Lecture 5: Membrane Transport and Electrical Properties

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• Lecture 5 Membrane Transport and Electrical
  Properties

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Ion concentrations inside and outside the cells
are often different
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The relative permeability of a synthetic lipid
bilayer
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Two main classes of membrane transport proteins
(aka carriers, permases, or transporters)
Conformation change
Both Specificity Multi-TM Hydrophilic solutes
cross membrane via hydrophilic protein pathway
Aqueous Pores (faster)
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Passive transport, active transport,
electrochemical gradient
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Ionophores are tools to increase permeability of
membranes to specfic ions
Made by microorganisms
Mobile ion carriers Channel formers
A23187
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Reversible conformational change in a carrier
protein
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Like an enzyme reaction--it saturates
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Three ways of driving active transport
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Active transport can be driven by ion gradients
Primary active transport ATP-driven Secondary
active transport ion-driven
Na is the usual co-transported ion
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Binding of Na and glucose is cooperative
Binding of Na and glucose is cooperative
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In bacteria, yeast and membrane organells proton
gradient is more predominant
Lactose permease
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Transcellular transport
Asymmetric distribution of carrier proteins
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Na-K pump
Both Na and K are transported to higher
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Reversible!
P-type transport ATPases
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F-type ATPasesATP synthases Work in reverse of
transport ATPases
Chapter 14
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MDR
ABC transporters
ATP binding dimerization
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• ABC transporters
• Amino acids, sugars, ion, polysaccharides,
• peptides, proteins, flipping of lipids
• MDR gene hydrophobic drugs, chemotherapy
• Malaria chloroquine
• Yeast mating pheromone
• Peptides from degration into ER
• Cystic fibrosisregulator of Cl- channel

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Channels form hydrophilic pores, selective
We will not talk about gap junctions here Porins
stay in outer membrane of bacteria, mitochondria
and Chloroplasts Channels in the plasma membrane
are narrow and selective And can open and
close--ion channels, only downhill
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Channels are gated
Phosphorylation, desensitized, inactived
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A typical vertebrate neuron
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Voltage-gated Na channels Voltage-gated K
channels
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The ball-and-chain model of voltage-gated K
channel inactivation
Driven by state of lowest energy
20 aa
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Axon myelination
Schwann cells
More mature
Just beginning To myelinate axon
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Patch-clamp recording of current flow through
Individual channels
All or nothing Conductance Duration Rate
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synapses
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Acetylcholine receptor is the first ion
channel to be purified, cloned, Reconstituted,
recorded single channel,3-D structure
Neuromuscular junction is one of the best studied
synapse
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Schematic structure of AchR
Ligand-gated ion channels
Cations Na, K, Ca2
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Five sets of channels are involved in
neuromuscular transmission
(from a nerve impulse to muscle contraction)
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• Summary
• Ions and larger polar molecules cannot cross the
  lipid bilayer
• Two types of transport proteins carriers and
  channels
• Passive and active transport
• Three types of active transport
• Mechanisms of cotransport
• Na-K pump and ABC transporters
• Channels voltage-gated and ligand gated
• Selectivity of K channels
• Action potential, voltage-gated Na channel,
  voltage-gated
• K channel, myelination
• 10. Single-channel recording
• 11. Neuromuscular junction as an example of
  synapse,
• functions of ion channels.