Membrane Dynamics:- Acyl Groups in the Bilayer Interior Are Ordered to Varying Degrees

Although the lipid bilayer structure is quite stable, its individual phospholipid and sterol molecules have some freedom of motion (Fig. 11–15). The structure and flexibility of the lipid bilayer depend on temperature and on the kinds of lipids present. At relatively low temperatures, the lipids in a bilayer form a semisolid gel phase, in which all types of motion of individual lipid molecules are strongly constrained; the bilayer is paracrystalline (Fig. 11–15a). At relatively high temperatures, individual hydrocarbon chains of fatty acids are in constant motion produced by rotation about the carbon–carbon bonds of the long acyl side chains. In this liquid-disordered state, or fluid state (Fig. 11–15b), the interior of the bilayer is more fluid than solid and the bilayer is like a sea of constantly moving lipid. At intermediate temperatures, the lipids exist in a liquid-ordered state; there is less thermal motion in the acyl chains of the lipid bi layer, but lateral movement in the plane of the bilayer still takes place. These differences in bilayer state are easily observed in liposomes composed of a single lipid, but biological membranes contain many lipids with a variety of fatty acyl chains and thus do not show sharp phase changes with temperature. At temperatures in the physiological range (about 20 to 40 C), long-chain saturated fatty acids (such as 16:0 and 18:0) pack well into a liquid-ordered array, but the kinks in unsaturated fatty acids (see Fig. 10–1) in terfere with this packing, favoring the liquid-disordered state. Shorter-chain fatty acyl groups have the same effect. The sterol content of a membrane (which varies greatly with organism and organelle; Table 11–1) is another important determinant of lipid state. The rigid planar structure of the steroid nucleus, inserted between fatty acyl side chains, reduces the freedom of neigh boring fatty acyl chains to move by rotation about their carbon–carbon bonds, forcing acyl chains into their fully extended conformation. The presence of sterols therefore reduces the fluidity in the core of the bilayer, thus favoring the liquid-ordered phase, and increases the thickness of the lipid leaflet (as described below). Cells regulate their lipid composition to achieve a constant membrane fluidity under various growth conditions. For example, bacteria synthesize more un saturated fatty acids and fewer saturated ones when cultured at low temperatures than when cultured at higher temperatures (Table 11–2). As a result of this adjust ment in lipid composition, membranes of bacteria cultured at high or low temperatures have about the same degree of fluidity.

FIGURE 11–15 Two states of bilayer lipids. (a) In the paracrystalline state, or gel phase, polar head groups are uniformly arrayed at the surface, and the acyl chains are nearly motionless and packed with regular geometry; (b) in the liquid disordered state, or fluid state, acyl chains undergo much thermal motion and have no regular organization. Intermediate between these extremes is the liquid-ordered state, in which individual phospholipid molecules can diffuse laterally but the acyl groups remain extended and more or less ordered.

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مواضيع ذات صلة

Membrane Dynamics:- Transbilayer Movement of Lipids Requires Catalysis

The Composition and Architecture of Membranes:- Lipids and Proteins Diffuse Laterally in the Bilayer

The Composition and Architecture of Membranes:- Covalently Attached Lipids Anchor Some Membrane Proteins

The Composition and Architecture of Membranes:- The Topology of an Integral Membrane Protein Can Be Predicted from Its Sequence

The Composition and Architecture of Membranes: -Integral Proteins Are Held in the Membrane by Hydrophobic Interactions with Lipids

The Composition and Architecture of Membranes:- All Biological Membranes Share Some Fundamental Properties

The Composition and Architecture of Membranes:- Each Type of Membrane Has Characteristic Lipids and Proteins

Working with Lipids:- Mass Spectrometry Reveals Complete Lipid Structure

Working with Lipids:- Gas-Liquid Chromatography Resolves Mixtures of Volatile Lipid Derivatives

Working with Lipids:- Adsorption Chromatography Separates Lipids of Different Polarity

Working with Lipids:- Lipid Extraction Requires Organic Solvents

Lipids as Signals, Cofactors, and Pigments: -Plants Use Phosphatidylinositols, Steroids, and Eicosanoidlike Compounds in Signaling