Lecture 6 - Channels and Transporters Flashcards
True or False?:
Conductance is a combined measure of permeability and number of ions. Permeability is a property of the cell membrane (function of the number of open channels). Conductance is the ability of current (ions) to pass across the membrane.
True
What shape do microscopic currents take?
Square Wave
What determines the slope of the current-voltage (I-V) plot? Why?
The slope is equal to conductance (g) because I/V=g (as per Ohm’s law).
How can you use an I-V plot to determine what type of ion channel you are studying?
You can experimentally produce an I-V plot, find its x-intercept, and see which ion channel’s Ex is equal to that.
What is a activation curve (in terms of gating of ion channels)?
An activation curve is a plot of Po vs Vm. At negative potentials, the channels are likely to be closed and vice versa for positive. It has the shape of a hill function.
What is gating?
Gating is the transition of channels between open and closed states.
True or False?:
The Na+/K+ pump is electrogenic. It extrudes 3 cations for every 2 it imports. As such, it generates a very small, slow, outward, hyperpolarizing current.
True
What does the macroscopic I-V plot for non-voltage gated (leak) channels look like? What does the macroscopic I-V plot for voltage-gated channels look like?
What is the name of the seal made between the membrane and the pipette during patch-clamp recordings?
Giga Ohm Seal
True or False?:
There are about 100 different K+ channel genes.
True
What Nobel prize-winning method is used to conduct single channel recordings?
Patch-Clamp
Consider that you are performing an outside-out patch-clamp recording with a K+ “leak” channel. There is no concentration gradient. In what direction will the current be (if any) at 0 mV.
No Current
Describe what is happening in the attached figure showing microscopic currents of sodium ions.
A command voltage (a) is applied to a cell. Since the voltage increases, the probability of sodium channels opening increases. The individual channels open to allow current through (b). The individual microscopic currents can be summed (c) to produce a macroscopic current that displays the net current for the cell.
How do ion exchangers work?
Ion exchangers harness energy from concentration gradient(s) in order to pump ions.
What is the name of the segment that forms the selectivity pore in ion channels?
P-Segment
True or False?:
In sodium channels, the S4 domain acts as the voltage sensor. It functions by moving up and down across the membrane as the membrane potential changes.
True
How does the equilibrium potential for a channel affects its I-V plot?
It shifts it so that the x-intercept is at (Ex, 0).
What two types of ions transporters are there?
ATPases and Ion Exchangers
How does the selectivity filter in ion channels work?
The selectively filter works by replacing the hydration shell with residues along the channel.
Describe the structure of a Na+ channel.
Na+ channels have 4 repeat motifs of 6 transmembrane spanning regions. They also can incorporate β-subunit adaptors.
Consider that you are performing an outside-out patch-clamp recording with a K+ “leak” channel. There is no concentration gradient. In what direction will the current be (if any) at -20 mV. How will the current at -40 mV compare to this?
Since the inside of the pipette is negative, current flows in. At -40 mV, the current is twice as large as at -20 mV due to Ohm’s law.
How does one deduce the structure of ion channels?
The oldest way to do this is to divide pools of mRNAs until you find single channels. You express the mRNA in Xenopus oocytes. You can also use X-ray crystallography or electron microscopy imaging.
Consider that you are performing an outside-out patch-clamp recording with a K+ “leak” channel. There is no concentration gradient. In what direction will the current be (if any) at 20 mV.
Since the inside of the pipette is more positive, current flows out.
Probability of open channels (Po) is dynamic. What regulates it?
Modulation and Gating
What are rectifiying channels?
Recitifying channels are channels that exhibit non-ohmic I-V relationships. In essence, current passes in one direction better than it does in another. They can be inward rectifiers (current flows better inward) or outward rectifiers (current flows better outward).
How can you predict the reversal potential for a K+ channel when there is a concentration gradient and a membrane potential?
You can use the Nernst equation.
The Na+/K+ pump and Ca2+ pump are what type of ion transporter?
ATPases
True or False?:
Individual channel currents are called microscopic currents. They summate to produce the macroscopic current.
True
What shape does the probability of Na+ channel opening vs. membrane potential graph take?
It resembles the Hill function.
What blocks the Na+/K+ ATPase?
Ouabain
What is the relationship between macroscopic current (I), single channel current (micro) (i), # of channels (n), and probability of open channels (Po)?
I = i n Po
True or False?:
There is only 1 human Na+ channel gene and 1 human voltage-gated Ca2+ channel gene.
False
There are 9 or 10 human Na+ channel genes and about 16 human voltage-gated Ca2+ channel genes.
When producing an I-V plot from single-channel recordings, if you increase the number of ions but keep the relative ratio (inside:outside), what happens? If you alter the relative concentration gradient, what happens?
If you increase the ions but keep the same ratio (inside:outside), the slope changes. If you alter the relative concentration gradient, the reveral potential changes (and the graph shifts) because the Nernst equation changes..
What are the different types of gating?
- Voltage
- Ligand
- Temperature
- Stretch
- Light
True or False?:
There are 20 human Cl- channel genes. The Cl- channel has 10-12 transmembrane regions.
True
