A Closer Look at Membranes

1. A Closer Look at Membranes Chapter 5

2. Cystic Fibrosis • Caused by defective protein channel (CFTR) in epithelial cells • Not enough Cl- and water cross membranes • Thick mucus clogs airways and ducts throughout body • Usually fatal by age 30

3. Lipid Bilayer • Main component of cell membranes • Gives membrane its fluid properties • Fatty acid tails sandwiched between hydrophilic heads lipid bilayer fluid fluid Figure 5.2cPage 82

4. Fluid Mosaic Model • Membrane is a mosaic of • Phospholipids • Glycolipids • Sterols • Proteins • Most phospholipids and some proteins can drift through membrane

5. Membrane Proteins adhesion protein receptor protein recognition protein communication protein passive transporter active transporters Figure 5.5 Page 85

6. Membrane Experiments • Split membranes reveal embedded proteins In-text figurePage 86

7. Membrane Experiments • Hybrid human-mouse cell shows some proteins drift within membrane human cell mouse cell hybrid cell Figure 5.6Page 86

8. Selective Permeability O2, CO2, and other small, nonpolar molecules; some water molecules glucose and other large, polar, water-soluable molecules; ions, water molecules

9. Membrane Crossing Mechanisms Diffusion across lipid bilayer Passive transport Active transport Endocytosis Exocytosis

10. Concentration Gradient • Means the number of molecules or ions in one region is different than the number in another region • In the absence of other forces, a substance moves from a region where it is more concentrated to one where it’s less concentrated - “down” gradient

11. Diffusion • The net movement of like molecules or ions down a concentration gradient • Although molecules collide randomly, the net movement is away from the place with the most collisions (down gradient)

12. Factors Affecting Diffusion Rate • Steepness of concentration gradient • Steeper gradient, faster diffusion • Molecular size • Smaller molecules, faster diffusion • Temperature • Higher temperature, faster diffusion • Electrical or pressure gradients

13. Example of Diffusion

14. Transport Proteins • Span the lipid bilayer • Interior is able to open to both sides • Change shape when they interact with solute • Move water-soluble substances across a membrane

15. Passive and Active Transport Passive Transport Active Transport • Doesn’t require energy inputs • Solutes diffuse through a channel inside the protein’s interior • Net movement is down concentration gradient • Requires ATP • Protein is an ATPase pump • Pumps solute against its concentration gradient

16. Figure 5.10Page 88 Passive Transport glucose transporter solute (glucose) high low Stepped Art

17. Active Transport Figure 5.11Page 89 higher calcium concentration lower calcium concentration ATP Pi ADP Stepped Art

18. Osmosis • Water molecules tend to diffuse down water concentration gradient • Total number of molecules or ions dictates concentration of water • Tonicity - relative solute concentrations

19. Tonicity 2% sucrose solution 2% sucrose solution 10% sucrose solution distilled water Hypotonic Conditions Hypertonic Conditions Isotonic Conditions Figure 5.13Page 90

20. Fluid Pressure • Hydrostatic pressure • Turgor pressure • Osmotic pressure normal plant cells after plasmolysis

21. Exocytosis Endocytosis and Exocytosis plasma membrane cytoplasm Endocytosis cytoplasm

22. Endocytosis Pathways • Bulk phase • Receptor-mediated • Phagocytosis clathrin Figures 5.17, 5.18 Pages 92, 93

23. Membrane Cycling clathrin Exocytosis and endocytosis continually replace and withdraw patches of plasma membrane Golgi body Figure 5.19Page 93 lysosome