co05 - CHAPTER 5 MEMBRANE STRUCTURE AND FUNCTION Chapter...

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CHAPTER 5 M EMBRANE S TRUCTURE AND F UNCTION Chapter Outline 5.1 Membrane Models A. Early Observations 1. At turn of the century, researchers noted lipid-soluble molecules entered cells more rapidly than water-soluble molecules, suggesting lipids are component of plasma membrane. 2. Later chemical analysis revealed the membrane contained phospholipids. 3. Gorter and Grendel (1925) found the amount of phospholipid extracted from a red blood cell was just enough to form one bilayer; they also suggested the nonpolar tails were directed inward and polar heads outward. 4. To account for permeability of membrane to nonlipid substances, Danielli and Davson proposed sandwich model (later proved wrong) with phospholipid bilayer between layers of protein. 5. With an electron microscope available, Robertson proposed that proteins were embedded in an outer membrane and that all membranes in cells had similar compositions—the unit membrane model. 6. However, additional research showed a high diversity in membrane structure and function. B. Fluid-Mosaic Model 1. In 1972, S. Singer and G. Nicolson introduced the currently accepted fluid-mosaic model . 2. A plasma membrane is phospholipid bilayer in which protein molecules are partially or wholly embedded. 3. Embedded proteins are scattered throughout membrane in an irregular pattern; this varies among membranes. 4. Electron micrographs of freeze-fractured membrane support the fluid-mosaic model. 5.2 Plasma Membrane Structure and Function A. Separating the Internal and External Environment 1. A membrane’s structure has two components: lipids and proteins. 2. Phospholipids are arranged in a bilayer. 3. Plasma phospholipids spontaneously arrange themselves into a bilayer. 4. This bilayer has a fluid consistency similar to light oil. 5. Nonpolar tails are hydrophobic and directed inward. 6. Polar heads are hydrophilic (water-loving) and are directed outward to face both extracellular and intracellular fluids. 7. Glycolipids have a structure similar to phospholipids except the hydrophilic head is a variety of sugar; they are protective and assist in various functions. 8. Cholesterol is a lipid found in animal plasma membranes; it stiffens and strengthens the membrane. 9. Glycoproteins have an attached carbohydrate chain of sugar that projects externally. 10. The plasma membrane is asymmetrical; glycolipids and proteins occur only on outside and cytoskeletal filaments attach to proteins only on the inside surface. B. Carbohydrate Chains 1. In animal cells, the glycocalyx is a sugar coat of carbohydrate chains used in cell recognition. 2. Cells are unique in a “fingerprint” of highly varied carbohydrate chains. 3. The immune system recognizes foreign tissues that have strange carbohydrate chains. 4.
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co05 - CHAPTER 5 MEMBRANE STRUCTURE AND FUNCTION Chapter...

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