Membrane Transport Channel Proteins

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Membrane TransportChannel Proteins
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General Characteristics of Channel Proteins

• Most transport inorganic ions only (mainly Na,
  K, Ca2, Cl-) - thus a.k.a. ion channels. Note
  recent discovery of water-transporting channels
  though (see later).
• Narrow, highly-selective pores - specificity
  based on size and charge.
• Function is to allow for rapid diffusion of
  specific ions across the lipid bilayer of
  membranes.
• Direction of transport is down electrochemical
  gradient - thus passive transport only.

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The Structure of An Ion Channel
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Examples of Ion Channels

• K leak channels
• structure and mechanism of ion selectivity
• ungated - e.g., unregulated flicker between open
  and closed states
• play greatest role in determining membrane
  potential - thus greatest contribution to
  membrane potential is passive ion flow
• Gated channels - opening of channel is
  stimulus-driven
• Voltage-gated open when membrane potential
  changes and exceeds threshold only present in
  "excitable" cells
• Ligand-gated open following noncovalent binding
  of an intracellular ligand (e.g., cAMP) or
  extracellular ligand (e.g., neurotransmitters)
• Stress-activated physical force pulls open
  channel

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Types of Gated Ion Channels
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A Bacterial K Channel
Fig. 21-25(a)
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Mechanism of Ion Selectivity
Fig. 21-25(b)
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Fig. 15-8(a)
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Fig. 15-8(b)
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Fig. 15-8(c)
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Generation of Resting Membrane Potential
Fig. 21-9
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Establishing the resting membrane potential
across the plasma membrane of animal cells

• The Na/K-ATPase maintains a high intracellular
  concentration of K ions.
• K flows out of cell down its electrochemical
  gradient through K-leak channels.
• As a K ion leaks out, it takes a charge with
  it inside of membrane becomes more negative,
  creating an attractive force for K ions to stay
  in, which counteracts force of K ions to flow
  out due to concentration gradient.
• The value of the resting membrane potential is
  the voltage when there is no net flow of ions
  across the membrane.
• In most cells, this value is -70 mV, which is
  close to the equilibrium value for K (EK -91
  mV). Therefore, passive flow of K ions through
  K-leak channels is the major determinant of the
  resting membrane potential of animal cells.
• The resting membrane potential does not equal EK
  because of other factors (e.g., the Na/K-ATPase
  is electrogenic, a few Na-leak channels are
  open, etc.).

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Fig. 21-10
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Fig. 15-34
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Venus fly trap
Folding of mimosa leaves
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The Gating and Cation Selectivity of the
Vertebrate Acetylcholine Receptor
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A Voltage-Gated K Channel from Drosophila
Fig. 21-24
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Hair cells of the vertebrate cochlea have
stress-gated ion channels.
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The Structure of Aquaporin
Fig. 15-33
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Experiment that shows aquaporin is a
water-transporting channel protein!!
Fig. 15-32