Equilibrium Potentials + Ion channels

Question 1

T/F

The resting potential of a cell is negative because there is more Na+ outside the
cell than inside the cell, and Na+ is a positive ion.

Answer 1

False
- There are equal #’s of negative ions, the inside of the cell is electrically neutral
- Concentration of Sodium outside the cell remains the same, and its tiny fluxes of ions across the membrane that creates this potential difference
- The charge of the cell is not changed the difference is found right at the membrane

Question 2

The number of ions needed to create the electrical potential is…

Answer 2

• very small!
  -> (For K+ = 10-12 moles of K+ per cm2 of membrane)

Question 2

Why is the resting membrane potential negative if there’s more K+ inside the cell?
Isn’t K+ a positive ion?

Answer 2

• More Cl-, and K+ would flow out if the K+ is open

Question 3

The concentration of ions on each side remains essentially …

Answer 3

constant

Question 4

Tiny fluxes of ions do not disrupt …

Answer 4

• chemical electroneutrality
• (Each ion has an oppositely charged counter-ion, such as chloride)
• The intracellular and extracellular fluids are electrically neutral

Question 5

Cell membrane:

Answer 5

• Lipid molecules arranged in a bilayer.
  1) Polar, hydrophilic heads face outwards.
  2) Hydrophobic tails extend into the middle.
  3) Proteins are embedded in the bilayer and make
  contact with both the extracellular fluid and the
  cytoplasm.
  4) Can be defined in terms of their ion selectivity and the factors that control their opening and closing

Question 6

General characteristics of ion channels:

Answer 6

• Ion channels contain:
  1) Aqueous pore
  2) Selectivity filter
  3) Gate

Question 7

Aqueous pore:

Answer 7

• allows substances such as ions which are soluble in water to pass into the cell.

Question 8

Selectivity filter:

Answer 8

• restricts ion permeability based on size and ionic charge.

Question 9

Gate:

Answer 9

• Most channels are gated. They fluctuate between open and closed states. At rest, K+ channels are open. They determine the resting membrane potential. At rest, the remainder of channels are predominantly closed.

Question 10

Channels can also be “gated”:

Answer 10

• they switch between being open and closed

Question 11

Three different models for channel gating:

Answer 11

• 1: A discrete conformational change occurs in one region of the channel
• 2: A general conformational change occurs along the length of the channel
• 3: A blocking particle swings in and out of the channel mouth

Question 12

Channel gating is controlled by several types of stimuli

Answer 12

• 1: Binding of a chemical to the channel (ligand-gated channels)
• 2: Phosphorylation of the channel
• 3: Changes in membrane voltage (voltage-gated channels)
• stretch or pressure of the channel

Question 13

Structure of the K+ channel pore and selectivity filter:
K+ channel:

Answer 13

• details of a bacteria
• Subunits that each cross the membrane twice.
• “pore-loop”

Question 13

Channels may enter a — or inactive state in which they are closed and incapable of being opened.

Answer 13

• “refractory”
• The inactive state can be relieved only when the membrane returns
  to its original resting membrane potential

Question 14

Four subunits together form the…

Answer 14

• complete K+ channel
• selective filter
• Outer helix
• Inner helix
• Pore helix

Question 15

Structure of the pore:

Answer 15

• it is well suited for conducting K+ ions.

Question 16

Selectivity filter:

Answer 16

• The narrowest part is near the outside mouth of the channel.
  Only “nonhydrated” K+ ions can enter

Question 17

Start of the K+ channel

K+ moving through the K+ channel selectivity filter

Answer 17

• Draws in Positively charged ions
• With these aminos:
• Aspartic acid, glutamic acid (- charge)

Question 18

The water-filled cavity at the center of the channel

Answer 18

• Allow K+ to interact with water molecules (for stabilization)
• With the following:
• Threonine, Isoleucine, Phenylalanine

Question 19

Dehydration of K+

Answer 19

• Glycine pulls H+ atoms from water molecules
• It’s at either end, so it allows K+ to move through either end of the channel.

Question 20

Why can Na+ not go through K+ channel

Answer 20

• It is attracting Na+ ions, but the precise diameter of this channel means that K+ ions (dehydrated) can be stabilized in the watr configuration. A Na+ atom and its associated water molecules would be smaller and so the glycine would not be able remove the water, thus the Na+ is not sufficiently dehydrated and the size of the channel is precise.

Question 21

The opening and closing of this K+ channel is not dependent…

Answer 21

• on the voltage difference across the membrane (membrane potential).
• But other K+ channels are voltage-dependent

Question 22

Voltage-gated channels have …

Answer 22

• sensors that detect the electrical potential across the membrane.
  Example: positively charged alpha-helices

Question 23

In the mammalian K+ channel, the voltage-sensor for a K+ channel:

Answer 23

• S4 = voltage sensor
• S3b-S4 = paddle (+ charge)
• S5-S6 = pore

Question 24

When the membrane is depolarized, the paddle-like sensor…

Answer 24

• moves towards the extracellular surface, opening the pore.
• at rest, the membrane is - charged and the paddles are pulled down to a close

Question 25

There are nearly a hundred
kinds of potassium channels.

How they differ + Their function

Answer 25

• They differ in their gating
  properties: how they open and
  close.
• Their most important function:
  maintaining the membrane
  resting potential!

Question 26

The Na+ channel has 4 subunits.

Answer 26

• Each subunit contains an alpha helix S4 which functions as a voltage sensor
  -> sensitive to changes in membrane potential
• There is a pore loop between S5 and S6

Question 27

Na+ Channel

Answer 27

• Selectivity filter
• Voltage Sensor
• Gate

Question 28

Properties of the voltage-gated Na+ channel

Answer 28

• They open with little delay
• They stay open for about 1 ms.
• They quickly inactivate.
• They can’t be opened again until the membrane returns to near-threshold.

Question 29

The Na+ channel twists open when the membrane depolarizes

Answer 29

• The channel has alpha-helix domains which are positively charged.
• When the membrane potential changes, the shape of the channel changes. This allows the pore to open.
• The closed pore is pulled down towards the floor of the cell

Note how the channel is closed when the membrane potential is -65mV

Question 30

States of the Na+ channel

Answer 30

• Closed: no ions can pass through
• Open: when the membrane becomes depolarized, the channel opens and Na+ ions can enter the cell
• Inactive: as the membrane remains depolarized, the pore of the channel becomes blocked. No Na+ ions can enter
• Closed: When the membrane returns to rest, the block is removed and the channel becomes closed.

Question 31

Diversity of ion channels:

Channel gating is controlled by several types of stimuli

Answer 31

1: Binding of a chemical to the channel
2: Phosphorylation of the channel
3: Changes in membrane voltage
4: Stretch or pressure of the channel

Question 32

How can we study the activity of
individual channels?

Answer 32

-The patch-clamp technique
- Whole-cell: recordings from the entire cell.
- Inside-out: record from a single channel. Can look at how chemicals affect the cytoplasmic side of the channel.
- Outside-out: record from a single channel. Can look at how chemicals affect the extracellular side of the channel.

Question 33

Advantages of the patch-clamp technique:

Answer 33

1) Can observe activity of only a few channels
rather than the thousands in an entire cell.
2) Can observe very small amounts of current.
3) Can examine the effects of adding ions or
chemicals to either the internal or external
environment.

Question 34

Patch-Clamp

Voltage and Current

Answer 34

• Voltage: the experimenter controls this
• Current: the amount of current flowing
  through the channel

Question 35

The probability of a Na+ channel opening depends on…

Answer 35

membrane potential