02/27/2007 05:16 PMLEC20-21_MembraneTransportPage 1 of 11http://www.biochem.arizona.edu/classes/bioc460/spring/460web/lectures/LEC20-21_MembraneTransport/LEC20-21_MembraneTransport.htmlLectures 20-21: Biological Membranes 3-4 -- Transport [PDF]Reading: Berg, Tymoczko & Stryer, Chapter 13, pp. 345-364problems in textbook: chapter 13, pp. 379-380: #1, 3 (done in lecture notes), 14a, 20 Updated on: 2/27/07 at 4:15 pm Key ConceptsFree energy of transporting material across membrane depends on concentration gradient across membrane:For uncharged solutes, Solutes move spontaneously (ΔGt < 0) from compartment of higher concentration to compartment of lower concentration.Equilibrium: ΔG = 0 when C1 = C2Charged solutes: presence of a membrane potential as well as the chemical concentration gradient influences the distribution of ions:Passive transport: spontaneous passage of solute "down" its concentration and/or electrical potential gradient -- no input of free energy required.Simple diffusion (no assistance)Facilitated diffusion (rate enhanced by carrier or channel, generally an integral membrane protein (transporter or permease)rapid diffusion, "down" a concentration gradientsaturable (reaches a maximum velocity that depends on transporter concentrationspecific (depends on interaction of solute with transporter)Example: GLUT1 glucose transporter in erythrocytesGated ion channels (ligand-gated or voltage-gated)VERY rapid, ~107-108 ions/sec, "down" a concentration gradientnot saturabledegree of specificity/ion selectivity variesExamples:Acetylcholine receptor of motor neuronsbacterial potassium channelEukaryotic sodium, potassium, and calcium channelsUniport (system in which one solute transported)Cotransport (system in which transport of one solute is coupled to transport of another)Symport (different solutes transported in same direction)Antiport (different solutes transported in opposite directions)Active transportPrimary active transport (transport of solute against its concentration gradient, coupled directly to an exergonic chemical reaction, e.g., ATPhydrolysis)ExamplesCa2+ ATPase of muscle cell sarcoplasmic reticulumNa+-K+ ATPase of animal cell plasma membranesSecondary active transport (energy from ATP hydrolysis is used to generate a gradient of another solute, and the transport/"flow" of that othersolute "down" its concentration gradient is used to drive transport of a different solute against its concentration gradient)ExamplesE. coli lactose permease (H+-lactose symporter)Na+-glucose symporter (in some animal cells)Transport processes involving membrane proteins usually involve protein conformational changes.ObjectivesTerminology: membrane potential, passive transport, simple diffusion, facilitated diffusion, active transport, gated channel, P-type ATPase,secondary active transport, cotransport, symport, antiportWhat defines equilibrium for a transport process involving an uncharged solute? (Express it in words, and in terms of concentrations at02/27/2007 05:16 PMLEC20-21_MembraneTransportPage 2 of 11http://www.biochem.arizona.edu/classes/bioc460/spring/460web/lectures/LEC20-21_MembraneTransport/LEC20-21_MembraneTransport.htmlthe "origin" and at the "destination", C1 and C2, and also in terms of ΔGtransport.)Explain free energy changes in biological transport reactions in terms of whether they are "favorable" (solute is moving in direction to gotowards equilibrium, ΔGtransport < 0) or "unfavorable" (would have to go in other direction to go towards equilibrium, ΔGtransport > 0). Given the equation for free energy of transport, be able to calculate ΔGtransport for an uncharged solute (e.g., glucose), given theconcentrations on both sides of the membrane and the direction of transport, and be able to explain the 2 terms involved in calculating ΔGtransport for a charged solute, under what conditions the first term (concentration gradient term) would be favorable, and under whatconditions the 2nd term (electrical gradient term) would be favorable.Describe the GLUT1 erythrocyte glucose transporter with respect to its biological function; kinetics of transport ("saturation" behavior(plots of Vo vs. [glucose] and 1/Vo vs. 1/[glucose]); and proposed mode of action.Explain whether ion channels mediate passive or active transport, and use the acetylcholine receptor to explain how one ion channel mightcontrol the flow of ions.Name 2 P-type ATPases and briefly explain their biological functions. Which one consumes over 30% of the ATP in animal cells in orderto establish/maintain the [Na+] and [K+] concentration gradients and electrical potential across the plasma membrane? Which oneestablishes/maintains the Ca2+ gradient involved in control of muscle contraction?Outline the proposed mechanism by which P-type ATPases couple ATP hydrolysis with "uphill" transport of solutes, with thesarcoplasmic reticulum Ca2+-ATPase as the example. Briefly explain how the E. coli lactose permease couples H+ transport and lactose transport, as an example of a membrane protein thatfunctions as a secondary active transporter, coupling "downhill" transport of one solute to "uphill" transport of another solute.Membrane transport: How polar molecules and ions cross biological membranesThermodynamics of Transport Processes:EQUILIBRIUM FOR A TRANSPORT PROCESS (AS FOR ANY PROCESS): CONDITIONS UNDER WHICH ΔGt = 0.uncharged solutes: free energy change for transporting an uncharged solute across a membrane depends only on concentration gradient across themembrane:For uncharged solute:where C1 = concentration at "origin"and C2 = concentration at "destination"Uncharged solutes move from region of higher concentration to region of lower concentration (the direction in which C1 > C2) soΔGt = 0, when C1 = C2 (Equal concentrations defines equilibrium for a transport process involving uncharged solutes.)charged solutesElectrical potential (charge gradient across membrane) influences distribution of ions.For ion of electrical charge Z: where F = Faraday constant [96.5 kJ/(V•mol)] and ΔV = membrane electrical potential (charge gradient across membrane), in Volts. ΔV (charge gradient, i.e., electrical potential gradient)can result from concentration differences across membrane for OTHER ions than the one you're looking atcan work either with or against concentration gradient (first term)ΔV termcan make an already unfavorable process (from conc. gradient) even less favorable, orcan actually make an otherwise
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