9 16 13 Chapter 12 Membrane Transport Membranes are selective barriers 1 9 16 13 Be Able To Compare contrast simple diffusion facilitated diffusion and active transport Identify whether a molecule can passively diffuse through a membrane Explain the difference between a channel and transporter Calculate which net direction molecules would be transported given specific concentrations of molecules across the membrane Describe the Na K ATPase cycle Define the different gating mechanisms for ion channels Describe the different properties of a single ion channel including its ion conduction and gating characteristics Explain why some molecules require energy for transport across a membrane while other do now Be Able To Describe how an action potential is initiated and terminated at a nerve terminus Identify the basic components of a nerve cell Identify the primary ions and channels involved in each phase of an action potential Describe how the electrical signal of a cell is converted to a chemical signal and back again Predict the shape of an action potential if the gating or inactivation properties of a channel are altered Define the role of inactivation in the action potential What happens if channel do not inactivate Describe how an action potential propagates long an axon 2 9 16 13 Diffusion of molecules across a synthetic lipid bilayer is dependent on size and solubility in lipid Charged molecules rarely diffuse across a lipid bilayer no matter how small Membrane transport proteins are responsible for the selective transfer of water soluble molecules solutes across the membrane 3 9 16 13 Selective transport can lead to the differential distribution of solutes inside and outside the cell and also between the cytosol and organelles 4 9 16 13 Simple Diffusion Movement of molecules from high to low concentration Often referred to as down hill movement of molecules 150 mM Na 15 mM Na Diffusion is energetically favorable Negative G G RT ln solute inside solute outside The concentration of K inside the cell is much greater than outside How is this accomplished 5 mM K 150 mM K 5 9 16 13 Classification of membrane transport proteins Mechanism of Solute Discrimination Carrier proteins Channels Transport Energetics Passive Active Mechanisms of solute discrimination Difference between transporter and channel 6 9 16 13 Transport Energetics Facilitated diffusion Carriers may be passive or active Channels passive only Passive Transport Net rate of uptake Membrane transporters increase the rate of diffusion facilitated diffusion Simple diffusion External solute concentration Why does the rate of facilitated diffusion plateau 7 9 16 13 Carrier proteins passive transport Uncharged molecules Reversible For an uncharged molecule like glucose the direction of transport is dependent solely on concentration Carrier proteins passive transport Charged molecules Cell membranes have a voltage across them called the membrane potential E that exerts a force on charged molecules 150 mM Na 15 mM Na Net driving force electrochemical gradient Sum of the concentration and electrical forces G RT ln solute inside solute outside FEM 8 9 16 13 Electrochemical Gradient 2 Components 12 08 electroch gradient jpg Active Transport Transport of solutes against their electrochemical gradient Uphill transport 5 mM K 1 mM glucose 150 mM K 10 mM glucose 9 9 16 13 Mechanisms of Active Transport Active Transport ATP driven pumps Na K ATPase also called Na K pump 3 2 The energy from ATP hydrolysis is used to drive Na out and K in both against their electrochemical gradients Establish membrane potential 10 9 16 13 Cyclic mechanism of the Na K pump 3 Na 2 K exchange 30 total cellular ATP consumption Active transport coupled transporters Facilitated diffusion 11 9 16 13 The Na gradient generated by the Na K pump is used in animal cells as an energy source to drive transport of many other solutes by coupled transport The uphill transport of glucose can be driven by the downhill transport of Na Some cells use both active and passive transport mechanisms on the same solute asymmetric distribution of transport proteins 12 9 16 13 The Na K pump also helps maintain osmotic balance Na and indirectly Cl concentrations outside the cell are kept high Ion Channels Control Many Types of Behavior In Both Plants and Animals 13 9 16 13 Nerve cell neuron structural components Signals transmitted Signals received A signal is communicated as change in membrane potential Ion channels Selective gated passive transport 14 9 16 13 Defects in Ion Channels A Cause of Inherited Diseases A number of inherited disorders are the result of defects in ion channels Ex Migraines Ca 2 Myasthenia gravis Na Cystic Fibrosis Cystic Fibrosis CF Cl transporter Perturbed H2O balance increases and CFTR itself increases susceptibility to bacterial infections esp Pseudomonas aeruginosa Ion channels Channels flicker between the closed and open states Channels allow very high transport rates Much higher than carriers How could this process be regulated 15 9 16 13 Patch clamp recording of single ion channel activity Current through a single ion channel measured by patch clamp 16 9 16 13 Ion channels are gated triggered by a specific stimulus 17 9 16 13 Different ion concentrations on either side of the lipid bilayer results in the membrane potential K leak channels help maintain the membrane potential 5 mM K 150 mM K E 60 mV The Na K pump establishes a high inside K concentration K leak channels allow K to move down its chemical gradient out of the cell As K flows out it leaves behind a net negative charge the membrane potential This membrane potential opposes further K movement The electrochemical gradient for K is zero 18 9 16 13 Your book fubars the Nernst equation EM membrane potential in volts EM RT zF ln inside outside Recognize the similarity to the free energy equation relating to differential concentrations across a membrane and how charge affects this potential energy 19 9 16 13 Nerve cell neuron structural components Signals transmitted Signals received A signal is communicated as change in membrane potential A stimulus causes a localized membrane depolarization Resting membrane potential 60 mV Stimulus signal membrane potential 40 mV Spreading this signal long distances is difficult 20 9 16 13 A localized membrane depolarization large enough to pass a critical threshold will activate voltage gated Na channels Stimulus signal membrane potential 40 mV Na floods into