Week 4 Flashcards
Number of Na+ out for every ATP hydrolyzed in the Na+/K+ ATPase
3 Na+ out
Number of K+ in for every ATP hydrolyzed in the Na+/K+ ATPase
2 K+ in
Why is the Na+/K+ ATPase electrogenic?
Every ATP cycle gives a net movement of one positive charge out of the cell.
Width of the plasma membrane
5 nm
Three mechanisms for gating ion channels
- Voltage-gated
- Ligand-gated
- Mechanically-gated
What conformation are leak channels always in?
They are always open.
Give examples of important extracellular ligands for ligand-gated ion channels.
Neurotransmitters such as acetylcholine, glutamate, serotonin, GABA, and glycine.
What forms the selectivity filter on the bacterial K+ channel?
Carbonyl (C=O) oxygens from peptide bonds form the selectivity filter. The precise distance between oxygen atoms is critical for ion selectivity.
On the molecular level, how is the K+ ion channel opened and closed?
The movement of the pore alpha helix in a bacterial K+ channel moves oxygen atoms in the channel that open and close the channel.
Why can K+ ions pass through a K+ channel but Na+ ions can’t?
The carbonyl oxygens of the selectivity filter are perfectly positioned to displace the K+ water shell. They cannot displace the Na+ water shell; therefore, Na+ does not pass through the channel. When Na+ tries to enter the channel, more bonds are broken than formed, causing an energy barrier (+∆G)
What primarily determines the resting potential of the plasma membrane of neurons?
The presence of K+ leak channels.
What do open K+ leak channels set the resting membrane potential to?
~70 mV
Action potential
The electrical signal that propagates down an axon.
What type of channels generate action potentials?
Voltage-gated Na+ and K+ channels generate the action potential.
About how much of the ATP in the brain goes towards maintaining the Na+ and K+ ion gradients?
Nearly 50% of the ATP in the brain.
Why does the resting membrane potential stop at 70 mV?
When the potential reaches around 50-70 mV, the K+ ions experience an attractive force into the cell (electrical gradient pulls in, chemical gradient pushes out). No net ∆G here. This is an example of a homeostatic interaction where two forces are balanced.
Refractory period
Time where the channel is silenced and will not reopen in response to a change in the membrane potential.
What is the time scale of the propagation of an action potential?
On the scale of milliseconds.
First two steps of an action potential.
- Na+ channel opens then inactivates .
- K+ channel opens then inactivates (voltage-gated K+ channel)
What model explains the rapid inactivation of the voltage-gated K+ channel?
The ball and chain model.
When the membrane is polarized, what conformation is the voltage-gated K+ channel in?
It is closed (ball is not in the channel).
When the membrane is depolarized, what conformation is the voltage-gated K+ channel in?
Inactivated: ball is in channel
Open: ball is not in channel but channel is open.
How can the activity of a single ion channel be measured?
Using patch-clamp technique.
What type of cells wrap nerves with a myelin sheath?
Glial cells called Schwann cells or oligodendrocytes
