11 - Chapter 11. STRUCTURE AND PROPERTIES OF MEMBRANES...

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Chapter 11. STRUCTURE AND PROPERTIES OF MEMBRANES Biologic membranes delimit compartments of various sizes in the body. Of greatest importance here is the plasma membrane, or the limiting membrane that separates cytoplasm from interstitium. Naked cytoplasm does not exist. There is a long history of various models proposed to account for the functional, biochemical, and structural properties of these membranes. The most useful model, and currently the most generally acceptable, is Singer's fluid mosaic (Figure 11-1), which provides for proteins and lipids coexisting in a dynamic structure. Figure 11-1. Membrane structure: the fluid mosaic. In this illustration of the belayed membrane, proteins “float” in a sea of lipids. Note the presence of the channel protein, providing a hydrophilic “pore” through the thickness of the membrane. Other proteins are shown exposed on the external face (upper) and on the cytoplasmic face (lower). Two of the externally exposed proteins are decorated with carbohydrate chains; these are glycoproteins. The carbohydrates contribute to the glycocalyx, an external coat over the cell surface. The membrane lipids are shown with polar heads on the surfaces and their apolar tails extending into the inner matrix of the membrane. Lipids, including a high proportion of distinctly polar molecules, are oriented into hydrophilic surfaces with a hydrophobic interior. Interspersed in this lipid matrix are various proteins, as shown in Figure 11-2. Some of the proteins are peripheral and exposed on either the external or internal faces. Others traverse the lipid matrix and, in the special case of channel proteins, or pores, provide hydrophilic domains through the essentially hydrophobic inner region of the membrane. Many transmembrane proteins (e.g. some hormone receptors) traverse the membrane multiple times. These have a distinct interstitial “head” and a cytoplasmic “tail” separated by five or seven hydrophobic domains (embedded in the lipid bilayer) that are in turn separated by four or six hydrophilic “connectors”, half of which are extracellular and the others are cytoplasmic (Figure 11-3). Figure 11-2. Categories of membrane proteins. Proteins fitting a variety of functional classes, as described in the text, are found as integral components of the cell surface membrane. 2007 version – page 82
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Figure 11-3. Membrane receptor with seven membrane-spanning domains . This depiction of the oxytocin receptor shows the hydrophobic domains within the thickness of the plasmalemma (the two horizontal lines) with more hydrophilic linker regions on both the interstitial side (upper) and the cytoplasmic face (below). The amino acid sequence was derived from a cDNA sequence then portions with more hydrophobic amino acids were identified and arranged as shown above. From Gimpl, G and Fahrenholz, F. Physiol. Rev. 81 : 629-683, 2001. A. Membrane proteins
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11 - Chapter 11. STRUCTURE AND PROPERTIES OF MEMBRANES...

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