lipid membranes

Question 1

what are membranes? what do they do (generally speaking)?

Answer 1

Proteins associated with lipid bilayers, all cells have membranes
separate cell from its surroundings
regulate entry/exit from cells
divide internal space into compartments
cell-cell communication

Question 2

what is a micelle?

Answer 2

an arrangement of wedge shaped (single tailed) fatty acids where the hydrophobic tails congregate together to increase hydrophobic interactions. The result is an enclosed sphere with tails inside and heads outside.

Question 3

what makes up a bilayer?

Answer 3

two leaflets of lipid monolayers. Each leaflet is made up of cylindrical (two tailed) fatty acids arranged with hydrophobic tails packed together inside and hydrophilic heads together outside.

Question 4

what is a vesicle (liposome)?

Answer 4

small bilayers will spontaneously seal into spherical vesicles with an aqueous cavity in the middle that is able to carry molecules

Question 5

functions of membranes

Answer 5

define cell boundaries
allow import/export (selectively)
retain metabolites and ions within cell
sense external signals and transmit info into cell
provide compartmentalization within cell
produce/transmit nerve signals
store energy as a proton gradient
support ATP synthesis

Question 6

common features of membranes

Answer 6

sheet-like flexible structure, 3-10 nm thick
structure is composed of two leaflets of lipids
form spontaneously in aqueous solution (stabilized by non covalent forces especially hydrophobic effect)
protein molecules span the bilayer
asymmetric with lipids preferably outside/inside
electrically polarized
fluid structures, lipids freely move laterally

Question 7

what is the fluid mosaic model?

Answer 7

lipids form a viscous, 2D solvent into which proteins are integrated, often spanning the bilayer. peripheral proteins are weakly associated and can be removed easily

Question 8

is lipid composition of membranes always the same?

Answer 8

no. composition varies in different organisms, tissues, and organelles. ratio of lipid to protein, type of phospholipid, and abundance/type of sterols varies

Question 9

which lipids are found on which leaflets?

Answer 9

phosphatidylethanolamine and phosphatidylserine are inner monolayer and phosphatidylcholine and sphingomyelin are outer monolayer.

Question 10

what does phosphatidylserine in the outer monolayer mean?

Answer 10

it has a special meaning in platelets where it activates blood clotting. in other cells it marks the cell for destruction.

Question 11

what are the functions of proteins in membranes?

Answer 11

receptors
channels, gates, pumps
enzymes

Question 12

what are the three types of proteins?

Answer 12

peripheral
GPI anchored
integral

Question 13

how can you determine what type of protein a membrane protein is?

Answer 13

by what process is needed to remove it from the membrane
peripheral: removed by disrupting ionic interactions with high salt or change in pH
integral: removed by detergents that disrupt the membrane (lipids still associated)
GPI anchored: phospholipases cut protein from anchor

Question 14

what are peripheral proteins? how do they interact with the membrane?

Answer 14

they associate with the polar head groups of membranes loosely through ionic interactions with lipids or integral proteins.

Question 15

what are integral membrane proteins? how do they interact with the membrane?

Answer 15

they span a large part of the membrane (entire membrane = transmembrane protein). they are tightly associated with the membrane, protein hydrophobic stretches interact with hydrophobic membrane

Question 16

six types of integral membrane proteins

Answer 16

1. spans the membrane, N outside and C inside
2. spans membrane, C outside and N inside
3. multiples through membrane, C/N can be either side
4. multiple individual N/C termini (more than 1 protein)
5. lipid anchored, not actually integrated
6. type 1/2 with a fatty acid tail in membrane

Question 17

how do you read a hydropathy plot?

Answer 17

the regions above the x-axis are hydrophobic and therefore are more likely to be inserted into the hydrophobic membrane. So by counting thick regions above the axis we can estimate the number of transmembrane domains

Question 18

where would we expect to see certain amino acids in membrane proteins?

Answer 18

transmembrane region: hydrophobic amino acids
non polar/polar interface: Tyr and Trp
aqueous domain: charged amino acids

Question 19

in what scenario would a hydropathy plot not be helpful in determining transmembrane domains?

Answer 19

in Beta sheets. these are not continuous in primary sequence and so hydropathy plot doesn’t work

Question 20

what are the phases a membrane can exist in? what conditions cause phase changes?

Answer 20

Gel phase: individual molecules do not move around. cold temps
Fluid phase: individual molecules can move around. hot temps
physiological conditions are more fluid like

Question 21

what composition of fatty acids are required at high and low temps to maintain functional membranes?

Answer 21

high temps: more saturated FA and longer tails cause increased rigidity (increased melting temp) which helps the membrane retain integrity as the high temp makes it more fluid
low temps: more unsaturated FA and shorter tails cause increased fluidity (decreased melting temp) to help maintain fluidity as the cold temp makes it more gel phase

Question 22

what do sterols do in membranes?

Answer 22

increase rigidity and permeability. they consume space between phospholipids but don’t take up head space, so the membrane is more rigid. they do still allow permeability as they are hydrophobic and leave enough space.

Question 23

what helps transverse diffusion to occur?

Answer 23

flipping from one monolayer to another is unfavorable as the hydrophilic head must pass through the hydrophobic region. Flippase (PE and PS from outer to inner), Floppase (phospholipids from inner to outer), and Scramblase (lipids in either direction, equilibrium) allow for flipping.

Question 24

what are lipid rafts?

Answer 24

areas on membranes with clusters of glycosphingolipids with longer tails (thicker membrane) that allow for increased proteins in the membrane. often a target of bacteria

Question 25

how can a membrane be curved?

Answer 25

Caveolin curves membranes.
proteins with intrinsic curvature cause curving as its + charges interact with membrane - charges
a protein with amphipathic helices that insert into one monolayer
proteins with BAR domains polymerize together into a superstructure favoring curvature

Question 26

how do membranes fuse? (vesicle + PM)

Answer 26

vesicles v-SNAREs intertwine with PM t-SNAREs, winching the two membranes closer together. they are brought close enough to favor hemifusion and then complete fusion. cargo is released and vesicle membrane is integrated into PM

Question 27

membranes are permeable to

Answer 27

small non polar molecules like O2 and CO2

Question 28

what are transporters?

Answer 28

proteins that facilitate transport of polar molecules across membrane

Question 29

types of transport?

Answer 29

simple diffusion: non polar compounds down gradient
facilitated diffusion: down gradient
primary active transport: against gradient, uses ATP
secondary active transport: against gradient, driven by ion moving down its gradient
ion channel: down gradient, can be gated
ionophore mediated ion transport: down gradient

Question 30

three classes of transport systems and examples

Answer 30

uniport: moves one substrate. glucose uniporter releases Glc into blood
symport (cotransport): moves two substrates in same direction. Na+ - glucose symporter moves Glc into cell with Na+ gradient
antiport (ecotransport): moves two substrates in opposite directions. Bicarbonate anti porter releases HCO3- as Cl- moves in with gradient in respiring tissues, vice versa in lungs

Question 31

how is ATP used in transport?

Answer 31

ATP can drive conformational changes needed for active transport, like its binding can cause a transmembrane protein to open or close

Question 32

what are Aquaporins?

Answer 32

they allow rapid water passage through membranes. Their side-chain charge (+) is important for H20 specificity as they reduce H bonding opportunities for H20

Question 33

what causes cystic fibrosis?

Answer 33

mutation to an ion channel in epithelial cells where the regulatory domain is dephosphorylated and rearrangement occurs that blocks the ATP binding site