Section 1: Lipid Bilayers

Lipids

• Associates w/ other molecules

• Aggregate to form micelles and bilayers

  • aggregates form the structural basis for biological membranes

Micelles

• Single-tailed amphiphiles form spheroidal or ellipsoidal micelles

• Globular aggregates

• Hydrocarbon groups are out of contact w/ water

• Arrangement

  • eliminates contact b/w water and hydrophobic tails

  • permits the solvation of the polar head groups

In micelle, what happens if there are too few lipid molecules

It exposes the hydrophobic core of the micelle to water

In micelle, what happens if there are too many lipid molecules

It would give the micelle an energetically unfavorable hollow center

What happens if there is a very large micelle

• Could flatten out to eliminate this hollow center

• As a result, there is a decrease of curvature at the flattened surfaced which would generate empty spaces

Micelle structure w/ glycerophospholipid and sphingolipids

• Two hydrocarbon tails

• rectangular cross section

• form large disk-like micelles

• resemble extended bimolecular leaflet

Liposomes

• Similar to cell membranes; however, there are no proteins embedded, just lipids

• sever as models of biological membranes

• Formed by suspension of phospholipids

• Closed, self-sealing solvent-filled vesicles

• Bound by only a single bilayer

• Quite stable

• absorbed by many cells through fusion w/ plasma membrane

  • hold promise as vehicles for drug delivery

How can lipsomes be purified

1. dialysis

2. gel filtration chromatography

3. centrifugation

Transverse diffusion (flip-flop)

• Transfer of lipid molecules across bilayer

• very slow process

• rare

• hydrated, polar head groups of the lipid have to pass through the anhydrous hydrocarbon core of the bilayer

• only half of the membrane does this during several days

Lateral diffusion

• Pairwise exchange of neighboring phospholipid molecules in the same bilayer leaflet

• lipids are highly mobile

• Rapid diffusion

Lipid bilayer viscosity (fluid resistance to flow)

• Viscosity depends on saturation and length of the fatty acid

• The ability of C-C bonds of the lipid tails to rate allows for constant motion of the lipid bilayer interior

• Viscosity of the bilayer increases dramatically when it is closer to the lipid head group

  • the further away fatty acids are from their head group, the more mobile they are

• Rotation of head groups is limited

Lipid bilayer viscosity changes with transition temperature

• Lipid bilayer cools below a characteristic transition temperature causing it to become a gel-like solid (fluidity decreases)

  • low temperatures → stiffening of the hydrocarbon tails

  • bilayer is thicker in the gel state

• Lipid bilayer above the transition temperature causes them to be more mobile

  • High temps have mobile tails

Bilayer above transition temperature

Bilayer below transition temperature

How does the transition temperature increase with?

1. chain length of its fatty acid residues

2. degree of saturation of its fatty acid residues

• Temperature ranges b/w 10-40˚C for most biological membranes

• Bacteria and cold-blooded animals modify fatty acid compositions of their membrane lipds

  • this maintains a constant level of fluidity

Cholesterol in lipid bilayers

• DO NOT form bilayer

• decreases membrane fluidity

• rigid steroid ring system in cholesterol interferes w/ the motions of the fatty acid side chains in other membrane lipids

• Cholesterol inhibts the ordering of fatty acid side chains by fittinng in between them → this broadens the temperature range of the phase transition