Biological Motivation Leaflet composition is important in cell

Biological Motivation • Leaflet composition is important in cell signaling, cell-to-cell adhesion, and ion transport • Lipid domains are though to be associated with many health problems including – Diseases – Alzheimer’s, Parkinson’s athsma, Tay. Sach’s, autoimmune – Bacterial infections – E. coli, Salmonella – Viral infections - Influenza, HIV, measles

Phospholipids • One main component of cell membranes • polar head • ionic phosphate group • attracted to water (hydrophilic) • hydrocarbon tail • fatty acid side chain groups • repelled by water (hydrophobic) • Saturation of fatty acid chains determines shape of tail

Membrane Composition • Phospholipids • Proteins • Used for transportation of substances across the membrane , cell communication, and adhesion • Integral proteins – may span both leaflets • Peripheral proteins – only associated with polar heads • Cholesterol • in some membranes • Involved in membrane fluidity • Hydroxyl group of cholesterol is oriented to interact with water inside and outside of cell • May also contain • Glycoproteins • Glycolipids • Lipoproteins • Channels and Pumps

Lipid Bilayer • Leaflet – one layer of the phospholipid bilayer • Arrangement – spontaneous and due to amphipathic nature of phospholipids • Fluid Mosaic Model • lipids are not in a fixes position • free lateral diffusion of lipids • flippase enzymes involved in movement from one leaflet to the other • less common due to the energy expense • Selectively permeable • About 7 nm thick

Domain Formation • Lipids of a certain type congregate together in the membrane • Domains are evaluated experimentally using fluorescence micrographs • Formation of domains seems to dependant on leaflet composition, temperature, and other factors • “Lipid rafts” are domains that are rich in cholesterol • Associated with sphingolipids • “rafts” are thicker than the surrounding leaflet

Questions to be answered with model: • Why do lipid domains form in some leaflet compositions and not in others? • Is there a biological advantage to lipid domain formation? • Is there a way to predict the probability of domain formation with given parameters?