Lecture 8: Ion Channels

Question 1

Do ion pumps have a reversal potential?

Answer 1

no – because it only works one way, and works regardless of what the equilibrium potential is

Question 2

Do ion channels have a reversal potential?

Answer 2

yes – reversal potential at the predicted Eion

Question 3

What is an ion channel?

Answer 3

transmembrane protein that forms a pore through the membrane, allowing for rapid passive flow of one or a few species of ions down their electrochemical gradient

Question 4

What is a macroscopic current?

Answer 4

total current generated by many ion channels in one patch of a cell membrane

Question 5

What is a microscopic current?

Answer 5

tiny fluctuations

Question 6

What are binary openings of ion channels?

Answer 6

individual channels are either fully open or fully closed, and the switch from one state to the other is essentially instantaneous

Question 7

What are stochastic openings of ion channels?

Answer 7

length of time a channel stays open, once it does open, can vary considerably even in a single trial

Question 8

How do macroscopic currents vary?

Answer 8

appear to vary in a smooth, graded manner because they detect the stochastic openings and closings of many thousands of ion channels

Question 9

What type of signal is each ion channel?

Answer 9

binary signal

Question 10

What happens when there are many channels present in an area?

Answer 10

the currents produced by their stochastic openings sum together and overlap in time

Question 11

Patch clamp doesn’t actually tell you anything about how a membrane protein can produce all these diverse functions. How do we fix this?

Answer 11

need to determine amino acid sequence of these proteins

Question 12

Patch clamp works best when there are a high density of channels, all of the same type, in a large, accessible cell compartment. How can we fix this?

Answer 12

could just study only the most common ion channels found in very large cells (like muscle fibres), OR could find the genes and then put any ion channel into cell types suited for patch clamp recordings

Question 13

What can’t ion channel sequences answer? What would be needed to answer it?

Answer 13

questions about how the structure-function relationships – how you can have a pore that is selective for a larger ion (ie. K+), but manages to bar entry to smaller ions (ie. Na+)

crystal structure of the ion channel would be needed

Question 14

Crystal structure of the ion channel would be needed to answer questions that ion channel sequences can’t. Why was this a problem?

Answer 14

• no ion channel physiologist knew anything about crystallography
• no one had ever crystallized a transmembrane protein – no crystallographer wanted to try because phospholipid membranes do not make good crystals