Biochemistry and Molecular Biology - Belyasova N.A. 2002

Structure and Functions of Cellular Components
Biomembranes
Properties of Polar Lipids and Their Aggregates

Lipids that form Biomembranes are amphiphilic compounds (Fig. 4.1, 4.2). This structural feature is reflected in their properties: in an aqueous environment, polar lipid molecules spontaneously aggregate to form structures in which the hydrophobic PARTS OF THE molecules are packed inward and protected by hydrophilic heads facing the Water. Such aggregates can vary in shape and Structure, which depends on The structure of the lipid molecule and the size ratio of its polar and nonpolar parts.

The simplest aggregates of amphiphilic molecules are called micelles. Depending on The Nature of the solvent, lipids can form either conventional micelles or so-called "reverse" micelles (Fig. 4.5). In water, conventional micelles are formed, in which hydrophobic hydrocarbon chains are isolated from the aqueous environment by hydrophilic polar heads.

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Fig. 4.5. Lipid micelles in water and nonpolar Solvents

In nonpolar solvents (benzene, hexane, etc.) containing trace amounts of water, reverse micelles are formed, in which the orientation of the lipid molecules is reversed (Fig. 4.5). Micelle-forming lipids include salts of Higher Fatty acids, lipid forms with only a single hydrocarbon chain per molecule, and many detergents. Lipid micelles can have various shapes: spherical (Fig. 4.5), cylindrical, ellipsoidal, or disk-shaped. A special variety of lipid aggregates are bilayers (bimolecular layers). As a rule, these structures are formed by lipids incapable of forming micelles, such as Phospholipids. The ability to organize into bilayers is determined, as in the case of micelles, by the size ratio of the polar and nonpolar parts of the molecule.

Due to their elasticity and flexibility, bilayers can close in on themselves to form lamellar vesicles called Liposomes (Fig. 4.6).

Liposomes serve as convenient model systems for studying membranes and are also used for drug delivery to various Organs and toys. In this case, it becomes possible to isolate the drug from Enzymes that degrade it and target it directly to the disease site. Liposomes and micelles can be obtained by treating aqueous dispersions of polar lipids, such as phosphatidylcholine, with ultraviolet light or ultrasound.

The Forces Stabilizing the structure of Cell/29.html">The Lipid Bilayer are hydrophobic interactions, Hydrogen Bonds, and Van der Waals forces. Hydrophobic interactions make the greatest contribution to the stabilization of bilayers: under the action of these forces, the system adopts such

Fig. 4.6. Structural representation of liposomes and lipid bilayers structural Organization that minimizes contacts between the nonpolar regions of lipid molecules and water. The net Free energy change upon transferring a nonpolar substance from a nonpolar solvent to water is thermodynamically unfavorable due to Entropy effects associated with the disruption of water structure as a solvent. It can be said that unfavorable interactions between the nonpolar solute and water constitute "hydrophobic forces." Hydrogen bonds are formed between the polar heads of certain lipids. Van der Waals forces are short-range, weak attractive forces between adjacent hydrophobic chains.

The thickness of lipid bilayers is determined by the length of hydrocarbon chains, as well as the presence of double bonds and substituents, i.e., the packing density of hydrophobic tails. Typically, this parameter ranges between 4–5 nm.

The lipid bilayer is a fluid medium with low viscosity (the consistency of vegetable oil). Depending on Temperature, the lipid bilayer can exist in two main states: crystalline (solid or gel) and liquid-crystalline (liquid). Usually, this transition occurs at a temperature of 15–40° C, but for each specific lipid, this parameter is strictly defined and is called the phase transition temperature (t_n). The phase transition temperature depends on the STRUCTURE OF THE hydrocarbon tails and polar heads. Fig. 4.7 provides a schematic representation of the phase transition of lipids in bilayers. When membranes transition from the liquid-crystalline state to the gel phase, fluidity decreases by approximately two orders of magnitude. Maintaining the liquid-crystalline state is crucial for membrane function, and Cells possess mechanisms to regulate membrane fluidity based on altering the Lipid Composition of bilayers. This mechanism is of particular importance for poikilothermic organisms, which cannot maintain a constant BODY TEMPERATURE AND depend on the ambient temperature.

Fig. 4.7. Phase transitions in the state of the lipid bilayer