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9.4 What Are the Dynamic Processes That Modulate Membrane Function? 261 (a) (b) 100 Inner leaflet Outer leaflet Outer leaflet 1% 1% 5% 11% 11% 14% Distribution 0 26% 42% 45% 44% Inner leaflet 100 PI PIP PIP, PA SM - = SM 27 PC 29 PS 13 PE 27 3 PC = sphingomyelin phosphatidylcholine phosphatidylethanolamine PS = phosphatidylserine PI/PA = phosphatidylinositol/ phosphatidic acid = PE = FIGURE 9.24 Phospholipids are distributed asymmetrically in most membranes, including the human erythro- cyte membrane, as shown here. (a) The distribution of phospholipids across the inner and outer leaflets of human erythrocytes. The x-axis values show, for each lipid type, its percentage of the total phospholipid in the membrane. (b) The phospholipid compositions of the inner and outer leaflets. All percentages in (a) and (b) are weight percentages. (Adapted from Zachowski, A., 1993. Phospholipids in animal eukaryotic membranes: Transverse asymmetry and movement. Biochemical Journal 294:1–14; and from Andreoli, T. E., 1987. Membrane Physiology, 2nd ed. Chapter 27, Table I. New York: Springer.) Loss of transverse lipid asymmetry has dramatic (and often severe) consequences for cells and organisms. For example, appearance of PS in the outer leaflet of the plasma membrane triggers apoptosis, the programmed death of the cell. Similarly, ag- ing erythrocytes and platelets slowly externalize PS, culminating in engulfment by macrophages. Many disease states, including diabetes and malaria, involve microvas- cular occlusions that may result in part from alterations of transverse lipid asymmetry. 9.4 What Are the Dynamic Processes That Modulate Membrane Function? Lipids and Proteins Undergo a Variety of Movements in Membranes Mis of diesel funci Led moments gure 35) range from bood vibrations 10"per wc), sowie wundulations (1 to 10perc), to tranne motion called "liptop (roughly one கல கால OOOOOO Law FIGURES Nos the membrane and the Com them tad- ...
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