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LEQ: What are the mechanisms that move materials into and out of a cell? Cell Transport part 1 Pages 81 to 83
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Passive Transport Diffusion ◦ Is the tendency of particles to spread out evenly ◦ Molecules move randomly from areas of high concentration to area of low concentration ◦ No expenditure of energy
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Fig. 7-11 Molecules of dye Membrane (cross section) WATER Net diffusion Equilibrium (a) Diffusion of one solute Net diffusion Equilibrium (b) Diffusion of two solutes
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Passive Transport Facilitated Diffusion ◦ Transport proteins (channel proteins) allow hydrophilic substances to move across the cell membrane from high concentration to low concentration ◦ No expenditure of energy
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Fig. 7-15 EXTRACELLULAR FLUID Channel protein (a) A channel protein Solute CYTOPLASM Solute Carrier protein (b) A carrier protein
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Passive Transport Osmosis ◦ The diffusion of water across a selectively permeable membrane ◦ Water moves from high water (low solute) to low water (high solute) ◦ Facilitated by aquaporins (transfer channels that allow water to move from high water to low water)
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Lower concentration of solute (sugar) Fig. 7-12 H2OH2O Higher concentration of sugar Selectively permeable membrane Same concentration of sugar Osmosis
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Water Balance of Cells Without Walls Tonicity is the ability of a solution to cause a cell to gain or lose water Isotonic solution: Solute concentration is the same as that inside the cell; no net water movement across the plasma membrane Hypertonic solution: Solute concentration is greater than that inside the cell; cell loses water Hypotonic solution: Solute concentration is less than that inside the cell; cell gains water Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
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Fig. 7-13 Hypotonic solution (a ) Animal cell (b ) Plant cell H2OH2O Lysed H2OH2O Turgid (normal) H2OH2O H2OH2O H2OH2O H2OH2O Normal Isotonic solution Flaccid H2OH2O H2OH2O Shriveled Plasmolyzed Hypertonic solution
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Water Balance Hypertonic or hypotonic environments create osmotic problems for organisms Osmoregulation, the control of water balance, is a necessary adaptation for life in such environments The protist Paramecium, which is hypertonic to its pond water environment, has a contractile vacuole that acts as a pump Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
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Fig. 7-14 Filling vacuole 50 µm (a) A contractile vacuole fills with fluid that enters from a system of canals radiating throughout the cytoplasm. Contracting vacuole (b) When full, the vacuole and canals contract, expelling fluid from the cell.
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