1. Membranes and Transport Chapter 6

2. 6.1 Membrane Structure  Biological membranes contain both lipid and protein molecules  Fluid mosaic model explains membrane structure  Fluid mosaic model is fully supported by experimental evidence

3. Biological Membranes  Membrane phospholipids, membrane proteins Both have hydrophobic and hydrophilic regions Dual solubility properties

4. Phospholipid Bilayer  Membranes are based on fluid phospholipid bilayer  Polar regions of phospholipids lie at surfaces of bilayer  Nonpolar tails associate together in interior

5. Phospholipid Bilayer Fig. 6-2, p. 120

6. Cholesterol in Bilayers Fig. 6-3, p. 121

7. Membrane Proteins  Membrane proteins are suspended individually in the bilayer  Hydrophilic regions at the membrane surfaces  Hydrophobic regions in the interior

8. Structure of Membrane Proteins Fig. 6-4, p. 121

9. The Lipid Bilayer  Forms the structural framework of membranes  Serves as a barrier that prevents passage of most water-soluble molecules

10. Functions of Membrane Proteins  Proteins embedded in the phospholipid bilayer perform most membrane functions Transport of selected hydrophilic substances Recognition Signal reception Cell adhesion Metabolism

11. Types of Membrane Proteins  Integral membrane proteins Embedded deeply in the bilayer Can’t be removed without dispersing the bilayer  Peripheral membrane proteins Associate with membrane surfaces

12. Lipid Bilayer Organization  Membranes are asymmetric Different proportions of phospholipid types in the two bilayer halves

13. Membrane Structure Fig. 6-5, p. 122

14. Frye-Edidin Experiment Fig. 6-6, p. 124

15. 6.2 Functions of Membranes in Transport: Passive Transport  Passive transport is based on diffusion  Substances move passively through membranes by simple or facilitated diffusion  Two groups of transport proteins carry out facilitated diffusion

16. Passive Transport  Depends on diffusion Net movement of molecules with a concentration gradient (from region of higher concentration to region of lower concentration)  Does not require cells to expend energy

17. Transport Mechanisms Table 6-1, p. 125

18. Simple Diffusion  Passive transport of substances across lipid portion of cellular membranes with their concentration gradients  Proceeds most rapidly for small molecules that are soluble in lipids

19. Facilitated Diffusion  Passive transport of substances at rates higher than predicted from their lipid solubility Depends on membrane proteins Follows concentration gradients Specific for certain substances Becomes saturated at high concentrations of the transported substance

20. Channel Proteins: Aquaporin Fig. 6-8a, p. 127

21. Carrier Proteins Fig. 6-8b, p. 127

22. Transport Control  Most proteins that carry out facilitated diffusion of ions are controlled by “gates” that open or close their transport channels

23. 6.3 Passive Water Transport and Osmosis  Osmosis can operate in a purely physical system  Free energy released by osmosis may work for or against cellular life

24. Osmosis  Net diffusion of water molecules Across a selectively permeable membrane In response to differences in concentration of solute molecules

25. Osmosis Fig. 6-9, p. 129

26. Tonicity  Water moves From hypotonic solution (lower concentrations of solute molecules) To hypertonic solution (higher concentrations of solute molecules)  When solutions on each side are isotonic No osmotic movement of water in either direction

27. Tonicity Fig. 6-10, p. 130

28. Turgor Pressure and Plasmolysis in Plants Fig. 6-11, p. 131

29. 6.4 Active Transport  Active transport requires a direct or indirect input of energy derived from ATP hydrolysis  Primary active transport moves positively charged ions across membranes  Secondary active transport moves both ions and organic molecules across membranes

30. Active Transport  Moves substances against their concentration gradients; requires cells to expend energy Depends on membrane proteins Specific for certain substances Becomes saturated at high concentrations of the transported substance

31. Active Transport Proteins  Primary transport pumps Directly use ATP as energy source  Secondary transport pumps Energy source: Concentration gradient of positively charged ions (created by primary transport pumps)

32. A Primary Active Transport Pump Fig. 6-12, p. 132

33. Secondary Active Transport  Symport Transported substance moves in same direction as concentration gradient used as energy source  Antiport Transported substance moves in direction opposite to concentration gradient used as energy source

34. Coupled Secondary Active Transport Fig. 6-13, p. 133

35. 6.5 Exocytosis and Endocytosis  Exocytosis releases molecules outside cell By means of secretory vesicles  Endocytosis brings materials into cells In endocytic vesicles

36. Transporting Larger Substances  Exocytosis and endocytosis Move large molecules, particles in and out of cells  Mechanisms allow substances to leave and enter cells without directly passing through the plasma membrane

37. Exocytosis  Vesicle carries secreted materials Fuses with plasma membrane on cytoplasmic side  Fusion Vesicle membrane joins plasma membrane Releases vesicle contents to cell exterior

38. Exocytosis Fig. 6-14a, p. 134

39. Endocytosis  Encloses materials outside cell in plasma membrane Pockets inward and forms endocytic vesicle on cytoplasmic side  Two main forms Bulk-phase (pinocytosis) Receptor-mediated endocytosis

40. After Endocytosis  Most materials that enter cells are digested into molecular subunits Small enough to transport across vesicle membranes

41. Endocytosis: Pinocytosis Fig. 6-14b, p. 134

42. Receptor-Mediated Endocytosis Fig. 6-14c, p. 134

43. Phagocytosis Fig. 6-15, p. 136