Lecture 19 - Txpt II

Question 1

Compare ion channel speed to ion pump speed

Answer 1

Channel 1000x > than pump

Question 2

The opening and closing of ion channels are responsible/shape the membrane _____.

Answer 2

Potential (flow down [gradient] changes electrical potential)

Question 3

Voltage gated ion channels respond to _____.

Answer 3

changes to membrane potential

Question 4

Chemically/ligand gated channels respond to _______.

Answer 4

ligands (or “signals”) binding to them.

Question 5

Mechanically gated channels respond to _____.

Answer 5

mechanical forces that deform them (P, Temp, pain, etc).

Question 6

Describe the “patch clamp” method of channel isolation.

Answer 6

A small (1 micron) pipette is placed against a cell’s plasma membrane. Depending on the amount of suction applied, either the entire cell (low suction) is removed, or just a portion of the membrane with embedded channels is torn away (high suction).

Question 7

Explain how homology of a protein sequence across species indicates the importance of said protein.

Answer 7

If a protein is homologous, it is a highly conserved protein, and therefore highly important to cell function. There is homology among sodium channels, potassium channels, etc.

Question 8

Describe the mechanism of “-caine” anesthetics.

Answer 8

Transmembrane (“TM”) domains 5-6 act as a pore when activated by voltage sensing domain 4 on the Na+ channel. Though the Na+ channel may be activated, -caine anesthetics block the pore formed by TM5-TM6

Question 9

Tetrodotoxin acts as a (permanent/transient) blocker of Na+ channels.

Answer 9

permanent

Question 10

K+ channels are comprised of 6 transmembrane domains (S1-S6). Which act as a voltage sensor, and which form the K+ channel pore?

Answer 10

S1-S4: voltage sensor

S5-6

Question 11

Explain the K+ selectivity filter’s “Mechanism of Action.”

Answer 11

K+ is solvated in ICF. The large (10Angstrom) K+ channel pore will allow solvated K+ to enter. However, as the solvated K+ approaches the extracellular leaflet, the width decreases to 3 angstroms. The K+ is desolvated by the carbonyl groups there, and each subsequent positive, desolvated K+ is “repelled” out of the channel.

DRAW OUT!!!

Question 12

What is the AA sequence for the K+ selectivity filter

Answer 12

TVGYG

Question 13

Explain the Gibbs free energy basis for ion conductance through a K+ channel and how it affects selectivity.

Answer 13

To pass through a K+ channel, ions must be desolvated, then resolvated once through the channel.

For K+, the total change in E* for desolvation/solvation via the K+ channel is (-), or favorable.

For Na+, the total change in E* for desolvation/solvation via the K+ channel is (+), or unfavorable.

Question 14

Voltage gated channels change formation due to _____

Answer 14

changes in membrane potential.

Question 15

Inactivation of a voltage gated channel occurs through an “inactivation domain.” What end of the channel protein is this located (C-term/N-term)?

Answer 15

C-terminus

Question 16

Describe the steps of voltage gate inactivation:

Answer 16

1) An action potential (depolarization) opens the channel

2) Eventually, hyperpolarization moves the ball into the intracellular portion of the pore opening

Question 17

Describe the structure of the ACH-R

Answer 17

2xα; β; γ; δ

Question 18

Differing [Na+] and [K+] are generated by _____.

Answer 18

Na+/K+/ATPase pumps in the plasma membrane

Question 19

Outline the steps of an action potential

Answer 19

1) Transient changes in membrane potential reach a threshold
2) Conductance of Na+ rises by opening of v-gated Na+ channels secondary to the S4 “switch” domain
3) Depolarization occurs by Na+ flowing through S5-S6 pores
4) At +35mV, Na+ inactivation gate closes, and K+ channel opens
5) K+ efflux hyperpolarizes the cell
6) At -90mV (appx.) the K+ inactivation gate closes and hyperpolarization stops
7) Na/K/ATPase pumps return cell to RMP

Question 20

Explain the transmission of an action potential at the NMJ.

Answer 20

1) An action potential reaches the telodendria
2) Depolarization opens voltage gated-Ca2+ cells
3) Calcium enters the cell and causes fusion of vesicle PM/telodendria PM (SNAP protein)
4) Exocytosis of NT
5) Binds to receptor
6) Opening of ligand-gated channels on post-synpatic tissue causes Na+/K+ movement (Na+ influx > K+ efflux)
7) If depolarization is large enough, end plate potential produced (is this just in muscle?)
8) PMCA removes Ca2+ from telodendria (Ca2+ is cytotoxic)

Question 21

“Cell to cell” channels important to action potential/cell communcation.

Answer 21

Gap junctions

Question 22

Describe structure of gap junctions

Answer 22

One channel is comprised of 12 connexins. 6 connexins make 1 connexon. 2 connexons make one f(x) channel.*

*1 connexon on one cell, 1 connexon on the other

Question 23

Explain the selectivity of gap junctions

Answer 23

Gap junctions are

Question 24

Explain why connexons are important in tissue like the lens of the eye and bones.

Answer 24

These tissues lack abundant vasculature. Therefore, gap junctions provide a means of nourishment.

Question 25

What closes gap junctions?

Answer 25

High [Ca2+] and [H+]. These molecules are cytotoxic/inducers of apoptosis.

Question 26

How are connexons regulated

Answer 26

Hormone induced phosphorylation/membrane potential changes

Question 27

Describe how a gap junction is different from an ion channel.

Answer 27

Ion channel:            
Spams 1 PM
ICF to ECF connection
Synthesized by 1 cell
Rapid open/close

Gap:
Spans 2 PM
Cytosol to cytosol connection
Synthesized by 2 cells
Open for seconds/minutes

Question 28

Aquaporins are found in RBCs, kidneys, and corneas. Why?

Answer 28

These tissues require large amounts of water to be extruded past cell membranes.

Question 29

Describe the structure of aquaporins.

Answer 29

6 transmembrane spanning alpha domains

Question 30

Transport channels are responsible for ______ transport.

Answer 30

facilitated, passive

Question 31

What molecules are gap junctions permeable to?
A. Proteins, polysaccharides, and nucleic acids
B. Large, hydrophillic (> 1kDa) molecules
C. Small hydrophillic (

Answer 31

C. Small hydrophillic molecules