Section 1 Part B Lipid Bilayers

Glycolipid

lipid with sugar residue attached (exterior of bilayer)

Glycoprotein

protein with sugar residue attached (exterior of bilayer)

Channel Protein

facilitate removal of waste/ export substances (embedded in bilayer)

Cholesterol

within membrane helps with rigidity

Lipids

• usually associate with other molecules

• aggregate to form micelles and bilayers

Lipid Aggregates

form the structural basis for biological membranes

Micelles

globular aggregates

• hydrocarbon groups out of contact with water

Micelle Arrangement

• eliminates contact between water and hydrophobic tails

  • permits the salvation of the polar head groups

Micelle Composition

• single-tailed amphiphiles form spheroidal or ellipsoidal micelles

  • # of molecules in micelle depends on the amphiphile

Micelle # of lipid molecules importance

• too many: energetically unfavorable hollow center

  • too few: expose hydrophobic core to water

Large Micelle Effects

• flatten out to eliminate hollow center

  • result in decrease of curvature at the flattened surfaces would generate empty space

plasma membrane looking

Glycerophospholipids and Sphingolipids

• 2 hydrocarbon tails

• rectangular cross section

• disk-like micelles

• resemble extended biomolecular leaflet

Liposomes characteristics

• formed by suspension of phospholipids

• closed, self-sealing solvent-filled vesicles

• single bilayer

• uniform in size

• stable

Liposome Functions

• models for biological membranes

• absorbed by many cells through fusion with plasma membrane

• used to deliver drugs to membrane

Liposomes purified by:

• dialysis

• gel filtration chromatography

• centrifugation

Liposome Drug Delivery System

• many systems depending on what we want to transport

Phospholipid Diffusion

• Transverse

• Lateral

transverse Diffusion

• flip-flop

• slow (rare)

  • lipid molecule across bilayer

Lateral Diffusion

• highly mobile in the bilayer plane (rapid)

• bilayer is a 2D fluid

• exchange of neighboring phospholipid molecules in the same bilayer leaflet

Lipid Bilayer Viscosity

• C-C bonds of lipid tails to rotate: constant motion of lipid bilayer interior

• viscosity of light machine oil and increases closer to the lipid head groups

How does head group rotation affect mobility

limited rotation

• father from head group increases mobility

• lateral constrained by interactions between head groups

Transition Temperature

• Below: bilayer becomes gel-like solid ( decrease in fluidity) stiff tails

• Above: liquid crystal (highly mobile lipids) kinked tails

In liquid crystal state (above transition temp)

• lipids are ordered in some directions but not in others

Transition temp increases with

• chain length of FA resides

• degree of unsaturation of FA residues

Transition temp range for most biological membranes

10-40 degrees celsius

Which organisms can modify FA compositions of their membranes to maintain constant fluidity?

• bacteria and cold blooded animals

Cholesterol as a membrane plasticizer

• does NOT form a bilayer

• decreases membrane fluidity

• rigid steroid ring interferes with motions of FA resides

• inhibits order of FA residues by fitting between them

• broadens temp range of phase transition