Biology 1201-Chapter 7-Membranes

Biology 1201-Chapter 7-Membranes -...

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Biology 1201   (MWF 11:30-12:30 PM) Chapter 7 Membrane Structure and Function Membranes: Basic Structure – 6.2.4 *Membrane structural models have evolved as new data is acquired. Earliest models were deduced from indirect evidence. Membrane —A microscopic double layer of lipids and proteins that bounds cells and  organelles and forms structures within cells. 1.  Membranes are made of lipid. Lipid —Any of a class of organic compounds that are fatty acids or their derivatives  and are insoluble in water but soluble in organic solvents. They include many natural  oils, waxes, and steroids. 2.  Phospholipids are  amphipathic  molecules that can form membranes. Amphipathic —(of   a   molecule,   esp.   a   protein)   having   both   hydrophilic   and  hydrophobic parts. 3.  Cell membranes are phospholipid bilayers. Phospholipid Bilayers —They have a hydrophilic head and two  hydrophobic  tails.  When phospholipids are exposed to water, they arrange themselves into a bilayer  with all of their tails pointing toward the center of the sheet.  The center of this bilayer contains almost no water and also excludes molecules  like  sugars  or salts that dissolve in water but not in oil. 4.  Biological membranes contain proteins. 5.  Biological membranes are coated with proteins that generally absorb water.
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Davson-Danielli Model —A  model  of the  cell membrane  in which the  phospholipid   bilayer  lay between two  layers  of  globular   protein .   phospholipid (PL) bilayer globular   protein   coating   ( non  soluble ) Hydrophilic zones  o polar head groups + globular  proteins Hydrophobic zone o  2 x hydrophobic tails o about 8 mm thick Confirmed in 1950's Problems with this model: All membranes are not the same. How do the proteins stay attached? Singer  and  Nicolson:  Fluid Mosaic Model —In this model, the proteins are thought to  be globular and to float within the lipid bilayer rather than form the layers of the  sandwich-type model. Proteins embedded in the lipid bilayer. Hydrophilic   zones   =   polar   head   groups   +  hydrophilic portion of proteins. Hydrophobic   zone   =   2   x   hydrophobic   tails   +  hydrophobic portion of proteins. Confirmed   by   freeze-fracture   and   freeze-etch  micrographs. Fluid — a substance that has no fixed shape and yields easily to external pressure; a  gas or (esp.) a liquid. Mosaic
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Biology 1201-Chapter 7-Membranes -...

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