Week 5 Fitzakerley - Ion Channel Physiology

Question 1

What is Ohm’s Law?

Answer 1

V = IR

Question 2

What substances are least likely to diffuse through a pure phospholipid bilayer?

Answer 2

Anything charged.

Na⁺, K⁺, Cl^-

Question 3

What three things provide routes for charged molecules to cross the cell membrane?

Answer 3

1. Gap junctions
2. Membrane transporters
3. Ion channels

Question 4

Why does the cell’s phospholipid bilayer limit the passage of charged molecules (especially ions) across the cell membrane?

Answer 4

The lipid part of the cell membrane has high electrical resistance.

Question 5

What are four differences between ion channels and transporters?

Answer 5

• Ion channels:
  
  • holes through membranes, BIDIRECTIONAL
  • passage of small things (ions)
  • always passive (with electrochemical gradient)
  • high rates of transport
• Transporters:
  
  • protein translocates (not open to both sides at once)
  • passage of big things (glucose)
  • passive & active
  • slower rate of transport

Question 6

Do ion channels or transporters set up concentration gradients?

Answer 6

Transporters set up concentration gradients.

Ion channels use concentration gradients.

Question 7

Do ion channels or transporters have a larger impact on Voltage (V=IR)?

Answer 7

Ion channels.

Increased rate of flow (larger I)

V = I x R

Question 8

What happens when red blood cells are placed in a hypotonic solution?

Answer 8

Water molecules enter the RBCs, causing them to lyse.

Question 9

What happens when a cell with no aquaporins is put in a hypotonic solution?

Answer 9

Nothing changes.

(Cell with aquaporin allows water to rush in and eventually lyses.)

Question 10

What are the two fundamental characteristics of ion channels?

Answer 10

• Selectivity
  
  • number/types of ions allowed through
• Gating​
  
  • leak (always open)
  • voltage
  • ligand
  • mechanically

Question 11

In addition to ion channel gating, what mechanism can prevent the flow of ions through the channel?

Answer 11

Inactivation

Question 12

What is the difference between closing and inactivating a channel?

Answer 12

• All gated channels can close, only some channels can inactivate
• Inactivation is random

Question 13

Live cells have a resting membrane potential (RMP) that is negative with respect to what?

Answer 13

the Extracellular fluid

Question 14

Why do electrically excitable cells such as neurons and myoctyes have a much larger negative RMP?

Answer 14

Because they have a larger number of K⁺ channels open at rest.

Question 15

What things determine the extracellular ion concentrations?

Answer 15

Kidney

Diet

Question 16

What things determine the intracellular ion concentration?

Answer 16

Transporters

(especially Na⁺/K⁺ ATPase)

Question 17

What effect does the ELECTRICAL gradient have on ions?

Answer 17

Positive ions move toward negatively charged areas.

Positive ions move away from positively charged areas.

(V_m & E_rev)

**Based on charge

***Electrical gradients will determine direction and magnitude of ion flow.

Question 18

What effect does the CHEMICAL gradient have on ions?

Answer 18

Ions moved based on concentration gradient

High → Low

• Set up by transporters
• Used by ion channels

Question 19

What is the equilibrium (reversal) potential?

Answer 19

The membrane potential where the net flow through any open channel is 0.

Question 20

What two forces are in balance at E_rev?

Answer 20

Chemical and Electrical forces

Question 21

In order to determine the resting membrane potential (RMP), you must first determine what about each channel type?

Answer 21

Permiability (P)

The relative contribution of each channel type.

Question 22

What value will the resting membrane potential (RMP) be close to?

Answer 22

The reversal potential (E_rev) for the ion that carries the majority of the resting current.

Question 23

Define depolarized in terms of the reversal potential.

Answer 23

More positive than the reveral potential.

Question 24

Define hyperpolarized in terms of the reversal potential.

Answer 24

More negative than the reversal potential.

Question 25

True or False: In neurons, the resting membrane potential is a constant negative value that is determined by the activity of the Na⁺/K⁺ ATPase.

Answer 25

False.

Homeostasis is dynamic, not constant.

ATPase sets up concentration gradients for ion channels to use.

*Ion channels determine resting membrane potential in neurons.

Question 26

How do you re-establish the resting membrane potential, when extracellular K⁺is rising due to tissue damage?

Answer 26

Do not shock with defibrillator!

• Ca²⁺Gluconate
  
  • regenerates ability to produce action potential
• Hyperpolarize cells by giving Insulin
  
  • pumps K⁺ back into cells
• Dialyze
  
  • reduce extracellular K⁺ concentration

Question 27

What regulates the lysis of RBCs, and how does this change during Plasmodium vivax (Malaria) infection?

Answer 27

• Lysis of RBCs:
  
  • change in osmolarity across the cell
  • aquaporins allow water flow
• Lysis of RBCs in Malaria infection:
  
  • malaria parasite eats hemoglobin to prevent a change in osmolarity initially as the parasite reproduces
    
    • Hb is also a source of energy
  • sequential activation of large anion transporter, K⁺ channels, Na⁺ channels

Question 28

What happens to the RBC in response to activation of the large anion transporter?

Answer 28

• Cl^- flows out of the cell
  
  • RMP of RBC = -15
  • E_revof Cl^- = -7.6
• Depolarization of cell until V_{m ~} -7.6
• Small change, but cell still able to maintain homeostasis

Question 29

What happens to the RBC in response to activation of large anion transporter AND K⁺ channels?

Answer 29

• K⁺ flows out of cell
  
  • E_rev of K⁺ = -85
• Cell hyperpolarizes to V_{m ~} -85
  
  • lose more Cl^-
• cell contracts due to increasing change in osmolarity

Question 30

What happens to the RBC in response to activation of large anion transporters AND K⁺channels AND Na⁺channels?

Answer 30

• Na⁺ flows into cell
  
  • E_rev of Na⁺= +83
• Cell depolarizes
• Cl^- enters once V_m is greater than -7.6
• Too much for cell to handle
  
  • Not able to maintain homeostasis
  • CELL LYSIS!!!

Question 31

What events happen before the Plasmodium vivax enters RBCs?

Answer 31

Bite → blood → liver (dormant 3-4 mths) →

*blood* → RBCs

*Replication of malaria and RBC lysis does not occur until the second time it enters the blood.