Ch. 11 - Membrane Transport of Small Molecules - Part 2 Summary of chapter and lecture on membrane transport of small molecules, part 2. Includes illustrations and graphics.

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16 Oct 2011
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BIOL 2021- March 19 2009
ION CHANNELS
ALWAYS PASSIVE
Undergoes conformational change; can be open or closed
When open, channel acts as a selectivity filter (permits some inorganic ions to pass but not
others)
Energy source: passive transport
Much faster transport than transport proteins
The solute doesn’t undergo any conformational changes unlike with transport proteins
Gating of ion channels:
1. Voltage gated
- Open when voltage across membrane changes
- Sensitive to voltage change
2. Ligand gated
- Opens when ligand binds (ligand could be neurotransmitter, nucleotide etc.)
- Ligand: anything that binds
3. Mechanically gated: Open when subjected to mechanical stress
i.e. Auditory hair cells (in ear); mircovilli: finger like projections responsible for
conversion of mechanical stimuli to electrical signal in nerve
- Tilting action that turns into action potential
Membrane potential: arises when there is a difference in the electrical charge on the two sides of the
membrane due to a slight excess of positive ions over negative ones on one side and a slight deficit on
the other. Measured in voltage.
In animal cell typical membrane potential is -20 to -120V (negative inside cell)
- Passive ion movements are biggest contributor to the electrical potential across the plasma
membrane.
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- Moving ions across membrane creates membrane potential
- 100mV= 1/1000 of K ions in cytosol
- Potential only exists near membrane.
WHAT GENERATES POTENTIAL
a) Na K ATP ase
Pumps 3Na out; 2 K in
Generates 10% of potential
- Reminder: Na K pump maintains the osmotic balance across the animal cell membrane by
keeping the intracellular concentration of Na low. Because there is little Na inside the cell other
cations must be plentiful inside in order to balance the charge carried by the cell’s fixed anions
(negatively charged molecules stuck inside the cell). The balancing role is performed by K.
b) K leak channels: K channels that are open even in an unstimulated (resting) cell. They have a
crucial role in maintaining the membrane potential across all plasma membranes.
Responsible for 90% of potential
Open all the time
K comes to equilibrium
OUT IN
Na/K pump + -
3 Na+ + -
+ - ADP
A ATP
2 K+
+ -
+ -
+ -
K leak channel K +
+ -
Resting membrane potential: no net flow of ions across the plasma membrane
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