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Biology Chapter 5

Biology Chapter 5 - 5.1 Membrane Models 1 In the early...

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5.1 Membrane Models 1. In the early 1900s, researchers noted that lipid-soluble molecules entered cells more rapidly than water-soluble molecules, suggesting lipids are component of plasma membrane. 2. Later, chemical analysis revealed that the membrane contained phospholipids. 3. Gorter and Grendel (1925) found that 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 the permeability of membrane to nonlipid substances, Danielli and Davson (1940s) proposed the "sandwich" model, with a phospholipid bilayer between layers of protein. 5. Robertson (1950s) proposed that proteins were embedded in an outer membrane and that all membranes in cells had similar compositions—the "unit membrane" model . 6. Additional research showed great diversity in membrane structure and function. B. Fluid-Mosaic Model 1. In 1972, Singer and Nicolson introduced the currently accepted fluid-mosaic model. a. The plasma membrane is a phospholipid bilayer, in which protein molecules are embedded. b. Embedded proteins are scattered throughout membrane in an irregular pattern; this varies among membranes. 5.2 Plasma Membrane Structure and Function 1. The plasma membrane is a phospholipid bilayer with embedded proteins. 2. Phospholipids have both hydrophilic and hydrophobic regions; nonpolar tails (hydrophobic) are directed inward, polar heads (hydrophilic) are directed outward to face both extracellular and intracellular fluid. 3. The proteins form a mosaic pattern on the membrane. 4. Cholesterol is a lipid found in animal plasma membranes; it stiffens and strengthens the membrane. 5. Glycolipids have a structure similar to phospholipids except the hydrophilic head is a variety of sugar; they are protective and assist in various functions. 6. Glycoproteins have an attached carbohydrate chain of sugar that projects externally. 7. 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; it has several functions. 2. Cells are unique in that they have highly varied carbohydrate chains (a "fingerprint"). 3. The immune system recognizes foreign tissues that have inappropriate carbohydrate chains. 4. Carbohydrate chains are the basis for A, B, and O blood groups in humans. C. Fluidity of the Plasma Membrane 1. At body temperature, the phospholipid bilayer has the consistency of olive oil. 2. The greater the concentration of unsaturated fatty acid residues, the more fluid the bilayer. 3. In each monolayer, the hydrocarbon tails wiggle, and entire phospholipid molecules can move sideways.
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