Week 4

Question 1

Number of Na+ out for every ATP hydrolyzed in the Na+/K+ ATPase

Answer 1

3 Na+ out

Question 2

Number of K+ in for every ATP hydrolyzed in the Na+/K+ ATPase

Answer 2

2 K+ in

Question 3

Why is the Na+/K+ ATPase electrogenic?

Answer 3

Every ATP cycle gives a net movement of one positive charge out of the cell.

Question 4

Width of the plasma membrane

Answer 4

5 nm

Question 5

Three mechanisms for gating ion channels

Answer 5

1. Voltage-gated
2. Ligand-gated
3. Mechanically-gated

Question 6

What conformation are leak channels always in?

Answer 6

They are always open.

Question 7

Give examples of important extracellular ligands for ligand-gated ion channels.

Answer 7

Neurotransmitters such as acetylcholine, glutamate, serotonin, GABA, and glycine.

Question 8

What forms the selectivity filter on the bacterial K+ channel?

Answer 8

Carbonyl (C=O) oxygens from peptide bonds form the selectivity filter. The precise distance between oxygen atoms is critical for ion selectivity.

Question 9

On the molecular level, how is the K+ ion channel opened and closed?

Answer 9

The movement of the pore alpha helix in a bacterial K+ channel moves oxygen atoms in the channel that open and close the channel.

Question 10

Why can K+ ions pass through a K+ channel but Na+ ions can’t?

Answer 10

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)

Question 11

What primarily determines the resting potential of the plasma membrane of neurons?

Answer 11

The presence of K+ leak channels.

Question 12

What do open K+ leak channels set the resting membrane potential to?

Answer 12

~70 mV

Question 13

Action potential

Answer 13

The electrical signal that propagates down an axon.

Question 14

What type of channels generate action potentials?

Answer 14

Voltage-gated Na+ and K+ channels generate the action potential.

Question 15

About how much of the ATP in the brain goes towards maintaining the Na+ and K+ ion gradients?

Answer 15

Nearly 50% of the ATP in the brain.

Question 16

Why does the resting membrane potential stop at 70 mV?

Answer 16

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.

Question 17

Refractory period

Answer 17

Time where the channel is silenced and will not reopen in response to a change in the membrane potential.

Question 18

What is the time scale of the propagation of an action potential?

Answer 18

On the scale of milliseconds.

Question 19

First two steps of an action potential.

Answer 19

1. Na+ channel opens then inactivates .
2. K+ channel opens then inactivates (voltage-gated K+ channel)

Question 20

What model explains the rapid inactivation of the voltage-gated K+ channel?

Answer 20

The ball and chain model.

Question 21

When the membrane is polarized, what conformation is the voltage-gated K+ channel in?

Answer 21

It is closed (ball is not in the channel).

Question 22

When the membrane is depolarized, what conformation is the voltage-gated K+ channel in?

Answer 22

Inactivated: ball is in channel
Open: ball is not in channel but channel is open.

Question 23

How can the activity of a single ion channel be measured?

Answer 23

Using patch-clamp technique.

Question 24

What type of cells wrap nerves with a myelin sheath?

Answer 24

Glial cells called Schwann cells or oligodendrocytes

Question 25

What is the purpose of nodes of Ranvier and how do they work?

Answer 25

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.

Question 26

Cause of multiple sclerosis (MS)

Answer 26

The loss of myelination in the central nervous system.

Question 27

First event at the neuromuscular junction that triggers muscle contraction.

Answer 27

1. 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.

Question 28

Second event at the neuromuscular junction that triggers muscle contraction.

Answer 28

Acetylcholine binds to its receptor on the muscle, which is a ligand-gated Na+ channel. The channel opens and depolarizes the muscle membrane.

Question 29

Third event at the neuromuscular junction that triggers muscle contraction.

Answer 29

The initial depolarizing event is propagated along the muscle membrane by voltage-gated Na+ channels.

Question 30

Fourth event at the neuromuscular junction that triggers muscle contraction.

Answer 30

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).

Question 31

Fifth event at the neuromuscular junction that triggers muscle contraction.

Answer 31

The Ca++ release channel of the SR opens and cytosolic Ca++ increases, causing contraction of muscle.