MCB 2210 001 2 6 2015 Membrane Transport Continued Diffusion of ions gives rise to electrical potentials o As K ions flow out Cl ions helped maintain charge balance are left behind Cl ions distribute themselves beneath the membrane attracted to cations outside Exert a force across the membrane o The electrical potential create across membranes is called the membrane potential Vm o When the density of negative charges increases enough it exerts a negative inside electrical force that s sufficient to COUNTER the concentration gradient K ion stop flowing out o Only relatively few K ions acutally have to flow for there to be enough Cl ions to generate the electrical force So the concentrations DON T APPRECIABLY CHANGE Nerst Equation gives the membrane potential that s sufficient to counter a given concentration gradient for an ion equilibrium potential o Eion 60 z log ion out ion in z is the charge of the ion o Equilibrium no energy is required to maintain the status quo No net flow of ions No net currents are flowing with or without a concentration gradient In real cells Vm is determined by the conductances of multiple ions o Typically resting cells have K conductance Gk that s 10x higher than the Na and Cl conductances Open K channels make the cells much more permeable to K o Vm Gk Ek GCl ECl GNa ENa GK GCl GNa o Vm 75 mV o Vm will tend to be close to the Eion for the ion that has the most conductances open in the membrane I e Vm will be closest to the most permeable ion o In real cells multiple conductances for different ions are active simultaneously At any given Vm the driving force for an ion is its total electrochemical potential in mV Vm reflects not equilibrium but a steady state There is no net ion flow o Na current is equal and opposite to K current Cells use energy from ATP hydrolysis to pump Na out and pump K in The electrochemical gradient for Na is being used to do useful work o The Na conductance represents the activity of thousands of these transporters Ion Channels o Transmembrane proteins that form a pore through the membrane Allows passage of ions o Multispanning o Typically bidirectional Net direction is down an electrochemical gradient o Ion specific o Interactions with the ion are limited to determining selectivity o Regulated they can be opened and closed gated o Voltage gated K channels Homotetramer Both N and C termini are intracellular Opened by membrane depolarization Selective for K Voltage gated change in membrane charge causes a minor conformational change in the base of the channel opening the pore o Passive transport Uniport Carriers o Multipass transporter that acts more like an enzyme o Bind the substrate to undergo REVERSIBLE CONFORMATIONAL CHANGES Brings about the transport from one side of the membrane to the other No energy input is required other than random thermal energy to drive the change o Bidirectional movement can be in both directions Net movement is down concentration gradient Passive o GLUT transporters 12 membrane domains Intracellular N and C termini Affinity for glucose Active Transporters o Protein complexes that act as pumps to move molecules against a gradient o Require an extra energy input o Primary Active Transporters ATP dependent pumps ATP used as the direct energy source Hydrolyze ATP and use the energy to move one or more molecules across the membrane P type become phosphorylated by phosphate from ATP during transport o Na K ATPase moves 2 K inwards and 3 Na out of the cells using energy from ATP Maintains distribution of these ions in cells Conformational change driven by ATP and binding of ions o K H ATPases stomach acidification o Ca2 ATPases pump Ca2 put of the cells or into the ER V type o Vesicular H ATPases Doesn t become phosphorylated Pumps H into membrane compartments o ABC transporters Light driven pumps light used as the direct energy source o Secondary Active Transporters Coupled transporters that run off ion gradients Use an electrochemical gradient generated by primary active transporters is used to power movement of another molecule against a gradient Symporters cotransporters move two molecules in the same direction One down and one up gradient 2Na 1 glucose symporter works against glucose gradient using Na gradient o accumulates glucose at high concentrations inside the cell Antiporter exchangers move two molecules in opposite directions One down and another up a gradient Na H exchangers Na Ca2 exchangers Action Potentials electrical signals caused by rapid changes in Vm o They are all or none either they fire or they don t o Propagate actively down nerve axons o Neurons encode info by the frequency of action potentials o Coordinated gating of voltage gated Na channels and voltage gated K channels o Vm will tend to be closest to Eion for the ion that has the most channels open in the membrane o How is it propagated Voltage gated Na channels are closed with open inactivation gates Stimulation starts to open voltage gated Na channels causing further depolarization as Na ions flow in At threshold opening of voltage gated Na channels triggers a runaway positive feedback cycle triggering further depolarization and channel opening As it depolarized Vm rockets toward ENa because GNa now dominates o How is it terminated Na channel inactivation gates close with a delay following depolarization No Na current flows even if the inactivation gates stay open Vm would tend to return to rest However voltage gated K channels open speeding up repolarization and causing a slight undershoot of Vm hyperpolarization Repolarization causes Na channel inactivation gate to reopen and causes the voltage gated K channels to close The system is now reset o How is the action potential propagated along the nerve cell membrane Local influx of Na during depolarization moves further down the axon Opens neighboring Na channels as it moves along causing action potential Doesn t propagate backwards because Na channels haven t yet recovered from inactivation o What initiates action potentials Stimuli initiate the opening of a gated channel that depolarizes the membrane potential Voltage gated Ligand gated Mechanically gated Inhibitory signals can feed into neurons to prevent them from firing The net sum of the stimulatory and inhibitory current inputs causes Vm to reach threshold Neurons talk to each other via chemical synapses o Neurotransmitters are released by neurons into synapses between neurons to signal the next neuron to fire