A typical cell contains myriad types of membranes, each with unique properties bestowed by its particular mix of lipids and proteins. The data in Table 5-1 illustrate the variation in lipid composition among different biomembranes. Several phenomena contribute to these differences. For instance, differences between membranes in the endoplasmic reticulum (ER) and the Golgi are largely explained by the fact that phospholipids are synthesized in the ER, whereas sphin-golipids are synthesized in the Golgi. As a result, the proportion of sphingomyelin as a percentage of total membrane lipid phosphorus is about six times as high in Golgi membranes as it is in ER membranes. In other cases, the translocation of membranes from one cellular compartment to another can selectively enrich membranes in certain lipids.
Differences in lipid composition may also correspond to specialization of membrane function. For example, the plasma membrane of absorptive epithelial cells lining the intestine exhibits two distinct regions: the apical surface faces the lumen of the gut and is exposed to widely varying external conditions; the basolateral surface interacts with other epithelial cells and with underlying extracellular structures (see Figure 6-5). In these polarized cells, the ratio of sphin-golipid to phosphoglyceride to cholesterol in the basolateral membrane is 0.5:1.5:1, roughly equivalent to that in the plasma membrane of a typical unpolarized cell subjected to mild stress. In contrast, the apical membrane of intestinal cells, which is subjected to considerable stress, exhibits a 1:1:1 ratio of these lipids. The relatively high concentration of sphingolipid in this membrane may increase its stability
TABLE 5-1 Major Lipid Components of Selected Biomembranes
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