Membrane Transport of Small Molecules and the Electrical Properties of Membranes

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Chapter 11

• Membrane Transport of Small Molecules and the
  Electrical Properties of Membranes

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Membrane permeability

• Small molecules and those less strongly
  associated with water will pass across membrane
• Charged molecules will not cross membrane

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Ficks Law of diffusion
Permeability Coefficients

• RDA (?P/d)
• R Rate of diffusion
• D Diffusion constant
• A Area for diffusion
• ?P Difference in concentration
• d Distance

• Rate of diffusion across bilayer dependent on
  concentration difference and permeability

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Passive and active transport compared

• Passive transport requires concentration gradient
• Active transport requires input of energy

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Electrochemical gradients affect diffusion

• Combined effects of concentration and electrical
  gradients alter ionic movement across a membrane
• Stable (Donnan) equilibrium

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Passive transport Transporter proteins and
channel proteins

• 2 classes of membrane transport proteins
• Transporter proteins
• Alternating conformations
• Permeases
• Transporters
• Channel proteins
• Water filled pore

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Passive transport Carrier proteins

• 2 conformations- A and B
• Transition is random and reversible
• Transition not dependent on state of binding site

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Kinetics of membrane transport

• Simple diffusion/channel proteins proportional to
  concentration
• Transporter mediated diffusion saturable
• Carriers can be blocked by competitive or
  non-competitive inhibitors
• Kinetics will
• increase rate at low concentrations
• regulate rate at high concentrations

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Active transport

• Coupled-transporter transport links uphill
  transport of one solute to downhill transport of
  another
• ATP and light energy used to drive uphill
  transport
• Transported molecule/Energy Source

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Three types of transporter mediated movement

• Uniport
• Single solute
• Passive
• Symport
• Coupled
• Two solutes in same direction
• Active
• Antiport
• Coupled
• Two solutes in different directions
• Active

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An example of Symport

• Glucose transport
• Binding is cooperative
• Thus glucose is more likely to bind when sodium
  is bound
• Glucose is more likely to bind to state-A than
  state-B
• Both solutes required for transport

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An application of Na driven symport glucose
transport
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Ion pumps provide energy for symport or antiport

• Sodium/Potassium-dependent ATPase
• Both ions pumped against concentration gradient

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Active transport of calcium ions
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A typical ion channel which fluctuates between
open and closed conformations (passive)
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The gating of ion channels
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Ionophores-used as experimental tools to increase
ion transport across membranes

• Small hydrophobic molecules that increase
  membrane permeability to ions (passive)
• A23187 Ca2 and Mg2 mobile
• Valinomycin K mobile
• FCCP H mobile
• Gramicidin monovalent cations channel

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Passive and active transport compared

• Passive transport requires concentration gradient
• Active transport requires input of energy

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Membrane permeability

• Small molecules and those less strongly
  associated with water will pass across membrane
• Charged molecules will not cross membrane