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Unformatted text preview: 11-1Cell and Molecular Biology (Biol. Chem. 410A)Lecture #11Harry R. Matthews, Ph.D.October 10, 1996MembranesClinical correlations:drug delivery systemcystinuriamulti-drug resistancecystic fibrosisLearning objectives:membrane structurethe molecules of membraneshow proteins cross membranesproteins in membranesOptional reading:Stryer IV: Chapter 11any molecules are small enough and hydrophilic enough to move freely through the aqueous environment of the cell, even under the molecular crowding conditions found in the cytoplasm. Hence, to keep the required molecules in and the un-wanted molecules out, all cells are surrounded by a membrane called the plasma membrane. Eukaryotic cells also have extensive intra-cel-lular membrane structures to regulate the movement of molecules between intra-cellular compartments. Since the molecules moving in the aqueous environment are hydrophilic, the barrier used by the cell is a continuous hydro-phobic sheet that is impermeable to hydro-philic molecules. To prevent the two-dimen-sional hydrophobic sheet folding up into a 3-dimensional ball, the sheet is surrounded on both sides by a hydrophilic layer that allows the membrane to take up an extended shape. The hydrophobic barrier is made from lipids and the hydrophilic cover is made from the head-groups of the lipid molecules. This struc-ture is called a lipid bilayer. Since the barri-er has to regulate the passage of molecules through it, the membrane contains protein structures that carry ions, molecules or sig-nals across the membrane. MLipids, phospholipids, membrane components.The hydrophobic part of the membrane is made from a class of lipids that comprises fatty acids attached covalently to hydrophilic, i.e. polar, heads, often joined through a glycer-ol derivative. Thus, diacyglycerols have two fatty acids esterified to glycerol and triacyl-glycerols have three fatty acids. The main groups of membrane lipids that share this overall structure are phospholipids and glycolipids. The major constituents of mem-branes are phospholipids such as phos-phatidyl choline (figure111a) sphingomyelin, phosphatidylserine and phosphatidylethano-lamine. Glycolipids have sugar residues as their polar head groups and include ganglios-ides whose polar groups include sialic acid. Glycolipids are found in the extracellular lay-er of the bilayer in plasma membranes and in-ositol phospholipids (figure 111b) are found on the cytoplasmic side and are important in cell signaling mechanisms. Eukaryotic plasma membranes also contain lipids that lack a large hydrophilic part, particularly cholester-ol, figure 111c. ol, figure 111c....
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- Spring '03