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Biochem > Lecture 9: Passive Membrane Transport > Flashcards

Lecture 9: Passive Membrane Transport Flashcards

1
Q

General Requirements

A
  1. molecule must be able to cross a hydrophobic barrier
  2. metabolic energy source must power the movement
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2
Q

How do lipophilic molecules pass through a membrane’s hydrophobic interior?

A
  • simple diffusion
  • nonmediated process
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3
Q

How do polar or charged molecules pass through a membrane?

A
  • facilitated diffusion (aka passive-mediated transport)
  • active transport

* both mediated transport processes requiring the activities of specific membrane proteins

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4
Q

Which way do molecules move in simple and facilitated diffusion?

A
  • move down gradient
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5
Q

which way do molecules move in active transport?

A
  • 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
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6
Q

Energy of an uncharged solute molecule

A

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

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7
Q

electrochemical potential for charged solute molecule

A

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

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8
Q

Free energy change imposed by a concentration ratio of 10 is equivalent to what membrane potential?

A

60mv

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9
Q

Delta G and passive transport?

A

negative

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10
Q

delta g and active transport

A

positive

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11
Q

simple diffusion versus mediated transport

A

simple diffusion has much higher energy input

delta G with a trasnporter is lowered

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12
Q

permeability of a membrane

A

tendency to allow a given substance to pass (translocate) across this structure

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13
Q

permeability of lipid bilayers?

A
  • selectively permeable
  • small or nonpolar molecules move (diffuse) across lipid bilayers relatively quickly
  • charged or large polar substance cross slowly, if at all
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14
Q

Net rate of diffusion

A

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

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15
Q

permeability coefficient

A

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

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16
Q

Ex: small nonpolar molecules

A

high permeability/no barrier from mem

10^0

O2, CO2, N2

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17
Q

small uncharged polar molecules

A

H2O, glycerol

10^-4 (about 100x slower than small nonpolar molecules)

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18
Q

large, uncharged polar molecules

A

glucose and sucrose

do not go through on their own

10^-8

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19
Q

Ions and permeability

A

Cl-, K+, Na+

CANNOT pass through

10^-10

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20
Q

Factors determining the integrity and permeability of biological membranes

A
  1. temperature
  2. number of double bonds between carbons in the lipids hydrophobic tails (more DBs for lower temps)
  3. lengths of tails (shorter tails are more sensitive to vibration)
  4. number of cholesterol molecules
  5. presence of transport proteins (transmembrane proteins that translocate specific moelcules)
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21
Q

Phases of the lipid bilayer

A
  • 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
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22
Q

Physical comparison of gel and fluid phase

A
  • gel looks LESS compact, tails are nicely aligned
  • fluid/disordered loks more compact, but tails are less aligned
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23
Q

Maintaining the membrane fluidity

Fluid mems?

Higher temps?

Lower temps?

A
  • 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
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24
Q

Cholesterol and permeability

A
  • 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

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25
Q

uncatalyzed lateral diffusion

A
  • individual lipids undergo fast and free (uncatalyzed) lateral movement within a membrane leaflet

1 um/s speed = FAST

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26
Q

Uncatalyzed transbilayer diffusion (flip flop)

A
  • 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)
27
Q

Special enzymes to catalyze transverse diffusion

A

Flippase (I –> in is result)

Floppase (O –> outside is result)

Scramblase

28
Q

Flippase

A

Moves PE and Ps from outer to cytosolic leaflet

outside –> inside

29
Q

Floppase

A

moves phospholipids from cytosolic to outer leaflet

inside –> outside

30
Q

Scramblase

A
  • move lipids in either direction

out –> in

in –> out

31
Q

simple diffusion

A
  • small molecules and ions in solution (solutes), have thermal energy and are in constant random motion

–> diffusion

32
Q

Concentration gradient (simple diffusion)

A
  • 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
33
Q

Equilibrium

A
  • reached once the molecules or ions are randomly distributed throughout the solution
34
Q

What can diffuse across a lipid bilayer?

A
  • water and lipid soluble molecules
  • not electrically charged molecules, polar molecules
35
Q

osmosis

A
  • 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
36
Q

hypertonic solution

A
  • outside concentration of solute is higher than inside
  • water moves out of cell and cell shrinks
37
Q

hypotonic solution

A
  • outside concentration is lower than inside
  • water moves into cell by osmosis and cell swells
38
Q

isotonic solution

A
  • outside concentration equals inside
  • no NET movement and cell size remains the same
39
Q

mediated transport processes depend on…

A

carrier proteins

channel proteins

40
Q

mediated: carrier proteins

A
  • 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
41
Q

mediated: channels

A
  • 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

42
Q

Structures of channels

A

single channel pores formed from dimers, trimers etc

or

multimeric assemblies in which each subunit has its own pore

*NEED pore

43
Q

carriers: passive transporter family

A
  • simply facilitate diffusion down a concentration gradient
44
Q

carriers: active transport family

Types?

A

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

45
Q

stoichiometry of transporters

A
  • 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
46
Q

facilitated diffusion:

A
  • 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
47
Q

facilitated diff by channel proteins - Types:

A
  • 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)
48
Q

Aquaporins

A
  • hist, asn on one side
  • arg, asn on other

3 Positive charges from Arg, Asn and Asn

2 negative charges from a-helix ends

49
Q

Potassium channels

A

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

50
Q

ion selectivity filter of potassium channel

A

Gly

Tyr (OH)

Gly

Val

Thr (OH)

51
Q

Ion selectivity filter differences

A

changing a few aas will alter the sleectivity for a different ion

52
Q

ion selectivity filter of K+

A

TVGYGDLYP

53
Q

ion selectivity filter of Na+ and K+

A

TVGDGNFSP

54
Q

ion selectivity filter of Ca2+

A

LTGEDWNSV

55
Q

voltage gating of K+ channels

A

very low affinity binding site

but once it is bound and reaches critical concentration, channel opens

56
Q

acetylcholine receptor

A

5 subunits

  • extracellular domain
  • membrane spanning seg
  • segments inside cell
57
Q

ligand gating of acetylcholine receptor

A
  • 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
58
Q

acetylcholine ligand binding mechanism

A
  • 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;
59
Q

action potentials pf channel proteins

A
  • integrate the activities of several ion channels working in concert
60
Q

action potential fetaures

A
  • resting (-60)
  • rising/depolarization (threshold around -56, rises to +40)
  • falling/repolarization (falls to around -60)
  • hyperpolarization (to -80, then rises back to -60)
61
Q

voltage gated and ligand gated ion channels in neuron transmission

A
  • NEED both
  • just voltage gated –> the signal dissipates
  • need to make new signals
  • start over with aligand (between synapses)
62
Q

cystic fibrosis

A
  • 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
63
Q

Where are aquaporins needed?

A
  • kidneys
  • liver –> produces urea and needs to remove it (then gets sent to kidneys)
  • large intesting

*water reabsorption

Biochem (15 decks)

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