1.1 - Membrane Lipids Flashcards
cell membrane system functions
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
phospholopid structure
what are the main phospholipids in mammalian cell membranes
1) PE
2) PS
3) PC
how long are the alkyl chains usually
14-22 carbons long
why do kinks form in the hydrocarbon tails of phospholipids (what kind of bonds)
due to cis-double bonds
why do fatty acid molecules cluster together in micelles
expose smallest possible hydrophobic surface surface area to water
how is the structure of the micelle stabilized
energy gained by freeing the immobilized water molecule stabilizes the micelle (the water molecules are less ordered and thus entropy of the system increases)
what shape do fatty acids form in water and why
micelles
- due to wedge-shape (head size greater than the side chain)
- acyl side chains protected from water in the core
what shape do phospholipids form in water and why
bilayers
- cylinder shaped (head and side chain similar sizes)
- acyl side chains protected from water (except for at ends)
can lipid bilayer have edges
no, form sealed compartments or vesicles
describe the possible movements of individual lipid molecules in a lipid bilayer
1) rotate on themselves
2) freely move within same leaflet
3) flip-flop (rare)
why can the lipid bilayer be seen as 2D fluid
the individual phospholipid molecules can be so fluid/flexible in their movements within the same lipid bilayer
how is lipid bilayer fluidity measured, and how does the process work
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
what happens in the gel phase
individual molecules do not move around much
what happens at fluid phase
individual molecules can move around
draw a typical fluidity vs temperature graph and label the melting point
(insert graoh from slide 18)
what happens if you heat a lipid bilayer in the gel phase
will go from gel phase to fluid phase
under physiological conditions, are membranes more gel-like or more fluid-like?
more fluid-like
what factors influence fluidity/freezing point of a phospholipid bilayer
1) acyl chain length
2) degree of saturation
what happens to phospholipid bilayer at the melting point
transitions between gel phase and fluid phase
how does acyl chain length affect melting point
longer chain = higher melting point (more VDW interactions with tail)
how does degree of saturation affect melting point
more unsaturated (more double bonds) = lower MP
- more double bonds = worse packing and less VDW interactions
as temperature decreases, will you see more unsaturated or saturated fatty acids?
more unsaturated fatty acids (as temperature decreases, membrane becomes less fluid so you want to counter by making it more fluid)
as temperature increases, will you see more unsaturated or saturated fatty acids?
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)
does lipid bilayer viscosity affect membrane transport + enzymatic activities?
yes, that’s why cell membrane fluidity needs to be precisely regulated
structure of cholesterol + features
(insert picture of cholesterol)
describe effect of cholesterol below Tm
increase fluidity
(short acyl chain of cholesterol disrupts the local packing)
describe effect of cholesterol above Tm
decrease fluidity
(interferes with freedom of acyl chains: rigid sterol rings stabilize the hydrocarbon chains, increase rigidity and intereferes with lateral diffusion)
in general, what is the function of cholesterol
decreases the temperature dependancy of lipid bilayer fluidity
draw graph of fluidity vs temperature with and without cholesterol
(insert graph of slide 24 graph)
charge character of phosphatidolethanolamine
zwitterionic
charge character of phosphotidylchlorine
zwitterionic
charge character of phosphatidylserine
negatively charged
inositol phospholipids
phospholipids with a 6-membered ring head group with hydroxyl
how are different phosphatidylinositides produced
phosphorylation and dephosphorylation of inositol sugar head group
why do different organelles have different PiP’s?
different organelles have different PiP kinases and phosphatases
describe sphingolipid structure (backbone, head groups)
backbone: amino alcohol with sphingosine (acyl chain) and a second acyl chain attached to the sphingosine
headgroups: sphingomyelin or glyco group
where is sphingomyelin most abundant in
membrane of axonal membranes (5-10% total lipid mass)
describe phospholipid distribution of RBC and how this helps it
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)
function of PS in RBC
signals macrophages to phagocytose dead cell
describe relationship between cholesterol and sphingolipids
cholesterol interacts preferentially with sphingolipids due to structure and saturation of hydrocarbon chains
lipid composition of a liposome with
a) 1:1 PC and SM
b) 1:1:1 PC and SM and Chol
a) forms uniform bilayers
b) forms two phases due to aggregation of SM and Chol
