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Biology with MasteringBiology™ (8th Edition)

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1. Label this diagram of the plasma membrane with the following terms: carbohydrate chain cholesterol cytoskeleton filaments glycolipid glycoprotein phospholipid bilayer protein molecule The sandwich model and its associated unit membrane model preceded the currently accepted fluid-mosaic model of membrane structure. According to the fluid-mosaic model, phospholipid molecules provide a fluid, lipid bilayer with their polar heads at the membrane surfaces. Hydropho- bic portions of protein molecules are in the lipid bilayer; their hydrophilic portions are at the surfaces. The proteins form channels and function as receptors, enzymes, and carrier molecules. Molecules move across a membrane in several ways. By diffusion, molecules move down their concentration gradient. Osmosis is the diffusion of water through a dif- ferentially permeable membrane. When cells are in a hy- potonic solution, they gain water; when they are in a hypertonic solution, they lose water. In isotonic solu- tions, cells neither gain nor lose water. Both diffusion and osmosis are passive processes that do not require energy. Facilitated transport is also passive and involves carrier molecules moving substances from higher to lower con- centrations. Active transport moves substances in the op- posite direction, with the function of a carrier molecule and energy. Larger substances pass through cells by en- docytosis and exocytosis. An extracellular matrix is a meshwork of insolu- ble proteins and carbohydrates that is now known to in- fluence animal cell differentiation and cellular metabolism. Plant cells are bounded by a cell wall that is external to the plasma membrane. This boundary is freely permeable. Several types of junctions exist between animal cells: desmosomes, gap junctions, and tight junc- tions. Between plant cells, plasmodesmata perform this function. 30 4 M EMBRANE S TRUCTURE AND F UNCTION C HAPTER R EVIEW S TUDY E XERCISES Study the text section by section as you answer the questions that follow. 4.1 P LASMA M EMBRANE S TRUCTURE AND F UNCTION ( P . 68) The plasma membrane regulates the passage of molecules into and out of the cell. The membrane contains lipids and proteins. Each protein has a specific function.
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