A Closer Look at Membranes Chapter 5

Main component of cell membranes Gives membrane its fluid properties Fatty acid tails sandwiched between hydrophilic heads 5.1 Lipid Bilayer Figure 5.2c Page 82 lipid bilayer fluid

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

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

5.4 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 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

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 5.6 Transport Proteins

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

5.7 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 cellsafter plasmolysis

5.8 Endocytosis and Exocytosis plasma membrane cytoplasm Exocytosis Endocytosis

Endocytosis Pathways Bulk phase Receptor-mediated 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