Membrane Transport and Ion Channels Flashcards
Types of Transport across a cell membrane:
- 4.
- simple diffusion
- facilitated diffusion
- osmosis
- active transport
simple diffusion
- examples
- energy req’d?
- saturatable?
- Equation and Driving Force
- DMSO crossing a lipid membrane; small, uncharged molecules that don’t need channels/pumps
- No- molecules travel down conc. gradient
- no
- Ji (flux) = Pi (permeability of memb) * A (area of memb) * Ci (concentration gradient)
facilitated diffusion
- examples
- energy req’d?
- saturatable?
- Equation and Driving Force
- anything using a receptor/transporter to move down conc. gradient (ie: glucose carrier)
- No (moving down conc. gradient)
- Yes! Only have so many transporters
- 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
Osmosis
- examples
- energy req’d?
- saturatable?
- Equation and Driving Force
- kidney reabsorbing water from urinary space into blood
- No
- Not really (maybe somewhat due to aquaporins?)
- 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
Active transport:
- Types
- examples
- energy req’d?
- saturatable?
- Equation and Driving Force
- primary: is an ATPase; 2ndary: uses a gradient made from a different ATPase
- ATPases, pumps, etc used to move large, charged molecules against conc gradient
- YES- moving against conc gradient
- Yes- only so many transporters
- Varies
electrogenic co-transporters
affect and affected by Vm (ie: ion channels, Na+/K+ ATPase, H+ ATPase)
electroneutral co-transporters
Do not affect or affected by Vm (solely concentraiton gradient). (e.g.. Na+/Cl- co-transporter, Na+, H+ antiporter
hydrostatic pressure
water moves preferrentially from high pressure to low pressure (e.g.: water in high-pressure artery moves outwards into surrounding tissue)
osmotic pressure
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.
urea is hypotonic because
its large proteins move freely into the cell, drawing lots of water into the cell, causing it to swell
hypotonic fluids
have low tonicity relative to the cell, so water moves into the cell, making cells swell
hypertonic fluids
fluid has high tonicity relative to the cell, so water moves out of cell, causing cell shrinking
isotonic fluids
have same tonicity as teh cell, so water does not move one way or the other
____moves H20 across tissues
osmotic pressure by Na+ pumps and water channel aquaporins (create high permeability for water)
Nernst Equation:
Nernst equilibrium:
Nernst/Reversal Potential:
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
Goldman equation is used for______. It is a modified ____.
many ions at deltaMu =0. modified Nernst equation
Edema in legs:
Na+ retained, water follows, legs swell
Flux = 0 when
deltaMu = 0 or permeability =0
Why is the inside of a cell negative?
- Na+ K+ ATPase: 3 Na+ leave the cell and 2K+ come in, so change of -4mV per cycle
- K+ channels which transport K+ out of the cell because PK >PNa
- Cl channels: Inward gradient of Cl-
To find Nernst potential for element i (Vi):
To find driving force on element i (deltaMu)
Z(Vi) = -61*log(c1/c2)
deltaMu = 61*log(c1/c2)+ zF(V)
general structure of ion channels
- large glycoprotein tubes
- multiple protein subunits
- hydrophobic AAs face lipid membrane, hydrophilic AAs face pore or intra/extracellular space
can determine # of transmembrane passes an ion channel has by:
counting the # of hydrophobic regions
Common functions of ion channels
- 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
*
ohmic conductance
linear relationship between Vm and i (current)
rectifying conductance
non-linear relation between Vm and i because channel is altered at different potentials; rectifying channels have a preference for direction of ion flow
saturation of ion channels:
like traditional catalysts, ion channels are saturable and have a max speed
determines ability of an ion to travel through a channel
- size
- ion charge
- channel flexibility
different kinds of ion channel gates (gating opens and closes the channel)
- 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
Inactivation of the channel:
state in which no amount of depolarization of the membrane will open up the channel
- prolonged exposure to necessary ligand causes desensitization
Curare
reversible, competitive agonist of AcCh receptor (closes channel temporarily)
BTx:
irreversible, non-competitive antagonist of ACh receptor (closes channel permanently)
phenobarbital
keeps GABA channel open, more Cl- comes in –> sleepy time.
PCP
blocks glutamate-activated channel
trimeric ion channels
purinergic (sense intracellular ATP levels)
quatrameric ion channels
glutamate-bindin
pentameric ion channels
Ach, GABA, Serotonin, Glycine (all neurotransmitters)
voltage-gated ion channels:
K+, Na+, Ca++ channels
TRP ion channels:
cation non-specific, respond to mechanical force
CLC ion channels
CL- permeable channels, gated by voltage, pH, and swelling
problems that can occur w ion channels
- mutations (as in cystic fibrosis)
- abnormal expressions
- autoimmune attack on the channels (as in myasthenia gravis, when antibodies attach AcCh receptors)
