Lecture 9: Passive Membrane Transport Flashcards Preview

Biochem > Lecture 9: Passive Membrane Transport > Flashcards

Flashcards in Lecture 9: Passive Membrane Transport Deck (63):
1

General Requirements

 

1. molecule must be able to cross a hydrophobic barrier

2. metabolic energy source must power the movement

2

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

- simple diffusion

- nonmediated process

3

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

4

Which way do molecules move in simple and facilitated diffusion?

 

- move down gradient

5

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

6

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

7

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

8

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

 

60mv

9

Delta G and passive transport?

 

negative

A image thumb
10

delta g and active transport

 

positive

A image thumb
11

simple diffusion versus mediated transport

 

simple diffusion has much higher energy input

delta G with a trasnporter is lowered

12

permeability of a membrane

 

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

13

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

14

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

15

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

16

Ex: small nonpolar molecules

high permeability/no barrier from mem

10^0

 

O2, CO2, N2

17

small uncharged polar molecules

 

H2O, glycerol

 

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

18

large, uncharged polar molecules

glucose and sucrose

 

do not go through on their own

 

10^-8

19

Ions and permeability

 

Cl-, K+, Na+

 

CANNOT pass through

10^-10

20

Factors determining the integrity and permeability of biological membranes

 

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

6. presence of transport proteins (transmembrane proteins that translocate specific moelcules)

21

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

22

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

23

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

24

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

25

uncatalyzed lateral diffusion

 

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

 

1 um/s speed = FAST

26

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)

27

Special enzymes to catalyze transverse diffusion

 

Flippase (I --> in is result)

Floppase (O --> outside is result)

Scramblase

28

Flippase

 

Moves PE and Ps from outer to cytosolic leaflet

 

outside --> inside

29

Floppase

 

moves phospholipids from cytosolic to outer leaflet

 

inside --> outside

30

Scramblase

 

- move lipids in either direction

 

out --> in

in --> out

31

simple diffusion

 

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

--> diffusion

32

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

33

Equilibrium

 

- reached once the molecules or ions are randomly distributed throughout the solution

34

What can diffuse across a lipid bilayer?

 

- water and lipid soluble molecules

- not electrically charged molecules, polar molecules

35

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

36

hypertonic solution

 

- outside concentration of solute is higher than inside

- water moves out of cell and cell shrinks

37

hypotonic solution

 

- outside concentration is lower than inside

- water moves into cell by osmosis and cell swells

38

isotonic solution

 

- outside concentration equals inside

- no NET movement and cell size remains the same

39

mediated transport processes depend on...

 

carrier proteins

channel proteins

40

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

41

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

42

Structures of channels

 

single channel pores formed from dimers, trimers etc

or

multimeric assemblies in which each subunit has its own pore

 

*NEED pore

43

carriers: passive transporter family

 

- simply facilitate diffusion down a concentration gradient

44

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

45

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

46

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

 

47

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)

48

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

A image thumb
49

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

A image thumb
50

ion selectivity filter of potassium channel

 

Gly

Tyr (OH)

Gly

Val

Thr (OH)

A image thumb
51

Ion selectivity filter differences

 

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

52

ion selectivity filter of K+

 

TVGYGDLYP

53

ion selectivity filter of Na+ and K+

 

TVGDGNFSP

54

ion selectivity filter of Ca2+

 

LTGEDWNSV

55

voltage gating of K+ channels

 

very low affinity binding site

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

56

acetylcholine receptor

 

5 subunits

- extracellular domain

- membrane spanning seg

- segments inside cell

57

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

58

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;

A image thumb
59

action potentials pf channel proteins

 

- integrate the activities of several ion channels working in concert

60

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)

 

A image thumb
61

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)

62

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

63

Where are aquaporins needed?

 

- kidneys

- liver --> produces urea and needs to remove it (then gets sent to kidneys)

- large intesting

 

*water reabsorption