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Unformatted text preview: 1 UNIT TWO: THE CELL Chapter Eight: Membrane Structure and Function Chapter Eight: Membrane Structure and Function Chapter Eight: Membrane Structure and Function Chapter Eight: Membrane Structure and Function (Text from Biology , 6 th Edition, by Campbell and Reece) Membrane Structure and Function (Chapter Eight) MEMBRANE STRUCTURE The selectively permeable selectively permeable selectively permeable selectively permeable plasma membrane allows certain substances easier passage than others. Lipids and proteins are the staple ingredients of membranes. Phospholipids are especially important. A phospholipid is an amphipathic molecule amphipathic molecule amphipathic molecule amphipathic molecule, meaning it has hydrophilic and hydrophobic regions. The fluid mosaic model fluid mosaic model fluid mosaic model fluid mosaic model states that the membrane is a fluid structure with various proteins embedded in or attached to a bilayer of phospholipids. Membrane Models Have Evolved to Fit New Data Scientists worked to discover how the membrane was composed long before membranes could be seen with electron microscopes. The Dutch scientists E. Gorter and F. Grendel reasoned that cell membranes must be phospholipid bilayers, with the hydrophobic tails facing each other and the hydrophobic heads in contact with water. The Davson-Danielli model shows a phospholipid bilayer sandwiched in between two protein layers. It was also thought that all cell membranes were the same. However, scientists later recognized problems with these conclusions. With further usage of the electron microscope, it was discovered that not all membranes look alike. Membranes with different functions differ in chemical composition and structure. Additionally, membrane proteins are not very soluble in water, and are amphipathic as well. If they were layered on top of the phospholipid bilayer, their hydrophobic regions would be in contact with an aqueous environment. As a result, S. J. Singer and G. Nicolson advocated a revised model in which proteins were dispersed and inserted into the phospholipid bilayer. According to this model, the membrane is a mosaic of protein molecules bobbing in a fluid bilayer of phospholipids. Freeze-fracture is a method of preparing cells for electron microscopy that splits a membrane along the middle of the phospholipid bilayer. When the halves are viewed, small bumps are visible, with protein particles interspersed in the matrix. Membranes Are Fluid Membranes are held together by hydrophobic interactions, which are much weaker than covalent bonds. As a result, lipids and proteins can move around in the membrane. Some membrane proteins drift, while others move along with more purpose. Furthermore, other membrane proteins seem to be held immobile by their attachments to the cytoskeleton. 2 UNIT TWO: THE CELL Chapter Eight: Membrane Structure and Function Chapter Eight: Membrane Structure and Function Chapter Eight: Membrane Structure and Function Chapter Eight: Membrane Structure and Function...
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- Spring '08
- AP Biology