Membrane Transport and Ion Channels

Question 1

Types of Transport across a cell membrane:

3. 4.

Answer 1

1. simple diffusion
2. facilitated diffusion
3. osmosis
4. active transport

Question 2

simple diffusion

1. examples
2. energy req’d?
3. saturatable?
4. Equation and Driving Force

Answer 2

1. DMSO crossing a lipid membrane; small, uncharged molecules that don’t need channels/pumps
2. No- molecules travel down conc. gradient
3. no
4. Ji (flux) = Pi (permeability of memb) * A (area of memb) * Ci (concentration gradient)

Question 3

facilitated diffusion

1. examples
2. energy req’d?
3. saturatable?
4. Equation and Driving Force

Answer 3

1. anything using a receptor/transporter to move down conc. gradient (ie: glucose carrier)
2. No (moving down conc. gradient)
3. Yes! Only have so many transporters
4. Uncharged molecule: Ji (flux) = Pi (permeability of memb) * A (area of memb) * Ci (concentration gradient); **charged ion: **Ji (flux) = Pi (permeability of memb) * A (area of memb) * (61log(c1/c2) +Vm) ; this includes chemical and electrical gradients

Question 4

Osmosis

1. examples
2. energy req’d?
3. saturatable?
4. Equation and Driving Force

Answer 4

1. kidney reabsorbing water from urinary space into blood
2. No
3. Not really (maybe somewhat due to aquaporins?)
4. J-h20 = L(p)*A*(deltaP-(sigmaRTC1-sigmaRTC2)) –> hydrostatic (delta P) and osmotic pressures (sigma RT(C1-C2))

sigma is the reflection coefficient: sigma = 1 for a completely impermeant solute and 0 for water

Question 5

Active transport:

1. Types
2. examples
3. energy req’d?
4. saturatable?
5. Equation and Driving Force

Answer 5

1. primary: is an ATPase; 2ndary: uses a gradient made from a different ATPase
2. ATPases, pumps, etc used to move large, charged molecules against conc gradient
3. YES- moving against conc gradient
4. Yes- only so many transporters
5. Varies

Question 6

electrogenic co-transporters

Answer 6

affect and affected by Vm (ie: ion channels, Na+/K+ ATPase, H+ ATPase)

Question 7

electroneutral co-transporters

Answer 7

Do not affect or affected by Vm (solely concentraiton gradient). (e.g.. Na+/Cl- co-transporter, Na+, H+ antiporter

Question 8

hydrostatic pressure

Answer 8

water moves preferrentially from high pressure to low pressure (e.g.: water in high-pressure artery moves outwards into surrounding tissue)

Question 9

osmotic pressure

Answer 9

water will be drawn across a membrane to higher concentrations of solute in order to reach equilibrium, but only if the solute itself cannot move across the membrane first.

Question 10

urea is hypotonic because

Answer 10

its large proteins move freely into the cell, drawing lots of water into the cell, causing it to swell

Question 11

hypotonic fluids

Answer 11

have low tonicity relative to the cell, so water moves into the cell, making cells swell

Question 12

hypertonic fluids

Answer 12

fluid has high tonicity relative to the cell, so water moves out of cell, causing cell shrinking

Question 13

isotonic fluids

Answer 13

have same tonicity as teh cell, so water does not move one way or the other

Question 14

____moves H20 across tissues

Answer 14

osmotic pressure by Na+ pumps and water channel aquaporins (create high permeability for water)

Question 15

Nernst Equation:

Nernst equilibrium:

Nernst/Reversal Potential:

Answer 15

deltaMu (chemical and electrical potential put together) = 61*log(c1/c2) +Vm = 61*log(c1/c2) +zF(V)

Chemical and electrical gradients come to a standstill, so deltaMu =0 .

Vk = -61log(c1/c2), assuming only k crosses membrane

Question 16

Goldman equation is used for______. It is a modified ____.

Answer 16

many ions at deltaMu =0. modified Nernst equation

Question 17

Edema in legs:

Answer 17

Na+ retained, water follows, legs swell

Question 18

Flux = 0 when

Answer 18

deltaMu = 0 or permeability =0

Question 19

Why is the inside of a cell negative?

Answer 19

1. Na+ K+ ATPase: 3 Na+ leave the cell and 2K+ come in, so change of -4mV per cycle
2. K+ channels which transport K+ out of the cell because PK >PNa
3. Cl channels: Inward gradient of Cl-

Question 20

To find Nernst potential for element i (Vi):

To find driving force on element i (deltaMu)

Answer 20

Z(Vi) = -61*log(c1/c2)

deltaMu = 61*log(c1/c2)+ zF(V)

Question 21

general structure of ion channels

Answer 21

• large glycoprotein tubes
• multiple protein subunits
• hydrophobic AAs face lipid membrane, hydrophilic AAs face pore or intra/extracellular space

Question 22

can determine # of transmembrane passes an ion channel has by:

Answer 22

counting the # of hydrophobic regions

Question 23

Common functions of ion channels

Answer 23

• action potentials
• hormone release
• cell motility
• other FAST cellular processes (ion channels are one of the fastest modes of transport into/out of the cell
  *

Question 24

ohmic conductance

Answer 24

linear relationship between Vm and i (current)

Question 25

rectifying conductance

Answer 25

non-linear relation between Vm and i because channel is altered at different potentials; rectifying channels have a preference for direction of ion flow

Question 26

saturation of ion channels:

Answer 26

like traditional catalysts, ion channels are saturable and have a max speed

Question 27

determines ability of an ion to travel through a channel

Answer 27

• size
• ion charge
• channel flexibility

Question 28

different kinds of ion channel gates (gating opens and closes the channel)

Answer 28

• ligand binding (bound drug/molecule opens/closes channel)
• phosphorylation (2nd messenger cascade)
• pressure/stretch-gated (cytoskeleton regulation)
• temperature gated

ligand, ball and chain, or structural changes in the channel

Question 29

Inactivation of the channel:

Answer 29

state in which no amount of depolarization of the membrane will open up the channel

• prolonged exposure to necessary ligand causes desensitization

Question 30

Curare

Answer 30

reversible, competitive agonist of AcCh receptor (closes channel temporarily)

Question 31

BTx:

Answer 31

irreversible, non-competitive antagonist of ACh receptor (closes channel permanently)

Question 32

phenobarbital

Answer 32

keeps GABA channel open, more Cl- comes in –> sleepy time.

Question 33

PCP

Answer 33

blocks glutamate-activated channel

Question 34

trimeric ion channels

Answer 34

purinergic (sense intracellular ATP levels)

Question 35

quatrameric ion channels

Answer 35

glutamate-bindin

Question 36

pentameric ion channels

Answer 36

Ach, GABA, Serotonin, Glycine (all neurotransmitters)

Question 37

voltage-gated ion channels:

Answer 37

K+, Na+, Ca++ channels

Question 38

TRP ion channels:

Answer 38

cation non-specific, respond to mechanical force

Question 39

CLC ion channels

Answer 39

CL- permeable channels, gated by voltage, pH, and swelling

Question 40

problems that can occur w ion channels

Answer 40

• mutations (as in cystic fibrosis)
• abnormal expressions
• autoimmune attack on the channels (as in myasthenia gravis, when antibodies attach AcCh receptors)