Transport of Molecules Aross Membranes: Transporters & Ion Channels Flashcards
How do Transporters / Carriers transport molecules?
Through cycles of conformational changes
Do Transporters / Carriers have a high or low rate of transport?
Low
Do Uniporters transport molecules UP or DOWN their concentration gradients?
Down
How do Symporters and Antiporters transport molecules?
Couple the transport of one type of molecule UP its concentration gradient to the transport of another molecule(s) DOWN an electrochemical gradient
What is an example of a Uniporter?
GLUT1
* glucose transporter
What is an example of a Symporter?
Two-Na+ / One-Glucose Symporter
* transports Na+ DOWN its concentration gradient
* transports Glucose UP its concentration gradient
Why are TWO Na+ ions used in the Two-Na+ / One-Glucose Symporter?
- Transport of 2 Na+ provides twice as much energy
- Allows symporter to transport Glucose against a much steeper concentration gradient
What is an example of a Uniporter and Symporter working together?
2-Na+ / 1-Glucose Symporter + GLUT2 glucose uniporter
* work together to take up glucose from the gut
* 2-Na+ / 1-Glucose symporter in the microvilli transports glucose UP its concentration gradient from intestine into cells
* Glucose uniporter transports glucose into blood by facillitated diffusion
What are THREE Antiporters involved in regulating cellular pH?
- Na+ / H+ antiporter
- Na+ HCO3- / Cl- antiporter
- Cl- / HCO3- antiporter
How does the Na+ / H+ antiporter regulate cellular pH?
Exports H+ which Raises pH
How does the Na+ HCO3- / Cl- antiporter regulate cellular pH?
Imports HCO3- which Raises pH
How does the Cl- / HCO3- antiporter regulate cellular pH?
Exports HCO3- which Lowers pH
How is the opening/closing of Resting (Non-Gated) Ion Channels regulated?
Not regulated by any external factors
- open probability is an intrinsic property of the channel
How is the opening/closing of Voltage Gated Ion Channels regulated?
Regulated by the electrical potential across the membrane
* open probability depends on Vm (voltage across membrane)
How is the opening/closing of Ligand Gated Ion Channels regulated?
Regulated by binding of one or more ligands to channel
* open probability depends on ligand
What is an example of a Resting (Non-Gated) Ion Channel?
Resting K+ channel from Streptomyces lividans
How is the Resting K+ Ion Channel selective for the larger K+ ion over the smaller Na+ ion?
- K+ is normally coordinated to 8 water molecules
- Must lose water to pass through channel
- 8 backbone Carbonyl oxygens from conserved Glycine residues coordinate K+ in the pore
- Several K+ ions move “in step” through the selectivity filter
- Na+ is too small to be coordinated properly by the Carbonyl oxygens and so can’t shed its water and can’t enter pore
What is an example of a Voltage-Gated Ion Channel?
Voltage-Gated Channels in neurons
What is Mytonia and what causes it?
- Delayed relaxation of muscle after contraction
- Characterised by decreased Cl- conductance of muscle cells
What mutation causes congenital myotonia in fainting goats?
Ala 855 Pro mutation in voltage gated Cl- channel gClC-1
* decreased opening probability at physiological Vm
What does a mutation in the sodium channel SCN1A cause?
Epilepsy
What is an example of a Ligand-Gated Ion Channel?
Nicotinic Acetylcholine Receptor (nAChR) at the NMJ
How many subunits does an nAChR have?
5 (pentameric)
How many binding sites does an nAChR have?
Two acetylcholine binding sites
Are nAChR channels fast or slow acting?
Fast.
* channel opens within milliseconds of ACh binding
What forms the gate on an nAChR channel?
Kinked M2 helices to constrict the pore / gate
* conformational change on ligand binding causes helices to rotate, opening the gate
How wide is the gate on an nAChR channel when open?
0.6 - 0.8 nm
* allows cations to pass through WITH water
