1. A Closer Look at Cell Membranes Chapter 5 Part 2

2. 5.5 Membrane Trafficking  By processes of endocytosis and exocytosis, vesicles help cells take in and expel particles that are too big for transport proteins, as well as substances in bulk

3. Exocytosis and Endocytosis  Exocytosis The fusion of a vesicle with the cell membrane, releasing its contents to the surroundings  Endocytosis The formation of a vesicle from cell membrane, enclosing materials near the cell surface and bringing them into the cell

4. Three Pathways of Endocytosis  Bulk-phase endocytosis Extracellular fluid is captured in a vesicle and brought into the cell  Receptor-mediated endocytosis Specific molecules bind to surface receptors, which are then enclosed in an endocytic vesicle (Example – Cholesterol)  Phagocytosis Pseudopods engulf target particles and merge as a vesicle, which fuses with a lysosome in the cell (Example – WBC’s)

5. Membrane Cycling  New membrane proteins and lipids are made in the ER, modified in Golgi bodies, and form vesicles

6. Exocytic Vesicle

7. Fig. 5-12, p. 86 F Some vesicles and their contents are delivered to lysosomes. lysosome B The pits sink inward and become endocytic vesicles. C Vesicle contents are sorted. Exocytosis D Many of the sorted molecules cycle to the plasma membrane. E Some vesicles are routed to the nuclear envelope or ER membrane. Others fuse with Golgi bodies. Golgi Endocytosis A Molecules get concentrated inside coated pits at the plasma membrane. coated pit Stepped Art

8. 5.6 Which Way Will Water Move?  Water diffuses across cell membranes by osmosis  Osmosis is driven by tonicity

9. Osmosis  Osmosis The movement of water down its concentration gradient (high to low) – through a selectively permeable membrane from a region of lower solute (higher water) concentration to a region of higher solute concentration (lower water)  Tonicity The relative concentrations of solutes (NaCl or sucrose) in two fluids separated

10. Tonicity  For two fluids separated by a semipermeable membrane, the one with lower solute concentration is hypotonic, and the one with higher solute concentration is hypertonic  Isotonic fluids have the same solute concentration

11. Fig. 5-16, p. 88 hypotonic solution hypertonic solution solutions become isotonic selectively permeable membrane A Initially, the volume of fluid is the same in the two compartments, but the solute concentration differs. B The fluid volume in the two compartments changes as water follows its gradient and diffuses across the membrane.

12. Animation: Tonicity and water movement

13. Fig. 5-17a, p. 89 2% sucrose 10% sucrosewater A What happens to a semipermeable membrane bag when it is immersed in an isotonic, a hypertonic, or a hypotonic solution?

14. Fig. 5-17 (b-d), p. 89 B Red blood cells in an isotonic solution do not change in volume. Study these figures C Red blood cells in a hypertonic solution shrivel because water diffuses out of them. D Red blood cells in a hypotonic solution swell because water diffuses into them.

15. Effects of Fluid Pressure  Hydrostatic pressure (turgor) The pressure exerted by a volume of fluid against a surrounding structure (membrane or cell wall) Plant (hypertonic) – wilt, limp, plasmolysis, shrivel (5.18c) Plant (hypotonic) – increase in turgor pressure, cell wall prevents the cell from exploding.

16. Animation: Solute concentration and osmosis

17. Video: Diffusion of dye in water

18. Video: Contractile vacuole