1 / 17

Membranes and Transport

Membranes and Transport. Topic II-1 Biophysics. Nernst Equation. www.kcl.ac.uk/teares/gktvc/vc/lt/nol/Nernst.htm. F = 96,400 Coulomb/mole Simplest equation for membrane potential – one ion. Goldman-Hodgkin-Katz Equation.

hendrixj
Télécharger la présentation

Membranes and Transport

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Membranes and Transport Topic II-1 Biophysics

  2. Nernst Equation www.kcl.ac.uk/teares/gktvc/vc/lt/nol/Nernst.htm • F = 96,400 Coulomb/mole • Simplest equation for membrane potential – one ion

  3. Goldman-Hodgkin-Katz Equation • There is one membrane potential that counters all concentration gradients for permeable ions • Pi = (mikBT)/d = Di/d, with D – diffusion constant [cm2/sec]

  4. Example – simple Neuron • Note: Can define Nernst potential for each ion, DVk = -80 mV; DVNa = +58 mV. Relative permeabilities make membrane potential closer to DVk with b = PNa/PK b = 0.02 for many neurons (at rest). [K]i = 125 mM [K]o = 5 mM [Na]i = 12 mM [Na]o = 120 mM What is DV? Do Soma 1 (Nernst)

  5. Electrical Model g is like conductance (=1/R) and like permeability This equation is equivalent to Godman-Hodgkin-Katz equation. (from Kirchoff’s laws)

  6. [K]in =125 mM ds E E [K]out = 5 mM [K]in =125mM VK = -75 Vm = -60 [K]out = 5 mM Example squid axon • IK = gK (Vm – VK), Vm = -60 mV • IK = gK (-60 – (-75)) mV = gK(+15 mV). • g always positive. • DV = Vin – Vout • positive current = positive ions flowing out of the cell. • Vm not sufficient to hold off K flow so ions flow out. When Vm = Vk then no flow. • Remember electric field points in direction of force on positive test charge • E always points from higher potential (for ions) Do Soma 3 (Resting Potential)

  7. Donnan Rule and other considerations Example of two permeable ions and one impermeable one inside • KoClo = KiCli Donnan Rule • Electroneutrality • Osmolarity • Goldman- Hodgkin-Katz • Apply electroneutrality inside and out and plug in Donnan rule and get

  8. Animal Cell Model • * Should really use Molality (moles solute/ kg solvent) instead of per liter – accounts for how molecules displace water (non-ideality). • ** More on this later

  9. Maintenance of Cell Volume • Osmolarity must be same inside and out • Concentration of permeable solutes must be same inside and out • Si = So and Si + Pi = So (Osmolarity) • Solutions: cell wall, Pwater = 0, Pextra cellular solutes = 0 Cell impermeable to sucrose http://www.himalayancrystalsalt.com/html/images/PAGE-osmosis.gif www.lib.mcg.edu/.../section1/1ch2/s1ch2_25.ht

  10. Animal Na impermeable model • Apply electroneutrality outside, Donnan, and osmolarity • Get unknowns and Vm = -81 mV

  11. Active Transport • Na-K Pump • Two sets of two membrane spanning subunits • Phosphorylation by ATP induces a conformational change in the protein allowing pumping • Each conformation has different ion affinities. Binding of ion triggers phosphorylation. • Shift of a couple of angstroms shifts affinity. • Exhibits enzymatic behavior such as saturation. with a = (n/m)(PNa/PK),n/m = 2/3  Vm VK. student.ccbcmd.edu/.../eustruct/sppump.html

  12. Electrical Model Now include current for pump, Ip as well as input current I. Do Soma conductance and Na pump)

  13. Patch Clamping www.essen-instruments.com/Images/figure2.gif • Invented by Sakmann and Neher [Pflugers Arch 375: 219-228, 1978] • Can be used for whole cell clamp (measure currents in whole cell, placing electrode in cell) like on left or pulled patch as on right (potentially measure single channel). • Can control [ions]. • Usually voltage clamp (command voltage or holding voltage) and observe current (I = gV). Ix = g(Vh-Vx) where x is for each ion and Vx is Nernst potential for that ion. With equal concentration of permeable ion on both sides, get g easily

  14. Voltage Gated Channels • There is a degree of randomness in opening and closing of channels • Proportion of time open is proportional to Voltage for some channels • Average of many channels is predictable • When [ions] not limiting, can get nernst potential when current reverses I = gx*(Vh – Vx)

  15. Multiple Channels I • Get several channels on a patch • Gives quantized currents • Parallel: geq = S gi; Series: 1/geq = S 1/gi • g = 1/R t

  16. Ligand gated channels • When Ach binds, gate opens and lets in Na+ and K+. • I is proportional to [Ach]2 (binds 2 Ach) Nicotine also binds to Ach receptor – called nicotinic receptor Do Patch Cl, K, Multiple K, ligated)

  17. Jmax Co Facilitated Transport • Get Saturation Kinetics • Lower activation energy Jmax = NYDY/d2 NY = number of carriers DY = diffusion constant of Carrier d = membrane thickness

More Related