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Lecture 14 - Ch 7 - Chapter 7 Membrane Structure and...

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Chapter 7 – Membrane Structure and Function 7.1 Cellular membranes are fluid mosaics of lipids and proteins The most abundant lipids in most membranes are phospholipids. The ability of phospholipids to form membranes in inherent in their molecular structure. A phospholipid is an amphipathic molecule. Other types of membrane lipids are also amphipathic. Furthermore, most of the proteins within membranes are amphipathic. In a fluid mosaic model , the membrane is a fluid structure with a “mosaic” of various proteins embedded in or attached to a double layer (bilayer) of phospholipids. The Fluidity of Membranes Membranes are not static sheets of molecules locked rigidly in place. A membrane is held together primarily by hydrophobic interactions, which are much weaker than covalent bonds. Most of the lipids and some of the proteins can shift about laterally. It is quite rare for them to flip-flop; to do so, the hydrophilic part of the molecule must cross the hydrophilic part of the molecule most cross the hydrophobic core of the membrane. A membrane remains fluid as temperature decreases until finally the phospholipids settle into a closely packed arrangement and the membrane solidifies, much as bacon grease forms lard when it cools. The membrane reminds fluid to a lower temperature if it is rich in phospholipids with unsaturated HC tails. Because of kinks in the tails where double bonds are located, unsaturated HC tails cannot pack closely and this makes the membrane more fluid. The steroid cholesterols has different effect on membrane fluidity at different temperatures. - At relatively high t e m p er atures, cholesterol m akes the m e m bran e less fluid by restrai phospholipid m ove m e nt. - Because it also hinders the close packing of phospholipids, cholesterol lowers the te m required for the m e m br an e to solidify. - Thus, cholesterol is called a “te m p erature buffer.” Membran es m ust be fluid to work properly.
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When a membrane solidifies, its permeability changes, and enzymatic proteins in the   membrane may be inactive. Membrane Proteins and Their Functions Phospholipids form the main fabric of the membrane, but proteins determine most of the  membrane’s functions. Integral proteins  penetrate the hydrophobic core of the lipid bilayer. Many are  transmembrane proteins , which span the membrane; other integral proteins  extend only partway into the hydrophobic core. The hydrophobic regions of an integral protein consist of one or more stretches of  nonpolar amino acids, usually coiled into alpha helices.  The hydrophilic parts of the  molecule are exposed to the aqueous solutions on either side of the membrane.
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