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Unformatted text preview: -1-LECTURE 1424 September 2010 (P. J. Hollenbeck)BIOL 231The structure & function of cellular membranesRead: pp. 361-385; DVD 11.2Problems: 37, 38; ECB Q11-9<Lets start building the complete cell. We understand macromolecules, catalysis, the genome and thestructure and synthesis of proteins. Now we need to look at the structures of lipids, and theirinteraction with proteins to form plasma membranes and organelles.>I. Putting lipids together(A) Three kinds of long chain lipids are made by cells [see panel 2-4 and figs 11-6, 11-7](1) Phospholipids. Recall that these are composed of a glycerol backbone with esterlinkages to 2 long chain fatty acids to form the hydrophobic portion, and a phosphoesterlinkage to a polar head group. The most common polar head groups are choline andethanolamine (both amino alcohols), serine and inositol. Phospholipids are named afterthe polar head group such that phosphatidylcholine has a choline head group,phosphatidylserine has a serine head group, etc. Sphingolipids have a serinebackbone, one or two long acyl tails and various head groups. Lipids with head groupsthat include one or more sugars linked together are called glycolipids. Studyfigure 11-7, but dont worry about the details of the structure of sphingo- or glycolipids, justunderstand the basic form of the molecules.(2) Asymmetry: most long-chain lipids are located asymmetrically in cellularmembranes. For example, in most cells, most of the phosphatidylcholine is located inthe extracellular leaflet of the plasma membrane, while most of the phosphatidylserineis in the cytoplasmic leaflet (Fig 11-17). All of the glycolipids are in the extracellularleaflet. These asymmetries arise from both synthesis and sorting of lipids.(B) Generating & maintaining lipid asymmetry. (1) Scramblases and Flippases are 2 classes of enzymes that catalyze the movement ofphospholipids between leaflets of the bilayer (Fig 11-18, study pp. 370-2). In thepresence of these enzymes, the rate of phospholipid flipping between leaflets isincreased >10 -fold over the spontaneous rate, from a time scale of days to one of4seconds. (Of course, enzymes cannot make this energetically unfavorable event anyless unfavorable - so you know that it must be a coupled reaction.)-2-Fig 11-16 from ECB shows the size and position ofcholesterol in one leaflet of the membrane relative tophospholipid molecules....
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This note was uploaded on 09/18/2011 for the course BIOLOGY 231 taught by Professor Petethollenbeck during the Fall '10 term at Purdue University-West Lafayette.
- Fall '10