section 1: part 2 - lipid bilayers

generally describe what lipids associate with

other lipids

aggregate to form micelles and bilayers

form the structural basis of bio. membranes

Describe micelles

globular aggregates

hydrocarbon groups made of contact with water

single tail amphiphiles form spheroidal or ellipsoidal micelles

describe micelles arrangement

arrangement eliminates contact between water and hydrophobic tails

arrangement permits solvation of the polar head groups

Describe the importance of the number of lipid molecules

too few expose hydrophobic micelle core to H2O

too many give micelle an energetically unfavorable hollow center

What could happen with a very large micelle

could flatten out to eliminate a hollow center

resulting decrease in curvature at flattened surface would create empty spaces

Describe glycerophospholipids and sphingolipids generally

2 hydrocarbon tails

rectangular cross section

form large disk-like micelles

micelles resemble extended biomolecular leaflet

Describe liposomes

formed by suspension of phospholipids

closed, self-sealing solvent filled vesicles

bound by a single bilayer

uniform in size

stable

absorbed by cells through fusion with plasma membrane

why do liposomes hold promise for drug delivery

they are able to directly fuse with cell plasma membranes

how can liposomes be purified

dialysis

gel filtration chromatography

centrifugation

Describe transverse diffusion

transfer of lipid molcs. across a bilayer

extremely rare

hydrated, polar head group of lipid has to pass through anhydrous core

flip-flop rates have t(1/2) of several days or more

Describe lateral diffusion

lipids are highly mobile in the bilayer plane

lipids in a membrane can diffuse the 1 μm length of a bacterial cell in ~1 sec

describe lipid bilayer viscosity

ability of C-C bonds of lipid tail to rotate allowing for constant bilayer interior motion

viscosity like light machine oil

viscosity increases closer to head groups

head group rotation limited

lateral mobility head group constrained by interactions of other head groups

Describe transition temperature in relation to lipid bilayer viscosity

lipid bilayer cools below a characteristic temp. then it becomes a gel like solid (fluidity decreases)

above transition temp it becomes a highly mobile liquid (liquid crystal state)

describe the liquid crystal state

lipids ordered in some directions but not in others

What does transition temp increase with

chain length of FA residues

degree of FA residue sat.

what is the transition temp range of most biological membrane

10 and 40 Celsius

why do bacteria and cold blooded animals have to modify their FA compositions of membrane lipids

to maintain a constant level of fluidity

Describe how cholesterol functions as a membrane plasticizer

does not form a bilayer

decreases membrane fluidity

its steroid ring interferes with FA SCs motion in other membrane lipids

inhibits ordering of FA SCs by fitting between them -> broadens temp range of phase transition