1. The Cell Membrane & Passive Transport

2. The cell membrane is the boundary that separates the living cell from its surroundings Life has an inside and an outside

3. The Phospholipid Bilayer Phospholipids are the most abundant molecule in the cell membrane Hydrophobic tails & a hydrophilic head

4. The Fluid Mosaic Model The membrane is a fluid structure with a mosaic of proteins, glycolipids and sterols embedded in it

5. The Fluid Mosaic Model Phospholipids in the plasma membrane can move within the bilayer

6. Selective Permeability A cell must exchange materials with its surroundings, a process controlled by the plasma membrane Plasma membranes are selectively permeable, they only allow some substances to cross the membrane.

7. Hydrophobic (nonpolar) molecules, can dissolve in the lipid bilayer and pass through the membrane rapidly

8. Polar molecules, such as sugars, do not cross the membrane easily

9. Passive Transport Passive transport is diffusion of a substance across a membrane with no energy investment

10. Concentration Gradient A difference in the number per unit volume of ions or molecules of a substance between adjoining regions

11. Diffusion The net movement of like molecules or ions down a concentration gradient

12. (b) Diffusion of two solutes Fig. 7-11b Net diffusion Equilibrium

13. Diffusion Rate Determined by steepness of gradient, molecular size, temperature & electric or pressure gradients

14. Osmosis Osmosis is the diffusion of water across a selectively permeable membrane Water diffuses across a membrane from the region of lower solute concentration to the region of higher solute concentration

15. Lower concentration of solute (sugar) Fig. 7-12 H2OH2O Higher concentration of sugar Selectively permeable membrane Same concentration of sugar Osmosis

16. Tonicity Tonicity is the ability of a solution to cause a cell to gain or lose water

17. Hypertonic Solution Solute concentration is greater than that inside the cell; cell loses water

18. Hypotonic Solution Solute concentration is less than that inside the cell; cell gains water

19. Isotonic Solution Solute concentration is the same as that inside the cell; no net water movement across the plasma membrane

20. 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 Video: Chlamydomonas Video: Chlamydomonas Video: Paramecium Vacuole Video: Paramecium Vacuole

21. 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.

22. Water Balance of Cells with Walls Cell walls help maintain water balance A plant cell in a hypotonic solution swells until the wall opposes uptake; the cell is now turgid (firm) If a plant cell and its surroundings are isotonic, there is no net movement of water into the cell; the cell becomes flaccid (limp), and the plant may wilt Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

23. Video: Plasmolysis Video: Plasmolysis Video: Turgid Elodea Video: Turgid Elodea Animation: Osmosis Animation: Osmosis In a hypertonic environment, plant cells lose water; eventually, the membrane pulls away from the wall, a usually lethal effect called plasmolysis