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 Molecules of dye Membrane (cross section) WATER Net diffusion Equilibrium (a) Diffusion of one solute Net diffusion Equilibrium (b) Diffusion of two solutes

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 EXTRACELLULAR FLUID Channel protein (a) A channel protein Solute CYTOPLASM Solute Carrier protein (b) A carrier protein

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)

Lower concentration of solute (sugar) Fig H2OH2O Higher concentration of sugar Selectively permeable membrane Same concentration of sugar Osmosis

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 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

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 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.