BSCI 230 Today, 3/15/01

Active Transport

  • Uniport

    • -One solute, one direction
  • Symport

    • -Two solutes, same direction
  • Antiport

    • -Two solutes, opposite directions

    ATPases - Table 8-3

  • Type P

    •     -Phosphorylated intermediate
        -Na+, K+, Ca++, and H+
  • Type V -Vesicle

    •     -Pumps protons (H+) into vesicles
  • Type F - ATP synthases
  • Type ABC - ATP binding cassette

    •     -Various solutes in prokaryotes & eukaryotes

    Na+ - K+ ATPase Na+ - K+ Ratios

    in mammalian cells

    [Na+]inside : [Na+]outside = 0.08 : 1 

    [K+]inside : [K+]outside = 35 : 1



    Why pump Na+ and K+ ?

  • Use Dc for energy in symport

    • -e.g., glucose and 3 Na+
    -1 glucose = 36 ATP
  • Create Dc to set up ion gradients necessary to create DE across membrane

    • -Cell membrane potential for signaling
     

    Electrical Signals in Cells

    Chapter 9


    Design a Control System
    to maintain
    Homeostasis

  • Sensor
  • Reference + Comparator
  • Effector for output

    • Homeostatic Loop


    Homeostatic Systems:
    Closed Loop,
    Negative Feedback
    Signal Molecules:
     
     

    Output From Effector Organs
    Hormones
    Neurotransmitters
    Local mediators


    ëNerve & Neuroní


    Equilibrium Membrane Potential:
    Requires selective permeability


    Membrane Potential (Vm)
    Made Easy
    Membrane Potential (Vm)
    Made Easy


    To Approximate Vm:
    NERNST Equation

                 E Ion =  R T ln [Ion]1
                               zF    [Ion]2

    R = gas constant = 1.987 cal/mol-deg
    T = degrees K
    z = ion valence
    F = Faradayís Constant 23,062 cal/V-mol

    \
    Constants in the Nernst Equation

  • Temperature
  • R
  • F
  • Log10 = (ln/2.303)

    • At 37 o C,

    EIon (in mV) = 61.5 log10 [Ion]1
                            z             [Ion]2
     
     

    At 20 o C,

    EIon (in mV) = 58 log10 [Ion]1
                            z          [Ion]2
     

     


    What does EIon mean?

  • Eion is the Vm at which there is no net movement of that ion across the membrane
  • DE is equal to and opposite of Dc
  • Eion = the Vm that results if that ion is the only permeable ion.

    • Equilibrium Potential (Vm)
    When does Eion approximate Vm?
    Predicting the sign of Vm:
    Use the permeable ion in the Nernst
    If it doesnít diffuse across the membrane, it doesnít contribute to Vm



     
     

    Questions:

  • If GNa+ increases, Vm will . . . .?
  • If GK+ decreases, Vm will . . . .?
  • If GNa+ decreases, Vm will . . . .?
  • If GK+ increases, Vm will . . .?
  • If [K+]out increases, Vm will . . .?
  • If [K+]in increases, Vm will . . . .?
  • If [K+]out decreases, Vm will . . . .?
  • If [Na+]out increases, Vm will . . . .?
  • If [Na+]out decreases, Vm will . . . .?

    • Ohm's Law

    V = I R
    I = V/R
  • When ions (charges) move through membrane channels, current flows!

    •     -Amperes (A)
  • The flow of ions (I) is impeded by R
  • We use 1/R or conductance (g) to express the permeability of a membrane to an ion

    • Ohm's Law
    I = V/R
    I = VG
    Conductance is expressed as Siemens (S)
    For membranes, values are small so:
         pS (10-12 S)
         pA (10-12 A)
    Neher and Sackmanís Patch Clamp technique demonstrates:
  • Voltage -gated channels in membranes of excitable cells
  • Open <--> Closed states, no ëintermediateí
  • When open, each channel exhibits specific conductance and current flow

    •     -For Na+ channel ~ 1 pA/channel or 6 x 106 Na+ ions per second through the channel.

    Voltage-gated ion channels:

  • Open <---> Closed states determined by voltage difference across membrane (Vm)

    • -Threshold
  • Closed state often referred to as channel inactivation
  • Has a voltage sensor region
  • Inactivation region

    •  

    Action Potential:

  • Excitable cells have voltage gated Na+ channels
  • At resting Vm, these channels are closed, Vm is close to E of K+
  • Voltage gated Na+ channels open when Vm depolarizes to threshold
  • The G of Na+ increases many fold and Vm goes towards E of Na+ (usually around +30 to +60 mV)


    •  
     

    Action Potential

  • Na+ channels close and inactivate
  • Voltage gated K+ channels open
  • Vm returns towards E of K+ , usually ~ - 90 mV

    • Action Potential

  • All or None (threshold)
  • Cannot be summed
  • One direction (dendrites to terminal)

    • -Same amplitude
  • Propagated 

    • -Refractory period

    How is a depolarization generated at the dendrites of a neuron?
  • Neurotransmitter binding to receptor which is linked to an ion channel
  • Mechanical or other stimulus activating a sensory structure which leads to opening an ion channel

    		•