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
What is the purpose of nodes of Ranvier and how do they work?
The action potential will jump from node to node, greatly increasing the speed and efficiency of the signal transduction. The ap jumps at 1 mm segments along the axon. Ion channels are clustered in the nodes in order to save energy.
Cause of multiple sclerosis (MS)
The loss of myelination in the central nervous system.
First event at the neuromuscular junction that triggers muscle contraction.
- Action potential arrives at axon terminus depolarizing the membrane and causes opening of a voltage-gated Ca++ channel. Ca++ rushes into the presynaptic cell and causes fusion of synaptic vesicle with the membrane, spraying acetylcholine on the muscle.
Second event at the neuromuscular junction that triggers muscle contraction.
Acetylcholine binds to its receptor on the muscle, which is a ligand-gated Na+ channel. The channel opens and depolarizes the muscle membrane.
Third event at the neuromuscular junction that triggers muscle contraction.
The initial depolarizing event is propagated along the muscle membrane by voltage-gated Na+ channels.
Fourth event at the neuromuscular junction that triggers muscle contraction.
A voltage-gated Ca++ channel then opens, which is physically coupled to a Ca++ release channel in the sarcoplasmic reticulum (what muscle ER is called).
Fifth event at the neuromuscular junction that triggers muscle contraction.
The Ca++ release channel of the SR opens and cytosolic Ca++ increases, causing contraction of muscle.
