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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 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 Fig. 7-11 Molecules of dye Membrane (cross section) WATER Net diffusion Net diffusion Equilibrium (a) Diffusion of one solute Net diffusion Net diffusion (b) Diffusion of two solutes Net diffusion Net diffusion Equilibrium Equilibrium 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 Fig. 7-15 EXTRACELLULAR FLUID Channel protein Solute CYTOPLASM (a) A channel protein Carrier protein (b) A carrier protein Solute 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) Fig. 7-12 Lower concentration of solute (sugar) Higher concentration of sugar H2O Selectively permeable membrane Osmosis Same concentration of sugar 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 Fig. 7-13 Hypotonic solution H2O Isotonic solution Hypertonic solution H2O H2O H2O (a) Animal cell Lysed H2O Normal H2O Shriveled H2O H2O (b) Plant cell Turgid (normal) Flaccid Plasmolyzed 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 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.
