Fluidity of membranes

The most common sterol in membranes is cholesterol (Chapter 14), which resides mainly in the plasma membranes of mammalian cells but can also be found in lesser quantities in mitochondria, Golgi complexes, and nuclear membranes. Cholesterol intercalates among the phospholipids of the membrane, with its hydroxyl group at the aqueous interface and the remainder of the molecule within the leaflet. Its effect on the fluidity of membranes is discussed subsequently. 

Cholesterol modifies the fluidity of membranes. At temperatures below the T, it interferes with the interaction of the hydrocarbon tails of fatty acids and thus increases fluidity. At temperatures above the T, , it limits disorder because it is more rigid than the hydrocarbon tails of the fatty acids and cannot move in the membrane to the same extent, thus timiting fluidity. At... 

Carotenoids are hydrophobic molecules and thus are located in lipophilic sites of cells, such as bilayer membranes. Their hydrophobic character is decreased with an increased number of polar substitutents (mainly hydroxyl groups free or esterified with glycosides), thus affecting the positioning of the carotenoid molecule in biological membranes. For example, the dihydroxycarotenoids such as LUT and zeaxanthin (ZEA) may orient themselves perpendicular to the membrane surface as molecular rivet in order to expose their hydroxyl groups to a more polar environment. In contrast, the carotenes such as (3-C and LYC could position themselves parallel to the membrane surface to remain in a more lipophilic environment in the inner core of the bilayer membranes (Parker, 1989 Britton, 1995). Thus, carotenoid molecules can have substantial effects on the thickness, strength, and fluidity of membranes and thus affect many of their functions. 

Increasing fluidity makes lateral diffusion faster. Fluidity increases with increased temperature, increased content of short-chain fatty acids, and increased content of m-fatty acids. Cholesterol increases the fluidity of membranes that are not very fluid, but decreases the fluidity of membranes that are already fluid. 

An interesting observation is tiiat tile larger tiie amount of unsaturated fatty acids in the diet of hibernating animals, prior to hibernation, tiie lower tile body temperature falls during hibernation. The lower tiie temperature, tiie lower is the metabolic rate, which is important in survival from a prolonged period of hibernation. It is suggested that this is caused by an increase in fluidity of membranes but the mechanism is not known. 

Post mortem studies have shown that the level of polynn-satnrated fatty acids in some areas of the brain is decreased (particnlarly the hippocampus, striatum and cortex). In addition, it is known that the fluidity of membranes in... 

The fluidity of membranes primarily depends on their lipid composition and on temperature. At a specific transition temperature, membranes pass from a semicrystalline state to a more fluid state. The double bonds in the alkyl chains of unsaturated acyl residues in the membrane lipids disturb the semicrystalline state. The higher the proportion of unsaturated lipids present, therefore, the lower the transition temperature. The cholesterol content also influences membrane fluidity. While cholesterol increases the fluidity of semicrystalline, closely-packed membranes, it stabilizes fluid membranes that contain a high proportion of unsaturated lipids. 

Unsaturated fatty acid chains do not pack together in the bUayer as tightly as saturated fatty acid chains these properties contribute to different degrees of fluidity of membranes of different lipid composition. 

Bastiaanse EM, Jongsma H, van der Laarse A, Takens-Kwak BR Heptanol-induced decrease in cardiac gap jnctional conductance is mediated by a decrease in the fluidity of membranous cholesterol-rich domains. J Membr Biol 1993 136 135-145. 

A fully extended 19-unit dolichol (dolichol-19) would have a length of about 10 ran, twice that of the bilayer in which it is dissolved. It has been suggested that the central part of the molecule has a helical structure, while the ends are more flexible. Dolichols also appear to increase the fluidity of membrane bilayers.61 Bacterial undecaprenyl diphosphate, which has a similar function, contains only one E and ten Z double bonds62-633 (see p. 1152). 

Cholesterol - an essential component of mammalian cells - is important for the fluidity of membranes. With a single hydroxy group, cholesterol is only weakly am-phipathic. This can lead to its specific orientation within the phospholipid structure. Its influence on membrane fluidity has been studied most extensively in erythrocytes. It was found that increasing the cholesterol content restricts molecular motion in the hydrophobic portion of the membrane lipid bilayer. As the cholesterol content of membranes changes with age, this may affect drug transport and hence drug treatment. In lipid bilayers, there is an upper limit to the amount of cholesterol that can be taken up. The solubility limit has been determined by X-ray diffraction and is... 

Membranes, Fluidity of Membrane Proteins, Properties of Membrane Fusion, Mechanisms of Lipid BUayers, Properties of Lipid Rafts Biosensors... 

Cholesterol also affects the fluidy of lipid bilayers while increasing disorder in the low temperature plrase, and the order in the high temperature phase. The phase transition and the intensity are reduced. Generally speaking cholesterol reduces the fluidity of membranes above the phase transition temperature, and the permeability to aqueous solutes. 

T he use of spin-label probes to investigate cell membrane structure and function clearly demonstrates the fluidity of membrane lipid structures (1,2,3,4) however, a spin-label probe sees only its immediate environment. Predictions (5, 6, 7, 8) and data (9, 10, 11, 12) show that the introduction, for example, of a substituted oxazolidine ring as part of a typical amphiphatic lipid molecule can also significantly perturb a normal lipid environment. Consequently, some quantitative observations that used spin-label techniques need revision while others may be reduced to the level of qualitative predictions. 

Although the hydrophobic barrier created by the fluid lipid bilayer is an important feature of membranes, the proteins embedded within the lipid bilayer are equally important and are responsible for critical cellular functions. The presence of these membrane proteins was revealed by an electron microscopic technique called freeze-fracture. Cells are frozen to very cold temperatures and then fractured with a very fine diamond knife. Some of the cells are fractured between the two layers of the lipid bilayer. When viewed with the electron microscope, the membrane appeared to be a mosaic, studded with proteins. Because of the fluidity of membranes and the appearance of the proteins seen by electron microscopy, our concept of membrane structure is called the fluid mosaic model (Figure 18.13). 

SFAs (Cronan and Gelmann, 1973). As a result, the fluidity of membrane lipids returns to their original state, or close to it, with restoration of normal cell activity at the lower temperature. 

Explain the roles of the fatty acid chains of membrane lipids and cholesterol in controlling the fluidity of membranes. 

Polyunsaturated fatty acids affect the fluidity of membranes, thereby affecting cell membrane functions. On the other hand, increasing the proportions of saturated and monounsaturated fatty acids in membrane phospholipids decreases fluidity. In this study, we observed a positive correlation between phase angle and all of the n-3 PUFA but an inverse correlation between phase angle and palmitate and oleic acid. These results suggest that the phase angle may reflect some property of cell membranes that is related to fluidity. 

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