lecture14 - LECTURE 14 25 September 2009 (P. J. Hollenbeck)...

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-1- LECTURE 14 25 September 2009 (P. J. Hollenbeck) BIOL 231 Read: pp. 361-385; DVD 11.2 Problems: 37, 38; ECB Q11-9 <Let’s start building the complete cell. We understand macromolecules, boundaries, and the structure and synthesis of proteins. Now we need to look at the structures of lipids, and their interaction with proteins to form 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: ester linkages to 2 long chain fatty acids to form the hydrophobic portion; a phosphoester linkage to a polar head group. The most common polar head groups are choline and ethanolamine (both amino alcohols), inositol, and serine. Phospholipids are named after the polar head group such that “phosphatidylcholine” has a choline head group, “phosphatidylserine” has a serine head group, etc. Sphingolipids have a serine backbone, one or two long acyl tails and various head groups. When the head groups include one or more sugars linked together, they are called glycolipids. Study figure 11-7 , but don’t worry about the details of the structure of sphingo- or glycolipids, just understand the basic form of the molecules. (2) Asymmetry: most long-chain lipids are located asymmetrically in cellular membranes. For example, most of the phosphatidylcholine in the plasma membrane of most cells is located in the extracellular leaflet, while most of the phosphatidylserine is in the cytoplasmic leaflet ( Fig 11-17 ). Also, all of the glycolipids are in the extracellular leaflet. These asymmetries arise from both synthesis and sorting of lipids. (B) Generating & maintaining lipid asymmetry. (1) Flippases are a class of enzymes that catalyze the movement of phospholipids between leaflets of the bilayer ( Fig 11-18 , study pp. 370-2 ). They are not fully understood at present. In the presence of flippases the rate of phospholipid flipping is increased greatly, from a time scale of days to one of seconds or less. (Of course, the enzyme cannot make this energetically unfavorable event any less unfavorable - so you know that it must be a coupled reaction.)
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-2- Fig 11-16 from ECB shows the size and position of cholesterol in one leaflet of the membrane relative to phospholipid molecules.
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This note was uploaded on 12/18/2009 for the course BIOL 101 taught by Professor Wormer during the Fall '08 term at Purdue University-West Lafayette.

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lecture14 - LECTURE 14 25 September 2009 (P. J. Hollenbeck)...

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