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Lecture 15 Passive and active transport Channels and transporters Osmosis

Lecture 15 Passive and active transport Channels and transporters Osmosis. Diffusion across exchange epithelium. “random walk”. Einstein eqn:. < x 2 > - mean square distance (cm 2 ) D – diffusion coefficient (cm 2 /s) t – time interval (s). Need for circulation!.

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Lecture 15 Passive and active transport Channels and transporters Osmosis

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  1. Lecture 15 Passive and active transport Channels and transporters Osmosis

  2. Diffusion across exchange epithelium “random walk” Einstein eqn: <x2> - mean square distance (cm2) D – diffusion coefficient (cm2/s) t – time interval (s)

  3. Need for circulation! The Einstein relationship is non-liner: For a small molecule diffusing in the cytoplasm: D = 0.5·10-5 cm2/s x = 1 mm t = 1 ms x = 10 mm t = 100 ms x = 100 mm t = 10 s x = 1 m t = 107 s = 3.2 yr

  4. Ability to Move by Diffusion

  5. 1 2 concentration gradient Diffusion rate Flux J C1 (2) rate time Diffusion across membranes 14C-glycerol  J = P C1 J = P C2 Does rate change with C2? Jnet = P ΔC

  6. The NET flux is the difference of the two unidirectional fluxes Independent diffusion Single-file diffusion through a channel Electric potential DE DE where n is the maximal number of ions interacting in the pore H.H. Ussing, 1949

  7. volume of substance ability to dissolve into membrane oil Partition coef. water Permeation Through the Phospholipid Membrane defect propagation or solubility diffusion membrane: Jnet = P ΔC bulk: Jnet = D ΔC/Δx

  8. e2=2-6 Born energy e1=80

  9. Enough to cause cell lysis Poorly permeable

  10. + Flux with Force 0 5 10 15 20 25 mV (voltage φ) Electric field = dφ/dx Direction of force on ion? Force causes….? Acceleration? No…velocity…? friction Velocity, v = Force × mobility = u×Force; u is mobility J = v × concentration = u×c×Force……general flux equation

  11. Free Energy/ mole = chemical potential (μ=dG/dc) μ = μo + RT lnc + zFφ + VP + mgh +…. For simple diffusion of uncharged substance… z = 0; P=0; ignore gravity …same as Fick’s Law if D = uRT

  12. S S 1 2 Transport…catalyzed translocation across membranes Simple diffusion is not a transport process Passive: energy independent Active: energy dependent • Coupled to an energy source: light, ATP, redox, gradient • Transport against an electrochemical gradient Equilibrium: ΔμS = 0 Note: [S]1 not necessarily equal to [S]2 at equilibrium!!

  13. Nernst Equation...valid at equilibrium initial final S+1 -60 mV -60 mV S+1 S+1 [S+1]out = 1 mM [S+1]in = 0 mM [S+1]out = 1 mM [S+1]in = 10 mM Active or passive transport?

  14. Equilibrium (reversal) potential #2 let K+ cross #1 K+ K+ K+ K+ Cl- Cl- Cl- Cl- #3 equilibrium K+ - + K+ - + Cl- Cl- - + (Boltzmann) (Nernst) At 37oC:

  15. passive passive Which are passive?

  16. Solute transport Channels and Facilitators • Water channels (aquaporins) • Intercellular gap junctions (connexins) • Mitochondrial channels (ATP/ADP exchange) • ABC transporters (MDR proteins, CFTR) • Diffusion Facilitators: Glucose transporters (GLUT1-12)

  17. Non-specific water-filled channels Example: Bacterial PORIN, OmpF Water-filled pore (the first crystallized membrane protein, b-barrel)

  18. Porin OmpX Permeation of solutes by size and/or charge

  19. MscL open MscL closed WT MscL has one single Tyrosine (Y) per subunit in position 79. If we insert second aromatic residue (Y or W) in position 93, the channel becomes non-functional. If we move the second Y (or W) to position 102, this partially rescues the defect. (from Chiang et al., 2005)

  20. Gap junctions connexins (from Sosinsky)

  21. Gap Junction Channel From Unger et al., 1999

  22. Oocyte injected with aquaporin mRNA

  23. C1 = C2 C1 > C2 C1 > C2 H2O P1 = P2 P1 = P2 P1 > P2 Water flows into the left compartment through the semi-permeable membrane down its own concentration gradient. It tends to dilute the contents of the left compartment raising the level of fluid at the same time. The increased hydrostatic pressure eventually counters the water influx and at equilibrium the net water flow is zero.

  24. C1 > C2 C1 > C2 pressure gauge pressure gauge H2O H2O P1 = P2 P1 = P2 P1 > P2 P1 > P2 Equilibrium is achieved quicker if we close the left compartment Hydrostatic pressure difference at equilibrium: P2-P1 = RT(C2-C1) Osmotic pressures of individual solutions: p1 = RTC1 p2= RTC2 A difference of C = 1 mOsm creates pressure of 18.4 mm Hg 100 mOsm is equivalent to 1840 mm Hg or 2.42 atm

  25. Aquaporins and aquaglyceroporins

  26. Aquaporin = water channel The salient property of aquaporins is that pass only water (occasionally glycerol), but NO ions! From Agre and Kozono, 2004

  27. The Grotthuss mechanism Proton-hopping mechanism is prevented in aquaporins by strict orientation of water in each half of the channel Proton has abnormally high mobility in water and other dissociating fluids because it does not diffuse all the way, protons are re-distributed by binding and dissociation. Cation Mobility cm2 V-1 s-1 in water NH4+ 0.763×10-3 Na+ 0.519×10-3 K+ 0.762×10-3 H+ 3.62×10-3 T. Grotthuss, 1806

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