Phospholipids phosphoacylglycerols

Phospholipids are hydrolyzable lipids that contain a phosphorus atom. There are two common types of phospholipids phosphoacylglycerols and sphingomyelins. Both classes are found almost exclusively in the cell membranes of plants and animals, as discussed in Section 3.7. 

Phosphoacylglycerols containing choline (Figure 14-8) are the most abundant phospholipids of the cell mem-... 

Sphingomyelins, the second major class of phospholipids, are derivatives of the amino alcohol sphingosine, in much the same way that triacylglycerols and phosphoacylglycerols are derivatives of glycerol. Other notable features of a sphingomyelin include ... 

Fig. 5.21. Phosphoacylglycerols. Phospholipids found in membranes, such as phosphatidylcholine, have a polar group attached to the phosphate.

Fig. 10.3. Common phospholipids in the mammalian cell membrane. The polar head groups shown for ethanolamine and serine replace the choline in phosphatidylcholine to form phos-phatidylethanolamine and phosphatidylserine, respectively. Phosphatidylcholine, phos-phatidylethanolamine, and phosphatidylserine are phosphoacylglycerols. In contrast, sphingomyelin does not contain the glycerol backbone but has a sphingosine backbone and is a sphingolipid.

Cholesterol, which is interspersed between the phospholipids, maintains membrane fluidity. In the phosphoacylglycerols, unsaturated fatty acid chains bent into the cis conformation form a pocket for cholesterol, which binds with its hydroxyl group in the external hydrophilic region of the membrane and its hydrophobic steroid nucleus in the hydrophobic membrane core (Fig. 10.5). The presence of cholesterol and the cis unsaturated fatty acids in the membrane prevent the hydrophobic chains from packing too closely together. As a consequence, lipid and protein molecules that are not bound to external or internal structural proteins can rotate and move laterally in the plane of the leaflet. This movement enables the plasma membrane to partition between daughter cells during cell division, to... 

The arachidonic acid present in membrane phospholipids is released from the lipid bilayer as a consequence of the activation of membrane-bound phospholipase A2 or C (see Fig. 33.31 and Fig. 35.2). This activation occurs when a variety of stimuli (agonists), such as histamine and the cytokines, interact with a specific plasma membrane receptor on the target cell surface. Phospholipase A2 is specific for the sn-2 position of phosphoacylglycerols, the site of attachment of arachidonic acid to the glycerol moiety. Phospholipase C hydrolyzes phosphorylated inositol... 

Phospholipids, or phosphoacylglycerols, as they are more properly named, are the second most abundant group of naturally occurring lipids. They are found almost exclusively in plant and animal membranes, which typically consist of about 40% to 50% phospholipids and 50% to 60% proteins. The most abundant phospholipids are derived from aphospha-tidic acid (Figure 19.5). 

Phosphoacylglycerols, sphingomyelins and cholesterol are incorporated into the lipid bilayer because of their amphipathic character (Chapter 8). The plasma membrane of human erythrocytes has a high cholesterol content, about 45% of the total lipid content. The bilayer formed from natural phospholipids is essentially a liquid in that it exhibits random motions consistent with the liquid state. Their hydrocarbon tails may move more freely about in the plane of the monolayer without losing their hydrophobicity which is responsible for their mutual attraction. Membranous proteins embedded within the membrane can also move laterally but are limited in the magnitude of such migrations by their interactions with other components located internal to the membrane. 

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