Plasma membrane structure lipid composition

The lipid compositions of plasma membranes, endoplasmic reticulum and Golgi membranes are distinct 26 Cholesterol transport and regulation in the central nervous system is distinct from that of peripheral tissues 26 In adult brain most cholesterol synthesis occurs in astrocytes 26 The astrocytic cholesterol supply to neurons is important for neuronal development and remodeling 27 The structure and roles of membrane microdomains (rafts) in cell membranes are under intensive study but many aspects are still unresolved 28... 

Several aspects of Schwann cell metabolism emerge as potential points of vulnerability to toxicants. The Schwann cell perikaryon (cell body) supports an enormous peripheral structure, the myelin sheath, which, if unwrapped, would dwarf the body of the Schwann cell (see Figure 30.4). Thus, as in the case of axonal transport in neurons, there may be specialized processes involved in supporting the topologically distant myelin. Furthermore, myelin has a specialized lipid and protein composition and a relatively rigid and ordered structure as compared to other membranes. Metabolic perturbations that potentially cause alterations in the composition of lipids and proteins assembled to form this membrane may cause destabilization and collapse of the myelin membrane. In this context, myelin might be much more vulnerable than plasma membranes of other cells. 

Anacystis nidulans has a rather simple membrane structure (mainly plasma membrane and thylakoid) furthermore, these membranes have fairly simple lipid compositions. The latter changes distinctly with the growth temperature, the lipids being more unsaturated (more fluid) when the algae are grown at lower temperatures (Table 2, columns 1 and 3). By careful hydrogenation of... 

The plasma membrane of the brush border microvilli is characterized by certain distinctive structural features, which may be related to the specialized functional properties that distinguish it from plasma membranes of other cells. The width of the microvillus membrane (measured by electron microscopy) is 10-11 nm, whereas the average eukaryotic plasma membrane is only 7-9 nm. This is probably due to the biochemical composition of the membrane, which is characterized by a high protein to lipid ratio (1.7 1) and a unique lipid composition. The cholesterol-phospholipid ratio and the molar ratio of glycolipid to phospholipid are both about 1 1, which is consistent with the low values for membrane fluidity determined in microvillus membranes. This should be compared with the corresponding ratios... 

Cholesterol and its derivatives constitute the third important class of membrane lipids, the steroids. The basic structure of steroids is a four-ring hydrocarbon. Cholesterol, the major steroidal constituent of animal tissues, has a hydroxyl substituent on one ring (Figure 5-5c). Although cholesterol is almost entirely hydrocarbon in composition, it is amphlpathlc because its hydroxyl group can interact with water. Cholesterol is especially abundant in the plasma membranes of mammalian cells but is absent from most prokaryotic cells. As much as 30-50 percent of the lipids in plant plasma membranes consist of certain steroids unique to plants. 

As the plasma membrane of the glial cell is converted into myelin, the lipid composition of the brain changes (Table 48.2). The lipid-to-protein ratio is greatly increased, as is the content of sphingolipids. The myelin is a tightly packed structure. 

The currently accepted structure of B. is the fluid mosaic model. Lipid molecules and membrane proteins are free to diffuse laterally and to spin within the bilayer in which they are located. However, a flip-flop motion from the inner to the outer surface, or vice versa, is energetically unfavorable, because it would require movement of hydrophilic substituents through the hydrophobic phase. Hence this type of motion is almost never displayed by proteins, and it occurs much less readily than translational motion in the case of lipids. Since there is little movement of material between the inner and outer layers of the bilayer, the two faces of the B. can have different compositions. For membrane proteins, this asymmetry is absolute, and, at least in the plasma membrane, different proportions of lipid classes exist in the two monolayers. Attached carbohydrate residues appear to be located only on the noncytosolic surface. Carbohydrate groups extending from the B. participate in cell recognition, cell adhesion, possibly in intercellular communication, and they also contribute to the distinct immunological character of the cell. 

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