Membrane Functions Cell Membranes 1. boundaries 6. Cell-cell Today’s Topics 2. Localize adhesion • Finish Cell Structure and Movement specific – Cytoskeleton functions – Motor Proteins • Membrane Structure – Fluid Mosaic 5. Cell-cell – How appropriate fluidity is maintained communication • Diffusion and Osmosis http://library.thinkquest.org/C004535/media/cell_membrane.gif 3. Transport Sept 19, 2011 4. Signal detection Figure 7.5 Phospholipid bilayer Fibers of extra- cellular matrix (ECM) Glyco- Carbohydrate protein Glycolipid Fig. 7-2 EXTRACELLULAR SIDE OF MEMBRANE WATER! Hydrophilic head! Cholesterol Microfilaments Peripheral of cytoskeleton proteins Hydrophobic Integral protein tail! CYTOPLASMIC SIDE OF MEMBRANE WATER! Membrane Structure: The Fluid Mosaic Model 1972 Singer & Nicholson • Proteins embedded and floating in a sea hydrophobic of phospholipids A B Phospholipid bilayer hydrophilic Protein and Lipid raft Figure 7.3 1 • Membrane proteins and lipids are Figure 7.6 synthesized in the ER and Golgi The Fluidity of Membranes apparatus 1 Transmembrane glycoproteins ER Secretory protein ~25% of known genes code for membrane Glycolipid Golgi 2 proteins apparatus Vesicle Most drugs target Lateral movement occurs Flip-flopping across the membrane membrane proteins !107 times per second. is rare (! once per month). 3 Plasma membrane: Cytoplasmic face 4 Extracellular face Transmembrane glycoprotein Secreted protein Membrane glycolipid Figure 7.10 Figure 7.7 Evidence for integral membrane proteins: Evidence for membrane fluidity? Freeze-Fracture Electron Microscopy Extracellular layer A cell is frozen and fractured with a knife. The fracture plane often follows the hydrophobic interior of a membrane, splitting the phospholipid bilayer into two separated layers. The membrane proteins go wholly with one of the layers. Mixed proteins Knife Proteins Mouse cell after 1 hour Human cell Hybrid cell Plasma membrane Cytoplasmic layer Illustrates: asymmetry of membrane components External Leaflet Cytoplasmic Leaflet Figure 7.4 Extracellular layer Cytoplasmic layer Fluid Viscous Unsaturated hydrocarbon Saturated hydro- Cholesterol tails with kinks Carbon tails Figure 7.5 (c) Cholesterol within the animal cell membrane (b) Membrane fluidity Figure 7.5 B 2 Figure 7.13a 1. Lipid bilayers are selectively permeable Simple Diffusion: • small,nonpolar • small uncharged, polar • larger uncharged, polar molecules • ions Net diffusion Net diffusion Equilibrium (a) Diffusion of one solute Decreasing Size – polarity - ions permeability Osmosis Low High [Solute]! [Solute]! H2O! Semi- permeable membrane More Less free free water water Osmosis! 3 .