CH 5 - Ch 05 Membranes BIO 1610 The plasma membrane:...

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1 Ch 05 –Membranes BIO 1610 The plasma membrane: separates the living cell from its nonliving surroundings. This thin barrier, 8 nm thick, controls traffic into and out of the cell. It is selectively permeable S.J. Singer and G. Nicholson (1972) Æ the membrane proteins are dispersed and individually inserted into the phospholipid bilayer. Hydrophilic region of protein Phospholipid bilayer Hydrophobic region of protein The Fluid mosaic 3 Membrane proteins contain phospholipids. Amphipatic molecules Hydrophilic, polar Phospholipid structure consists of - glycerol –a3- carbon polyalcohol acting as a backbone for the phospholipid 4 Hydrophobic, non polar -2 fatty acids attached to the glycerol - phosphate group attached to the glycerol lipid bilayer fluid Actual membranes adhere more strongly to water than do artificial membranes composed only of phospholipids Æ proteins on the surface increased adhesion. Phospholipids spontaneously form bilayers phospholipids one layer of lipids one layer of lipids fluid The partially hydrophilic, partially hydrophobic phospholipid spontaneously forms a bilayer: -fatty acids are on the inside -phosphate groups are on both surfaces of the bilayer 6
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2 Cellular membranes have 4 components: 1. phospholipid bilayer (in animals also cholesterol) 2. transmembrane proteins (integral membrane proteins) 3. interior protein network (includes peripheral membrane proteins) 4. cell surface markers (glycoproteins, glycolipids) The transmembrane 7 The transmembrane domain of an integral membrane protein Is composed of hydrophobic amino acids, often forms an alpha helical structure and can cross the membrane multiple times Structural evidence Æ Freeze-Fracture 8 Extracellular layer middle layer The Fluid mosaic Phospholipid bilayers are fluid. -hydrogen bonding of water holds the 2 layers together -individual phospholipids and unanchored proteins can move through the membrane -saturated fatty acids make the membrane less fluid than unsaturated fatty acids -warm temperatures make the membrane more fluid than cold 9 temperatures The steroid cholesterol is wedged between phospholipid molecules in the plasma membrane of animals cells. At cool temperatures, it maintains fluidity by preventing tight packing Bacterial and plant cells don’t have cholesterol The Fluid mosaic (c) Cholesterol within the animal cell membrane Cholesterol At warm temperatures, it restrains the movement of phospholipids and reduces fluidity Do membrane proteins move? EXPERIMENT Researchers labeled the plasma membrane proteins of a mouse cell and a human cell with two different markers and fused the cells. Using a microscope, they observed the markers on the hybrid cell. Membrane proteins RESULTS Mouse cell Human cell Hybrid cell Mixed proteins after 1 hour CONCLUSION The mixing of the mouse and human membrane proteins indicates that at least some membrane proteins move sideways within the plane of the plasma membrane.
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This note was uploaded on 09/28/2010 for the course BIO 1620 taught by Professor Bybee during the Fall '06 term at Utah Valley University.

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CH 5 - Ch 05 Membranes BIO 1610 The plasma membrane:...

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