Membrane Transport

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Biochemistry & Molecular Bio.
David Gross

Chapter 9 Notes: Membrane Transport • all animal cells maintain intracellular ion concentrations that differ from those outside the cell • the plasma membrane prevents their diffusion 9.1 The Thermodynamics of Membrane Transport • inside of cell more negatively charged than outside o generates a voltage across membrane, membrane potential ( ∆φ ) [ion] ∆φ= FT ln ¿ • in simplest case ZF [ion]out o R = gas constant o T = temperature (K) o Z = net charge per ion o F = Faraday constant o units = volts • ion movements alter membrane potential o most animal cells have membrane potential of -70mV o in a nerve:  nerve stimulated, Na+ channels open, sodium moves in w/ gradient  membrane potential becomes more positive (up to +50mV)  this is called action potential  triggers opening of nearby voltage gated K+ channels, K+ diffuses out of cell  restores membrane potential to -70mV  also stimulates opening of Na+ channels farther along axon, action potential travels down axon o action potentials propagate very rapidly b/c of myelin sheath  several layers of membrane coiled around axon  rich in sphingomyelins, contains little protein  prevents ion movement except @ nodes between myelinated segments of axon  action potential jumps from node to node, 20x faster than unwrapped axon • transporters mediate transmembrane ion movement o passive transporters provide a means for ions to move down the concentration gradient  thermodynamically favorable o the free nrg change for the transmembrane movement of a substance from outside to in: [X] ¿  ∆G=RTln [X] out  process is spontaneous when X moves from high to low concentration o for ions, membrane potential must be added: ∆G=RTln [X] ¿+ZF∆φ  [X] out o the protein that initially established the ion gradient of the cells is an active transporter  requires free nrg ofATP to move the ions 9.2 Passive Transport • Porins are β barrel proteins o porins are trimers where each subunit forms 16 – 18 stranded membrane-spanning β barrel  has a water filled core lined w/ hydrophilic side chains  forms transmembrane passageway for molecules w/ molec. mass up to 1000 D o porins can show solute selectivity depending on geometry of barrel interior & nature of side chains projecting into it o porins are always open and solute can travel through in either direction • ion channels are highly selective o usually multimers w/ identical subunits composed of membrane spanning - helices o K+ ion channel well known  tetrameric protein w/ each subunit composed of 3 helices • 1 forms wall of transmembrane pore • 1 faces hydrophobic membrane interior • 1 on extracellular side of protein  10,000 X more permeable to K+ than Na+  high selectivity b/c of geometry, protein folds to accommodate only K+  K+ channels in neurons is larger, 6 subunits • gated
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