A Closer Look at Membranes Chapter 5

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

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

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

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

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

Hybrid human-mouse cell shows some proteins drift within membrane Hybrid human-mouse cell shows some proteins drift within membrane Membrane Experiments human cellmouse cellhybrid cell Figure 5.6 Page 86

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

Membrane Crossing Mechanisms Diffusion across lipid bilayer Passive transport Active transport EndocytosisExocytosis

Concentration Gradient Means the number of molecules or ions in one region is different than the number in another region 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 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 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) 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 Steepness of concentration gradient –Steeper gradient, faster diffusion Molecular size Molecular size –Smaller molecules, faster diffusion Temperature Temperature –Higher temperature, faster diffusion Electrical or pressure gradients Electrical or pressure gradients

Example of Diffusion

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

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

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

ATP ADP PiPi higher calcium concentration lower calcium concentration Figure 5.11 Page 89 Stepped Art Active Transport

Osmosis Water molecules tend to diffuse down water concentration gradient Water molecules tend to diffuse down water concentration gradient Total number of molecules or ions dictates concentration of water Total number of molecules or ions dictates concentration of water Tonicity - relative solute concentrations 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 Hydrostatic pressure Turgor pressure Turgor pressure Osmotic pressure Osmotic pressure normal plant cellsafter plasmolysis

Endocytosis and Exocytosis plasma membrane cytoplasm Exocytosis Endocytosis

Endocytosis Pathways Bulk phase Bulk phase Receptor-mediated Receptor-mediated Phagocytosis Phagocytosis clathrinFigures 5.17, 5.18 Pages 92, 93

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