MCB 2210 001 2 5 2015 MAKEUP LECTURE Announcements Don t forget to sign up for the exam Link is in HuskyCT Make sure if taking with the CSD to get that sorted out Membranes and Proteins Continued Beta Sheets can also interact with membranes o The R groups from the sheets are organized so that the non polar are on one side and polar on the other o Sheet is rolled into a tube Beta barrel o Hydrophobic parts are out toward the bilayer o Inside is polar o Forms a pore though the membrane that s a hydrophilic environment The fluid mosaic model o Lipid bilayer is a flexible 2D fluid sheet o Membrane proteins are embedded and float in this sheet o Proteins can move laterally in the plane of the membrane but A protein cannot easily leave the membrane once inserted Too much energy needed to do so Thermodynamically unfavorable Topology of a protein cannot easily change once inserted in the membrane The C terminus will want to stay where it is and the N terminus will want to stay where it s at The conformation can change To allow proteins to pass signals from outside to inside Many proteins are constrained and cannot move freely within the membrane o Restricted in their location to a particular region of the membrane membrane domain o Some proteins may be evenly distributed while others are restricted o The same protein may be restricted in one membrane but not in another o Cells employ a variety of mechanisms to non randomly distribute proteins Link them to membrane proteins Link them to outside molecules Link them to inside molecules Cytoskeleton has some static characteristics Prevent their diffusion to parts of the membrane Methods for measuring protein mobility o Cell fusion fuse 2 different types of cells with 2 different nuclei Label proteins of one cell with red Label those of a 2nd with green dye Fuse the membranes of both to form a heterokaryon You have to force this to happen Result over time they become mixed Shows that the proteins do move o FRAP of fluorescently labeled membrane protein Bleach out a section of membrane and see if fluorescent proteins move into the non fluorescent area o Single particle tracking Track them via antibodies that linked to quantum dots nano crystals that emit light that can be measured Show a time lapse video to actually see where the labeled protein goes Lipid Rafts composed of sphigolipids cholesterol and proteins o Tend to accumulate different proteins than the non raft areas o Microdomains of membranes little patches o GPI linked proteins are on some rafts Extracellular Membrane Transport Membranes with their proteins are selectively permeable o The proteins determine what the membrane is permeable to Molecules diffuse across the membrane at different rates depending on size and chemical properties o For those that cannot cross easily there are transporters to control their entry Passive Diffusion pass through the membrane directly no protein involvement o Dependent upon the gradient of the molecule o Small uncharged nonpolar molecules o Gases CO2 and O2 o H2O and urea but slowly o Rate decreases as molecules get larger and more polar o Small charged molecules cross very slowly and may be effectively impermeable without help o Rate of movement is directly related to solubility of molecule in lipid bilayer partition coefficient More soluble faster diffusion Transport Proteins enzymes that catalyze movement of specific substances across the membrane o Passive Transporters allow net movement down a chemical electrical electrochemical gradient high to low without requiring energy beyond thermal motion Uniporter carrier proteins facilitated diffusion Ion channels o Active Transporters move molecules against a gradient low to high but DO require extra energy input for transport ATP dependent pumps Symporter co transporter Antiporter Carrier proteins are only open to one side of the membrane at a given time o They have binding sites for certain solutes so they have affinities for certain solutes o When bound to the solute it undergoes a conformational change to move that solute to the other side of the membrane o Interacts directly with solute Channel proteins form aqueous pores that don t have much interaction with the channel itself Facilitated Diffusion diffusion by carrier proteins that can be saturated due to binding of solute to carrier o Unlike simple diffusions Cells use transport proteins to generate an internal ionic environment that s different than the external environment and to exploit the resulting gradients to perform critical functions o Ionic composition isn t the same inside and out o When the total anions total cations electroneutrality o When the total number of particles inside and out osmolarity is the same Hypotonic solution cell has higher solute concentration than the environment and lower water concentration o Net water gain cell swells Hypertonic solution cell has lower solute concentration and higher water concentration than the cell o Net water loss cell shrinks Isotonic solution the cell has the same concentrations as the environment o No net water movement Electrochemical gradients the combination of the chemical concentration gradient and the electrical gradient created by charged ions and molecules determines the rate and direction of transport of a charged molecule o So concentration gradients are the only forces moving charges species
View Full Document
Unlocking...