A Closer Look at Membranes Chapter 5 Cystic

A Closer Look at Membranes Chapter 5

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

Lipid Bilayer • Main component of cell membranes • Gives membrane its fluid properties • Fatty acid tails sandwiched between hydrophilic heads lipid bilayer fluid Figure 5. 2 c Page 82

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

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

Membrane Experiments • Split membranes reveal embedded proteins In-text figure Page 86

Membrane Experiments • Hybrid human-mouse cell shows some proteins drift within membrane human cell mouse cell hybrid cell Figure 5. 6 Page 86

Selective Permeability O 2, CO 2, glucose and other large, polar, and other small, nonpolar water-soluable molecules; ions, molecules; some water molecules

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

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

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)

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

Example of Diffusion

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

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

Passive Transport glucose transporter solute (glucose) high low Stepped Art Figure 5. 10 Page 88

higher calcium concentration Active Transport lower calcium concentration ATP Pi ADP Stepped Art Figure 5. 11 Page 89

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

Tonicity 2% sucrose solution distilled water 10% sucrose solution 2% sucrose solution Hypotonic Conditions Hypertonic Conditions Isotonic Conditions Figure 5. 13 Page 90

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

Exocytosis Endocytosis and Exocytosis plasma membrane cytoplasm Endocytosis cytoplasm

Endocytosis Pathways • Bulk phase • Receptor-mediated • Phagocytosis clathrin Figures 5. 17, 5. 18 Pages 92, 93

Membrane Cycling clathrin Exocytosis and endocytosis continually replace and withdraw patches of plasma membrane Golgi body Figure 5. 19 Page 93 lysosome