, a zwitterion at pH 7, the cross the membrane 103 time as slowly as walk indole, a structurally associated molecule that lacks ionic groups. In fact, the permeability coefficients of tiny molecules are correlated with their solubility in a nonpolar solvent relative to your solubility in water. This relation says that a small molecule can traverse a lipid double layer membrane in the complying with way: first, that sheds its solvation shell of water; then, the becomes dissolved in the hydrocarbon core of the membrane; finally, that diffuses with this main point to the various other side that the membrane, whereby it becomes resolvated by water. An ion such together Na+ tra-verses membranes very slowly since the instead of of that is coordination covering of polar water molecule by nonpolar interactions v the membrane inner is highly unfavorable energetically.

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Figure 12.15

Permeability Coefficients (P) of Ions and Molecules in a Lipid Bilayer. The ability of molecule to cross a lipid bilayer spans a wide selection of values.


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