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lecture 1

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University of Toronto St. George
Doug Mac Kay

Membrane TransportSeptember 10 2007 Cell Membranephospholipid bilayergases and lipidsoluble molecules diffuse through readilyimpermeable to organic anions proteinspermeability depends on molecular size lipid solubility and charge o large and charged molecules are impermeablewater diffuses via aquaporin channels Membrane Lipids polar endglycerolphosphate hydrophilicamino acid cholineserineetcnonpolar endtwo long fatty acid chains hydrophobic o one fatty acid chain is saturated and the other polyunsaturated kinkyunsaturated chains determine properties of the membranescholesterol is inserted among fatty acid chains Membrane Fluidity determined by lipid composition proportion of unsaturated and saturated fatty acids o essential for efficient carrier and channel protein function3 fatty acids are necessary for maintaining membrane fluiditycholesterol stabilizes membraneprevents diffusion of polar molecules across it o cholesterol maintains membrane integrityreduced by aging and poor diet o lose cholesterol with agemembrane becomes brittlemembrane is too fluid and falls apart o a deficiency in 3 fatty acids due to improper diet of trans fats causes negative effects on membrane fluidity and transporter action and reduces the available vesicle pool for some neurotransmitters at the synapsesMembrane Proteins functions of membrane proteins o structural support for cytoskeletonex anchor points for organelles in the cells o transmembrane channels for diffusion of selected moleculesionsact as pores o selective transporters carriers of molecules across membraneex openclose gateo receptors on the outside of membranes to detect signaling molecules and signal specific reactions within the cell o enzymesion pumps for active transportex ATP pumps to move molecules across their concentration gradientMembraneSpanning Proteinsglobular proteins crisscross the membrane 67 timescarbohydrate chains can be attached to the proteins in the extracelluar space Glycocalyx matrix of long carbohydrate filaments o fill extracellular space outsideo slows diffusionanchored to membrane proteins or lipidscontain hyaluronic acid chondroitin sulfate and heparinnet negative charge due to negative charge of the phosphate groups and of the glycocalyx2 o sequesters Cao stabilizes membrane potential o creates a low pH on the extracellular surface of the membraneimportant for cell identification and communication triggering immune responses cell adhesion o ex immune system recognizes invaders because they have the wrong glycocalyxextracellular nets of chondroitin sulfate proteoglycans ensheath clusters of neurons in parts of the brain ex cerebral cortex o leticans crosslink the glycocalyx filaments to create a net 1
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