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Lecture 5: Membrane Transport and
Electrical Properties
Ion concentrations inside and outside the cells are often different
The relative permeability of a synthetic lipid bilayer
Two main classes of membrane transport proteins
(aka carriers, permases, or transporters)
Conformation
change
Aqueous
Pores (faster)
Both:
Specificity
Multi-TM
Hydrophilic
solutes cross
membrane via
hydrophilic protein
pathway
Passive transport, active transport, electrochemical gradient
Ionophores are tools to increase permeability of
membranes to specfic ions
Made by microorganisms
Mobile ion carriers
Channel formers
A23187
Reversible conformational change in a carrier protein
Like an enzyme reaction--it saturates
Three ways of driving active transport
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
Binding of Na and glucose is cooperative
Binding of
Na and glucose
is cooperative
In bacteria, yeast and membrane organells
proton gradient is more predominant
Lactose
permease
Transcellular transport
Asymmetric
distribution of
carrier proteins
Na-K pump
Both Na and K are transported to higher [ ]
P-type transport ATPases
Reversible!
F-type ATPases=ATP synthases
Work in reverse of transport ATPases
Chapter 14
ABC transporters
MDR
ATP binding=
dimerization
ABC transporters:
1. Amino acids, sugars, ion, polysaccharides,
peptides, proteins, flipping of lipids
1. MDR gene: hydrophobic drugs, chemotherapy
2. Malaria: chloroquine
3. Yeast mating pheromone
4. Peptides from degration into ER
5. Cystic fibrosis:regulator of Cl- channel
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”
Channels are “gated”
“Phosphorylation”, “desensitized”, “inactived”
A typical vertebrate neuron
Voltage-gated
Na+ channels
Voltage-gated
K+ channels
The “ball-and-chain” model of voltage-gated K+ channel inactivation
Driven by state of lowest energy
20 aa
Axon myelination
Schwann cells
More
mature
Just
beginning
To myelinate
axon
Patch-clamp
recording of
current flow
through
Individual
channels
All or nothing
Conductance
Duration
Rate
synapses
Neuromuscular junction
is one of the best studied
synapse
Acetylcholine receptor
is the first ion channel
to be purified, cloned,
Reconstituted, recorded
single channel,3-D structure
Schematic structure of AchR
Ligand-gated ion channels
Cations: Na+, K+, Ca2+
Five sets of channels are involved in neuromuscular transmission
(from a nerve impulse to muscle contraction)
Summary
1. Ions and larger polar molecules cannot cross the lipid
bilayer;
2. Two types of transport proteins: carriers and channels;
3. Passive and active transport;
4. Three types of active transport;
5. Mechanisms of cotransport;
6. Na-K pump and ABC transporters;
7. Channels: voltage-gated and ligand gated;
8. Selectivity of K channels;
9. Action potential, voltage-gated Na channel, voltagegated
K channel, myelination;
10. Single-channel recording;
11. Neuromuscular junction as an example of synapse,
functions of ion channels.