Chapter 5 Membrane Structure and Function

Chapter 5 Membrane Structure and Function - Chapter 5...

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Chapter 5 Membrane Structure and Function I. Water, Water Everywhere A. Because the concentration of ions and other substances outside a cell may rapidly become too high or low, a mechanism is needed to selectively permit substances to enter or leave the cell. B. The plasma membrane—a surface of lipids, proteins, and some carbohydrate groups—regulates exchange of materials between cytoplasm and surroundings. C. Within the cytoplasm, exchanges are made across internal membranes of the organelles. II. Fluid Membranes in a Largely Fluid World A. The Lipid Bilayer 1. The “fluid” portion of the cell membrane is made of phospholipids. a. A phospholipid molecule is composed of a hydrophilic head and two hydrophobic tails. b. If phospholipid molecules are surrounded by water, their hydrophobic fatty acid tails cluster and a bilayer results; hydrophilic heads are at the outer faces of a two- layer sheet. 2. Bilayers of phospholipids are the structural foundation for all cell membranes. B. Fluid Mosaic Model of Membrane Structure 1. Three types of lipids are common in cell membranes: a. Phospholipids all have a hydrophilic head and hydrophobic tails but both of these regions show considerable variation. b. Glycolipids have sugar monomers attached at the head end. c. Cholesterol is abundant in animal—but not in plant—membranes. 2. Within a bilayer, phospholipids show quite a bit of movement; they diffuse sideways, spin, flex their tails to prevent close packing and promote fluidity, which also results from short-tailed lipids and unsaturated tails (kink at double bonds).
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