Lecture 9: Passive Membrane Transport Flashcards
General Requirements
- molecule must be able to cross a hydrophobic barrier
- metabolic energy source must power the movement
How do lipophilic molecules pass through a membrane’s hydrophobic interior?
- simple diffusion
- nonmediated process
How do polar or charged molecules pass through a membrane?
- facilitated diffusion (aka passive-mediated transport)
- active transport
* both mediated transport processes requiring the activities of specific membrane proteins
Which way do molecules move in simple and facilitated diffusion?
- move down gradient
which way do molecules move in active transport?
- move against their concentration gradient with external energy source
- electrochemical potential measure the combined ability of a concentration and an uneven distribution of charge to transport molecules across membrane
Energy of an uncharged solute molecule
delta G = RTln (c2/c1)
c2/c1 = conc ratio from side 1 to side 2
R = gas constant (8.314)
T = T in kelvin
*energy required to generate a concentration gradient
electrochemical potential for charged solute molecule
delta G = RTln (c2/c1) + zF (deltaV)
z = electrical charge of transported species
delta V = potential in volts across the membrane
F = faraday constant (9.65 Kj/ vxmol)
sum of concentration and electrical terms is called electrochemical potential or membrane potential
Free energy change imposed by a concentration ratio of 10 is equivalent to what membrane potential?
60mv
Delta G and passive transport?
negative
delta g and active transport
positive
simple diffusion versus mediated transport
simple diffusion has much higher energy input
delta G with a trasnporter is lowered
permeability of a membrane
tendency to allow a given substance to pass (translocate) across this structure
permeability of lipid bilayers?
- selectively permeable
- small or nonpolar molecules move (diffuse) across lipid bilayers relatively quickly
- charged or large polar substance cross slowly, if at all
Net rate of diffusion
net rate of diffusion of diffusional flux, J, is porportional to the concentration difference out - in of solute across membrane
Ja = (Dm {[A]out - [A]in})/lm
Dm = effective diffusion coefficient of solute A inside the membrane
lm = membrane thickness
permeability coefficient
permeability coefficient Pa is based on linear relationship between diffusional flux J and the concentration difference [A]out - [A]in across a membrane and can be measured experimentally
Ja = Pa ([A]out - [A]in)
Ja vs [A]s, positive slope Pa
Ex: small nonpolar molecules
high permeability/no barrier from mem
10^0
O2, CO2, N2
small uncharged polar molecules
H2O, glycerol
10^-4 (about 100x slower than small nonpolar molecules)
large, uncharged polar molecules
glucose and sucrose
do not go through on their own
10^-8
Ions and permeability
Cl-, K+, Na+
CANNOT pass through
10^-10
Factors determining the integrity and permeability of biological membranes
- temperature
- number of double bonds between carbons in the lipids hydrophobic tails (more DBs for lower temps)
- lengths of tails (shorter tails are more sensitive to vibration)
- number of cholesterol molecules
- presence of transport proteins (transmembrane proteins that translocate specific moelcules)
Phases of the lipid bilayer
- can range from gel to fluid phase
Gel (liquid ordered, Lo): individual molecules do not move areound and the bilayer is paracrystalline
Fluid (Liquid disordered, Ld): individual moelcules move freely in lateral plane of bilayer
- heating causes transition from gel to fluid
- under phys conditions membranes are more fluid than gel
Physical comparison of gel and fluid phase
- gel looks LESS compact, tails are nicely aligned
- fluid/disordered loks more compact, but tails are less aligned
Maintaining the membrane fluidity
Fluid mems?
Higher temps?
Lower temps?
- more fluid membranes require shorter and more unsaturated fatty acids
- at higher temperatures, cells with more sat fatty acids to maintain integrity
- at lower temps cells need more unsaturated fatty acids to maintain fluidity
Cholesterol and permeability
- more cholesterol reduces permeability to glycerol
- for all conditions, permeability increases with temp
* cholesterol regulates permeability and keeps it constant, without out it cells would be TOO permeable
uncatalyzed lateral diffusion
- individual lipids undergo fast and free (uncatalyzed) lateral movement within a membrane leaflet
1 um/s speed = FAST
Uncatalyzed transbilayer diffusion (flip flop)
- spontaneous flips from one leaflet to another are rare because the charged head group must transverse the hydrophobic tail region of the membrane
- very slow (t 1/2 in days)
Special enzymes to catalyze transverse diffusion
Flippase (I –> in is result)
Floppase (O –> outside is result)
Scramblase
Flippase
Moves PE and Ps from outer to cytosolic leaflet
outside –> inside
Floppase
moves phospholipids from cytosolic to outer leaflet
inside –> outside
Scramblase
- move lipids in either direction
out –> in
in –> out
simple diffusion
- small molecules and ions in solution (solutes), have thermal energy and are in constant random motion
–> diffusion
Concentration gradient (simple diffusion)
- difference in solute concentration (in aq medium) generates a concentration gradient
- solutes move randomly when a concentration gradient exists, but there is a ned movement from regions with high concentration to those with low
Equilibrium
- reached once the molecules or ions are randomly distributed throughout the solution
What can diffuse across a lipid bilayer?
- water and lipid soluble molecules
- not electrically charged molecules, polar molecules
osmosis
- movement of water (special case)
- if two solutions are separated by a membrane that allows water, but not solutes to pass through, water will move from regions of low solute concentration to regions of high solute conc, thus equalizing the conc on both sides
hypertonic solution
- outside concentration of solute is higher than inside
- water moves out of cell and cell shrinks
hypotonic solution
- outside concentration is lower than inside
- water moves into cell by osmosis and cell swells
isotonic solution
- outside concentration equals inside
- no NET movement and cell size remains the same
mediated transport processes depend on…
carrier proteins
channel proteins
mediated: carrier proteins
- bind substrate with high (stereo)specificity
- catalyze transport at rates well below the limits of free diffusion
- exhibit (like enzymes) substrate saturation kinetics
- often monomeric
mediated: channels
- generally allow transmembrane movement at rates orders of magnitude greater than those typical of carriers (rates often approaching the limit of unhindered diffusion)
- usually show less (stereo)specificity than carriers and exhibit no saturation kinetics
- often oligomeric complexes
*essentially just a hydrophilic hole in lipid bilayer –> lots can flow through
Structures of channels
single channel pores formed from dimers, trimers etc
or
multimeric assemblies in which each subunit has its own pore
*NEED pore
carriers: passive transporter family
- simply facilitate diffusion down a concentration gradient
carriers: active transport family
Types?
drive substances across the membrane against conc gradient
primary - driven by ATP
secondary - driven by coupled flow of 2 solutes, one of which flows down its gradient and the other pulled up against its gradient
stoichiometry of transporters
- uniport - carriers on subs at a time in one direction
- symport - moves 2 substrates simultaneously in same direction
- antiport - translocated 2 subs in opposite directions
facilitated diffusion:
- transport proteins speed the passive movement of molecules across cell mem
- channel proteins - central pores provide corridors and hence allow specific molecules to pass
- carrier proteins bind specific substances to increase their diff rate through bilayer
facilitated diff by channel proteins - Types:
- aquaporins (water)
- ion channels (ion selective)
- ligand gated (stimulus/signal binds)
- voltage gated (electrical charge from ions)
- mechanically gated (eg phosph or dephosph of critical serines)
Aquaporins
- hist, asn on one side
- arg, asn on other
3 Positive charges from Arg, Asn and Asn
2 negative charges from a-helix ends
Potassium channels
a helices?
- backbone carbonyl oxygen forms cage that fits K+ precisely, replacing waters of hydration sphere
- helix dipole stabilizes K+
- water filed vestibule allows hydration of K+ at end
*fits size of K+ and anythign smaller
ion selectivity filter of potassium channel
Gly
Tyr (OH)
Gly
Val
Thr (OH)
Ion selectivity filter differences
changing a few aas will alter the sleectivity for a different ion
ion selectivity filter of K+
TVGYGDLYP
ion selectivity filter of Na+ and K+
TVGDGNFSP
ion selectivity filter of Ca2+
LTGEDWNSV
voltage gating of K+ channels
very low affinity binding site
but once it is bound and reaches critical concentration, channel opens
acetylcholine receptor
5 subunits
- extracellular domain
- membrane spanning seg
- segments inside cell
ligand gating of acetylcholine receptor
- acetylcholine binds
- transient opening
allows Na+, K+, Ca2+ to pass through but other cations and anions cannot
- acetylcholine is degraded to allow channel to close again
acetylcholine ligand binding mechanism
- 4 subunit each of 4 transmembrane helices
- amphipathic helices surround the channel
- 2 acetylcholine binding sites
M2 is chain lining the channel
- bulky LEUCINE side chains of M2 helices close channel
- binding of 2 acetylcholine causes twisting of M2 helices
- now smaller, polar reaidues line the channe;
action potentials pf channel proteins
- integrate the activities of several ion channels working in concert
action potential fetaures
- resting (-60)
- rising/depolarization (threshold around -56, rises to +40)
- falling/repolarization (falls to around -60)
- hyperpolarization (to -80, then rises back to -60)
voltage gated and ligand gated ion channels in neuron transmission
- NEED both
- just voltage gated –> the signal dissipates
- need to make new signals
- start over with aligand (between synapses)
cystic fibrosis
- caused by mutation to a chloride-specific ion channel in epithelial cells
- need chloride to exit so water follows
- makes mucous less sticky and able to cough up
- with cystic fibrosis it stays sticky and cant be coughed up
Where are aquaporins needed?
- kidneys
- liver –> produces urea and needs to remove it (then gets sent to kidneys)
- large intesting
*water reabsorption
