1.1 - Membrane Lipids

Question 1

cell membrane system functions

Answer 1

1) form cell limits
2) retain cell content (metabolites + ions) within cell
3) provide compartments within cell
4) regulate exchange
5) sense signals (hormones, light)
6) conduct electrical currents

Question 2

phospholopid structure

Question 3

what are the main phospholipids in mammalian cell membranes

Answer 3

1) PE
2) PS
3) PC

Question 4

how long are the alkyl chains usually

Answer 4

14-22 carbons long

Question 5

why do kinks form in the hydrocarbon tails of phospholipids (what kind of bonds)

Answer 5

due to cis-double bonds

Question 6

why do fatty acid molecules cluster together in micelles

Answer 6

expose smallest possible hydrophobic surface surface area to water

Question 7

how is the structure of the micelle stabilized

Answer 7

energy gained by freeing the immobilized water molecule stabilizes the micelle (the water molecules are less ordered and thus entropy of the system increases)

Question 8

what shape do fatty acids form in water and why

Answer 8

micelles

• due to wedge-shape (head size greater than the side chain)
• acyl side chains protected from water in the core

Question 9

what shape do phospholipids form in water and why

Answer 9

bilayers

• cylinder shaped (head and side chain similar sizes)
• acyl side chains protected from water (except for at ends)

Question 10

can lipid bilayer have edges

Answer 10

no, form sealed compartments or vesicles

Question 11

describe the possible movements of individual lipid molecules in a lipid bilayer

Answer 11

1) rotate on themselves
2) freely move within same leaflet
3) flip-flop (rare)

Question 12

why can the lipid bilayer be seen as 2D fluid

Answer 12

the individual phospholipid molecules can be so fluid/flexible in their movements within the same lipid bilayer

Question 13

how is lipid bilayer fluidity measured, and how does the process work

Answer 13

FRAP (fluorescence recovery after photobleaching)

1) fluorescent probe on lipids
2) shine intense laser to bleach small area
3) measure how fast the fluorescence returns to the area

faster = more fluid membrane

Question 15

what happens in the gel phase

Answer 15

individual molecules do not move around much

Question 16

what happens at fluid phase

Answer 16

individual molecules can move around

Question 17

draw a typical fluidity vs temperature graph and label the melting point

Answer 17

(insert graoh from slide 18)

Question 18

what happens if you heat a lipid bilayer in the gel phase

Answer 18

will go from gel phase to fluid phase

Question 19

under physiological conditions, are membranes more gel-like or more fluid-like?

Answer 19

more fluid-like

Question 20

what factors influence fluidity/freezing point of a phospholipid bilayer

Answer 20

1) acyl chain length
2) degree of saturation

Question 21

what happens to phospholipid bilayer at the melting point

Answer 21

transitions between gel phase and fluid phase

Question 22

how does acyl chain length affect melting point

Answer 22

longer chain = higher melting point (more VDW interactions with tail)

Question 23

how does degree of saturation affect melting point

Answer 23

more unsaturated (more double bonds) = lower MP

• more double bonds = worse packing and less VDW interactions

Question 24

as temperature decreases, will you see more unsaturated or saturated fatty acids?

Answer 24

more unsaturated fatty acids (as temperature decreases, membrane becomes less fluid so you want to counter by making it more fluid)

Question 25

as temperature increases, will you see more unsaturated or saturated fatty acids?

Answer 25

more saturated fatty acids

(as temperature increases, membrane becomes more fluid so want to counter by making it less fluid ie increase number of VDW interactions between tails for tigher packing)

Question 26

does lipid bilayer viscosity affect membrane transport + enzymatic activities?

Answer 26

yes, that’s why cell membrane fluidity needs to be precisely regulated

Question 27

structure of cholesterol + features

Answer 27

(insert picture of cholesterol)

Question 28

describe effect of cholesterol below Tm

Answer 28

increase fluidity

(short acyl chain of cholesterol disrupts the local packing)

Question 29

describe effect of cholesterol above Tm

Answer 29

decrease fluidity

(interferes with freedom of acyl chains: rigid sterol rings stabilize the hydrocarbon chains, increase rigidity and intereferes with lateral diffusion)

Question 30

in general, what is the function of cholesterol

Answer 30

decreases the temperature dependancy of lipid bilayer fluidity

Question 31

draw graph of fluidity vs temperature with and without cholesterol

Answer 31

(insert graph of slide 24 graph)

Question 32

charge character of phosphatidolethanolamine

Answer 32

zwitterionic

Question 33

charge character of phosphotidylchlorine

Answer 33

zwitterionic

Question 34

charge character of phosphatidylserine

Answer 34

negatively charged

Question 35

inositol phospholipids

Answer 35

phospholipids with a 6-membered ring head group with hydroxyl

Question 36

how are different phosphatidylinositides produced

Answer 36

phosphorylation and dephosphorylation of inositol sugar head group

Question 37

why do different organelles have different PiP’s?

Answer 37

different organelles have different PiP kinases and phosphatases

Question 38

describe sphingolipid structure (backbone, head groups)

Answer 38

backbone: amino alcohol with sphingosine (acyl chain) and a second acyl chain attached to the sphingosine
headgroups: sphingomyelin or glyco group

Question 39

where is sphingomyelin most abundant in

Answer 39

membrane of axonal membranes (5-10% total lipid mass)

Question 40

describe phospholipid distribution of RBC and how this helps it

Answer 40

Different phospholipids in outer and inner monolayers:

Outer: PC + SM

Inner: PE + PS

Fn: allows distinguishing between dead and live cells (PS relocates to outside during apoptosis)

Question 41

function of PS in RBC

Answer 41

signals macrophages to phagocytose dead cell

Question 42

describe relationship between cholesterol and sphingolipids

Answer 42

cholesterol interacts preferentially with sphingolipids due to structure and saturation of hydrocarbon chains

Question 43

lipid composition of a liposome with

a) 1:1 PC and SM
b) 1:1:1 PC and SM and Chol

Answer 43

a) forms uniform bilayers
b) forms two phases due to aggregation of SM and Chol